37? N Bid

Mo.asif

KINETICS AND MECHANISMS OF LIGAND EXCHANGE

REACTIONS OF CHELATE COMPLEXES

DISSERTATION

Presented to the Graduate Council of the

University of North Texas in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Jose E. Cortes, B.S.

Denton, Texas

May, 1989

Cortes, Jose E., Kinetics and Mechanisms fif Lj.gafld

Exchange Reactions of Chelate Complexes. Doctor of

Philosophy (Physical Chemistry), May 1989, 150 pp., 17

tables, 50 figures, bibliography, 125 titles.

The ligand substitution reactions of (fts-DTHp)W(COU,

(fP-DTD)W(COU, and (fF-DTU)W(COU, (DTHp = 2,2,6,6-tetra-

methy1-3,5-dithiaheptane, DTD = 2,2,9,9-tetramethyl-3,8-

dithiadecane, DTU - 2,2,10,10-tetramethyl-3,9-dithia-

undecane), with L, (L = phosphites and phosphines), proceed

to a complete displacement of the chelate ligand. During the

course of the reactions there is appreciable formation of

cis-(n*-DTA) (L)W(CO)*, (DTA - DTHp, DTD, DTU).

The reactions of <n«-DTA)W(COU, (DTA = DTD, DTU),

with L to produce cis-(n*-DTA)(L)W(COU proceed through a

mechanism involving an initial fission of the tungsten-sulfur

bond to afford a coordinatively-unsaturated intermediate,

cis-[(n*-DTA)W(COUJ, which is rapidly attacked by

chlorobenzene. The resulting solvated intermediate

establishes an equilibrium which involves desolvation-

solvation. Then, cis-C(fl1 -DTA)W(C0U3 undergoes ring-closure

and attack by L.

The reactions of (fl®-DTHp)W(CO)^ with phosphites and

tri(n-butyl) phosphine proceed through a mechanism involving

an initial interaction between the incoming ligand, L, and

the substrate. Activation parameters for the reactions

leading to the ring-opened 1igand-substituted intermediate,

cis-(ft1-DTHp)(L)W(CO)^, are consistent with this mechanism.

The thermally-generated intermediate cis- {ft* -DTA) -

(L)W(COU, (DTA = DTHp, DTD, DTU), will undergo further

reaction with L to produce cis-(L)aW(CO)<•. The rate law for

these reactions, first order with respect to the

concentration of cis- (fl* -DTA) (L)W(CO)^. and zero order with

respect to the concentration of the incoming 1igand, and the

activation parameters, indicate rate-determining dissociation

of the anchored end of the chelate 1igand. Cis-(L)aW(COU

complexes, (L = tri(isopropyl) phosphite, trimethyl

phosphite, and tri(n-butyl) phosphine) undergo cis-trans

isomerization to produce a mixture of trans- and cis-

(L)eW(COU.

The kinetics evidence for the cis-trans isomerization

of the final product, {(L)eW(CO)<», L = tri(n-butyl)

phosphine) suggests that it is non-dissociative.

TABLE OF CONTENTS

Page

v LIST OF TABLES

LIST OF ILLUSTRATIONS v 1 1

Chapter

I. INTRODUCTION 1

History and General Aspects Metal Carbonyls Reactions The Problem Chapter Bibliography

II. EXPERIMENTAL 2 8

General Purification of Solvents Purification of Ligands Syntheses of Bidentate Ligands Syntheses of Metal Complexes Identification of Intermediates Identification of Reaction Products Recrystalization of Metal Complexes Kinetics Runs Flash Photolysis Chapter Bibliography

III. REACTIONS OF (fla-DTA)W(COU 54

General Reactions of (fle-DTHp)W(COU Formation of Cis-{fl*-DTHp) (L)W(CO)** Ring-Closure Cis- [ (fl1-DTHp) (S)W(C0)«*3 Mechanism for the Formation of Cis-(ni-DTHp) (L)W(COU Reactions of Cis-(H*-DTA) (L)W(CO)*-Chapter Bibliography

xxx

Chapter

TABLE OF CONTENTS-Continued

Page

f **• IV. REACTIONS OF CIS- (fl'-DTA) (L)W(CO)-DTA = DTD, DTU 1 1 1

Formation of Cis-(ni_DTA) (L)W(CO)*. Ring-Closure of Cis-DTA) (CB)W(CO)«» Summary Chapter Bibliography

V. CIS-TRANS ISOMERIZATION OF (L)sW(COU 132

Exper imenta1 Cis-trans Isomerization 3 1P NMR Studies Chapter Bibliography

VI. CONCLUSIONS 1 4 7

VII. BIBLIOGRAPHY 1 5 2

IV

LIST OF TABLES

Table Page

I.

II.

Ill

IV.

V.

VI.

VII.

VIII

IX.

Carbonyl Stretching Frequencies of Cia-(n*-DTHp) (L)W(COU in Chlorobenzene 41

First-Order Rate Constants for the Reactions of (D®-DTHp)W( CO)*. with Phosphites in Chlorobenzene at Various Temperatures 57

Rate Constants and Activation Parameters for the Reactions of (fle-DTHp)W(CO)<=» with Phosphites in Chlorobenzene at Various Temperatures 64

Rate Constants for the Ring-Closure of (fl1-DTHp) (Solvent)W(COU in

Bromobenzene and Chlorobenzene at Various Temperatures 68

Rate Constants for the Reactions of £i£-(n*-DTA)(L)W(COU with L in Chlorobenzene at Various Temperatures 76

Rate Constants for DTHp-Dissociation from £i£-(ni-DTHp) (L)W(COU in Chlorobenzene at 44.5 °C 79

Activation Parameters for the Dissociation of DTA from £i£-(fl 4-DTA)(L)W(C0U 87

Rate Constants for the Displacement of Chlorobenzene from QiSr [ (P(O-i-Pr)a) (CB)W(CO)*.] by Tri(isopropyl) Phosphite at 35.2 °C 91

Pseudo-First Order Rate Constants for the Reactions of (n*-DTA)W(CO)«. with Phosphites in Chlorobenzene at Various Temperatures 96

Table

LIST OF TABLES-Continued

Page

X.

XI

XII.

XIII

XIV.

XV.

XVI,

XVII.

Rate Constants for the Ring-Opening of (rF-DTA)W(CO)^ in Chlorobenzene at Various Temperatures 105

Rate Constants for the Ring-Closure of Sia~[(Hl-DTA)(Chlorobenzene )W( CO)*.] in Chlorobenzene at Various Temperatures

Rate Constants and Activation Parameters for the Displacement of DTA from (rF-DTA)W(COU at 35.2 °C

110

116

Rate Constants for the Isomerization of (P(n-Bu)a)aW(C0U in Chlorobenzene at Various Temperatures 128

CIS : Trans Ratios of (P(n-Bu)s ) aW(C0U in Chlorobenzene at Various Temperatures 134

Rate Constants for CIS-Trans Isomerization of (P(n-Bu)a)aW(CO)^ in Chlorobenzene at Various Temperatures 136

Rate Constants Involved in Mechanism Described in Figure 49 141

Rate Constants for the Overall Mechanism Described in Figure 50 142

vi

Figure

LIST OF ILLUSTRATIONS

Page

1. Fenske Direct Donation Model 5

2. Substitution Reaction Mechanism Involving Rate-Determining Dissociation of CO; Interchange or Associative Interaction with L 7

3. Generation of Coordinatively-Unsaturated Cis- [LW(C0U1 and Attack by CH, CB, and Pip. CH = Cyclohexane, CB = Chlorobenzene, Pip = Piperidine

4. Displacement of Bidentate Ligand, Bonded Through Nitrogen to the Central Metal, Involving a Competitive Me chanism

5. Displacement of Bidentate Ligand, Bonded Through Sulfur to the Central Metal, Involving a Competitive Mechanism 1?

6. Carbonyl Stretching Spectrum of Cis-(na-DTHp)W(CO)^ in Chlorobenzene 3 3

7. Carbonyl Stretching Spectrum of Cis - (PI1 -DTHp) (CP)W(CO )<» in Chlorobenzene 3 5

8. Carbonyl Stretching Spectrum of £i£-(fF-DTD)W(COU in Chlorobenzene 3 6

9. Carbonyl Stretching Spectrum of Cis-(fl1-DTD) (CP)W(COU in Chi orobenzene 3 ?

10. Carbonyl Stretching Spectrum of Cis-(ne-DTU)W(COin Chlorobenzene 3 9

v n

LIST OF ILLUSTRATIONS-Continued

Figure Page

11. Carbonyl Stretching Spectrum of £is- {P(n-Bu)a)eW( CO)«. in Chlorobenzene 40

12. Carbonyl Stretching Spectrum of Cis-and Trans-(P(n-Bu)a)EW(C0U in Chi or obenz ene 42

13. Carbonyl Stretching Spectrum of Cis-(CP)BW(COU in Chi orobenz ene 43

14. Carbonyl Stretching Spectrum of Cis- and Trans-(P(Q-i-Pr)a)aW(C0)*» in Chlorobenzene 44

15. Carbonyl Stretching Spectrum of Cis- and Trans- (P(0Me)A)EW(C0)«» in Chlorobenzene 45

16. Carbonyl Stretching Spectrum of Cis- (P(QCaHb )A )E»W(C0)*. in Chlorobenzene 46

17. Schematic of Flash-Laser Photolysis Equipment: SS, Slow Shutter; FS, Programmable Fast Shutter; Ft Filter; L, Lenses; S, Sample; A, Attenuator; MC, Monochromator; P, Photomultiplier Tube; D, Photodiode Energy Monitor; Mf Mirror; G, Quartz.... 50

18. Plot of (A*-A.) vs. Time for the Reactions of (fP-DTHp )W( CO )*. with Tri( isopropyl) Phosphite at 21.1 °C. [L] = 0.2008 M. Ordinate = (At - A-), Abscissa = Time X 10-** Sec. 55

19. Plots of kob«d 3£s. [L] for the Reactions of (na-DTHp)W(CO)«. with Tri (isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = kob.d X 10® Sec"1, Abscissa = [L] X 10 M 59

Vlll

Figure

LIST OF ILLUSTRATIONS-Continued

Page

20. Plots of kobmd vs. [L] for the Reactions of (ne-DTHp)W(CO) ** with L, L — Tri (isopropyl) Phosphite, Trimethyl Phosphite, in Chlorobenzene at 21.1 °C. Ordinate = kcf.ci X 10®, Abscissa = [L] X 10 M 60

21. Competitive Mechanism for the Displacement of One End of DTHp from ( n ® - D T H p ) W ( C O U by L Involving Initial Rate-Determining Ring-Opening; Bimolecular Attack by L 61

22. Eyring Plot of l n ( W T ) yg. 1/T for the Reactions of (fF-DTHp)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ln(ke/T), Abscissa = 1/T X 10s K"1 6 3

23. Ring-Closure of £is-[(n*-DTHp)(CB)W(CO)*] Involving a Bimolecular Displacement of CB. (CB = Chlorobenzene) 67

24. Eyring Plots of the Infk-^/T) vs. l/T for the Ring-Closure of Cis- Ufl1 -DTHp)(solvent )W( CO U 3 at Various Temperatures. BB = Bromobenzene, DCE = 1,2-Dichloroethane. Ordinate = ln(k-i/T), Abscissa * 1/T X 10s K-1

6 9

25. Plausible Mechanism for the Reactions of (rF-DTHp)W(COU with L to afford £i£-(n*-DTHp) (L)W<CO)* 73

26. Eyring Plots of ln(k*/T) vs. 1/T for the Reactions of fiiS-m^-DTHp) (L)W(COU with L, L = Tri(isopropyl) Phosphite, Trimethyl Phosphite, and CP in Chlorobenzene at Various Temperatures. Ordinate = ln(k*/T), Abscissa = 1/T X 10s K-1 qj

IX

Figure

LIST OF ILLUSTRATIONS-Continued

Page

27. Eyring Plot of ln(k*»/T) vs. 1/T for the Reactions of Cis-tn^-DTD)(L)W(COU with L, L = Tri(isopropyl) Phosphite, in Chiorobenzene at Various Temperatures. Ordinate = ln(k*/T), Abscissa = 1/T X 103 82

28. Eyring Plot of ln(k>»/T) vg,. 1/T for the Reactions of Cis-(n*-DTU) (L)W(COU with L, L = Tri < isopropyl) Phosphite, in Chlorobenzene at Various Temperatures. Ordinate = ln(k*./T), Abscissa = 1/T X 10s 83

29. Mechanism for the Displacement of DTA from Cis-<n1-DTA) (L)W(COU by L, L = Phosphites, Phosphines 84

30. Plot of Ink* vs. Tolman Cone Angles for the Reactions of Cis- (ni~DTHp) (L)W(C0)«. with L, L = 1, Tri(n-butyl) phosphine; 2, Triphenyl Phosphite; 3, Tri(isopropyl) Phosphite; 4, Trimethyl Phosphite; 5, CP 85

31. Photochemical Generation of Cis-[LW(COU1 in Chlorobenzene and Attack by L 89

32. Plot of kob.d va- CL3 f°r the Reactions of Cis- [ (L) (CB)W(COU] with L at 35.2 °C in chlorobenzene. L = Tri(isopropyl) Phosphite. Ordinate = k X 103 sec-1, Abscissa = [L] X 10 M 92

33. Plots of kob.« vs. CL] for the Reactions of (fF-DTD)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various ^ Temperatures. Ordinate = kob>d X 10® Sec-1, Abscissa = [L] X 10 M 98

Figure

LIST OF ILLUSTRATIONS-Continued

Page

34. Plots of keb«d vg. [L] for the Reactions of (rF-DTTJ)W(COU with Tri( isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ]<«**,.* x 103 Sec"1

Abscissa = [L] X 10 M [ 99

35. Plots of 1/kob.ci vs. 1/[L3 for the Reactions of (fF-DTD)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = 1/koe-c X 10~B

Sec, Abscissa = 1/[L] M_1 10i

36. Plots of 1/kofa.d vs. 1/[L] for the Reactions of (n®-DTU)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = l/k=>b_d X 10~« Sec, Abscissa = 1/[L] M-1

102

37. Mechanisms for the Displacement of One End of DTA from (fla-DTA)W(COU by L 103

38. Plots of l/kot.««j vs. 1/[L] for the Reactions of (f^-DTD)W(CO)<» with L in Chlorobenzene at 21.1 °C. Ordinate = l/kob«d X 103 Sec, Abscissa = 1/[L] M-1

104

39. Eyring Plots of ln(k/T) v§. 1/T for the Reactions of (rF-DTD)W(COU with Tri(isopropy1) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ln(k/T) Abscissa = 1/T X 10s It-1 ' 106

40. Eyring Plot of ln(k/T) l/T for the Reactions of (rF-DTU)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. A = k*, B = ka/k-a Ordinate = In{k/T), Abscissa = 1/T X 10s K _ 1

107

41. Eyring Plot of lndc-^/T) vs. 1/T for the Ring-Closure of Cis-[(n*-DTD)(CB)W(CO)^] in Chlorobenzene at Various Temperatures. Ordinate = ln(k~VT), Abscissa = 1/T X 10s kr1 1 1 3

XI

LIST OF ILLUSTRATIONS-Continued

Figure Page

42. Eyring Plot of ln(k-i/T) v&. 1/T for the Ring-Closure of £i§r [ (fl^-DTU) (CB)W(COUl in Chlorobenzene at Various Temperatures. Ordinate = ln(k-i/T), Abscissa = 1/T X 10s K"1 114

43. Plots of In (A* - Sbiank) HS- Time for the Reactions of (fF-DTN)W(COU with P(OMe)a in Chlorobenzene at 43.3 °C at Two Concentrations of P(OMe)a 121

44. Plot of Absorbance 2££. Time for the Reaction of (fF-DTHp)W(COU with P(n-Bu)a (0.1071 M) in Chlorobenzene at 44.5 °C. Ordinate = Absorbance, Abscissa = Seconds X 10~a 125

45. Plots of (Top) In (A* - A i*,-,*) Time for the Third Segment of This Plot, Obtained by Monitoring at 415 run, for Reaction of (rF-DTHp)W(COU with P(n-Bu)a (0.1084 M) at 44.5 °C and (bottom) These Data Plotted as In(A. - At) ys. Time. Ordinate = Absorbance, Abscissa = Seconds X 10~3.. 127

46. 3 1P NMR Spectrum of Cis- and Trans-(P(n-Bu)a )aW(C0U in Chlorobenzene at 35.2 °C 131

47. 3 1P NMR Spectrum of Cis- and Trans -(P(n-Bu)a )eW(C0)<* in Chlorobenzene at 44.5 °C 132

48. 3 1P NMR Spectrum of Cis- and Trans-(P(n-Bu)a )eW(C0U in Chlorobenzene at 54.6 °C 133

49. Proposed Mechanism for the Overall Displacement of DTA, DTA = DTD and DTU, from (fla-DTA)W(COU by L 140

50. Proposed Mechanism for the Overall Displacement of DTHp from (fla-DTHp)W(COU by L 143

XXX

CHAPTER I

INTRODUCTION

History sod general Aspects

The first organometallic compound was prepared in 1827

by W. C. Zeise through a reaction of ethanol with a mixture

of PtCls and PtCl* in the presence of KC1 (D .Zeise

suggested PtCle (CeH*. )KC1 .BiaO as the formula for this new

compound (2, 2). The first organometallic compound having a

direct carbon-metal bond was synthesized by E. Frankland

(it). This compound was obtained by accident. Heating ethyl

iodide with zinc to probe the presence of organic radicals,

Frankland obtained a volatile, colorless liquid that roughly

analyzed as CeH*. At first, he thought that the reaction

product proved the presence of organic radicals. Later,

molecular weight determinations revealed that this compound

was not the ethyl radical but butane that was formed from

decomposition of ethylzinc iodide. Despite their flammable

nature, these compounds were extensively used as alkylating

agents until replaced by Grignard reagents, which were

easier to handle. Later discoveries followed, such as that

of nickel carbonyl by Mond (5.) in 1890, and Grignard

reagents in 1900 (ft).

2

After 1950, with the steady growth of chemical knowledge and

the discovery of ferrocene (2-If)) in 1952 and the invention

of the Ziegler process in 1953 (H), organometallic

chemistry was firmly established as a chemical discipline.

Although main group organometallic chemistry has received a

great deal of attention, this discussion will be centered in

organotransition metal chemistry, in particular, metal

carbonyls.

Metal Carbonvls

Metal carbonyl complexes (12, 11) are molecules

containing a central metal atom coordinated to at least one

carbon monoxide molecule. The coordination number, the

number of 1igauds bonded to the central metal, is determined

by both the nature of the metal and the ligand. Properties

such as the oxidation state, the size of the metal, and the

bulkiness of the ligand will affect the stoichiometries of

these complexes. Furthermore, their molecular formulas can

be predicted by use of the noble gas formalism or

eighteen-electron rule (12). This requires that the number

of valence electrons residing in the metal plus the number

of electrons donated by the ligands equal the number of

electrons of the next noble gas in the periodic table. This

is a consequence of the metal atoms use of its valence nd,

(n + 1)s and (n + D p orbitals as fully as possible when

forming bonds with ligands.

