New perspectives in the tailoring of hetero (bi- and tri-) metallic alkoxide derivatives

Poh'hedronVol. 17, No. 5 6, pp. 1005 1034, 1998

~ Pergamon PII : S0277-5387(97)00272-6

~! 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0277 5387/98 $19.00+0.00

New perspectives in the tailoring of hetero (biand tri-) metallic alkoxide derivativest Michael Veith,* Sanjay Mathur* and Charu Mathur Institute of Inorganic Chemistry, University of Saarland, P.O. 151150, D-66041 Saarbrticken, Germany

(Received 22 April 1997)

Abstract--Metal alkoxides containing more than one sort of metallic element are predominantly known in the field of heterobimetallic derivatives. In order to pass to heterotrimetallic homoleptic alkoxides bimetallic alkoxides have to be prepared which are at the same time heterobimetallic and heteroleptic. They serve as starting materials in salt elimination reactions leading to heterotrimetallic homoleptic alkoxides. The structural entity serving as a building block in these reactions must be kinetically stable in order to survive in the synthetic procedures. Besides heterotermetallic alkoxides, heterobimetallic alkoxides with halides and cyclopentadienyl as ancillary ligands and transition metal complexes of new mixed-metal alkoxy derivatives are presented. Besides the syntheses and the structural characterization reactivity, spectral aspects in view of dynamic or static structures and applications of hetero(bi- and tri-)metallic alkoxides are discussed. © 1997 Elsevier Science Ltd. All rights reserved.

Keywords: heterobimetallic alkoxides; heterotrimetallic alkoxides; heteroleptic alkoxides; transition metal complexes of heterometal alkoxides.

1. INTRODUCTION Within the realm of M-O C derivatives, metal alkoxide chemistry has seen an impressive growth over the last decade. The importance of metallo-organic derivatives as versatile precursors to ultra-pure oxideceramics [1] in conjunction with the fact that most of the advanced ceramics are multimetallic in nature e.g., YBa2Cu3Oy_a [2], HgBa2CuO4 [3], Bi2Sr2CaCu2Os [4] has led to the present interest in the tailoring of heterometal alkoxides. The chemical synthesis routes provide better control over purity, composition, homogeneity and microstructure which are crucial parameters for high performance materials. In addition, the precursors used are generally readily purifiable (via crystallization/distillation) and the possibility of variable atomic ratios in the precursors allow for novel compositions not easily obtainable, by other conventional routes. The versatility of chemical routes largely depends on the availability ofheterometal molecules as precursors and in view of the metal corn-

t Dedicated to Prof. D. C. Bradleyfor his pioneering work on metal alkoxides and on the occasion of his 73rd birthday. * Authors to whom correspondence should be addressed.

binations being immense, a large number of new heterometal alkoxides and allied derivatives with novel structural and bonding features have been characterized in the recent years [5]. However, the formation of a thermodynamically favoured heterometal alkoxide assembly is subject to various factors such as the atomic size of the metallic elements and steric bulk of the alkoxo ligands and the synthesis of a heterometal precursor with targeted ratio of metallic elements cannot be achieved by simply mixing the two or more components in required molar ratios. The metal stoichiometry in the isolated product is thus often inappropriate for the material science requirements. The appropriate metal ratio is of prime consideration in assessing the feasibility of heterometal alkoxides as precursors to useful multi-component metal oxides. Moreover, the growing realization that a "single-source' precursor with different metals assembled in one molecule in the desired metal ratios (for instance 'YBa2Cu3(OR)I 3' for YBa,Cu307 ~) can lead to morphologically pure samples of multi-component ceramics, constitutes one of the continuing synthetic challenges in heterometal alkoxide chemistry. Of central importance to this theme are the developments of new synthetic principles (e.g., the use of novel alkoxometallate units or metallic elements in

1005

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M. Veith et al.

unusual oxidation states) which will allow the rational synthesis of stable heterometallic systems with desired metal combinations. It is relevant to mention that the existence of heterotermetallic alkoxides as discrete molecular species in the solution and solid state has been recently established [6], employing coordinatively unsaturated species {M'(OPr%} (M = Ba, Sr, Ca) towards well-characterised halide heterobimetallic alkoxides, ICd{M2(OPr%} (M = Sn, Ti, Zr, Hf). A comprehensive discussion of the various aspects of metal alkoxide chemistry has been made in some recent reviews [5]. In view of the above and the limited scope of the present article, this review is not intended to be an all-inclusive treatment of heterometal alkoxide chemistry and we confine to the research carried out in our laboratories on heterometal alkoxide derivatives, however, a relevant reference is made to other recent examples not covered in earlier reviews. Owing to the pronounced tendency of heterometal alkoxides to undergo redistribution reactions, dynamic rearrangement of the ligands around the metallic centres and rapid site exchange among alkoxy groups, the spectroscopic pattern of heterometal systems are often deceptive and present an over-simplified picture of metal stoichiometry and molecular symmetry [5b]. In view of the above, special emphasis is given in this article, to the heterometal alkoxide derivatives whose identity has been established by Xray crystallography. We outline in this review our recent research on three new classes of heterometal alkoxide derivatives namely (i) heterotermetallic alkoxides (ii) heterobimetallic alkoxides with halide(s) and o,clopentadienyl as ancillary ligand(s) and (iii) the transition metal complexes of new mixed-metal alkoxy derivatives. The subject matter entails the synthesis, structure, reactivity, spectral aspects and applications of the hetero (bi- and tri-) metallic alkoxides. 2. HETEROBIMETALLIC ALKOXIDES

2.1 Homoleptic heterobimetallic alkoxides The reactions involving component alkoxides (Lewis acid base interactions) or metathesis of all the metal halogen bonds in metal halides by alkali metal alkoxometallates (salt elimination reactions) represent the most general synthetic routes to heterobimetallic alkoxy derivatives based on only one type of alkoxy ligands (homoleptic).

the equations illustrated below. Some of the reactions (eqs 2, 4, 6 and 7) do not represent pure Lewis acid base interactions as the product formation involves addition or release of neutral molecules (alcohol, THF, dioxane), however, in view of their similar formation (donor-acceptor interaction of two component alkoxides), they are included in the following discussion. M'OBu~+M(OBu')2 ~ 1/n{M'M(OBu')~}. (1) M = Ge, Sn, Pb M' = Li, Na, K, Rb, Cs KOPri + AI(OPrl) 3

2PrlOH

1/n {KAI(OPr94 (PffOH) 2},,

(2)

M'OBu'+M(OBu93 ~ 1/n{M'M(OBut)4}~ (3) M = S b ; M ' = K, Rb, Cs M = Bi;M' = K C4H8O 2

2KOBu' + Sb{OBu') 3 K2Sb{OBu')5 "(C4H8O 2

(4)

M ' O P f +Ti(OPr')4 ~ 1/n{MTi(OPr%},,

(5)

M' = Li, Na, K KOPri + 2Ti(OPri)4

2THF

} (THF)2KTi2 (OPr99 {6)

2LiOPr~+ [Hf(OPr94 • PfOH]2 --* {LiHf(OPr% }2 +2Pr/OH

(7)

KOBu'+M(OBu')4 --+ I/n{KM(OBu')5},

(8)

M = Sn, Zr LiOEt+Nb(OEt)5 ~ 1/n{LiNb(OEt)6},,

(9)

In analogy to alkali metal alkoxometallates, a large number of molecular heterobimetallic alkoxides of various di-, tri-, tetra- and penta-valent metallic elements have been obtained following the Lewis acid-base principle. Some typical examples are shown below (eqs 10-15) [18 22]. 2/m[M(OBu')2]m + 1/n[M'(OBu92],

2.1.1 Synthesis (0 Lewis acid-base reactions. The ability of metal alkoxides (M(OR)x) to form mixed-metal species MM'(OR)x+~. is well-known [7] and is exemplified among the alkali metal derivatives of a large number of metal alkoxides (eqs 1-9) [8-17]. The oligomeric nature of the constituent alkoxides is not considered in

M'{M(OBu93 }2 M = G e , Sn

m=2;

M=Pb

(10)

m=2

M' = Mg, Ca, Sr, Ba 6/n[Pb(OPri)2]n + 6/n[Zr(OPr94]n ~ Pb4Zr2 (OPr916 +Pb2Zr4(OPri)2o

(11)

Hetero (bi- and tri-) metallic alkoxide derivatives

4/n[Pb(OPrl)2]. + 2/n[Zr(OPr')4]. -~ Pb4Zr2 (OPf) ~6

[6M'X2 +4NaeM2 (OBu')6

(112)

{Ti(OPr% }Ba{Ti2 (OPr~)9 }

(13)

A1.Hf(OPff) l0 + P r i O H

(14)

M=Pb

M'C12 +Na2M2(OBu')6 + M'M2(OBut)6 +2NaCI (18) M ' = M g , Cr, Mn, Zn;

(15)

The heterometal alkoxides based on Lewis acid base reactions generally involve the reaction of an alkoxide of molecular nature with unsaturated metal alkoxides [M(OR)x]n [5@ However, mixing two component metal alkoxides do not always result in a heterobimetallic compound and this synthetic strategy is found to be inoperative in many cases, for instance. K. Rb and Cs tert-butoxides react with Sn(OBut)4 to form compounds MSn(OBu% (M = K. Rb, Cs), however, no heterometal derivatives are formed with lithium and sodium tert-butoxides [15]. Similarly, attempts made to use As(OBu')3 as a Lewis acid in the reaction with alkali metal tert-butoxides MOBu' (M = LL Na, K, Rb. Cs) failed even at elevated temperatures and after long reaction times [10]. Furthermore, no heterometal derivative results in the reaction involving Sn" and Ge" tert-butoxides whereas, Sn2(OBu'h and Pb3(OBut)6 react in appropriate molar ratio to yield PbSn2(OBut)6, which has been characterised in the solid state [18b]. Although depolymerization of [Pb(OPffh]. takes place in the presence of [Zr(OPf)4],, to yield Pb-Zr mixed metal alkoxides, no mixed-metal species is formed when Ti(OPri)4 is used [19]. [Sr(OBu')2]. undergo a facile reaction with Sn2(OBut)4 to form SrSn2(OBut)6 while [Ca(OBu')2]. do not react even under drastic conditions; this discriminating reactivity of calcium and strontium tert-butoxides toward Sn2(OBut)4 is shown to be selective for the quantitative separation of Sr and Ca [18]. (ii) Salt elimination reactions. Substitution of all halide ligands in a metal halide by anionic alkoxometallates is another synthetic pathway for the construction of heterometal alkoxide molecules (eqs I G 20) [5a.c,d,23-25]. The alkali metal derivatives (eqs 1-9) described in the preceding sub-section are interesting reagents to transfer metallo-ligands of the type {M"(OR)3} {Mm(ORh} , {M~V(OR)s~ [MY(OR)6}- and {M~(OR)9}- to an electropositive metal centre and have been used as building blocks in the construction of a variety of heterometal alkoxides. 2M'Br+Na2M2(OBu')6 ~ 2M'M(OBu')3 + 2 N a B r (16) M = G e , Sn, Pb;

