POLYSTANNANES - Reaction Mechanism and Products

Research Collection

Doctoral Thesis

Polystannanesreaction mechanism and products

Author(s): Trummer, Markus

Publication Date: 2011

Permanent Link: https://doi.org/10.3929/ethz-a-006741780

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DISS. ETH No. 19 795

POLYSTANNANES - Reaction Mechanism and Products

A dissertation submitted to

ETH ZURICH

for the degree of

Doctor of Sciences

presented by

MARKUS TRUMMER

DI

22.03.1980

citizen of Austria

accepted on the recommendation of

Prof. Paul Smith, examiner

Prof. Walter Caseri, co-examiner

Prof. Frank Uhlig, co-examiner

PD Dr. Wolfram Uhlig, co-examiner

2011

Contents

Summary I

Zusammenfassung V

Chapter I 1

Introduction

Chapter II 17

Diorganostannide Dianions (R2Sn2−) as Reaction Intermediates Revisited:

In-situ 119Sn NMR Studies in Liquid Ammonia

Chapter III 39

Reaction Products of Dichlorodiorganostannanes with Sodium in Liquid

Ammonia: In-situ Investigations with 119Sn NMR Spectroscopy and

Usage as Intermediates for the Synthesis of Tetraorganostannanes

Chapter IV 73

Poly(dialkylstannane)s and Poly(diarylstannane)s Homo- and Copolymers

Synthesized in Liquid Ammonia

Chapter V 107

From Poly(dialkylstannane)s to Poly(diarylstannane)s:

Comparison of Synthesis Methods and Resulting Polymers

Chapter VI 133

Stability of Polystannanes Towards Light

Chapter VII 151

Conclusions and Outlook

Acknowledgements 165

Curriculum Vitae 167

I

Summary

Polystannanes are specified by a polymer main chain consisting of

covalently interconnected tin atoms, which is to our knowledge unprecedented for

other metals and therefore of fundamental interest. Due to the delocalization of

the electrons in the polymer backbone (σ-delocalization), polystannanes are

potentially appealing materials concerning their chemical, optical, thermal and

electrical properties.

The first synthesis providing pure high molar mass polystannanes was based

on dehydropolymerization of dialkylstannanes (H2SnR2) with the catalyst

[RhCl(PPh3)3] (Wilkinson’s catalyst). This route allows to obtain and isolate pure

linear poly(dialkylstannane)s without cyclic oligomers; but on the other hand, has

some substantial drawbacks. In particular, this method has so far not been suited

to synthesize poly(diarylstannane)s. Hence, to create such materials a new

synthetic route is required, for instance reaction of dichlorodiorganostannanes in

liquid ammonia.

It has frequently been proposed that diorganostannide dianions, SnR22-,

form during reactions of dihalodiorganostannanes with sodium in liquid ammonia.

The formation of this intermediate has been advanced to be an important step in

the synthesis of polystannanes. However, our investigations conducted with 119Sn

NMR spectroscopy in liquid NH3 of reaction intermediates formed in-situ during

the exposure of dichlorodiphenylstannane, dichlorodibutylstannane and dichloro-

dioctylstannane to a stoichiometric amount of sodium (i.e. 4 molar equivalents

sodium per tin atom) unveiled that the proposed SnR22- dianion was not present.

Tetraorganodistannides, (R2Sn-SnR2)2-, and hydrodiorganostannides (tin hydri-

II

des), R2SnH-, were detected instead. Also, products resulting from mixtures of

R2SnCl2/Na ratios of 1:3 to 1:10 were soluble and, hence, could be studied in-situ

in liquid ammonia with 119Sn NMR spectroscopy. The composition of the

respective compounds was found to be essentially independent of the R2SnCl2/Na

ratio. Our experiments showed that the chemical structure of the in-situ generated

species did not permit to draw conclusions about the composition of the

corresponding reaction products with bromoethane and vice versa – a practice

commonly employed. Furthermore, we observed migration of butyl groups both

in-situ during the reaction of dichlorodibutylstannane with sodium in liquid

ammonia, as well as in the final reaction products. By contrast, in the case of

phenyl substituents, migration was not detected in liquid ammonia, unless a large

excess of sodium was present. These observations imply a different mechanism for

butyl and phenyl group migration.

At a molar ratio of R2SnCl2/Na of 1:2, polystannanes precipitated from the

reaction mixture, in some cases accompanied by cyclic oligostannanes. Therefore,

in the case of dichlorodibutylstannane, Bu2SnCl2, and dichlorodiphenylstannane,

Ph2SnCl2, two different reaction pathways could be applied: the monomers were

either directly treated with 2 molar equivalents of sodium, or the reactive

organostannides formed in-situ were further converted with the respective

R2SnCl2. The polymers obtained with the new synthesis route were compared to

the products obtained in polymerization with Wilkinson’s catalyst and tetra-

methylethylenediamin (TMEDA). The route employing Wilkinson’s catalyst was

most beneficial for preparation of poly(dibutylstannane) and TMEDA for

polystannanes containing at least one aromatic group per Sn atom, whereas

synthesis in Na/NH3 yielded best results for polystannanes comprising two

aromatic groups per Sn atom – poly(diarylstannane)s.

III

To expand the range of polystannanes, poly(diarylstannane)s and copoly-

mers of dialkylstannanes (butyl, octyl and dodecyl) and diarylstannanes were

synthesized and characterized. UV/Vis absorption spectroscopy unveiled the

presence of σ-delocalization and σ-π-delocalization in the copolymers with the

σ-π-delocalization originating from the SnPh2 moieties in the polymer. The

copolymers were mainly soluble, dichroic materials which could easily be oriented.

Depending on the length of the alkyl side chain, the orientation was parallel or

perpendicular to the direction of external stimuli.

Finally, the influence of pendant side groups on the stability towards light

of polystannanes in solution was studied; more specifically poly[bis(4-butyl-

phenyl)stannane] and poly(dibutylstannane) in solutions of tetrahydrofuran and

dichloromethane. In both solvents, the poly(diarylstannane) was found to be more

resistant towards light than the poly(dialkylstannane). Experiments with laser flash

photolysis and gel permeation chromatography (GPC) analysis of irradiated

polymer solutions resulted in the conclusion that two different decomposition

mechanisms can occur: either random scission of polymer chains or unzipping,

depending on the polymer architecture and the nature of the solvent.

V

Zusammenfassung

Polystannane sind Verbindungen mit kovalent gebundenen Zinn-Atomen

im Polymerrückgrat. Solche Substanzen sind für andere Metalle bislang unbekannt

und deswegen von fundamentalem Interesse für die Wissenschaft. Die Delokalisa-

tion der Elektronen entlang der Polymer Hauptkette macht Polystannane zu

attraktiven Materialien in Bezug auf ihre chemischen, optischen, thermischen und

elektronischen Eigenschaften.

Die erste Synthese reiner Polystannane mit hoher molarer Masse gelang

mittels der katalytischen Dehydropolymerisation von Dialkylstannanen Alk2SnH2

mit dem Katalysator [RhCl(PPh3)3] (Wilkinson Katalysator). Dieser Weg

ermöglicht es, reine Polymere ohne zyklische Nebenprodukte zu erhalten, birgt

allerdings den Nachteil, dass Diarylstannane Ar2SnH2 nicht zu Poly(diaryl-

stannane)n umgesetzt werden können. Zur Herstellung dieser erstrebenswerten

Materialien, musste ein neuer synthetischer Ansatz gefunden werden: Die

Reaktion von Dichlordiorganostannanen mit Natrium in flüssigem Ammoniak.

Generell wurde bis anhin in der Literatur davon ausgegangen, dass die

Umsetzung von Dichlordiorganostannanen mit Natrium in flüssigem Ammoniak

zur Bildung des Dianions R2Sn2- führt. Die entstehenden Produkte sind ein

wichtiger Schritt bei der Umsetzung zu Polymeren und wurde deswegen mittels

in-situ 119Sn NMR Messungen in flüssigem Ammoniak genauer untersucht. Dabei

zeigte sich, dass bei eine stöchiometrischen Umsatz (4 molare Equivalente

Natrium pro Zinn-Atom) das Dianion R2Sn2- nicht gebildet wird. In der Lösung

detektiert wurden Tetraorganodistannide (R2Sn-SnR2)2- und Diorganohydro-

stannide R2SnH-. Auch durch die Veränderung des Stannan : Natrium

VI

Verhältnisses zwischen 1:3 und 1:10 konnte das Dianion nicht erzeugt werden,

vielmehr wurde festgestellt, dass die entstehenden Zwischenprodukte größtenteils

unabhängig von der eingesetzten Menge an Natrium sind. Weiters wurden die

Reaktionslösungen mit Bromethan umgesetzt. Diese Experimente zeigen keinen

direkten Zusammenhang zwischen den in-situ gebildeten Stanniden und den

später gebildeten Reaktionsprodukten, welche früher öfters zur Identifizierung der

Zwischenprodukte herangezogen wurden. Bei der Reaktion von Dichlordibutyl-

stannan mit Natrium wurde eine Wanderung der Alkylgruppen sowohl in

flüssigem Ammoniak als auch bei den Reaktionsprodukten mit Bromethan

festgestellt. Im Gegensatz dazu wurden bei aromatischen Substituenten ähnliche

Phänomene nicht gefunden, außer es wurde ein sehr großer Überschuss an

Natrium eingesetzt. Dies lässt auf unterschiedliche Mechanismen bei der Alkyl-

und Arylgruppen Migration schließen.

Ein Mischungsverhältnis von 1:2 zwischen R2SnCl2 und Natrium führt zur

Bildung von Polystannanen, teilweise begleited von zyklischen Oligostannane.

Dadurch konnte zur Herstellung von Polystannanen zwischen zwei Wegen

gewählt werden: einerseits die direkte Route mit zwei molaren Equivalenten

Natrium oder die Umsetzung der in-situ gebildeten, reaktiven Zwischenstufen mit

weiterem R2SnCl2. Die erhaltenen Polymere wurden mit den Produkten der

Dehydropolymerisation mit dem Wilkinson Katalysator und Tetramethylethylen-

diamin (TMEDA) verglichen. Es stellte sich heraus, dass der Wilkinson Katalysa-

tor die besten Resultate für die Synthese von Poly(dialkylstannan)en liefert,

hingegen der Weg mit TMEDA für Polymere mit mindestens einer Arylgruppe

pro Zinn die beste Leistung zeigte. Die Synthese mit Natrium in flüssigem

Ammoniak lieferte die besten Resultate für Stannane mit zwei aromatischen

Gruppen pro Zinn-Atom.

VII

Durch die Herstellung von Poly(diarylstannan)en und Copolymeren

zwischen Dialkylstannanen (Butyl, Octyl, und Dodecyl) und Diarylstannanen

wurde der Bereich der zugänglichen Materialien deutlich erweitert. Mittels

UV/Vis Spektroskopie wurde die -Delokalisierung des aliphatischen- und die -

-Delokalisierung des aromatischen Anteils in den Polymeren verdeutlicht. Die

Copolymere sind größtenteils lösliche, dichroitische Materialien, die leicht

orientierbar sind. Je nach Länge der Alkyl-Seitenkette orientiert sich die Sn-Sn

Hauptkette parallel oder senkrecht zur Orientierungsrichtung.

Am Schluss wurde noch der Einfluss der Seitenketten auf die Lichtstabilität

in Lösung erforscht. Dazu wurden Lösungen von Poly[bis(4-butylphenyl)stannan]

und Poly(dibutylstannan) in THF und Dichlormethan untersucht. In beiden

Lösungsmitteln war das aromatische Polymer deutlich stabiler gegenüber Licht-

einflüssen, verglichen mit Poly(dibutylstannan). Blitzphotolyse und gelperme-

ations-chromatographische Untersuchungen an bestrahlten Proben enthüllten

zwei unterschiedliche Abbaumechanismen: zufällige Hauptkettenspaltung und

fortlaufender Kettenabbau.

Chapter I

Introduction

3

Preface

Tin is located in group 14 of the periodic table and the first metal therein. In

fact, the elements in this group - carbon, silicon, germanium, tin and lead -

represent the transition from non-metallic elements (C), to semi-metals (Si, Ge)

and metals (Sn, Pb). Notably, elemental tin exists in two allotropes that also

indicate this change. Grey tin, or the -form is a semiconductor with the same

diamond type crystal structure like the prominent crystalline semiconductors

silicon and germanium, whereas the second modification, white – or - tin is a

silvery, ductile metal with a distorted octahedral structure and a melting

temperature of 232 °C. It is a typical metallic conductor. The metallic form slowly

converts into grey tin below 13 °C (tin disease or tin pest) [1-3].

Organotin Compounds

First reports of organotin compounds, i.e. species comprising both Sn and

organic moieties, date back to 1849, when Frankland heated iodomethane in the

presence of metallic tin [4, 5]. Thereafter, formation of these so-called

organostannanes was also reported by reaction of tin/sodium with iodoalkanes [6]

and by exposure of organozinc or organomercury compounds to tin halides [7].

The yields of these reactions were not satisfying due to the occurrence of secondary

reactions. This could be improved by application of Grignard reactions as shown

by Pope and Peachey for tetraalkyltin derivatives [8], as well as by Pfeifer et al. for

alkyl- and arylstannanes [9, 10]. Synthesis of chlorotriphenylstannane by Krause in

1920 via Grignard reaction and further conversion with Na to hexaphenyldi-

stannane Ph3Sn-SnPh3 in benzene and absolute alcohol resulted in high purities

and good yields of the species [11]. The compound was identified by its elemental

composition and was the first example of an aromatic distannane. The existence of

4

Scheme 1. Preparation of organotin (IV) compounds starting from SnO2 by subsequent alkylation

of SnCl4 with organometallic reagents (RMgX or RLi) and Kocheshkov comproportionation [1].

hexaethyldistannane was already shown with vapor-density measurements by

Ladenburg in 1870 [12], and related work was continued by the synthesis of

various hexaalkyldistannanes by Grüttner [13]. The oxidation number of tin in the

distannanes amounts to +III, in contrast to the most common organic and

inorganic compounds which comprise Sn(II) or Sn(IV). In 1964, Neumann and

König [14] published an excellent overview about compounds with the proposed

structure (SnPh2) obtained by various available methods; in addition, these authors

reported the synthesis of pure dodecaphenylcyclohexastannane in excellent yields

by catalytic condensation of diphenyltin dihydride and by coupling of dichlorodi-

phenylstannane with sodium naphtalide.

Nowadays, aromatic and aliphatic organostannanes are extensively used in

organic chemistry for the formation of C-C bonds in small molecules, as well as

polymers, by palladium-catalyzed Stille cross-coupling [15-18]. Typical industrial

production of organostannanes utilizes the most common tin source, i.e. the oxide-

ore-mineral cassiterite (SnO2), as illustrated in Scheme 1. Industrial applications of

such compounds are the stabilization of poly(vinylchloride) [19-21] (PVC, 20 000

tons of tin/year [22]) and antifouling agents [22]. While the market of PVC stab-

ilization is still growing, that of antifouling is decreasing due to environmental

issues.

SnCl4SnO2

C

CO2

SnCl4

SnCl2

SnCl4 SnR4

MR

MClRxSnCl4-x

+200 °C

+ 2

+ 4

4

+

5

Liquid Ammonia as Medium for the Synthesis of Organostannanes

Reactions of sodium and ammonia were first described by Davy in 1807 [23].

Subsequently, Weyl dissolved metallic sodium and potassium together with

mercury in liquid ammonia in 1864 (“Weylsche Flüssigkeiten”) [24, 25]. In 1878,

Bleekrode [26] mentioned liquid ammonia as a good electric conductor and

observed a blue coloration by applying an electric current on pure liquid ammonia,

and Cady [27] reported in 1897 a high conductivity of sodium solutions in liquid

ammonia. Kraus explained this behavior and the intense blue color of the solutions

by advancing the concept of solvated electrons – i.e. sodium dissolved in liquid

ammonia acts like metal cations and solvated electrons in equilibrium with metal

atoms [28]. This approach gained large interest in organic chemistry after Birch

introduced reduction of aromatic compounds, e.g. benzene to 1,4-cyclohexadiene,

by the action of electrons dissolved in liquid ammonia and ethanol [29-34].

First reports of reactions of organostannanes with sodium in liquid ammonia

were published by Kraus in 1925 [35, 36] with chloro- and bromomethylstannanes

and Chambers 1926 [37] with chloro- and bromophenyltin compounds. Attempts

to produce free SnR2 moieties failed in both cases due to the formation of

polymeric species. As Kraus stated: “In no case was the molecular weight found to

correspond, even roughly, to the monomolecular formula” [35]. Reactions of mono- and

dichloroorganostannanes with sodium in liquid ammonia and subsequent exposure

to haloalkanes resulted in the formation of tetraorganostannanes and, depending

on the sodium : stannane ratio, also di-, tri- and pentastannanes were proposed

[35, 38, 39]. Therefore, interest arose on products of the reaction between

haloorganostannanes and sodium in liquid ammonia. The reaction intermediates

were analyzed by conductivity measurements in liquid ammonia [40-42], or by

synthetic experiments and characterization of the obtained products after removal

6

of the liquid ammonia [38, 39, 43-52]. It was shown that sodium salts of

stannanes behave like strong electrolytes with a high degree of dissociation in

liquid ammonia. However, neither the reaction intermediates that were formed in-

situ with sodium in liquid ammonia, nor the obtained polymeric products were

explored and characterized to satisfaction until now.

Polystannanes

Polymeric materials, i.e. species consisting of a backbone of covalently bound

group 14 elements are known for all corresponding elements, starting from the

traditional organic polymers based on a backbone of carbon atoms (e.g.

polyethylene -CH2-) [53, 54] to polysilanes [55-57], polygermanes [58],

polystannanes as well as (poly)plumbanes (diplumbanes) [59]. The characteristics

of the -bond in the polymer backbone change significantly from the C-C bond to

the Sn-Sn bond. The interaction between the Sn sp3 orbitals in the chain give rise

Scheme 2. Common synthesis methods of polystannanes starting from

dichlorodiorganostannanes, R2SnCl2, with Wurtz-type coupling or electropolymerization, and

from dihydrodiorganostannanes, R2SnH2, by catalytic dehydropolymerization with release of

hydrogen, H2.

Sn

R

R

ClCl

Sn

R

R

ClCl

Sn

R

R

HH

Sn

R

R

Sn

R

R

Sn

R

R

Wurtz-Type Coupling

Electrochemical Synthesis

Catalytic Dehydropolymerization

x

x

Na / Tol.

e-

catalyst

- NaCl

- Cl2

- H2

x

x

x

7

to delocalization of the electrons which is described as linear combination of

* Sn-Sn orbitals [22, 60]. Due to the electronic configuration of Sn, [Kr] 4d10 5s2

5p2, also overlapping with d-orbitals is possible, which could lead to σ--deloca-

lization of electrons, which is not very pronounced for silicon and not possible for

carbon (C: 1s2 2s2 2p2).

Polystannanes were first described by Löwig already in 1852 [61] by reaction

of Sn/K and Sn/Na alloys with iodoethane. The elemental composition of the

obtained materials corresponded to the formula (SnEt2). Also Cahours found

similar products by heating of iodoethane with metallic tin [62-65].

Today, Wurtz-type coupling with Na in organic solvents is still used to obtain

high molar mass poly(dialkylstannane)s (Scheme 2) [66-69], even though low

yields and (cyclic) oligomeric byproducts are commonly reported. Electrochemical

synthesis of dichlorodiorganostannanes, R2SnCl2, has also been applied to create

poly(dialkylstannane)s (Scheme 2), as well as polystannane-polysilane and

polystannane-polygermane copolymers and polystannane networks [70-73].

Formation of poly(dialkylstannane)s by catalytic dehydropolymerization of

R2SnH2 with R = alkyl was often mentioned in literature [74-78], but only

recently, Choffat et al. developed a synthesis route that resulted in linear

poly(dialkylstannane)s in high yields and without the occurrence of cyclic

oligomers, by reaction of Alk2SnH2 with Wilkinson’s catalyst [RhCl(PPh3)3]

(Scheme 2) [79-81]. This procedure allowed for systematic investigation of the

materials properties of polystannanes without the influence of those of the

oligomers. It was found, for instance, that poly(dibutylstannane) is highly

birefringent at room temperature [80], it could easily be oriented by various

methods [82] and possesses a high mobility of charge carriers along the tin atoms

in the polymer chain [83]. Unfortunately, the stability towards light of the

8

materials produced was limited [84]. One possible option to enhance that stability

could be to employ aryl moieties [85, 86] due to the enhanced delocalization of the

electrons (-delocalization), which was also observed for poly(diarylsilanes) by

West [55]. However, in the case of poly[bis(-phenylalkyl)stannane]s [87] this

also resulted in poor resistance.

Only a limited number of reports refer to the synthesis of

poly(diarylstannane)s. Unfortunately, experiments with polymerization via the

catalyst Cp2ZrMe2 by Lu and Tilley yielded only mixtures with cyclic- and low

molar mass oligomers.

9

Objective and Scope

The main objective of this thesis was to extend the available range of

polystannane compounds from poly(dialkylstannane)s to polystannanes with

aromatic side groups. This study also included a range of materials properties of

those materials, especially concerning their interaction with light. A high stability

towards light might be associated with a bathochromic shift in the absorption

maximum, i.e. a lower band-gap and therefore potentially higher conductivity,

compared to the poly(dialkylstannane)s produced so far. As catalytic dehydropoly-

merization with Wilkinson’s catalyst was not successful for the synthesis of poly-

stannanes with aromatic side groups [87], the objective was to develop an adequate

synthesis route for such polymers and thoroughly characterize them.

Thus, in Chapter II and III the reaction of sodium in liquid ammonia and di-

chlorodibutylstannane and dichlorodiphenylstannane is revisited to evaluate the

applicability of this route to produce polystannanes. Reaction intermediates were

characterized in-situ by 119Sn NMR spectroscopy and exposed to bromoethane to

analyze the reaction products.

In Chapter IV poly(dibutylstannane) and poly(diphenylstannane) prepared

with sodium in liquid ammonia are presented, and the synthesis of copolymers of

poly(diphenylstannane) and various dialkylstannanes, ranging from butyl to

dodecyl explored.

A study of the applicability of different synthetic routes to poly(dialkyl-

stannane)s and poly(diarylstannane)s, including the catalytic dehydropolymeriza-

tion, the reaction of sodium and dichlorodiorganostannanes in liquid ammonia and

of dihydrodiorganostannanes with N,N,Nʹ,Nʹ-tetramethyl-1,2-ethylendiamin,

10

TMEDA, is presented in Chapter V. These experiments were performed in

cooperation with Prof. Frank Uhlig and Dr. Marie-Luise Lechner (TU-Graz).

In Chapter VI the stability of poly(diarylstannane)s and poly(dialkylstannane)s

in solution towards light was explored by laser flash photolysis and irradiation

experiments, in collaboration with Dr. Thomas Nauser from the Laboratory of

Inorganic Chemistry at ETH Zürich.

Finally, general conclusions and an outlook on the prospering future of poly-

stannanes are presented in Chapter VII.

11

This thesis is based on manuscripts, which have been published, have been

submitted for publication or are in preparation:

Chapter II:

M. Trummer, W. Caseri, Diorganostannide Dianions (R2Sn2−) as Reaction

Intermediates Revisited: In-situ 119Sn NMR Studies in Liquid Ammonia,

Organometallics, 29 (2010) 3862-3867.

Chapter III:

M. Trummer, J. Zemp, C. Sax, P. Smith, W. Caseri, Reaction Products of

Dichlorodiorganostannanes with Sodium in Liquid Ammonia: In-situ

Investigations with 119Sn NMR Spectroscopy and Usage as Intermediates for the

Synthesis of Tetraorganostannanes, J. Organomet. Chem., (2011) accepted.

Chapter IV:

M. Trummer, D. Solenthaler, P. Smith, W. Caseri, Poly(dialkylstannane)s and

Poly(diarylstannane)s Homo- and Copolymers Synthesized in Liquid Ammonia,

(2011) submitted

Chapter V:

M.-L. Lechner, M. Trummer, I. Bräunlich, P. Smith, W. Caseri, F. Uhlig, From

Poly(dialkylstannane)s to Poly(diarylstannane)s: Comparison of Synthesis

Methods and Resulting Polymers, Appl. Organomet. Chem., (2011) submitted.

Chapter VI:

M. Trummer, T. Nauser, M.-L. Lechner, F. Uhlig, W. Caseri, Stability of

Polystannanes Towards Light, Polym. Degrad. Stab., (2011) submitted.

In addition, the following publication is related to this work:

M. Trummer, F. Choffat, M. Rämi, P. Smith, W. Caseri, Polystannanes –

Synthesis and Properties, Phosphorus, Sulfur, and Silicon, 186 (2011) 1-3

12

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Chapter II

Diorganostannide Dianions (R2Sn2-) as Reaction

Intermediates Revisited:

In-Situ 119Sn NMR Studies in Liquid Ammonia

19

1. Introduction

The in-situ formation of organotin anions in liquid ammonia is a key step in

the synthesis of a wide spectrum of organic tin compounds [1-7]. In this context,

diorganostannides R2Sn2- have been advanced as intermediates for different

organometallic syntheses, including deuteration of carbonyl compounds [8],

preparation of asymmetric stannanes [9, 10] and formation of polymers [11].

However, conclusive spectroscopic evidence for the formation of diorganostannide

dianions is not available; for instance 119Sn Mössbauer spectroscopy [12] did not

lead to identification of specific compounds. Driven by our interest in the synthesis

of macromolecular polystannanes [13-19], we revisited the purported existence of

diorganostannide dianions. Diorganostannides are commonly generated in-situ by

reaction of dihalodiorganostannanes with sodium in liquid ammonia; the

intermediate products are further converted for synthetic purposes to tetra-

organostannanes according to Scheme 1. Early reports from Kraus et al. [20, 21]

propose the formation of trimethylstannide, SnMe3-, and dimethylstannide

dianion, SnMe22-, by reaction of sodium with bromotri methylstannane, Me3SnBr,

or dibromodimethylstannane, Me2SnBr2, respectively. Depending on the applied

ratio between the dibromodimethylstannane and sodium, also oligomeric stannides

of the type (Me2Sn)x2- were postulated. Similar conclusions were drawn for the

formation of monostannides and distannides upon conversion of the corresponding

Scheme 1. Formation of the previously proposed diorganostannide dianions by conversion of

diorganostannanes with sodium in liquid ammonia and subsequent reaction with haloalkanes [8-

10, 21-25] (Y = Cl, Br, H; X = Cl, Br, I).

Sn

R

R

YY Na Sn2-

R

R

NH3Sn

R

R

R'R'+, 195 K R'X+, ℓ

20

phenyl [22] and ethyl halostannanes [23]. More recently, synthesis of substituted

diaryldimethylstannanes was proposed to proceed via the intermediate Me2Sn2-;

but the formation of the dimeric species (Me2Sn)22- was anticipated, since the

dimer (PhMe2Sn)2 emerged among the reaction products [25]. In-situ formed

tetraorganodistannides were suggested to arise on the basis of crystallization of

salts comprising the dimeric dianion (Ph2Sn)22- from lithium-treated dichlorodi-

phenylstannane in liquid ammonia in the presence of tetraammine lithium [26] or

(18-crown-6)diammine potassium [27] counterions.