3

Many of these metal atoms are in a low positive,

zero or negative oxidation state. Thus, a high electron

density will develop on the central metal atom when sigma

donation from a bonding ligand takes place. This apparently

anomalous situation is what sets apart the chemistry of

transition metal elements. It can be rationalized by a

close examination of the interacting orbitals between the

central metal and the ligand. Carbon monoxide belongs to

the so-called u-acid ligands and is by far the most

important one. These ligands can act as sigma-electron

donors (Lewis bases) and can also act as Lewis acids in the

sense that energetically accessible u-orbitals are

available to accept electron density from the metal.

Bonding between CO and the central metal thus involves

dative sigma-bond formation through overlap of a slightly

antibonding lone pair on carbon with a dssp3 hybrid orbitals

on the metal. This sigma-interaction is reinforced by "it-

back bonding" between the filled metallic orbitals and the

it-antibonding molecular orbitals on CO. This bonding

mechanism is "synergic" since there is donation of electrons

from the metal to CO via "backbonding"; at the same time,

there is donation of electrons by carbon to the metal.

Thus, sigma-bond formation and it-backbonding enhance each

other and the concerted drift of electrons is such that the

M-C bond approaches electroneutrality (13).

4

Vibrational spectra and Cotton-Khraihanzel force

constants are evidence of the multiple nature of the M-C

bond-lengths (lit). It is expected that as the extent of

bonding increases, the M-C bond becomes stronger at the

expense of the CO bond, which becomes weaker. Thus, as

antibonding orbital occupancy increases, carbonyl

stretching frequencies should become lower.

Although a wide variety of u-acid ligands, L, such

as phosphines, phosphites, arsines, stilbenes, and sulfides

can replace CO, they are relatively poor u-acceptors. Thus,

a replacement of CO by L should lead to a decrease of the

carbonyl stretching frequencies and the force constants.

According to Cotton (Hi), the influence of the substituted

ligand should be twice as great for trans (axial) carbonyls

relative to cis (equatorial) carbonyls, since L and the cis

carbonyls share only one dit orbital while the trans

carbonyls shares two.

Fenske (!&, 12.) has proposed a model to describe the

bonding in substituted carbonyl complexes. This model

involves a through-space direct donation between the p* lone

pair on the ligand and a proper linear combination of the

it*-orbitals of the equatorial carbonyls (Figure 1). The

effect of the substituted ligand on the stretching

frequencies and on force constants of the cis carbonyls is

expected to increase with increasing covalent radius and

L ,0 9

^rf 0

^ 7 ^ "

& ^ / / M

o ° ° C

° 0 °

o O

/

' V o %

Fig. l--Fensko direct donation model

6

decreasing nuclear charge on the donor atom (U£). Dobson

(il-lfl), Keeling (22) and MacDiarmid (22) have presented

experimental evidence that supports a direct

substituent-carbon monoxide interaction.

Reactions

Octahedral metal carbonyls, M(CO)sL, (L = CO,

amines, M = group VIB metals), exhibit a wide variety of

reactions (2k_25.)- These reactions may be classified as

follows: 1. Ligand coordination and dissociation (in this

category ligand-substitution reaction are included). 2.

Oxidative addition and reductive elimination. 3. Insertion

and deinsertion of olefins and other ligands. 4. Reaction

of coordinated ligands. Our attention will be focused on

ligand substitution reactions. Octahedral metal carbonyls

may undergo ligand-substitution reactions through four

different pathways: dissociative (D), dissociative

interchange (Id), associative interchange (I«) and

associative (A).

Dissociative pathway (D).-- This pathway assumes a rate

determining loss of L (figure 2 path a). The rupture of the

M-L bond proceeds far enough to give an intermediate which

can recombine with L governed by k-i or react with the

incoming nucleophile with rate constant ke. The rate law,

assuming that the concentration of the intermediate 2-a is

steady-state, is given by equation (1).

& o

o'

M

C 0

PATH B k2

°o

PATH A

< — ' - 1 a

o c

M

•o

c o

v

-o

k 8 » <? /L

, / .oO N I ' *

^ 1/V M

^l\ c o 0

C O 0

Fig. 2--Substitution reaction mechanism involving rate-determining dissociation of CO; interchange or associative interaction with L.

-dtS]/dt = kikeCL'3)tS]/(k_itL] + k.[L']> (1)

S = M(CO)sL

Although the step governed by k* is the rate-determining

step, the rate will show a dependence on CL'] since the

intermediate will competitively be attacked by L and L'.

Detection of the intermediate would provide the only

unambiguous proof of the mechanism.

Id and la mechanisms.-- These mechanisms may be described as

involving diffusion-controlled cage formation which then

proceeds to the transition state [M L], which positions

the incoming ligand to enter the coordination sphere on

departure of L. The fundamental difference between Id and

I« is that Id involves considerable bond breaking in the

transition state, while I« involves a more advanced M L'

interaction. The assumption made for both Id and I* is that

the initial interaction between the complex and

L1, (governed by k'e in Figure 2), is the rate-determining

step. The rate law is given by equation (2).

-dtS3/dt = kdx-r^k'stS][L'] (2)

9

mechanism (A).Like the interchange mechanism,

this mechanism proceeds to involve diffusion-controlled cage

formation. There is formation of an intermediate of an

increased coordination number. The rate law for this

mechanism is indistinguishable from the rate law of the

interchange pathway. Unambiguous proof for this mechanism

will come from observation of the intermediate.

Reactions of hexacarbonvl complexes.-- Hexacarbonyl

transition metal complexes are relatively inert toward

ligand-substitution reactions. This property makes them

ideal for thermal kinetics studies since reactions will

take place at convenient rates. The most thermally labile

of these complexes is V(CO )<s , f which undergoes CO exchange at

25 °C with a half-life of several hours (£2)• On the other

hand V(CO)a- is inert to CO substitution even in molten

P(CaH= )a (30). Metal carbonyl complexes of the group VIB

metals undergo ligand-exchange in solution and in gas phase

(29). Activation parameters are insensitive to the nature

of the solvent, consistent with a dissociative mechanism

(22)- The order of M-CO lability as indicated by , Mo >

Cr > W, does not parallel the one for the mean bond energies

for M-CO dissociation; W (42.1 kcal/mole) > Mo (35.9) > Cr

(26.1) (31). However, a closer correlation is found with

the M-C force constants: W (2.32 mdyn/A) > Cr (2.10) > Mo

(2.00) (12.).

10

Substitution reactions of M(CO)*,, (M = Cr, Mo, W),

with amines (31), phosphorus ligands (M. !£). and

acetonitrile (36), showed a two term rate law consistent

with a mechanism involving competitive dissociative and

ligand-dependent interchange (Id) pathways. The activation

parameters for the ligand-independent pathway are very

similar to those obtained for CO exchange, which suggests a

common reaction path for both reactions. Two important

trends associated with k'E (figure 2) are observed for the

ligand-dependent pathway. First, there is a progressive

increase of ks as the basicity of L is increased (as given

by &HNP, AHNP = half neutralization potential) (32). The

order of reactivity was found to be: P(OGd>Hs )3 < P(CU,Ife)a <

P(OCHe)aCEt < P(OEt)3 < P(n-Bu)a. Amines and acetonitrile

showed a reactivity comparable to that of P(OCHe)aCMe and

P(C*Hs)a, respectively. Second, the order of reactivity, Cr

< Mo ~ W, parallels the order of increasing covalent radius

of the metal, Cr < Mo ~ W (M) •

Flash photolysis of cis-W(C0)«.LL' , (L = CO,

piperidine (pip), L' = CO, tri(isopropyl) phosphite), and

matrix isolation studies have probed the nature of the

intermediate generated through the dissociative pathway

(Jl£-.5Ji) • The initial photoproduct has not been observed,

but it is presumed to be an excited state of C<w symmetry.

In the matrix, the C-w nature of the intermediate has been

demonstrated (39. 40).

11

The dependence of the visible spectrum on the nature

of the matrix strongly suggests a solvated intermediate.

However» recent work by Dobson et al. (41) and Simon et al.

(42) suggests that upon the flash, a coordinatively-

unsaturated intermediate is initially formed and is rapidly

attacked by the solvent at the unsaturation site.

Competition studies in cyclohexane (Jtl) between a bona fide

ligand, piperidine, and chlorobenzene (CB), a second ligand,

showed that the five coordinate intermediate will

competitively be attacked by chlorobenzene and piperidine.

upon reaction with chlorobenzene, the cyclohexane-

coordinated intermediate will yield the chlorobenzene-

substituted intermediate, which then will undergo further

substitution by piperidine. According to figure 3 and

assuming that the concentration of intermediate 3-a is

steady-state, the rate law for the displacement of CB by pip

from cis-C(CB)(P(O-i-Pr))W(COU] is given by equation (3).

-d[ (L) (CB)W(COU]/dt = kob.d[ (L) (CB)W(CO)^.] (3)

kob.d = k-ask-i [pip]/(k^i [pip] + k=[CB] (4)

1/kot .c = (kss[CB]/k-ssk-i Cpip]) + 1/k-a (5)

According to equation (4), plots of the observed rate

constants vs. [pip]/[CB] are anticipated to be curved. The

reciprocal relationship as dictated by equation (5) predicts

a linear plot of 1/kob.c vs. [CB]/[pip] with intercept equal

to 1/k—ss and slope k=/k-ik-=.

12

Pi p.

o c

W - V'V

y \ k-1 pip ^ \ 1 o co

J? hv

c o

o c

w

J& *

31 C H CH.

O c

w

x?

R5CB

-5

o

c \ l ^

co c o

k : 3

b s

Fig. 3--Generation of coordinatively unsaturated cis-[LW(COU] and attack by CH, CB and pip. CH = cyclohexane, CB = chlorobenzene, pip = piperidine.

13

Curved plots of kot>.c« vs. [pipl/ECB], indicative of the

mechanism depicted in figure 3, were observed for the

reaction of cis-[ (pip) (CB)W(CO)**] with pip. The rate

constant for M-CB bond rupture, obtained as 1/intercept from

the reciprocal plot at 20.2 °C, is 1920 sec-1 (<tl). The

competition ratios, k-i/k«, which describe the relative

rates of attack at 3-a by pip and CB, reflect the poor

selectivity of 3-a toward incoming nucleophiles. For

example, at 20.2 °C kr-i/kas is 2.36. Activation parameters

for k» are consistent with an initial dissociation of the

coordinated solvent (&H^= 13.0(4) kcal/mol, 5.6(11)

cal/deg-mol) (41).

Reactions of chelate complexes.-— Chelate complexes may

undergo ligand-substitution reactions involving either

CO-displacement or complete removal of the bidentate ligand

(56. 60-75). In some instances these two reactions take

place competitively. Early works by Angelici and Graham

showed that Cr(CO)«*(dipy) (dipy = 4,4'-dimethyl-2,2'-

dipyridyl) reacts under fairly mild conditions with a

variety of phosphites, L, in organic solvents to yield

cis-Cr(CO)s(dipy)(L) <jL£). This was an unexpected result in

view of the inertness of Cr(CO)& toward CO exchange (jiZ,

58). The labilizing effect by the hard base, dipy, was

proposed to explain the enhancement in reactivity of the

Cr-C bond ( 59. (jO) .

14

Analogous species , W(CO )* (dipy) and Mo (CO) * (dipy) showed a

more complex behavior (jjl). Reaction of M(CO)*(dipy) (M =

W, Mo) with a variety of phosphites yielded three products

(il): M(C0)3(dipy)L, M(CO)*Ls, and M(CO)3La. The rate law

contains two terms, ascribable to a ligand-dependent pathway

and a ligand-independent pathway (figure 4).

The first term, independent of [L]> suggested a

simple dissociation process analogous to the one observed

for Cr(CO)*(dipy). Although activation parameters (AH = 25

kcal/mole, As^ = 12 to -12 cal/deg-mol) are very similar to

those reported for similar reactions of Cr(CO)*(dipy) (M),

a slight dependency of ki on the nature of L was found.

Despite this observation a pathway involving initial

fission of the M-CO bond was proposed (.61). For the ligand-

dependent pathway, an associative mechanism involving a

7-coordinate intermediate was proposed. Large negative

entropies of activation and small positive enthalpies of

activation are consistent with this mechanism. Dobson, et

al. reinvestigated the reactions of Cr(CO)*(dipy) (£2.) and

W(CO)*(dipy) (63) with L, (L = triethyl phosphite) and

proposed an additional competing reaction pathway which

involves fission of the N-M bond to produce a ring-opened

intermediate. According to the mechanism shown in figure 4,

the rate law, assuming that the concentration of the

intermediate 4-a is steady-state, is given by equation (6).

o c

0 c

M

/ l \ c o

fc 1

M

c o

15

S i -o \

/

/ | \ M

C O

O c

k

c o

M

c o

2 I

'M

% C o

Fig. k--Displacement of bidentate ligand, bonded through nitrogen to the central metal, involving a competitive mechanism.

16

dtS3/dt = k'ltS] + k'.CLHS] + (kikatL] [S] )/(kx + ks[L]) (6)

S = (dipy)Cr(COU

At high ligand concentration, equation (6) reduces to

equation (7).

-d[S]/dt = (k'x + kx )CS] + k'a CL3 CS3 (7)

Equation (7) is consistent with the kinetics behavior for

the reactions of Cr(COU(dipy) with phosphites.

Chelate complexes bonded through gulfuy--. Ligand

substitution reactions of chelate complexes bonded through

sulfur atoms proceed through a complete displacement of the

chelate ring (M-71). However, contrasting kinetics

behavior has been found for systems which are closely

related. The reaction of (De-DTH)M(CO, (DTH =

2,5-dithiahexane), (M = Mo, Cr), with a variety of

phosphites was found to follow a second-order rate law (M)-

Under pseudo-first order conditions, plots of the observed

rate constant vs. [L] were found to be linear with zero

intercept. This behavior is consistent with two closely

related mechanisms which have been shown in some instances

to be competitive (figure 5 path a and b).

4 s

w

c o

PATH B k. f a s t

O C

/ / ' y *

/ | \ + &

PATH A

fast

17 o c

w

I?

4- c I o

o c

w

+ + c o

o c

w

c o

f as t

O c ?

/

w l\ c o

Fig. 5--Displacement of bidentate ligand, bonded through sulfur to the central metal, involving a competitive mechanism.

18

One involves an initial rate-determining ring-opening to

afford a coordinatively-unsaturated five membered

intermediate (path A); the other involves an initial

associative or interchange attack at the metal center

(path B). In order to distinguish between these two

mechanisms an extra-kinetics probe has to be used. The

criterion of entropy of activation was employed to

distinguish between the possible mechanisms, since a

dissociative pathway is expected to show a more positive

entropy of activation. Activation parameters for the

reaction of (fF-DTH)Cr(CO)«. with various phosphites, P(OR)s

(R = methyl, ethyl, isopropyl), in 1,2-dichloroethane were

consistent with a dissociative mechanism <M) (average AH =

24.5 kcal/mole, average As^= 3.39 cal/deg-mol), while

activation parameters for the reaction of (fla-DTH)Mo(CO)«.

under similar conditions suggested an associative^or

interchange mechanism being operative (average Ah = 16.2

kcal/mole, average As* = -19.7 cal/deg-mol) (M>- The

reaction of (fF-BTE)Cr(COU (££) and (ne-BMTB)Cr(COU (46)

(BTE = cis-bis(t-butylthio)ethylene, BMTB = 4-methyl-l,2-

bis(methylthio)benzene), with L = P(OR)a (R = ethyl,

isopropyl) in chlorobenzene and 1,2-dichloroethane showed

similar behavior to that of (De-DTH) Cx*(CO)** in the sense

that linear plots of the observed rate constants vs. EL]

were observed.

19

However, slightly negative entropies of activation were

observed; for (na-BTE)Cr(COU, the average As^(~3-2 cal/deg-

mol); for (fF-BMTB)Cr(CO)*., the average As^(-8.20). This

discrepancy prompted the kinetics studies of the reaction of

(fF-BMTB) Cr(CO)«• with triethyl phosphite over a wide range

of temperatures. This experiment was performed to probe the

possibility of a competing mechanism, since deviations from

linearity for Eyring plots of ln(k/T) vs. 1/T (T = absolute

temperature) are anticipated if a competitive mechanism was

operative. A linear Eyring plot was obtained over an 80

degree temperature range, which suggests a single pathway.

Despite the observed negative entropy of activation a

dissociative mechanism was suggested. The basis for this

interpretation rests on the fact that the observed As^from

the composite rate constant (path a in figure 5), kike/k—x,

is given by As^ = Asi + Ast - Asl*. The rigid backbones of

(n®-BTE)Cr(COW and (fl®-BMTB)Cr(CO)-=» causes few degrees of

freedom to be gained upon ring-opening. Thus, making As»t

small and positive. In addition, upon ring-closure, few

degrees of freedom are lost making A s ! small and negative.

A highly negative ASe is expected for two colliding bodies.

Thus, the summation of all the individual _S might result

in a negative value. Further evidence supporting a

dissociative mechanism comes from volumes of activation

studies of the reaction of (He-BTE)Cr(CO ) . with L (jiZ).

20

The reactions of (fF-DTO)W(COU ( M ) and (n«-DTN)W(C0U (££)

(DTO = 2,2,7,7-tetramethy1-3,6-dithiaoctane, DTN = 2,2,8,8-

tetramethyl-3,7-dithianonane) with a variety of phosphites

have been extensively studied. (fF-DTOJWfCOK undergoes

substitution reactions involving a rate-determining ring-

opening to afford a solvated five-coordinate intermediate

(intermediate 5-a), which then undergoes a competitive ring-

closure and attack by an incoming ligand (figure 5). Plots

of kobad vs. [L] with all ligands employed were curved.

Linear reciprocal plots of 1/kot—c. vs. 1/tL], indicative of

a complex behavior, have common intercepts for all ligands

studied. According to figure 5 the rate law for the

disappearance of the substrate, assuming that Jo-e << ke and

that the concentration of the intermediate 5-a is steady-

state , is given by equation (8).

-dtS]/dt = ((ktke[L])/(k_t + ketL])) [S] (8)

Analogous reactions of ( rF-DTN)W(COU also exhibited

curvature for plots of kob.«. vs. [L] (£2). The reciprocal

plots, although linear, did not show a common intercept for

different ligands as predicted by equation (8). Common

intercepts are anticipated since L is not involved in the

ring-opening step which is governed by k*. An alternative

mechanism was proposed which involves a competitive

rate-determining ring-opening and an initial attack at the

metal center by L (figure 5).