M=Sn

Pr~OH

BaNb2(OPri)t2(priOH)2

M ' = I n , TI

(17)

M=Ge

M ' = M g , Cr, Mn, Co, Ni; M'=Co;

l/2[Al(OPri)3]4 + 1/2[Hf(OPf)4" PriOH]2 -*

1/n[Ba(OPri)2]. + Nb2(OPr")lo

--* 3M2Me(OBu')8 +8NaX] M ' = C o , Ni:

l/n[Ba(OPr')2]. + 3Ti(OPr')~

1007

M ' = M n , Zn;

M=Ge

M=Pb

ErC13 + 3/n[KAI(OPr')4],, ~ Er{Al(OPri)4} 3 + 3KCI (19) MCI_, + 2/n[KAI(OR)4],, ~ M{AI(OR)4}2 + 2KCI

(2o) M = Be, Mg, Zn, Cd, Hg, Mn, Fe, Co, Ni, Cu ; OR = OPf, OBu' The transmetallation reactions using large, soft and polarizable metal cations (e.g., T1 +, Ba -~+) represent another means of inducing salt-elimination reactions as shown in the following examples (eqs 21-23) [5a,26]. T1Sn(OBu')3 + InBr ~ InSn(OBu')3 +TIBr (21) ZnCI2 + T12Sn(OEt)6 ~ (EtO)ZnSn(OEt)5 + 2TICI (22) 2Ba{OCH(CF3)2}2 +CuC12 ---+BaCu{OCH(CF3)2} 4 + BaC12

(23)

Although anion-exchange reactions between a mixed metal alkoxide MM'(OR)x (M = an alkali metal) and a metal halide (M'X,,) appears a possible synthetic route to build heteronuclear species, the transfer of {M'(ORL}- moiety does not always take place as desired and in a number of cases, no reaction occurs when an alkali metal alkoxide is treated with a metal halide (eqs 24-26) [10,20]. CuCl 2 +2/n[KTi(OPr%]. ~ Cu(OPri)2 ÷2Ti(OPri)4 +2KC1

(24)

CaC12 + 2/n[KSb(OBu')4]. ~ Ca(OBu~)2 +2Sb(OBu')3 +2KC1

(25)

CuI + 1/n[KSb(OBu')4]. ~ l/4[Cu(OBu')]4 + Sb(OBu')3 + KI

(26)

(iii) Mis'cellaneous methods. Other common preparative routes to heterobimetallic alkoxides involve

1008

M. Veith et al.

accessible heterometal alkoxo clusters (Table 1). The bridging propensity of the alkoxide groups and the strength of alkoxy bridges between similar as well as different metal atoms generally preclude the coordination expansion of metallic constituents by any alternative mechanism, for instance, adding up solvent molecules (containing a donor atom) as donor ligands. In view of the above, the number of doubly Mg + 2AI + 10PrgOH (lt2-) and triply (#~-) bridging alkoxide groups are (27) important in determining the nuclearity of the result(P¢OH)2MgA12 (OPr98 ÷4H2 ing complex and their presence can result in different frameworks e.g., the potassium penta-tert-butoxy 4PriOH 2Ba+ 2[Zr(OPrl)4" PriOH]2 stannate, [KSn(OBu%] [15] devoid of alkoxy groups in #3-configuration is a one-dimensionalpolymer (Fig. Ba2Zr4(OPf)20+2H2 (28) 9) whereas, zirconium compound with an identical formulation [KZr(OBu%] [16] but containing a triply M + 4Ti(OEt) 4 2EtO., M{Ti2(OEt)9}2 +H2 (29) bridging alkoxy group is observed as a dimer in the solid state (Fig. 10). The effect of the size of metal M = Ba, Ca atoms is evident in the solid state structures of 6Pr~OH (DME)KZr2(OPr% [31] and (PfOH)LiZr2(OPr% 6Pb[N(SiMe3)2]2 + 3[Zr(OPri)4 "PfOH]2 [29]; whereas potassium derivative shows a closed triangular (KZr2) structure, the smaller lithium atom Pb2 Zr4 (OPt i) 20 -~-Pb4Zr2 (OPrl) j 6 (Li + = 0.70 A ; K + = 1.33 A) fails to adopt a closed + 12HN(SiMe3)2 (30) MM2 structural motif and displays an open structure. A remarkable variety of structures [5,8,18,34] is 2CuX+ [Zr(OPr94 • PriOH]2 observed among the tris-(tert-butoxo)stannate (-gerCuzZr2 (OP¢)10 + 2XH (31) manate or plumbate) derivatives of the alkali (eq. 1) and alkaline earth (eq. 10) metals. Whereas the X = mesityl stannates of lithium and sodium form cage-like molecular structures (I), the similar derivatives of heavier KH + [Zr(OPff)4 • Pr~OH] z --* KZr2 (OPr% K, Rb and Cs are polymeric (II) (see Fig. 1). The Li + PriOH + H 2 (32) and Na compounds are dimers of formula M:Sn2(#3OBff)2(#2-OBff)4 (I) with no terminal ligands on Y[N(SiMe3)2]3 + 2Ba[N(SiMe3)2]2 + 8Bu'OH either of the metallic elements and pyramidal tin(II) presumably possesses a stereochemically active elecYBa2 (OBu') 7(BffOH) + 7HN(SiMe3)2 (33) tron lone pair. The polycyclic Sn_,O6M2 (M = Li, Na) In an interesting synthetic approach, heterometallic core consists of two analogous seco-norcubane subCe ~v alkoxides (DME)2Na2Ce(OBu')6 and NaCe2(O- units, sharing a face. Alternatively, the structure can Bur)9 have been obtained by the reaction of ceric be viewed as two pyramidal Sn(OBu')3 units linked ammonium nitrate with appropriate amounts of by two alkali metal atoms. The heavier congeners (K, Rb, Cs) form a one-dimensional chain of interlinked sodium tert-butoxide (eqs 34~35) [33]. SnO3M units (|I), the alkali metals are additionally THF/DME coordinated by two oxygen atoms of a second cage to (NH4)2Ce(NO3)6 + 8NaOBu' give a polymeric arrangement in the crystal. The (DME)2Na2Ce(OBu')6 + 2NH3 + 6NaNO3 higher coordination number observed for heavier alk+ 2Bu'OH (34) ali metal atoms is possibly favoured by their larger size. The room temperature ~H NMR spectra of the alkali metal alkoxo-stannates show a single alkoxide 2(NH4)2Ce(NO3)6 + 13NaOBu' ~ environment. In case of M2Sn2(OBu% (M = Li, Na) NaCe2(OBu% +4NH3 + 12NaNO3 derivatives, the cryoscopic measurements indicate that a dimeric structure is maintained in the solution. How+ 4Bu'OH (35) ever, the gas phase nuclearity as determined by mass spectroscopy, shows the sodium compound to be dimer while lithium analog is a monomer. 2.1.2 Structural and spectral features The general structural type (Fig. 2) observed in the The binding mode (terminal, #2- or #3-) of alkoxy interaction of Lewis basic tert-butoxides of divalent ligands and the nature of metal atoms play a gov- Ge, Sn and Pb with polymeric alkoxides of alkaline erning role in the structural frameworks (e.g., molec- earth metals (eq. 10) can be designated as a bis (Janusular or polymeric linear arrays or closed polyhedral head) structural motif (III) [34]. The metal-oxygen units) formed among the various thermodynamically framework, as found in the X-ray crystallographic

(i) the reaction of two different metals with an alcohol (eq. 27) [27] (ii) the reaction of a metal with a metal alkoxide in the presence of parent alcohol (eqs 28 29) [28,29] (iii) reaction of a metal amide [19], alkyl [30] or hydride [31] with a metal alkoxide alcoholate (eqs 30-32) and (iv) alcoholysis of a mixture of two different metal amides (eq. 33) [32].

Hetero (bi- and tri-) metallic alkoxide derivatives

1009

Table 1. The thermodynamically accessible metal stoichiometries as observed in the crystallographically charaeterised homoleptic hetero-(bi- and tri-) metallic alkoxides Coordination number Cluster

M

M'

M"

/I2-OR ~

[MM'X.] [MM'X.]2

3 4 4 3 5 5 6 4 4 5 4 4 6

3 3 5 6 5 3 4 4 5 5 4 6 3

--

3 4 4 4 4 3 4 4 4 3 5 4 6

5 5 6 4 4 4

6 5 4 4 6 6

[M_.M~X.I

4 4 4 6 6 3,4 4 6 5 4 6 6 4

6 6 6 6 6 6 6 6 4 6 4 6,5 3

2 4,5 8 4

6 4 6 6

--

[M2M~X.] [MM~X.] [M2M2M~X.] [M2M'M~X.]

6

6

[MM'X~].

[MM~X,,]

[MM~X.]2

[MM~X.],, [MM;X.]