The dianion SnMe22- is believed to be formed by reaction of dihydrodimethyl-

stannane (dimethyltin dihydride, dimethylstannane), Me2SnH2, with sodium in

liquid ammonia. Conductivity measurements [28-30] and conductometric titration

of the sodium with dihydrodimethylstannane [24] (and also stannane, SnH4 [31])

were interpreted to indicate the formation of ionic products. A conductivity

minimum was found at a Me2SnH2:Na ratio of ca. 0.5. Up to this ratio,

development of one molar equivalent hydrogen gas per dimethylstannane was

reported, which might at first glance point to the formation of dimethylstannide

dianions by replacement of the hydrogen atoms. At larger Me2SnH2:Na ratios,

however, less than one equivalent hydrogen per dimethylstannane evolved, and it

was assumed – but not proven – that dimethylstannide reacted with dimethyl-

stannane also to form tetramethyldistannide, (SnMe2)22-, or hydrodimethyl-

stannide, Me2SnH-, respectively.

In order to bring to light the nature of the intermediate form of stannides

resulting from the reaction of dihalodiorganostannides or dihydrodiorgano-

stannides with sodium, we employed 119Sn NMR spectroscopy in liquid ammonia

to identify the in-situ formed species – previously claimed to be R2Sn2- – and

examined their reactivity.

21

2. Results and Discussion

In-situ electric conductivity. Reactions were carried out by treatment of dichloro-

dibutylstannane, Bu2SnCl2, and dichlorodiphenylstannane, Ph2SnCl2, with four

molar equivalents of sodium in liquid ammonia. This stoichiometric ratio is just

required for the formation of the hypothetical SnR22- under release of two

equivalents of NaCl as byproduct (upon application of two molar equivalents of

sodium, yellow precipitates emerged, apparently oligostannanes or polystannanes).

In order to determine the time needed for completion of the reactions, its course

was monitored in-situ by recording the electric conductivities of the reacting

solutions (Figure 1). The conductivity reached a plateau value within 30 min

during the conversion of both stannanes with sodium – implying that the reaction

had terminated. The conductivities at the plateau value corresponded to 30 % and

20 % of the initial value for dichlorodiphenylstannane and dichlorodibutyl-

Figure 1. Electric conductivity of reaction mixtures upon addition of dichlorodiphenylstannane,

Ph2SnCl2 (□), or dichlorodibutylstannane, Bu2SnCl2 (■) to a solution of sodium in liquid

ammonia at 195 K (t = 0 indicates the moment of stannane addition; conductivity levels are

normalized for comparison to 100 % at t = 0). Insert: Schematic illustration of the experimental

setup of the in-situ conductivity measurements.

0 20 40 60 800

20

40

60

80

100

Ele

ctric

co

nd

uct

ivity

/ %

Time / min

N2

0 4020 60 80

Time / min

100

60

40

20

0

80

Ele

ctric

alco

nduc

tivity

/ %

22

stannane, respectively. Qualitatively, a pronounced decrease in conductivity is

expected indeed, as highly mobile and conductive solvated electrons, generated

upon dissolution of sodium in liquid ammonia [32], are consumed by the reaction

with the stannanes to form negatively charged stannides.

In-situ 119Sn NMR spectroscopy. In order to characterize the in-situ formed

stannides, the solutions in liquid ammonia were transferred to NMR tubes and

119Sn NMR spectra were recorded at 200 K and 220 K. The higher of these

temperatures resulted in a significantly better resolution of 119Sn-1H and 119Sn-2D

couplings, but was relatively close to the boiling temperature of ammonia;

therefore a temperature of 200 K was adopted for measurements which were

unrelated to the aforementioned couplings. In particular at very long measurement

times, often additional signals emerged, probably due to slow diffusion of water or

oxygen from the atmosphere through the cap of the NMR tube lead to subsequent

reactions.

Reaction of dichlorodiphenylstannane and dihydrodiphenylstannane with Na in

liquid ammonia. 119Sn NMR spectra of dichlorodiphenylstannane treated with

sodium in liquid ammonia featured two strong signals at -132 ppm and -197 ppm

when measured at 200 K (Figure 2a). The chemical shifts showed a pronounced

temperature dependence (Δδ = 0.2 to 0.8 ppm K-1; Table 1). Notably, the same

signals were found when dihydrodiphenylstannane, Ph2SnH2, was exposed to four

molar equivalents of sodium (Figure 2b), i.e. under conditions where the formation

of diorganostannide dianions was expected [24], indicating that in the cases of

dichlorodiphenylstannane and dihydrodiphenylstannane the same products are

formed, although possibly via different processes. The signal at -132 ppm detected

23

Figure 2. 119Sn NMR spectra recorded in-situ in liquid ammonia (at 200 K) of reaction products

of (a) Ph2SnCl2, (b) Ph2SnH2 and (c) Bu2SnCl2 formed during treatment with four molar

equivalents of sodium.

at 200 K (-116 ppm at 220 K) is accompanied by those of satellites of tin (totally

~8 % of the integrated intensity of the main signal, corresponding to the natural

abundance of 117Sn) with a coupling constant 1J(119Sn,117Sn) = 1950 Hz (Figure 3a)

This indicates the presence of a binuclear species in the solution, with the coupling

constant being in the range common to distannanes [33]. Hence, we attribute this

signal – which remained unaffected in hydrogen-coupled 119Sn NMR spectra – to

tetraphenyldistannide, (Ph2Sn)22- [1]. Remarkably, the signal at -193 ppm (measured at

220 K) split up into a well-defined doublet in the hydrogen coupled spectra indicating the

presence of a monohydrostannide (Figure 3a; the coupling typically poorly resolved at 200

K); the coupling constant 1J(Sn,H) = 145 Hz is in good agreement with the coupling

constant for hydrostannides reported earlier [34]. Therefore, we assign this signal to

hydrodiphenylstannide, Ph2SnH-. Obviously, we failed to detect any evidence for the

presence of diphenylstannide, Ph2Sn2- – in surprising variance with literature.

a

b

c

300 200 100 0 -100 -200

Chemical shif t δ / ppm

-300

24

Figure 3. (a) Proton-coupled 119Sn NMR spectrum of Ph2SnCl2/Na in liquid ammonia (at 220

K). The signal at -116 ppm shows a 119Sn-117Sn coupling with a coupling constant of 1950 Hz

and the signal at -192 ppm a 119Sn-1H coupling with a coupling constant of 145 Hz. (b)

Proton-coupled and (c) proton-decoupled 119Sn NMR spectrum showing the hydride region of

Ph2SnD2/Na in liquid ammonia after 60 min.

Reaction of dideuterodiphenylstannane with Na in liquid ammonia. In order to

elucidate if ammonia is the hydrogen source for the hydrodiphenylstannide

resulting from dihydrodiphenylstannane, or if the corresponding hydrogen atom

remained from the starting compound, dideuterodiphenylstannane, Ph2SnD2, was

-190 -192 -194 -196 -198 -200

c

b

-100 -120 -140 -160 -180 -200

a

Sn Sn Sn H



25

exposed to four equivalents of sodium in liquid ammonia. Water and other highly

active hydrogen donating impurities can be excluded as hydrogen source, since

they are expected to react rapidly with sodium in liquid ammonia, before the

addition of the stannanes.

After exposure of dideuterodiphenylstannane to four equivalents of sodium in

liquid ammonia for 60 min, the 119Sn NMR spectrum at 220 K showed, as

expected, on one hand the signal at -116 ppm of tetraphenyldistannide (see above),

and on the other hand two signals in the hydride region. A small signal, which

appeared as a singlet at -192.9 ppm in the proton-broadband-decoupled spectrum

(Figure 3c), split into a doublet in the proton-coupled spectrum (Figure 3b; a

series of additional spectra recorded at reaction times between 30 min and 10 h are

displayed in Figure 4), which is indicative of the above-mentioned hydroidphenyl-

stannide. Further, a pronounced three line feature with 1J(Sn,D) = 25 Hz at

-196.8 ppm (center peak) is indicative of the presence of deuterodiphenylstannide,

Ph2SnD-. (NB the three signals in the proton-coupled spectra are poorly resolved

due to additional couplings with the phenyl protons). The intensity of the

Ph2SnH- signal observed after one hour is by far too strong to be due to residual

hydrogen atoms in the starting compound Ph2SnD2 (as evident from analysis of 1H

and 119Sn NMR spectra of Ph2SnD2 in organic solvents). The ratio between

hydrodiphenylstannide and deuterodiphenylstannide increased steadily during a

period of more than one week, indicating that further H-D exchange occurred very

slowly (~25 % exchange after 12 h at 220 K and 45 % after one week, monitored

directly in the NMR test tube). It appears, therefore, that at least two processes

contribute to the H-D exchange: one largely advancing within one hour or less,

probably along the reaction path from Ph2SnD2 to Ph2SnD-, and another one

lasting for days or weeks. Since Ph2SnD2 itself is essentially insoluble in liquid

ammonia (it only dissolves upon treatment with sodium) it seems unlikely

26

Figure 4. 119Sn NMR spectra of in-situ formed products resulting from Ph2SnD2 exposed to four

molar equivalents of sodium in liquid ammonia, measured at 220 K at different reaction times in

order to monitor Ph2SnH-. The series shows certain fluctuations of the position of the main

signal, attributed to Ph2SnD-, most likely reflecting temperature fluctuations during the

experiment. Temperature fluctuations, which are expected to be more pronounced in the initial

phase where the temperature of the sample is adjusted, are assumed to cause the broadness of the

signal obtained after 30 min (note that 2000 scans were accumulated which took about 20 min;

the indicated times refer to the time at the middle of the accumulation process).

30 min

100 min

200 min

300 min

400 min

500 min

600 min

-180 -190 -200


-210

27

that the faster of the two processes is due to a H-D exchange of Ph2SnD2 with the

solvent (ammonia). Besides, it is worth to note that the percentage of tetraphenyl-

distannide in relation to the sum of the two monostannides remained constant

over time, within experimental error.

Since the hydrogen atoms of hydrodiphenylstannide extracted from dichloro-

diphenylstannane or dihydrodiphenylstannane at least partially stem from different

sources, hydrodiphenylstannide is formed by different processes. This might be

due to the higher reactivity of sodium to Sn-Cl than Sn-H groups at initial stages

of the reaction. Dichlorodiphenylstannane may initially lose both chlorine atoms

and quickly react with hydrogen atoms from the solvent (liquid NH3) to form the

hydrodiphenylstannide. Dihydrodiphenylstannane preferentially loses only one

hydrogen atom initially, while the other remains bound to the tin atom for a long

time. The exchange of the remaining hydrogen (deuterium) atom with hydrogen

atoms from the solvent is a second reaction step proceeding at a different time

scale and with complex reaction order. The initial reactions which lead to the

distannide and the hydrostannide (or deuterostannide) are completed within less

than 60 min, whereas the second step takes days.

Reaction of dichlorodibutylstannane with Na in liquid ammonia. 119Sn NMR

spectra of dichlorodibutylstannane exposed to sodium in liquid ammonia were

more complex than those of the diphenylstannanes. In the case of

dichlorodibutylstannane, four major signals emerged (Figure 2c; additional spectra

in Figure 5), i.e. two additional signals featured when compared to those resulting

from experiments with dichlorodiphenylstannane (occasionally, a minor signal at

27 ppm was also present, probably as a result of a reaction with traces of oxygen).

The two main signals (with respect to the integrated intensities) were linked to the

28

Figure 5. 119Sn NMR spectra of in-situ formed products resulting from Bu2SnCl2 converted with

four molar equivalents of sodium in liquid ammonia, measured at 200 and 220 K with and

without proton broadband decoupling as indicated (see main text). An additional peak at -139.7

in the spectra at 220 K is probably caused by a reaction product of stannides with atmospheric

water or oxygen which diffused via the cap into the NMR tube; note that the spectra at 220 K

were measured after those at 200 K in the same NMR tube.

binuclear species tetrabutyldistannide, (Bu4Sn)22- (-161 ppm at 200 K), and

hydrodibutylstannide, Bu2SnH-(-228 ppm at 200 K). Accordingly, the latter signal

splits up into a doublet in proton-coupled 119Sn NMR spectra

(1J(119Sn-1H) = 96 Hz at 220 K), but the resolution is relatively low due to the

coupling with the methylene protons of the butyl groups. The resolution decreased

even more upon very long measurement periods (>7 h), which is probably a result

of the limits in temperature control: the chemical shift strongly depends on the

temperature (Δδ = 0.5 ppm K-1; cf. Table 1); compare also to the fluctuations of

chemical shifts of phenylstannides during NMR measurements as displayed in

Figure 4. The additional signals are presumably due to butyl group migration (see

also below, in the section describing reactions with bromoethane). The signal at

-136 ppm (at 200 K) obviously represents tributylstannide, SnBu3-, since the

decoupled200 K

coupled200 K

decoupled220 K

coupled220 K

-50 -100 -150 -200 -250 -300 -350

Chemical Shif t δ / ppm

29

Table 1. Chemical shifts (), coupling constants (J) and full-widths at half-maximum (fwhm) in

proton-decoupled and proton-coupled 119Sn NMR spectra of different diorganostannanes exposed

to four molar equivalents of sodium in liquid ammonia.

reaction mixture of chlorotributylstannane and two equivalents of sodium in liquid

ammonia resulted in a single peak at the same chemical shift. The signal at

-212 ppm showed pronounced broadening in the proton-coupled 119Sn NMR

spectra, at least partially due to non-resolved couplings to protons of the butyl

groups. Yet the coupling of 119Sn nuclei to protons of Sn-H bonds is significantly

stronger and, therefore, the line broadening increases additionally in species with

non-resolved Sn-H bonds. The full-width at half-maximum (fwhm) of the peak at

-212 ppm (-208 ppm at 220 K) extended in the proton-coupled spectra by 140 %

(170 % at 220 K) compared to the decoupled spectra (cf. Table 1). This increase in

fwhm is even more pronounced than that seen for hydrostannide at -228 ppm

proton decoupled proton coupled

Measurement Temp

K

δ 119Sn

ppm

fwhm

Hz

1J

Hz

δ 119Sn

ppm

fwhm

Hz

1J

Hz

Ph2SnCl2 200 -131.7 s 100 -131.7 s 124

-197.2 s 58 -197.1 d 111 147 (Sn,H)

220 -115.9 s 38 2027 (Sn,Sn) -116.2 s 45 2018 (Sn,Sn)

-192.7 s 30 -191.8 d 35 145 (Sn,H)

Ph2SnH2 200 -132.2 s 102 -131.5 s 130

-197.2 s 75 -197.1 d 90 148 (Sn,H)

220 -116.1 s 44 2029 (Sn,Sn)

-192.8 s 28

Ph2SnD2 220 -116.4 s 18 2023 (Sn,Sn) -116.5 s 26 2023 (Sn,Sn)

-192.9 s 9 -192.9 d 22 144 (Sn,H)a)

-196.8 m 9 24 (Sn,D) -196.8 m 26 23 (Sn,D)b)

Bu2SnCl2 200 -136.3 s 104 -136.3 s 92

-161.1 s 125 -161.1 s 120

-212.0 s 57 -212.0 m 155

-228.0 s 118 -228.0 m 220

220 -136.7 s 67 -137.4 s 80

-143.0 s 75 -141.6 s 86

-207.9 s 60 -208.1 m 144

-219.5 s 120 -218.6 m 195 96 (Sn,H)b)

a) Hydrostannide formed by D-H exchange (see text); b) determinded by peak deconvolution.

30

(-218 ppm at 220 K) which was broadened by about 80 % (60 % at 220 K). For

comparison, the fwhm of the two peaks at -136 ppm and -161 ppm (-137 ppm

and -142 ppm at 220 K), which represent species without Sn-H bonds, were only

weekly influenced by the change from proton-coupled to proton-decoupled

spectra; the broadening amounted only to 15 - 20 % at 220 K and even less at

200 K. Thus, the signal at -212 ppm may represent, for instance, a mononuclear

dihydrostannide or, since the signal intensity was not sufficiently high to allow

detection of tin satellites, a binuclear hydrostannide.

Reaction of the stannide intermediates with bromoethane. As mentioned above, it

has been postulated that the intermediate diorganostannide dianion can be trapped

by reaction with organohalides (Scheme 1) [20-23]. Accordingly, we transferred

solutions with the in-situ prepared stannides in the final state into a large excess of

precooled bromoethane. In the case of the intermediates resulting from conversion

of dichlorodiphenylstannane, only diethyldiphenylstannane, Et2Ph2Sn, was found

after reaction (Figure 6a; for chemical shifts see Experimental Section). The

stannides resulting from conversion of dichlorodibutylstannane with bromoethane

yielded two additional products compared to the analogous conversion with di-

chlorodiphenylstannane. Note that in the former case also two additional reaction

intermediates were detected (see above). Besides the expected main product

dibutyldiethylstannane, Bu2Et2Sn, also tributylethylstannane, Bu3EtSn, and

butyltriethylstannane, BuEt3Sn, were found by 119Sn NMR analysis (Figure 6b;

chemical shifts see Experimental Section), in line with the alkyl group migration

implied by the reaction intermediates. Consequently, reaction experiments with

1-bromobutane yielded only one product, i.e. tetrabutylstannane. Thus, the two

different stannides (R2Sn)22- and HR2Sn- react with bromoethane to the same

product (Et2R2Sn) which would be expected from a reaction of R2Sn2- with

31

Figure 6. 119Sn NMR spectra of (a) Ph2SnCl2 and (b) Bu2SnCl2 converted with 4 molar

equivalents of sodium in liquid ammonia and subsequently reacted with an excess of

bromoethane.

bromoethane. These findings show that the reaction products of the intermediates

in liquid ammonia with haloalkanes allow only limited conclusions on the

composition of the tin species present in liquid ammonia, although starting from

Bu2SnCl2 they appear to reflect at least the in-situ migration of organic groups. As

a final remark, note that the quantity of sodium in the system corresponds to the

stoichiometry of the overall reaction according to Scheme 1 (Na:Ph2SnCl2 = 4:1);

i.e. some sodium is also involved in the reaction of the stannides with

bromoethane, since sodium is only partially consumed upon formation of

tetraorganodistannide and hydrodiorganostannide. In the case of R2SnH2, there is

sufficient sodium for a reduction under formation of NaH (it is not evident if H2

formed, in this case sodium would be present in excess).

300 200 100 0 -100 -200

a

b


-300

32

3. Conclusions

119Sn NMR measurements in liquid ammonia showed that, in contrast to the

generally accepted view, diorganostannide dianions are not formed significantly by

exposure of dichlorodiorganostannanes or dihydrodiorganostannanes with four

equivalents of sodium in liquid ammonia, as deduced previously from indirect

experiments. The species that are present in the reacting medium, as a matter of

fact, are tetraorganodistannide, (R2Sn-SnR2) 2- and hydrodiorganostannide,

HR2Sn-. The latter slowly exchanges hydrogen atoms with the solvent. In

addition, butyl group migration takes place in liquid ammonia, while significant

phenyl group migration does not occur under the applied reaction conditions.

All experiments showed that, in contrast to previous assumptions, reaction

products of the in-situ generated diorganostannides with haloalkanes do not

represent the chemical nature of the intermediates in liquid ammonia.

33

4. Experimental Section

Materials. Ammonia was purchased from PanGas, (Dagmarsellen, Switzerland,

99.999 %), dichlorodibutylstannane from ABCR GmbH (Karlsruhe, Germany)

and dichlorodiphenylstannane from Sigma Aldrich (Buchs, Switzerland). Both

substances were recrystallized twice by dissolving in boiling pentane and

subsequent precipitation of the product at 250 K. Deuterated dichloromethane

(99.9% D) was purchased from Cambridge Isotope Laboratories (ReseaChem

GmbH, Burgdorf, Switzerland), and organic solvents from Fluka (Buchs,

Switzerland).

Conductivity measurements. Electrical conductivity measurements in liquid

ammonia solutions were performed with a TetraCon 325/Pt electrode from WTW

(Weilheim, Germany) in combination with a WTW MultiLab 540 instrument. In

a typical reaction, 150 mL of ammonia were condensed with a cold finger

condenser in a flame-dried 200 mL three-neck flask with flat bottom under

nitrogen atmosphere. The conductivity cell was immersed into the cold solution

(195 K) and 8 mmol of sodium were introduced in a nitrogen counter flow. The

reaction mixture was stirred until the conductivity level was constant, which took

about 15 min. Subsequently 2 mmol of the respective dichlorodiorganostannane

were added at 195 K and the electrical conductivity was monitored as a function of

time.

NMR spectroscopy. 119Sn NMR spectra were recorded with a Bruker UltraShield

300 MHz/54 mm Fourier-transform spectrometer at a frequency of 112 MHz

with either inverse-gated decoupling or without decoupling, as indicated in the

text. In both cases a delay time of 0.5 s, an acquisition time of 0.1 s and a pulse

angle of 3 μs (90°) was applied. The sweep width was 700 ppm with a 16 k data

34

point acquisition range resulting in a digital resolution of 4.78 Hz. Chemical shifts

(δ) are reported in ppm referring to tetramethylstannane (δ(Me4Sn) = 0 ppm).

Syntheses of dihydrodiphenylstannane and dideuterodiphenylstannane. The com-

pounds were synthesized according to the literature [35] but with LiAlD4 instead

of LiAlH4 for the deuterated compound. NMR analysis (in CD2Cl2, room

temperature, chemical shifts in ppm, coupling constants in Hz, q: quintet, m:

multiplet): 1H: = 8.07 (m, 2 H), 7.81 (m, 3 H), 13C: = 129.3 (J(C,117Sn/119Sn)

51.7/54.2), 129.6, 129.6 (J(C,Sn) 11.8), 138.2 (J(C,117Sn/119Sn) 39.3/40.7), 119Sn:

= -233.6 (q, 1J(119Sn/D) 296.6), proton-coupled 119Sn: -233.6 (qq, 3J(119Sn,H)

53.9, 4J(119Sn,H) 11.4).

Reactions in liquid ammonia. The conversion of dichlorodiorganostannanes or

dihydrodiorganostannanes with sodium in liquid ammonia was conducted in a

flame-dried three-neck flask with flat bottom under nitrogen atmosphere in which

ca. 100 mL of ammonia were condensed with a cold finger condenser. A quantity

of 8 mmol of sodium were introduced in a nitrogen counter flow and dissolved by

stirring with a magnetic glass stirring bar, resulting in a homogeneous blue

solution (15 min). Thereafter, 2 mmol of dichlorodibutylstannane,

dichlorodiphenylstannane or dihydrodiphenylstannane were dissolved in 1 mL of

THF (dried over molecular sieve) and slowly added to the sodium/ammonia

solution with a syringe through a septum (to examine the influence of the THF in

the reaction mixture, some reactions were conducted by directly adding solid

dichlorodiorganostannane under a nitrogen counter flow, which led to the same

results). The color of the solution changed from clear blue to dark red. The

reaction mixture was stirred for 30 min in order to complete the reaction (as

previously determined with conductivity measurements). Syntheses of

phenylstannides from dideuterodiphenylstannane were performed in the same way.

35

For in-situ investigations with low-temperature NMR spectroscopy in liquid

ammonia, the reaction mixture containing the final stannides was transferred via a

bended glass tube into a flame dried and precooled NMR tube (195 K, Type 5UP

5×178 mm; ARMAR AG, Döttingen, Switzerland) equipped with a sealed

capillary with deuterated dichloromethane. The NMR tube was stored under

argon atmosphere in a 250 mL Schlenk tube and the transfer of the reaction

mixture was performed with nitrogen overpressure by carefully excluding oxygen

(argon counterflow from the Schlenk tube). The first 5-10 mL were poured into

the Schlenk tube before filling about 0.5 mL into the NMR tube. The filled NMR

tube was flushed with argon and stored at 195 K before it was inserted into the

precooled NMR spectrometer.

Conversion of the in-situ prepared stannides with bromoethane. For the reactions

of the in-situ prepared stannides with bromoethane, 5-10 mL of the ammonia

solutions containing the final stannides were transferred at 195 K to 20 mL of

precooled bromoethane (195 K) in a 100 mL two-neck round bottom flask via a

bended glass tube with nitrogen overpressure. The ammonia was evaporated by

warming the flask to room temperature in a N2 stream. The remaining products

were dried in vacuum (ca. 0.1 mbar) for 24 hours, and the solids thus obtained

were dissolved in deuterated dichloromethane for analysis with NMR

spectroscopy. 119Sn NMR analysis (CD2Cl2, Me4Sn), chemical shifts in ppm

(discussion of the products see text): Bu4Sn = -11.8, Bu3EtSn = -7.9, Bu2Et2Sn

= -4.1, BuEt3Sn = -0.5, Et2Ph2Sn = -65. Selected literature values for

comparison: Bu4Sn = -11.5[33], Et2Ph2Sn = -66 [36]; since we did not find

chemical shifts of Bu2Et2Sn, EtBu3Sn and Et3BuSn, we also quote the value of

Et4Sn ( = 1.4 [36]) which discloses that the chemical shifts of the stannanes

comprising mixed alkyl groups are located between the chemical shifts of Bu4Sn

and Et4Sn. Further, the chemical shifts reported for Et3PhSn = -34 [36] and

36

EtPh3Sn = -98 [36] reveal that these products did not appear in the spectra

obtained by conversion of the related phenylstannides with bromoethane. The

same reaction procedure with 1-bromobutane gave only one product, Bu4Sn.

References

[1] C. Beermann, C. Hartmann, Über Metallorganische Derivate des Acteylens. I, Z. Anorg.

Allg. Chemie, 276 (1954) 20-32.

[2] T.L. Brown, G.L. Morgan, Infrared and Nuclear Magnetic Resonance Spectra of Methyltin

Compounds, Inorg. Chem., 2 (1963) 736-740.








[6] C.A. Kraus, A.M. Neal, Studies Relating to Methyl Tin Derivatives. IV. The Reaction

between Chloroform and Sodium Trimethyl Stannide in Liquid Ammonia, J. Am. Chem. Soc.,

52 (1930) 4426-4433.



[8] K. Kühlein, W.P. Neumann, H. Mohring, A Versatile Method for Preparation of

C-Deuterated Compounds, Angew. Chem., Int. Ed. Engl., 7 (1968) 455-456.

[9] R.H. Bullard, F.R. Holden, Action of Hydrogen Chloride on Stannanes of the Type

R2SnR'2, J. Am. Chem. Soc., 53 (1931) 3150-3153.

[10] R.K. Ingham, S.D. Rosenberg, H. Gilman, Organotin Compounds, Chem. Rev., 60 (1960)

459-539.

[11] R.K. Ingham, H. Gilman, Inorganic Polymers, Academic Press, New York, 1962.

[12] T. Birchall, J. Vetrone, Organotin Anions in Solution and in the Solid State, Hyperfine

Interact., 40 (1988) 291-294.



37




Chem., 15 (2005) 1789-1792.







18 (2006) 44-47.




Soc., 47 (1925) 2568-2575.


(1925) 2361-2368.

[22] R.F. Chambers, P.C. Scherer, Phenyltin Compounds, J. Am. Chem. Soc., 48 (1926)

1054-1062.


(1939) 302-313.




9063-9066.

[26] N. Scotti, U. Zachwieja, H. Jacobs, Tetraammin-Lithium-Kationen zur Stabilisierung

phenylsubstituierter Zintl-Anionen: Die Verbindung, Z. Anorg. Allg.Chem., 623 (1997)

1503-1505.

[27] K. Wiesler, C. Suchentrunk, N. Korber, Syntheses and Crystal Structures of Ammoniates

with the Phenyl-Substituted Polytin Anions Sn2Ph42-, cyclo-Sn4Ph4

4- and Ph6Ph122-, Helv. Chim.

Acta, 89 (2006) 1158-1168.