21

According to figure 5 and employing the assumption of the

steady-state concentration of intermediates (5-a) and

(5-b), the rate law for the reaction of (fF-DTN)W(CO)«• with

L is given by equation (9).

-d[S]/dt = kot»d[S] (9)

S = (fF-DTN)W(CO)^

ko b « c

k* (kike + k-ik-a ) tL] + kak.2[LJe

(k-i (k~a + k'alu ) ) + ke (k'a + k*. ) [L]

Assuming that ki >> k'a

ke(k's + k*) k—i(k—a + k-3 + k^)

(10)

(11) kob.et k^.(kik-a + k-ik'e) k^(kike + k-ik'e) [L]

Assuming that k^i >> ke this rate law will assume the same

form as the one observed for (n*-DT0)W(C0U and thus curved

plots of kct>m« vs. tL] are expected. The intercept for the

reciprocal plot is given by equation (11). Since ke, k'a,

k'a, and k*. are ligand-dependent terms and are included in

the intercept of the reciprocal plot, non-common intercepts

for the reciprocal plot are anticipated. It is rather

intriguing that molecules which are closely-related in

structure will show such dramatically different kinetics

behavior. This fact prompted crystal and molecular

structure studies of both (ne-DT0)W(C0U and (ne-DTN)W(C0U

(7(2) .

22

Both systems were found to contain a distorted octahedral

W( CO )«»Se fragment but are quite similar in their structural

parameters.

The problem

As mentioned above, very small but statistically-

significant differences in structural parameters may account

for the dramatic differences in reactivity and reaction

pathway by which (fP-DTO)W(CO)^. and (na-DTN)W(CO)^ undergo

ligand exchange reactions. The kinetics and mechanisms

studies for the reactions of (ns-DTHp)W{CO)«.,

(fF-DTD)W(COU, <fle-DTU)W(COU, (DTHp = 2,2,6, 6-tetramethyl-

3,5-dithiaheptane; DTD = 2,2,9,9-tetramethyl-3,8-dithia-

decane; DTU = 2,2,10,10-tetramethyl-3,9-dithiaundecane) with

phosphites and phosphines could provide a better

understanding about how the molecular structure of these

sulfur chelate complexes may influence the reactivity and

the reaction mechanisms by which the ligand-exchange

reactions take place. These complexes provide an

opportunity to probe this problem since the only difference

between the complexes in the series is the size of the

chelate ring.

CHAPTER BIBLIOGRAPHY

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25

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41. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E.; Awad, H. H.; Dobson, G. R. J. Am. Chem. Soc.. 1987, 109, 5386.

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43. Geoffroy, G. L. ; Wrig-iton, M. S. Qrganometallic Photochemistry: Academic Press: New York, 1979, pp 68-78.

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45. Stolz, I. W.; Dobson, G. R.; Sheline, R. K. J. Am. Chem. Soc.. 1962, 84, 3589.

46. Perutz, R. N.; Turner, J. J. Inorg. Chem.. 1975, 14, 262 .

47. Boylan, M. J.; Braterman, P. S.; Fullarton, A. J. Organomet. Chem.. 1971, 31, C29.

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49. Boylan, M. J.; Black, J. D.; Braterman, P. S. J_. Chem. Soc. Dalton Trans.. 1980, 1646.

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26

52. Langford, C. H.; Moralejo, C.; Sharma, D. K. Inorg. Chim. Acta. 1987, 126, Lll.

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56. Angelici, R. J.; Graham, J. R. J. Am. Chem. Soc.. 1965, 87, 5586.

57. Basolo, F.; Wojcicki, A. J.Am. Chem. Soc.. 1961, 83, 520.

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J. M - Chem. Soc. . 1961, 83, 3200.

59. Stiddard, M. H. B. J. Chem. Soc.. 1963, 756.

60. Pearson, R. G. J. Am. Chem. Soc.. 1963, 85, 3533.

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62. Memering, M. N.; Dobson, G. R. Inorg. Chem.. 1973, 12, 2490.

63. McKerley, B. J.; Faber, G. C.; Dobson, G. R. Inorg. Chem., 1975, 14, 2275.

64. Faber, G. C.; Dobson, G. R. Inorg. Chem.. 1968, 7, 584.

65. Halverson, D. E.; Reisner, G. M.; Dobson, G. R.; Bernal, I.; Mulcahy, T. L. Inorg. Chem.. 1982, 21, 4285.

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68. Schultz, L. D. ; Dobson, G. R. jJ. Organomet. Chem. . 1977, 124, 19.

27

69. Dobson, G. R.; Schultz, L. D. J. Organomet. £hem., 1977, 131, 285.

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74. Dobson, G. R.I Basson, S. S.j Dobson, C. B. Inorg. Chim. Acta. 1985, 105, L17.

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CHAPTER II

EXPERIMENTAL

General

Infrared spectra were obtained employing a Nicolet

20 SXB Fourier transform infrared spectrometer and where

indicated a Perkin-Elmer model 621 grating spectrometer.

Unless indicated, NMR spectra were obtained using a JEOL

FX90Q Fourier transform NMR spectrometer. Reaction rates

were monitored following a decrease in absorbance at 415 nm,

using either a direct reading Beckman DU-2 spectrophoto-

meter, or an in-house built optical rail spectrophotometer.

This in-house built spectrophotometer will be described in a

later subsection. A Haake D8 temperature controlled water

circulator and a refrigerated and heated Forma-Temp Jr model

2095 bath and circulator were employed as temperature

control devices. Elemental analyses were performed by

Midwest Microlab, Indianapolis, IN.

28

29

pirrification of gQlvgPtg

Chlorobenzene (CB; Fisher), and 1,2-dichloroethane (DCE;

Fisher) were stirred and refluxed in the presence of PeOs

under nitrogen for at least twenty-four hours before they

were fractionally distilled. Bromobenzene (BB; Matheson,

Coleman and Bell) was stirred for three hours over anhydrous

MgSO » and then refluxed for four hours over PeOas. It was

then fractionally distilled under nitrogen.

pirrification of lisands

In a typical experiment, the ligands (L) were stirred in

the presence of sodium metal under nitrogen for at least

twenty-four hours, followed by a fractional or a vacuum

distillation.

Constrained phosphite.— The constrained phosphite (CP),

4-methyl-2,6,7-trioxa-l-phosphabicyclo-[2.2.2]octane,

prepared according to the published method (i, j2.), was

twice sublimed under reduced pressure into a wide-mouthed

condenser, and was then recrystallized under nitrogen from

hot n-hexane.

Tri(isopropvl) phosphite.-- To avoid air and moisture upon

breaking the vacuum, tri(isopropyl) phosphite (Aldrich) was

distilled at reduced pressure from sodium metal and under a

bleed of nitrogen, (b 60 °C at 11 torr).

30

Trvimethvl phosphite.— Trimethyl phosphite (Aldrich) was

fractionally distilled from sodium metal under nitrogen

(b 111 °C).

TV\phenyl phosphite.-- Triphenyl phosphite (Aldrich) was

distilled at reduced pressure from sodium metal under a

bleed of nitrogen (b 190 °C at 11 torr).

Tri(n-but.vl) phosphine.-- Tri(n-butyl) phosphine was

distilled at reduced pressure from sodium metal, under a

bleed of nitrogen (b 80 °C at 11 torr).

Syntheses of bidentate liRapds

DTA = DTHp. DTD. DTU.-- DTA were prepared following a

similar method reported by Dobson and others (1, it). In a

typical experiment, 23 g (1 mol) of sodium metal was

allowed to react with 500 mL of ethyl alcohol. After all

the sodium was consumed, 90 g (1 mole) of t-butyl mercaptan

(Aldrich) was slowly added and after addition was completed,

the reaction mixture was stirred for two hours. A pale

yellow solution resulted. Then, 0.5 mol of the appropriate

dibromoalkane (Aldrich) was slowly added under cooling

(dibromomethane for DTHp, 1,4-dibromobutane for DTD, 1,5-

dibromopentane for DTU).

31

Upon addition of the dibromoalkane a milky suspension was

obtained due to the formation of sodium bromide. The

reaction mixture was stirred for several hours, then the

sodium bromide was removed by suction filtration. Ethyl

alcohol was removed from the reaction mixture by

distillation and the reaction product was vacuum distilled.

DTHP.-- DTHp was prepared according to the method described

above. Upon distillation at reduced pressure (1 torr), a

colorless liquid was obtained (b 52-53 °C). The purified

yield was 58%. *H NMR (CDCla) 5 1.28 (18 H, s), 3.62 (2 H,

s); 13C NMR (CDCla) 6 27.83 (t), 30.81 (q), 43.39 (s).

DTD.-- DTD was prepared according to the method described

above. Upon distillation at reduced pressure (1 torr), a

colorless liquid was obtained (b 119-20 °C). The purified

yield was 42%. *H NMR (CDCla) 6 1.24 (18 H, s), 1.67 (4 H,

m), 2.53 (4 H), (t, J = 9 Hz); 13C NMR (CDCla) 6 27.540 (t),

28.970 (t), 30.758 (q), 41.425 (s).

DTU.-- DTU was prepared according to the method described

above. Upon distillation at reduced pressure (1 torr), a

colorless liquid was obtained (b 134-35 °C). The purified

yield was 38%. *H NMR (CDCla) 5 1.251 (18 H, s), 1.50 (6

H, m), 2.45 (4 H), (t, J = 9 Hz); l3C NMR (CDCla) 6 27.778

(t), 28.434 (t), 29.208 (t), 30.70 (q), 41.246 (s).

32

Syntheses of metal complexes

(ng-DTHpiWCCO)^.— Two g of DTHp were added to a solution

of 3 g of W(CO)<s, (Pressure Chemical) in 350 mL of hexanes

(Fisher). The reaction mixture was irradiated using a

Hanovia 450 watt immersion UV lamp. The reaction was

monitored by infrared spectroscopy by observing the growth

of a band at 2017 cm-1. A yellow solid precipitated as the

reaction proceeded. The crude product was recrystallized

from toluene-hexanes. Bright yellow crystals were obtained.

The yield was 20%. The carbonyl stretching spectrum is

shown in figure 6. Anal. Calcd for CiaHsoO^SsW: C, 31.98;

H, 4.13. Found: C, 32.01; H, 4.30.

Cis-(fl1 -DTHP) (CP?W(COU.-- In a 100 mL volumetric flask, 0.3

g (0.61 mmol) of (CF-DTHpJWtCOU were allowed to react with

0.15 g (one mmol) of CP in 1,2-dichloroethane at room

temperature. The reaction was monitored by observing the

relative intensities of the bands at 2017 cm-1 and

2031 cm-1, corresponding to (fle-DTHp)W(CO)^ and

(0*-DTHp)(CP)W(CO)^, respectively. When the intensity of

the band at 2031 cnr1 no longer increased, the reaction

mixture was quenched by immersing the flask in an ice-water

bath. The solvent was evaporated under vacuum and a pale

yellow solid precipitated, which was then recrystallized

from toluene-hexanes. The yield was 53.4%.

33

a

2 0 6 7 2 0 0 0 1933 1866 WAVENUM8ER

1 79S

*>--Carbonyl stretching spectrxim of <2ia-<n«-DTHp)W(C0U in chlorobenzene.

34

The carbonyl stretching spectrum is shown in figure 7.

Anal. Calcd for CisHs^O^SePW: C, 33.97; H, 4.60. Found:

C, 33.90; H, 4.35.

(nB-nTD)W(COU.-- (fle-DTD)W(C0)4 was prepared in a manner

analogous to the one used for (DE!-DTHp)W( CO) *•. The reaction

progress was followed by observing the growth of a band at

2013 cm-1. A yellow solid precipitated as the reaction

proceeded. The crude product was recrystallized from

toluene-hexanes and bright yellow crystals were obtained.

The yield was 25.2%. The carbonyl stretching spectrum is

shown in figure 8. Anal. Calcd for Ci«,He«»04.SeW: C, 36.23;

H, 4.94. Found; C, 36.01; H, 4.99.

Cis-(n*-DTD) (CP)W(COU.-- Cis-(fl1 -DTD) (CP)<CO)<• was

prepared following a method similar to the one used

for cis- (fl'-DTHp) (CP)W(CO)*.. The progress of the reaction

was monitored by observing the disappearance of the band at

2013 cm-1 and the growth of the band at 2028 cm-1. After

completion of the reaction, the reaction mixture was

quenched by immersing the reaction flask in an ice-water

bath. The solvent was then evaporated by bubbling a gentle

stream of nitrogen through the solution. A yellow solid was

obtained. Recrystallization from toluene-hexanes yielded

pale yellow crystals. The yield was 43%. The carbonyl

stretching spectrum is shown in figure 9.

35

(O o <Ti

2077 2015 1953 1891 WAVENUMBER

8 2 ?

Fig. 7--carbonyl stretching spectrum of cis- (n'-DTHp) (CP)W(CO)*. in chlorobenzene.

36

m OD CO

2 0 1 5 1 9 5 3 1 8 9 1

W A V E N U M B E R

1 8 2 9

, ~ ® * ® " ~ C a r b o n y l s t r e t c h i n g s p e c t r u m o f £ i S - < f l s - D T D ) W ( C O ) » i n c h l o r o b e n z e n e .

37

CD O c n

2 0 1 5 1953 1891 WAVENUMBER

1829

^ o f

38

Anal. Calcd for C«iH3*<>*SePW: C, 37.18; H, 5.20. Found:

C, 37.06; H, 5.20.

fne-rmnw(COU.-- (ns-DTU)W(C0U was prepared in a manner

analogous to the one used for (n®-DTD)W(C0)^ and

(fie-DTHp)W(CO)<». The yield was 22%. The carbonyl

stretching spectrum is shown in figure 10. Anal. Calcd for

Ci-rHsoO^SeW: C, 37.51; H, 5.18. Found: C, 37.52; H, 5.13.

Hia-tn^-nTTTWCPiWCCQ)^..-- Cis-(n1-DTU) (CP)W(CO)^ was

prepared in chlorobenzene following a method similar to the

one used for cis-Cfl'-DTHp) (CP)W(COU and cis-Cf^-DTD)

(CP)W(CO)*+. However, this intermediate was not isolated.

It was characterized in situ by infrared spectroscopy. The

carbonyl stretching spectrum is shown in figure 11.

Trfantification of intermediates

The carbonyl stretching spectra of the intermediates

[cis-{n*-DTHp) (L)W(COU] (L = tri(n-butyl) phosphine,

4-methyl-2,6,7-trioxa-1-phosphabicyclo-[2.2.2]octane (CP),

tri(isopropyl) phosphite, trimethyl phosphite, and

triphenyl phosphite) were recorded in situ employing a

Perkin-Elmer 621 grating infrared spectrometer. The

carbonyl stretching frequencies are given in table I.

39

tn m

&

2 0 7 7 2 0 1 5 1 9 5 3 1 8 3 ! i 8 2 9

W A V E N U M B E R

£ i a - ( f t®-DTU?t f^C0^U°in 1 chlorobenzenef e C ^* r U m ° f

40

to o 0)

2 0 7 7 2 0 1 5 1 9 5 3 1 8 9 1

w a V £ N U M B E R

1 8 2 9

c i s - r n ^ l w l T w ™ ^ ™ ? 1 s t r e t c h i n « s p e c t r u m o f H2SL T U ) ( C P ) W ( C O ) ^ x n c h l o r o b e n z e n e .

41

TABLE I

CARBONYL STRETCHING FREQUENCIES OF CIS- (fl1 -DTHp) (L)W(CO)*. IN CHLOROBENZENE

(CO) cnr1

P(n-Bu)s 2008 (w) 1901 (sh) 1980 (s) 1869 (m)

CP 2031 (w) 1925 (sh) 1907 (s) 1883 (m)

P(0-i-Pr)3 2025 (w) 1919 (sh) 1902 (s) 1886 (m)

P(0Me)a 2025 (w) 1917 (sh) 1900 (s) 1872 <m)

PtOCeHsJa 2030 (w) 1927 (sh) 1910 (s) 1886 (m)

THtan-hi fication of reaction products

Reaction products were identified by comparing the

carbonyl stretching spectra of reaction solutions to the

spectra reported for those complexes (JL, &, !)• The

carbonyl stretching spectra for cis-(L)eW(COU, (L =

P(n-Bu)s, CP, tri(isopropyl) phosphite, trimethyl

phosphite, and triphenyl phosphite), are given in figures

12, 13, 14, 15, and 16, respectively (1).

42

M O r-09

2 0 3 3 1977 1921 WAVENUMBER

1865 1809

Fig. 12--Carbonyl stretching spectrum of cis- and trans- (P(n-Bu)a )eW(C0)«» in chlorobenzene.

43

(0 r*. CO N 0)

2 0 8 9 2 0 3 3 1977 1921 1865 WAVENUMBER

eis - (cpf * vfrni C a f b o n^ | stretching spectrum of £A§. v CF )eW( CO )i» m chlorobenzene.

44

o 0 01

2 0 3 3 1977 1921 WflVENUMBER

1865 1809

Fig. 14--Carbonyl stretching spectrum of cis-and trans-(P(O-i-Pr)a)eW(CO)^ in chlorobenzene.

45

O) o O)

2089 2033 1977 1921 WAVENUMBER

1865

Fig. 15--Carbonyl stretching spectrum of cis and trans -(P(OMe)a)eW(CO )*+ in chlorobenzene.

46

CNJ <n <D

2089 2033 1977 1921 WAVENUMBER

1865 1809

CIS Pig. 16--Carbonyl stretching spectrum of

{PfOC ffa )a )rW(C0)<. in chlorobenzene.

47

Recrvstallizatior} of metal complexes

( n e - D T A ) W ( C Q ) ^ . - - All the < n e - D T A ) W ( C O U complexes were

recrystallized from a toluene-hexanes solution. In a

typical experiment, 3g of (fle-DTA)W(COU were dissolved in

25 mL of hot toluene. The solution was then filtered

through celite, and hexanes were added until crystallization

commenced. The solution was shaken and then placed in the

refrigsrator. After 24 hours, yellow crystals were

obtained. After filtration, these crystals were rinsed with

five portions of 10 mL of hot hexanes and dried under vacuum

for 6 hours.

gig- (I11-PTA) (CP)W(CO)^.— All the cis-(n*-DTA) (CP)W(CO)*.

were recrystallized from a toluene-hexanes solution. In a

typical experiment, 0.2 g of cis-(fl1-DTA) (CP)W(COU were

dissolved in 10 mL of toluene. The solution was then

filtered through celite, and hexanes were added until

recrystallization commenced. The solution was shaken and

then placed in the refrigerator. After 24 hours, a pale

yellow solid was obtained. After filtration under a stream

of nitrogen, the product was rinsed with five portions of

hot hexanes and dried under vacuum for 6 hours.