--

---

m

,u3-OR b

2 2 2 3

Examples [MSn(OBu')3] (M = T1, In) [39] [MSn(OBu')3]2 (M = Li, Na) [8] [LiTi(OPri)5]2 [11] [LiHf(OPr%]2 [14] [KZr(OBu')d2 [16]

[KSn(OBu')3]. [8] --

1 --

2 3 4 4 2 3

2 2

3 3 4 4 5 8 8 8 3 4 6 5 6

2 2

2 2

2 4 2 2 2

--

6

6 8 6 12

6

6

4

4 4

[KAI(OPr')4(Pr~OH)2]. [9] [KM'(OBu'h]o (M = Bi, Sb) [10] [NaTi(OPr%]. [12] [KTi(OPr')5]. [12,13] [KSn(OBu')do [151 [LiNb(OEt)6]. [17] [MM~(OBu')6] (M = Mg, Ca, Pb, Cd, Mn, Eu, M ' = G e ; M = S r , Ba, Ca, Pb, Cd. M' = Sn; M = Sr, M' = Pb) [18b] [(DME)2Na2Ce(OBu')6] [33] [YBa2(OBu')7(Bu'OH)] [32] [MgAI2(OPrgs(Pr'OH)2] [27] [NiAI2(OBu')8] [13], [MgAI2(OBu')~] [25] [LiZr2(OPr%(PriOH)] [29] [(DME)KZr2(OPr)9] [31], [(THF)2KTi2(OPr%] [13] [KU2(OBu')9I [451 [NaM~(OBu')9] (M' = Ce [33], Th [34]7 [A12Hf(OPr'),o] [21] [BaNb2(OPr~)L2(Pr~OH)2] [22] [(Pr~O)La [Nbz(OPr ~)L2}] [22] [Pb2Zr(OPri)s]2 [19] [PbZr2(OPr~)to]2 [19] [BaZr2(OPriho]2 [46] [K2Sb(OBu')5" (C4H802)]. [101 [TI2Sn(OEt)6],, [26c] [Er {AI(OPr~)4}3] [24] [BaTi3(OPr~)L4] [20] [M2M~(OBu')s] (M = Co. M' = Ge: M = Mg, Cr. Mn, Co, Ni. M' = Sn) [23] [CuzZr2(OPr~),o] [30] [Mg2AI~(OPr~),3] [37] [M{Ti2(OEt)9}2] (M = Ba, Ca) [42] [{Cd(OPr')3}M'{M'2'(OPr~)9}]2 (M' = Ba, Sr, Ca; M~ = Sn, Ti, Zr, Hf) [6,13,14] [Ba2Ti2Zr2(OPr~)2o] [13]

#2-OR = doubly bridging alkoxo ligands. = triply bridging alkoxo ligands.

~' p 3 - O R

studies [18], displays two trigonal bipyramids joined together at a c o m m o n apex, defined by the heterometal partner. Consequently in all structures, the central element is six-fold c o o r d i n a t e d by the alkoxide oxygens while the outer metals occupying the term i n a t o r positions are three-fold c o o r d i n a t e d a n d possess p r e s u m a b l y a lone pair o f electrons. The detailed c o m p a r i s o n of b o n d lengths a n d angles in the different derivatives show t h a t the ability of the m o n o a n i o n i c M ( O B u ' ) ; - unit to a c c o m m o d a t e a metal a t o m with

respect to its size follows the order S n ( O B u ' ) 3 < Ge(OBu')3 < Pb(OBut)3 [18b]. This trend is especially evident in the transition metal derivatives (see sub-section 2.1.2). A l t h o u g h a large n u m b e r o f heterometallic alkoxides containing a l u m i n i u m are based on the {A1 (OPr)4} unit [5c], the ligating properties of this m o n o a n i o n i c alkoxometallate unit is not well-established. M o s t of the studies propose a bidentate ligating m o d e which indeed has been observed in the solid

M. Veith et al.

1010

R I

R--O

Sn ""

S'h

~ M / O ~

s

O'eR

S.-

M~ R

R

[

I

I1

(R = But) Fig. 1. Schematic representation of the metal oxygen core observed in the structures of [M{Sn(OBu%}], (M = Li, Na) (I) and [M{Sn(OBu')3}], (M = K, Rb, Cs) (II) [8].

o0,, ttt 'tJtt

IO#l~,#

,oOs,,

ttl ~*0

I#1*ae

RO \ go

..A,

10 \

R

R O

~ / o ~ ....~...~ K ~

o

/ ROH

HI Fig. 2. Schematic representation of the metal oxygen skeleton observed in M'M2(OBu% derivatives [18].

state structures of [C1Pr{AI(OPri)4}2(PriOH)]2 [35], and [Mg{Al(OPri)4}2(PriOH)2] [27]. However, other structurally characterised examples with an {A1 (OPri)4} unit bridging (i) two metals through single alkoxide bridges (e.g., [Mo2(O2CCH3)2{AI(OPri)4}2] [36] or (ii) two metals by a double OR bridge to one and a single bridge to the second (e.g., [Mg2A13 (OPri)~3] [37]) are also known. The attempts made to establish the identity and formulation of heterometal alkoxides by single crystal X-ray diffraction analysis are generally frustrated by their organic periphery (which makes them extremely soluble in common organic solvents) and physical state (viscous liquids or waxy solids). These problems can be overcome, in some cases, by using coordinating solvents (alcohol, THF, ether, pyridine, DME) which serve as neutral ligands and complete the coordination sphere of involved metal(s) to offer more tractable (coordinatively saturated) systems suitable for single crystal X-ray crystallography. A structural study performed on the single crystals obtained from an isopropanol solution of potassium tetraisopropoxyaluminate, KAI(OPr% (eq. 2), revealed a polymer of composition [(PffOH)2K{AI(OPri)4}], [9] with chains of alternating K and A1 atoms joined by doubly bridging isopropoxy groups (Fig. 3). Aluminium is present in a tetrahedral environment while potassium additionally coordinated by two neutral PriOH mol-

"

/ A|

/ ROH

R

OR

OR

_K

,o R

(R = Pri) Fig. 3. Representation of a section of the solid state structure of [(Pr~OH)2K{AI(OPrg)4}],,polymer [9].

ecule displays a severely distorted octahedral coordination sphere. The alkali metal alkoxy antimonates and bismuthates obtained according to eq. 3 exhibit only one signal in their ~H N M R spectra. The X-ray structural study performed on crystalline samples of KSb(OBu')4 [10] and KBi(OBu~)~ [10] reveal the compounds to be made up of one-dimensional arrangements of M(OBu')4 (M = Sb "r, Bi m) units linked through potassium atoms situated at inversion centres and coordinated by four alkoxo groups in a single plane. The coordination figure of both Sb m and Bi "~ resembles a O-trigonal bipyramid where one of the equatorial site is occupied by the electron lone pair. Although identical in stoichiometry, the two compounds crystallise with slightly different structures (Fig. 4), whereas the pseudo-trigonal bipyramid Sb(OBu')4 unit possesses a two fold axis of symmetry [Fig. 4(a)], the analogous Bi(OBu')2 unit lacks it [Fig. 4(b)]. The crystal structure of K2Sb(OBut)~ • dioxane [10] obtained (eq. 4) on varying the K : S b metal stoichiometry in eq. 3 reveals an infinite array of molecules linked through the oxygen atoms of dioxane molecules (Fig. 5). The repeating unit consists of an 05 trigonal bipyramid, the equatorial edges of which

Hetero (bi- and tri-) metallic alkoxide derivatives

1011

0(2l

(a)

(b)

0111~ ~ I I ~

0121

Fig. 4. A section of the one-dimensional structures of (a) [KSb(OBu~)4].and (b) [KBi(OBu')4].[10].

are centred by two potassium and one antimony atom. Antimony(III) bonded to four oxygen atoms exhibits a pseudo-trigonal bipyramidal coordination while potassium atoms are trigonally bipyramidally coordinated. Among the alkali metal titanium alkoxide derivatives M{Ti(OPr%} (M = Li, Na, K), (eq. 5) described by Hampden-Smith et al. [11], the lithium derivative is found to be dimeric in the solid state. The X-ray structure of [LiTi(OPr%]2 (Fig. 6) shows two approximately trigonal-bipyramidal titanium atoms joined by lithium bridges. If the terminal O P f ligands on titanium atoms are removed, the core structure parallels the metal-oxygen framework observed for [LiSn (OBu')312 (Fig. 1). All the derivatives undergo an alkoxide ligand exchange process that is rapid on the IH NMR time scale at ambient temperature. However, low-temperature (-20':'C) NMR studies reveal two types of ligand environments for [LiTi

(OPr%] 2 in intensity ratio 4 : 1 which is consistent with the empirical formula [LiTi(OPr%] but not with the solid state structure which shows three different alkoxide environments. A limiting spectrum displaying the three unique isopropoxy environments could not be obtained even at - 120~C. The use of alkali metal NMR in examining the solution behaviour of [MTi(OPr%] derivatives along with the X-ray structural characterization of Na and K substituted titanium isopropoxide derivatives is recently reported [12]. Both [NaTi(OPr%], and [KTi (OPr%],, are polymeric species wherein the alkali metal acts as a bridge between discrete mononuclear trigonal-bipyramidal Ti(OPd)5 units. The onedimensional polymer [NaTi(OPr%], is linear and exhibits alternating five coordinate titanium moieties and tetrahedrally surrounded sodium atoms. With respect to the coordination numbers of the metallic elements, the structure is comparable to that of

1012

M. Veith et al.

0(4ol~ ' 0

'l

o,

f41 "-" Fig. 7. A portion of the one-dimensional structure observed for [KTi(OPr%]o [13]. Fig. 5. A section of the dioxane-bridged structure of [K2Sb(OBu')5 • dioxane] [10].

[KSn(OBu')5],. In contrast to the sodium derivative, the potassium analog exhibits a penta-coordination for the K atoms. The non-linear continuous chain found in the solid state is a reminiscent of the structure observed for [KSn(OBu')3]~ with K atoms being present in a highly distorted trigonal-bipyramidal geometry. We have simultaneously solved the structure for potassium derivative (Fig. 7) [13]. The seco-norcubane core (M2M206) (I) is a recurrent structural feature among heterometallic alkoxides based on M : M ' = l : l stoichiometry; a modification of this framework has been observed in the solid state structure of [LiHf(OPr~)s]2 [14] (eq. 7) which displays a dimeric arrangement with two Hf w atoms present in a distorted octahedron of OPr i ligands while the lithium atoms are three-fold coordinated and display a trigonal pyramidal geometry (Fig. 8). The central Li2Hf206 unit in [LiHf(OPr%]2 (Fig. 8) is an interesting variant of the metal-oxygen core observed in [LiSn(OBu')3]2 (Fig. 1) and [LiTi(OPr~)s]2 (Fig. 61) with lithium occupying the position of tin (titanium) and hafnium that of lithium. Thus in

--.,.... o.