[28] C.A. Kraus, E.G. Johnson, Properties of Electrolytic Solutions. VII. Conductance of

Sodium Trimethylstannide and of the Sodium Salts of Certain Phenols and Thiols in Liquid





38

[30] C.A. Kraus, P.B. Bien, Properties of Electrolytic Solutions. VIII. Conductance of Some

Ternary Salts in Liquid Ammonia, J. Am. Chem. Soc., 55 (1933) 3609-3614.


2448.

[32] C.A. Kraus, Solutions of Metals in Non-metallic Solvents. VI. The Concuctance of the


[33] B. Wrackmeyer, G.A. Webb, Annu. Rep. NMR Spectrosc., 16 (1985) 73-186.

[34] R.E. Wasylishen, N. Burford, Large Isotope effects on the 119Sn NMR parameters of the

Stannyl Ion, J. Chem. Soc.,Chem. Commun., (1987) 1414-1415.


Secondary Stannanes to High Molecular Weight Polystannanes, J. Am. Chem. Soc., 117 (1995)

9931-9940.

[36] W. McFarlane, J.C. Maire, M. Delmas, Heteronuclear Magnetic Double Resonance Studies

of 119Sn Chemical Shifts in Ethytin Derivatives, J. Chem. Soc., Dalton Trans., (1972) 1862-1865.

Chapter III

Reaction Products of Dichlorodiorganostannanes

with Sodium in Liquid Ammonia:

In-situ Investigations with 119Sn NMR Spectroscopy

and Usage as Intermediates for the Synthesis of

Tetraorganostannanes

41

1. Introduction

The reaction of haloorganostannanes with sodium in liquid ammonia has

attracted attention since the early part of the past century [1-11]. The resulting

products have been used in-situ as intermediates for the preparation of organotin

compounds (organostannanes). In particular, conversion of such intermediates

with haloalkanes and haloarenes yielded tetraorganostannanes [12-25]. In this

context, also dihalodiorganostannanes, R2SnCl2, have been exposed to sodium, and

were regarded to yield diorganotin dianions (Scheme 1, diorganostannide dianions,

R2Sn2-), which can subsequently act as reaction intermediates for the synthesis of

tetraorganostannanes, with the respective sodium halides as reaction byproducts

[18, 26-30]. However, surprisingly, our recent studies performed in-situ with 119Sn

NMR spectroscopy experiments indicated that diorganostannide dianions were not

formed when stoichiometric ratios of dichlorodiorganostannanes and sodium are

present in liquid ammonia (i.e. 4 molar equivalents of sodium per mol stannane)

[31], Chapter II.

Scheme 1. Previously advanced reactions of dihalodiorganostannanes with sodium.

SnCl

R

R

ClSn

R

RNa

+Cl

SnCl

R

R

ClSn

R

RSn

R

R

Na+

Cl

SnCl

R

R

ClSn

R

R n

Na+

Cl

+ 4 Na + +4 2

+ 3 Na + +3 2

2-

1/2

+ 2 Na + +2 2 (1)

(2)

(3)2- -

-

-

42

Further, it has been suggested (but not spectroscopically verified) [5, 26] that

the composition of the reaction intermediates changes upon variation of the

diorganostannane/sodium ratio. In addition, treatment of dihalodiorganostannanes

with less than 4 molar equivalents of sodium (preferentially two equivalents) was

claimed to give rise to the formation of oligostannanes or polystannanes

(Scheme 1) [1, 4, 5]; however, the resulting products were not thoroughly

characterized. Polystannanes are a unique class of polymers as their backbone

consists of covalently bound metal atoms; they were first prepared by Löwig [32]

and then mainly during the last two decades in various laboratories by different

methods [33-52].

In order to resolve the various discrepancies alluded above, we carried out in-

situ investigations of the species arising from reactions of dichlorodiorgano-

stannanes and sodium at different molar ratios in liquid ammonia, and explored

their applicability as reaction intermediates for the synthesis of tetraorgano-

stannanes by conversion with bromoethane.

43

2. Results

For the experiments, dichlorodibutylstannane and dichlorodioctylstannane

were employed as representatives for alkylstannanes, and dichlorodiphenylstannane

as the most convenient arylstannane. All those compounds were exposed to 2, 3, 4

and 10 equivalents of sodium, respectively. A summary of the resulting products is

presented in Table 1.

Dihalodiorganostannane/Sodium 1:2. A general feature of reactions with a di-

chlorodiorganostannane/sodium ratio of 1:2 was the formation of polymers (cf.

Table 1). Below, the results obtained for the different systems are described in

more detail.

Table 1. Overview of the detected products that emerged from reaction of dichlorodiorgano-

stannanes, R2SnCl2, with different ratios of sodium in liquid ammonia.

Dichlorodibutylstannanes

Exposure of dichlorodibutylstannane to two molar equivalents of sodium in

liquid ammonia caused immediate precipitation of a yellow product, which is

R2SnCl2 : Na ratio R = butyl R = octyl R = phenyl

1:2 (SnBu2)n polymer and cyclic oligomers

(SnOct2)n polymer and cyclic oligomers

(SnPh2)n polymer

1:3 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-a)

1:4 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-

1:10 Bu2SnH-, (Bu4Sn2)2-, Bu3Sn-, one unidentified product Oct2SnH- Ph2SnH-, (Ph4Sn2)2-

a) Additional signals are attributed to degradation products formed during transfer of the reaction solutions to the NMR tubes that could not be avoided.

44

typical for poly(dibutylstannane). The gel permeation chromatography (GPC)

diagrams revealed the presence of polymer with a molar mass of 8 kg/mol. In

addition, products in a mass range of cyclic byproducts (cyclopentastannane and

cyclohexastannane) were detected. 119Sn NMR spectra showed a broad signal

at -190 ppm characteristic for poly(dibutylstannane) and signals at -202 ppm

and -203 ppm, which are typical for cyclic byproducts [50]. The elemental

composition was consistent with products of the composition (SnBu2)n, which is in

agreement both with the composition of linear polymers and cyclic oligomers.

Dichlorodioctylstannane

Conversion of dichlorodioctylstannane with two molar equivalents of sodium

was performed in the same manner as the above described reaction with dichloro-

dibutylstannane, resulting in precipitation of a yellow, pasty material. Analogously,

GPC analysis indicated the generation of poly(dioctylstannane) (molar mass

around 6 kg/mol), together with cyclic pentamers and hexamers. The elemental

composition was in agreement with that of (Oct2Sn)n. 119Sn NMR spectroscopy in

deuterated dichloromethane revealed a broad signal at -192 ppm which correspon-

ds to the value of poly(dioctylstannane), and signals at -203 ppm and ‐205 ppm for

the cyclic byproducts [50].

Dichlorodiphenylstannane

Also treatment of dichlorodiphenylstannane with two molar equivalents of

sodium resulted in immediate precipitation of a yellow, shiny product. The

material obtained was insoluble in all tested organic solvents at room temperature,

as well as at elevated temperatures (close to the boiling point of the solvents).

Therefore, it was not possible to determine its molar mass. The product was

45

washed with a water/ethanol mixture (9:1) to remove sodium chloride and,

thereafter, extracted with hot dichloromethane to dissolve potential byproducts, in

particular cyclic oligo(diphenylstannane)s. 119Sn NMR analysis of the concentrated

extracts indicated that no significant amounts of cyclic byproducts were formed.

Elemental analysis of the material was consistent with that of (Ph2Sn)n.

Dichlorodiorganostannane/Sodium 1:3, 1:4 and 1:10. In the previous Chapter

and [31], we reported that in contrast to general views (Scheme 1),

dichlorodibutylstannane and dichlorodiphenylstannane do not react with four

equivalents of sodium to yield the respective diorganostannide dianions. Instead,

the anions HSnR2– and (R2Sn-SnR2)2– formed in quantities of a similar order of

magnitude, and in the case of dichlorodibutylstannane additionally R3Sn– and a

fourth unidentified product (e.g. H2RSn– or (HRSn-SnHR)2-) arose by alkyl group

migration. Considering that Na is in fact present in liquid ammonia as Na+ and

solvated electrons, the latter causing the typical blue color of the corresponding

solutions [53], the existence of (R2Sn-SnR2)2– may be somewhat surprising, as two

of the highly reactive solvated electrons per dianion may remain under a dichloro-

diorganostannane/sodium ratio of 1:4. In order to investigate if larger quantities of

sodium would ultimately lead to cleavage of Sn-Sn bonds in the dianions and if

lower amounts influence the ratio between the aforementioned stannides,

dichlorodiorganostannanes were exposed to 3, 4 and 10 molar equivalents of Na.

According to the literature [5, 26] substantial differences in the in-situ formed

reaction products are to be expected at these different ratios.

Since the solvated electrons produced upon dissolution of metallic sodium are

consumed when dichlorodiorganostannanes react - due to the generation of

stannides and chloride ions - the electric conductivity of the reaction mixture is

46

expected to decrease during the reaction of dichlorodiorganostannanes with

sodium, as highly mobile electrons are removed from the system. Thus, we

employed in-situ measurements of the electric conductivity in liquid ammonia to

qualitatively monitor the course of the reaction.

An overview of the soluble products detected by in-situ 119Sn NMR

spectroscopy in liquid ammonia obtained with the different compounds is

displayed in Table 2; the results are described in more detail in the following

sections.

Table 2. Overview of the soluble products that emerged after treatment of dichlorodiorgano-

stannanes with sodium in liquid ammonia identified by in-situ 119Sn NMR spectroscopy.

Dichlorodibutylstannane

The electric conductivity of mixtures of dichlorodibutylstannane/sodium 1:4

versus reaction time is presented in Figure 1a. The data show that the conductivity

reached a constant value after 30 min, indicating that the reaction was terminated

within this period (Figure 1a). Accordingly, in-situ NMR measurements in liquid

ammonia were performed after a reaction time of 30 min. Remarkably, regardless

of the dichlorodibutylstannane/sodium molar ratio in the range of 1:3 up to 1:10,

δ 119Sn (ppm) 200 K 220 K

Bu2SnH- -228 -219 (Bu4Sn2)2- -161 -143 Bu3Sn- -136 -137 Oct2SnH- -223 -219 Ph2SnH- -197 -192 (Ph4Sn2)2- -132 -116

47

Figure 1. (a) Electrical conductivity of a mixture of dichlorodibutylstannane (■), dibromodibutyl-

stannane (□), dichlorodioctylstannane (●) and dichlorodiphenylstannane (▲) and 4 equivalents of

sodium measured in-situ in liquid ammonia as a function of reaction time. (b) 119Sn NMR spectra

recorded in-situ of dichlorodibutylstannane exposed to 10, 4 and 3 molar equivalents of sodium in

liquid ammonia.

not only characteristic signals of the same intermediates emerged, but also their

relative distribution did not alter strikingly (Figure 1b). These intermediates are

attributed to dibutylhydrostannide Bu2SnH- (-228 ppm), tetrabutyldistannide

0 20 40 600

10

20

30

40

50

60

70

80

90

100

elec

tric

cond

uctiv

ity /

%

Time / min

100

20

40

0

80

60

Ele

ctric

alco

nduc

tivity

/ %

10 20 30 40 50 60

Time / min

0

0 -100 -200 -300


1:10

1:4

1:3

a

b

48

(Bu4Sn2)2- (-161 ppm), tributylstannide Bu3Sn- (-136.3 ppm) and a fourth

unidentified product that arose by alkyl group migration (e.g. H2RSn– or (HRSn-

SnHR)2–) (-212 ppm) [31]. Thus, dichlorodibutylstannane was found to undergo

alkyl group migration at molar stannane/sodium ratios between 1:3 and 1:10.

Dichlorodioctylstannane

Conversion of dichlorodioctylstannane with sodium in liquid ammonia was

slow compared to that of dichlorodibutylstannane, as revealed by conductivity

measurements. It took well over one hour until equilibrium was established in a

reaction mixture of dichlorodioctylstannane/sodium at a ratio of 1:4 (Figure 1a).

Notably, the reaction mixture of dichlorodioctylstannane with three, four and ten

equivalents of sodium gave rise to only one signal in in-situ 119Sn NMR spectra

within the detection limits, at -223 ppm at 200 K (-219 ppm at 220 K,

Δσ = 0.2 ppm/K), i.e. in the range of the above mentioned Bu2SnH- anion. This

signal is present as a singulet in broadband proton-decoupled 119Sn NMR spectra,

in contrast to the corresponding signal in proton-coupled spectra, where a doublet

is visible, due to a 119Sn-1H coupling (Figure 2). Therefore we assign the signal to

dioctylhydrostannide Oct2SnH-. The coupling constant of 1J(Sn,H) = 95 Hz is in

good agreement with values reported earlier for Bu2SnH- [31, 54]. I would like to

note that the transfer of the reaction products in liquid ammonia into NMR tubes

was more challenging for the octyl than for the butyl compounds. Due to the lower

solubility of the octyl compounds in liquid ammonia, they tend to precipitate

during the transfer to the NMR test tube (yellow polymeric residue). To avoid this

problem, lower stannane concentrations were used - leading to a decreased

sensitivity of the in-situ 119Sn NMR spectra and therefore increased acquisition

times.

49

Figure 2. (a) In-situ 119Sn NMR spectra of dichlorodioctylstannane exposed to 10, 4 and 3 molar

equivalents of sodium in liquid ammonia. The spectrum of the 1:3 ratio was recorded at 220 K

whereas the other measurements were performed at 200 K. (Δσ = 0.2 ppm/K). (b) 1H-decoupled

and 1H-coupled 119Sn NMR spectra of the signal at -223.3 ppm (1J(Sn,H) = 95 Hz).

Dichlorodiphenylstannane

The reaction of dichlorodiphenylstannane with sodium was terminated within

30 min, as evident from conductivity measurements (Figure 1a). After this period,

two strong signals were visible in 119Sn NMR spectra of in-situ formed

intermediates at dichlorodiphenylstannane/sodium ratios of 1:4 and 1:10

(Figure 3), representing diphenylhydrostannide, Ph2SnH-, (-197.2 ppm) and tetra

-100 -200-150 -250-50Chemical shif t δ / ppm

a

1:10200 K

1:4200 K

1:3220 K

-220 -240


b

1:4, 200 KH-decoupled

1:4, 200 KH-coupled

-220 -240

50

Figure 3. In-situ 119Sn NMR spectra of dichlorodiphenylstannane exposed to 10, 4 and 3 molar

equivalents sodium in liquid ammonia including a mixture of 1:3 ratio that was aged for 2 h.

phenyldistannide, (Ph4Sn2)2- (-131.7 ppm), respectively [31]. Reliable in-situ

measurements of 1:3 mixtures were somewhat more difficult to perform with

reaction mixtures based on dichlorodiphenylstannane as compared to those with

dichlorodibutylstannane, since a higher tendency to form precipitates (polymers or

other products) was observed in the former. When such solutions (still dark red)

were investigated by 119Sn NMR spectroscopy, several signals emerged. In addition

to the peak of diphenylhydrostannide at -197.2 ppm, two other strong signals

at -65 ppm and -125 ppm arose (Figure 3). Unfortunately, the species causing the

latter signals could not be identified. The solutions lost their characteristic dark red

color after about 2 h at 200 K; a yellow precipitate was formed, but the NMR

-100 -200

1:10

1:3aged

1:3

1:4

-150 -250-50


51

spectra still featured the diphenylhydrostannide signal at -197.2 ppm together with

the strong signal at -75 ppm, which could not be attributed to a particular

compound.

In-situ prepared alkylstannides as intermediates: Reactions with bromoethane.

The above described invariability of detected products arising at dichlorodialkyl-

stannane/sodium ratios between 1:3 and 1:10, as well as in dichlorodiphenyl-

stannane/sodium 1:4 and 1:10 mixtures are not in agreement with previous

suggestions in the literature [5, 26]. In fact, it has been argued that after reactions

of in-situ formed products with haloalkanes, the composition of the formed

products should change upon variation of the dihalodiorganostannane/sodium

ratio (note that these conclusions were not based on direct spectroscopic data).

Hence, in the following we investigated if related products with haloalkanes can

Table 3. Summary of the compounds that resulted from reaction of in-situ prepared dichlorodi-

organostannane/sodium mixtures in liquid ammonia with bromoethane (30 min equilibration

time for butyl and phenyl; 90 min for octyl).

R2SnCl2 : Na ratio R = butyl R = octyl R = phenyl

1:10 Bu3SnOH Bu2SnEt2 Bu3SnEt Bu4Sn2Et2 Bu3Sn2Et3

Oct2SnEt2a)

OctxSn2Et6-x unident. prod. (-125 ppm)

Ph2SnEt2 Ph4Sn2Et2 unident. prod. (-28 ppm)

1:4 BuSnEt3

Bu2SnEt2 Bu3SnEt

OctSnEt3 Oct2SnEt2 Oct3SnEt OctxSn2Et6-x Oct6Sn2O

Ph2SnEt2

1:3 BuSnEt3

Bu2SnEt2 Bu3SnEt

Oct2SnEt2 OctxSn2Et6-x Oct6Sn2O

PhSnEt3 Ph2SnEt2 Ph3SnEt

a) Minor quantities of OctSnEt3 and Oct3SnEt might be present but were not observed due to a relatively low signal/noise ratio in the spectra.

52

indeed provide information about in-situ formed products and vice versa,

exemplarily with reactions with bromoethane. An overview of the results is

presented in Table 3, and will be detailed below.

Conversion of butylstannides

Butylstannides were exposed to an excess of bromoethane and products studied

with 119Sn NMR spectroscopy. At dichlorodibutylstannane/sodium ratios of 1:4

and 1:10, the spectra did not differ considerably (Figure 4a). A main species

accompanied by two side products emerged in a region of 0 to -10 ppm (Table 4),

i.e. in the typical range of tetraalkylstannanes, which is consistent with the

chemical shifts of the Sn-CH2 signals in 13C NMR spectra (around 0 ppm,

Table 5). Experiments with 13C-labeled bromoethane, Br-13CH2CH3, allowed

identification of the reaction products via the signal splitting pattern caused by

119Sn-13C couplings (Figure 5 and Table 4 and 5). Thus, it was concluded that the

primary product of the mixtures with 1:4 and 1:10 ratios was dibutyldiethyl-

Figure 4. 119Sn NMR spectra of the reaction products of (a) dichlorodibutylstannane, (b)

dichlorodioctylstannane and (c) dichlorodiphenylstannane exposed first to 10, 4 and 3 molar

equivalents of sodium and then converted with bromoethane.

Chemical shift δ / ppm0100 -100

1:10

1:3

1:4

Chemical shift δ / ppm

1:10

1:3

1:4

0100 -100Chemical shift δ / ppm

1:10

1:3

1:4

0100 -100

a b c

53

Figure 5. (a) 119Sn NMR spectrum of the reaction products of dichlorodibutylstannane/sodium

1:4 exposed to Br13CH2CH3. (b) magnification of the region of the monostannanes.

SnC13 H2

BuBuBu

SnC13 H2

C13 H2

Bu C13 H2

SnC13 H2

C13 H2

BuBu

0 -2 -4 -6 -8 -10 -12246


50 0 -50 -100100


a

b

54

Table 4. 119Sn NMR data of the products resulting from conversion of a mixture of Bu2SnCl2/Na

1:4 with 13C labeled bromoethane, Br-13CH2CH3.

Table 5. 13C NMR data of the reaction products of a mixture Bu2SnCl2/Na, 1:4, with 13C labeled

bromoethane Br-13CH2CH3.

δ 1J(Sn,C) 2J(Sn,C) 1J(Sn,Sn) ppm Hz Hz Hz Bu3 Sn (13CH2CH3) d -7.9 318.8 Bu2 Sn (13CH2CH3)2 t -4.1 319.2 Bu Sn (13CH2CH3)3 q -0.6 319.3 Bu2 Sn (13CH2CH3) - Bu2 Sn (13CH2CH3) dd -75.3 245.3 41.7 2580 Bu2 Sn (a)(13CH2CH3) - Bu Sn (b)(13CH2CH3)2 a dt -73.8 246.0 42.1 b td -69.1 245.3 41.4 Bu2 Sn (13CH2CH3) OH d 99.9 374.0 Bu2 Sn (13CH2CH3) - O - Bu2 Sn (13CH2CH3) d 90.1 374.9 Bu Sn (a) (13CH2CH3)2 - O - Bu2 Sn(b) (13CH2CH3) a t 90.4 375.4

b d 90.6 374.6

δ 1J(119Sn,C) 1J(117Sn,C) ppm Hz Hz Bu3 Sn (13CH2CH3) 0.11 321.4 309.0 Bu2 Sn (13CH2CH3)2 0.28 319.1 305.0 Bu Sn (13CH2CH3)3 0.67 318.8 304.6 Bu2 Sn (13CH2CH3) - Bu2 Sn (13CH2CH3) 1.53 245.5 234.2 Bu Sn (13CH2CH3)2 - Bu2 Sn (13CH2CH3) 1.88 245.4 234.5 Bu2 Sn (13CH2CH3) - O - Bu2 Sn (13CH2CH3) 7.73 373.8 358.8 Bu Sn (13CH2CH3)2 - O - Bu2 Sn (13CH2CH3) 8.23 376.3 358.8

55

stannane, Bu2SnEt2, and the side products were butyltriethylstannane, BuSnEt3,

and tributylethylstannane, Bu3SnEt, which had been established by alkyl group

exchange reactions [31]. The very weak signal at -75 ppm in the spectra

representing the 1:4 ratio is probably due to tetrabutyldiethyldistannane.

Tributylethylstannane and dibutyldiethylstannane were found in similar quantities

for dichlorodibutylstannane/sodium ratios of 1:3, but in addition distannanes

formed (Figure 4a). The latter were identified not only by the chemical shifts

(about -70 ppm, typical for distannanes [55]), but also by the signal splitting

patterns in 119Sn NMR spectra (Table 4 and Table 5). The broad signal at

105 ppm was attributed to Bu3SnOH [31].

Conversion of octylstannides

In contrast to the other stannanes, the product composition resulting from the

reaction of bromoethane with dichlorodioctylstannane/sodium mixtures at molar

ratios between 1:3 and 1:10 strongly depended on the sodium ratio (Figure 4b).

The corresponding products were designated by 119Sn NMR spectroscopy on the

basis of the above discussed values of the butylstannanes (Table 6). Mono- and

distannanes were found in all cases, yet the ratio between these species shifted to

the distannanes with decreasing dichlorodioctylstannane/sodium ratio.

Distannanes dominated at a ratio of 1:3, while dioctyldiethylstannane, Oct2SnEt2,

was the main product at 1:4 and 1:10. Alkyl group exchange was little pronounced

at a 1:4 ratio, while at a 1:3 ratio strong NMR signals at -69.4 ppm and -70.1 ppm

indicated exchange of alkyl groups in distannanes, resulting in derivatives of the

general composition EtxOct6 - xSn2 (Figure 4b). The compound causing the signal

at about -125 ppm observed in the case of a 1:10 ratio could not be identified. NB

56

oxidation products, Oct3Sn-OH (99 ppm) and Oct3Sn-O-SnOct3 (96 ppm), were

observed in some cases.

Evolution of the octylstannides

The low reaction rate of dichlorodioctylstannane with sodium (Figure 6a) was

beneficially employed to convert portions of dioctylstannane/sodium 1:4 mixtures

Figure 6. (a) Time-dependent conductivity of a mixture of dichlorodioctylstannane and four

equivalents of sodium in liquid ammonia including marks at the times selected for addition of

bromoethane. (b) 119Sn NMR spectra of the reaction products with bromoethane after the times

indicated in Figure 3a.

-40 -20 0 20 40 60 80 100 120 140

20

30

40

50

60

70

80

elec

trica

l con

duct

ivtiy

/ m

S c

m-1

time / minTime / min

Ele

ctric

alco

nduc

tivity

/ mS

cm-1

40

20

30

70

50

60

7 min

14 min21 min

100 min

-40 0 40 80 120

7 min

14 min

21 min

100 min

300 0 -300150 -150Chemical shif t / ppm

a

b

57

with bromoethane, already before the equilibrium of the intermediate products was

established in-situ. The 119Sn NMR spectra of the reaction products (Figure 6b)

indicate that during the decrease of the conductivity in the reaction mixtures

various intermediates formed, which, however, could not be detected in-situ by

119Sn NMR during the formation and after the equilibrium was reached (see

above). For example, besides the signals of diethyldioctylstannane (-4.4 ppm),

tetraoctyldiethyldistannane and derivatives of the composition EtxOct6 - xSn2 at

about -70 ppm were detected, and the spectra of the reaction products after 7 and

14 minutes equilibration time (Figure 6b) showed the signals of chloroethyl-

dioctylstannane (148 ppm [55]), together with numerous signals

between -150 ppm and -200 ppm. After 21 minutes a group of signals

around -210 ppm was present. These signals might represent oligostannanes

(dimers -70 ppm, polystannanes -192 ppm and cyclic oligostannanes -203 ppm

[55]). Finally, after 100 min equilibration time the number of significant species

was reduced.

Conversion of diphenylstannides

In the case of conversion of in-situ prepared diphenylstannides, the ratio of

dichlorodiphenylstannane/sodium in the initial solution had a significant influence

on the products formed after subsequent reaction with bromoethane. The simplest

situation occurred at a dichlorodiphenylstannane/sodium ratio of 1:4 where only

one product was observed in 119Sn NMR spectra after conversion with

bromoethane, namely at -65.4 ppm – diethyldiphenylstannane [56] (Figure 4c)

(occasionally a small signal around -90 ppm was detected, which was not due to

either Ph3SnEt, δ 119Sn = -98 ppm or to dimeric species Ph4Et2Sn2,

δ 119Sn = -116 ppm). A stannane/sodium ratio of 1:3 might favor formation of

58

distannanes due to stoichiometric considerations (equation 2 in Scheme 1), and

indeed a adominating additional signal at -116 ppm was observed in 119Sn NMR

spectra (Figure 4c, identified by the accompanied tin satellites 1J(Sn,Sn) = 3472 Hz

in other experiments), attributed to diethyltetraphenyldistannane (Table 6). When

dichlorodiphenylstannane was exposed to ten equivalents of sodium and

subsequently converted with bromoethane, the main product was again diethyldi-

phenylstannane. But the additional sodium seemed to trigger the breakage of the

tin-carbon bond, as the signals at -34 ppm and -98 ppm in the 119Sn NMR spectra

can be attributed to triethylphenylstannane (-34 ppm) and ethyltriphenylstannane

(-98 ppm) [56, 57]. A signal at -28 ppm could not be identified.

Table 6. 119Sn NMR data of the products resulting from conversion Oct2SnCl2/Na and

Ph2SnCl2/Na mixtures with bromoethane.

δ 119Sn (ppm)

Oct3SnEt -8.3 Ph3SnEt -34 Oct2SnEt2 -4.4 Ph2SnEt2 -65 OctSnEt3 -0.7 PhSnEt3 -98

Oct4Et2Sn2 -75.7 Ph4Et2Sn2 -116 Oct3Et3Sn2 -70.1 / -70.2 Oct2Et4Sn2 -69.7

59

3. Discussion

In the range of dichlorodiorganostannane/sodium ratios of 1:2 to 1:10 in

liquid ammonia, polymers formed only at a ratio of 1:2, with all three stannanes

investigated. The polystannanes with aliphatic side groups were accompanied by

cyclic oligo(dialkylstannane)s, while no evidence for cyclic oligo(diphenylstanna-

ne)s was found. Indeed, polymer as well as cyclic oligomer formation agrees with a

1:2 ratio of dichlorodiorganostannane and sodium (cf. reaction scheme in Figure

1). Already at a ratio of 1:3 formation of polymeric material was completely

suppressed. Yet this does not necessarily imply that polymers never formed at this

ratio, as macromolecules might have been generated in an early stage of the

reaction and decomposed later.