48

Kinetics runs

Fast reactions were monitored employing an in-house

built optical rail spectrophotometer. This spectrophoto-

meter employed a 40-W tungsten lamp, powered by a

Hewlett-Packard (Harrison 6274 A) DC power supply, a Bausch

& Lomb (33-86-20) monochromator, a side-on photomultiplier

tube (Hamamatsu R 136), and an Aminco linear-log-photo

meter, powered by an Aminco dual power supply. The

spectrophotometer's output was digitized employing a Nicolet

2090 Ila oscilloscope.

In a typical experiment, approximately 1.5 mL of a

ligand-solvent solution was placed in a thermostated cell.

Then approximately 2 mg of substrate were dissolved in the

thermally-equilibrated solution. The oscilloscope was then

triggered, and the absorbance of the reaction solution was

measured as a function of time. For slower reactions, a 25

mL volumetric flask containing 25 mL of solvent-ligand

solution was placed in a thermostated bath in which the

temperature was the same as that of the thermostated cell.

Approximately 5 mL of the solvent-ligand solution was

rapidly added to 5 mg of the substrate. The resulting

yellow solution was then quickly placed in the thermostated

cell, and when thermal equilibrium was reestablished

(approximately 10 seconds), the absorbance as a function of

time was recorded.

49

During the early stages of this investigation, the progress

of the slow reactions was monitored by withdrawing samples

and by measuring the absorbance at 415 nm employing a

Beckman DU-2 direct reading spectrophotometer. In a 50 mL

volumetric flask, the solvent was added to a weighed amount

of ligand, which together occupied a volume of approximately

47 mL. The flask was then placed in the thermostated bath

until the solution was thermally equilibrated. Additional

solvent was then added up to the calibration mark.

Approximately 30 mg of the substrate were placed in a 100 mL

volumetric flask equipped with a stopcock and rubber septum.

The solution was added to the substrate and a gentle stream

of nitrogen was bubbled through the solution. The. reaction

mixture was then placed in the thermostated bath and allowed

to equilibrate. Sampling was then begun by withdrawing

samples of 3 mL each and by measuring the absorbance at 415

nm. Three mL of nitrogen were injected into the reaction

flask before removal of the sample. This insured a positive

pressure within the reaction vessel.

Pulsed-laser flash photolysis studies were carried

out using the facilities of the Center for Fast Kinetics

Research (CFKR), University of Texas at Austin (£., 10 ). A

schematic diagram of the equipment employed is shown in

figure 17.

50

LASER (Nd: YAG)

XENON LAMP

• A

- X M

TRIGGER SEQUENCE

GENERATOR

MC

P P

POP 11/70 COMPUTER

BIOMATION 8100

WAVEFORM DIGITIZER

P; P h o t o m u l t i o l i S ? ' k A ' ® t t e " u a t o r : MC, monoch roma to r ; ' D ' P h ° t 0 d i 0 , l e - o n i t c r ; M,

51

A Quantel Q-switched Nd:YAG laser (355 nm irradiating

wavelength, 11 ns FWHI) was used as a source of high

intensity radiation.

In a typical experiment, a solution containing the

substrate, the ligand, and the solvent were placed in a

jacketed observation cell. The temperature of this cell was

controlled by an external circulating bath (Forma Jr. model

2095) and monitored by a Keithley 872 digital thermometer

(FeCuNi thermocouple). Concentrations of the substrate were

in the vicinity of 5 X lO"* M. After the flash, the decay

of the photogenerated transient was followed by a

conventional optical rail spectrophotometer which was

controlled by a pulse generator. The photolysis cell had a

monitoring path of 1.0 cm. Transient absorptions were

monitored at a right angle to the photolyzing beam with an

Oriel xenon lamp (150-W). A Bausch & Lomb monochromator and

a Hamamatsu Corp. 928 photomultiplier were employed as

detection devices. The output wave forms from the

photomultiplier were fed into a Biomation 8100 waveform

digitizer.

During all the experiments the sample and the

detector were protected from the intense monitoring beam by

shutters which were controlled by the programmable sequence

generator.

52

Pseudo-first order rate constants were evaluated

employing an interactive linearized least-square analysis

program (DEC PDP-11/90 computer) and each numerical value is

the average of five or more kinetics runs employing the same

sample.

CHAPTER BIBLIOGRAPHY

1. Wadsworth, W. S.t Jr.; Emmons, W. C. iv Am- £hsm> Soc.,

1962, 84, 610.

2. Verkade, J. G. Inors. Ghem-, 1962, 1, 392.

3. Dobson, G. R.; Faber, G. C. I nor a. Chim. Acta, 1970, 4, 87 .

4. Federov, B. P. ; Savel' eva, I. S. Igvest . Ak^d• » SSSR. 1950, 223.

5. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E.; Awad, H. H. ; Dobson, G. R. J.. Am. Chem. Soc.. 1987, 109, 5386.

6. Dixon, D. T. ; Kola, J. C.;Howell, J. A. S. jl. Chgffl. Soc. Dal ton Trans.., 1984, 1307.

7. Vandenbroucke, A. C.; Hendricker, D. G.; McCarley, R. E.; Verkade, J. G. Inors. Sbfiffl-. 1968, 1, 1825.

8. The experimental assistance by Mr. David Dumond is gratefully acknowledged.

9. Lindig, B. A.; Rodgers, M. A. J. «I. Phys. Chem. , 1979, 83, 1683.

10. The experimental assistance by the staff of the CFKR is gratefully acknowledged.

53

CHAPTER III

REACTIONS OF (fF-DTA)W(COU

General

The thermal reactions of (na-DTA)W(CO)«., (DTA

2,2,6,6-tetramethyl- 3,5-dithiaheptane, (DTHp), 2,2,9,9

tetramethy1-3,8-dithiadecane, (DTD); 2,2,10,10-tetramethyl-3,9-

dithiaundecane, (DTU)), with L = phosphites and phosphines take

place according to equation (12).

(n*-DTA)W(CO)<. + L > (L)eW(COU + DTA (12)

cis-(n1-DTA) (L)W(CO)*. -> cis-(L)eW(COU + DTA

Plots of In(A* - A») vs. time, in which At and A- are the

absorbances of the reaction solution at a given time (t) and the

absorbances at over ten half-lives, respectively, consisted of

two linear segments. Figure 18 illustrates this plot for the

reaction of (n*-DTHp)W(COU with tri(isopropyl) phosphite (0.2008

M) in chlorobenzene (CB) at 21.1 °C.

During the course of reaction (12), appreciable

formation of cis-(fl1 -DTA) (L)W(COU, (governed by k„*«c), <L =

phosphites, and phosphines) was observed. These intermediates

were characterized when L = CP.

54

55

10 ^

9

8

7

6 -

5 -

4 -<

3 -

1 - i 2

Fig. 18--Plot of (A* - A-.) vs. time for the reaction of (n®-DTHp)W(C0)^ with tri(isopropyl> phosphite in chlorobenzene at 21.1 °C. [L] =0.2008 M. Ordinate = (A* - A-), Abscissa = time X 10—* sec.

56

Further reaction of cis-(fl*-DTA) (L)W(CO)«. with L, governed by

k'ob.d afford the disubstituted product cis-(L)eW(CO)^.

Both the rates at which cis-(fl1 -DTA) (L)W(CO)^ is produced, and at

which it will undergo subsequent reaction to produce

cis-(L)eW(CO)^, are within the same time-scale. Thus these

reactions exhibit biphasic behavior. A discussion of the

reactions of (fle-DTD)W(CO)^. and (ne-DTU)W(CO)*. with L to produce

cis-lfP-DTA)(L)W(CO)^ will be deferred until next chapter.

Reactions of (fF-DTHp)W(CO)^

Formation of Cis-(fl*-DTHp) (L)W(COU

The reactions of (f1e-DTHp)W(C0)^. with L (L =

phosphites, and phosphines) in chlorobenzene (CB) to produce

cis-(fU-DTHp) (L)W(CO)^ were studied under pseudo-first order

conditions. The concentrations of L were at least a twenty-fold

excess relative to that of the substrate. Pseudo-first order

rate constants are given in table II. Plots of the observed rate

constants vs. [L] are linear over a wide range of concentrations.

These plots are illustrated in figure 19 and 20 for L =

tri(isopropyl) phosphite and trimethyl phosphite, respectively.

This behavior is consistent with two previously proposed

mechanisms which are shown in figure 21 (J.-5.).

57

TABLE II

FIRST-ORDER RATE CONSTANTS FOR THE REACTIONS OF ( f F_ D T Hp)W<COU WITH PHOSPHITES IN

CHLOROBENZENE AT VARIOUS TEMPERATURES

ligand O O [L]

(M) 10e kob.d (sec-1)

P(0-i-Pr)a 44.5

0.9230 0.6922 0.6571 0.5455 0.3310 0.2493 0.1316 0.05378

7.19(17) 5.73(6) 5.41(21) 4.89(4) 2.39(1) 2.44(3) 1.28(3) 0.575(11)

35.2 1.1547 0.9315 0.7536 0.6132 0.4769 0.3231 0.1424

4.16(3) 3.45(2) 2.74(2) 2.342(7) 1.556(6) 1.123(5) 0.555(5)

31.1 0.9131 0.6488 0.3984 0.3084 0.1331 0.1021 0.09935 0.07905 0.07321 0.06392 0.05734 0.02837

2.45(3) 1.63(1) 0.996(10) 0.701(7) 0.330(4) 0.278(2) 0.292(3) 0.1815(6) 0.182(1) 0.158(1) 0.159(1) 0.0909(4)

21.1 0.9730 0.7727 0.4848 0.3487 0.3010 0.2674 0.2008 0.1675

1.037(8) 0.840(9) 0.541(6) 0.380(2) 0.317(3) 0.262(2) 0.212(1) 0.164(1)

TABLE II CONTINUED

58

T, [L] (M)

1 0 S kob«d (sec-1)

P(OMe)a 21.1

0.1263 0.125(1) 0.07528 0.0731(6) 0.05462 0.0529(6) 0.04800 0.0449(6)

0.9831 2.72(2) 0.5970 1.64(2) 0.4474 1.23(1) 0.3519 1.04(1) 0.3140 0.841(5) 0.1565 0.463(3)

One involves an associative, or perhaps a dissociative

interchange process (path B), to form a seven-coordinate

transition state or intermediate. The other (path A) involves

rate-determining ring-opening to yield an unsaturated five-

coordinate intermediate (21-a). This intermediate then

undergoes rapid solvation and desolvation governed by ka and

3c-a, respectively, and either ring-closure or competitive attack

by L at the vacant site.

Given the assumption that the concentration of the

intermediates 21-a and 21-b is steady-state, the rate law for

the dissociative pathway is given by equation (14).

59

k T " " [ L I f o r the reactions of in" DTHp)W(CO)-, with tri(lsopropyl) phosphite in cniorobenzene at various temperatures. Ordinate = koo.d X 10E sec-1, Abscissa = [L] X 10 M.

60

2 8

2 6

2 4

2 2 -

2 0

t 8 -

1 6

1 4

1 2 -

1 0 -

Z -

1 0

Fig. 20--Plots of kob.d \£s. [L] for the reactions of (fF-DTHp)W(CO)with L, L = tri( isopropyl) phosphite, trimethyl phosphite, in chlorobenzene at 21.1 °C. Ordinate = ko*..* X 10a sec"1, Abscissa * [L] X 10 M.

s.

0 c

w.

/

- / N S

c - 0

PATH A • *2

r 0 l ' r » * / yCo

\l','

Is*

PATH B

k1

M

CBV

0 C

.W

c 0

cP

'O

b

*3 C 8

- 3

0 C r O

w '

/ \ c 0

2 L

Lv

0 C

\ I • w

S ^ l \ c °o 0

61

Fig. 21--Competitive mechanism for the displacement of one end of DTHp from (fle-DTHp)W{CO)«. by L involving initial rate-determining ring-opening; bimolecular attack by L.

62

-d[S]/dt = ktketLHSl/Oc-i + ka[L]) (14)

Where S = (fF-DTHp)W(COU

kobsd = kike [L]/(k-i + k*[L]) (15)

When k-i >> ke

kob«d kike [L]/k-i

Three cases can be envisioned for this mechanism

(figure 21). The first case involves a competitive ring-

closure and a bimolecular ligand attack at the vacant site,

i.e., k-i ~ ke. The second case involves a rate of ring-

closure which is much faster than the one for the rate of the

bimolecular ligand-attack at the vacant site, i.e, k-i >> ks.

The third case is operative when k-i << ka. The rate law for

the second case is indistinguishable from the rate law for an

associative mechanism.

Entropies of activation have been used as criteria to

distinguish between the two mechanisms just described, because a

dissociative mechanism could show a more positive entropy of

activation (£, 2). The second order rate constants for the

reactions of (fF-DTHp)W(COU with phosphites in CB are given in

table III. The Eyring plot of ln(k/T) vs. 1/T (T = absolute

temperature) for L= tri(isopropyl) phosphite is depicted in

figure 22. An enthalpy of activation of 15.00(4) kcal/mol is

much lower than the enthalpies of activation observed in closely

related systems {5, 8.-JJ2.).

63

•8 -

-9 -

-10

•11 -

1 3.0 3.1 3 2

1 3.3

—r-3 .4

Fig. 22--Eyring plot of ln(k«/T) vg. l/T for the reactions of (f^-DTHpJWtCO)*. with tri( isopropyl) phosphite in chlorobenzene at various temperatures. (Ordinate = ln(ke/T)f Abscissa = (l/T) X 10

3 K~l).

64

This finding should not be surprising, since the

rigidity that the four-membered ring may experience will be

released upon ring-opening. However, a highly negative entropy

of activation is inconsistent with an initial ring-opening.

TABLE III

RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE REACTIONS OF (fF-DTHp )W( CO )<• WITH PHOSPHITES IN

CHLOROBENZENE AT VARIOUS TEMPERATURES

Ligand T, °C k 10®, M_1. sec-1

P(0-i-Pr)a 44.5 7.8(3) P(0-i-Pr)a 35.2 3.67(11) 31.1 2.61(5) 21.1 1.09(1)

P(MeO)a 21.1 2.73(5)

activation parameters for L = tri(isopropyl) phosphite A S1 = -16.1(1) cal/deg-mol. A H*= 15.00(4) kcal/mole

One can envision a situation in which a negative

entropy of activation will be observed for a dissociative

mechanism, provided there is a competition for the intermediate

21-a between ring-closure and bimolecular ligand attack. For

example, in the limiting case for the dissociative mechanism,

depicted in figure 21, in which k -i >> ks, the observed rate

constant will be given by kiks/k—i (e<juation (15)). The entropy

of activation for such a process will be given by As^ = As^i +

AsJ - A S L .

65

Given a highly constrained four-membered ring, it is not

unreasonable to expect a small and positive entropy of

activation for the ring-opening step, since few degrees of

freedom are gained upon ring-opening. Therefore, few degrees of

freedom are lost upon ring-closure; thus, the observed entropy of

activation for this elementary step should be small and negative.

Hence, the overall entropy of activation is expected to be

negative.

One can see that the nature of the mechanism for the

ring-opening of (rF-DTHp)W(COU cannot be assessed employing the

criterion of the entropy of activation alone. Therefore, a

competitive mechanism cannot be ruled out. This fact prompted

the ring-closure studies of cis-1 (01 -DTHp) (BB)W(CO)^.], and

cis-[ (fl1-DTHp) (DCE)W(CO)*. (BB = bromobenzene and DCE =

1,2-dichloroethane). Both species could be generated via pulsed

laser flash photolysis of (ne-DTHp)W(COU in BB and DCE,

respectively. It is known that upon the flash a coordinatively-

unsaturated intermediate, illustrated in figure 21 (intermediate

21-a), is produced from analogous species (rF-DTA)W(CO )*•, (DTA =

DTO, and DTN) (1&-JL2) • In this case, this species is attacked by

the solvents BB and DCE with rates close to those of diffusion

control C13-18). Since the attack by the solvent at the initial

photoproduct is very fast, the species that is being monitored is

the solvated transient.

66

fl-jpg-nlnsure of cis~ [ (0* -DTHp ) (S )W( CO )^1

The photochemical generation of cis-[(D1-DTHp)(S)

W(CO)4], S = solvent, has been described in chapter II.

According to figure 23, this transient may undergo ring-closure

through a concerted expulsion of the solvent, governed by k'-i ,

or through unimolecular solvent-dissociation followed by ring-

closure. The rate constants for the ring-closure of

cis- [ (H* -DTHp) (BB)W(CO)<»] and cis- [ (H^-DTHp) (DCE)W(CO)*.] are

given in table IV. Plots of ln(k'-i/T) vs. 1/T are given in

figure 24, from which activation parameters have been obtained.

The activation parameters for the ring-closure of cis-nfl1-

DTHp)(BB)W(COU] over eight temperatures suggest that

bromobenzene is either weakly bonded to tungsten (&H = 6.4(3)

kcal/mole) or the W-BB bond fission is assisted by the free end

of the chelate ligand upon ring-closure (figure 23). ¥

Furthermore, a highly negative entropy of activation (&S =

-11.2(8) cal/deg-mol suggests that there is substantial

bond-formation in the transition state leading to the

ring-closed product. •

+ g

A, ,G o w

^ l \ c c o

o

0 c

w

• o

- 1

CB.

0 c

v / w

c o

r O I co C 0

67

0 c

w

s>

/ \ c 0

co

F i g • 2 3 - - R i n g - c l o s u r e o f £ i s ~ [ {f)1 -DTHp) (CB)W(COU] i n v o l v i n g a b i m o l e c u l a r d i s p l a c e m e n t o f CB. (CB = c h l o r o b e n z e n e ) .

