R

Za

Da

b

C

a

ARRAA

KZAHOHS

tM2pm

A

0h

Coordination Chemistry Reviews 270–271 (2014) 112–126

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

j ourna l h omepage: www.elsev ier .com/ locate /ccr

eview

inc hydroxides and oxides supported by organic ligands: Synthesisnd structural diversity

aniel Prochowicza, Kamil Sokołowskib, Janusz Lewinskia,b,∗

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, PolandInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1132. Synthesis and structure of organozinc hydroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

2.1. Heteroleptic alkylzinc hydroxides of the type [RZnOH]n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1132.2. Alkylzinc hydroxides supported by multifunctional ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1152.3. Zinc hydroxides derived from the activation of dioxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

3. Zinc oxide clusters: structural types and synthetic methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.1. Zincoxane compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.2. Zinc oxide species entrapped by organozinc moieties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1203.3. Zinc oxide complexes supported by organic ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

4. Oxozinc carboxylates as the predesigned molecular platform for modelling the reactivity of prototypical Zn-MOFs towards H2O and donorsolvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

r t i c l e i n f o

rticle history:eceived 8 November 2013eceived in revised form 5 December 2013ccepted 5 December 2013vailable online 21 December 2013

eywords:

a b s t r a c t

This review focuses on the synthesis, reactivity and structural chemistry of zinc hydroxides and oxidesderived essentially from organozinc compounds. It demonstrates the advantages of the organometallicroute to these classes of compounds that is time and atom efficient. We have surveyed the preparation oforganozinc hydroxide complexes via direct hydrolysis of both R2Zn compounds and alkylzinc complexesof the type RZn(L) (where L = monoanionic, mono- or multidentate organic ligand). The variety of theirmolecular structures is discussed with an emphasis on how the character of zinc bonded alkyl substituents

inclkylydroxidexideydrolysistructure

or L ligands can influence the stability and the mode of aggregation of the resulting zinc hydroxides. Thesection dedicated to synthetic procedures and the solid state structures of zinc oxides focuses on the fol-lowing aspects: (i) zincoxane compounds; (ii) the encapsulation of ZnO species into organozinc moietiesand (iii) zinc oxide complexes supported by organic ligands. Finally, utility of oxozinc carboxylates asa predesigned molecular platform for modelling of the reactivity of prototypical Zn-MOFs towards H2Oand donor solvents is presented.

Abbreviations: Ac, acetate; Ant-H, 9-anthracenecarboxylic acid; Ar, aryl; box, bisoxaBu, tert-buthyl; CA, carbonic anhydrase; DOSY, Diffusion Ordered Spectroscopy; DAB, 1

onte Carlo; hppH, 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidine; iPr, iso-prop,4,6-trimethylphenol; MOFs, metal organic frameworks; nhc, 1,3-bis(2,4,6-trimethylpheyridine; pyr2,5-Ph

2, 2,5-diphenylpyrrolyl; SBU, secondary building unit; SCXRD, single-cethylpyrazolyl)borate; tptm, tris(2-pyridylthio)methyl; qH, 8-hydroxyquinoline; Me2Z∗ Corresponding author at: Faculty of Chemistry, Warsaw University of Technology,

cademy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.E-mail address: lewin@ch.pw.edu.pl (J. Lewinski).

010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ccr.2013.12.003

© 2013 Elsevier B.V. All rights reserved.

zoline; H2bpc, N,N′-bis(5-ethyl-1,3,4-thiadiazol-2-yl)-2,6-pyridinedicarboxamide;,4-diazabutadiene; dpds, 4,4′-dipyridyldisulfide; Et, ethyl; GCMC, Grand Canonicalyl; IR, infrared spectroscopy; Me, methyl; 1-Meim, 1-methylimidazole; MesOH,nyl)imidazol-2-ylidene; NPM, noncovalent porous materials; Npy, pyrrolidine; py,

rystal X-ray diffraction; THF, tetrahydrofuran; TpCum,Me, hydrotris(3-p-cumenyl-5-n, dimethylzinc; Et2Zn, diethylzinc; tBu2Zn, di-tert-buthylzinc.Noakowskiego 3, 00-664 Warsaw, Poland; Institute of Physical Chemistry, Polish

emistry Reviews 270–271 (2014) 112–126 113

1

icbmta–rtaisoowTeeoggimipstNcc(

idztats

2

aghrostsiouRotpimmd

ZnO

ZnO

H

HEt

Et

O Zn

OZnZnO

OZn

HEt

Et

H

Et

Et

H

H2 EtZn(OZn)3OH

- 3 Et H

L = dioxane

L

L

D. Prochowicz et al. / Coordination Ch

. Introduction

Interactions of zinc complexes with water have attracted majornterest due to their implications in bioinorganic and materialshemistry. For example, in the last decades much attention haseen paid to mono- or dinuclear zinc complexes supported byultidentate ligands with terminal or bridging hydroxide liga-

ion. These complexes have been widely investigated as syntheticnalogues of many zinc enzymes including hydrolases and lyases

this chemistry was nicely summarized in a few elegant tuto-ial reviews describing examples of synthetic analogues relevanto the structure and function of zinc enzymes [1]. More recently,side from fundamental novelty, there has been a discernible shiftn interest towards zinc hydroxide and oxide clusters as tran-ient species or potentially efficient precursors in the preparationf zinc oxide (ZnO) nanoparticles [2]. Particularly this concernsrganometallic approaches towards monodisperse ZnO colloidshere a solution of organozinc precursor is exposed to air [3].

he formation of ZnO nanoparticles in these systems undoubt-dly involves a number of consecutive in-cage transformations,ach of which requires the occurrence of fundamental processesn the molecular level, followed by nanoparticle nucleation androwth. A thorough understanding of the hydrolysis and/or oxy-enation processes involving organozinc compounds may providenformation on the problem of controlling the composition and

orphology of ZnO-based (nano)materials produced via wet chem-cal processes. Moreover, zinc oxide clusters have appeared asotentially very attractive secondary building units (SBUs) forupramolecular chemistry. Such units can self assemble solelyo form interesting materials (e.g. noncovalent porous materials,PMs) [4] or can act as predesigned SBUs that can be inter-onnected by di- and multi-topic ligands to form 2D and 3Doordination polymers including porous metal organic frameworksMOFs) [5].

This review is focused on advances in the synthesis, reactiv-ty and structural chemistry of zinc hydroxide and oxide systemserived from organozinc compounds; pure inorganic systems likeinc hydroxyl salts are essentially excluded from the present con-ribution. The organization of this review should be regarded as

flexible guide to the classification of these systems. In addition,he relevance of these molecules or higher aggregates to materialsynthesis will be briefly discussed.