Surprisingly, and in contrast to concepts advanced in the literature, the

differences in the distribution of the in-situ formed products at dichlorodiorgano-

stannane/sodium ratios of 1:3 to 1:10 were not significant. Stannides of the

composition R2SnH- were detected in all cases and dinuclear compounds of the

type R4Sn22- with R = butyl and R = phenyl. In non of the solutions was found

evidence of the presence of the frequently proposed SnR22- dianion. In fact, SnR2

2-

is expected to act as a very strong base, which may readily be able to undergo an

acid-base reaction with ammonia, thus forming NH2- and R2SnH-. Therefore, it

cannot be excluded that the detected R2SnH- developed from very short-living

SnR22-.

Migration of alkyl groups in the in-situ formed species was found to occur

with butyl but not with octyl moieties. Also, formation of tetraalkyldistannides was

observed with butyl but not with octyl groups. Obviously, octyl moieties impede

60

both formation of dimers and alkyl group migration in liquid ammonia, which

might be due to steric hindrance.

Further, it is evident that, again in contrast to previous concepts proposed in

the literature, the use of the reaction products of dichlorodiorganostannane with

sodium as soluble intermediates for subsequent reactions with haloalkanes only

allows limited conclusions about the nature of the intermediates. In particular the

existence of the intermediate hydrodiorganostannides, R2SnH- (which were

present in all reaction solutions of dichlorodiorganostannanes with sodium), as

well as the appearance of the dinuclear species R4Sn22- in the in-situ reactions of

dichlorodibutylstannane and dichlorodiphenylstannane with sodium were not

reflected in the products arising after subsequent reaction with bromoethane. On

the other hand, the chemical structure of the intermediates does not allow definite

conclusions for the composition of the products emerging after subsequent

reaction with bromoethane. For example, the presence of in-situ generated

R2SnH- does not result in formation of tin hydrides after conversion with

bromoethane. Also, solutions with virtually the same in-situ prepared products can

lead to different reaction products after conversion with bromoethane if different

ratios between dichlorodiorganostannane and sodium were initially present. Note

that the formation of the detected in-situ formed products requires less than four

equivalents sodium per dihalodiorganostannane, i.e. unreacted sodium was present

at least in the case of four or ten equivalents. This leads to the conclusion that

unreacted sodium is involved in the conversion of intermediate stannides with

bromoethane.

Remarkably, even an excess of sodium is not able to cleave Sn-Sn bonds in

R4Sn22- (detected with R = butyl and R = phenyl), while subsequent cleavage of

those Sn-Sn bonds after addition of bromoethane was observed - in particular

61

when four equivalents of sodium per tin atom were present. This implies that

bromoethane or reaction products of bromoethane, respectively, were involved in

Sn-Sn cleavage. Contrariwise, the occurrence of binuclear species in reaction

products with bromoethane does not allow to conclude that the in-situ generated

intermediate products also contained binuclear stannanes, as especially evident

from the solutions with dichlorodioctylstannane/sodium 1:3, where Oct2SnH-

dominated in the in-situ generated reaction solution, while binuclear species

dominated after conversion with bromoethane.

However, an exchange of organic side groups in in-situ generated species was

also reflected in the reaction products with bromoethane. Yet the reverse

conclusion about side group exchange in in-situ generated species derived from the

presence of side group exchange after exposure to bromoethane, is not valid. While

considerable breakup of tin-carbon bonds happened during the reaction of

dichlorodibutylstannane with sodium, exchange of phenyl groups was observed

only after the reaction of bromoethane with the 1:10 mixture of dichlorodiphenyl-

stannane/sodium; phenyl group exchange was not detected in the in-situ 119Sn

NMR measurements. This implies that exchange of phenyl groups took place

simultaneously with the reaction of the in-situ generated intermediates with

bromoethane.

As a consequence of the above considerations, conclusions regarding the

composition of solutions of dichlorodioctylstannane and sodium at different

reaction times by analysis of the reaction products observed after addition of

bromoethane should be taken with care. These experiments may only indicate that

numerous intermediates are formed initially which ultimately converge to few

products.

62

Finally, it appears that the halogen atoms present in the system do not exert a

pronounced influence on the product distributions. Substitution of dichlorodi-

butylstannane by dibromodibutylstannane did not considerably change the course

of the conductivity of reaction solutions with four equivalents sodium (Figure 1a),

and there was no fundamental alteration in the reaction products after subsequent

conversion with bromoethane. Moreover, when bromoethane was substituted by

iodoethane added to reaction solutions of dichlorodibutylstannane and four

equivalents of sodium, product formation essentially did not differ.

4. Conclusions

Reaction of dichlorodiorganostannanes with two equivalents of sodium in

liquid ammonia resulted in the formation of polystannanes, which were

accompanied by cyclic byproducts in the case of the alkylstannanes. However,

conversion of dichlorodiorganostannanes with three to ten molar equivalents of

sodium yielded soluble intermediates. Instead of the frequently proposed R2Sn2-

dianions, R2SnH- and in the case of R = butyl and R = phenyl also dinuclear

species of the type R4Sn22- emerged. These species did not allow reliable

conclusions about the reaction products resulting from exposure to bromoethane

and vice versa. Migration of organic groups in in-situ formed reaction products was

reflected also in products resulting after conversion with bromoethane, while the

reverse conclusion was not true. Finally, when in-situ formed reaction products are

employed as intermediates for the preparation of tetraorganostannanes by

conversion with haloalkanes, a large number of products can arise when the

haloalkanes are added before that equilibrium is reached.

63

5. Experimental

Materials. Ammonia was purchased from PanGas (Dagmarsellen, Switzerland,

99.999 %), dichlorodibutylstannane, dichlorodioctylstannane from ABCR GmbH

(Karlsruhe, Germany) and dichlorodiphenylstannane from Sigma Aldrich (Buchs,

Switzerland). All substances were recrystallized twice by dissolving in boiling

pentane and subsequent precipitation of the products at 200 K. Bromoethane was

purchased from Acros Organics (Basel, Switzerland), Br13CH2CH3, deuterated

dichloromethane (99.9% D) from Cambridge Isotope Laboratories (ReseaChem

GmbH, Burgdorf, Switzerland), and organic solvents from Fluka (Buchs,

Switzerland).

Methods. 119Sn NMR spectra were recorded with a Bruker UltraShield 300

MHz/54 mm Fourier-transform spectrometer at a frequency of 112 MHz with

either inverse-gated decoupling or without decoupling, as indicated in the text. In

both cases a delay time of 0.5 s, an acquisition time of 0.1 s and a pulse angle of

3 μs (90°) were applied. The sweep width was 700 ppm with a 16 k data point

acquisition range resulting in a digital resolution of 4.78 Hz. Chemical shifts (δ)

are reported in ppm referred to tetramethylstannane (δ(Me4Sn) = 0 ppm).

Electrical conductivity measurements in liquid ammonia solutions were

performed with a TetraCon 325/Pt electrode from WTW (Weilheim, Germany)

in combination with a WTW MultiLab 540 instrument. A 100 mL three-neck

flask was filled to the top with ammonia (about 150 mL) to ensure complete

coverage of the electrodes. After ~200 mg of sodium were dissolved in the

ammonia the conductivity cell was immersed under nitrogen counter flow. The

dichlorodiorganostannanes were introduced through the side necks of the flask and

the conductivity monitored as a function of time. Conductivity was recorded

64

during the reaction of dichlorodiorganostannanes with four molar equivalents of

sodium until a plateau value was reached. At this stage of the reaction only soluble

products were present.

Elemental analysis was performed by the Microelemental Analysis Laboratory

of the Department of Chemistry at ETH Zürich. For differential scanning

calorimetry (DSC), a DSC822e instrument (Mettler Toledo, Greifensee, Switzer-

land) equipped with an intracooler was applied, and the measurements were

carried out under nitrogen atmosphere.

Gel permeation chromatography was performed with a Viscotek VE7510

equipped with degasser, VE1121 solvent pump, VE520 autosampler and Model

301 triple detector array. A PL gel 5 μm Mixed-D column from Polymer

Laboratories Ltd. (Shropshire, United Kingdom) was used, calibrated with

poly(styrene) standards.

Liquid ammonia was condensed in a three-neck flask with flat bottom

equipped with a magnetic glass-coated stirring bar (Teflon-coated stirring bars

were attacked by the sodium). The flask was evacuated, flame-dried and flushed

with nitrogen three times. A cold finger condenser equipped with a calcium

chloride drying tube was mounted under nitrogen counter flow and flushed with

nitrogen for another 10 min. Subsequently the ammonia gas bottle was connected

via a plastic tube, which was flushed with ammonia for 30 sec. The nitrogen flow

was arrested and the equipment was flushed with ammonia for 5 min. The drying

tube outlet was closed with a balloon before the cold finger condenser was filled

with a dry ice/isopropanol mixture. Also, the reaction vessel was surrounded by a

dry ice/isopropanol bath to cool the condensed ammonia to 200 K. When the

desired amount of ammonia was condensed (90 mL for NMR experiments and

150 mL for conductivity measurements), the ammonia flow was stopped and a

smooth nitrogen flow restarted.

65

Preparation of reaction mixtures for in-situ NMR investigations. For reactions of

dichlorodibutylstannane and dichlorodiphenylstannane typically 8 mmol of sodium

were introduced in a nitrogen counterflow into 90 mL condensed ammonia and

dissolved by stirring for 15 min to result in a homogeneous blue solution.

Thereafter 2.67 mmol, 2 mmol and 0.6 mmol respectively of dichlorodiorgano-

stannane dissolved in 1 mL THF were added slowly. The color changed from blue

to dark red. The mixtures were stirred for 30 min in order to complete the

reactions and subsequently transferred via a bended glass tube directly into

precooled (200 K) and dried NMR tubes (Type 5UP 5x178mm; ARMAR AG,

Döttingen Switzerland) equipped with a sealed capillary with deuterated dichloro-

methane. The transfer was performed with nitrogen overpressure and argon

counterflow by carefully excluding oxygen. The filled NMR tubes were flushed

with argon and stored at 200 K before they were inserted into the precooled NMR

spectrometer. To avoid precipitation during the transfer to the NMR tube, the

applied concentration of dichlorodioctylstannane was significantly lower than in

the cases of the other two dichlorodiorganostannanes. 2 mmol sodium were

dissolved in liquid ammonia by stirring for 15 min followed by the addition of

0.67 mmol (1:3 ratio) or 0.5 mmol (1:4 ratio) dichlorodioctylstannane. The

mixtures with the 1:10 ratio were prepared by dissolving 4 mmol sodium together

with 0.4 mmol dichlorodioctylstannane. Apart from the differences in concen-

tration, the reaction was performed as described above for the other stannanes.

Reactions with two molar equivalents of sodium. About 8 mmol sodium (detailed

quantities see Table 7) were stirred in 90 mL liquid ammonia at 200 K for 15 min

to obtain a homogeneous solution. The flask was wrapped with white soft tissue

and aluminum foil to exclude light and 4 mmol of the respective

dichlorodiorganostannane dissolved in 10 mL THF were added dropwise to the

66

Table 7. Quantities applied for the conversion of dichlorodiorganostannanes with 2 molar

equivalents of sodium.

sodium solution under continuous stirring, whereupon the deep blue color

disappeared and a yellow precipitate formed. After about two minutes, the

ammonia was removed under a constant nitrogen flow by warming the flask to

room temperature, and THF was evaporated at room temperature in vacuo

(0.1 mbar, 12 h). For gel permeation chromatography (GPC) analysis of the

obtained material was dissolved in THF; this solution was directly used after

filtration with syringe filters (0.45 μm PTFE filters). The residues stemming from

the reactions of the different stannanes were further processed as follows:

Dichlorodibutylstannane: The residue was dissolved in 50 mL

dichloromethane and the insoluble parts (sodium chloride) were filtered off.

Subsequently the solvent was removed in a rotary evaporator and the product dried

in vacuo (0.1 mbar, 24 h). Elemental analysis (in % w/w, in brackets values

calculated for (SnBu2)n): C 40.75 (41.25), H 7.56 (7.79).

Dichlorodioctylstannane: The remaining product was dissolved in 50 mL

dichloromethane, filtered off to remove insoluble NaCl, the solvent was thereafter

removed in a rotary evaporator and the product dried in vacuo (0.1 mbar, 24 h).

Elemental analysis (in % w/w, in brackets values calculated for (SnOct2)n): C 55.15

(55.68), H 9.17 (9.93).

sodium R2SnCl2 mg mmol mg mmol

Bu2SnCl2 151.5 6.59 1000.7 3.29 Ph2SnCl2 181.9 7.91 1360.5 3.96 Oct2SnCl2 227.0 9.87 2053.6 4.94

67

Dichlorodiphenylstannane: The resulting product was washed with 50 mL of a

water/ethanol (9:1) mixture until no chloride could be detected in the washing

solution (usually 3-4 times, until the addition of 5 mL saturated AgNO3 solution

did not lead to the visible formation of AgCl precipitates). Then, the material was

washed three times with 50 mL dichloromethane, and finally the product was

dried in vacuo (0.1 mbar, 24 h). Elemental analysis (in % w/w, in brackets values

calculated for (SnPh2)n): C 51.79 (52.81), H 3.76 (3.69).

Reactions of organostannide intermediates with bromoethane. In a typical

reaction, 218 mg of sodium (9.45 mmol) were dissolved in 90 mL liquid ammonia

and stirred for 15 min. Thereafter 720.9 mg of dichlorodibutylstannane

(2.37 mmol, or other dichlorodiorganostannanes, respectively) dissolved in 5 mL

tetrahydrofuran (THF) were added. The deep red mixture of

dichlorodiorganostannanes and sodium was transferred after establishment of the

equilibrium (30 min for dichlorodibutylstannane and dichlorodiphenylstannane,

90 min for dichlorodioctylstannane) via bended glass tubes by nitrogen

overpressure directly into a 100 mL 2-neck round bottom flask containing a stirred

solution of 5 mL bromoethane diluted with 20 mL THF kept at 200 K. The

solution instantaneously turned colorless and a white precipitate formed. The

ammonia was evaporated by warming the reaction mixture to room temperature in

a nitrogen flow, and THF was removed at room temperature in vacuo (about

0.1 mbar). The reaction products were dissolved in deuterated dichloromethane

and analyzed by means of 119Sn NMR spectroscopy.

68

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173-186.

Chapter IV

Poly(dialkylstannane)s and Poly(diarylstannane)s

Homo- and Copolymers Synthesized in Liquid

Ammonia

75

1. Introduction

Polystannanes are defined as polymers of which the main chain consists of

covalently connected tin atoms. This, to our knowledge, has not been reported for

other metallic elements and, therefore, is of fundamental interest. Due to delocal-

ization of electrons along the polymer backbone (σ-delocalization) [1-3], poly-

stannanes were regarded to be appealing materials with respect to their chemical,

optical, thermal and electrical properties [4-10]. Moreover, polystannanes with

aromatic side groups were reported to feature σ-π-delocalization of the electrons

[11]. While some research has been addressed to copolymers of stannanes and

silanes or germanes (Scheme 1) [12-15], little is known about polystannane

copolymers constituted of tin atoms with different organic side groups. Only a

copolymer comprising the repeat units dibutylstannane and bis(3-phenylpropyl)-

Scheme 1. Schematic of the reported synthesis of copolymers with diorganostannane moieties in

the main chain: poly(dibutylstannane-co-methylphenylsilane), poly(dibutylstannane-co-dibutyl-

silane), poly(dibutylstannane-co-dibutylgermane) and poly[dibutylstannane-co-di(ω-phenyl-

propyl)stannane]. Refs. indicated.

Sn

Bu

Bu

ClCl Si

Me

ClCl

Ph

Sn

Bu

Bu

ClCl E

Bu

Bu

ClCl

Sn

Bu

Bu

ClCl Si

Me

ClCl

Ph

Sn

Bu

Bu

HH

Sn

Bu

Bu

Si

Me

Ph

Sn

Bu

Bu

E

Bu

Bu

Sn

Bu

Bu

Si

Me

Ph

Sn

Bu

Bu

Sn

Ph

Ph

Sn HH

Ph

Ph

+

+

+

+

[12]

[15]

[13]

[4]

x Y

x Y

x Y

x Y

Na / 120 °C

e-

Na / tol.

[RhCl(PPh3)3]

E = Si, Ge

76

stannane (i.e. with the propyl and not the phenyl groups bound to the tin atoms)

has been described [4]. Hence, in order to further explore the spectrum of

polystannanes and their material properties, copolymers comprising

dialkylstannane and diarylstannane moieties are aimed for in this study, as

indications exist that incorporation of the latter may enhance the environmental

stability of these materials.

Efficient synthesis of polystannanes was found to be the dehydropoly-

merization of R2SnH2 with Wilkinson’s catalyst [5], resulting in pure linear poly-

(dialkylstannane)s. This route enabled to uncover material properties of

polystannanes without the influence of undesirable byproducts, in particular cyclic

oligomers. Unfortunately, however, Wilkinson’s catalyst was of limited

applicability for polymerization of diarylstannanes, (e.g. Ph2SnH2), and thus likely

to be of limited use for the synthesis of copolystannanes comprising this moieties.

Therefore, we adopted the reaction of dichlorodiorganostannanes with sodium in

liquid ammonia for the preparation of poly(dialkylstannane-co-diarylstannane)

copolymers of the general composition (SnAlk2)x(SnAr2)y, with Alk representing

an alkyl- and Ar an aryl group and, for reference purposes, the corresponding

homopolymers (Scheme 2). Conveniently, reactions of this type have found

attention in the literature for some time [16-19] with a focus to form tin-carbon

bonds as well as tin-tin bonds [20-35], and have been investigated in more detail

more recently [36].

Scheme 2. Overall reaction scheme of dichlorodialkylstannanes and dichlorodiarylstannanes with

sodium for the synthesis of homo- and copolymers in liquid ammonia.

Sn ClCl

R

R

Sn

R'

ClCl

R'

Sn Sn

R'

R'

R

R

+Na / NH3

-78°C

x Y

Homopolymerization: R = R' = alkyl, arylCopolymerization: R = alkyl, R' = phenyl

77

2. Results and Discussion

Polymerization. Reactions of dichlorodiorganostannanes, R2SnCl2, with

sodium in liquid ammonia offer two different pathways to create polymers – either

in a one-step polymerization by application of two molar equivalents of sodium

(Scheme 3), or a two-step reaction of dichlorodiorganostannane with four

equivalents of sodium to generate a mixture of soluble stannides as intermediates

[36], followed by addition of another equivalent of dichlorodiorganostannane

(two-step polymerization, Scheme 3). The latter quantities result again in an

overall dichlorodiorganostannane : Na stoichiometry of 1 : 2 (one sodium atom per

chlorine), which corresponds to the theoretical ratio to yield “free (SnR2) groups”

and the polymer (SnR2)n, respectively.

Scheme 3. Schematic of two pathways towards poly(diorganostannane)s by reaction of dichlorodi-

organostannanes with sodium in liquid ammonia.

Sn

R

R n

SnCl

R

R

Cl

+ 2 Na; NH3 (l)- 2 NaCl

+ 4 Na; NH3 (l)- 2 NaCl

intermediate stannides

+ R2SnCl2- 2 NaCl

one-steppolymerization

two-steppolymerization

78

In the present study poly(dibutylstannane) and poly(diphenylstannane) and

copolymers comprising their respective repeat units were selected as representatives

for polystannanes with aliphatic and aromatic substituents. A priori it should be

noted that chemical structures, and, hence the properties of polymers resulting

from treatment of dichlorodiorganostannanes with sodium using the one- or

two-step synthesis route may differ significantly. For instance, homopoly-

merization of dichlorodibutylstannane Bu2SnCl2 by the two-step process is expec-

ted to result in branched polymer chains, due to the exchange of alkyl groups in

the intermediate stannides [36]. However, such phenomena have not been

observed for reactions with dichlorodiphenylstannane, Ph2SnCl2, i.e. there was no

evidence for exchange of aryl groups in the liquid ammonia solutions.

Thus, copolymers with Bu2SnCl2 and Ph2SnCl2 and their reference

homopolymers were synthesized by the above mentioned one- and two-step

polymerization protocols. Since poly(diphenylstannane) is insoluble - at least in

common solvents - and since increasing the length of alkyl side groups is generally

reported to show positive influence on the solubility of polymers [37-43], also

copolymers of dichlorodiphenylstannane with dichlorodialkylstannanes comprising

longer alkyl groups were synthesized, i.e. with dichlorodioctylstannane, Oct2SnCl2,

(Oct = octyl) and dichlorodidodecylstannane, Dod2SnCl2 (Dod = dodecyl). An

overview of the homo- and copolymers explored is shown in Table 1.

Table 1. Overview of the suitability of the applied methods for the synthesis of homo- and

copolymers discussed in the present work: + good; - bad due to crosslinking/branching; x no

polymer formed/no reaction.

Phenyl Butyl Octyl Dodecyl 1step 2step 1step 2step 1step 2step 1step 2step

Phenyla) + + + + x + x + Butyla) + - + - not applied not applied a)present in intermediate stannides in the case of two-step synthesis

79

Poly(dibutylstannane)

When the one-step synthesis was applied, polymeric material precipitated

within 10 to 20 s after addition of dichlorodibutylstannane to the solution of Na in

liquid ammonia. After isolation, the molar mass of the polymer dissolved in THF

was estimated by gel permeation chromatography (GPC). This analysis revealed a

weight-average molar mass MW of 8 kg/mol (Table 2). The molar mass of the

polymer obtained by two-step polymerization was higher (MW = 15 kg/mol).

Reassuringly, elemental analysis of the two products did not differ significantly

(Table 2).

In subsequent experiments, using the two-step procedure, the stoichiometric

ratios of the two Bu2SnCl2 portions of the first and the second addition were

varied between 0.9 and 1.1. Isolation and dissolution of the products in THF

allowed the determination of the molar mass as a function of this stoichiometry.

However, no significant influence of the stoichiometric ratios on the molar masses

was observed (Mw = 15 kg/mol at a ratio of 1.1, Mw = 13 kg/mol at a ratio of 0.9).

Importantly, a pronounced decrease in molar mass is expected for a poly-

condensation type reaction of the in-situ formed stannides in the 1st step with

dichlorodiorganostannane added in the 2nd step, according to Carother’s equation

[44]. As this was not the case, a chain-growth polymerization is presumed to

prevail. Radical reactions of organostannanes in liquid ammonia are well known

[29-34] indicating that the chain-growth polymerization could possibly be

initiated by radicals. Since the intermediates formed in-situ in the first step did not

react with each other to yield polymers, the species which initiated polymerization,

therefore, must have been generated upon addition of the second portion of

Bu2SnCl2 - perhaps by reaction with Na, since the entire quantity of Na is not re-

quired for the formation of the observed stannides generated in the first step [36].

80

Table 2. Calculated and found elemental compositions (in % m/m, products analyzed after

extraction of NaCl from reaction mixtures) and molar masses MW of the polymers obtained. The

theoretical elemental compositions of the copolymers were calculated on the basis of the applied

equimolar ratio of the monomers.

Poly(diphenylstannane)

One-step polymerization of dichlorodiphenylstannane, Ph2SnCl2, with two

molar equivalents of sodium resulted in precipitation of a yellow product. The

reaction appeared to be completed - as far as could be detected by visual

observation - within a few seconds after the addition of Ph2SnCl2. Interestingly,

rapid addition triggered inhomogeneity in the solution yielding, after evaporating

the ammonia, a product with red, pasty parts which were soluble in toluene and

dichloromethane to a certain degree. Drop-wise addition of Ph2SnCl2 to the

sodium solution in ammonia, by contrast, resulted in a yellow, bright precipitate.

However, this reaction appears to represent an intermediate stage between one-

and two-step polymerization, as the first quantities of Ph2SnCl2 reacted with a

high excess of sodium to form soluble intermediates. Therefore, the resulting

polymer showed the same characteristics as the material obtained by the two-step

Polymer C (% m/m) H (% m/m) MW

found calculated found calculated kg/mol (SnBu2)n 1 step 41.09 41.25 7.61 7.79 8 2 step 40.75 7.56 15 (SnPh2)n 2 step 51.87 52.81 3.74 3.69 insoluble (SnBu2)n(SnPh2)m 1 step 46.88

47.48 5.37

5.59 10

2 step (Bu-stannides) 37.87 5.19 insoluble 2 step (Ph-stannides) 47.74 5.50 10 (SnOct2)n(SnPh2)m 53.36 54.24 7.32 7.48 10 (SnDod2)n(SnPh2)m 57.86 59.21 8.89 8.28 12

81

polymerization – i.e. a homogeneous bright yellow powder that was insoluble in all

tested organic – low and high boiling – solvents, also at elevated temperatures.

Nonetheless, elemental analysis of the purified species were in good agreement

with the composition of pure (Ph2Sn)n (Table 2).

Poly(dibutylstannane-co-diphenylstannane)

Mixtures of dichlorodibutylstannane and dichlorodiphenylstannane in various

ratios were introduced into a homogeneous mixture of sodium in liquid ammonia

(Na : total stannane = 2 : 1, one-step synthesis). This procedure resulted in

precipitation of polymeric species within 10 and 40-50 seconds. The isolated

polymers were largely soluble in toluene and dichloromethane (Mw = 10 kg/mol,

Table 2), but also contained an insoluble fraction. Since the corresponding homo-

polymers poly(dibutylstannane) and poly(diphenylstannane) differ in their solubility

(see above), it would be reasonable to assume that the insoluble part consisted of

copolymers of a high content of diphenylstannane. Application of mixtures of 75 %

mol/mol Bu2SnCl2 and 25 % mol/mol Ph2SnCl2, as well as 25 % mol/mol

Bu2SnCl2 and 75 % mol/mol Ph2SnCl2 also resulted in the formation of soluble

and insoluble copolymer.

The two-step copolymerization started with the in-situ formation of stannides.

Accordingly, there are two possibilities to produce a copolymer. If in the first step

Bu2SnCl2 was applied, followed by the addition of Ph2SnCl2 in the 2nd step, the

product obtained was largely insoluble in toluene and dichloromethane. Elemental

analyses deviated significantly from theoretical values (Table 2). Oxygen contents

of up to 5 % w/w were found in the elemental compositions; obviously the

materials formed were sensitive to ambient oxygen. On the other hand, employing

Ph2SnCl2 in the 1st step, followed by addition of Bu2SnCl2 led to the formation of

82

partly soluble reaction products. The soluble fraction possessed a molar mass

Mw = 10 kg/mol, and the elemental analysis were in good agreement with the

theoretical composition (Table 2).

Poly(dioctylstannane-co-diphenylstannane)

One-step synthesis of Oct2SnCl2 and Ph2SnCl2 was not successful in liquid

ammonia since Oct2SnCl2 precipitated on the bottom of the flask and was inert to

further reactions. Hence, two-step polymerization was conducted by reaction of

Ph2SnCl2 in the 1st step followed by slow addition of Oct2SnCl2 to circumvent the

monomer to precipitate prior to reaction. About 60 s after the addition the liquid

reaction mixture turned yellow, and a yellow compound precipitated and could be

isolated by removing NH3. The polymeric material was partly soluble in THF and

dichloromethane. The soluble fraction was of a molar mass Mw = 10 kg/mol with

an elemental composition close to the theoretical values of the copolymer (cf.

Table 2).