68

TABLE IV

RATE CONSTANTS FOR THE RING-CLOSURE OF CIS- t (fl1 -DTHp) ( S )W( CO U ] IN BROMOBENZENE AND CHLOROBENZENE AT VARIOUS TEMPERATURES

S T, °C k-i10~s

(sec-1) (kcal/mole) Asii (cal/deg-mol)

BB

DCE

4 5 . 5 8 . 8 ( 3 ) 4 5 . 5 8 . 2 ( 3 ) 3 9 . 7 7 . 3 ( 6 ) 3 9 . 7 6 . 6 ( 3 ) 3 4 . 6 6 . 1 ( 4 ) 3 4 . 6 5 . 5 ( 4 ) 2 9 . 3 5 . 1 ( 4 ) 2 5 . 7 4 . 5 ( 3 ) 2 5 . 7 4 . 0 ( 2 ) 2 5 . 7 3 . 9 ( 3 ) 1 9 . 9 3 . 2 7 ( 1 0 ) 1 9 . 9 3 . 1 8 ( 2 ) 1 9 . 9 3 . 0 1 ( 1 0 ) 1 6 . 4 2 . 8 5 ( 1 0 ) 1 6 . 4 2 . 8 5 ( 8 )

3 0 . 3 1 0 . 6 ( 3 ) 3 0 . 3 1 1 . 1 ( 4 ) 2 5 . 2 8 . 5 ( 7 ) 2 5 . 2 7 . 8 ( 3 ) 2 5 . 2 7 . 4 ( 6 ) 1 9 . 5 7 . 6 ( 2 ) 1 9 . 5 6 . 3 ( 8 ) 1 9 . 5 6 . 2 ( 6 ) 1 9 . 5 6 . 1 ( 4 ) 1 9 . 1 7 . 2 ( 2 ) 1 6 . 3 6 . 6 ( 4 ) 1 6 . 0 6 . 1 ( 4 ) 1 5 . 9 5 . 8 ( 4 ) 1 5 . 9 5 . 5 ( 4 ) 1 5 . 9 5 . 5 ( 1 ) 1 2 . 3 5 . 2 ( 1 ) 1 2 . 2 5 . 8 ( 1 ) 9 . 5 4 . 7 ( 5 ) 9 . 5 4 . 7 ( 3 ) 9 . 5 4 . 4 ( 3 )

6 . 4 ( 3 ) -11.2(8)

5 . 6 ( 5 ) 1 2 . 7 ( 7 )

S = solvent BB = bromobenzene, DCE 1,2-dichloroethane

69

8 - 5

8.0

7 . 5 H

DC E

. ^ ; s u ^ r ; ^ e n n ? - S T H p , ( s o l v e n t ) w " f c 0 ) ^ a t ° v a r i o u s t e m p e r a t u r e s . b I ^ b U o b e n L n e . DCE - 1 J - d i c h l o r o e t h a n e ( O r d i n a t e = l n ( k - i / T ) , A b s c i s s a = 1 / T X 10 K ) .

70

If there is an initial dissociation of bromobenzene from

cis-[ (fl'-DTHp) (BB)W(CO)*-] to produce the coordinatively-

unsaturated intermediate 23-a, then a low and positive entropy

of activation would be observed (12).

The displacement of CB, CB = chlorobenzene, from

cis-[(CB)(P(0-i-Pr)®)W(C0U] has been shown to be dissociative

in nature (13). The enthalpy of activation (AH = 13.0(4)

kcal/mol) for the CB-W bond-fission suggests there is a strong

interaction between CB and the tungsten metal.

One would expect bromobenzene to be a better Lewis base

than chlorobenzene (11). Thus, there should be an observably

stronger bromobenzene-tungsten interaction. Therefore, a

smaller enthalpy of activation for the displacement of

bromobenzene upon ring-closure argues in favor of an assisted

displacement of BB by the free end of the chelate ligand. This

proposal is further supported by the activation parameters

observed for the ring-closure of cis-[ (ni-DTHp) (DCE)W(CO)*.] (AH*

= 5.6(5) kcal/mole, &S*= -12.7(7) cal/deg-mol. If, indeed, the

solvent is coordinated to the tungsten metal through the halogen

atom, then the dichloroethane-solvated species should resemble

the behavior of its chlorobenzene-solvated analog. Differences

in enthalpies of activation for the ring-closure (DCE = 5.6(5),

BB = 6.4(3)), if significant, can be attributed to the fact that

1,2-dichloroethane is a poorer Lewis base than bromobenzene (19).

However, this is not supported by the entropies of activation.

71

The previous discussion of ring-closure studies clearly indicate

that some kind of an associative pathway is operating during the

ring-closure of the transient 23-b.

It is quite intriguing that the rate for ring-closure

is much faster than the rate for the bimolecular ligand attack

at the solvent coordinated species, especially when one

envisions the formation of a highly strained ring. However,

because DTHp is only seven atoms in length, the free end is

located conveniently close to the reaction site. The

possibility of competition between ring-closure and ligand

attack at cis-[ (fl1 -DTHp) (BB)W(CO)*.] by L = tri( isopropyl)

phosphite thus was tested. The transient cis-[{fl1-DTHp)

(BB)W(CO)*.] was generated in the presence of a large excess of

tri(isopropyl) phosphite (> 1M). In the absence and presence of

tri(isopropyl) phosphite similar rates for the disappearance of

this transient were observed, indicating that ligand attack at

cis-[fl1-DTHp(BB)W(CO)<*.] is not competitive with ring-closure.

With the presently available information, one may propose a

concerted expulsion of the solvent <BB, DCE) upon ring-closure of

cis-(fl1 DTHp) (solvent)W(CO)

Furthermore, one may propose that this associative

pathway takes place because the rate constant for ring-closure,

k'-i, is much larger than k—ak—i/(k—i + kstCB]) (assuming the

steady-state concentration of 23-a). Therefore, if ring-

closure is to take place, the free end of the chelate ligand

must assist the departure of the solvent molecule.

72

MftP-hanism for QlS. formation &£ ?>\ S~ (0* -PTflP) < L )W(CO )<*

The proposed mechanism by which (0s-DTHp)W(CO)^ reacts

with L to afford cis- (fF-DTHp) (L)W(CO)*» (25-c) is shown in

figure 25. According to the principle of microscopic

reversibility and to the results of ring-closure studies of

cis-[ (fl1-DTHp) (S)W(CO)*.], S = solvent, one can propose an

initial solvent-substrate interaction to produce the solvated

intermediate 25-b (figure 25). In view of L being a better

Lewis base than the solvent, another pathway, which involves an

initial ligand-substrate interaction to produce cis-(fl1 -DTHp)

(L)W(COU, can be proposed.

The rate law for the formation of 25-c, assuming that

the concentrations of intermediates 25-a and 25-b (figure 25)

are steady-state and that k'-i >> k-a, k'i, ke; k-x >> ks, k e,

I k ' x / k ' - x ) » k'e >> k'xWk'-i, is given by equation (17).

d[25-cl/dt = R* + R« <17>

Where;

substrate = (ns-DTHp)W(C0)^

S\\ I I co

o

/ i \ c 0

°o

73

Fig. 25--Plausible mechanism for the reactions of (ne-DTHp)W(CO)^ with L to afford cis-(D»-DTHp)(L)W(COU

74

Ri = k'a[L][substrate] (18)

Ra = (k'l/k'-i )ka[L3 [substrate] (19)

Although the parallel photochemical studies showed that

all the steps governed by k'i and k1 —* are operative during the

course of the formation of intermediate 25-c, the most likely

pathway by which intermediate 25-c is formed is by that step

which involves an initial interaction of L with the substrate

governed by k'a.

The basis for this interpretation rests on the relative

magnitudes of the rate constants for the different pathways

involved. One would expect that the rate constant k'» for the

formation of the solvated intermediate 25-b, via association of

chlorobenzene with the substrate, will be similar to k'a, the

rate constant for the process involving the ligand-substrate

interaction. Furthermore, the rate constant for ring-closure of

intermediate 25-b, via a concerted displacement of the solvent

molecule will be much larger than the rate constant for the

expulsion of the ligand coordinated at the metal in 25-c (20).

Therefore, k'a >> k'l/k'-i. Since (k'./k'-i) is included in the

equation (18 ), equation (17) reduces to equation (20), which

describes an associative pathway for the reactions of

(fF-DTHp)W(CO)^. with tri(isopropyl) phosphite.

d[25-c]/dt = k'etL][substrate] (20)

75

pactions of CIS-(n1-DTA) (li)W(QO?**

During the course of "the displacement of DTA, (DTA =

DTHp, DTD, and DTU) from (fle-DTA)W<COU a p p r e c i a b l e formation of

cis- (01 -DTA) (L)W(C0)4, (L = phosphites and phosphines) has been

observed. Cis-(n*-DTA)(L)W(COU will further react with L to

afford cis-(L)eW(COU according to equation (21).

cis- (fl1 -DTA) (L)W(CO)^ + L > cis-(L)«W<COU + DTA (21)

These reactions have been studied under pseudo-first order

conditions. The pseudo-first order rate constants are given in

table V.

The rates of reaction of cis-(n*-DTA) (L)W(CO)*. with all

ligands studied were found to be first-order with respect to the

concentration of cis-(fl1-DTA) (L)W(CO)*., and zero-order with

respect to the concentration of L. The rate law for these

reactions is given by equation (22).

-d[S]/dt = k'ob.ct [S] (22)

k(ob»d — ^4

Where S = cis-(fl*-DTA) (L)W(COU

76

TABLE V

RATE CONSTANTS FOR THE REACTIONS OF CIS- (fl1 -DTA) (L)W(CO). WITH L IN CHLOROBENZENE AT VARIOUS TEMPERATURES

L T, °C [L], M 103 k V , sec"3

P(0-i-Pr)a 44.5 1.235 1.43(1) P(0-i-Pr)a 44.5 0.8958 1.51(2) 0.6922 1.427(6) 0.5565 1.395(5) 0.4376 1.390(4) 0.2259 1.48(1) 0.1985 1.51(2) 0.09449 1.43(2) 0.04127 1.61(5) 0.03640 1.6996) 0.02264 1.64(1)

35.2 1.5065 0.323(3) 0.9998 0.9315

0.326(2) 0.341(5)

33.9 0.2721 0.1736 0.09028

0.290(2) 0.285(1) 0.308(2)

31.1 1.550 1.085 0.5520

0.1750(8) 0.164(2) 0.1714(7)

21.1 0.5840 0.3731 0.1168

0.0392(5) 0.0386(3) 0.0374(3)

P(OMe): 68.1 0.4894 0.3664 0.1438

2.78(3) 3.01(4) 2.52(2)

57.0 0.4925 0.3625 0.2005

0.757(6) 0.707(5) 0.743(3)

44.5 1.929 1.278 0.3325

0.156(1) 0.153(2) 0.151(2)

77

TABLE V CONTINUED

DTA L T, °C [ L J , M 1 0 3 k ^ , s e c - 1

DTHp CP 68.1

5 7 . 5

4 4 . 5

P(OC*tks)3 4 4 . 5

P ( n - B u ) a 4 4 . 5

DTD P ( 0 - i - - P r ) 3 5 2 . 3

4 4 . 5

3 5 . 2

3 1 . 6

21.1

0 . 09346 0 . 7 2 7 ( 5 ) 0 . 0 5 8 3 1 0 . 7 3 6 ( 5 ) 0 . 03186 0 . 6 6 0 ( 6 )

0 . 03222 0 . 1 9 9 ( 2 ) 0 . 02177 0 . 2 0 4 ( 1 ) 0 . 08897 0 . 2 1 3 ( 1 )

0 . 4872 0 . 0 4 5 4 ( 2 0 ) 0 . ,3927 0 . 0 4 4 4 ( 5 ) 0 . 1183 0 . 0 3 9 8 ( 4 )

0 . 9801 1 . 1 5 ( 4 ) 0. ,1555 1 . 2 8 ( 2 ) 0 . ,05351 1 . 1 0 ( 1 )

1. ,059 2 . 8 1 ( 2 ) 0, .7729 3 . 0 4 ( 2 ) 0. ,3958 2 . 8 3 ( 1 )

0. .09968 1 . 7 3 ( 1 ) 0. . 08736 1 . 8 0 ( 6 )

0, .7709 0 . 6 3 9 ( 2 ) 0 .5629 0 . 6 0 1 ( 6 ) 0, .5554 0 . 6 8 7 ( 5 ) 0 . 4494 0 . 6 7 5 ( 6 ) 0, . 2904 0 . 6 5 9 ( 7 ) 0 . 2 1 6 9 0 . 6 3 8 ( 7 )

1 .080 0 . 1 4 8 8 ( 5 ) 0 . 9 0 5 2 0 . 1 8 0 7 ( 9 ) 0 . 7 8 8 3 0 . 1 5 0 0 ( 6 )

0 . 1 3 2 1 0 . , 0 9 6 1 ( 1 0 ) 0 . 0 9 9 9 3 0 . 1 0 8 ( 1 ) 0 . 0 7 5 1 0 0 . , 0 9 7 7 ( 6 )

1 . 3 0 4 0 . , 0 2 2 0 ( 3 ) 0 . 9862 0, . 0 2 2 0 ( 2 ) 0 . 8 3 7 6 0 . , 0 2 4 7 ( 3 ) 0 . 4 4 9 2 0, . 0 2 5 0 ( 3 ) 0 . 1 5 3 2 0. . 0 2 5 8 ( 5 )

78

TABLE V CONTINUED

DTA L o O

[L], M 103 k'^, sec-1

DTD P(0-i~Pr)s 11-1

P(OMe)3 21.1

DTU P(0-i~Pr)a 52.3

44.4

35.2

33.9

21.0

14.2

P(OMe)a 14.2

0. 6584 0. 00422(8) 0. 6347 0. 00398(6) 0. 5310 0. 00457(6) 0. 4457 0. 00432(4) 0. 3892 0. 00447(18) 0. 2938 0. 00481(9)

1. 509 0. 00428(6) 0. 6040 0. ,00356(7) 0. 4354 0. 00349(14) 0. 2823 0. ,00405(6)

0. 5750 1. ,77(1) 0. ,4113 1, .81(3) 0. 3364 1. ,73(2)

0. .2602 0, .625(6) 0. ,1218 0. .63(2)

1. .201 0 .183(1) 0. ,8010 0. .177(1)

1. .123 0 .1516(8) 0. .5418 0 .1612(9) 0 .1524 0 .158(1)

1, .371 0 .0241(3) 0 .2388 0 .02319(2) 0, .2015 0 .0233(2)

1 .051 0 .00780(7) 0 .4471 0 .00749(14) 0 .2468 0 .00746(12)

1 .463 0 .00165(2) 0 .7828 0 .00152(2)

79

The reactions of cis-(fl1-DTHp) (L)W(CO)*. with a wide

variety of phosphites and P(n-Bu)a were studied. A strong

dependency of the rate constant on the steric nature of L was

observed.

In table VI, the rate constants, the Tolman cone

angles, and the Tolman electronic parameters (20.) are given.

The cone angles are an empirical measure of the steric

requirements of the ligands. There are an increase in the

values of the rate constants as the cone angle of the ligand

coordinated cis to DTHp increases.

TABLE VI

RATE CONSTANTS FOR DTHp-DISSOCIATION FROM CIS- (fl1 -DTHp) (L)W(COU IN

CHLOROBENZENE AT 44.5 °C

L 1 0 3 e , Cone Angle , (CO) (seer1 ) (Deg.) cm-1

P(n-Bu)a 2 . 8 9 ( 1 2 ) 132 2060 . 3

P(0-i-Pr)a 1 . 4 3 ( 3 ) 130 2075 . 9

P(OC*ife)a 1 . 1 8 ( 9 ) 128 2085 . 3

P (OMe )a 0 . 1 5 3 ( 4 ) 107 2079 . 3

CP 0 . 0 4 4 8 ( 8 ) 101 2087 . 3

80

Plots of l n ( W T ) vs. 1/T for the reactions of

cis- (DTHp) (L)W(CO)*. with L, L = tri(isopropyl) phosphite,

triinethyl phosphite, and CP are given in figure 26. Analogous

plots for the reactions of cis-ff^-DTD) (L)W(COU and

cis- (nx -DTU) (L)W(CO)* with tri.(isopropyl) phosphite are given in

figures 27 and 28, respectively.

Enthalpies of activation in the vicinity of 25 kcal/mol

and positive entropies of activation strongly suggest a

rate-determining unimolecular dissociation of the anchored end

of the chelate ligand to afford cis~[LW(C0U] (ZX~2k) • Figure

29 shows the proposed mechanism.

The activation parameters for the systems studied are

given in table VII. The enthalpies of activation for the

reactions of cis-<n*-DTHp)(L)W(COU with L, (L = tri(isopropyl)

phosphite, trimethyl phosphite, and CP) are very similar to one

another. But the entropies of activation significantly increase

with the sxze of L. This outcome and the fact that the plot of

Ink* vs. the cone angle of the coordinated ligand is linear

(figure 30) suggest that the increase of the rate constant for

the dissociation of DTHp from cis-(DTHp) (L)W(COU is entropic in

nature. There have been instances in which both the electronic

and steric parameters of the coordinated ligands cis to the

departing ligands are simultaneously affecting the rates of the

unimolecular dissociations (25-21). An empirical equation has

been used by Poe and co-workers to assess the contribution of

each parameter (29).

- 1 0 -

- 1 1

- 1 2

-1 3

-1 4

-1 5 -

-1 6

P(OM«)

2 . 9

P(OP'r),

r e a c t i o n s o f o i ^ n ? - D T O p ) ( L ^ ' S ' r V T f o r t h e

L " t r x { i s o p r o p v l ) ( C ? , < ' W l t h L , S? ,fn chlorobenzene at v a r i o u s P h o s p h i t e , and O r d i n a t e = I ,

- 1 4

- 1 5

- t 6

- 1 7

1 8

- 19

- 2 0 -

82

3*0 3.1 3.2 3 3 3 4 3 5

. ^ g * 27--Eyring plots of ln(k/T) vs. l/T for the reactions of £ A S - ( n i-DTD) (L)W(COU with L

=. ^isopropyl) phosphite, in chlorobenzene at various temperatures. Ordinate = l n < W ? ? Abscissa = l/T X 1053 K~% .

83

-1 2

-1 4

-16-

3 0 32 34

Fig. 28--Eyring plots of ln(k>»/T) ss. 1/T for the reactions of cis- (fi1 -DTU) (L)W(CO )*• with L, L = tri(isopropyl) phosphite( in chlorobenzene at various temperatures. Ordinate = ln(k«»/T), Abscissa = 1/T X 10s K—1.

o

84

^ i/ 'vv

* r / l x

o c

V '

- / i \ v-5

c °o o

o c

u p°

\ /

w

\ c o

Ji£L

8 l \ 1 /

-> W

/ l \ C 0

Fig. 29--Mechanism for the displacement of DTA from cis- (I"!1 -DTA) (L) W (CO)«. by L, L = phosphites, phosphines .

85

- 8

- 9

10

100 11 0 1 2 0 1 30

Fig. 30--Plots of In k*» vs. Tolman cone angles for the reactions of £±£-< fl1-DTHp) <L)W(e0U with L, L = 1, tri(n-butyl) phosphine; 2, triphenyl phosphite; 3, tri(isopropyl) phosphite; 4, trimethyl phosphite; 5, CP.

86

This empirical relationship is given by equation (23).

log k*. = a6 + b2) + c (23)

where:

0 = The Tolman cone angle

y = The Tolman electronic parameter

c = Constant

a = Coefficient for the steric parameter

b = Coefficient for the electronic parameter

The numerical values of a (a = 0.0479 deg-1), b (b = -0.0101

cm), and c (c = 10.9) were determined employing a multivariable

linear regression program (3H). Substitution of these

coefficients into equation (23) yields equation (24).

log k<* = 0.0479 © - 0.01012/ + 10.9 (24)

According to Tolman (2£). the relative contribution of steric

and electronic influence to the value of the rate constant are

given by equation (25) and (26), respectively.

contribution of steric influence = ta/(a + b)] (25)

contribution of electronic influence = [b/[(a + b)] (26)

87

In the present study, it was found that the steric influences of

the coordinated L predominate, with a positive correlation

observed between the rate of departure of DTHp from c i s - < n * -

DTHp) (P(0-i-Pr )a )W(CO)<* and the Tolman cone angle.