. Synthesis and structure of organozinc hydroxides

The interaction of organozinc compounds with water remainsn area essentially little explored thus far and, likewise, thereat potential of such reactions for synthesizing useful zincydroxides and oxides remains undeveloped. Due to the higheactivity of Zn C bond towards acidic protons, the stabilizationf organozinc hydroxides remains a challenging task. In general,uch systems need highly encumbered environment or/and addi-ional supporting ligands and can be sorted into those consistingolely of alkyl groups or are stabilized also by additional aux-liary ligands. In this section, we summarize the preparation ofrganozinc hydroxide complexes and the variety of their molec-lar structures. They can be synthesized by direct hydrolysis of2Zn and RZnCl compounds or by hydrolysis of alkylzinc complexesf the type RZn(L) (where L = monoanionic, mono- or multiden-ate organic ligand). These will be discussed with an emphasis

laced on how the character of R substituents or L ligands can

nfluence the stability of the resulting zinc hydroxides and theode of aggregation. Additionally, the synthetic approach to theixed aggregates consisting of RZnOH and RZnOR species is

escribed.

Scheme 1. The proposed equilibrium between dimeric and tetrameric ethylzinchydroxide species and their decomposition with subsequent elimination of ethane.

2.1. Heteroleptic alkylzinc hydroxides of the type [RZnOH]n

The homoleptic main-group metal alkyls, including zinc alkyls,are extremely reactive towards water. To avoid side reactions inthe preparation and handling of these organometallic compoundschemists commonly use anaerobic conditions and moisture freereaction media in their daily laboratory work. Simultaneously, it isa considerable challenge to bring the hydrolysis reactions undercontrol to favour the more rapid design and implementation ofH2O-based reaction systems. Surprisingly, despite the rich his-tory of R2Zn compounds their interactions with water remain alargely unexplored area [6]. Early interest in the R2Zn/H2O sys-tems stem from their application as most effective initiators inthe ring-opening polymerization of various heterocyclic monomers[7]. In 1963 spectroscopic studies of Herold et al. implied thatEtZnOH species are unstable and decompose slowly (in minutes)above −60 ◦C to zinc oxide clusters with concomitant evolutionof ethane [8]. In 1984 Kuran et al. applied a cryoscopic methodfor molecular weight determination to the Et2Zn/H2O reactionsystem with 1,4-dioxane as a solvent [9]. This study indicatedthat hydrolysis of Et2Zn initially leads to the dimeric (EtZnOH)2and tetrameric (EtZnOH)4 species with the subsequent eliminationof ethane and the formation of ethylzinc(oxo)hydroxide aggre-gates (Scheme 1). Undoubtedly the high reactivity of low-alkylzinchydroxides towards further alkane elimination precludes their iso-lation.

There is a very limited number of well-characterized zinccomplexes with an incorporated hydroxide group derived fromthe intentional or accidental hydrolysis of organozinc complexes.Alkylzinc hydroxide species exhibit a significant tendency to max-imize their coordination number by self-association to give aggre-gates containing trigonal or tetrahedral zinc centres. So far, thereare only three structurally characterized RZnOH-type compoundsdetermined by the steric demand of the substituents bondedto the zinc, a trigonal planar dimeric form [(Me2PhSi)3CZn(�2-OH)]2 (1) [10], a four-coordinate tetrameric arylzinc hydroxide[ArZn(�3-OH)]4 (2) (where Ar = 2,4,6-(CF3)3C6H2) [11] and hexam-eric tert-buthylzinc hydroxide [tBuZn(�3-OH)]6 (3) [12].

Compound 1 was synthesized by the metathesis reaction of(Me2PhSi)3C-ZnCl with NaOH [10]. The molecular structure of 1consists of two three-coordinate zinc centres that are bridgedby two hydroxyl groups, forming a Zn2O2 four-membered ring(Fig. 1a). The tetrameric arylzinc hydroxide 2 was prepared inlow yield by the hydrolysis of ArZnCl. Compound 2 consists ofZn4O4 slightly distorted cubic structure with alternating the four-coordinate zinc and oxygen atoms in the corners (Fig. 1b) [11].More recently, Lewinski and co-workers provided the first reactionsystem leading to well-defined, stable representative of RZnOH-type compounds via direct hydrolysis of homoleptic R2Zn [12]. Thetwo-phase reaction system involving tBu2Zn with 1 equivalent ofdegassed water in toluene afforded the alkylzinc hydroxide cluster,[tBuZn(�3-OH)]6 (3), in an essentially quantitative yield (isolated

yield 78%) (Scheme 2).

The DOSY experiments indicate that 3 exists in equilibriumbetween trimeric and hexameric species in a d-8 tetrahydrofuransolution [13]. This observation is distinct from cryoscopic studies

114 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

Fig. 1. The molecular structure of (a) [(Me2PhSi)3C-Zn(�-OH)]2

ftndtZadTai[otbiwZIfi

one, two or three hydroxide groups, respectively. Interestingly,

So

Scheme 2. Schematic representation of the synthesis of 3.

or EtZnOH compound in a donor solvent [9] as well as the relatedert-buthyl(tert-buthyloxide)zinc compound, (tBuZnOtBu)n (where

= 3,4) [14]. In the solid state compound 3 forms a hexamericrum-like structure [tBuZn(�3-OH)]6 (Fig. 1c). The molecular struc-ure of 3 comprises of two six-membered and six four-memberedn O rings. Each hydroxide ligand bridges up three zinc centresnd adopts the �3-OH coordination mode. The Zn6O6 core can beescribed as a hexagonal prism with alternating Zn and O atoms.he hexameric zinc hydroxide 3 was further successfully employeds an excellent precursor of nano-ZnO that decomposes smoothlyn one step at only 120 ◦C with the formation of wurzite-ZnO NPs12]. The authors argued that such low decomposition temperaturef 3 can be rationalized in terms of the presence in the struc-ure of Zn OH groups which can hydrolyze proton-reactive Zn Conds (Scheme 3). The breaking of three Zn O bonds in 3 resulted

n the formation of a flexible quasi-molecule (transition state),hich can be further easily stabilized as a sub-nanometre scale

nO cluster with a concomitant evolution of iso-butane (Scheme 3).nterestingly, very recently 3 was successfully utilized for thexation of CO2, which led to isolation of the mesoporous solid

cheme 3. (a) Thermogravimetric analysis (TGA, DTG) showing the decomposition of 3 in af 3.