Poly(didodecylstannane-co-diphenylstannane)

This copolymer could be synthesized only by the two-step procedure, since

Dod2SnCl2 accumulated at the bottom of the flask without reaction in the

one-step reaction. Therefore, in-situ created phenylstannide intermediates were

exposed to Dod2SnCl2. The resulting polymer precipitated immediately during

addition of Dod2SnCl2. The material thus produced was partly soluble in toluene

and dichloromethane, and its molar mass amounted to Mw = 12 kg/mol (Table 2).

83

Characterization

NMR Spectroscopy

All polymers were analyzed by NMR spectroscopy. The results are

summarized in Table 3 and will be discussed below.

Table 3. 1H and 119Sn NMR data for the polymers produced; 119Sn NMR chemical shifts δ refer

to tetramethylstannane (δ(Me4Sn) = 0 ppm); t = triplet, m = multiplet.

Poly(dibutylstannane)

119Sn NMR spectra of poly(dibutylstannane) recorded for

poly(dibutylstannane) from the one-step synthesis dissolved in deuterated

dichloromethane, CD2Cl2, featured a relatively broad signal at -190 ppm and two

sharp signals at -202 ppm and -203 ppm. The former corresponds to that of linear

poly(dibutylstannane), while the others correspond to decabutylpentastannane,

(Bu2Sn)5, and dodecabutylhexastannane, (Bu2Sn)6 respectively [5, 6]. The product

obtained from two-step synthesis showed a somewhat broader signal at -190 ppm,

attributed to linear poly(dibutylstannane) and signals of the cyclic oligomers at

Polymer δ 1H (ppm) Ratio Ar/Alkyl δ 119Sn (ppm)

-CH3 -CH2- Ar-H Sn

(SnBu2)n 0.9-1.0 [m, 3H] 1.1-1.8 [m, 6H] - - -190, -202, -203 -410 to -430a)

(SnPh2)n - - - - -197b)

(SnBu2)n(SnPh2)m 1.1 [m, 3H] 1.3-2.3 [m, 6H ] 6.7-8.1 [m, 3.7H] 0.7-0.9 -160 to -190 -200 to -220

(SnOct2)n(SnPh2)m 0.9 [t, 3H] 1.0-1.55 [m, 14] 6.7-7.6 [m, 5H] 1 -157 to -195 -200 to -220

(SnDod2)n(SnPh2)m 0.9 [t, 3H] 1.0-1.65 [m, 22 H] 6.7-7.6 [m, 5H] 1 -155 to -180 -187 to -211

a) the signal at -410 to -430 ppm was only observed in the two-step synthesis; b) determined by solid-state MAS NMR spectroscopy.

84

-202 ppm and -203 ppm. In addition, weak signals around -420 ppm were found,

which most likely originate from tertiary tin atoms (i.e. Sn atoms bound to three

other Sn atoms) in the polymer chain [45]. This also explained the broadening of

the polymer signal at -190 ppm, as a result of variances in the neighboring tin

atoms of the main chain.

Poly(diphenylstannane)

The soluble fractions extracted from the inhomogeneous product resulting

from the one-step synthesis did not feature any signal in 119Sn NMR experiments.

As the product from the two-step polymerization was insoluble, solid-state magic-

angle-spinning (MAS) 119Sn NMR spectra were acquired, which revealed one

(broad) signal at about -197 ppm, i.e. in the range typical for polystannanes [5, 6].

Poly(dibutylstannane-co-diphenylstannane)

1H NMR spectra of the soluble products (one- and two-step synthesis)

showed the typical signals of phenyl, as well as alkyl groups (Table 3). Integration

of the aliphatic and the aromatic signals indicated that the butyl content was

higher than expected – typical ratios between 0.9 and 0.7 were found (calculated

on the basis of the total number of hydrogen atoms). 119Sn NMR spectra featured

two very broad signals, one ranging from -160 ppm to -190 ppm and another from

-200 ppm and -220 ppm – the first is attributed to SnBu2 and the latter to SnPh2

moieties.

Poly(dioctylstannane-co-diphenylstannane)

The soluble fraction of this material featured signals in 1H NMR spectra of

the hydrogen atoms of the aromatic and aliphatic units; integration of these signals

85

were consistent with a 1 : 1 ratio between octyl and phenyl groups (Table 3). 119Sn

NMR spectra displayed broad signals ranging from -157 ppm to -195 ppm

resulting from SnOct2 and from -200 ppm to -220 ppm resulting from SnPh2

moieties.

Poly(didodecylstannane-co-diphenylstannane)

1H NMR spectra of this compound featured evidence of a 1 : 1 ratio of

dodecyl- and phenyl groups in the copolymer (Table 3). 119Sn NMR spectra of the

soluble polymer showed two broad peaks ranging from -155 ppm to -180 ppm,

assigned to SnDod2 moieties, and from -187 ppm to -211 ppm representing SnPh2

units.

UV/Vis spectroscopy

Delocalization of σ−electrons in the backbone of polystannanes was found to

be responsible for the characteristic yellow color and the absorption maxima

around 390 nm for poly(dialkylstannane)s (σ-delocalization) [5, 10]. In the case of

aromatic side groups, additional delocalization is expected to result in a

bathochromic shift of the absorption maximum, due to σ-π-delocalization [1, 11].

A main goal of the UV/Vis investigation was to investigate, if σ-π-delocalization

also occurs in copolymers with phenyl and alkyl side groups.

Homopolymers

UV/Vis absorption spectra of poly(dibutylstannane) featured an absorption

maximum at 390 nm independent of the synthesis method (one- or two-step),

consistent with values reported in the literature [5, 6, 10]. Absorption spectra of

poly(diphenylstannane), however, displayed an absorption edge around 480 nm,

86

which was attributed to enhanced delocalization of the electrons along the

backbone and the aromatic side chains (σ-π-delocalization) [1, 11].

Copolymers

UV/Vis spectra of poly(dibutylstannane-co-diphenylstannane) synthesized by

one-step reactions featured two peaks - one around 400 nm originating in the

σ-delocalization of the electrons of dibutylstannane units, and one around 470 nm

related to the σ-π-delocalization in diphenylstannane units. Comparing the

UV/Vis spectra of the insoluble and the soluble part of poly(dibutylstannane-co-

diphenylstannane), obtained from a 1 : 1 mixture of dichlorodibutylstannane and

dichlorodiphenylstannane (Figure 1a) indicated that the fraction of Ph2Sn moieties

was larger in the insoluble part, as indicated by the obervation that the maximum

at 470 nm was more pronounced in the absorption spectra of this part (Figure 1).

The normalized spectra of the insoluble part showed an intense shoulder at a level

of about 75 % compared to the signal at 400 nm, whereas in the soluble part this

signal was of only ~50 % of the corresponding intensity. Taking into account that

the same initial molar amount of Bu2SnCl2 and Ph2SnCl2 was employed, it is

obvious from the spectra that the extinction coefficient of the dibutyl stannane

units was higher than that of the diphenylstannane units, as absorbance at 400 nm

was higher in the soluble - as well as the insoluble fractions.

UV/Vis absorption spectra of the soluble polymer produced with mixtures

containing 25 to 75 % mol/mol Bu2SnCl2 also featured the two absorption

maxima at 400 nm and 470 nm. The signal at 400 nm, attributed to dibutyl-

stannane units, increased with increasing amount of dichlorodibutylstannane in the

starting mixtures concomitant with a decrease of the intensity of the signal at

470 nm, representing a decreasing fraction of diphenyltin moieties (Figure 2).

87

Figure 1. UV/Vis absorption spectra of the insoluble (dashed) and soluble (solid) fraction of poly-

(dibutylstannane-co-diphenylstannane) obtained by one-step polymerization (a) and two-step

polymerization (b).

Surprisingly, analysis of UV/Vis spectra indicated that the soluble polymer

resulting from the mixture with 75 % dichlorodiphenylstannane comprised a higher

aromatic content than the insoluble part of the 50 % mixture. Thus, it appears that

not only the amount of phenyl groups, but also their arrangement had an influence

on the solubility – larger blocks of poly(diphenylstannane) could lead to a decreased

solubility, whereas randomly distributed SnPh2 moieties, even at a higher overall

diphenyltin content, can still result in solubility of the respective copolymer.

A shift of the absorption maxima towards higher wavelengths was observed in

the soluble part from 459 nm (25 % Ph2SnCl2) to 465 nm (75 % Ph2SnCl2),

0.0

a

Rel

. Abs

orba

nce

0.5

1.0

b

Wavelength / nm400 600500

Rel

. Abs

orba

nce

0.5

0.0

1.0

88

Figure 2. UV/Vis absorption spectra of poly(dibutylstannane-co-diphenylstannane) produced by

one-step polymerization with a molar dichlorodibutylstannane content of 75 % (solid), 50 %

(dashed) and 25 % (dotted) in the starting mixtures.

indicating an increasing σ-π-delocalization of the electrons in the phenyl fraction in

the latter. At the same time the absorption maxima assigned to dibutylstannane

units shifted from 399 nm (75 % Bu2SnCl2) to 383 nm (25 % Bu2SnCl2) indicative

of decreasing delocalization and the influence of the (Bu2Sn) moieties (Table 4). In

the case of pure poly(dibutylstannane) only σ - delocalization of the electrons

occurs. The results of the above analysis represent conclusive evidence of the

formation of copolymers with both diarylstannane (SnAryl2) and dialkylstannane

(SnAlkyl2) moieties in the polymer main chain, as the absorption maxima shifted

depending on the composition of the copolymer. This, would of course not be the

case for blends of poly(dialkylstannane) and poly(diarylstannane) homopolymers.

UV/Vis absorption spectra of the materials obtained in two-step synthesis

(Figure 1b) showed a high phenyl content (assessment see above) in the insoluble

part, whereas the shoulder at 470 nm in the soluble polymer, indicating the

presence of diphenylstannane units, was detected, but not very pronounced.

Rel

. Abs

orba

nce

Wavelength / nm

400 600500

89

Figure 3. UV/Vis absorption spectra of the insoluble (dashed) and soluble (solid) fraction of poly-

(didodecylstannane-co-diphenylstannane).

Also the UV/Vis absorption spectra of poly(dioctylstannane-co-diphenylstannane)

featured an absorption maximum at 400 nm and a shoulder at 460 nm,

representative of dioctylstannane and diphenylstannane moieties, respectively.

Again, the intensity of the latter was more pronounced in the spectrum of the

insoluble part. UV/Vis absorption spectra of the soluble and insoluble parts of

poly(didodecylstannane-co-diphenylstannane) are shown in Figure 3. Both spectra

show an absorption maximum at 400 nm, together with an intense shoulder at

473 nm, which represents a bathochromic shift of the diphenylstannane related

absorption maxima compared to the corresponding copolymers comprising butyl

Table 4. Absorbance E and absorption maxima λmax of the soluble poly(dibutylstannane-co-di-

phenylstannane) synthesized by copolymerization with different molar fractions X of dichlorodi-

butylstannane.

Wavelength / nm400 600500

Rel

. Abs

orba

nce

0.5

0.0

1.0

dibutylstannane

segments diphenylstannane

segments XButyl EButyl λmax EPhenyl λmax

0.75 0.434 399 0.080 459 0.50 1.835 398 0.934 461 0.25 0.226 383 0.2422 465

90

and octyl groups, respectively. The shoulder was more pronounced in the insoluble

part. Furthermore, also the second absorption maximum showed a shift towards

higher wavelengths when compared to the poly(dibutylstannane-co-

diphenylstannane) polymers (400 nm to 406 nm).

Overall, the UV/Vis spectra are consistent with the incorporation of aromatic

moieties in the copolymer. However, it was not evident if the σ−π-delocalization is

caused by the existence of randomly distributed aromatic groups in the copolymer,

or by the occurrence of aromatic blocks of several (SnPh2) units in sequence.

Thermal Properties

Previously, distinctly different thermal characteristics have been reported for

poly(dialkylstannane)s [5, 6] and poly[di(ω-phenylalkyl)stannane]s [4]. For

instance, poly(dibutylstannane) synthesized with Wilkinson’s catalyst was found to

exhibit a phase transition at ca. 0 °C on heating and -25 °C on cooling, whereas

poly[bis(4-phenylbutyl)stannane] revealed a glass transition at -52 °C [4]. Also the

thermal stability differed significantly between polystannanes with aliphatic and

aromatic side groups. Therefore, thermal analysis should yield indications whether

a mixture of two homopolymers or a copolymer was formed in the present

synthetic efforts. Accordingly, thermal properties were investigated by differential

scanning calorimetry (DSC) and thermogravimetric analysis (TGA).

Poly(dibutylstannane) synthesized with sodium in liquid ammonia did not

feature the characteristic phase transition at 0 °C from a low temperature

crystalline solid phase to a liquid-crystalline mesophase [5, 7] (Table 5). This was

attributed to the presence of cyclic oligomers and the occurrence of branching

points in the products obtained from liquid ammonia, due to exchange of alkyl

91

Table 5. Peak transition temperatures and enthalpies of poly(dialkylstannane) and poly(diaryl-

stannane) homo- and copolymers, determined by differential scanning calorimetry (DSC).

groups [36], which would supress crystallization of the polymer. The

decomposition temperature (270 °C), however, was found to be virtually

independent of the synthesis method employed; i.e. no significant differences in

degradation temperatures were detected between polymers synthesized with

Wilkinson’s catalyst or sodium in liquid ammonia.

Poly(diphenylstannane), displayed no phase transition between -50 °C and

200 °C in DSC thermograms. This observation is consistent with polarization

microscopy studies which revealed no change in birefringence upon heating

polymer films up to 200 °C. Thermogravimetric analysis (TGA) indicated a

decomposition temperature of 350 °C with an onset at about 270 °C, leading to

the conclusion that poly(diphenylstannane) decomposes prior to melting.

Polymer Peak Transition Temperature °C

Transition Enthalpy J/g

1st trans. 2nd trans. 1st trans. 2nd trans. (SnBu2)n heating -1 - 10.1 - Wilkinson’s Cat.[2] cooling -26 - -9.3 - (SnBu2)n heating - - - - cooling - - - - (SnPh2)n heating - - - - cooling - - - - (SnBu2)n(SnPh2)m heating - - - - cooling - - - - (SnOct2)n(SnPh2)m heating -5 58 1.25 0.65 cooling -16 52 -1.28 -0.64 (SnOct2)n heating 29 74 14.3 5.2 Wilkinson’s Cat. [2] cooling 13 58 -13.5 -5.8 (SnDod2)n(SnPh2)m heating 94 - 43.0 - cooling 83 - -43.7 - (SnDod2)n heating 55 91 12.7 2.1 Wilkinson’s Cat. [2] cooling 39 80 -24.9 -3.8

92

Figure 4. Differential scanning calorimetry (DSC) thermograms of poly(dioctylstannane-co-

diphenylstannane) recorded at heating- and cooling rates of 10 °C/min. Two phase transitions are

observed at -5 °C and 58 °C upon heating and at 52 °C and -16 °C upon cooling in the second

respective thermograms.

Poly(dibutylstannane-co-diphenylstannane) copolymers did not show any phase

transition (Table 5), independent of the synthesis route (one- or two-step

procedure) and the butyl/phenyl ratio (1 : 3 to 3 : 1). Also these polymers reminded

birefringent in the entire temperature range. Similar to poly(dibutylstannane),

thermal decomposition occurred at 270 °C.

By contrast, poly(dioctylstannane-co-diphenylstannane) featured two weak

transitions in DSC thermograms at -5 °C and at 58 °C on heating (73 °C in the

first heating) and at -16 °C and 52 °C upon cooling (Figure 4). The crystallization

enthalpy of the first transition was about 1.25 J/g and the second 0.65 J/g

(Table 5). In the corresponding homopolymer poly(dioctylstannane), two phase

transitions were reported at 29 °C and 74 °C upon heating with melting enthalpies

of 14.3 and 5.2 J/g, respectively [5]. The decrease in the phase transition

temperatures and enthalpies of the copolymers was ascribed to the lower content of

octyl groups in the copolymer compared to that in the homopolymer, and to the

presence of rather amorphous segments of randomly dispersed diorganostannane

Hea

t flo

w W

/g

100500

endo

1st heating

2nd heating

1st cooling

2nd cooling

0.01 W/g

93

Figure 5. DSC thermograms of poly(didodecylstannane-co-diphenylstannane). In contrast to poly-

(didodecylstannane), only one phase transition upon heating and cooling were detected.

units. Thermogravimetric analysis unveiled a degradation temperature of 300 °C,

i.e. slightly higher than that of the dibutylstannane-diphenylstannane copolymer.

Poly(didodecylstannane) homopolymer synthesized with Wilkinson’s catalyst

from didodecylstannane (Dod2SnH2) showed two phase transitions in DSC

diagrams – at 55 °C upon heating and at 91 °C and upon cooling at 39 °C and

80 °C, with melting enthalpies of 13 J/g for the first and 2.1 J/g for the second

transition. For the copolymer poly(didodecylstannane-co-polydiphenylstannane)

only one, intense transition was observed at 94 °C on heating and 83 °C on cooling

with a melting enthalpy of 43 J/g (Table 5 and Figure 5). The temperature of this

transition corresponds well with the second transition of the homopolymer. It

seems, therefore, that the aromatic moieties suppressed the first phase transition at

55 °C, while melting occured at the same temperature as for the poly(didodecyl-

stannane) homopolymer – most likely due to the long alkyl chains. The

decomposition temperatures (330 °C) were found to increase compared with the

corresponding homopolymers.

Hea

t flo

w W

/g

100500-50

0.5 W/g

endo

1st heating

2nd heating

1st cooling

2nd cooling

94

Orientation

Previously, molecular orientation of poly(dialkylstannane)s was studied [10],

since orientation of polymeric materials is of great relevance, e.g. with regard to

mechanical, electrical and optical properties. For instance, poly(dibutylstannane)

was oriented by shearing, drawing of blends with ultra-high molecular weight

polyethylene (UHMWPE) and by crystallization onto friction-deposited layers of

uniaxially oriented poly(tetrafluoroethylene) (PTFE) molecules. The same

methods were also used for poly(dioctylstannane) and poly(didodecylstannane)

[10]. Remarkably, it has been described earlier [10] that the polymer main chain

can orient either preferentially parallel or perpendicular to the orientation direction

of the external stimulus, depending on the orientation method employed and the

side group of the polystannanes. Therefore, also in this study we investigated the

influence of the side group on the orientation direction of copolymers.

Poly(diphenylstannane), poly(dibutylstannane-co-diphenylstannane), poly(di-

octylstannane-co-diphenylstannane) and poly(didodecylstannane-co-diphenyl-

stannane) were examined. Orientation was induced by shearing on a glass slide

with a spatula, by drawing of blends with UHMWPE on a hot-stage at 110 °C

and, in the case of the soluble polymers, by crystallization onto PTFE orientation

layers. Alignment of the polymer was analyzed by means of polarized optical

microscopy, UV/Vis spectroscopy with polarized light and wide-angle X-ray

diffraction (WAXD).

Shearing the polymers at room temperature induced orientation of the main

chain for all materials parallel to the direction of shear, as clearly evident from

Figures 6 and 7 - shown by the example of poly(diphenylstannane) and

poly(dibutylstannane-co-diphenylstannane) (synthesized by the two-step synthesis).

95

Figure 6. Polarized optical microscopy images taken between crossed polarizers of poly(diphenyl-

stannane) prepared by two-step polymerization and oriented by shearing on a glass slide (top row)

and by drawing of a blend with UHMWPE (second row), at 0° and 45 ° angles between the

orientation axis of the sample and polarization direction of the light. At the bottom wide angle

X-ray diffraction patterns of the oriented materials are shown; left, oriented by shearing; right, by

drawing of blends with UHMWPE. Arrows indicate signals due attributed to the polystannane,

whereas the other signals originate from UHMWPE; orientation direction vertical.

Tensile deformation of blends with UHMWPE provided the same results for

poly(diphenylstannane), poly(dibutylstannane-co-diphenylstannane) and poly(di-

octylstannane-co-diphenylstannane). The orientation of these polymers was parallel

to the drawing direction. Drawing of blends with poly(didodecylstannane-co-

diphenylstannane), on the other hand, showed unexpected results. UV/Vis

spectroscopy with polarized light yielded different results compared to those

recorded in the shearing experiments. The development of the shoulder at 470 nm,

1 mm

45° 0°

96

Figure 7. Polarized UV/Vis absorption spectra (angles relative to the orientation direction

indicated) and wide angle X-ray diffraction pattern of poly(dibutylstannane-co-diphenylstannane)

oriented by shearing (orientation direction vertical).

attributed to the aromatic content of the copolymer, with the angle between

polarization direction of incident light and drawing direction was essentially the

same for both methods (Figure 8). By contrast, that of the signal around 400 nm

differed strikingly (cf. Figure 8b). Dichroism at 400 nm was more pronounced and

the absorbance perpendicular to the drawing direction and polarization plane of

incident light was higher, i.e. the dichroic ratio changed from 1.2 to -1.7 (the

negative sign indicates that absorption perpendicular to the polarization direction

is higher than parallel). This result implies that the copolymerization of dichloro-

diphenylstannane and dichlorodidodecylstannane did not yield an alternating

copolymer, but contains didodecylstannane blocks which cause orientation of side

groups parallel to the drawing axis thus forcing orientation of the corresponding

main chain segment to be perpendicular to the drawing axis, as was observed with

poly(didodecylstannane) homopolymer [5]. Inducing orientation by crystallization

from solution onto pre-oriented PTFE layer was not possible for poly(diphenyl-

stannane) due to its insolubility and was not successful for the copolymers.

Wavelength / nm

400 500

Abs

orba

nce

0.4

0.0

0.2

0.6

0

9045

97

Figure 8. Polarized optical absorption spectra of poly(didodecylstannane-co-diphenylstannane)

oriented by shearing on a glass slide at room temperature (a) and by drawing of a blend with

UHMWPE (b). Note that for orientation by shearing the absorption at 0° is higher for both

signals, whereas in the drawing experiments the absorption around 400 nm is higher at 90°.

Rel

. Abs

orba

nce

Wavelength / nm350 400 450 500

Rel

. Abs

orba

nce

a

b

0

9045

0

9045

98

3. Conclusions

We demonstrated that formation of polystannane homopolymers and copoly-

mers with sodium in liquid ammonia can be achieved via two different reaction

paths. That is, starting from dichlorodiorganostannanes, R2SnCl2, one-step

synthesis by reaction with two molar equivalents sodium and two-step synthesis,

where in the first step reactive intermediates are generated by the reaction with

four molar equivalents sodium. The two-step polymerization does not follow a

step-growth mechanism, but rather chain growth polymerization, probably

initiated by radicals generated during the second synthetic step. Migration of alkyl

groups- but not of phenyl groups - during the reaction with sodium in liquid

ammonia was confirmed, which can lead to branched polymers if alkylstannide

intermediates are formed.

By means of 119Sn NMR spectroscopy, UV/Vis spectroscopy and thermal

analysis it is demonstrated that copolymers were created as opposed to blends of

the respective homopolymers. UV/Vis spectra indicated the presence of both σ-π-

delocalization and pure σ-delocalization in poly(dialkylstannane-co-diphenyl-

stannane). From the spectra it could not be unequivocally concluded if the

corresponding signals were just due to occurrence of aromatic moieties in the

polymer or due to formation of block-copolymers.

Phase transitions observed in homopolymers were not, or only partially,

reflected in the related copolymers. The decomposition temperatures of poly(di-

alkylstannane-co-diphenylstannane)s increased with increasing length of the alkyl

group.

Like the homopolymers, the copolymers could readily be oriented by shearing

or by drawing of blends with ultra-high molecular weight polyethylene

(UHMWPE). In most cases, the polystannane main chains oriented preferentially

99

parallel to the orientation direction of the external stimulus. However,

interestingly, the orientation of poly(didodecylstannane-co-diphenylstannane)

depended on the orientation method. Whilst in simple shearing experiment the

copolymer chain axis oriented into the direction of shear, this polymer revealed

preferential orientation of segments with dodecylstannane perpendicular and

segments with diphenylstannane parallel to the drawing direction of blends with

UHMWPE. This observation appears to indicate the presence of blocks of

dodecylstannane and phenylstannane units, respectively, where the dodecyl groups

oriented parallel to the direction of the external stimulus, therewith forcing the

main chain to a perpendicular orientation.

100

4. Experimental


99.999 %). Dichlorodibutylstannane and dichlorodioctylstannane was acquired

from ABCR GmbH (Karlsruhe, Germany) and dichlorodiphenylstannane from

Sigma Aldrich (Buchs, Switzerland). The substances were recrystallized twice by

dissolving in boiling pentane and subsequent precipitation at -78 °C. Deuterated

dichloromethane CD2Cl2 (99.9% D) was purchased from Cambridge Isotope

Laboratories (ReseaChem GmbH, Burgdorf, Switzerland), and organic solvents

from Fluka (Buchs, Switzerland). Dichlorodidodecylstannane was synthesized

according to literature [5].

Characterization. NMR spectra were recorded on a Bruker UltraShield 300

MHz/54 mm Fourier transform spectrometer with standard 5 mm broad band

probe. All soluble samples were dissolved in CD2Cl2. Solid-state magic-angle

spinning experiments were executed on a Brucker UltraShield 500 MHz/54 mm

Fourier transform spectrometer, at a spinning rate of 14’000 rpm. Samples were

prepared in Bruker Ph MAS ZrO2 Rotors with Kel-F caps.

Elemental analyses were performed by the Microelemental Analysis

Laboratory of the Department of Chemistry at ETH Zürich. Gel permeation

chromatography was conducted with a GPC instrument from Viscotek (VE7510)

equipped with degasser, VE1121 solvent pump, VE520 autosampler and Model

301 triple detector array. A PL gel 5 μm Mixed-D column from Polymer

Laboratories Ltd. (Shropshire, United Kingdom) calibrated with atactic

poly(styrene) standards from Fluka (Buchs, Switzerland) were used. Samples were

dissolved in THF containing 2.5 % v/v toluene, which served as a marker. THF

eluent flow amounted to 1 mL/min. For optical microscopy, a Leica DMRX

101

microscope equipped with two polarizers and a Mettler Toledo FP82 HT hot

stage was used. UV/Vis spectra were recorded in transmission with a Perkin Elmer

Lambda 900 spectrophotometer equipped with rotating polarizers. Thermal

analysis were carried out with a differential scanning calorimeter (DSC) DSC822e

instrument (Mettler Toledo, Greifensee, Switzerland) equipped with an

intracooler, and thermal gravimetric analysis (TGA) with a TGA/SDTA851e

from Mettler Toledo under nitrogen atmosphere. The heating and cooling rates

were 10 °C/min. Maximum decomposition was determined by the maximum of

the 1st derivative of the TGA thermogram. Wide-angle X-ray diffraction pattern

were taken with a Diffraction Xcalibur™ PX (Oxford Instruments, Scotts Valley,

USA), using MoKα radiation.

Polymerization

One-step polymerization

Typically, around 350 mg (15 mmol) of sodium were added under a nitrogen

counterflow to 70 mL of liquid ammonia at -78°C and stirred for 15 min to obtain

a homogeneous solution (quantities for each synthesis in Table 6). Subsequently,

the flask was wrapped with soft tissue and aluminum foil to exclude light, and

dichlorodiorganostannane (half of the molar amount of sodium, or mixtures of two

dichlorodiorganostannanes which together contained half of the molar amount of

sodium) dissolved in about 5 mL THF was added with a syringe through a

septum. The reaction mixture was stirred for a few minutes to complete the

reaction before the flask was allowed to warm up to room temperature under a

gentle nitrogen stream. Thereafter, the nitrogen outlet was closed and the products

dried in vacuum (0.1 mbar, 12 h). Subsequently, the resulting solids were washed

102

Table 6. Synthesis parameters for the one- and two-step preparation of homo- and copolymers.

with 50 mL of a water/ethanol (9:1) mixture until no chloride was detected in the

washing solution (usually 3-4 times, until the addition of 5 mL saturated AgNO3

solution did not lead to the formation of visible AgCl precipitates) and again dried

in vacuum (0.1 mbar, 24 h).