The rate constants for the reactions of cis-(H1-DTA)-

(P(0-i-Pr)» )W(CO)«. with L, (L = tri(isopropy 1) phosphite) at

35.2 °C are given in table V. The rate constants for DTA = DTD,

DTU are very similar, 16.0(14) X 10~s sec-1, 18.0(3) X 10

sec-1, respectively. However, the rate constants observed for

DTA = DTHp are almost twice as large (k* = 32.7(1) X 10~s sec"1)

as the ones observed for DTA = DTD, DTU. This outcome is

somewhat surprising, since all the complexes are very similar at

the reaction site.

TABLE VII

ACTIVATION PARAMETERS FOR THE DISSOCIATION OF DTA FROM CIS-((l1 -DTA) (L)W(CO W

DTA Mil AS* He*

(kcal/mole) (kcal/mole)

DTHp P(0-i-Pr)» 28.2(6) 18.1(2)

P(OMe)a 25.8(1) 6.1(2)

CP 25.1(6) 1.4(11)

DTD P(0-i-Pr)» 26.0(3) 11.1(11)

DTU P(0-i-Pr)3 25.8(2) 10.4(5)

88

From table VII it can be seen that the enthalpies of

activation for these reactions are similar.

However, there is a greater difference in the observed entropies

of activation: 18.1(2) cal/deg-mol for the dissociation of DTHp

vs. 11.1(11) and 10.4(5) for the dissociation of DTD and DTU,

respectively.

Cis- [ (P(Q-i-Pr)a) (CB)W(CO)^.]

The generation of cis-[(P(0-i-Pr)3)(CB)W(CO)^] via

pulsed-laser flash photolysis of cis- (pip) (P(0-i-Pr)3 )W(CO)<*

(pip = piperidine) in chlorobenzene provides the opportunity to

study the steps subsequent to DTA dissociation.

Previous studies have firmly established that both the

intermediate which is produced during the thermal dissociation

of pip from cis-(pip) (P(O-i-Pr)a )W(C0)*», and the transient that

is produced after the flash at cis-(pip)(P(O-i-Pr)a)W(CO)^ are

the same (25.). Furthermore, this transient has been

characterized by time-resolved spectroscopy in n-heptane solvent

(11).

As shown in figure 31, the initial ground-state photo

product will undergo very fast attack by CB to yield

cis-C(P(0-i-Pr)3)(CB)W(CO)^]. This transient will establish an

equilibrium involving desolvation-solvation governed by k-= and

k=, respectively.

o ? r ° PiP I

x. / W

/ I V c uo o

h v

89

CB

O C

W

/ v

k5'CB

-5

I y\

,o°

o

\ i w

• i \ o

Fig. 31--Photochemical generation of sis- [LW(COU] in chlorobenzene and attack by L.

90

Assuming that the concentration of intermediate 31-a (figure 31)

is steady-state and that k»[CB] » k*CL], the rate law for the

disappearance of cis-[(P(0-i-Pr).(CB)W(C0U] is given by

equation (27).

-d[42-b]/dt = [42-b] (27)

= K-ck^CLl/tCB] (28)

K-c = k-o/kb (29)

Table VIII contains the pseudo-first order rate

constants for the displacement of CB from cis-t(P(0-i-Pr)»)

(CB)W(CO)*»] by tri(isopropyl) phosphite at 35.2 °C, and figure

32 shows a linear plot of ko*-* vs. [L] from which the value of

the composite rate constant K-ck*,/[CB]. (7.2(3) X 103 M x,

sec-4), is obtained.

Dobson and his co-workers have reported the value of

k*/ks (1.12(20) at 31.1 °C) in CB to be nearly independent of

the temperature (25). The approximate value for the rate

constant for W-CB bond dissociation, k-», can be calculated from

the value of the "competition ratio" k^/ks, and the value of

k-«,k*,/[CB]. This value calculated as k-=s = ([CB]/(k*/k») ) X

k.ck* equals 6.3(13) X 10s sec"1.

It is interesting to note that the calculated value of k-=

is very similar to the value of k—is, (6.3 X 10 sec )

calculated (extrapolated) for the CB dissociation from cis-

t(P(O-i-Pr)a)(CB)W(COU] at 35.2 °C in cyclohexane-CB solvent

(mixed solvents) (2_5).

91

This supports that indeed displacement of CB by L from cis-[P(0-

i-Pr)s)(CB)W(CO)^] takes place involving initial reversible CB

dissociation and followed by attack by L at the non-solvated

intermediate. Activation parameters for CB-W bond dissociation

from cis- E (P(0-i-Pr)a (CB)W(CO)^.] are 13.0(4) kcal/mol for / H—a A

and 5.6(11) cal/deg-mol for AS-s (U.)-

TABLE VIII

RATE CONSTANTS FOR THE DISPLACEMENT OF CHLOROBENZENE FROM CIS-[(P(0-i-Pr)3(CB)W(C0U] BY TRI(ISOPROPYL) PHOSPHITE AT 35.2 °C

[L], M 10~3 kob.d sec-1

1.367 10.05(4) 1.367 9.83(12) 1.367 9.465(8) 1.367 8.65(3)

0.9036 6.6(2) 0.9036 6.1(2) 0.9036 6.13(5) 0.9036 5.28(12) 0.9036 5.76(2)

0.7193 4.24(2) 0.7193 4.21(5)

0.4428 2.89(13) 0.4428 6.68(2)

0.1987 1.28(1) 0.1987 1.146(3)

K«ck<s./[CB] = 7.2(3) X 103 M — 1 , sec"1

92

10 12 14 16

Fig. 32--Plot of kob.d vg.. [L] for the reaction of cis- [ (L) (CB)W(CO)*»] with L at 35.2 °C in chlorobenzene L = tri(isopropyl) phosphite. Ordinate = k X 10~3 sec"1

Abscissa = [L] x 10 M. '

CHAPTER BIBLIOGRAPHY

1. Pardue, J. E.; Dobson, G. R. Inorg. Chim. Acta, 1976, 20, 207.

2. Graham, J. R.; Angelici, R. J. Inorg. Chem., 1967, 6, 2082 .

3. Angelici, R. J.; Graham, J. R. J. Am- Chgm. Sfic. » 1966, 88, 3658.

4. Al-Kathumi, K. M.; Kane-Maguire, L. A. P. J. Inorg. NucI. Chem., 1972, 34, 3759.

5. Dobson, G. R. et al. J. Coord. Chem., 1978, 7, 253.

6. Faber, G. C.; Dobson, G. R. Inprg. Chem., 1968, 7, 584.

7. Dobson, G. R.; Binzet, C. S.; Cortes, J. E. J. Coord. Chem.. 1986, 4, 215.

8. Schultz, L. D.; Dobson, G. R. J. Qrganomet. Chem., 1977, 131, 285.

9. Schultz, L. D.; Dobson, G. R. J. Qrganomet. Chem-, 1977, 124, 19.

10. Dobson, G. R. ; Basson, S. S.; Dobson, C. B. Inorg. CMm-Acta. 1985, 105, L17.

11. Dobson, G. R.; Dobson, C. B.; Halverson, D. E.; Mansour, S. E. J. Drganomet. Chem.. 1983, 253, C27.

12. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inorg. Chgm., 1985, 24, 2179.

13. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H. ; Dobson, G. R. J. Aju. Chem. Soc.. 1987, 109, 5386.

14. Simon, J. D.; Peters, K. S. Chem. Phys. Lett., 1983, 98, 53.

15. Simon, J. D.; Xie, X. J. J. Phys. Chem-, 1986, 90, 6715.

16. Langford, C. H.; Moralejo, C.; Sharma, D. K. Inorg. Chim. Acta. 1987, 126, Lll.

93

94

17. Seder, T. A. ; Church, S. P.; Weitz, E. J. Am- Ciiem. Sua., 1986, 108, 4721.

18. Bonneau, R. ; Kelly, J. M. J. J. Am. Chem. Sop. , 1980, 102, 1220.

19. Carey, F. A.; Sunberg, R. J. Advanced Organic Chemistry part B: Reactions and Mechanisms: Plenum Publishing Corporation: New York, 1980, pp 281-28 2.

20. Tolman, C. A. Chem. Rev.. 1977, 77, 345.

21. Schultz, L. D.; Dobson, G. R. J. Qreanomet. Chem., 1977, 131, 285.

22. Schultz, L. D.; Dobson, G. R. J. Qreanomet. Chem., 1977, 124, 19.

23. Dobson, G. R.; Basson, S. S.; Dobson, C. B. Triors. Chim. Acta. 1985, 105, L17.

24. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inorg• Chem.. 1985, 24, 2179.

25. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H. ; Dobson, G. R. J. Sin• Chem.

Soc.. 1987, 109, 5386.

26. Basolo, F. Inorg. Chim. Acta, 1985, 100, 33.

27. Basolo, F. Coord. Chem. Rev., 1982, 43, 7.

28. Darensbourg, D. J.; Graves, A. H. Inorg. Chem., 1979, 18, 1257.

29. Dahlinger, K.; Falcone, F.; Poe, A. Inorg. Chem., 1986, 25, 2654.

30. SAS Institute Inc., Cary, N C

31. Dobson, G. R.; Hodges, M. P.; Healy, A. M.; Poliakoff, M.; Turner, J. J.; Firth, S.; Asali, K. J. Am- Chem.

Soc.. 1987, 109, 4218.

CHAPTER IV

FORMATION OF (fl4 -DTA) (L )W(CO)-DTA = DTD, DTU

Of nis-tn^-DTA) (L)W(COU

The reactions of (fF-DTA)W(COU (DTA = DTD, DTU) with

L = phosphites in CB are similar to the corresponding reactions

Q£ (ns-DTHp)W(CO)^ in the sense that a biphasic behavior is

observed.

Plots of In(At - A„) vs. time are linear up to one half

life, then after curving, another linear segment is observed.

The overall stoichiometric reaction is given by equations 30 and

31.

(fp-DTAJWfCO).* + L > cis-(fl1-DTA) (L)W(CO)^ (30)

k' obwd cis-(IP-DTAWCO)** + L > cis - (L )eW( CO )*. + DTA (31)

Cis-(fl1-DTD) (L)W(CO)*-, L = CP, has been isolated and

characterized by infrared spectroscopy and elemental analysis

(see chapter II).

The reactions were carried out under pseudo-first order

conditions. The first section of the biphasic decay showed rate

constants (kot»-ci) with a strong dependence on the concentration

of L. Observed rate constants are given in table IX.

95

96

TABLE IX

PSEUDO-FIRST ORDER RATE CONSTANTS FOR THE REACTIONS OF (fF-DTA)W(COU WITH PHOSPHITES IN CHLOROBENZENE AT VARIOUS TEMPERATURES

DTA Ligand T, °C CL] (M)

103koto«d (sec-1)

DTD P(0-i-Pr)a 35.2

P(0-i-Pr)a 21.1

P(0-i-Pr)a 11.1

P(OMe): 21.1

DTU P(0-i-Pr)3 35.2

1. 584 5, .05(6) 1. 248 4 .81(3) 0. 9052 3, .90(4) 0. 7883 3 .74(2) 0. 6542 3 .27(3 ) 0. 5198 2 .78(1) 0. 3968 2 .35(1)

1. 7113 0 .772(5) 0. 9862 0 .627(3) 0. 8376 0 .554(5) 0. 6001 0 .462(3) 0. 5395 0 .449(3) 0. 4032 0 .363(3) 0. 3164 0 .315(1)

1. 550 0 .148(2) 0. 6584 0 .1075(9) 0. 6347 0 .1014(7) 0. 5310 0 .1037(6) 0. 4457 0 .0884(9) 0. 3892 0 .0796(7) 0. 2938 0 .0693(8) 0. 2370 0 .0586(4)

1. 509 0 .958(15) 0. 7247 0 .778(11) 0. 6040 0 .681(6) 0. 4354 0 .587(7) 0. 2823 0 .471(3) 0. ,2410 0 .415(4)

1. 4877 8.02(5) 0. .8928 6.60(4) 0. 7316 6.32(2) 0. ,5412 5.48(4) 0. ,3351 3.92(2) 0. .2365 3.05(2)

TABLE IX Continued

97

DTA LI GAUD T, [L] (M)

X 03 ko b*d (sec-1)

P(0-i-Pr)a 21.0

P(0-I-Pr)s 14.2

P(OMe): 14.2

1.592 1.34(2) 1.254 1.278(6) 1.088 1.142(7) 0.6459 0.934(10) 0.5155 0.846(5) 0.3611 0.674(6) 0.2388 0.541(5) 0.2015 0.465(1)

1.150 0.398(3) 0.7282 0.330(4) 0.5216 0.277(3) 0.4471 0.250(2) 0.2468 0.181(1)

1.463 0.419(2) 0.7828 0.382(2) 0.6349 0.330(2) 0.5499 0.308(2) 0.4423 0.293(3) 0.3235 0.251(1)

Plots of the observed rate constants vs. the ligand

concentration, as depicted in figures 33 and 34, are curved.

This kinetics behavior is suggestive of a complex reaction

mechanism. From the previous discussion, one can rule out an

interchange pathway since its rate law will predict linear plots

of vs. [L3. This is an example in which a plausible

mechanism could be ruled out based solely on the form of the rate

law.

98

s -

3 -

1 2 11

3 5 2

Fig. 33--Plots of kob.d vs. [L3 for the reactions of (n®-DTD)W(CO)^ with tri(isopropyl) phosphite in chlorobenzene at various temperatures. Ordinate = kob.d X 10

3 sec-*, Abscissa = [L] X 10 M.

99

2 10

Fig. 34--Plots of kob.d \rs„ [LI for the reactions of (ft®-DTU)W(CO)*. with tri{isopropyl) phosphite in chlorobenzene at various temperatures. Ordinate = kob.d X 10a sec-1, Abscissa = [L] X 10 M.

100

Plots of l/ke»t,„cj vs. 1/[L] (reciprocal plot) were

linear over a wide range of concentrations (figures 35 and 36),

a behavior which is consistent with the mechanism depicted in

figure 37 and predicted by equation (35). The rate law,

assuming the steady-state concentration of the intermediates

37-a and 37-b, is given by equation (32).

d[S]/dt = kiketL][S]/(k—% + ke[L]) (32)

S = (fF-DTA)W(COU

-d[S]/dt = kofcCS] (33)

= kikeEL]/(k-i + ke[L]) (34)

And

1/kcto.a = l/ki + ((Jc-t/kike)/CL3) (35)

Common intercepts for the reciprocal plots were

observed (figure 38) , for two ligands with different steric

properties, (L = tri(isopropyl) phosphite, trimethyl phosphite).

This mechanistic outcome is expected since L is not involved in

the ring-opening step.

101

1 5

1 0

1 11

35--Plots of l/kot».rt vg. 1/[L] for the reactions °£i 0 U w i * h tri^ isopropyl > phosphite in ch 1 orobenz ene at. various temperatures . (Ordinate = l/kot>«d X 10""e sec. Abscissa = 1/[L] M~~l).

102

2 1 . 0

Fig. 36--Plots of l/ko.*»«.<a V £ . 1/[L] for the reactions of <fF-DTU)W(COU with tri(isopropyl) phosphite in chlorobenzene at various temperatures. (Ordinate = l/k^.c X lCTe sec, Abscissa = 1/[L] M~l ).

o o . .0

o 103

o u -

CD a

-uo

CO"

00-

CD

O

00 CO j*:1

- h

° o

CM J*

- > o o -/

J ?

• o O

if)"

CO-

y-4

o

a Q) 0) c o M-4 o

4-> a a> g 0 O a* rH CU to

0

5 . U ^ 0 >1 M-l XX

£ 4 w ~ •jo S H 1 2 2 3 • P 1 I

co e

O O O O

bO £ •H O t*4 U

M-l

Q

104

P I O P r ' ) 3

P f M « o r

4= 3 8 — P l o t s o f l / k r > t > « c j v s . 1 / [ L ] f o r t h e r e a c t i o n s o f ( f l ® - D T D ) W ( C O ) « • w i t h L i n c h l o r o b e n z e n e a t 2 1 . 1 ° C . ( O r d i n a t e = 1 / k o b . d X 1 0 3 s e c , A b s c i s s a = 1 / [ L ] M ~ ~ l )

105

Table X gives the rate constants for ring-opening (k*)

and the competition ratios, ka/k-i, for the reactions of

(0a-DTA)W(C0)^ with L, (L = tri(isopropyl) phosphite and

trimethyl phosphite). The plots of ln(ki/T) vs. 1/T are given

in figures 39 and 40.

TABLE X

RATE CONSTANTS FOR THE RING-OPENING OF (fF-DTD)W(CO)-IN CHLOROBENZENE AT VARIOUS TEMPERATURES

DTA Ligand T, °C 103 kx (seer1 )

ks/k-i M-1

DTD P(0-i-Pr)3 35.2 8.1(3) 1.03(9)

21.1 1.13(4) 1.21(8)

11.1 0.203(11) 1.74(17)

P(OMe)a 21.1 1.26(4) 2.19(20)

DTU P(0-i-Pr)a 35.2 12.1(5) 1.44(9)

21.0 1.78(6) 1.77(9) )

14.2 0.57(3) 1.87(14)

P (OMe )3 14.2 0.53(3) 2.78(33)

Activation parameters for L = tri(isopropyl) phosphite

4 DTD: AJHx = 26.0(7) kcal/mole

&St* = 18.0(23) cal/deg-mol

DTU: &hJ= 24.9(7) kcal/mole 14.7(23) cal/deg-mol

106

- 1 0

- 1 1

- 1 2

-t 3 -

- t 4

6

4

- 2

- 0

3-2 3 .3 3 .4 3-5

Fig. 39—Eyring plots of ln(k/T) v£. i/T for the reactions of (fF-DTD)W(COU with tri < i^propyl) phosphite in chlorobenzene at various temperatures. A: k = ki; B = ke/k-i. Ordinate = ln(k/T). Abscissa = 1/T X 10® Kr*\

107

-1 o

- 11

-1 2

1 3

3.2 3.3 f

3.4

- 1 0

0*5

3 5

Fig. 40--Eyring plots of ln(k/T) vs. 1/T for the reactions of ( r F - D T l D W t C O ) * . with tri( isopropyl) phosphite in chlorobenzene at various temperatures. A = ki, B » ka/k-x. Ordinate = ln(k/T), Abscissa = 1/T X 10a K-t.

108

Highly positive entropies of activation are consistent

with a unimolecular dissociative process, involving a great deal

of bond breaking in the transition state. A comparison of the

activation parameters for the ring-opening of (fle-DTD)W(C0) and

(fF-DTU)W(CO)^ with those of (ne-DT0)W(C0U and (fle-DTN)W(C0)«.

shows that there is an increase in the entropy of activation as

the size of the chelate ring increases (1).