Fig. 8. Molecular structure of [LiHf(OPr%]2. Unlabelled atoms are carbon [14].

contrast to the four coordinate lithium atoms in Sn" and Ti ~v derivatives, the lithium centres in Hf w compound are three coordinate. Further, the hafnium derivative is stereochemically rigid and the spectral data indicate that the dimeric form persists in solution.

/

.o.Vo. ........................ R ' ..............

'" O R ( R = P r i)

Fig. 6. Schematic representation of the molecular structure of [LiTi(OPr%]2 [11].

Hetero (bi- and tri-) metallic alkoxide derivatives

1013

o,3,Lso/ ?

Fig. 9. A section of the crystal structure of [KSn(OBu%0.5CTHs]n[15]. The toluene molecule is not shown.

The ligand geometries at heterometal centres in the above examples based on MzM~O6 core can be attributed to the tendency of the metallic element to achieve a preferred configuration or higher coordination numbers. Thus tin(II), devoid of any terminal ligands in [M{Sn(OBu')3~]2 (M = Li, Na) derivatives possesses a stereochemically active lone pair and the preferred pyramidal configuration whereas the tendency of larger tetravalent metals to be in a hexa-coordination is reflected in [LiHf(OPr)5]2 where a bond to lithium is sacrified to attain an octahedral ligand environment for Hf 'v. The structural characterisation of [KSn(OBu')5], [15] (eq. 81)reveals that the compound crystallises with a half molecule of toluene per repeating unit. In the crystal, four- and five-fold coordinated potassium and tin(IV) atoms, respectively are interlinked via alkoxy bridges to give a one dimensional helical chain (O_,SnO2K), (Fig. 9). Alternatively, the polymer results from the trigonal bipyramids of tert-butoxy ligands built about Sn w which are interlinked by two structurally different potassium atoms. In contrast to the polymeric structure observed for [KSn(OBu')5],, (Fig. 9), the zirconium analogue [16] is dimeric in the solid state (Fig. 10). Both the metals (K and Zr) are five-coordinate, however, the disposition of two Zr(OBu')5 unit is asymmetric with one Zr atom sharing four of its OBu'-groups in bridg-

.....-....

/ ~Zr'"'"'O

,.o

-.o. \ R O ~ Z r ~ O R

.o7\ ',...l// .o (R = Bu t)

Fig. 10. Schematic representation of the molecular structure of [KZr(OBu')d2 [16].

ing the "KZr(OBu')/fragments while the other shares only three. This makes Zr Iv centres inequivalent (Fig. 10), however potassium centres are equalised by a mirror plane. The molecule is highly fluxional and even at - 7 5 ' C , the 'H NMR spectrum remains a singlet. On the basis of solubility differences, two structurally and stoichiometrically different products [Pb2Zr4(OPri)20] and [Pb4Zr2(OPri)16] can be isolated from a 1:1 mixture of lead and zirconium isopropoxides (eq. 11) [19]. Both the products were also obtained by mixing lead amide and zirconium isopropoxide in 2 : 1 molar ratio, in the presence of isopropanol (eq. 30). The latter derivative could be obtained quantitatively on employing the correct stoichiometry &component alkoxides (eq. 12). A rigorous centrosymmetric structure is observed in Pb4Zr2(OPri)16 where two (priO)3Zr(it2-OPr%Pb are linked by a Pbz(OPri)4 unit (Fig. 1la). The solid state structure of Pb2Zr4(OPri)20, with Pb : Zr ratio 1 : 2, can be derived from that of Pb4Zr2(OPri)16 by replacing the Pb(OPri)2 units with a Zr(OPri)4 moiety on either side of the Pbz(OP/)4 core (Fig. lib). Formation of the alkali and alkaline earth metal alkoxometallates of compositions M2M 3(OPr9~4 and M"M~(OP/)I4 (M = Na, K ; M ' = Zr w, HfW; M" = Mg, Ca, Sr, Ba), respectively is reported and the proposed structures of these derivatives suggest a hexa-coordination for the tetravalent metals [7]. However, there is no report available on the existence of a M3 (OPff)~4 substructure in the solid state, although a modification of M3 (OR)14 structural unit has been observed in the solid state structure of heterobimetallic acetato-oxo-alkoxide PbZr30(OAc)2 (OP/)10 [le]. In order to examine the formation of a dianionic sequestering unit {Ti3(OPr~)~4}-'-, a B a T i mixed metal alkoxide was prepared by reacting Ba (OPr~)2 with Ti(OPr% (eq. 13) [20] in 1:3 stoichiometry. The analytical and cryoscopic data of the crystalline product indicate the formation of a monomeric heterometal species with the anticipated Ba : Ti (1:3) stoichiometry and possibly the Ti3(OPri)~4 unit. The solid state structure, however, revealed a

M. Veith et al.

1014 OR ~

.o7, \

Pb

I

......o.

OR

.,,OR

°.o*•

°*%e

Pb "

OR

"~'Pb

"~OS~

OR

\ / OR ........

.""

I OR

"~Zr~ - /

..-" . ~ O R

OR OR

(a)

o. [

OR

O~

OR %

/

Zr "\ ~ .~ '

OR

"". / .n ~"Zr ~" IV',,

IX "'... . . . . . . 1 ~ . "0s ...... OR X "

.OR

/

................

I

Pb'

~ X ...... OR

j " Pb .......

[ ~o. /

~"

~ OR

,

OR/

".Oo

t /

~"'-,,I/ z~.

/ RO

~

"......

OR

OR

"'%" OR OR

(b) (R = Pr i) Fig. 11. Schematic representation of the molecular structures of (a) [Pb4ZL,(OPri)~6]and (b) [Pb_,Zr4(OPr~)20][19].

different geometry and arrangement of ligands but with the desired Ba : Ti metal ratio. The X-ray structure (Fig. 12) shows Ba 2+ in a distorted trigonal prismatic geometry formed by the tetra- and bi-dentate interaction of monoanionic {Ti2(OPrg)9} - and {Ti(OPri)5} units, respectively. The ~H N M R spectrum at - 5 3 ~ C shows seven sets of methyl resonances

~0Ill)

)'""~

Fig. 12. The molecular structure of [BaTL(OPr ~)14][20].

whose relative and total intensities are consistent with the retention of heterometal framework in solution. The above observations are corroborated by ~3C CP MAS N M R data. Recently, the X-ray structure determination [21] of a previously reported hafnium-aluminium heterometal derivative, Al2Hf(OPri)10, (eq. 14) confirmed the proposed structure of Mehrotra et al. [38]. The molecule (Fig. 13) consists of an octahedrally coordinated Hf Iv atom bearing two bidentate Al(OPrl)4 sub-units and two terminal OPr i ligands. The aluminium atoms are slightly distorted from a regular tetrahedral geometry. The barium-niobium heterobimetallic isopropoxide obtained from the isopropanol solution of two alkoxides ( B a : N b = 1 : 2, eq. 15) is shown by Xray crystallographic study to have the composition [Ba [Nb(OPE)6}2(Pr/OH)2] [221. The molecule consists of a chain of three octahedra built around one barium(II) and two niobium(iV) atoms. The central BaO6 octahedron shares two neighbouring edges with the peripheral NbO6 units to give a molecular framework shown in Fig. 14. The l a n t h a n u m - n i o b i u m isopropoxide LaN-

Hetero

(bi- and tri-) metallic

Fig. 13. A ball and stick representation

alkoxide

of the molecular

1015

derivatives

structure

of [A12Hf(OPr’),,]

[21]

Cl271 C(361

if231

C(371

t Fig. 14. Molecular

structure

b,(OPr’)r3 1221 crystallises from the solutions containing two alkoxides in La :Nb = 1 : (l-3) molar ratios. The molecule (Fig. 15) in the solid state reveals

of [Ba{Nb(OPr’),},(Pr’OH),]

[22].

the central La atom in a strongly distorted octahedron of OPr’ ligands which shares a face with one NbO, octahedra and an edge with the other. The noteworthy

M. Veith et al.

1016

0(6}

0(10) 0(3) 0(7)

0(5)

,

~

0113)

0(11)

0(81

0(12)

Fig. 15. Metal-oxygen core of[(PfO)La{Nb(OPr%}2] [22].

structural feature is the ambidentate (bi- and tri-) ligating behaviour of {Nb(OPr)6}- moiety. Following the salt elimination principle, we have reported on the synthesis, structural characterization and reactivity of a variety of heterometal derivatives formed by the use of chelating {M(OBu')3}(M -- Ge", Sn n, Pb n) anions toward main group and transition metals [5a,18]. Unlike the solid state structures observed for the alkali metal tris-(tert-butoxy) stannates (-germanates or -plumbates) (Fig. 1), the analogous derivatives of monovalent indium and thallium (eq. 16) are obtained as monomeric cage (IV) of the type M(#-OBu')3M' (M = Sn, Ge; M' -- In TI; M = Pb, M' = T1) (Fig. 16). The M'MO3 core represents a distorted trigonal bipyramid where the oxygen and metal atoms occupy the equatorial and apical positions, respectively. The cationic fragment {Sn(pOBut)3Pb} + with a structure similar to that of MM'(OBu'L (I) compounds has been observed as a component of the multimetallic compound PbvSn6Is(OBut)ls [25]. Depending upon the size of metal and the geometrical softness of the {M(OBut)3} - ligands, two structural forms are observed for the transition metal derivatives; the smaller 3d cations form compounds of the general formula (OR)M(/~-OR)2M'(p-OR)2M' (#-OR)2M(OR) (V) (M = Sn, Ge, Pb; M' = Ni, Cr, Co, Mn) (eq. 17) with the main group and transition metal stoichiometry being 1:1. The metal-oxygen

,,*~ •

tst,,** /

•

IV Fig. 16. Schematicrepresentation of the metal oxygencore of the M'(OBu')sM (M = Ge, Sn, Pb ; M' = In, TI) compounds [391.