Two-step polymerization

Approximately 350 mg (15 mmol) of sodium were dissolved in 70 mL liquid

ammonia (N2 atmosphere) by stirring for 15 min, before a quarter of the amount

of substance (3.75 mmol) dichlorodiorganostannane dissolved in 5 mL THF was

slowly added with a syringe through a septum (exact quantities see Table 6). The

# steps Na

(mmol) 1st step(mmol)

2nd step(mmol)

Homopolymers

Poly(dibutylstannane) 1 15.21 7.610 Bu

2 3.72 1.860 Bu 1.865 Bu

Poly(diphenylstannane) 1 13.24 6.625 Ph

2 19.42 4.850 Ph 4.852 Ph

Copolymers

Poly(dibutylstannane -co- diphenylstannane)

1 14.58 3.645 Bu+ 3.64 Ph

2 15.11 3.780 Bu 3.775 Ph

2 16.94 4.242 Ph 4.239 Bu

Poly(dioctylstannane -co- diphenylstannane)

2 14.84 3.710 Ph 3.710 Oct

Poly(didodecylstannane -co- diphenylstannane)

2 19.88 4.970 Ph 4.968 Dod

103

reaction mixture was vigorously stirred for 30 min to ensure complete and

homogeneous mixing. Thereafter, another portion of dichlorodiorganostannane

(3.75 mmol) dissolved in 5 mL THF was added with a syringe through a septum,

whereupon the polymer precipitated. The ammonia was evaporated by warming

the flask to room temperature under a gentle nitrogen stream. Afterwards the flask

was closed and the material dried in vacuum (0.1 mbar, 12 h). Results from

elemental analysis of the products obtained are listed in Table 2, and 1H and 119Sn

NMR data in Table 3. The resulting solids were washed with 50 mL of a

water/ethanol (9:1) mixture until no chloride was detected in the washing solution

(as before, usually 3-4 times, until the addition of 5 mL saturated AgNO3 solution

did not lead to the formation of AgCl precipitates) and again dried in vacuum

(0.1 mbar, 24 h).

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[17] C.A. Kraus, W.V. Sessions, Chemistry of the Trimethyltin Group, J. Am. Chem. Soc., 47 (1925)

2361-2368.

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[19] C.A. Kraus, W.N. Greer, The Dimethyltin Group and Some of its Reactions, J. Am. Chem. Soc., 47

(1925) 2568-2575.

[20] T. Harada, On the Metallo-Organic Compounds, Sci. Papers Inst. Phys. Chem. Res., 35 (1939) 302-

313.

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Res. Inst. Tokyo, 38 (1940) 146-166.

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Ammonia, J. Sci. Res. Inst. Tokyo, 43 (1948) 31-33.


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Triorganotin Hydrides Through New (Diorganostannyl)lithiums, Organometallics, 13 (1994) 24-25.

[28] C. Beermann, C. Hartmann, Über metallorganische Derivate des Acteylens. I, Z. Anorg. Allg.

Chemie, 276 (1954) 20-32.

[29] E.F. Córsico, R.A. Rossi, Sequential Photostimulated Reactions of Trimethylstannyl Anions with

Aromatic Compounds Followed by Palladium-Catalyzed Cross-Coupling Processes, J. Org. Chem., 67

(2002) 3311-3316.

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[31] C.C. Yammal, J.C. Podesta, R.A. Rossi, Reactions of Triorganostannyl Ions with Haloarenes in

Liquid Ammonia. Competition between Halogen-Metal Exchange and Electron-Transfer Reactions, J.

Org. Chem., 57 (1992) 5720-5725.






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100 (1978) 2779-2789.

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In-situ 119Sn NMR Studies in Liquid Ammonia, Organometallics, 29 (2010) 3862-3867.

[37] J. Majnusz, J.M. Catala, R.W. Lenz, Liquid Crystal Polymers-11. Structure-Property Relationships in

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1046.

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2,5-dialkoxyterephthalate)s, Makromol. Chem., Rapid Commun., 7 (1986) 407-414.

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Chapter V

From Poly(dialkylstannane)s to

Poly(diarylstannane)s: Comparison of Synthesis

Methods and Resulting Polymers

109

1. Introduction

Polystannanes are hitherto the only class of characterized, organometallic poly-

mers which comprise a linear polymer backbone of covalently interconnected metal

atoms. A number of methods have been advanced to synthesize poly(diorgano-

stannane)s, in particular Wurtz reactions [1-4], electrochemical reactions [5-7]

and hydrostannylation reactions [8, 9]. However, most of these reactions suffer

from drawbacks such as pronounced formation of cyclic stannanes as byproducts,

low yields, low molecular weights or poor reproducibility.

Recently, a facile synthesis route was developed for poly(dialkylstannane)s [10]

and poly[bis(ω-phenylalkyl)stannane]s [11]. Thereby, diorganostannanes R2SnH2

are polymerized in the presence of the catalyst precursor chlorotris(triphenylphos-

phine)rhodium(I) [RhCl(PPh3)3] by dehydrogenation to result in linear polymers

which can be isolated in high yields. Depending on the side groups, uncommon

thermal behavior (e.g. liquid crystallinity below room temperature) was observed.

Further, due to σ-delocalization of the electrons along the polymer main chain,

electric semiconductivity was anticipated and indeed demonstrated for

poly(dibutylstannane) so far [12].

Interesting properties are expected for polymers with extended σ-π-

delocalization of the electrons along the main chain and the side groups, as

reported earlier for poly(diarylsilane)s [13] and poly(diarylstannane)s [14]. These

aspects, together with the fact that polystannanes can be oriented by various

techniques to yield materials with anisotropic behavior, such as dichroism, attract

significant attention to such materials. Remarably, while poly(dialkylstannane)s

have been extensively studied [1-3, 5-7, 10, 12, 15-18], the synthesis of poly(di-

arylstannane)s was little considered due to the insolubility of typical representatives

110

Scheme 1. Overview of the reaction types for the preparation of polystannanes investigated in this

work.

like poly(diphenylstannane) [19], and therefore materials properties of well defined

poly(diarylstannane)s have still only been modestly explored.

Accordingly, in this study we compare the efficiency and applicability, respect-

ively, of polystannanes prepared by catalytic dehydropolymerization with two new

methods, i.e. polymerization of diorganostannanes, R2SnH2, under the action of

tetramethylethylenediamine (TMEDA) and polymerization of dichlorodiorgano-

stannes, R2SnCl2, with sodium in liquid ammonia (Scheme 1). All methods were

applied to monomers of the type R2SnX2, with X = H or Cl (as appropriate for the

particular route) and R = butyl, phenyl, 4-butylphenyl, as well as to Bu(Ph)SnX2.

This permits systematic investigation of the influence of alkyl and aryl groups on

the polymerization method and the properties of the resulting polymers, respecti-

vely. For instance, the presence of flexible chains bound directly or via aryl groups

to the polymer backbone may increase the solubility [20-26], while the presence of

aryl groups might increase the stability towards light, which has been reported to

be low for dissolved- and moderate for solid poly(dibutylstannane) [18].

R2SnH2[RhCl(PPh3)3]

- H2(R2Sn)n

R2SnH2TMEDA

- H2(R2Sn)n

R2SnCl2 - NaCl(R2Sn)n

Na / NH3 (l)

111

2. Polymerization

Diorganostannanes and dichlorodiorganostannanes were used as starting

materials for the synthesis of poly(diphenylstannane), poly[bis(4-butylphenyl)-

stannane], poly[butyl(phenyl)stannane] and poly(dibutylstannane). The

monomers, when not commercially available, were synthesized by Grignard

reaction of tetrachlorostannane with the corresponding organomagnesium halide

to obtain the tetraorganostannane, which was subsequently converted in a

Kozeschkow type reaction with additional tetrachlorostannane to result dichlorodi-

organostannane [27, 28]. To yield diorganotin dihydrides, the latter was treated

with an excess of LiAlH4 [29, 30].

Three different polymerization methods were employed (Scheme 1). The

reaction with Wilkinson’s catalyst, [RhCl(PPh3)3], probably proceeds by oxidative

addition of Sn-H bonds to Rh(I) centers [17]; the one with TMEDA most likely

via radicals as described for the reaction of R2SnXH with pyridine [31]; and that

with Na in liquid ammonia NH3 (l) via stannide ions [32, 33], and including a

radical process, as found in Chapter IV. The polystannanes indicated above could

indeed be synthesized; yet the schematic overview in Scheme 2 shows that, in fact,

each polymerization method is favorable for polymers with particular substitutents.

While polymerization with Na/NH3 (l) was especially appropriate for poly(diaryl-

stannane)s, the competence of Wilkinson’s catalyst was quite complementary to

that of TMEDA for the compounds explored. Obviously, phenyl and 4-butyl-

phenyl groups restrict the efficiency of the former and promote the performance of

the latter method. In the following, we will refer to characteristics of the individual

reactions. Note that molar masses of the polystannanes were estimated by GPC

analyses (see Experimental section and Table 2), as discussed elsewhere [17].

112

Scheme 2. Polymerization methods for the preparation of polystannanes. Symbols + : Mw above

8 kg/mol, no cyclic byproducts; the presence of polydiphenylstannane (not accessible to GPC

analysis due to insolubility) was deduced from other methods (elemental analysis, UV/Vis

spectra); - : molar masses below 8 kg/mol and/or cyclic byproducts; X : very slow or no reaction

and/or absence of polymeric products.

Polymerization with Wilkinson’s catalyst

Wilkinson’s catalyst is suited mainly for the polymerization of dibutylstannane.

The reaction proceeded rapidly in toluene at room temperature, and poly(dibutyl-

stannane) was isolated by precipitation from methanol at -78°C. 119Sn NMR

spectra showed one signal at -190 ppm, in agreement with literature values [18],

while signals of cyclic byproducts were absent in the spectra.

Poly[butyl(phenyl)stannane] also formed under the action of Wilkinson’s

catalyst; however, precipitation from methanol or other solvents was not successful.

Sodium in liquid ammonia + + ‐

Snn

Sn

n

Sn

n

Sn

n

Wilkinson’s catalyst X +X ‐TMEDA + + X

‐+

113

Thus, the solvent was evaporated to leave the reaction products. The resulting

compound featured a wide molar mass distribution (cf. Table 2). 119Sn NMR

spectra showed a broad signal at -197 ppm, in the common range of

polystannanes.

Polymers containing two aromatic substituents per tin atom essentially could

not be obtained with Wilkinson’s catalyst, apart from a minor fraction of

poly(diphenylstannane) when the reaction was performed at 70 °C. The reaction

mixtures showed the complete disappearance of Sn-H vibrations at 1854 cm-1 in

IR spectra, and of the signals associated to Ph2SnH2 in 1H and 119Sn NMR

spectra, but red or brown solids or viscous oils arose as main products. These

products featured no signal in 119Sn NMR spectra and we failed to consistently

interpret the results of other analyses.

Polymerization with TMEDA

Reactions in the presence of TMEDA were performed in diethyl ether at

room temperature. While the conversion of dibutylstannane was not efficient (see

Experimental section), the reaction of diphenylstannane with TMEDA yields a

yellow precipitate. Elemental analysis revealed only few impurities in the

poly(diphenylstannane). As this polymer was insoluble in all solvents tested, the

molar mass could not be determined. Extraction of the products with

dichloromethane CH2Cl2 did not give rise to any signals in 119Sn NMR spectra,

indicating that cyclic oligo(diphenylstannane)s did not form in significant

quantities, since those species dissolve in the solvent. Butyl(phenyl)stannane and

bis(4-butylphenyl)stannane polymerized to highly viscous oils which were soluble

in common organic solvents. The monomer concentration was varied between

10 g/L and 40 g/L and the reaction time between 10 min and 45 h. Suitable

114

conditions were found at a concentration of 10 g/L (26 mmol/L) and a reaction

time of 30 min for bis(4-butylphenylstannane). The vanishing of the Sn-H

vibration at 1850 cm-1 in IR spectra of reaction mixtures indicated complete

conversion of this monomer. For the polymerization of butyl(phenyl)stannane also

monomer concentrations of 10 g/L (39 mmol/L) was applied but the reaction

time was elongated to 40 min to yield the highest molar masses. Under these

conditions, weight-average molar masses (Mw) of 46 kg/mol and 13 kg/mol were

found for the two polymers (Table 2). The polydispersity index (PDI) amounted

for poly[butyl(phenyl)stannane] to 1.8 – 2.1, i.e. to a value frequently obtained for

radical polymerizations and polycondensations, while the PDI of poly[bis(4-butyl-

phenyl)stannane] had a value of 3.2 – 3.3. It appears that the molar mass, at least

of poly[butyl(phenyl)stannane], decreased upon increase of the reaction time

beyond 30 min. Higher monomer concentrations resulted in lower molar masses.

Poly[bis(4-butylphenyl)stannane] and poly[butyl(phenyl)stannane] featured a

signal at -197 ppm in 119Sn NMR spectra, i.e. in the typical region of

polystannanes. Again, no evidence for formation of cyclic oligomers was observed.

Polymerization with sodium in liquid ammonia

The monomers with two aromatic groups connected to the tin atom were

better suited for polymerization in liquid ammonia than the alkyl substituted

monomers. Conversion of dichlorodiphenylstannane with two molar equivalents of

sodium resulted in immediate precipitation of a shiny yellow product. The material

obtained was insoluble even at elevated temperature. The product was extracted

with hot CH2Cl2 to detect soluble reaction byproducts such as cyclic

oligostannanes by 119Sn NMR analysis, but no indication on their formation was

115

found. Elemental analysis of the material was in agreement with the composition

of poly(diphenylstannane).

Polymerization of dichlorobis(4-butylphenyl)stannane resulted in a product

with a bimodal molar mass distribution but an unsatisfactory elemental analysis.

119Sn NMR experiments displayed a signal at -197 ppm indicating the presence of

polystannane. Also in the case of poly[butyl(phenyl)stannane), 119Sn NMR

spectroscopy revealed a broad signal at -197 ppm and did not show the presence of

cyclic byproducts.

Treatment of dichlorodibutylstannane with two molar equivalents of sodium

in liquid ammonia caused immediate precipitation of a yellow product. Yet 119Sn

NMR spectra disclosed not only a broad signal at -190 ppm representing linear

polystannane but also signals of cyclic oligostannanes at -202 ppm and -203 ppm.

3. Materials Properties

The molar masses determined for the soluble polymers by GPC analysis are

summarized in Table 2.

Table 2. Weight-average molar mass (Mw kg/mol) and polydispersity indices (PDI) of polystan-

nanes prepared by different synthetic methods.

Wilkinson’s cat. TMEDA NH3/Na

Mw PDI Mw PDI Mw PDI

(SnBu2)n 57 2.2

no polymer obtained

5 b) 2.3

(SnBuPh)n a) a)

46 2

<5 ~2.5

[Sn(4-BuPh)]n no polymer obtained 13 3.2 8c) 1.5c)

a) Broad molar mass distribution with the highest value detected at about 30 kg/mol and a high fraction of low molar mass products as low as 1.5 kg/mol. b) Contains also cyclic oligomers. c) Bimodal molar mass distribution; the value only refers to that of the high molar mass fraction.

116

Figure 1. GPC traces of the products of the reactions intended to generate (a) poly(dibutyl-

stannane), (b) poly[butyl(phenyl)stannane] and (c) poly[bis(4-butylphenyl)stannane], synthesized

with Wilkinson’s catalyst (solid), TMEDA (dashed) and sodium in liquid ammonia (dotted).

The respective values (Mw between 5 kg/mol – 60 kg/mol) and polydispersity

indices (around 2 – 3), of the polystannanes obtained in this work are in the range

of those reported previously [11]. Clearly, the polymerization method strongly

influenced the molar mass, as reflected by the GPC traces shown in Figure 1. The

a

103 104 105 106 107

Inte

nsity

RI

Molar Mass / g mol-1

103 104 105 106 107

Inte

nsity

RI


103 104 105 106 107

Inte

nsity

RI


1 10 100 1000

1 10 100 1000

1 10 100 1000molar mass / kg mol-1

molar mass / kg mol-1

molar mass / kg mol-1

Inte

nsity

RI

Inte

nsity

RI

b

c

Inte

nsity

RI

Snn

Snn

Snn

117

highest molar masses were obtained for poly(dibutylstannane) synthesized with

Wilkinson’s catalyst (57 kg/mol) and poly[bis(4-butylphenyl)stannane] prepared in

the presence of TMEDA (46 kg/mol). Rather low molar masses (with respect to

the soluble polymers) were obtained with the Na/NH3 synthetic route.

In the case of poly(diphenylstannane), material properties were investigated for

the polymer that featured the best values in elemental analysis, i.e. the polymer

resulting from the Na/NH3 synthesis method. For studies of the other polymers,

those obtained with the method which yielded the highest molar masses were

used. Degradation temperatures, relative stability, wavelength at maximum

absorption in UV/Vis spectra (λmax) and dichroic ratio of polymer films produced

by shearing, solubility and phase transitions are summarized in Table 3, and

discussed in the following.

Table 3. Selected material properties of polystannanes synthesized with the suitable method.

compound: (SnBu2)n [SnBu(Ph)]n [Sn(4-BuPh)2]n (SnPh2)n

synthesized with: Wilk. Cat. TMEDA TMEDA Na/NH3

molar mass [kg/mol] 57 13 46 -

degradation temperature [°C] 250 300 320 350

thermal phase transitions [°C] 0 no phase transition observed

absorption max. UV/Vis [nm] 390 410 420 470

dichroic ratio after shearing 2 2.1 1 1.7

solubility common organic solventsa) not solublea)

a) e.g. dichloromethane, toluene, hexane, diethyl ether, THF

118

Thermal properties

Thermogravimetric analysis (TGA) of the polystannanes (Figure 2) clearly

showed increasing thermal stability with increasing aromatic content in the

polymers. The decomposition temperature increased markedly from 250 °C for

poly(dibutylstannane) to 350 °C for poly(diphenylstannane). At 400 °C,

decomposition appears to be complete for all polystannanes. However, it is evident

that the residual mass is clearly below the mass fraction of tin in the polymers; this

can be explained by formation of volatile organotin compounds at elevated

temperatures.

Differential scanning calorimetry revealed the previously described phase

transition of poly(dibutylstannane) synthesized with Wilkinson’s catalyst [10],

while no phase transition was observed between -50 °C and 200 °C for the other

polymers.

Figure 2. Thermogravimetric analysis (TGA) of poly(dibutylstannane) (solid), poly[butyl-

(phenyl)stannane] (dotted), poly[bis(4-butylphenyl)stannane] (dash-dot) and poly(diphenyl-

stannane) (dashed).

Temperature / °C

wei

ght /

%

100 200 300 400 500

20

40

60

100

80

0

119

Relative stability at ambient temperature

Visual examination of solutions and solid polymers showed again, as in the

case of thermal stability, that polystannanes with two aryl groups bound to each tin

atom were more stable at ambient than polymers which contained Sn-alkyl bonds.

Degradation, as judged by the loss of the characteristic yellow color, was far slower

for the poly(diarylstannane)s than for the poly(dialkylstannane)s and

poly[alkyl(aryl)stannane]s. Obviously, not only one but two aryl groups are

required to provide enhanced stability at ambient.

UV/Vis spectra and dichroic ratio of sheared poly(stannane)s

Poly(dibutylstannane) with pure σ-delocalization showed an absorption

maximum (λmax) at 390 nm, i.e. in the range common for polystannanes [16, 34].

UV/Vis absorption spectra reveal that phenyl groups induce a bathochromic shift

of the wavelength at maximum absorbance (Table 3 and Figure 3), which might be

associated with increasing delocalization of electrons with increasing number of

aromatic groups in the polymer. For instance a bathochromic shift of 20 nm was

found for poly[butyl(phenyl)stannane] in comparison to poly(dibutylstannane).

However, apart from the nature of the side chains, also the conformation of the

polystannane main chain may influence λmax. This phenomenon is well known for

polysilanes [35] and was also described for polystannanes, where delocalization was

found to be at a maximum for planar zig-zag structures [14, 36]. Thus, part of the

poly(diphenylstannane) might be present in the planar zig-zag conformation,

leading to a low band gap polymer with a pronounced bathochromic shift of the

absorption maximum. In contrast to poly(diphenylstannane) no pronounced

bathochromic shift was observed for poly[bis(4-butylphenyl)stannane]. Apparently

120

Figure 3. Optical absorption spectra of poly(dibutylstannane) (solid), poly[butyl(phenyl)stannane] (dotted),

poly[bis(4-butylphenyl)stannane] (dash-dot), and poly(diphenylstannane) (dashed) recorded for thin films

on glass slides and arbitrarily adjusted in intensity for facile comparison.

the 4-butylphenyl side group inhibits the planar zig-zag conformation and there-

fore a further bathochromic shift.

Optical microscopy investigations of samples placed between crossed polarizers

revealed that all polymers, except poly[bis(4-butylphenyl)stannane], could readily

be oriented (Figure 4); preferred orientation in the direction of shear was also

evident from UV/Vis spectroscopy using polarized light. Light was preferentially

absorbed for a parallel position of the polarization plane to the orientation

direction of the polymer, leading to dichroic ratios around 2 at the absorption

maximum (Table 3).

When subjected to shear, poly[bis(4-butylphenyl)stannane] featured hardly

any alignment at all (Figure 4b; Table 3). Notably, in contrast to the other

polymers which exhibited the consistence of soft powders, poly[bis(4-butyl-

phenyl)stannane] was a highly viscous oil. Apparently the quasi-liquid state of

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

[nor

mal

ised

]

Wavelength / nm

0.0

0.6

0.4

0.2

1.0

0.8R

el. A

bsor

banc

e

400 500 600 700 800Wavelength / nm

300

121

Figure 4. Polarized optical absorption spectra of sheared films of poly[butyl(phenyl)stannane] (a)

and poly[bis(4-butylphenyl)stannane] (b) at different angles φ between the polarization plane of

the light and the shearing direction (parallel: φ=0 °, perpendicular: φ=90 °). (c) Optical microscopy

images of sheared films taken between crossed polarizers at angles of 45 ° to the polarizers and 0 °

to one of the polarizers (i.e. 90 ° to the other polarizer). The films were deposited on glass slides.

this polymer allowed rapid rearrangement of the polymer chains back into the

isotropic state during shearing.

Solubility

As already indicated above, poly(dibutylstannane) , poly[butyl(phenyl)-

stannane] and poly[bis(4-butylphenyl)stannane] were soluble in common organic

solvents (e.g. CH2Cl2, toluene, hexane, THF). In fact, it is not uncommon that

alkyl groups provide solubility to polymers, mainly as a result of entropy gain upon

dissolution - due to an increase in mobility of alkyl groups upon transition from

the solid to the dissolved state. By contrast, however, no solvent for poly(diphenyl-

stannane), the only polymer without alkyl groups, could be found.

0°45°Wavelength /nm

Abs

.

300 500400 600

Wavelength /nm

Abs

.

300 500400 600

a b c

0°

90°

45°

0°

90°45°

122

4. Conclusions

Each of the three polymerization methods here explored is particularly suited

for the synthesis of specific polymers. The route employing Wilkinson’s catalyst is

most beneficial for the preparation of poly(dibutylstannane), TMEDA for

polystannanes containing at least one aromatic group per Sn atom and Na/NH3 for

polystannanes with two aromatic groups per Sn atom. With the most suited

method, polymers of weight-average molar masses in the range of roughly 10

kg/mol to 60 kg/mol were obtained, depending on the particular structure of the

macromolecules.

Not surprisingly, the material properties are strongly influenced by the

substituents along the polymeric chains. Poly(diarylstannane)s exhibited higher

thermal stability and were more resistant at ambient than the other two

polystannanes, while butyl groups, also in 4-butylphenyl, improved the solubility.

Finally, phenyl groups resulted in a bathochromic shift, which might be due to

delocalization of electrons as well as the particular conformations of the polymer

chains. Except in the case of poly[bis(4-butylphenyl)stannane], which was present

in a liquid-like state, the polystannanes could readily be oriented.

123

5. Experimental


99.999 %), dichlorodibutylstannane from ABCR GmbH (Karlsruhe, Germany)

and dichlorodiphenylstannane from Sigma Aldrich (Buchs, Switzerland). Both

substances were recrystallized twice by dissolving in boiling pentane and

subsequent precipitation of the product at -78°C. CD2Cl2 (99.9% D) was

purchased from Cambridge Isotope Laboratories (ReseaChem GmbH, Burgdorf,

Switzerland), and organic solvents from Fluka (Buchs, Switzerland). TMEDA was

dried with molar sieve; all other chemicals were used as received from the

respective chemical suppliers.

Methods. NMR spectra were recorded on a Bruker UltraShield 300 MHz/54 mm

Fourier transform spectrometer employing standard 5 mm broad band probes. The

samples were dissolved in CD2Cl2. For the investigation of reaction solutions,

D2O-capillaries was inserted into the NMR tube. In order to inhibit

decomposition of the samples by ambient light, the NMR tubes were wrapped in

white tissue and subsequently covered with aluminum foil which was only removed

immediately before inserting the samples in the spectrometer. Elemental analysis

were performed by the Microelemental Analysis Laboratory of the Department of

Chemistry at ETH Zürich. Gel permeation chromatography was performed with a

GPC instrument from Viscotek (VE7510) equipped with degasser, VE1121

solvent pump, VE520 autosampler and Model 301 triple detector array. A PL gel

5 μm Mixed-D column from Polymer Laboratories Ltd. (Shropshire, United

Kingdom) was used. Preliminary test showed that the refractive index detector

revealed the most reproducible values. Therefore, the reported data refer to molar

masses obtained with this detector. For calibration, atactic poly(styrene) standards

from Fluka were employed. Samples were dissolved in THF with 2.5 % v/v

124

toluene which served as marker. The THF eluent flow amounted to 1 mL/min.

For optical microscopy a Leica DMRX polarizing microscope was used at 20 fold

magnification. UV/Vis measurements were performed in transmission with a

Perkin Elmer Lambda 900 spectrophotometer equipped with rotating polarizers.

The polymers were applied as films on glass slides. Infrared spectra were recorded

with a Bruker Vertex 70 FTIR spectrometer with the attenuated total reflection

(ATR) technique by using a Si-crystal. The samples were directly deposited on the

crystal with a syringe or a spatula. Differential scanning calorimetry (DSC)

analysis were performed with a DSC822e instrument (Mettler Toledo, Greifensee,

Switzerland) equipped with an intracooler, and thermal gravimetric analysis

(TGA) with a TGA/SDTA851e from Mettler Toledo under nitrogen

atmosphere. The heating and cooling rates were 5 °C/min.

Synthesis of the starting materials

Tetrakis(4-butylphenyl)stannane

42.6 g (0.2 mol) 4-butylphenylbromide were treated with 5.8 g (0.24 mol)

magnesium in 300 mL THF and heated under reflux for 1 h to obtain 4-butyl-

phenylmagnesium bromide. Subsequently, the mixture was cooled to 0 °C and

4.7 mL (0.4 mol) SnCl4 suspended in 200 mL THF were added before heating for

another hour under reflux. Thereafter, the THF was removed in vacuo and the

product extracted with hexane in a soxhlet extractor. The solvent was removed in a

rotary evaporator and the resulting product dried in vacuum (0.1 mbar, 12 h).