Although, the reactions of (fF-DTO)W(CO)* (g) and

(rF-DTN)WCO)*. (.3.) were carried out in a different solvent

(xylenes), one would expect minimal influence of the solvent in

the step leading to the ring-opened unsaturated intermediate.

The parallel increase in the entropies of activation and the

size of the chelate ring, although expected, is quite high when

one would expect an increase of only four cal/deg-mol for an

increase of one carbon in the chelate ring (it).

The competition ratios, ka/k-i, as given in table X,

suggest an intermediate with a small discriminatory ability.

This small discrimination between incoming nucleophiles should

not be surprising, since these species react with an incoming

nucleophile with rates close to those of diffusion control

The work of Peters and Simon indicates that Cr(C0)s

undergoes solvation within 25 ps (£). Later, Simon and Xie

reported that Cr(C0)ss undergoes solvation by methanol molecules

within 2.5 ps and by cyclohexane within 0.8 ps (7.). The

pseudo-first order rate constant, k = 5 X 1010 sec-1, for the

solvation of W(CO)= by perfluoromethylcyclohexane, has been

109

reported (£).

Ring-closure fif cis-C (n*-DTA) (CB)W(CO)^1

The intermediates 37-bt cis-[ (fl1 -DTA) (CB)W(CO)^] f DTA =

DTO, DTN, have been generated via pulsed-laser flash photolysis

of (fle-DTA)W(CO)*. in chlorobenzene (.!» 5.. U2) • Upon the flash,

the transient 37-a is formed which will enter into a fast

equilibrium involving solvation/desolvation. Furthermore, it

will undergo unimolecular ring-closure. In the absence of an

incoming ligand and assuming that the concentration of the

transient 37-a is steady-state, the rate law is given in

equation (36).

-d[ ( 37-b) ]/dt = (k-slc-! + ka[CB]))[37-b] (36)

k-i ~ ka because of the poor discriminatory ability of 37-a, and

k -i << ka tCB] because tCB] is very high .

Thus equation (36) becomes:

-d[37-b]/dt = k'-i[37-b] (37)

where k'-i = k-iK«c/[CB] (38)

K » q — lt-3/ka

Rate constants for the ring-closure of cis-(fl1-DTA)-

(CB)W(CO)^ at various temperatures are given in Table XI.

110

TABLE XI

RATE CONSTANTS FOR THE RING-CLOSURE OF CIS-(ft*-DTA) (CB )W(CO). IN CHLOROBENZENE AT VARIOUS TEMPERATURES

DTA T, °C lO^kot-.d , (sec-1)

DTD 9.5 1.9(6) 9.5 1.75(11) 9.5 1.71(10) 9.5 1.54(11) 9.5 1.51(6) 9.8 1.54(14)

12.3 1.70(6) 12.3 1.46(9)

15.0 1.8(3) 19.1 2.77(4) 19.1 2.70(4)

19.7 2.52(12) 19.7 2.43(10) 19.7 2.29(13) 19.7 2.25(8)

21.1 2.98(7) 21.1 3.08(8) 21.1 3.04(9)

22.9 3.40(3) 22.9 3.14(4) 22.9 3.1(3)

26.0 5.69(20) 26.0 5.38(10) 26.0 4.6(2)

29.7 6.61(17) 29.7 6.59(13) 29.7 6.6(2)

30.5 6.5(6) 30.5 5.6(5)

Ill

DTU

TABLE XI CONTINUED

DTA T, °C 10^ko (sec-1)

DTD 31.1 5.4(5) 31.1 5.3(4)

35.2 10.6(7) 35.2 8.20(18)

9.5 1.04(7) 9.5 0.96(10)

12.3 1.49(14) 12.3 1.40(9) 12.3 1.14(11) 12.3 1.10(11) 12.3 1.08(8)

15.2 1.78(8) 15.2 1.78(2) 15.2 1.76(3)

19.1 2.21(7) 19.1 2.12(8)

22.1 2.59(9)

22.9 2.74(4) 22.9 2.63(3)

25.9 3.83(13) 25.9 3.44(6)

26.0 3.71(10) 26.0 3.24(10) 26.0 3.5(10)

112

TABLE XI CONTINUED

DTA O O

10"kob«d (sec-1)

28.6 28.6

3.5(2) 3.2(2)

30.2 30.2 30.2 35.2 35.2

4.4(3) 4.1(5) 3.5(4) 6.4(2) 6.01(17)

DTD: bflt = AS* =

11.1(6) kcal/mole 0.2(18) cal/deg-mol

DTU: LJI4 = kS* =

10.5(5) Kcal/mol -2.3(16) cal/deg-mol

The plots of ln(k'-i/T) vs. 1/T are given in figures 41 and 42.

The activation parameters (DTD, /NJT-i = 11.11(6) kcal/mole, * ± *

AS'-i = 0.2(18) cal/deg-mol; DTU, AJi'-! = 10.5(5), =

-2.3(16)) are consistent with a process which involves an initial

rupture of the chlorobenzene-tungsten bond to produce the

coordinatively-unsaturated intermediate cis- [ (fl1 -DTA)W( CO )*. J.

This intermediate will then undergo ring-closure. The value for

ks, the rate constant for the displacement of CB by L =

tri(isopropyl) phosphite from (fl1 -DTA) (CB)W(CO)*. at 35.2 °C, can

be calculated from the competition ratios, ke/3c-i, obtained from

the thermal reactions of (n«-DTA)W(C0U, (DTA = DTD, DTU), with L

and from the value of k-i for the ring-closure of cis-[ (fl1-DTA)-

(CB)W(COUJ.

113

3.3 3-4 3.5

Fig. 41--Eyring plot of ln(k-i/T) vs.. 1/T for the ring-closure of £i&-[ (m-DTD) (CB)W(CO)*.] in chlorobenzene at various temperatures. Ordinate = ln(k-i/T), Abscissa = 1/T x 103 k-1.

114

5-

3 6

Fig. 42--Eyring plot of in(k_,/T) vs. i/t for the

ainvarlm^rt ° f ^ [ ( n * " D T U > < C B > W ( C O U ] in chlorobenzene at various temperatures. (Ordinate = ln(k. ,/T) Abscissa = l/T x 103 k~*).

115

The values of the rate constants k 1 are in agreement

with its values in other systems, in the sense that a decrease

of k"-i is observed as the size of the chelate ring increases

(1). The difference in enthalpy of activation between ring-

closure and bimolecular attack by L suggests a slightly higher

energy barrier for the process involving ring-closure. This

barrier is also indicated by a decrease in the values of the

competition ratios, ke/k-i, as the temperature increases (1).

The difference in the entropy of activation for the

processes just described suggests a greater degree of bond

formation in the transition state for the bimolecular ligand

attack than for the process of ring-closure. Perhaps this

reflects a greater distortion of the octahedral geometry upon

ring-closure, hence there will be less bond formation in the

transition state. This tendency has also been observed in the

photochemically-generated species cis-(fl1-DTA)W(CO)^, (DTA =

DTO, DTN) (lfi.). Activation parameters for the ring-closure and

bimolecular attack by L at the photochemically-generated

transient reflect the same trend.

Summary

Rate constants and activation parameters for the

mechanism described in figure 37 for the reactions of

(f^-DTAWCOU, (DTA = DTO, DTN, DTD, DTU) , with L =

tri(isopropyl) phosphite at 35.2 °C in CB are summarized in

table XII.

116

TABLE XII

RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE DISPLACEMENT OF DTA FROM (fF-DTA)W(CO)^ at 35.2 °C

Complex "(DTO)W(COU (DTN)W(COU (DTD)W(COU (DTU)W(COU

10s ki (sec-1) -0.16(3 ) b6.5(5) 810(3) 1210(50)

10-= k'-i (sec-1 ) =5.67(2) *=2.04(3) 0.94(12) 0.62(2) 10"® k'a (sec-1) =1.2(2) =1.78(5) 0.97(22) 0.89(9) 10s k* (secr-M 16.0(14) 18.0(3)

ke/k~i (M-1) =0.21(5) =0.87(4) 1.03(9) 1.44(9) *0.28(7) *0.97(9)

*29.0(12) '='25.5(3) 26.0(7) 24.9(7) (kcal/mole)

AS?

(cal/deg-mol) *1.8(37) *-5.2(8) 18.0(23) 14.7(23)

+ i Affe - AHli *-2.3(24) *='-3.8(21) -3.6(8) -2.3(16) (kcal/mole) =-0.7(38)) =-4.9(11) AsJ - AS-!* =-5.6(129) =-15.5(36) -12.3(3) -7.0(5) (cal/deg-mol) *=•-4.5(60) * -11.1(65)

i Afl'-i =8.8(5) =11.5(3) 11.11(6) 10.5(5) (kcal/mole)

(cal/deg-mol)

AH'! (kcal/mole)

As't (cal/deg-mol)

AS'ii =-3.5(6) =2.6(15) 0.2(18) -2.3(16) :al.

AH'! =8.1(33) =6.6(8)

AS't =-9.1(113) =-12.9(21)

-Ref. 1, 2, 10 toRef. 1, 3, 10 =Ref. 10 * Complementary data from thermal ligand-exchange studies, rather than via flash photolysis studies (ref. 10).

117

The value of the rate constant for ring-opening, klf

increases as the size of the chelate ring increases. While

there is a 40-fold increase in the value of the rate constant

when the ring size is increased from five to six atoms, there is

a 120-fold increase when the ring size is increased from six to

seven atoms. However, there is a very small increase in the

value of the rate constant k* for the eight-membered complex in

relation to the seven-membered complex.

Since the evidence points to a transition state with

little bond formation, the acceleration in rates may come from

the degrees of freedom gained by the chelate ligand in the

transition state, plus the distortion that bigger chelate rings

may impose on the octahedral geometry. The conclusion that the

transition state resembles the unsaturated intermediate, cis-

(0* -DTA)W( CO )*», comes from the fact that rate constants for

ring-closure do not change as much as the rate constants for

ring-opening as the ring size is increased.

The values of the competition ratios, ka/k-i, show a

steady increase with increasing ring size. Because rate

constants for the displacement of CB from cis-(fl1-DTA) (CB)

W<C0U by tri(isopropyl) phosphite do not show a definitive

trend, one may conclude that the increase in the values of the

competition ratios as the ring increases is due almost

exclusively to a decrease in the value of ]c-* .

The activation parameters given in table XII can

provide insight into the nature of the transition states for the

118

different steps involved in this mechanism. For example,

negative values for (AHe - Afl-i) indicate that an energy barrier

is imposed upon the ring-closure in relation to the bimolecular

ligand attack. A smaller difference in the enthalpies of

activation, (Afia ~ )» for DTA = DTO suggests a greater

distortion of the octahedral geometry for the bigger rings when

ring-closure of cis-(fl1-DTA)W(CO)«. takes place. There is also

observed a more negative entropy of activation for the

bimolecular attack than for the ring-closure. Furthermore,

negative values for ASe - i S-i obtained from the thermal studies

reinforce these findings.

The rate-determining step for the reactions of (ft®-

DTA)W(CO)** with L will depend on the relative values of kob.d

and k'ob«d (equations (30) and (31)). If k»b.d >> k' ot.mci, the

reaction will be biphasic. Conversively it will be monophasic

when kob.d << k'ob«d. Changes in the rates of the step governed

by kot>.d are dominated by [L] and the value of ki. For a given

L, the value of k'obmd (k'o,tomci = k*.) is independent of [L].

Furthermore, the nature of DTA does not significantly affect the

value of k*. (16.0(14) X 10-= sec-1 and

18.0(3) X 10-= sec"1 for DTD and DTU at 35 °C for L =

tri(isopropyl) phosphite). Thus, for a given [L] the ratios

ki/k'cb.cj will dictate the form of the overall rate law for the

displacement of DTA.

Assuming that the value of k^ (L = tri(isopropyl)

phosphite at 35 °C) for DTO and DTN is 18 X 10"= sec-1, the

119

ratios ki/k'ota.c* can be calculated (DTO (0.0089), DTN (0.36),

DTD (51), DTU (67)). These values indicate that the rate-

determining step for the reactions of (n®-DTA)W(C0)«^ with

tri(isopropyl) phosphite changes as a function of the ring size.

The rate-determining step for the reactions of DTO and DTN is

governed by kob.cs, while for the reactions of DTD and DTU it is

governed by k'ob.d.

Another factor that may influence the rate law for the

reactions of a particular (fP-DTA)W(CO)^ is the effect of the

coordinated L on k*. (k'ot.»ci). The values of k* for the

reactions of cis-((V-DTHp)(L)W(CO)^ with various L at 44.5 °C

were found to depend on the steric properties of L (table VI).

The value of k*. increases with the Tolman cone angle (11) of L

(L = CP, k*. = 4.48(8) X 10~= sec-1, cone angle = 101°; P(0Me)a,

15.3(4) X 10"s sec"1, 107°; P(0-i-Pr)a, 143(3) x 10"= sec"1,

130°; P(OPh)a, 118(9) X 10"a sec-1, 128°; P(n-Bu)3, 289(12) X

10-® sec"1, 132°).

The form of the rate law for the reactions of DTN, for

which kob.cj/k'ote.d is closest to unity, will depend critically

on the steric nature of L. Monophasic behavior is expected for

bulky ligands because k ' o b . d >> kot>.<*. Biphasic behavior is

expected for non-bulky ligands since k'obsc* << kob.d. Biphasic

behavior was observed for the reactions of (rF-DTN)W(CO)«. with

CP (cone angle 101°) 12.). However, for larger ligands (P(OHe)s

(cone angle 107°), P(0Et)a cone angle 109°) non-common intercepts

were observed for the plots of 1/ko.b.c vs. 1/tL] (£).

120

The latter behavior was attributed to competition

between path A and B described in figure 5. For larger ligands

than P(OEt)s, (P(OPh)a (cone angle 128°), P(0-i-Pr)a (130°),

P(Ph)a (145°)), similar intercepts for plots of l/kob»«a vs.

1/[L3 were observed. Thus, indicating that k'eb«> >> kot,«d .

These inconsistencies prompted the preliminary

kinetics reinvestigations of the reaction of (ne-DTN)W(CO)*. with

trimethyl phosphite. The reactions were carried out following

the exact conditions of previous studies (2.), however

chlorobenzene was used as a solvent. In this study, more data

points (which improved the accuracy of the results) were

obtained.

Plots of In (A* - Atoimr,* ), (At, 1 ank s absorbance of the

ligand-solvent solution), vs. time showed a curvature indicative

of a biphasic behavior (figure 43). In previous studies biphasic

behavior was not observed, perhaps due to limitations of data

acquisition. Furthermore, if the data were acquired only at the

early stage of the reaction, the plots of lnfA* - Abx-r,*) vs.

time may have shown a very small curvature or none at all. In

addition, small curvature in this plot can be attributed to a

small difference between the experimentally-determined A. and the

true value (£2).

121

JQ < I

<

0.4

0.3

[P(OMe)3] = 0.3332

[P(OMe)3] = 0.8824

15 20 10"3 TIME, s

30 35

Fig. 43- -Plots of In (At - vs. time for reactions of (DTN)W(C0)<. with P(OMe)a in chlorobenzene at 43.3 °C at two concentrations of P(OMe)3.

122

From figure 43, it can be noticed that the value of

increases with the concentration of P(OMe)a. These plots

are concave downward at low [P(OMe)a] and concave upward at

higher [P(OHe)al. One may conclude, tentatively, that no

interchange pathway (path b in figure 5) is operative in this

system. The mechanism for the reactions of (ne-DTN)W(CO)*, will

be investigated further.

The fact that (ns-DTHp)W(CO)^. undergoes ring-opening

through a different pathway is surprising, especially, when the

kinetics evidence points to an interchange pathway involving a

great deal of bond formation in the transition state.

CHAPTER BIBLIOGRAPHY

1. Dobson, G. R.; Basson, S. S.; Dobson, C. B. Inorg. Chim. Acta. 1985, 105, L17.

2. Schultz, L. D. ; Dobson, G. R. J. Oreanomet. Chem. 1976, 124, 19.

3. Schultz, L. D.; Dobson, G. R. J. Oreanomet. Chem. 1977, 131, 285.

4. Illuminati, G.; Mandolini, L. Acc. Chem. Res.. 1981, 4, 95.

5. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E.; Awad, H. H.; Dobson, G. R. J. Am. Chem. Soc.. 1987, 109, 5386.

5. Simon, J. D.; Peters, K. S. Chem. Phvs. Lett.. 1983, 98, 53.

6. Simon, J. D.; Xie, X. J. J. Phvs. Chem.. 1986, 90, 6715.

7. Langford, C. H.; Moralejo, C.; Sharma, D. K. Inore. Chim. Acta. 1987, 126, Lll.

8. Seder, T. A.; Church, S. P.; Weitz, E. J. Am- Chem. Soc., 1986, 108, 4721.

9. Bonneau, R. ; Kelly, J. M. J. J. Ajq. Chem. Soc. . 1980, 102, 1220.

10. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inore. Chem.. 1985, 24, 2179.

11. Tolman, C. A. Chem. Rev.. 1977, 77, 343.

12. Espenson, J. H. "Chemical Kinetics and Reaction Mechanisms", McGraw-Hill Book Co., New York, 1981, pp. 65-71.

123

CHAPTER V

CIS-TRANS ISOMERIZATION OF (L)eW(COU

Experimental

The cis-trans isomerization of (P(n-Bu)3 )eW(CO)*» was

monitored following the increase of the absorbance at 415 nm

using the in-house built spectrophotometer described in chapter

II. 3 1P NMR spectra were recorded employing a Varian VXR 300

NMR spectrometer (i).

Cis-trans isomerjgation

The plot of absorbance vs. time for the reactions of

(fF-DTHp)W(CO)* with P(n-Bu)a is given in figure 44. The first

and the second segments of the plot can be ascribed to the

formation of cis-(n1-DTHP)(P(n-Bu)a)W(C0)^ and

cis-(P(n-Bu)a)eW(COU, respectively. The third segment, for

which an increase of the absorbance is observed as a function of

the time, can be assigned to the formation of the colored

trans-(P(n-Bu)a )eW(CO)+. (2). The carbonyl stretching spectrum

for the reaction product at t- is given in figure 12. The

strong band at 1870 cm-1 suggests a mixture of the cis and trans

isomers. This carbonyl stretching spectrum is consistent with

the ones reported by Mosbo and his co-workers (2.) arid by Howell

et al (2).