(1), (b) [ArZn(�3-OH)]4 (2) and (c) [tBuZn(�3-OH)]6 (3).

based on ZnCO3 nanoparicles and a discrete alkylzinc carbonate[(tBuZn)2(�5-CO3)]6 cluster just by a slight modification of thereaction systems [15].

Mixed aggregates consisting of RZnOH and RZnOR species arealso known. Polarz and co-workers studied the reaction of H2O with[MeZn(�3-OiPr)]4 and the spectroscopic data indicated that thealkoxide group becomes cleaved prior to the Zn Me group [16].The authors postulated formation of a relatively stable intermedi-ate in which OH represents one edge of the “Zn4O4” cubane core,however the resulting product was not isolated. Similarly, relatedstudies on the direct reaction of [tBuZn(�3-OtBu)]4 (4) with H2Odid not provided well-defined products [13]. In the latter studies,however, a series of [(tBuZnOH)x(tBuZnOtBu)y] aggregates wereprepared via the reaction of 3 and 4. For example, the mixture offreshly dissolved 3 and 4 in toluene in the molar ratio 1:3 (relative toZn centres) afforded the novel mixed alkyl(alkoxy)zinc hydroxide[(tBuZn(�3-OH)(tBuZn(�3-OtBu)3] (5) (isolated yield 76%; Fig. 2a,path 1). The analogous reaction systems involving 3 and 4 inthe 1:1 and 3:1 molar ratio (Fig. 2a, path 2 and path 3, respec-tively), led to the formation of [tBuZn(�3-OH)2(tBuZn(�3-OtBu)2](6) (isolated yield 88%) and [(tBuZn(�3-OH)3(tBuZn(�3-OtBu)] (7)(isolated yield 74%), respectively. The identity of compounds 5–7was confirmed by single crystal X-ray diffraction studies [13].These mixed aggregates are distinct from the above described hex-americ zinc hydroxide 3 and possess a distorted heterocubanestructure (Fig. 2b). Formally their composition can be viewed asderivatives of (tBuZnOtBu)4 where tBuO moieties are replaced by

from the reaction system leading to 7 as the main product, amixed aggregate [(tBuZnOH)4(tBuZnOtBu)2(ZnOH)2] (8) was alsoisolated in small quantities (Fig. 2a and b). The centrosymmetric

n inert atmosphere and (b) the schematic representation of thermal transformation

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 115

F p) ando

anhidrcdg

2

hvcaiicbobCzdtscdf

type RZn(L) have only been reported in the past few years. Forexample, the reaction of a sterically demanding dimeric com-plex [(Me3Si)2CHZn(�-N(CH2py)2]2 (11b) with H2O (Scheme 4)

ig. 2. (a) Synthetic pathways leading to alkyl(alkoxy)zinc hydroxides 5, 6 and 7 (tof 5–8.

ggregate 8 formally consists of two Zn4O4 cubanes sharing a cor-er to give a distorted Zn7O8 dicubane core (this structural motifas been observed earlier for various alkylzinc alkoxides obtained

n a direct reaction of R2Zn with corresponding alcohols [17] orioxygen [18]). The corner-sharing zinc centres in 8 are octahed-ally surrounded by six hydroxyl groups and adopts Zn(�3-OH)6oordination environment. All the other zinc atoms are tetrahe-rally coordinated by three oxygen atoms (two derive from OHroups and one from the alkoxide group) and one alkyl group.

.2. Alkylzinc hydroxides supported by multifunctional ligands

This subsection shows the literature data concerning zincydroxides derived from organozinc complexes supported byarious multifunctional ligands. A number of mono- or dinu-lear zinc complexes supported by multidentate organic ligandsnd terminal or bridging hydroxide ligation have been widelynvestigated as synthetic analogues of many zinc enzymes includ-ng carbonic anhydrase (CA). However, these zinc hydroxideompounds are essentially excluded from the present contri-ution as they have been reviewed elsewhere [1]. Herein wenly discuss briefly two examples of zinc hydroxides supportedy various multifunctional ligands which were investigated asA mimics: (i) one of the most extensively studied monomericinc hydroxide complex of the type [NNN]ZnOH and (ii) theimeric zinc hydroxide. Both of them enable recently first isola-ion of bicarbonate species (an intermediate in ZnOH/CO2 reaction

ystem) [19]. The tris(3-tert-butyl-5-methylpyrazolyl)hydroboratoomplex, [TptBu,Me]ZnOH (9) (Fig. 3) [20], provided the first well-efined example of a monomeric zinc hydroxide derivative thateatured a tetrahedral {[N3]ZnOH} motif akin to that in CA.

dicubane compound 8 as a side product (bottom) and (b) the molecular structures

Compound 9 has been synthesized by (i) the direct reac-tion between Zn(ClO4)2•6H2O, K[TptBu,Me] and KOH in methanol[20], and (ii) metathesis of [TptBu,Me]ZnI with nBu4NOH [21].More recently Parkin and co-workers during their investi-gations on CA mimics employed the tetradentate tripodaltris(2-pyridylthio)methyl ligand (Tptm) and synthesized a dimericzinc hydroxide {[�3-Tptm]Zn(�2-OH)}2 (10) by the direct hydrol-ysis of [�3-Tptm]ZnX (where X = H, NSiMe3, OSiMe3) (Fig. 4a and b)[19]. Thus, the treatment of [�3-Tptm]ZnX with H2O leads to elim-ination of the terminally coordinated X ligand irrespectively of itscharacter and generated the hydroxide complex 10.

The chemistry of organozinc hydroxide complex es supportedby multidentate ligands discussed below remains an essentiallyundeveloped field. A number of well-characterized zinc com-plexes with an incorporated hydroxide group derived from theintentional or accidental hydrolysis of alkylzinc complexes of the

Scheme 4. Synthesis of [{(Me3Si)2CHZn}2(�-OH)-{�-N(CH2py)2}] (12).

116 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

Fig. 3. (a) Schematic representation and (b) the molecular structure of [TptBu,Me]ZnOH (9).

nd (b)

aNXegbor1

t[(ya6acl

F

Fig. 4. (a) Schematic representation of the synthesis a

fforded a zinc hydroxide complex [{(Me3Si)2CHZn}2(�-OH){�-(CH2py)2}] (12) (estimated yield 5%; Fig. 5) [22]. Single crystal-ray diffraction revealed that the tridentate amide ligand wasliminated in the course of the hydrolysis instead of the alkylroup. The structure contains �-OH functionality shielded by theis(trimethylsilyl)methyl substituents which effectively preventsligomerization via hydroxide bridges. In contrast, the analogouseaction involving the sterically less shielded methylzinc complex1a lead to an intractable mixture of products.