OR

R

R

R

R

~

R

1

OR

V (R = Bu t) Fig. 17. Schematicrepresentation of the solid-state structures observed for [M~M2(OBu')8] (M = Ge, Sn, Pb; M' = Mg, Ni, Cr, Co, Mn) derivatives [23].

skeleton (Fig. 17) of these derivatives exhibits a bidentate ligating mode of {M(OR)3}- (M = Sn(II), Ge(II), Pb(II)) units in contrast to the tridentate behaviour observed in the structural types I-IV. The transition metal in the observed framework (V) occupies a four fold-coordination site which can be described as a spiro metallic centre in a distorted oxygen tetrahedron. The overall metal-oxygen skeleton (V) can be seen as three four-membered rings arranged in a perpendicular manner. The transition metals (M') are quasi-tetrahedrally coordinated while the group 14 elements bearing a dangling alkoxo ligand are in a pyramidal 3-fold oxygen environment. On the other hand, if tris-(tert-butoxy)-germanate or plumbate, is used against metal halides of Cr, Mn or Zn compounds with the general formula M (#-O R)3M ' (p-OR)3M are obtained (eq. 18) which have a structure similar to alkaline earth derivatives ( l i d based on tridentate {M(OR)3 } units. In all these derivatives (l-V), the terminal positions of main group elements present in the subvalent state (ns2 configuration) function as potential Lewis basic sites which have been used to obtain transition metal derivatives (see sub-section 2.1.3) [34]. The X-ray diffraction analysis performed recently on an erbium-aluminium (Er:A1 = 1:3) bimetallic isopropoxide (eq. 19) reveals a molecular formula

Hetero (bi- and tri-) metallic alkoxide derivatives ErA13(OPrg,2 [24] analogous to that proposed for LnAI3(OPr~),, [7] compounds (Ln = lanthanide atom). In the light of structure determination (Fig. 18), the molecular formula can be precisely expressed as Er{(/~2-OPr~)2Al(OPrl)2}3 where each of the three tetrakis-isopropoxy aluminate ligands coordinate the central atom Er "I via two OPr ~groups. The structural framework of Er{AI(OPr~)4}3 differs from the related compound AI{AI(OPr~)4}3 [39A0] in the coordination figures of hexa-coordinate central atoms. In case of aluminium isopropoxide, the central AI "* has almost an ideal octahedral geometry, whereas, the erbium complex displays a distorted trigonal prismatic coordination for Er "* which is in accordance with the tendency of lanthanide elements to prefer a prismatic geometry (over octahedral). It has been reported that the complex formation of Al(OPri)3 with [Ni(OPr~)2], do not give the previously reported [Ni{AI(OPr%},] and only the mixture of two corresponding alkoxides results [41]. However, we have been able to synthesise and structurally characterise the analogous tert-butoxide derivative. The sublimable nature of [Ni{AI(OBu')4}2] speaks for the stability of such derivatives. The compound i shown by the X-ray crystallographic study [13] is monomeric in the solid state and the structure (Fig. 19) [27] is similar to that proposed by Mehrotra et al. for M{AI(OR)4}2 derivatives [5c,d] (eq. 20). The molecular architecture can be viewed as bidentate interactions of two tetrahedral AI(OBu')2 moieties to the Ni 2+ centre. The two 'NiAl(OBu92' four-membered rings fused at a common end (Ni") are perpendicular to each other. Both Ni" and AI "* are quasi-tetrahedrally coordinated by the tert-butoxy ligands, the distortion being more significant in the case of nickel. The heterobimetallic compound [Mg{AI(OBu')4}2} [25] with Mg:A1 stoichiometry appropriate for MgAI_,O4 spinel is obtained from the reaction of

1017

Fig. 19. Ball and stick drawing of the molecular structure of [Ni{Al(OBu%}2] [13].

MgCI2 with NaAl(OBu')4 in 1 : 2 molar ratio. The Xray diffraction study shows a spirocyclic core made up of two planar AIOzMg four-membered rings joined at magnesium (Fig. 20). With two terminal ligands present on each of the aluminium atoms, all the metallic elements are present in a four-fold coordination of oxygen atoms. The angular distortions, due to the formation of four-membered AI(/~-OBu')2Mg rings, are reflected in the distorted tetrahedral geometries of Mg" and AI "* centres. The isopropoxide analogue of [Mg{AI(OBu')4},] derivative is synthesised by the reaction of the two metals in isopropanol (eq. 27). The compound crystallises as [(PriOH)2Mg{AI(OPr%} ~] [27] and the solid state structure (Fig. 21) exhibits two [Al(OPr94}

~R

Rg, .~ tt%°°

¢,,,, I ]

"OR

d Fig. 20. Molecular structure of [Mg{Al(OBu')4}:] [25].

/o,

.o.

/\ RO

.of ~H~oI

~

~RO.-.---H ~ ~ ~

OR

(R = Pr i) Fig. 18. Schematic representation of the molecular framework of Er{Al(OPr%}3 [24].

(R = pri)

Fig. 21. Schematic representation of the molecular structure observed for [(PriOH)2Mg{AI(OPr%}2] [27].

1018

M. Veith et al.

units coordinating the central atom Mg n in a bidentate fashion. The coordination sphere of magnesium is completed by two adduct alcohol molecules which show AI(R)--O. • • H(OR)M interactions with one of the terminal OPri-groups of each [AI(OPr%}moiety. The effect of the ramification of alkyl group on the coordination numbers of the metals present in the heterometallic alkoxides is nicely illustrated in the derivatives [Mg{AI(OBu')4}2] and [Mg{AI(OPri)4}, • (Pr~OH)2]. Whereas the central atom Mg n is tetracoordinate in the tert-butoxide derivative, it achieves a hexa-coordination by two coordinating alcohol molecules in the latter case. Although both the heterometal precursors have a Mg : AI ratio suitable for MgA1204 ceramic, the "base free' tert-butoxide compound appears to be a better choice as the presence of stabilising ligands in the latter case can lead to a higher percentage of carbonaceous impurities in the resulting mixed-metal oxide. The M'{M:(OR)9}2 ( M ' = Ba, Ca: M = Ti, Zr) derivatives [20,42,43] (eqs 28 and 29) contain a central eight coordinated metal atom bound to two facesharing biooctahedral Ms (OR)~ units. Although the X-ray data in the case of Ba Zr mixed-metal compound, could not be refined satisfactorily, the molecule is proposed to have a similar spiro MeM'M2 arrangement. A similar structure has been proposed for the derivative [Ba{Ti2(OPr%} 2] (Fig. 22) [20]. A Cu ~ Zr w mixed-metal species is obtained by the reaction of copper(I) mesityl and zirconium isopropoxide isopropanolate (eq. 31) [30]. The X-ray diffraction study shows the molecule, CueZre(OPri)~0, to result from a union of confacial bioctahedral Zre(OPri)~- substructure and a bent (Cu,(OR)) + unit bound together by the two bridging OP( groups of Zre(OPr% fragment. The eight membered metal-oxygen ring (Fig. 23) is essentially planar. The yttrium-barium (1 : 2) heterobimetallic YBaz(OBu')7(Bu'OH) [32] obtained by the alcoholysis of a mixture of yttrium and barium amides (eq. 33) displays a triangular structure (Fig. 24) commonly

OR

9R RO

"

RO

,c..¥'." d

R

"

--M'--

\\

/

.ol i \o /

R

==

O.

" h O R

"%

'2

Lx,

OR

,..Mr./" ,,, ....... g \ &xo.

a6

(R = Et, Pri) Fig. 22. Schematic representation of the structure of [M'{Me(OR)9}_~] (M' Ba, M = Ti, Zr; M' = Ca, M = Ti) derivatives [20,42,43]. =

/

~O~',,,,C Cu

RO

/

RO--"~

,-~n ..... "%.

. ~

/

~

~ O R / "

RO

u

\ OR

I

.

OR OR

(R -- Pr i)

Fig. 23. Schematic representation of the molecular structure of Cu_,Zre(OPri)m[30].

OR

goJ RO H

OR (R = Bu t)

Fig. 24. Schematic representation of the molecular structure of YBae(OBu')7(Bu'OH) [32].

adopted by M'M2 heterometallic alkoxides. All metals are five-coordinate, with the terminal ligand on one of the barium atoms being an alcohol molecule. The crystal structure comprises chains of YBae triangles held together by hydrogen bonds between adduct alcohol molecule of one YBa~ unit to a terminal alkoxo group of another unit. The sodium thorium heterometallic alkoxide NaThe(OBu')~, described by Clark et al., involves the deprotonation of the coordinated alcohol in The (OBu'),(Bu~OH) with NaN(SiMe3)2 [44]. The crystal structure (Fig. 25) shows discrete NaThe(OBu% molecules. There is no terminal ligand attached on the alkali atom, however, no intermolecular interactions between the sodium atom of one molecule and oxygen atoms of adjacent molecules are observed. A similar structure is observed for the related derivatives KUe(OBu')9 and NaC%(OBu')9, reported by Cotton et al. [45] and Evans et al. [33], respectively. Heterometal alkoxides of Ce w have been obtained, using ceric ammonium nitrate and sodium tert-butoxide as starting materials (eqs 34 and 35). The derivatives (DME)eNaeCe(OBu')6 [33] and NaCe,(OBu% [33] characterised by single crystal X-ray diffraction technique reveal molecular frameworks based on

Hetero (bi- and tri-) metallic alkoxide derivatives

,

RO .........M ~ ' ' -

/

/

j M ..........'OR

\

OR

OR (R = But)

Fig. 25. Schematic representation of the molecular structure observed for M'{M3(OBu')9} ( M ' - N a , M = Th~v, Ce~V; M' = K, M = UIv) derivatives [33,44,45].

[

~"'-.~

erobimetallic alkoxides labile species and neutral metal alkoxide molecules can be added as building blocks, to modify the coordination figure of the central metal atom or to change the nuclearity of the complex [5,7]. Three soluble barium-titanium containing heterobimetallic isopropoxides with manipulable Ba:Ti stoichiometries and nuclearity have been obtained by using Ti(OPf)4 as a building block towards polymeric [Ba(OPr;)2],, [20]. The dimeric compound [BaTiz(OPr;).0]2 with Ba: Ti ratio 1:2, reacts with one equivalent of Ti(OPf)4 to afford mononuclear species BaTi3(OPf)~4 which can be converted into BaTi4(OPf)lS by an additional mole of Ti(OPr;)4 (eq. 36). Ti(OPr;)4

OR

RO /

1019

1/2[BaTi2 (OPri) i 012

Ti(OPrJ }4

~BaTi3 (OPri) 14 BaTi4 (OPr i) i s

OR

......