124

125

1 4

OS

• • •

of 44

Fig. 44--Plot of absorbance vg. time for the reaction (fla-DTHp )W( CO)«, with P(n-Bu)a (0.1071 M) in chlorobenzene 5 °C. Ordinate = absorbance, Abscissa = seconds X 10~"-4

at

126

The reactions of (fF-DTA)W(COU, (DTA = DTHp, DTD, and

DTU) with L, (L = CP, trimethyl phosphite, tri(isopropyl)

phosphite, and triphenyl phosphite) did not show a third segment

for the plot of absorbance vs. time. This outcome is due to

either the lack of cis-trans isomerization of the final product,

or to the fact that the trans isomer is colorless. The carbonyl

stretching spectra (figures 13 and 16) of the product of the

reaction of cis-(fl1 -DTHp)W(CO)^. with L, (L = CP and triphenyl

phosphite) show that the final product is exclusively the cis

isomer. However, for L = trimethyl phosphite and tri(isopropyl)

phosphite the carbonyl stretching spectra (figures 14 and 15)

suggest there is a mixture of the cis and trans isomers.

The cis-trans isomerization of (L)sW(CO)*., (L = P(n-

Bu ) a ) , in CB was studied at 35.2 °C, 44.5 °C and 54.6 °C. The

reactions for the isomerization process were monitored after the

reactions for the formation of cis-(L)eW(CO)«* were completed (at

least eight half-lives). Plots of ln(A- - A*) vs. time were

linear over two half-lives (figure 45). These reactions were

carried out in the presence of a high excess of L. The observed

pseudo-first order rate constants, for the transformation

described by the chemical equation (39), were independent of the

concentration of L.

cis-(P(n-Bu)a )aW(CO)f trans-(P{n-Bu)3)aW(CO)*• (39)

127

(At -A b | )

(Aoo-At)

for ^Ki g;v, 4 57 P l o t S ° f ( t o p ) l n { 7 U - vs. time

the third segment of this plot, a t 415 rim, f o r r e a c t i o n o f (IT (0.1084 M) at 44.5 °C; and ln(A- - A*) vs.. t i m e . seconds X 10~3.

obteined by monitoring DTHp)W(C0U with P(n-Bu)3

(bottom) these data plotted as Ordinate = absorbance. Abscissa =

128

The rate law for the reaction reaction described by

equation (39) is given by equation (40).

-d[Substrate ]/dt - kot,«d [ Substrate ] (40)

Substrate = cis-(P(n-Bu)s )eW(CO)<»

kobad = (k? + k-7) (41)

These rate constants (k-7 + k—7) are given in table XIII.

These results are consistent with results observed for other

systems (.2., J., it) •

TABLE XIII

RATE CONSTANTS FOR THE ISOMERIZATION OF (P(n-Bu)a)eW<COU IN CHLOROBENZENE AT

VARIOUS TEMPERATURES

Temp [L] 1 0 " 1 zAt loin

54.6 0.3259 10.49(7) 54.6 0.3259 9.74(5)

44.5 1.616 2.20(2) 44.5 1.059 2.24(1) 44.5 0.9180 2.29(3) 44. 5 0.7729 2.31(1) 44.5 0.7041 2.21(4) 44.5 0.2372 2.34(3) 44.5 0.1080 2.26(1)

35.2 0.8702 0.481(4) 35.2 0.4301 0.471(6)

129

Dixon et al. (.2) have studied the cis-trans

isomerization of (P(n-Bu)a)eW(COK in n-heptane employing 3 1P

NMR spectroscopy. At 46 °C, the rate constant for the

isomerization was reported as 3.5(4) X lO-** sec-1, which is a

slightly higher value than the values obtained in this study at

44.5 °C in CB (2.28(3) X 10~+ sec"1).

The relative concentration of the trans and the cis

isomers can be calculated by 3 1P NMR spectroscopy.

This method is useful in the characterization of complexes of

the type (L)sW(CO)*, (L = phosphines, phosphites) since the cis

and the trans isomers show resonances which are well separated

(1, 1, £, £>•

3 1P NMR studies

In order to determine the ratio of cis and

trans-(L)eW(CO)«., L = P(n-Bu)a, by 3 1P NMR, the solutions

containing the products of the reaction of (fF-DTHp)W(CO)^ with

L were placed in NMR tubes and allowed to thermally equilibrate

in a constant temperature bath (haake ED) for 24 hours at their

respective temperatures (35.2 °C, 44.5 °C and 54.6 °C). To

ensure constancy in the temperature of the product mixture

during the progress of the experiment, the NMR probe was

pre-heated at the equilibrated temperature of the NMR tube which

contained the sample. Then, rapidly the NMR tube was placed in

the probe. The NMR tube was allowed to stand for at least 15

minutes in the NMR probe prior to recording.

130

The NMR spectra were recorded for 30 minutes (pulse intervals = 5

seconds) employing 85% HsaPCW as an external standard. The

spectra at 35 .2 °C, 44 .5 °C and 54.6 °C are shown in figures 46,

47 and 48, respectively.

The resonances at 6 -1.8 and 6 -9.5 were assigned to

phosphorus atoms in trans and cis- (P(n-Bu)a )aW(C0)*.,

respectively. These assignments were made by comparison of

these resonances to resonances of actual species reported by

Mosbo and co-workers (6-2.4, 6-10.6) (2), Grim and Wheatland

(6-2.5, 6-10.0) (5.) and McFarlane et al (6-2.6, 6 -10.4) (.£).

However, these resonances are in disagreement with those

reported by Dixon and co-workers (6-6.03, 6 2.4) (1).

Since, at equilibrium,

k-?[cis-(L)eW(CO)*] = k-v[trans-(L)eW(CO)^], (42)

the equilibrium constant for the cis-trams isomerization is

given by equation (43).

K»d = [trans3/[cis] = k-r/k-r (43)

The ratios, [trans3/[cis], can be calculated from the

relative integrated intensities of the resonances at 6(-1.9) and

6(-9.6), which have been assigned to the trans and the cis

isomers, respectively. Table XIV presents these values,

together with those for K»c*.

131

2: 0-_ IT. Q.

- hit-

L j

.CD I

-ID I

VLB'S*

. r i

91

O .(V U • j 2

to C A3 •

U o -P 0 TJ CM 13 «

A3 uo <T> i

<n -P •H fE3 0 M-l V a 0 a) N gj c p 4) H .0 -M 0 0 h 03 0 04 r-i W J3 a jg a 2 •H 04 J (!)

i o (!)

i u "W

V0 a < 8 dO i •H S3 to CO

i c «w 04

- k <\i

132

}

§ k •P O o

in a • rd <4*

i tO|4J •rfl fd

0

0) e 0) N § G

5 a)

-P 0

ft rH n X 0

ft 41 * 01 O i O I W ®s 5 <> (U

• o

•W 3 En PQ

1 c

13 3

.CO i

i

I- 3 o '

ifS. 5T

-J O)

OJ

CD

—

— d •

" ^ <TJ

H

o 0

<n +> mm <sO

_ 0 D XJ *

_ t d ^ ra m

» ,-p - to a

— 0 I 03 — d

a) 0 N

ru ~*ri

I

I a. CL-yV

o ' *

- I

-J 3

d g 0 3 A H 0 -p h a o 0) H a x to a e d S *H

1 8 I ^ CO 3

92

&! m

c

Ou

134

TABLE XIV

CIS : TRANS RATIOS OF (P(n-Bu)a)aW(eOU IN CHLOROBENZENE AT VARIOUS TEMPERATURES

T, °C [P(n-Bu)»] relative abundances ~K.«, trans cis ttrans]/[cis]

35.2 0.03983 132.3 45.3 2.92 0.03983 131.0 44.2 2.96

44.5 0.03983 89.5 33.4 2.68 0.03983 88.8 31.8 2.79 0.03983 - — 3.17

54.6 0.03983 94.9 34.7 2.73

Average trans/cis = 2.82 * Error limits ca. = ± 6%

The composite rate constant for the isomerization at

44.5 °C (2.28(3) X 10-* sec-1) observed in the present study is

in reasonable agreement with the rate constant under similar

conditions (3.5(4)X10—* sec-1 at 46 °C) reported by Dixon and

co-workers (3). However, the values of K_c found in the present

study (table XVIII), differs significantly from the value of

theirs (K-„ =8.93).

The determination whether this isomerization takes

place in the five coordinate intermediate, [LW(CO)*], on the

time-scale of the ligand-substitution reaction, or through

isomerization of cis-(L)aW(CO)*, has been a subject of

discussion (.2~k.t 7-20). Dobson, Awad and Basson's work

indicated that the five-coordinate intermediates, cis and

trans-[ (P(CAH= )A )W(C0) .3, produced via pulsed-flash photolysis

135

of cis- [ (pip) (PCGsIts )a )W(COU] in chlorobenzene, do not inter-

convert on the time-scale of the bimolecular ligand attack (7.).

a iP NMR studies of the thermal ring displacement of

(tmpa)W(COU, (tmpa = N.N.N',N'-tetramethyl-1,3-diamino-

propane), indicated that only the cis-disubstituted product is

initially produced, and that the production of the trans product

takes place subsequent to this (7). Furthermore, the reaction

rates of the thermal cis to trans isomerization of

cis-[(P(C«,H)a)BW(COU] has been studied and the results are

consistent with a process which does not involve W-P bond

fission (2). In addition, Dixon and co-workers (D have shown

that during the cis-trans isomerization of (P (n-Bu)3 )eW(CO )*. in

n-heptane under an atmosphere of 13C0 there is no formation of

(P(n-Bu)a) (*3CO)W(CO)<». This experimental observation suggests a

non-dissociative process. Similar results have been observed by

Darensbourg amd Gray (4). They reported that after heating

((Et)3 )PW(CO)<* under an atmosphere of iaC0, no incorporation of

iaC0 into (P(Et)a)W(COU was observed. In the present study, the

carbonyl stretching spectrum of a mixture of cis and trans

(P(n-Bu)a)sV(CO)*. in the presence of an excess of piperidine in

CB showed no indication of incorporation of piperidine into the

complex.

From equation (41) and (43) the individual rate constants k- and

1c--?- can be calculated. These rate constants are given in table

XV.

136

TABLE XV

RATE CONSTANTS FOR CIS-TRANS ISOMERIZATION OF (P(n-Bu)a )aW(COU IN CHLOROBENZENE

AT VARIOUS TEMPERATURES

T, °C klssnsr1 cat Ion 10s *kv 10s T, °C 10s (sec-1) (sec-1)

35.2 4.79(2) 3.56 1.21

44.5 22.8(5) 16.9 5.89

54.6 101(5) 74.7 26.4

fcfi-r = 30.9(5) kcal/mole, Atf--* = 31.3(5) kcal/mole A 2 2 . 4 ( 1 4 ) cal/deg-mol , A S ^ - 21.6(12) cal/deg-mol * Error limits for determination of the integrated intensities of the 3 1P NMR absorptions are estimated to be £&. ± 3%.

In this study, the highly positive entropies of i 4

activation (AS- = 22.4(14) cal/deg-mol, AS—^ ~ 21.6(12) cal/deg-

mol) suggest a great deal of reorganization in the transition

state for both pathways.

CHAPTER BIBLIOGRAPHY

1. The experimental assistance by Mr. T. Corby Young is gratefully acknowledged.

2. Boyles, M. L.; Brovm, D. V.; Drake, D. A.; Hostetler, C. K.; Maves, C. K.; Mosbo, J. A. Inore. Chem.. 1985, 24, 3126.

3. Howell, J. A. S.; Dixon, D. T.; Kola, J. C. vj. Chem• SQQ• Dalton Trans.. 1984, 1307.

4. Darensbourg, D. J.; Gray, R. L. Inore. Steffi., 1984, 23, 2993.

5. Grim, S. 0; Wheatland, D. A. Inore. Chem.. 1969, 18, 1716.

6. McFarlane, H. C. E.; McFarlane, W.; Rycroft, D. S. Chem. , Dalton Trans.. 1976, 1616.

7. Dobson, G. R.; Awad, H. H.; Basson, S. S. Inoxs. Ctum. Acta. 1986, 118, L5.

8. Majunke. W.; Leibfrits, T.; Mack, D.; Dieck, H. T. Chem• v., 1975, 108, 3025.

9. Darensbourg, D. J. Inore. Chem.. 1979, 18. 14.

10. Darensbourg, D. J.; Baldwin, B. J. J. Iffi. Chem. goo., 1979, 101, 6447.

11. Darensbourg, D. J.; Kudaroski, R.; Schenk, W. Inore.

Chem.. 1982, 21, 2488.

12. Bailar, J. C., Jr. Inore. Nucl. Chem.. 1958, 8, 165.

13. Cotton, F. A.; Darensbourg, D. J.; Kleins, S.; Kolthammer, B. W. S. Inore. Chem.. 1982, 21, 2661.

14. Darensbourg, D. J.; Kump, R. L. InorK. Chem-. 1978, 17, 2680.

15. Fischer, H.; Fischer, E. 0.; Werner, H. J. J. Qrsanomet. Chem.. 1974, 73, 331.

137

138

16. Hoffmann, R. ; Howell, J. M. ; Ross, A. R. J. Ain. Chem. Soc.• 1976, 98, 2484.

17. Pomeroy, R. K.; Vancea, L.; Calhoun, H. P.; Graham, W. A. G. Inorg. £hm-, 1977, 16, 1508.

18. Ray, P.; Dutt, N. K. £. Indian Chem. SQC., 1943, 20, 81.

19. Springer, C. S. Jr.; Sievers, R. E. Inorg. Chem-. 1967, 6, 852.

20. Vancea, L.; Pomeroy, R. K.; Graham, W. A. G. Chgm. Soc.. 1976, 98, 1407.

CHAPTER VI

CONCLUSIONS

The reactions of (n«-DTA)W<CO)«. with L , (L * CP and

triphenyl phosphite) are biphasic and with L (L = n-butyl

phosphine, tri(isopropyl) phosphite, and trimethyl phosphite)

the reactions are triphasic. During the course of these

reactions, appreciable amounts of cis-(fl1-DTA) (L)W(CO)^. are

produced.

The most likely mechanism by which (fle-DTA)W(CO)^ ,DTA

= DTD and DTU, react with L = phosphites is depicted in figure

49. There is an unimolecular ring-opening of (fle-DTA)W(CO)<* to

afford cis-tn^-DTAWCO)*, which then will undergo an

equilibrium involving solvation and desolvation governed by ka

and k-a, respectively. This intermediate will also undergo a

competitive ring-closure and bimolecular attack by L. Upon

attack of cis-(n*-DTA)W(COU by L, cis-(n*-DTA) (L)W(COU is

produced. Dissociation of DTA from cis-((V -DTA) (L)W(CO)*. will

produce cis-[LW(COU], which will undergo an equilibrium

involving solvation and desolvation governed by k-s and k-a,

respectively. Further attack by L at cis-[LW(CO)«»] will produce

cis-(L)aW(COU.

139

1 4 C

° o

\ x

4 -

r > I

\ 4-

° o

o o - X / >1

% 0° \ S

o o § o o

u +

o o

° a

V O O §

y ' \ +

h -JtL

1

O O

O O -\ / —Y— /\

O O \ /

^ C _ o O

Oo- u o

a o u

o r ;

r—4 c £ r~\ P - l

0 5 Q U t 0 ) -li > C 0 —

< D £ x; o - M p

u - i

0 -

6 a t n

x; q o CD I! C " <

T 3 £ h 0 ) Q c n o -D * c t 0 H M Q

CXf U - J

1 0 I

<x> 4 J C CD

• £ & 0 0

• H o Uu r f l

a J < n

• H " 0 X !

141

Furthermore, cis-(L)eW(COU, L = tri(n-butyl) phosphine,

tri(isopropy1) phosphite, and trimethyl phosphite will undergo a

cis-trans isomerization to afford a final mixture of

cis- (L)eW(COU and trans-(L)aW(COU. Since this isomerization

takes place within the time scale for the formation of

cis-(L)sW(CO)^ and its subsequent reaction with L, the overall

process involves three consecutive reactions. In table XVI, the

rate constants for the overall mechanism just described for L =

tri(isopropyl) phosphite are given.

TABLE XVI

RATE CONSTANTS INVOLVED IN MECHANISM DESCRIBED IN FIGURE 49

Complex (fF-DTD)W(COU (na-DTU)W(COU

ki (sec-1) 8.1(3)X10~3 12.1(5)xl0~3

k'-i (see-31 ) 94(12)X103 62(2)X103

k'e (sec-1) 97(22)X10a 89(9)X103

Jfe/k-! (M-M 1.03(9) 1.44(9)

k*. (sec-1) 0.160(4)X10-3 0.180(3)X10~3

kr-=s (sec-4) 6.17X10** 6.17X10**

ke> (k-=/k= [CB]) (sec-1 )

7.01(2)X103 7.01(2)X103

*k-7 (sec-1 ) 0.0356XZ10-® 0.03 56X10-3

* 1 ( s e c - 1 ) 0.0121X10-3 0.0121X10-3

L = P(n-Bu)3 Temperature = 35.2 °C

142

The proposed mechanism for the reactions of

(n®-DTHp)W(COU with L = phosphites and phosphines is described

in figure 50. According to figure 50, an initial interaction of

L with (na-DTHp)W(COU will afford cis-(n1-DTHp) (L)W(CO)*.. The

mechanism by which cis- (H* -DTHp) (L)W(CO)<* undergoes displacement

of DTHp by L is analogous to the mechanism just described for the

reactions of cis-tf^-DTA) (L)W(COU, DTA = DTD and DTU, with L.

In table XVII, the rate constants for the overall mechanism

described in figure 50 are given.

TABLE XVII

RATE CONSTANTS FOR THE OVERALL MECHANISM DESCRIBED IN FIGURE 50

10—3k'& 103k«. 10~5 k'-i 10-^k-a 10-3k'A 10®k-7 103k-7 sec-1 sec-1 sec-1 sec-1, M-1 sec-1, M 1 sec 1, M 1

36.7(11) 0.327(1) 6.591 6.5(14) 7.2(3) 0.0356s 0.01213

11.62®

*in BB solvent ®in DCE solvent 3L = P(n-Bu)a Temperature = 35.2 °C

0 c

\

.c?

w

c o

to0

143

o c

o °

o c

w

C B c o

*5 i C B

-5

o c

w

l \ c ° o

M .

o c

w

,o°

c o

Fig- 50---Proposed mechanism for the overall displacement of DTHp from (He-DTHp )W( CO )*• by L.

BIBLIOGRAPHY

Al-Kathumi, K. M.; Kane-Maguire, L. A. P. J. Inorg. Nucl.

Chem.. 1972, 34, 3759.

Angelici, R. J. Oreanomet. Chem. Rev.. 1968, 3, 173.

Angelici, R. J.; Graham, J. R. J. £ffi. Chem. Sog., 1965, 87, 5586.

Angelici, R. J.; Graham, J. R. 1. M - Chem. SQC., 1966, 88, 3658.

Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H.; Dobson, G. R. J. Ajn. Chem. Soc.. 1987, 109, 5386.

Atwood, J. D. Inorganic and Oreanometallie Reaction Mechanisms: Brooks/Cole Pub. Corp: Monterey, Cal., 1986.

Atwood, J. D.; Brown, T. L. M - Chem. SOC-. 1976, 98, 3160.

Bailar, J. C. Jr. Inore. Chem.. 1958, 8, 165.

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