Anantharaman et al. reported the synthesis and struc-ural characterization of dimeric organozinc hydroxide(NHCZn)(MesO)(�2-OH)]2 (13) obtained in the reaction ofNHC)ZnEt2 (NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-lidene) with 2,4,6-trimethylphenol (MesOH) in the presence of

deliberately added equivalent amount of H2O (isolated yield8%) (Scheme 5) [23]. In the molecular structure of 13 each zinc

tom possesses a tetrahedral geometry and coordinate to the NHCarbene carbon, the oxygen atom of the 2,4,6-trimethylphenoxideigand, and also to the two bridging hydroxide groups. As the

ig. 5. Molecular structure of [{(Me3Si)2CHZn}2(�-OH)-{�-N(CH2py)2}] (12).

molecular structure of {[�3-Tptm]Zn(�2-OH)}2 (10).

starting reaction mixture contains a Et2Zn adduct with the car-bene, the resultant zinc hydroxide 13 is probably a product ofhydrolysis of a putative aryloxide intermediate [(MesO)2Zn[(NHC)]as the authors assumed.

Recently Lewinski and co-workers reported for the first timethe synthesis of an organozinc hydroxide via direct hydrolysis,which is an effect of elimination of an alkyl group from the par-ent alkylzinc precursor of the type RZn(L) [24]. The reaction of[tBuZn(q)] (where q-H = 8-hydroxyquinoline) with H2O in the 2:1molar ratio afforded an alkylzinc hydroxide [Zn(q)2]2[tBuZn(�3-OH)]2 (14) supported by the quinolinate ligands (isolated yield79%) (Fig. 6a, path III). The molecular structure of 14 represents acentrosymmetric dimer formally formed by the association of twodinuclear [Zn(q)2][tBuZn(�3-OH)] moieties (Fig. 6b). There are twofour-coordinate Zn atoms with the terminally Zn-bound tert-butylgroups, the CZn(Oquinolinate)2(Ohydroxy) core, and two six-coordinateZn atoms with peripheral ligation provided by four chelating quino-linate groups. The hydroxide groups act as �3-bridging ligandsbetween the tetrahedral and octahedral zinc atoms. Interest-ingly, in the reaction system involving tBu2Zn and a commercialgrade 8-hydroxyquinoline (q-H), i.e. crude, as received (Aldrich),the authors repeatedly observed the formation of a mixture ofthe expected tert-butylzinc complex stabilized by the quinolinateligand, [tBuZn(q)]3 and surprisingly an organozinc oxo cluster,[(tBu)3Zn5(�4-O)(q)5] (15) (Fig. 6a, path II; the molecular struc-ture of 15 is described in Section 3.2). Notably, the self-assemblingprocess of 14 driven by the non-covalent interactions resulted in a

microporous supramolecular architecture [24]. Adjacent moleculesof 14 are connected by �-stacking interactions with the forma-tion of an array of a 1D noncovalent polymer. The polymer chainsare close-packed generating a network of quasi-grid 2D layers. The

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 117

N

N

Me

Me

2

es

es

ZnEt2

OH

2

O

ON

N

Mes

Mes

Zn2

OH2

2

2

N

N

Mes

Me

O

es

ZnZnO

O

H

H O

NMes

13

N

N

Mes

NHCZ

2am

rrt[odowaO(o

dtORi

Scheme 5. Synthesis of [(

D sheets further pack, through van der Waals interactions, into 3D network with rectangular 1D open channels filled by THFolecules.A route to the mixed hydroxyl/hydrazide aggregates was

ecently described by Mitzel and co-workers. The authorseported the formation of mixed hydrazide hydroxide clus-ers [(MeZn)4(NHNMe2)3(�3-OH)] (isolated yield 40%) and(EtZn)4(NHNMe2)3(�3-OH)] (16; isolated yield 49%) from thene-pot reaction between Me2Zn or Et2Zn, respectively, 1,1-imethylhydrazine and H2O (Fig. 7a) [25]. The molecular structuresf [(MeZn)4(NHNMe2)3(�3-OH)] and 16 contain Zn4N6O cages,here one hydroxide ligand bridges up three Zn centres and

dopts �3 coordination mode. Formally [(MeZn)4(NHNMe2)3(�3-H)] and 16 can be regarded as elusive alkylzinc hydroxide species

vide supra, Section 2) entrapped by the parent alkylzinc derivativesf hydrazide.

By carrying out the reaction under similar conditions withiisopropylzinc as the organometallic component the cocrys-alline compound [(iPrZn)4(NHNMe2)4]·[(iPrZn)4(NHNMe2)3(�3-

H)] (17) was formed and structurally characterized [26]. Recently,edshaw et al., have extended the family of Zn4N6O type cages by

ncluding the fluorinated aryl group C6F5 at Zn atoms [27].

Fig. 6. (a) Scheme of the synthesis of various alkylzinc derivatives

OOH

n)(MesO)(�2-OH)]2 (13).

2.3. Zinc hydroxides derived from the activation of dioxygen

The dioxygen activation mediated by organozinc complexesalso led to the isolation of zinc hydroxide clusters. The reac-tion of a trinuclear ethylzinc derivative of o-hydroxybiphenyl(ArOH), [(EtZn)2Zn(OAr)4], with O2 resulted in the formation ofcolourless crystals of a aryloxide(hydroxide) octanuclear clusterZn8(OAr)8(�2-OH)6(O2) (18) with an encapsulated peroxide moi-ety, which was deposited in high yield after several days [28].The identity of 18 was confirmed using X-ray crystallography. Themolecular structure of 18 can be formally viewed as a Zn6(O2)(�2-OAr)6 unit with zinc centres bridged by aryloxide ligands andencapsulated peroxide species with an �6-(�1)3-binding mode(Fig. 8). The octahedral arrangement of zinc centres is bicappedby ArOZn(OH)3 units. In this case a decomposition of unstablezinc alkyl peroxide moieties via homolysis of the O O bond isproposed as being responsible for the formation of ZnO• rad-icals and the latter oxyl radicals are likely responsible for theobserved formation of ZnOH species. These findings are also in line

with other recent reports on the long-overlooked decompositionpathway of zinc alkyl peroxides via homolysis of the O O bond[29].

of 8-hydroxyquinoline and (b) the molecular structure of 14.