(R = Bu t) Fig. 26. Schematic representation of the molecular structure of [(DME)_,Na2Ce(OBu;)6] [3].

(36)

A similar interconversion has been reported for Pb Zr mixed-metal alkoxides where the lead-rich derivative [Pb4Zr2(OPf)~6] can be converted into the zirconium-rich analogue [Pb2Zr4(OPr;)20] by reacting it with an appropriate amount of zirconium alkoxide and v i c e - v e r s a (eqs 37 and 38) [19]. Pb4Zr2 (OPr;) 16+ 6Zr(OPr;)4 --~ 2Pb2 Zr4 (OPf)20 (37) Pb3Zr4(OPr;)20 + 6/n[Pb(OPr;)2],

Na2Ce and NaCe2 triangles, respectively (Figs 25 and 26). Whereas cerium is hexa-coordinate in both the derivatives, the sodium atoms display penta- and tetra-coordination, respectively. 2.1.3

Reactions

Heterobimetallic alkoxides with subvalent main group metals (ns2 configuration) situated at the apices of one or two (fused) trigonal bipyramids act as excellent electron donors to various transition metal carbonyls. This synthetic strategy has allowed the construction of a large number of derivatives with one-dimensional arrays of metallic elements held together by direct metal metal and M - - O ( R ) - - M bonds [34]. The difference in the reactivity of the n s 2 elements follow the sequence In I >> Ge u > Sn u > T1~, Pb n. The efforts made to attach transition metal carbonyl at T1~ or Pb n ends were not successful. For a particular ns2 element, the reactivity follow the order: Ni(CO)4 > Fe2(CO)9 > Mo(CO)6 > Cr(CO),, > W(CO)6. A number of metal carbonyl complexes of mixed-metal alkoxides have been characterised in the solid state. Some representative frameworks of the heterometal assemblies containing three or four different metals are shown in Fig. 27. The pronounced tendency of metal atoms to increase their coordination numbers makes the her-

2Pb4Zr2 (OPff)16

(38)

Discrete heterometallic alkoxy derivatives have a defined stoichiometry at the molecular level and are thus attractive precursors in sol-gel processing of mixed-metal oxides. Although extensive studies have been made to understand the initial stages of hydrolysis and the mechanisms of conversion of homo-metallic alkoxides to metal oxides, the key studies using heterometallic precursors are missing, however, a few scattered reports available show interesting structural and nuclearity changes. The controlled hydrolysis reaction (eq. 39) of [LiTi(OPr%]2 proceeds with the retention of metal stoichiometry, however the degree of aggregation grows and the resulting molecule [LiTiO(OPr%]~ [46] is tetrameric, possibly due to the greater bridging ability of an oxo (02-) ligand in comparison to the alkoxide group. 2[LiTi(OPr;)5]2 + 4 H , O ~ [LiTiO(OPr;)3]4 + 8Pr;OH

(39)

However, the controlled hydrolysis of Ba{Zr2(OPr%}2 shows no change in the metal ratios or segregation of the molecule and BaZr4(OPr;)IT(OH) [46] is obtained with a structure similar to that of parent compound (Fig. 22).

M. Veith et al.

1020 R

R

,,O,, (CO)sM--Ge ~

,,O,,

O/

--M(CO)s

(CO)sC--Sn ~

O/

R

- - e(CO)4

R R

R

,,0,,

,,0,,, :TI

/

Sn--M(CO)4--In ~

- - O - R

~

TI:

- - O R

R

R

:Ge

- " ~ In--Cr(CO),,--In ~

~o

R

~

- " ~ Ge:

~o

R

olooo

(CO)sCr-- S n ' ~

R

~

R

#~%%

.,, o°l

~~

In~ Cr(CO,4---In ~

• ssa%

~~ S n

O

O

R

R

R

R . . ' ~ ° ' °~."" ' * ° R ° % %%...

...,.o**" ~n % *%°°.°

(CO),Fe-- S n ' ~

- Cr(CO)s

~

B~~ 0

R

~

Sn- - Fe(CO),

R

(R = But)

Fig. 27. Schematic representation of the frameworks observed in the transition metal derivatives of some mixed-metal alkoxides [34].

2.2 Heteroleptic heterobimetallic alkoxides In contrast to the large number of reports available on the structural characterization of homoleptic heterometal alkoxides, the structurally characterized examples of halide heterometallic alkoxides are scarce. Prior to our investigations, the well-characterised chloride heterobimetallic alkoxides were limited to [C1Mg{Ti2(OEt)sC1}]2 [47], [C1M {Zr2(OPrl)9}]2 (M = Cd [48], Cu [30]) and [CIPr{A1 (OPri)4}z(Pr~OH)]2 [35]. Although a large number of chloro alkoxometallates are reported, the absence of single crystal X-ray diffraction studies together with oxo forming and salt retaining [49] tendency of heterometal alkoxide derivatives has made their formulation and identity ambiguous. In our continuing investigations on halo-functionalised heterometallic alkoxides, we have found that these 'intermediates' are generally soluble and stable compounds (most of

them sublime/distill in vacuo without any extensive decomposition) and show enormous potential as attractive synthons for incorporating other ligands (OR , O S i M e ; , OSiPh3, CsH;-) in the heterometal assembly. It is worth noting that use of halide heterobimetallic alkoxides towards cyclopentadienyl reagents has furnished the first examples of an elusive class of cyclopentadienyl containing heterobimetallic alkoxides [50,51]. More interestingly, the metathesis of metal-halogen bonds using suitable alkalimetal alkoxometallates has offered novel molecular alkoxide assemblies containing three different metals (see Section 3) [6,14]. Although the class of heteroleptic alkoxides would, in principle, include the heterometal alkoxide derivatives based on other hetero-ligands such as hydride, oxide, //-diketonates, carboxylates, siloxides, aryloxides, etc., the present article is limited to heterobimetallic derivatives containing halide and cyclopentadienyl as ancillary ligands.

Hetero (bi- and tri-) metallic alkoxide derivatives 2.2.1 Synthesis

(C5 Hs)SnCI + KM2 (OPr/)9 ---,

(i) Salt elimination reactions. In view of our interest in the halo-alkoxometallates as interesting synthons (see Section 3) and dearth of information about their structural features, we have investigated the solution and solid state characterization of a number of halide heterobimetallic derivatives illustrated in eqns 40-46 [6,13,14,51,52] MX2 + K M ~ ( O P f ) 9 --, XM{M'(OPr')9~ + K X M = Cd ; X = I ; M = F e , Cu;

(40)

X=CI;

X=CI;

M=Pb:

X=I:

M ' = Z r , Hf

(49) BiC13 + OV(OPr;) 3 [BiCI30V(OC2H4OCH3)3] 2 + 3PfOH

(50)

2.2.2 Structural and spectral Jeatures

M = Sn, Ti, Zr, Hf 2Pr'OH

[ICd{Ta(OPf)6 } (PfOH):] + KI

(43)

C7H s

2CdI2 + 2KOPr~ + N b ( O P f ) 5

,

[I2Cd2Nb(OPr')7]2 + 2 K I

(44)

2MC13 + 2KM2 (OPf)9 ~ 2/n[C12M {M~ (OPf)9 }],, +2KC1 M ' = T i , Zr, Hf;

(45)

n=2

M ' = T i , Zr, Hf;

n= 1

SnL + nKTi (OPr ~)s -* [I4 nSn {Ti(OPf) s },] + nKI (46) n = 1-3 The first examples of cyclopentadienyl containing heterobimetallic alkoxides have been synthesised by treating various alkali metal alkoxometallates with cyclopentadienyl tin(II) chloride (eqs 47 and 481). Alternatively, these heteroleptic derivatives become accessible on reacting well-characterised halide heterobimetallic alkoxides with sodium cyclopentadienide [50,51] (CsHs)SnC1 + KM(OBu')3 --* (C5 Hs)Sn(p-OBu')2M(OBu') + KC1 M = Sn, Ge

MgClz + 2Ti(OEt)4 ~ ~ [CIMg{Ti2 (OEt)8C1}]z

M ' = T i , Zr

(42)

M = I n , Y;

(ii) Miscellaneous methods. The Lewis acid-base interaction of a metal halide and alkoxide (eqs 49 and 50) provides an alternative to the salt elimination reaction for the synthesis of halo functionalised heterometal alkoxides [47,53]

3HOC2H4OCH 3

Bal2 + KM2(OPf)9 --, ~,[IBa{M2 (OPff)9 }]2 + K I

CdI2 + KTa(OPr')6

(48)

M = Zr, Hf

M ' = T i w,Hp v

(41) M=Sn:

(CsHs)Sn{M2(OPr')9} + K C I

M' = Sn jr, Ti w, Zr TM,HP v

MX2 + K M I ( O P f ) 9 ~ ~ [XM{MI(OPf)9}]2 + K X

M=Bi;

1021

(47)

In the course of our ongoing investigations, we have synthesised and characterised a large number of heteroleptic heterometal alkoxides based on chelating alkoxometallate units of the type {M(OR)3} (M = Ge, Sn, Pb), {M(ORh} (M = A1, Ga, Sb, Bi), {M(OR)5} (M = Sn, Ti, Zr, Hf), {M(OR)6} (M = Nb, Ta) and {Ma2(OR)9} (M = Sn, Ti, Zr, Hf). The structurally characterised examples of various heteroleptic heterobimetallic alkoxides are assembled in Table 2. Salient features of the structurally characterised compounds based on face-sharing bioctahedral {M2(OR)9} sub-structure are (i) the gradation of the metal-oxygen bond lengths within the M2(OPri)9 unit [ M - - O (terminal) < M--(p2-OM) (doubly bridging, p2-)< M-(#3-OM) (triply bridging, p;)] which corroborates the loss of electron density at the oxygen with each successive bridge and (ii) the greater opening of the terminal M - - O - - C bond angles which is presumably a consequence of metal-oxygen multiple bond order and steric factors. The most commonly observed mode of ligation for M2 (OPf);- type sequestering anions is the use of four alkoxy oxygen atoms (two terminal and two bridging) to form a pocket to accommodate the cationic fragment. The size of the pocket depends on the size of the M ~vcation and M ... M vector and its compatibility to the size of heterometal partner decides the stability of the resulting complex. The chemistry of confacial bioctahedral M 2(OR)9 units is dominated by the use of zirconium and the formation of analogous bioctahedral substructure by tetravalent tin, titanium and hafnium was not investigated before. The solid state structures of ICdSnz(OPf)9 [52], ICdTi2(OPri)9 [6] and ICdHf2(OPri)9 [6] derivatives reported by us, provide the first structural evidence for the existence of a M2(OPf)9 unit, based on Sn ~v, Ti w and Hf Iv. Owing to the facts, such as (i) the monomeric and non-adduct forming behaviour of Ti(OPff)4 [7] (cf. isopropoxides

1022

M. Veith et al.

Table 2. The thermodynamically accessible metal stoichoimetries found in the crystallographically characterised heteroleptic heterobimetallic alkoxo clusters Coordination number Clusters

M

M'

/~2-OR"

/~3-OR~

[MM'X,, l

3

3

2

--

5

6

2

[MM'X,,]

4

4

4

5

6

3

2

6

6

3

2

6 3 7 6 6 6 4 5 6 4 5

5 6 4 6 6 6 6 6 6 6 4

4 3 8 6 6 6 6 4 4 4

[MM'X,]2

[M4M'_,X~]

[MM{~X.]