Yield: 80 %; 1H NMR (299.948 MHz, CDCl3, in ppm): δ = 0.4-1.0 [t, 12 H],

1.3-1.5 [m, 8 H], 1.6-1.7 [m, 8 H], 2.55-2.6 [t, 8 H], 7.2-7.3 [d, 8 H], 7.8-7.9 [d,

8 H]; 13C NMR (75.50 MHz, CDCl3, in ppm): δ= 13.93, 22.45, 33.73, 35.82,

135.2, 129.0, 137.5, 143.7; 119Sn NMR (111.96 MHz, CDCl3, in ppm): δ= -125.

125

Dichlorobis(4-butylphenyl)stannane

25.8 g (0.04 mol) tetrakis(4-butylphenyl)stannane and 4.6 mL (0.04 mol)

SnCl4 in 200 mL heptane were heated under reflux for 4 h. The hot solution was

filtered to remove inorganic side products and the solvent was removed in a rotary

evaporator. Thereafter, the product was dried in vacuum (0.1 bar, 12 h). Yield:

83 %; 1H NMR (299.948 MHz, CDCl3, in ppm): δ = 0.94-1.01 [t, 6 H], 1.2-1.4

[m, 4 H], 1.5-1.6 [m, 4 H]; 2.5 [t, 4 H]; 7.1-7.2 [d, 4 H]; 7.6 [d, 4 H]; 119Sn

NMR (111.96 MHz, CDCl3, in ppm): δ = -19.6.

Bis(4-butylphenyl)stannane

0.5 g (0.013 mol) LiAlH4 were suspended under nitrogen atmosphere in

150 mL degassed diethyl ether. 6 g (0.013 mol) dichlorobis(4-butylphenyl)-

stannane were placed in a dropping funnel and dissolved in 100 mL degassed

diethyl ether. This solution was added dropwise while cooling the reaction mixture

to 0 °C. Subsequently, the reaction mixture was stirred for one hour. Unreacted

LiAlH4 was neutralized by drop wise addition of 100 mL degassed water. The

organic phase was separated with a cannula and washed with 200 mL degassed

saturated aqueous disodium tatrate solution. Afterwards the organic phase was

dried with CaCl2 for 30 minutes. The solution was filtered and the diethyl ether

was removed. The product was purified by drying in vacuo (about 0.1 mbar) for 1h.

The product was a colourless liquid. No melting point could be measured. Due to

instability the compound was finally stored in brown-colored septum vials at 4 °C.

Yield: 51%; 1H NMR (299.948 MHz, D2O, in ppm): δ = 0.94-0.99 [t, 6 H],

1.3-1.4 [m, 4 H], 1.5-1.6 [m, 4 H]; 2.5-2.6 [t, 4 H]; 7.2 [d, 4 H]; 7.6 [d, 4 H],

6.3 [s, 2 H, 1J(H-119/117Sn) = 1907/1821 Hz]; 119Sn NMR (111.96 MHz, D2O, in

ppm): δ = -234.3.

126

Butyltriphenylstannane, dichlorobutyl(phenyl)stannane and butyl(phenyl)-

stannane were synthesized according to literature [37-39].

Reactions of diorganostannanes with Wilkinson’s catalyst

In a typical reaction 0.04 mmol (3 mol% with respect to diorganostannane)

Wilkinson’s catalyst were placed under nitrogen atmosphere in a Schlenk tube and

dissolved in 10 mL of toluene. The flask was wrapped with white tissue which was

subsequently surrounded by aluminum foil before 1.3 mmol of the respective

diorganostannane were added dropwise with a syringe through a septum.

Reaction mixtures with dibutylstannane were stirred for one hour before

cooling to -78 °C in an isopropanol/dry-ice bath and then poured into 50 mL of

precooled methanol at -78 °C. Poly(dibutylstannane) precipitated and was filtered

off under nitrogen atmosphere and dried in vacuo (24 h, 0.1 mbar). Elemental

analysis (in % w/w, calculated values in brackets): C 40.82(41.25), H 7.53 (7.79).

Obtained molar masses: 57 kg/mol (corresponds to approximately 250 n-Bu2Sn

units);

Applying the above conditions to diphenylstannane yielded a red solid with an

elemental composition well apart from that of poly(diphenylstannane). Elemental

analysis (in % w/w, calculated values in brackets): C 43.86 (52.81), H 3.42 (3.69).

If the reaction solution was heated to 70 °C for one hour after adding

diphenylstannane and stirring of about 5 min at room temperature, a minor

quantity of yellow precipitate formed which was filtered and dried under reduced

pressure (0.1 mbar). The composition of this product was in the range of the

corresponding polymer. Elemental analysis (in % w/w, calculated values in

brackets): C 51.89 (52.81), H 3.69 (3.69). When the filtrate was cooled to -78 °C

and poured into 100 mL methanol of the same temperature, a light yellow solid

127

precipitated which turned immediately red upon filtration. Elemental analysis (in

% w/w, calculated values in brackets): C 43.82 (52.81), H 3.42 (3.69).

The reaction mixture of bis(4-butylphenyl)stannane was also warmed to 70 °C

after stirring for one hour at room temperature. Subsequently the solution was

cooled to -78 °C and poured into 100 mL methanol; As no precipitate formed, the

solvents were removed completely in vacuo (0.1 mbar) resulting in a product with

an elemental analysis different from that of poly[bis(4-butylphenyl)stannane].

Elemental analysis (in % w/w, calculated values in brackets): C 52.69 (62.38),

H 5.53 (6.80).

Catalytic dehydrogenation of butyl(phenyl)stannane was performed at room

temperature. The reaction solution was stirred for two hours and subsequently

cooled to -78 °C and poured into 50 mL methanol at the same temperature. The

solvent was removed and the product dried under reduced pressure (0.1 mbar,

24 h). Elemental analysis (in % w/w, calculated values in brackets): C 46.05

(47.49), H 5.33 (5.58).

Reactions of diorganostannanes with N,N,N’,N’- tetramethylethylenediamine

(TMEDA)

In a typical experiment 0.5 mmol monomer were placed under nitrogen

atmosphere in a schlenk tube and dissolved in 10 mL diethyl ether. The flask was

wrapped with white tissue which was subsequently surrounded by aluminum foil

before 0.5 mmol of TMEDA were added with a syringe. After a given time

(between 10 min and 45 h) the solvent was evaporated. Polymers were dried in

vacuo (ca. 0.1 mbar, 24 h).

128

Poly(dibutylstannane): After stirring the reaction mixture over night the

polymer was obtained in 10 % yield according to 119Sn NMR spectroscopy. The

rest of the starting material remained unreacted.

Poly[butyl(phenyl)stannane]: Monomer concentrations between 39 mmol/L

(10 g/L) and 157 mmol/L (40 g/L) were used. Elemental analysis (in % w/w,

calculated values in brackets): C 46.85 (47.49); H 5.62 (5.58). Obtained molar

masses: 13 kg/mol (corresponds to approximately 51 BuPhSn units).

Poly(diphenylstannane): Elemental analysis (in % w/w, calculated values in

brackets): C 50.55 (52.81); H 3.90 (3.69).

Poly[bis(4-butylphenyl)stannane]: Monomer concentrations between

26 mmol/L (10 g/L) and 103 mmol/L (40 g/L) were used. Elemental analysis (in

% w/w, calculated values in brackets): C 63.05 (62.37); H 6.92 (6.80). Obtained

molar masses: 46 kg/mol (corresponds to approximately 120 (4-BuPh)2Sn units).

Reactions of dichlorodiorganostannanes with sodium in liquid ammonia

Sodium (8 mmol) was dissolved under nitrogen atmosphere in 90 mL of liquid

ammonia at -78 °C by stirring for 15 min. After the flask was wrapped with white

soft tissue and surrounded by aluminum foil, a quantity of

dichlorodiorganostannane (4 mmol) dissolved in 10 mL THF was slowly added

through a septum under continuous stirring. The polymer precipitated after about

10 to 15 seconds and the solution was stirred for another five minutes before the

ammonia was evaporated by warming the reaction solution to room temperature in

a nitrogen stream. Thereafter the THF was removed in vacuo (about 0.1 mbar).

The resulting solids were washed with 50 mL of a water/ethanol (9:1) mixture

until no chloride could be detected in the washing solution (usually 3-4 times,

129

until the addition of 5 mL saturated AgNO3 solution did not lead to the visible

formation of AgCl precipitates) and thereafter three times with 50 mL CH2Cl2.

Finally the product was dried in vacuo (about 0.1 mbar, 24 h).

Poly(dibutylstannane): Elemental analysis (in % w/w, calculated values in

brackets): C 40.75 (41.25), H 7.56 (7.79). Obtained molar masses: 5 kg/mol

(corresponds to approximately 22 Bu2Sn units).

Poly[butyl(phenyl)stannane]: Elemental analysis (in % w/w, calculated values

in brackets): C 44.90 (47.49); H 5.66 (5.58) Obtained molar masses: <5 kg/mol

(corresponds to approximately <13 BuPhSn units).

Poly(diphenylstannane): Elemental analysis (in % w/w, calculated values in

brackets): C 51.98 (52.81); H 3,66 (3.69).

Poly[bis(4-butylphenyl)stannane] : Elemental analysis (in % w/w, calculated

values in brackets): C 59.22 (62.37); H 6.43(6.80). Obtained molar masses:

8 kg/mol (corresponds to approximately 22 (4-BuPh)2Sn units).

130

References

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[2] N. Devylder, M. Hill, K.C. Molloy, G.J. Price, Wurtz Synthesis of High Molecular Weight

Poly(dibutylstannane), Chem. Commun., (1996) 711-712.



[4] D. Miles, T. Burrow, A. Lough, D. Foucher, Wurtz Coupling of Perflourinated

Dichlorostannanes, J. Inorg. Organomet. Polym., 20 (2010) 544-553.

[5] M. Okano, N. Matsumoto, M. Arakawa, T. Tsuruta, H. Hamano, Electrochemical Synthesis

of Dialkylsubstituted Polystannanes and their Properties, Chem. Commun., (1998) 1799-1800.

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Germane Copolymers, Electrochem. Commun., 2 (2000) 471-474.



[8] L.R. Sita, A New Strategy for the Synthesis of Homologously Pure Linear Polystannane

Oligomers, Organometallics, 11 (1992) 1442-1444.

[9] K. Shibata, C.S. Weinert, L.R. Sita, Deconvoluting Steric and Electronic Substituent Effects

on the Properties of Linear Oligostannanes: Synthesis and Characcterization of a New Series

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Room-Temperature Liquid-Crystalline Semiconductor Poly(dibutylstannane), Adv. Mater., 18

(2006) 44-47.


[14] V. Lu, T.D. Tilley, Low-Band-Gap, s-Conjugated Polymers: Poly(diarylstannanes),

Macromolecules, 29 (1996) 5763-5764.

[15] H.K. Sharma, K.H. Pannell, in: A.G. Davies, M. Gielen, K.H. Pannel, E.R. Tiekink (Eds.)

Tin Chemistry: Fundamentals, Frontiers, and Applications, John Wiley & Sons, Ltd, 2008, pp.

285-411.



131





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Macromolecules, 19 (1986) 1366-1374.

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phenylene 2,5-dialkoxyterephthalate)s, Makromol. Chem., Rapid Commun., 7 (1986) 407-414.

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[23] J. Majnusz, J.M. Catala, R.W. Lenz, Liquid Crystal Polymers-11. Structure-Property

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[24] H.-R. Dicke, R.W. Lenz, Liquid Crystal Polymers. 14. Synthesis and Properties of

Thermotropic Poly(1,4-alkylphenylene Terephthalates), J. Polym. Sci., Polym. Chem. Ed., 21

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(1957) 369-374.

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Hydride and Lithium Gallium Hydride, and Some of Their Applications in Organic and

Inorganic Chemistry, J. Am. Chem. Soc., 69 (1947) 1199-1203.

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Properties of Hydrides of Elements of the 4th Group of the Periodic System and of Their

Organic Derivatives, J. Am. Chem. Soc., 69 (1947) 2692-2696.

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Organomet. Chem., 474 (1994) C8-C10.

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132

[33] M. Trummer, W. Caseri, Diorganostannide Dianions (R2Sn2−) as Reaction Intermediates

Revisited: In-situ 119Sn NMR Studies in Liquid Ammonia, Organometallics, 29 (2010) 3862-

3867.


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9931-9940.

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Crys. Liq. Cryst., 190 (1990) 265-282.

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n-butylstannane) Observed by UV-Vis and Raman Spectroscopy, Macromolecules, 35 (2002)

1757-1761.

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Chapter VI

Stability of Polystannanes Towards Light

135

1. Introduction

Polystannanes are a unique class of polymers as their backbone consists of

covalently bound metal atoms, which, to our knowledge, has not been reported so

far with any other metal element. These species were first described by Löwig [1]

but gained wide interest in the field of organometallic polymers only in recent

years [2-11], often with a view to their thermal, optical and electronic properties

[2, 5, 6]. However, a considerable drawback of these materials is their limited

stability towards light, in particular when in solution [4]. The light stability and

degradation of linear polystannanes was studied comprehensively only recently [4],

exemplary with poly(dialkylstannane)s, after a facile synthesis method for such

polymers was developed (Scheme 1). Results from this study indicated that,

depending on the solvent, cyclic oligo(dialkylstannane)s or reaction products with

the solvent formed as degradation products.

The stability of poly(diarylstannane)s against light is likely to considerably

deviate from that of poly(dialkylstannane)s, as it was reported that the

photochemical behavior of silanes and polysilanes with aryl groups differs

Scheme 1. Schematic representation of the synthesis and structure of (a) poly(dibutylstannane)

and (b) poly[bis(4-(butylphenyl)stannane].

Sn

H

H

Snn

[RhCl(PPh3)3]

-H2

Sn

Cl

Cl

Snn

Na, NH3

-NaCl

a b

136

significantly from that of related alkyl-substituted compounds [12-15].

Furthermore, replacement of methyl by phenyl groups in polysiloxanes is known to

result in enhanced stability [16-21]. Thus, we devoted this study to a comparison

of stability against light of polystannanes with aromatic and aliphatic groups,

exemplary for poly[bis(4-butylphenyl)stannane] and poly(dibutylstannane), and

attempted to unveil the mechanism of degradation.

Poly[bis(4-butylphenyl)stannane] was selected since unsubstituted poly(di-

phenylstannane) – contrary to poly(dibutylstannane) which dissolves in common

organic solvents like dichloromethane, toluene, tetrahydrofuran (THF) and

hexane - is a rather intractable species that does not dissolve in common solvents.

The derivatized form allows to compare the stability of poly(dialkylstannane)s and

poly(diarylstannane)s under equivalent conditions.

In addition to irradiation experiments of the polystannanes in different

solvents with light of defined wavelengths in a UV/Vis spectrophotometer and

under a so-called daylight lamp, also laser flash photolysis was employed. This

method was previously used extensively for the characterization of compounds

based on elements of the group 14 in the periodic table. In these studies photo-

degradation intermediates were investigated ranging from low molar mass alkyl-

and aryl-substituted carbon-, silicon-, germanium- and tin-centered compounds

[12, 13, 22] to cyclic and linear oligomeric and polymeric silanes with aryl and

alkyl side groups [14, 15, 23-30].

137

2. Results

UV/Vis Light Irradiation. Prior to the light exposure experiments, it was

established that poly(dibutylstannane) dissolved in tetrahydrofuran (THF,

stabilized and unstabilized) and in dichloromethane (CH2Cl2) exhibited a

pronounced absorption maximum at 390 nm [2-4], and poly[bis(4-butylphenyl)-

stannane] dissolved in the same solvents at 422 nm (Figure 1a), which were, hence

subsequently employed to monitor the stability of both polymers.

The stability against light of both polystannanes was studied by exposing

solutions of both polymers directly to the analyzing light of the UV/Vis spectro-

photometer by repeated scanning between 500 nm and 300 nm. The decrease in

absorbance of the samples after a series of scans indicated that the intensity of the

incident light beam was sufficient to cause degradation of the dissolved polymers.

The extent of decomposition of the polymers depended both on the structure of

the polystannanes, as well as the chemical nature of the solvent. In all cases,

however, stannane containing the aromatic substituent poly[bis(4-butylphenyl)-

stannane] was more stable than poly(dibutylstannane).

In a first set of experiments the absorption of poly(dibutylstannane) was

reduced by more than 50 % as compared to the initial absorbance already after less

than three scans, whereas poly[bis(4-butylphenyl)stannane] approached that

degree of reduction after 24 scans (Figure 1b), which represents roughly a 10-fold

increase in stability when comparing poly(diarylstannane) and poly(dialkyl-

stannane) under these conditions. After 10 subsequent scans, the absorbance at the

absorption maximum of poly(dibutylstannane) amounted to ca. 2 % of the starting

value only, while the absorbance of poly[bis(4-butylphenyl)stannane] remained at

138

Figure 1. UV/Vis spectra of poly(dibutylstannane) (grey) and poly[bis(4-butylphenyl)stannane]

(black) recorded after subsequent scans in unstabilized THF (a), where (b) shows the relative

absorbance at the absorption maximum wavelength versus the number of scans. Corresponding

spectra in stabilized THF (c) and in dichloromethane (d). Note that for the solutions in

unstabilized THF the first 10 scans are displayed (a) whereas in spectra (c) and (d) the first 100

scans are shown.

77 % (Figure 1b). Evidently, though, THF turned out to be a remarkably poor

environment to protect polystannanes against light. Interestingly, commercial

THF is typically stabilized with 250 ppm of 2,6-di-tert-butyl-4-methylphenol

(butylhydroxytoluene, BHT). Remarkably, solutions of polystannanes in such

stabilized THF featured a drastic increase in their stability towards light. After 10

scans, over 99 % of the initial absorbance was recorded both for poly(dibutyl-

stannane) and poly[bis(4-butylphenyl)stannane] (Figure 1c); after 100 scans

0.2

0.0

0.8

0.6

1.0

0.4

40 60 80 1001 20Number of scans

Rel

. Abs

orba

nce

max

imum

300 400 500350 450Wavelength / nm

0.2

0.0

0.8

0.6

1.0

0.4A

bsor

banc

e

300 400 500350 450Wavelength / nm

0.2

0.0

0.8

0.6

1.0

0.4

Nor

mal

ized

Abs

orba

nce

300 400 500350 450Wavelength / nm

0.10.20.3

0.0

0.50.6

0.7

0.4

Abs

orba

nce

a b

c d

1

10

1

100

1

100

THFInhib. f ree

THF + BHT

CH2Cl2

THFInhib. f ree

139

Figure 2. Gel permeation chromatograms displayed the decrease in molar mass of (a) poly[bis-

(4-butylphenyl)stannane] (8.5 kg/mol to about 4 kg/mol) and (b) poly(dibutylstannane) (from

15 kg/mol to about 8 kg/mol) dissolved in THF (stabilized) after the different light exposure

times indicated.

96 and 98 %, respectively, remained. The stability of the polymers dissolved in

dichloromethane was in between that of the polymers dissolved in stabilized and

non-stabilized THF. For instance, 87 % of the initial absorption was observed

after 100 scans for poly(dibutylstannane) and 98 % for poly[bis(4-butylphenyl)-

stannane (Figure 1d).

The stability of both dissolved polystannanes against irradiation with a so-

called “daylight-lamp” was monitored by gel permeation chromatography

(Figure 2). Dichloromethane and stabilized THF were employed as solvents (in

non-stabilized THF, degradation was too fast to be followed by GPC). As in the

case of irradiation in the UV/Vis spectrophotometer, results obtained with the

“daylight lamp” showed that the exposure times to reach the same level of

degradation were higher for poly[bis(4-butylphenyl)stannane] than for poly(di-

butylstannane) and lower in dichloromethane than in stabilized THF.

Retention Time / min

Inte

nsity

RI

8 6 4

20 min

0 min

Retention Time / min

Inte

nsity

RI

8 6 4

5 min

0 min

a b

140

GPC analysis revealed that poly(dibutylstannane) and poly[bis(4-butylphenyl)-

stannane] dissolved in stabilized THF decomposed via a continuous reduction of

their molar mass at increasing light exposure time (Figure 2a and b). Solutions of

poly[bis(4-butylphenyl)stannane] showed the same behavior in CH2Cl2, whereas

in the case of poly(dibutylstannane) GPC indicated that the molar mass did not

decrease – only its intensity, consistent with earlier reports [4].

Laser flash photolysis. Laser flash photolysis experiments were conducted with the

setup shown in Figure 3 with solutions of both polymers dissolved in

dichloromethane and stabilized THF. These experiments unveiled the time-

resolved change in UV/Vis absorbance after a laser pulse was applied on the

dissolved polymers. As is evident from the data presented in Figure 4, the

polymers degraded rapidly by irradiation with a laser flash of 355 nm light (third

harmonic of the Nd:YAG laser): the polystannanes are bleached instantaneously

(<1 μs) upon irradiation with the laser pulse. The photochemical degradation is

extremely fast, as shown in Figure 4a and b. Absorption changes in the first milli-

seconds after the initial bleaching process are only minor. Importantly, samples

used for photostability determinations were prepared such, that their initial

Figure 3. Setup for flash photolysis experiments with a laser pulse energy of 60 mJ/pulse at

355 nm. The intensity of the analysis light beam was reduced with a cut off filter at 370 nm and

grey filter with 5 % transmission to avoid degradation of the sample upon irradiation of the

analysis light.

Laserbeam

Cuvette

Analysis light

Filter

Shutter

Detector

141

Figure 4. Time-dependent absorbance difference after a laser pulse applied to a solution of

poly(dibutylstannane) dissolved in (a) dichloromethane and (b) THF, compared to poly[bis-

(4-butylphenyl)stannane] in (c) dichloromethane and (d) in THF.

absorbance at 355 nm was similar by humble variations in the concentration.

Therefore our experiments allow for the comparison of the photosensitivity of the

compounds. This boundary condition was not strictly applied to experiments with

focus on the time resolved absorption change. The bleaching at 370 nm and

420 nm can be used to quantify the loss of poly(dibutylstannane) (50 % with 1

laser pulse) and of poly[bis(4-butylphenyl)stannane] (25 % bleaching with 5 laser

pulses) respectively. Clearly, the photo-stability of the latter is much better despite

the fact that the initial damage by the laser pulse seems to be comparable.

0 2 4

a

c d

b

ΔA

bs.

ΔA

bs.

Time / ms

Time / ms

Time / ms

0

0

-0.2

-0.2

BuCH2Cl2

4-BuPhTHF

ΔA

bs.

0

-0.1

4-BuPhCH2Cl2

0 2 4 6 0 2 4 6

Δ

Abs

.

Time / ms

0

-0.1

BuTHF

0 2 4 66

142

Figure 5. Absorbance difference at the respective absorption maximum wavelengths after laser

flash photolysis of poly[bis(4-butylphenyl)stannane] in dichloromethane (solid line) and THF

(dashed line).

Even though all traces look somewhat similar in the millisecond time range,

fundamental differences are visible if the acquisition time is prolonged three orders

of magnitude, which explains the very different overall stability: while the

bleaching of poly(dibutylstannane) by the laser pulse was largely irreversible,

bleaching of poly[bis(4-butylphenyl)stannane] was reversible to a considerable

extent: after each laser pulse, the measured absorbance of the samples recovered to

90 % of the original value, both with THF and with dichloromethane as solvents

(see Figure 5; data not shown for poly(dibutylstannane) as no significant recovery

was observed).

0 2 4 6Time / s

ΔA

bsor

banc

e

0

-0.2

143

3. Discussion

As demonstrated in this study, the stability of polystannanes against exposure

to light strongly depends on the nature of the organic side groups. Poly[bis-

(4-butylphenyl)stannane] was found to be more stable towards light than poly(di-

butylstannane), in THF as well as in dichloromethane. Although investigations by

laser flash photolysis reveal that the initial photochemical damage is comparable

for both polymers, poly[bis(4-butylphenyl)stannane] “recovered” to a large extent

(about 90 % under the applied experimenttal conditions) within a period of a few

seconds after irradiation. While poly[bis(4-butylphenyl)stannane] was efficiently

degraded by photolysis, its degradation products seem to be able to re-form

polymer chains to a remarkable extent - which results in an apparent stabilization

of this polymer. However, no comparable recombination process was observed for

poly(dibutylstannane). Radical mechanisms may explain our findings. Homolytic

cleavage of a Sn-Sn bond at a random position in a polystannane chain may lead to

two chain ends bearing a radical each. In fact, a chain end with two aromatic

groups bound to a tin atom could readily be more stable and therefore exhibit a

longer lifetime than a chain end with two aliphatic groups due to delocalization of

the radical throughout the aromatic groups. Thus, chain ends with Sn-aryl groups

may be long-lived enough to recombine to a polymer chain, in contrast to chain

ends with Sn-alkyl groups (Scheme 2). The latter more rapidly degrades to cyclic

oligostannanes or by reaction with the solvent [4]. A radical mechanism is further

supported by the fact that degradation in THF is inhibited by the radical scavenger

2,6-di-tert-butyl-4-ethylphenol (BHT), which is used in commercial THF for the

prevention of peroxide formation. Probably, the inhibitor stops the deploymeriza-

tion by radical trapping. Remarkably, different degradation mechanisms of the

polymer molecules themselves were observed. GPC measurements revealed that

144

Scheme 2. Schematic of the principal reactions involved in the decomposition of polystannanes:

Scission of polymer chains by incident light under formation of two radical end groups;

recombination of radicals is the case of poly(diarylstannane)s and the depolymerization in the case

of poly(dialkylstannane)s.

upon irradiation the molar mass of poly[bis(4-butylphenyl)stannane] in CH2Cl2

and THF is subsequently reduced. The same result was found for poly(dibutyl-

stannane) in THF. However, the decomposition of poly(dibutylstannane) in

dichloromethane lead to a decrease in the number of polymer molecules but no

substantial reduction in molar mass of the remaining polymer molecules. This

indicates two different degradation mechanisms – depolymerization by an unzip-

ping mechanism for poly(dibutylstannane) in CH2Cl2 as proposed earlier [4] and

random scission of polymer chains in the other cases. The unzipping of poly(di-

butylstannane) in dichloromethane could be favored by reaction of the tin radicals

with the solvent, as in this system related reaction products have been found [4].

Sn

R

R Sn

R

R Sn

R

R +n n - xx

Sn

R

R Sn

R

R Sn

R

R +nn - xx

Sn R

R Sn

R

R +

Chain scission

Recombination for R = Aryl

Unzipping for R = Alkyl

x x - ncyclo - (SnR2)n

hν

hν

Sn R

R Sn

R

R Sn

R

R

ClCl

+nhν

CH2Cl2 x - n

or

●

x

●

145

4. Conclusions

Poly[bis(4-butylphenyl)stannane], a representative of poly(diarylstannane)s,

was demonstrated to be more stable towards light than poly(dibutylstannane) - a

typical poly(dialkylstannane) – when dissolved in dichloromethane as well as in

THF. While degradation was found to proceed rapidly in unstabilized THF, the

radical scavenger 2,6-di-tert-butyl-4-methylphenol (BHT) strongly reduced the

rate of degradation, indicating that degradation of polystannanes proceeds via a

radical process. Results obtained with laser flash photolysis indicate that the

observed enhanced stability of the polymer with aromatic substituents is, in fact,

not due to a higher stability of Sn-Sn bonds but due to recombination of radicals.

By contrast less stable radicals generated in the polystannanes with aliphatic side

groups lead to rapid degradation of the macromolecular chains (Scheme 2).