118 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

Fig. 7. (a) Synthesis of mixed hydrazide hydroxide clusters [(MeZn)4(NHNMe2)3(�3

fw(y[tolbfh

3m

fbi

SO

hydroxide complex, TpCum,MeZn OH (isolated yield 0.6%). Themolecular structure of 20 reveals a linear [Zn O Zn = 180◦] zincoxide moiety with the Zn O distance of 1.854 A (Fig. 9).

Fig. 8. (a) Molecular structure and (b) representation of core of 18.

Another interesting example of zinc hydroxide cluster derivedrom the activation of dioxygen was described by Gade and co-orkers [30]. In situ aerobic oxidation of a bisoxazoline ligand

Me2box) in the presence of Zn(ClO4)2 and a base serendipitouslyielded the tetranuclear double-bowl shaped metallocavitand{(Me2box)O}4Zn4(�3-OH)][ClO4]3 19 as shown in Scheme 4. Inhe absence of O2 only the monomeric zinc(II) complexes werebserved, but, upon exposure of the monomers to O2, 19 was iso-ated. An interesting feature of 19 is the central �3-OH found in thease of the cavity, rendering the two cavities chemically distinctrom each other. Thus, the metallamacrocycle acts as a nest-likeydroxide receptor (Scheme 6).

. Zinc oxide clusters: structural types and syntheticethods

The synthesis of well-defined ZnO-based nanomaterials of uni-orm size and morphology is a considerable challenge. Therefore aetter knowledge of the reaction and aggregation sequences lead-

ng to a stepwise growth of ZnO species starting from molecular

cheme 6. Synthesis of the tetranuclear zinc hydroxide [{(Me2box)O}4Zn4(�3-H)][ClO4]3 (19).

-OH)] and [(EtZn)4(NHNMe2)3(�3-OH) (16) and (b) molecular structure of 16.

precursors is necessary. In this regard polynuclear zinc oxo clus-ters might serve as model compounds for gaining a fundamentalknowledge of the Aufbau principles of zinc oxo(nano)clusters. Untilnow, the knowledge about the structure and chemical propertiesof zinc oxide cluster supported by organic ligands is very limited[31]. In this section, we focus on their synthetic procedures andthe solid state structures. In particular, we divided these systemsinto three types, i.e. (i) zincoxane compounds, (ii) Zn O speciesentrapped by organozinc moieties, and (iii) zinc oxide complexessupported by organic ligands. The coordination number of oxideions can vary from two to four depending on the steric demand ofthe substituents on zinc. A survey of the literature shows that sev-eral structural motifs are known: Zn2O (I), Zn3O (II, III), Zn4O (IV),Zn7O2 (V), Zn6O2 (VI) and high nuclearity zinc oxides of the typeVII–X. (Scheme 7).

3.1. Zincoxane compounds

Zincoxanes are defined as molecular or higher aggregated com-pounds consisting of a Zn O Zn backbone with pendant organicsubstituents. The simplest zincoxane compound possessing �2-O2− anion (structural motif I) was obtained by Vahrenkamp andco-worker [32]. Compound [Zn2(�2-O)(TpCum,Me)] (20) (whereTpCum,Me = hydrotris(3-p-cumenyl-5-methylpyrazolyl)borate) wasprepared by thermal condensation of the corresponding zinc

Scheme 7. Schematic representation of the structural types of oxide ions in variousmolecular Zn–O assemblies.

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 119

Fig. 9. (a) Schematic structure of hydrotris(3-p-cumenyl-5-methylpyrazolyl)borate) (TpCum,Me) and (b) the molecular structure of 20.

i([2bZw

otrycs

Me(MeOO)Zn(tBu-DAB) led to the formation of two zincoxane

Fig. 10. The molecular structure of 21.

The example of the �3-O2− structural motif II was foundn compound {[pyr2,5-Ph2]Zn(THF)2(�3-O)Zn[pyr2,5-Ph2]}2 (21)pyr2,5-Ph2 = 2,5-diphenylpyrrolyl) prepared in the reaction of(pyr2,5-Ph2)2Zn(THF)] with H2O [33]. The molecular structure of1 may formally be considered as a dimeric zincoxane stabilizedy organic ligands (Fig. 10). The Zn O distances within the centraln2(�-O)2 core amounts 1.909 A and are elongated in comparisonith the terminal Zn O distance that equals 1.865 A.

Håkansson and co-workers described the synthesis of anctanuclear aggregate [Zn8(Et)8O4(py)8]·2py (22) containing aetrameric Zn O Zn backbone. Compound 22 was prepared in theeaction of Et2Zn in the presence of pyridine (py) and H2O (isolated

ield 65%) [4b]. The molecular structure of 22 consists of a hetero-ubane [Zn(�4-O)]4 core with four terminally bonded EtZn unitsupported by two pyridine ligands (Fig. 11).

Fig. 11. (a) Schematic structure and

Scheme 8. Stepwise transformations of Me(MeOO)Zn(tBu-DAB) complex into zin-coxane clusters 23 and 24.

A detailed crystal structure analysis revealed that the origi-nal shape of single molecules of 22 and specific intermolecularinteractions lead to the formation of an interesting supramoleculararchitecture with channels containing pyridine molecules orderedalong helices of the same handedness (Fig. 12).

Another interesting zincoxane type complexes were formedthrough the controlled oxygenation of a Me2Zn/tBu-DAB(DAB = 1,4-diazobutadiene) system as shown in Scheme 8[34]. The stepwise transformations of resulting intermediate

clusters 23 and 24 (Scheme 8). The molecular structure of 23consists of a discrete asymmetric hexanuclear unit, which maybe formally viewed as being formed by the association of two

(b) molecular structure of 22.

120 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

Fa

dThszpI2cflftm

3

atoipttc

tar[

ig. 12. Channels along the c axis in 22 are filled with pyridine molecules which arerranged in a helical manner, as inicated by the arrows.

imethylzincoxanes (MeZnOZnMe) and two MeZnOOMe moieties.he basic skeletal arrangement of 23 consists of a central Zn4O4eterocubane motif with the two terminally bonded MeZn unitsupported by tBu-DAB ligands. In this case the formation of theincoxane species was rationalized by initial homolysis of the O Oeroxide bond of a part of the methylperoxide groups at 20 ◦C.

nterestingly, further transformation of 23 to a bis(heterocubane)4 occurred at 0 ◦C (yield 82%). The molecular structure of 24onsists of two corner-sharing distorted oxo-methoxide cubanesanked by two MeZn units chelated by tBu-DAB. Thus, 24 may

ormally be considered as a bis(heterocubane) formed fromhe cubane 23 with concomitant classical decomposition of the

ethylperoxide groups to methoxide groups [17,18].