2 4 4 4 2 4 4 -8

Examples [CpSnGe(OBu')3] [51]' [CpPbSn(OBu')3] [50]' [ICd [Ta(OPr~)~} (PriOH)2] [13] [(CsHMe4)4Ti2(OMe)4Mg] [65]' [C1Cu[M'2(OPr~)ql] ( M ' = Ti [20], Zr [30], Hf [14]), [ICd{M{(OPr')9}] (M' = Sn, Ti, Zr, Hf) [6,52], [C1Fe[M;_(OPr~)~] (M = Ti, Hf) [13] [CI~Y{M~(OPr*)91] tM = Ti [20], Zr [13], [(Pr~O)CIY {Ti:(OPr%}] [13] [IeSn {Ti(OPr')s~ 2] [13] [CpSn [Zr,(OP¢)9}] [50]' [C1PrAI,(OPr~) dPffOH)]2 [35] [C1Mg [Ti_,(OEt)sCl~ ]2 [47] [IBa[M~(OPr~)g}], (M' = Sn, Ti, Zr, HI-) [13] [C1Cd {Zrz(OPr%l]2 [48] [CISn{M~(OPri),I]: (M' = Zr, HI') [50] [ISn{M~(OPr*)9 ~j]e (M' = Zr, Hf) [13,52] [CI_,Bi[Zr_,(OPr')9]2 [13] [leCd,_Nb(OPr*)7]_, [13] [LnNadOBu'),oC1] [60]

a/t2-OR = doubly bridging alkoxo ligands. ,u3-OR = triply bridging alkoxo ligands. 'The contribution of cyclopentadienyl ligand in the ligand count is being considered to be unity.

of tetravalent Sn, Zr and H f are dimeric alcoholates [M(OPri)4 • PriOH]2) (ii) the known heterometal compounds where the use of homometallic Ti and Zr alkoxides as building blocks results in the formation of products with different metal ratios as well as different structural features and (iii) the 'belief' that smaller Ti 4+ cation cannot achieve hexa-coordination due to the steric bulk of OPr' ligands [19b] which results from the paucity of structurally characterised six-coordinate titanium isopropoxide derivatives, the use of alkali metal alkoxo-titanates in the formation of heterometal derivatives remain unexplored. In view of above and the importance of titanium containing ceramics [54] we have synthesised a number of mixedmetal derivatives based on mono-anionic {Ti2(OPr)9}- and {Ti(OPr~)5}- units (Tables 1 and 2). The iodide cadmium nonaisopropoxo-distannate (-dititanate, -dizirconate and -dihafnate) derivatives [6,52] (eq. 40) are isostructural and the molecule can be viewed as a tetra-dentate interaction of the anionic con-facial bioctahedral {M2(OPr%}- (M = Sn, Ti, Zr, Hf) unit to a cationic CdI÷ fragment. The coordination polyhedron of cadmium bearing iodide as a terminal ligand resembles a distorted trigonal bipyramid whereas each tetravalent metal centre (Sn, Ti, Zr, HI) is hexa-coordinate and displays a distorted octahedral geometry.

In the case of ICd{Tiz(OPri)9} derivative (Fig. 28), the Cd-(#2-OTi) distances are found to be longer than Cd-(it3-OTi) distances. The relatively small size of titanium atom results in shorter T i - - O P r ' terminal distances which consequently show longer Cd--#2OTi contacts. The pocket formed by short /~3- and long Ft2-contacts is not suitable for binding Cd 2+ strongly and the poor fit of Cd 2+ in the 'arms' of

0?

Fig. 28. Molecular structure of ICd{Ti2(OPr%} [6b].

Hetero (bi- and tri-) metallic alkoxide derivatives Ti_,(OPr% unit is evident in the thermal lability of ICd{Ti2(OPr%}, whereas the ICd{Mz(OPr%} derivatives based on larger ZR~v and Hf ~v atom sublime almost quantitatively, when heated in vacuo [6b]. The cryoscopic and multinuclear spectroscopic data indicate the ICd{M2(OPr%} derivatives to be monomeric in solution which is corroborated by the solid state CP MAS 113CdNMR studies. The tH NMR spectra of these derivatives suggest a C2, molecular symmetry in the solution as displayed in a characteristic set of four signals in the intensity ratio 4 : 2 : 2 : 1. Further, the similar ~3Cd chemical shifts in solution and solid state indicate that the binding mode of M2(OPr*)9 unit to cadmium is retained in both the states. The mononuclear nature of the iodo-heterobimetallic derivatives in the vapour phase can be inferred in a representative case on recording a chemical ionization mass spectrum (M +) for ICd [Hf2(OPr%} which exhibits a molecular ion corresponding to the monomer [6b]. The presence of an NMR active heterometal partner, as shown for cadmium compounds, facilitates the probing of structural behaviour of heterometal alkoxide derivatives in the solution and solid state. This assumes special significance among heterometal alkoxides where the IH and t~C NMR spectra are often not structurally diagnostic, due to the high fluxionality of these derivatives in solution. In view of the above, our initial investigations were focused on metals (~t3Cd, ~gSn, 2°7pb) whose coordination state could be studied by NMR spectroscopy in both the solution and solid state. New bimetallic halide isopropoxides of divalent Sn and Pb are accessible by the metathesis reactions of their halides and alkali metal salts of M2(OPr~)9 anions containing Ti, Zr and Hf atoms (eq. 41) [50,52]. In contrast to the monomeric Cd" derivatives, ICd{M2(OPri)9}, the analogous derivatives of Sn" and Pb H are found to be dimeric species. The molecular structure of [1Sn{Mz(OPr%}]2 (M = Zr, HI) shows (Fig. 29) a tridentate interaction of Mz(OPr% unit with Sn n which deviates from the typical tetra-dentate ligating mode of M2(OR)7 moieties presumably due to the presence of the stereochemically active lone pair

0151

Q

O~

Fig. 29. Molecular structure of [ISn {Zr,(OPr%}]2 [52].

1023

at tin. The central Sn2I~O6 unit can be described as two SNI203 trigonal bipyramids joined along a common edge. Alternatively, when the lone pair of electrons at tin(II) is considered to be stereochemically active, the tin geometry resembles a distorted octahedron in which one of the axial sites is occupied by the non-bonding electron pair. Whereas [ISn{Zr2(OPr%}]2 [52] is a centrosymmetric dimer, the chloride tin-dizirconate (dihafnate) molecule [50] adopts, in the solid state, a dimeric form via long S n . . . CI interactions (Fig. 30) between chlorine of one molecule and the tin atom of a second molecule, nevertheless their deviation from Ci symmetry is small. The bidentate coordination of the M2(OPri)9 unit has also been observed in [PbeZr4(OPff)20] [12]. If the long S n . . . C1 interactions are taken into account, the coordination sphere of tin(ll) can be termed as "3 + 1'. Most striking is the change in the ligating behaviour of M2(OPr% units to similar SnX + (X = C1, I) fragments ; the M2(OPr')9 sub-structures act as bi- and tri-dentate ligands for chloride and iodide derivatives, respectively. As inferred by the cryoscopic studies (molecular complexity, ~/= 1.41.6), the dimeric form of [XSn {M2(OPr%]_, derivatives is not maintained in the solution. A 2 monomer ¢~ dimer equilibrium, is indicated in the solution by t~gSn NMR spectra which has been confirmed by variable temperature ~gSn NMR studies. The effect of the nature of metal atoms is seen among iodo derivatives of bivalent cadmium and tin. The ICd [Me(OPr%~ derivatives (Fig. 28) exhibit a closed triangular (CdM2) framework however, the replacement of Cd m~ by Sn" of almost similar size (Cd 2+ = 0.99 •, Sn~+ = 1.02/~) gives an open structure (Fig. 29). This possibly results from the tendency of tin(II) atom to achieve a pyramidal configuration and to retain a stereochemically active lone pair consequently, a dimeric framework is preferred (despite the bulk of iodide ligands) over a monomeric closed triangular structure. The role of halide ligands in the nuclearity and coordination numbers of metals has also been observed for Cd Zr mixed-metal alkoxides where the substitution of chloride, in the dimeric [CICd{Zr2(OPr%]]2 [45] containing a six-coordinate cadmium, by bulkier iodide ligand produces monomeric ICd{Zr2(OPr%} with a five coordinate cadmium. The discovery of the super-conducting properties of a number of copper containing oxide-ceramic [55] materials has resulted in an unprecedented search for soluble copper containing derivatives which can be used as precursors to ceramic materials via sol-gel processes. The reactions of copper(lI) chloride with KM2(OPri)9 (M = T i , HI-) in 1:1 stoichiometry offered halide heterobimetallic alkoxide of the type CICu{ M2(OPr')9} (eq. 40) [14,20]. The zirconium analogue CICu{Zr,(OPr%] is also known [30]. Their solid state structures resemble closely to the structures observed for ICd{M_,(OPr%} compounds, a perspective drawing is shown for the titanium analog

M. Veith et al.

1024

0 •

81

(~L~

Fig. 30. Molecular structure of [C1Sn[Zrz(OPr%}]2 [50].