Analyses of reaction solutions by GPC unveiled that two different decomposition

mechanisms may occur - random scission of polymer chains or unzipping; the

latter might be supported by reaction of polystannane radicals with solvent

molecules.

146

5. Experimental


99.999 %) and dichlorodibutylstannane from ABCR GmbH (Karlsruhe,

Germany). The latter compound was recrystallized twice by dissolution in boiling

pentane and subsequent precipitation at -20 °C. Organic solvents were acquired

from Fluka (Buchs, Switzerland), except the inhibitor-free tetrahydrofuran (THF),

which was ordered from Sigma-Aldrich (Buchs, Switzerland; typical commercial

THF is stabilized with 250 ppm of the radical scavenger 2,6-di-tert-butyl-4-

methylphenol).

Methods

Synthesis of poly[bis(4-butylphenyl)stannane]

Sodium (8 mmol) was dissolved in 90 mL of liquid ammonia at -78 °C by

stirring for 15 min. To this solution, a quantity of dichlorobis(4-butylphenyl)-

stannane (2 mmol) dissolved in 10 mL THF was added through a septum and

stirred for 30 min. Subsequently, the flask was completely wrapped with white soft

tissue and surrounded by aluminum foil; another portion of dichlorobis(4-butyl-

phenyl)stannane (2 mmol) dissolved in 10 mL THF was added through a septum.

The polymer precipitated after about 10 s to 15 s. After two minutes, the ammonia

was evaporated by warming the reaction solution to room temperature under a

nitrogen stream, and the THF was removed at room temperature in vacuo (about

0.1 mbar, 12 h). The resulting solid was dissolved in dichloromethane, insoluble

residues were filtered off, the solvent was removed and the residue dried again

(0.1 mbar, 24 h). The polymer (dissolved in THF) possessed a molar mass at the

peak maximum of gel permeation chromatography (GPC) diagrams (Mp) of

147

8.5 kg/mol, employing a PL gel 5 μm Mixed-D column from Polymer

Laboratories Ltd. (Shropshire, United Kingdom) with THF as eluent. The

calibration was performed with atactic poly(styrene) standards.

Synthesis of poly(dibutylstannane)

Poly(dibutylstannane) was synthesized according to previously reported

procedures [3, 5] by dehydropolymerization starting from dibutylstannane with

Wilkinson’s catalyst. It possessed a molar mass Mp of about 15 kg/mol according

to GPC analysis in THF.

UV/Vis Exposure

Solutions of ca. 0.01 mg/mL poly[bis(4-butylphenyl)stannane] or poly(di-

butylstannane) in THF (stabilized and unstabilized) or dichloromethane,

respectively, were prepared at room temperature and protected from light prior to

the exposure experiments. UV/Vis absorption measurements were performed with

a Perkin Elmer Lambda 900 (Schwerzenbach, Switzerland) spectrophotometer.

The experiments were conducted by means of scans between 500 nm and 300 nm

at a constant scan speed of 214.29 nm/min, a 5 nm slit, an integration time of

0.24 s and a data interval of 1 nm. All spectra shown in this report stem from a

series which was analyzed within the same week to ensure constant conditions

since the intensity of the lamp is also depending on the lamp’s life time.

148

Laser Flash Photolysis

Polymer solutions of a concentration of ca. 0.01 mg/mL in THF (stabilized)

and dichloromethane were used and thoroughly protected from light. 2 mL of the

solutions were transferred into quartz glass cuvettes and UV/Vis absorption spectra

were recorded. Subsequently, laser flash photolysis was carried out with the third

harmonic (355 nm) of a Brilliant B YAG laser (Quantel, Les Ulis, France) coupled

to an Applied Photophysics LKS 50 (Leatherhead UK) instrument. Briefly, in this

technique a sample is irradiated by a laser pulse while a single beam UV/Vis

spectrometer records time resolved spectral information, in our case with sampling

rates up to 100 MHz (setup see Figure 3). The 5 ns laser pulses had an energy of

60 mJ. Kinetic traces were recorded at 370 nm for poly(dibutylstannane) and

420 nm for poly[bis(4-butylphenyl)stannane]. Time resolved spectroscopy usually

utilizes strong light sources (here, a 150 W Xe-arc lamp) for the analyzing light

beam. Since this caused considerable photolysis, an electronic shutter, a 370 nm

cut-off filter and a grey filter with only 5 % transmission were set between light

source and sample to minimize polymer degradation by analysis light. After

irradiation of the samples by the laser pulse, UV/Vis spectra were recorded on a

dual-beam spectrophotometer to analyze the damage induced by the laser light.

“Daylight Lamp” Irradiation; GPC Analysis

Solutions of 2 mg/mL of poly[bis(4-butylphenyl)stannane] and poly(dibutyl-

stannane), respectively, were dissolved in the dark in stabilized THF and directly

measured with gel permeation chromatography. Subsequently the GPC vial with

the polymer solution was irradiated with an Osram Dulux S Luminux 7 W/860

(Daylight) lamp (Jeker Leuchten AT, Zurich, Switzerland) in a closed irradiation

149

box with a distance of 13 cm between sample and lamp, as also described

previously [4]. Irradiation times are indicated in the corresponding figures.

Measurements in CH2Cl2 were performed by irradiation of polystannanes

solutions (~20 mg/mL) in a schlenk tube and subsequent dilution of 0.2 mL in

2 mL THF in the GPC vial.

References

[1] C. Löwig, Ueber Zinnäthyle, Neue aus Zinn und Aethyl bestehende Organische Radicale, Mitt.

Naturforsch. Ges. Zürich, 2 (1852) 556-619.

[2] F. Choffat, S. Fornera, P. Smith, W.R. Caseri, D.W. Breiby, J.W. Andreasen, M.M. Nielsen, Oriented

Poly(dialkylstannane)s, Adv. Funct. Mater., 18 (2008) 2301-2308.

[3] F. Choffat, P. Smith, W. Caseri, Facile Synthesis of Linear Poly(dibutylstannane), J. Mater. Chem., 15

(2005) 1789-1792.

[4] F. Choffat, P. Wolfer, P. Smith, W. Caseri, Light-Stability of Poly(dialkylstannane)s, Macromol.

Mater. Eng., 295 (2010) 210-221.

[5] F. Choffat, D. Schmid, W. Caseri, P. Wolfer, P. Smith, Synthesis and Characterization of Linear

Poly(dialkylstannane)s, Macromolecules, 40 (2007) 7878-7889.

[6] M.P. de Haas, F. Choffat, W. Caseri, P. Smith, J.M. Warman, Charge Mobility in the Room-

Temperature Liquid-Crystalline Semiconductor Poly(di n-butylstannane), Adv. Mater., 18 (2006) 44-47.

[7] V. Lu, T.D. Tilley, Low-Band-Gap, σ-Conjugated Polymers: Poly(diarylstannanes), Macromolecules,

29 (1996) 5763-5764.

[8] V.Y. Lu, T.D. Tilley, Poly(diaryl)stannanes: Influence of Substituents on the σ−σ* Transition Energy,

Macromolecules, 33 (2000) 2403-2412.

[9] M. Okano, N. Matsumoto, M. Arakawa, T. Tsuruta, H. Hamano, Electrochemical Synthesis of

Dialkylsubstituted Polystannanes and Their Properties, Chem. Commun., (1998) 1799-1800.

[10] M. Okano, K. Watanabe, Electrochemical Synthesis of Stannane-Silane and Stannane-Germane

Copolymers, Electrochem. Commun., 2 (2000) 471-474.



[12] C. Chatgilialoglu, K.U. Ingold, J. Lusztyk, A.S. Nazran, J.C. Scaiano, Formation, Decay, and Spectral

Characterization of Some Alkyl- and Aryl-Substituted Carbon-, Silicon-, Germanium-, and Tin-centered

Radicals, Organometallics, 2 (1983) 1332-1335.

150

[13] A.G. Moiseev, W.J. Leigh, Comparison of the Reactivities of Dimethylsilylene (SiMe2) and

Diphenylsilylene (SiPh2) in Solution by Laser Flash Photolysis Methods, Organometallics, 26 (2007) 6277-

6289.

[14] H.P. Trommsdorff, J.M. Zeigler, R.M. Hochstrasser, Narrow-Band Laser-Induced Photochemical

Processes in Polysilane Solid Films at 1.4 K, Chem. Phys. Lett., 154 (1989) 463-467.

[15] A. Watanabe, M. Matsuda, Photodegradation of Alkyl- and Aryl-Substituted Polysilanes Studied by

Flash Photolysis, Macromolecules, 25 (1992) 484-488.

[16] C.M. Murphy, C.E. Saunders, D.C. Smith, Thermal and Oxidation Stability of

Polymethylphenylsiloxanes, Ind. Eng. Chem., 42 (1950) 2462-2468.

[17] R.R. McGregor, Structure and Properties, Ind. Eng. Chem., 46 (1954) 2323-2325.

[18] N. Grassie, I.G. Macfarlane, The Thermal Degradation of Polysiloxanes-I. Poly(dimethylsiloxane),

Eur. Polym. J., 14 (1978) 875-884.

[19] N. Grassie, I.G. Macfarlane, K.F. Francey, The Thermal Degradation of Polysiloxanes-II.

Poly(methylphenylsiloxane), Eur. Polym. J., 15 (1979) 415-422.

[20] N. Grassie, K.F. Francey, The Thermal Degradation of Polysiloxanes-Part III: Poly(dimethyl/methyl

phenyl siloxane), Polym. Degrad. Stab., 2 53-66.

[21] N. Grassie, K.F. Francey, I.G. Macfarlane, The Thermal Degradation of Polysiloxanes-Part IV:

Poly(dimethyl/diphenyl siloxane), Polym. Degrad. Stab., 2 67-83.

[22] F. Lollmahomed, W.J. Leigh, Laser Flash Photolysis Studies of Some Dimethylgermylene Precursors,


[23] J.R.G. Thorne, S.T. Repinec, S.A. Abrash, J.M. Zeigler, R.M. Hochstrasser, Polysilane Excited States

and Excited State Dynamics, Chem. Phys., 146 (1990) 315-325.

[24] J.R.G. Thorne, Y. Ohsako, S.T. Repinec, S.A. Abrash, J.M. Zeigler, R.M. Hochstrasser, The Excited

States of Linear Chain Polysilanes, J. Lumin., 45 (1990) 295-297.

[25] J.R.G. Thorne, R.M. Hochstrasser, J.M. Zeigler, Photophysics of the Phases of Poly(di-n-

hexylsilane), J. Phys. Chem., 92 (1988) 4275-4277.

[26] Y. Ohsako, J.R.G. Thorne, C.M. Phillips, R.M. Hochstrasser, J.M. Zeigler, Picosecond Transient

Absorption Spectroscopy of Polysilanes, J. Phys. Chem., 93 (1989) 4408-4411.

[27] Y.R. Kim, M. Lee, J.R.G. Thorne, R.M. Hochstrasser, J.M. Zeigler, Picosecond Reorientations of the

Transition Dipoles in Polysilanes Using Fluorescence Anisotropy, Chem. Phys. Lett., 145 (1988) 75-80.

[28] M. Ishikawa, M. Kumada, Photochemistry of Organopolysilanes, Adv. Organomet. Chem., 19 (1981)

51-95.

[29] J. Michl, J.W. Downing, T. Karatsu, K.A. Klingensmith, G.M. Wallraff, R.D. Miller, Poly(di-n-

hexylsilane) in Room-Temperature Solution. Photophysics and Photochemistry, ACS Symp. Ser., 360

(1988) 61-77.


Chapter VII

Conclusions & Outlook

153

Conclusions

A versatile method for the synthesis of polystannanes with aromatic groups

directly bound to the tin atoms was realized by coupling of dichlorodiphenyl-

stannane with sodium in liquid ammonia. The nature of the intermediates arising

in this reaction was explored in-situ by 119Sn NMR spectroscopy and exposure to

bromoethane. In contrast to the generally accepted view, diphenylstannide

dianions, Ph2Sn2-, were not formed in liquid ammonia. The species that were

present in the reacting medium were, as a matter of fact, tetraphenyldistannide,

(Ph2Sn-SnPh2)2- and hydrodiphenylstannide, HPh2Sn- (Scheme 1).

Scheme 1. Reactions of dichlorodiphenylstannane with sodium in liquid ammonia and subsequent

reaction with various compounds.

SnCl

ClPh

Ph

2 Na / NH3, (l)

4 Na NH3, (l)

Ph2SnCl2

R2SnCl2

EtBrSn

Et

EtPh

Ph

Sn Sn

Ph

Ph

Ph

Ph

Sn H

Ph

Ph

Sn

n

Ph

Ph

Sn

n

Ph

Ph

Sn

n

Ph

Ph

Sn

R

R m

154

Similar experiments with dichlorodibutylstannane yielded a mixture of four

compounds dissolved in the liquid ammonia, which were identified as HBu2Sn-

and (Bu2Sn-SnBu2)2- together with Bu3Sn- and another product - H2BuSn- or

(BuHSn-SnHBu)2- - that probably formed by exchange of alkyl groups (Scheme

2). Differences in the distribution of the in-situ formed products at dichlorodi-

organostannane : sodium ratios ranging from 1:3 to 1:10 were either not

significant (in the cases of the alkylstannanes) or moderate (in the case of the

investigated arylstannane). Migration of organic groups in in-situ formed

Scheme 2. Reactions of dichlorodibutylstannane with sodium in liquid ammonia and subsequent

reaction with various compounds. Note that the obtained polymers are not purely linear, but

branching occurs due to exchange of alkyl groups.

2 Na / NH3, (l)

4 Na NH3, (l)

Bu2SnCl2

R2SnCl2

EtBrSn

Et

EtBu

Bu

SnEt

BuBu

BuSn

Et

EtBu

Et

Sn

Bu

Bu n

Sn

Bu

Bu n

cyclo(SnBu2)

cyclo(SnBu2)

+

+

Sn Sn

Bu

BuBu

Bu

Sn

Bu

Bu

H

Sn

Bu

Bu

Bu

Sn SnBu

HH

BuSn H

H

Bu

Sn

n

Bu

Bu

Sn

R

R m

or

SnCl

ClBu

Bu

155

alkylstannides was observed in products resulting after exposure to bromoethane;

however, the reaction of the intermediates with bromoethane did not allow to

draw conclusions on the chemical structure of the intermediates, in contrast to

common practice.

Reaction mixtures comprising a dichlorodiorganostannane : sodium ratio of

1 : 2 (one-step synthesis), as well as conversion of the soluble intermediates with

dichlorodiorganostannanes (two-step synthesis) led to precipitation of

polystannanes. Characterization of the reaction products provided mechanistic

insight regarding the polymerization in liquid ammonia. The two-step

polymerization did not follow a step-growth mechanism but, rather, a radical

chain-growth process. Furthermore, the formation of cross-linked and branched

products, in the case of butyl intermediates, confirmed the migration of alkyl

groups during the reaction with sodium in liquid ammonia.

These polymerization routes allowed not only synthesis of poly(diaryl-

stannane)s, but also the formation of copolymers of the general structure poly-

(dialkylstannane-co-diarylstannane) (SnAlkyl2)n(SnAryl2)m. To create linear

polymers, it was essential to conduct the first reaction step, i.e. the formation of

stannides, with dichlorodiarylstannane to circumvent cross-linking and branching,

resulting from the migration of alkyl groups. Poly(diphenylstannane) thus

produced featured a UV/Vis absorption maximum at 480 nm assigned to σ-π-de-

localization of electrons and was a dichroic material which could easily be oriented.

The UV/Vis spectra of poly(dialkylstannane-co-diarylstannane)s displayed two

absorption maxima, attributed to the aromatic (~480 nm) and aliphatic (~400 nm)

moieties in the polymer chain.

The polymers obtained with the new synthesis route were compared to the

products produced with Wilkinson’s catalyst and TMEDA, in collaboration with

156

Scheme 3. Schematic of the principal reactions involved in the decomposition of polystannanes:

Scission of polymer chains by incident light under formation of two radical end groups;

recombination of radicals in the case of poly(diarylstannane)s and the depolymerization in the

case of poly(dialkylstannane)s.

Prof. Frank Uhlig and Marie-Luise Lechner from the TU-Graz. For this purpose,

different polymers were synthesized: poly(dibutylstannane), poly[butyl(phenyl)-

stannane], poly(diphenylstannane) and poly[bis(4-butylphenyl)stannane]. The

route employing Wilkinson’s catalyst was most beneficial for the preparation of

poly(dibutylstannane) and TMEDA for polystannanes containing at least one

aromatic group per Sn atom, whereas synthesis in Na/NH3 yield best results for

polystannanes with two aromatic groups per Sn atom – poly(diarylstannane)s.

Investigations on the stability of polystannanes in solution towards light were

performed with poly[bis(4-butylphenyl)stannane], which was demonstrated to be

Sn

R

R Sn

R

R Sn

R

R +n n - xx

Sn

R

R Sn

R

R Sn

R

R +nn - xx

Sn R

R Sn

R

R +

Chain scission

Recombination for R = Aryl

Unzipping for R = Alkyl

x x - ncyclo - (SnR2)n

hν

hν

Sn R

R Sn

R

R Sn

R

R

ClCl

+nhν

CH2Cl2 x - n

or

●

x

●

157

more stable than poly(dibutylstannane) - a typical poly(dialkylstannane) – when

dissolved in dichloromethane as well as in THF.

Results obtained with laser flash photolysis indicated that the observed

enhanced stability of the polymer with aromatic substituents was not due to a

higher stability of Sn-Sn bonds, but to recombination of the generated radicals. By

contrast, less stable radicals generated in the polystannanes with aliphatic side

groups led to rapid degradation of the macromolecular chains (Scheme 3).

Analyses of reaction solutions by GPC unveiled that two different decomposition

mechanisms may occur - random scission of polymer chains or unzipping; the

latter might be supported by reaction of the polystannane radicals with the solvent

molecules present.

158

Outlook

Enhanced stability and improved electronic properties could increase the range

of possible applications of polystannanes. To create such properties, a series of

structures are proposed in the following.

Promoted recombination of radicals in polystannanes

In Chapter VI, it was described that enhanced stability of poly(diaryl-

stannane)s in solution – when compared with poly(dialkylstannane)s - is due to the

recombination of stabilized aryl-tin radicals, that were formed by the action of

light. Therefore, to further increase the stability, either the life-time of the radicals

should be prolongued or their recombination promoted.

Recombination could be favored by external forces that keep the Sn radicals

close together. This could be realized by ligands that connect at least two tin

atoms. The geometry of the ligands should fit the Sn-Sn distance of 2.8 Å to avoid

strain in the molecule, which could actually decrease the chain stability. Recently,

the first syntheses of a six-membered ring with neighboring tin atoms was reported

(Scheme 4a) [1], as well as calculations of ring strain energies of three-, four- and

five-member rings were presented [2]. It was shown that the homodesmotic strain

energy decreases from 1,2-distannacyclopropane to 1,2-distannacyclobutane and

1,2-distannacyclopentane and that non of the rings is excessively strained.

Polymerization of these cyclic distannanes could be a first step to impart enhanced

stability to such polymers. Ideally, every tin-tin bond would be connected by a

ligand (Scheme 4b).

159

Scheme 4. (a) 1,2-distannnacycloalkanes reported in literature so far [1, 2]; (b) idealistic

schematic of a polystannane with each Sn-Sn bond additionally stabilized by an ligand.

Another concept would be incorporation of polymerizable side groups. This

could be achieved by either generating a polystannane with reactive side chains

which are polymerized (Scheme 5a), or reversely by producing a soluble polymer

with an active stannane group on each monomer unit which could be converted to

polystannanes subsequently (Scheme 5b). The latter is probably favorable due to

the higher stability of the C-C chains. With these attempts, also the mechanical

properties of the obtained products could be varied in a wide range, as they are also

influenced by the nature and molar mass of the supporting polymer.

Ladder polystannanes

Ladder components of the group 14 elements have been known already since

1927, when Zelinsky and Kozechkow first synthesized [2]ladderane (bicycle-

[2.2.0]hexane) by reduction of cis-1,4-dibromocyclohexane with sodium [3]

R R R R R R

R R R R R R

[ ]

a

b

160

Scheme 5. Schematic of polystannane stabilization concepts: (a) incorporation of reactive groups

in the polystannanes which are subsequently polymerized and (b) preliminary polymerization of

the supporting polymer with attached active stannane groups or stannane groups that can be

activated for subsequent polymerization.

Different derivates of [3]- and [4]ladderanes have been reported in 1964 [4], and

more recently longer ladderanes were synthesized by the repeated cycloaddition of

cyclobutadiene ([3], [5] and [7]ladderanes) or alternate cycloaddition of

cyclobutadiene and dimethylacetylenedicarboxylate ([n]ladderanes n=3,4,5,6,7,9).

The first silicon analogue, bicyclo[2.2.0]hexasilane ([2]ladder polysilane), was

reported in 1987 by Matsumoto et al.. In contrast to the carbon based systems the

Scheme 6. Structure of (a) [2]ladder stannane, (b) [3]ladder silane and (c) polyladderane.

a

b

Si

Si

Si

Si

Si

Si Si

Si

R R RR

RRRRR

RR

RSn

Sn

Sn

Sn

Sn

SnR R

RRR

RR

R R

R1 2 21 3

a b c

n

161

polycyclic silanes feature extended σ-electron delocalization along the backbone

and are, therefore, expected to possess intriguing electronic, optical and chemical

properties. Lowest energy absorption of ladder polysilanes are reported to arise at

464 and 483 nm for the [7]ladder polysilane and the [8]ladder polysilanes,

respectively. These molecules represent the most extended ladder polysilanes that

have been synthesized so far [5]. They show also a strong red shift in

photoluminescence compared to the corresponding linear silanes [6]. Besides their

electronic properties longer ladder polysilanes have the capability to generate

highly stable radical anions after reduction with alkali metals [7]. Only a few

ladder polygermanes are known so far. Surprisingly the UV/Vis absorption band

shows a blue shift from [3]ladder polysilane to [3]ladder polygermane [8, 9], but

the oxidation potentials of the germanium compounds are significantly lower than

those of the silicon analogous [9]. The first ladder oligostannane reported is

bicyclo[2.2.0]hexastannane ([2]ladder polystannane) [10]. It is air-stable in the

crystalline form and shows dramatic reversible thermochromic behavior, being pale

yellow at -196°C and orange-red at room temperature with an absorption

maximum at 360 nm.

Conjugated poly(carbo)stannanes

Polymers with covalent carbon-tin bonds in the backbone were first described

by Noltes and Van der Kerk 1961 [11], continued by a series of reports concerning

polycarbostannanes obtained by step growth polymerization of diorganotin

dihydrides and diolefins or diacetylenes [12-14]. With this structural concept, it

should be possible to create polycarbostannanes with σ-π-conjugation throughout

the polymer backbone. Probably, it would be necessary to insert some alkyl

sidechains to increase the solubility, as Noltes and Van der Kerk found insolubility

162

for some of the polymers produced [13]. Finally, conjugated polymer networks

could be synthesized by application of 1.3.5 – triethynylbenzene, as suggested in

Scheme 7.

Scheme 7. Synthesis routes towards (a) saturated polycarbostannanes, (b) conjugated polycarbostannanes

and (c) conjugated polycarbostannane networks.

Sn

H

R

R

H

SnR

Rn

SnR

R n

SnR

R

n

Sn

Sn

R

RR

R

+

+

+

163

References

[1] E. Zarl, J.H. Albering, R.C. Fischer, D. Flock, B. Genser, B. Seibt, F. Uhlig, Tin-containing

Indane and Tetralin Derivatives, Z. Naturforsch., 64b (2009) 1591-1597.

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Acknowledgements

The present work would not have been possible without contributions of

numerous people and institutions which helped to conduct the research described

in this thesis the way it appears now.

First I want to thank Walter Caseri for his assistance and support throughout

the whole work, as well as his patience and endurance during the last months. No

less sincere gratitude belongs to Paul Smith for giving me the opportunity to be

part of this special group in a fantastic environment. I deeply appreciated to work,

learn and live with the given liberties and provided confidence.

Frank Uhlig (Technische Universität-Graz, Austria) is acknowledged for his

help concerning the chemistry of tin, providing stimulating discussions and for

asking the right questions at the right time, that provoked us to dive deeper into

the world of tin molecules and intermediates. The cooperation with the TU-Graz

offered the great opportunity for me to keep in contact with my home university.

I thank Wolfram Uhlig for accepting to be a co-examiner for this thesis and

his interest in my work.

Particularly acknowledged are Thomas Nauser (Department of Chemistry,

ETH Zürich) for the work performed with laser flash photolysis and his

contribution to the understanding of some important properties and Marie-Luise

Lechner for the fruitful collaboration, as well as Aitor Moreno and Heinz Rüegger

for their help with NMR experiments and the confidence given, especially

concerning the measurements in liquid ammonia in their laboratories. Also

Thomas Schweizer, Werner Schmidheiny, Marc Simonet and Martin Colussi,

contributed to the outcome of this thesis and I am grateful for their efforts.

166

I would like to thank my students Jérôme Zemp, Cédric Sax, Debora

Solenthaler, as well as Antoine Dorcier, Georgius Sotiriou and students from the

various practica who helped me to discover the diversity of polystannanes.

And of course I would like to express my thanks to all the people that

contributed to good atmosphere inside and outside our lab, and for the good times

we spent together, especially Karin Bernland and Pascal Wolfer, my lab mates

Irene Bräunlich, Joanna Wong and Stefan Busato, Felix Koch, lab chief Kirill

Feldman, Mr. Polystannane Fabien Choffat, Christian Müller, Kurt Pernstich,

Jérôme Lefèvre, Andreas Brunner, Susi Köppl, Theo Tervoort, Sara Fornera, Jan

Giesbrecht, Harald Lehman, Ueli Suter, Wolfgang Kaiser and Vappu Hämmerli.

I thank Debora Solenthaler and Rahel Bohlen for providing their picture of

poly(dibutylstannane-co-diphenylstannane) as cover for this thesis.

The Swiss National Science Foundation (Nr.: 200021_126450/1) is

acknowledged for financial support.

Am Ende möchte ich mich noch ganz besonders bei meinen Eltern für Ihr

Vertrauen und Ihre Unterstützung bedanken, sowie bei meiner Schwester und

meinem Götibueb Aron, der mir sehr viel Freude bereitet.

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Curriculum Vitae

Markus Trummer was born on March 22, 1980 in Feldkirch, Austria. He grew up

in Schruns, attended the Gymnasium in Bludenz and graduated from the Höhere

Technische Lehranstalt (HTL) für chemische Betriebstechnik in Wels 1999. After

one year of civilian service he started his studies in Technical Chemistry at the

Technical University of Graz (TU-Graz) in 2000. During his study he completed

industrial internships at Getzner Werkstoffe GmbH (2002, 2003 and 2004) and

Chemson Polymer - Additive AG (2005). In 2006 he received the degree Diplom

Ingenieur (DI), after completing his Diploma thesis “Polyelectrolyte-clay

nanocomposites as dielectric materials” at the Institute of Chemical Process

Development and Control (Joanneum Research), in cooperation with the Institute

of Chemistry, University of Graz and the Institute for Chemistry and Technology

of Materials (TU-Graz). In 2007 he joined the Polymer Technology Group of

Prof. Paul Smith at the Department of Materials of the Eidgenössische

Technische Hochschule (ETH) Zürich where he conducted his doctoral studies

under the supervision of Prof. Walter Caseri.

Death and/or old age is coming.....we must live sweet.

After all, it is not only life, but the quality of this life.

Mike J. Libecki

POLYSTANNANES - Reaction Mechanism and Products

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