.2. Zinc oxide species entrapped by organozinc moieties

The encapsulation of ZnO species into organozinc moieties isn intriguing phenomenon. Overall, the isolation and full charac-erization of this type of products derived from the hydrolysis ofrganozinc complexes has proved very rare. As was demonstratedn Section 2.2, the controlled hydrolysis of the RZn(L)-type com-lexes is not a trivial issue as both the zinc bonded alkyl group andhe L ligand can be cleaved. Reaction systems to be described inhis section further substantiate that the latter issue is even morehallenging.

Until now one tetranuclear, two pentanuclear and one hep-

anuclear clusters bearing a monomeric ZnO unit templated bylkylzinc precursors have been reported. Lang and co-workerseported on the synthesis of a unique teranuclear aggregateZn4(�4-O)(dmpzm)3Et3] (25) from the reaction of H2O with the

Fig. 13. (a) Scheme of the synthesis of 25

Fig. 14. (a) Schematic synthesis of 26, and (b) and (c) the molecular structure of[EtZn(hpp)]4·ZnO 26 and [(tBu)3Zn5(�4-O)(q)5] (15), respectively.

Lewis acid–base adduct [Et2Zn(dmpzm)] [where dmpzm = bis(3,5-dimethylpyrazolyl)methane] (Fig. 13a) (isolated yield 20%) [35].The product can be viewed as a 3:1 adduct between theorganometallic precursor [EtZn(dmpzm)] and a pre-organized ZnOunit. The molecular structure of 25 consists of a Zn4(�4-O) tetra-hedron in which three Zn atoms are further saturated by two Natoms from dmpzm ligands and an ethyl group while one Zn centreis coordinated by three C atoms from the methylene group of thethree dmpzm ligands (Fig. 13b).

A novel pentanuclear oxo cluster [EtZn(hpp)]4·ZnO (26) wasobtained in a two step reaction system.[36] The reactions of Et2Znwith a bicyclic guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidine (hppH) results in the formation of [EtZn(hpp)]3that transforms further via exposure to undried air into pentanu-clear [EtZn(hpp)]4·ZnO (26) (isolated yield 70%). In this case theproduct can be viewed as a 4:1 adduct between the organometal-lic precursor EtZn(hpp) and a pre-organized ZnO unit (Fig. 14). Thecaptured oxide dianion in 26 resides at the centre of a distortedtetrahedron of Zn centres with Zn O distances ranging from 3.427to 1.884 A. The shortest interaction between the oxide ion and oneof the five Zn centres (otherwise bonded only to guanidinate lig-ands), constitutes the remainder of an encapsulated fragment ofthe in situ formed ZnO. The remaining zinc centres each retainone Zn C(Et) interaction, indicating the stability of these bonds in[EtZn(hpp)]3 under limited air exposure.

Similarly, the composition of the pentanuclear oxo cluster[(tBu)3Zn5(�4-O)(q)5] (15) (isolated yield 41%), for which the syn-thetic details are described in Section 2.2, may be viewed as a clusterin which ZnO molecule is entrapped by three parent tBuZn(q)

and (b) molecular structure of 25.

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 121

mcwiZc

wt[Ttaaona

eeEtwccRfa

i(ao

scopic methods [41]. Compound 33 was also studied as a molecular

Fig. 15. The molecular structure of 27.

oieties and one bischelate Zn(q)2 moiety (Fig. 14c) [15]. Theentral core of 15 consists of a tetrahedral [Zn4(�4-O)]6+ coreith the bond distances between the zinc and �4-O atoms falling

n a narrow range of 1.940–1.976 A (mean Zn O = 1.955 A, meann O Zn = 109.08◦). In this case all five zinc centres have a differentoordination environment.

During the investigations on zincophosphonates Roesky and co-orkers reacted tert-butylphosphonic acid in THF with ZnEt2 in

oluene which resulted in formation of a dodecanuclear aggregateZn2(thf)2(EtZn)6Zn4(�4-O)(tBuPO3)8] (27) (isolated yield 27%).he molecular structure of 27 consists of a [Zn4(�4-O)]6+ core andhe alkylzincophosphonate shell (Fig. 15) [37]. According to theuthors view, the presence of the �4-O atom cannot be attributed todventitious moisture because of the relatively high yield of 27. Itsrigin can likely be ascribed to the induced dehydration of phospho-ic acid by ZnEt2, which resulted in the formation of the phosphonicnhydride (tBuPO2)3.

Interesting clusters that formally consist of a [(ZnO)2] unitntrapped by the parent RZn(L) moieties were described by O’Brient al. [38]. Compounds Me4Zn6O2[NMe(CH2)3NMe2]4 (28) andt4Zn6O2[NMe(CH2)2NMe2]4 (29) were obtained from the reac-ions of Me2Zn with N,N,N-trimethylpropylenediamine and Et2Znith N,N,N-trimethylethylenediamine, respectively, in toluene

ontaining traces of water. The molecular structures of 28 and 29onsist of the central Zn2(�4-O)2 core encapsulated by the parentZn[NMe(CH2)3NMe2] moieties (Fig. 16). The Zn O bond lengths

all in a narrow range of 1.99–2.05 A and the Zn N bond distancesre between 1.99 and 2.22 A.

Another intriguing cluster based on fused double oxoz-nc core [Zn(�4-O)2(ZnEt)6(�3-OAc)4(�3

′-OAc)2{Zn(dmpzm)Et}2]

30) was obtained in the system involving [Et2Zn(dmpzm)], waternd acetic acid (isolated yield 17%) [35]. The molecular structuref this nonanuclear zinc oxocarboxylate cluster consist of two

Fig. 16. (a) Schematic representation and (b) the molecu

Fig. 17. The molecular structure of [Zn(�4-O)2(ZnEt)6(�3-OAc)4(�3′-

OAc)2{Zn(dmpzm)Et}2] (30).

pseudo-tetrahedral Zn(�4-O)(ZnEt)3(�3-OAc)2 units joined at acommon Zn vertex connected with two [Zn(dmpzm)Et]+ fragmentsvia a pair of �3

′-OAc ligands (Fig. 17).Steiner and co-workers investigated the reactions involving

aminophosphazenes, water and Et2Zn in various stoichiometricratios leading to encapsulation of ZnO species [39]. It was shownthat tris(ethylzinc) phosphazenate complexes can act as templatesfor trimeric (31) and hexameric (32) ZnO species through thebowl-shaped coordination surface of three Lewis basic N functionsand three Lewis acidic Zn centres (Fig. 18) [39a]. In particular,the resulting complexes consist of a planar {(ZnO)3} ring (31)and hexagonal {(ZnO)6} prism (32) stabilized by two bowl-shaped(EtZn)3(RNH)6P3N3 templating segments. The selection of eitherthe ring or prism assembly is largely determined by the stericdemand of the ligand substituents.

Despite the above developments, the reactivity of organozinccompounds towards water remains to be explored. In particular,the potential of such reactions so that to in terms of accessing zincoxide clusters of potential utility certainly deserves to be exploited.

3.3. Zinc oxide complexes supported by organic ligands

The most representative group of molecular zinc oxide com-pounds are aggregates consisting of [Zn4(�4-O)]6+ core (type IV).The oxygen ion in these complexes is located at the centre oftetrahedron formed by four Zn centres. Among this family of zincoxides, oxozinc clusters supported by organic carboxylate ligandshave been particularly intensely investigated. The first exampleof structurally characterized molecular zinc oxocarboxylate clus-ter was Zn4(�4-O)(O2CMe)6 (33) that consists of a tetrahedral[Zn4(�4-O)]6+ core with six acetate ligands bridging the zinc ionsin a �-1,2 fashion (Fig. 19a). This compound has been thoroughlyinvestigated by single-crystal X-ray diffraction [40] and spectro-

model for ZnO because of its luminescent properties [42]. Molec-ular zinc oxide complexes supported by organic ligands havebeen usually obtained serendipitously from the reaction systems

lar structure of Me4Zn6O2[NMe(CH2)3NMe2]4 (28).

122 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

of 31

iotw

ld[aao(yic

bfb

Fc

Fig. 18. (a) The synthesis pathways of the synthesis

nvolving moist solvents [43]. Moreover, a series of isostructuralxozinc clusters [Zn4(�4-O)(O2CR)6] have been synthesized byhe solvothermal reaction between ZnO, monocarboxylic acid andater [44].

The alternative atom-efficient and mild synthetic strategyeading to well-defined oxozinc carboxylate complexes from well-efined alkylzinc precursors and H2O was reported very recently4a]. This two-step procedure involving 2 equiv. of Et2Zn in THFnd 3 equiv. of the monocarboxylic acid and followed by theddition of 0.5 equiv. of H2O yielded a series of isostructuralxozinc clusters [Zn4(�4-O)(O2CR)6] (where R = Ph (34), p-PhC6H435), p-MeC6H4 (36), p-MeSC6H4 (37)) in an almost quantitativeield (Scheme 9). Compound [Zn4(�4-O)(O2CPh)6] (34) was alsondependently synthesized through oxygenation of the alkylzincarboxylate [EtZn(O2CPh)]6 with a dry dioxygen [29a].

A variety of structurally analogous oxozinc clusters supportedy different organic ligands, like carbamates [45], amides [46],ormamidinate [47], and phosphonates [48], ligands have alsoeen reported. For example, Wheatley et al. reported on the

ig. 19. The molecular structures of the selected zinc oxide complexes based on [Zn4(arboxylates (33) [40], (b) carbamates (39) [45b], (c) phosphonates [48a] and (d) formam

and 32; the molecular structure of 31 (b) and 32 (c).

isolation and structural characterization of a oxo-encapsulatedproduct Zn4[(�4-O){(2-C5H4N)-NBz}6] (Bz = benzyl) (38) fromthe reaction of O2 with mixed lithium-zinc system containing2-pyridylamide ligand, however the origin of a diversity of oxo-capture modes and the nature of the reactive intermediates hasessentially remained unclear [46]. Caudle and co-workers reportedthe synthesis, structure, and reactivity of a series of oxozinc carba-mates, i.e. [Zn4(�4-O)(O2CNpy)6] (Npy = pyrrolidine) (39) (Fig. 19b)[45b]. These complexes can be considered as analogues of the basiczinc carboxylate structure. It was also demonstrated that the car-bamate ligands in these complexes are readily interconverted fromone to another simply by reacting with the appropriate differentsecondary amine.

An interesting example of oxo zinc complexes with the [Zn4(�4-O)] core was recently described by Lewinski et al. The reaction of

[EtZn(Pyr-pyrl)] [where Pyr-pyrl 2,2′-(1′-pyrrolinyl)-pyrrol] witha dry dioxygen resulted in the formation of [Zn4(�4-O)(Pyr-pyrl)6] (40) (Fig. 20) [29b]. The molecular structure of 40 consistsof discrete monomeric oxo centred tetranuclear units with the

�4-O)]6+ core stabilized by the representatives of the following ligand types: (a)idinates [47].

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 123

Scheme 9. An atom-efficient synthetic method of oxozinc carboxylates.

FO

zcdkzOm

rbhtNdas

ig. 20. (a) Schematic representation and (b) the molecular structure of [Zn4(�4-)(Pyr-pyrl)6] (40).

inc centres paired up by the �-1,2-N,N-bridges and additionallyhelated by Pyr-pyrl ligand (Fig. 20). The observed diversity of coor-ination modes of the N,N-bifunctional ligand is striking as, to ournowledge, for all previously reported examples of the tetranuclearinc oxo-encapsulated clusters the metal centres within [Zn4(�4-)] units were connected by bifunctional ligands of �-1,2-bindingode.The [Zn4(�4-O)]6+ species can also be incorporated into the inte-

ior of organic macrocycles [49]. The reaction of conjugated Schiffase macrocycles (Fig. 21) with zinc acetate resulted in a series ofeptazinc clusters. Single-crystal X-ray diffraction studies revealedhat the complexes consist of a trimetalated macrocycle with each

2O2 pocket coordinated to a zinc ion and a distorted tetrahe-ral [Zn4O]6+ cluster fastened to the macrocycle through bridgingcetate ligands. A representative example of this type of cluster ishown in Fig. 21 (41) [49b].

Fig. 21. The molecular structure of conj

Fig. 22. The molecular structure of [Zn7O2(OAc)10(1-Meim)2] (42).

There are few compounds with the formula{[Zn7(�4-O)2(O2CR)10L2]}comprising of heptanuclear [Zn7(�4-O)2]10+

core in which two tetrahedral Zn4(�4-O) units joined at a com-mon Zn vertex. The central core is supported by ten carboxylategroups and two additional terminal monodentate ligands [50].For example, Manessi-Zoupa and co-workers reported on thesynthesis of a heptanuclear complex [Zn7O2(OAc)10(1-Meim)2](42) (1-Meim = 1-methylimidazole) from the reaction betweenZn(OAc)2·2H2O and 1-Meim in the 2:1 molar ratio (Fig. 22)[50d].

Another coordination mode of oxozinc core was revealedin compound [Zn3(�3-O)(H2bpc)(bpc)2] (43) (H2bpc = N,N′-bis(5-ethyl-1,3,4-thiadiazol-2-yl)-2,6-pyridinedicarboxamide) that has a

4+

cage-like structure with the [Zn3(�3-O)] core and two deproto-nated bdc2− ligands as well as one H2bdc (Fig. 23). In the molecularstructure of 43 each Zn atom has a distorted octahedral geometrywith a �3-O atom and five nitrogen atoms.[51]

ugated Schiff base macrocycle 41.

124 D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126

ecular structure of 43.

4pZ

amtab5taaoGpwthbctoTyc

hrmM[a

Ft

molecule by intermolecular hydrogen bonds forming the secondcoordination shell.

Moreover, the reaction system Et2Zn/Ant-H/H2O (Ant-H = 9-anthracenecarboxylic acid) afforded a direct picture of the

Fig. 23. The mol

. Oxozinc carboxylates as the predesigned molecularlatform for modelling the reactivity of prototypicaln-MOFs towards H2O and donor solvents

The effect of moisture on the stability of Zn-MOFs is importantnd extensively explored issue [52]. The disruptive effect of H2Oolecules on MOF structures was mentioned by Yaghi et al. in

heir early reports on the synthesis of prototypical MOFs [53,54]nd this observation was examined experimentally in more detaily others [55,56]. The molecular dynamics simulation on the MOF-/H2O system suggested that the coordination of H2O molecules tohe Zn4O core leads to cleavage of one Zn Ocarboxylate bond initi-ting the water-induced disruption mechanism. The metal-oxidecetate cluster Zn4O(O2CCH3)6 was used as model of the metalxide clusters in MOF-5 for quantum mechanical calculations [57].eometries of predicted product and transition state for ligand dis-lacement reaction are shown in Fig. 24. The authors noted thatater molecules in the reactant complexes tend to have their pro-

ons directed at the oxygen of the cluster complex, which formydrogen bonds with the complex. Hydrogen bonding appears toe an important part of the interaction between water in the secondoordination shell and the metal oxide cluster. The carboxylate inhe transition state has changed from �2 to �1 coordination and haspened up a coordination site on the metal for the water molecule.he water molecule ends up bridging the metal and the carbox-late with a proton pointing at the displaced oxygen atom of thearboxylate and the oxygen coordinated to the metal.

The above presented calculations for the MOF-5/H2O systemave appeared not consistent with an experimental top-downelationship involving oxozinc carboxylates as the predesigned

olecular platform for modelling the reactivity of prototypical Zn-OFs towards H2O and donor solvent have been recently reported

58]. In the equimolar reaction involving [Zn4(�4-O)(O2CPh)6]nd water from the post-reaction mixture colourless crystals of a

ig. 24. Ligand displacement reaction for MOF-5. Structures represent reactant,ransition state, and hydrolysis product.

Scheme 10. The revealed coordination mode of the oxo-zinc core with H2O and THFmolecules.

cluster of the formula [Zn4(�4-O)(O2CPh)6(H2O)(THF)]2(THF) (44)was isolated (Scheme 10 and Fig. 25).

Single crystal X-ray diffraction study of 44 revealed the pres-ence of an oxo-centerd tetranuclear unit in which three Zn centresretain a tetrahedral environment and the fourth one adopts dis-torted octahedral coordination geometry by coordinating one H2Oand THF molecule. The zinc oxo-cluster is additionally solvatedby two THF molecules that are linked to the coordinated water

Fig. 25. The molecular structure of 44.

D. Prochowicz et al. / Coordination Chemistry Reviews 270–271 (2014) 112–126 125

zinc c

ipa1tootTcdcbsfil

5

tfirthouvicybFocpoTyi

A

Cs

[

[

[

[

[

[

[

[[

[[

Fig. 26. (a) Schematic representation of coordination modes of oxo-

nteraction of a donor solvent with a prototypical Zn-MOF. Com-ound [Zn4(�4-O)(O2CAnt)6(�1-OTHF)2(�2-OTHF)] (45) consists of

discrete, monomeric oxo-centred tetranuclear unit with six �-,2-carboxylate bridges and three THF molecules coordinated towo Zn atoms (Fig. 26). Strikingly, two THF molecules act as mon-dentate ligands and the third one involves two electron lone pairsf the oxygen atom in the formation of a �2-O bridge betweenwo zinc centres forming a non-planar four-membered Zn2O2 ring.hese results demonstrated that the basic prototypical Zn4O corean coordinate H2O or other donor molecules with changing coor-ination geometry around zinc centres but preserving the oxozincore essentially unchanged. Such fundamental studies are vital for aetter understanding of how water can affect the prototypical MOFtructure and the revealed propensity of oxozinc nodes towards theormation of covalent as well as non-covalent coordination spheresndicates that Zn-MOF nodes are prone to act as cooperative cata-ysts.

. Conclusions

We have presented an overview of the preparation and struc-ural variety of zinc hydroxide and zinc oxide compounds derivedrom organozinc reagents. The provided literature survey clearlyndicates that the interaction of organozinc compounds with wateremains an area essentially little explored thus far and, likewise,he great potential of such reactions for synthesizing useful zincydroxides and oxides remains undeveloped. A significant numberf zinc hydroxides and oxides have been obtained serendipitously,sually in low yields, from the reaction systems involving moist sol-ents or adventitious hydrolysis or oxygenation. Systematic studiesn this field are essentially lacking and only a handful of the reportedompounds have been isolated in high yield. The controlled hydrol-sis of the RZn(L)-type complexes is especially not a trivial issue asoth the zinc bonded alkyl group and the L ligand can be cleaved.urthermore, both the high propensity of zinc hydroxides andxides to form a variety of structures make described families ofompounds very attractive candidates for materials chemistry. Inarticular, the potential of such reactions in terms of accessing zincxide clusters of potential utility certainly deserves to be exploited.o conclude, it is still a considerable challenge to bring the hydrol-sis reactions under control to favour the more rapid design andmplementation of organozincs/H2O-based reaction systems.

cknowledgement

The authors would like to acknowledge the National Scienceentre (Grant Maestro DEC-2012/04/A/ST5/00595) for financialupport.

[[

ore in 45 with THF molecules and (b) the molecular structure of 45.

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