(Fig. 31). The {(Pr~O)3M(#2-OPri)3M(OPri)3} unit coordinate Cu H through four oxygen atoms of the OPr i groups, two of which also bridge the two M w centres. The core of the molecules comprise a heterometallic isosceles CuM2 triangle with similar nonbonded C u ' " M distances; the triangular base is capped on both the sides by a /~3-OPr~ ligand, two C u - - M and the third M - - M side of the triangular core are each bridged by a #2-OPri group. The five-coordinate copper in C1Cu{Mz(OPr%} derivatives (M = Ti, Zr, HI) constitute a distorted trigonal bipyramid with four oxygen and one chlorine ligand where the axial (doubly bridging) OPr i ligands are bound to copper in a symmetrical fashion. Among the equatorial ligands, the average Cu--(#3-O) distance is longer than the copper contact to much larger chloride ion and chloride is apparently the most tightly bound ligand to copper. This observation can be understood in terms of Jahn-Teller distortion (Cu H, d 9) which makes copper coordination sphere flexible to display both normal coordinated (Cu--L) and longer semi-coordinated ( C u ' . . L) bonds [56,57]. In view of the above, the copper coordination in C1Cu-

{Mz(OPrq9} derivatives can be viewed as '3 + 2 ' with three strong (C1, O,1, O j and two weak (Oe~, Oe2) interactions. CI

I In view of the paramagnetic influence of Cu H, these derivatives exhibit simple Curie-Law behaviour deduced from a linear relationship between the change in ~H NMR isotropic shift with respect to the temperature. The monomeric nature of CICu {M2(OPri)0} derivatives is supported by the cryoscopic measurements which rules out the possibility of interacting paramagnetic centres to give dimeric diamagnetic species of the type "Cu(#-C1)2Cu'. Iodide heterobimetallic derivatives of barium of the formula [IBa{M2(OPr%}]2 (M = Sn, Ti, Zr, Hf) [13] (eq. 42) are dimeric in the solution and solid state. The molecular structure (Fig. 32) can be analysed as a union of three face-sharing octahedra built about

Cl

0(41

0(2)

Fig. 31. Molecular structure of CICu{Tiz(OPr%} [20].

Fig. 32. Molecular structure of [IBa{Ti2(OPr%}]_~[13].

Hetero (bi- and tri-) metallic alkoxide derivatives two Ti (Sn, Zr, Hf) and one Ba atom. The monomeric formulation 'IBa{Ti2(OPri)9} ' of these derivatives is a ligand deficient modification of M3LI, structural unit observed in the solid state structures of [C12Y{Ti2(OPri)9}] [20] and [(OPr~)CIY{Tiz(OPri)9}] [13]. Owing to the ligand deficit, the simplest structural change to attain a M3Lu motif involves the dimerisation via 'terminal' iodide ligands. The other MM'2L,o derivatives supporting the proposed dimerisation are [C1Cd{Zr2(OPr%}]2 [48] and [BaZr2(OPr"),0]2 [46]. Although structurally characterised halide heterobimetallic alkoxides based on tri-, tetra-, pentaand nona-alkoxometallate are known, reports available on halo-functionalised heterometal alkoxy derivatives based on M(OPr*)6 fragments (M = Nb, Ta) are relatively scarce. Equation 43 illustrates the synthesis of a monomeric monoiodo cadmium hexaisopropoxo-tantalate [13]. The molecule (Fig. 33) is formed by the bidentate ligation of Ta(OPr')6 unit to Cd n bearing an iodide ligand. The cadmium atom is additionally coordinated by two alcohol molecules to display a quasi-trigonal bipyramidal geometry whereas the ligand disposition around tantalum represents a slightly distorted octahedron. An attempted synthesis of a halide-free C d - - N b mixed metal alkoxide with 1 : 1 metal stoichiometry gave a dimeric iodo-complex with Cd-Nb stoichiometry 2:1 (eq. 44) [13]. The molecular structure reveals two tetrahedral Cd n fragments 'lCd(OPr~)3 ' and 'I2Cd(OPri)2 ' (Fig. 34) bound to the octahedral Nb(OPr')6 moiety in bi- and mono-dentate fashion, respectively. The observation of two resonances, corresponding to two chemically inequivalent Cd u centres in the solution and solid state U3Cd CP MAS NMR spectra provides evidence that the heterometallic framework observed in the solid state is also maintained in the solution. As indicated in eqs 40-43, the reactions of divalent

1025

Fig. 34. Molecular structure of [12Cd2Nb(OPr%]2 [131.

metal halides with alkali metal alkoxometallates in 1:1 stoichiometry offer mono-halo heterometallic derivatives. In a similar manner di- and tri-halo functionalised heterometallic alkoxides are obtained on employing tri- and tetravalent metal halides, respectively. Dichloro-yttrium-nonaisopropoxo-dimetallate compounds of the type ClzY.[Mz(OPri)9} (M = Ti, Zr, HI') [13,20] have been synthesised (eq. 45) quantitatively by the equimolar reaction of YCI3 and KM2(OPr)9 derivatives. The derivatives are stereochemically rigid and the N M R spectra are indicative of the structural pattern existing in the solution. The X-ray diffraction analysis performed on a representative titanate derivative reveals a monomeric species (Fig. 35) formed by a face-sharing fusion of three octahedra built about a yttrium and two titanium atoms. Each metal atom bears two terminal ligands which are chlorides in the case of yttrium. The central M309C12 unit of the compound represent a modified M3Xll structural type observed in other metal alkoxides e.g., La3(OBu')9 (Bu~OH)2 [58]. In contrast to the monomeric yttrium derivatives, the analogous bismuth derivatives [13] have a dimeric structure. The molecular structure of [CI2Bi{Zr2

Ti(% 2) ,,0

Fig. 33. Molecular structure of [ICd.[Ta(OPr%}(PriOH)d [131.

~1(2} C1(1)

Fig. 35. Molecular structure of [CI2Y{Ti2(OPr%}] [20].

M. Veith et al.

1026

\/o.

OR

.o /A~oR,

~,

/A, .o /

.o-

/i'~

.o jOR AI

./\ OR

/P'r-----'-°R

,I\o . . . . --'-"'AL

oR

~OR

Fig. 36. Molecular structure of [CI2Bi[Zr2(OPr')9}]2 [13]. (R = Vri;L = priOH)

Fig. 38. Schematic representation of the molecular structure of [CIPr{AI(OPf)4}2(PriOH)]2 [35].

(OPrl)9}]2 is shown in Fig. 36. One of the chlorine atoms present at bismuth interact with the bismuth atom of another 'C12Bi{Zr2(OPrl)9} ' unit to give the observed dimeric form. Similar to the solid state structures of [ISn{Mz(OPri)9}]2 compounds, the [C12Bi{Zr2 (OPr%}]2 derivative displays a tridentate behaviour of M2(OPrl)9 (M = Zr, Hf) units. A series of iodo-heterobimetallic SnW-Ti TM derivatives [13] are obtained by the interaction of SnI4 and KTi(OPr~)5 in appropriate molar ratios (eq. 46). The derivatives are highly fluxional at room temperature, however, structurally diagnostic spectra are obtained on lowering the temperature. The cryoscopic and VT ~gSn N M R studies indicate the monomeric behaviour in solution. The X-ray diffraction study performed on a diiodo-derivative shows (Fig. 37) a six-coordinate tin(IV) surrounded by two iodide and two bidentate {Ti(OPr%} ligands. The two titanium atoms are present in a distorted trigonal bipyramidal arrangement of alkoxo ligands. The chloro(propan-2-ol)bis(tetraisopropoxoaluminato)-prasesodymium(III) dimer, [CIPr{A1 (OPf)4}2(PfOH)]2 [35] represent the only halide heterometallic alkoxide based on a bidentate {AI (OPri)4} moiety (Fig. 38). The centrosymmetric dimer consists of two triangular A12Pr(/l-OPrl)4 (OPr~)4(Pr'OH) + units linked by two chloride bridges. Praseodymium is hepta-coordinate and the coordination polyhedron formed by five oxygens and two

0(21~ }

~

112}

0141

Fig.37.Molecularstructureof[12Sn{Ti(OPr%}2][13].

bridging chlorine atoms can be envisaged as a distorted capped trigonal prism. Although the reactions involving metal halides and an alkali metal reagent proceed quantitatively in most of the cases and the desired target product is obtained in a 'clean' way (no undesirable side products), examples are known where alkali metal or halide ions remain attached to give 'ate' complexes. This problem of salt retention is more frequently encountered in the homo- as well as hetero-metallic alkoxide chemistry of yttrium and lanthanide metals. Evans et al. have reported [59] on the simple chloro-alkoxometallates of the type Y3(OBut)sCI(THF)2 and Y3(OBu')7 C12(THF)2 obtained as the major products in the reactions of YC13 with 3 and 2 equiv, of NaOBu', respectively. Mixed-metal alkoxides representing a new class of LnNas(OBu')~0Cl (Ln = Y, Eu) complexes [60] are obtained in high yields (70 90%) in the reactions involving LnC13 and 10 equiv, of NaOBu'. Interestingly, a hydroxy derivative YNas(OBu')OH belonging to this class is also formed in a similar reaction between YC13 and NaOBu' (which reportedly was contaminated by impurities such as NaOH). Other typical examples include chloride heterobimetallic aryloxide of the type [C1M(OR)3Na] (M = La, Y; R = C6H,(CH2NMe2)-2-6-Me-4) [61] and [CIY2(OR')6Na] ( R ' = C6H4(CH2NMe2)) [61] obtained in the reaction of lutetium or yttrium trichloride with three equivalents of sodium aryloxides. The problem of salt retention is more frequently observed in the case of smaller alkali-metal ions (Li +, Na + ) and can be overcome to a certain extent by using potassium derivatives presumably due to the higher insolubility of KC1, in common organic solvents, in comparison to that of LiC1 and NaC1. However, this problem is not always eliminated by the use of heavier alkali metals, e.g. the reaction of anhydrous LnC13 (Ln = Nd, Er, Lu) with potassium arlyoxide results, irrespective of the amount (3 or 4 equiv.) of KOAr used, in the formation of KLn(OAr)4 derivatives [62].

Hetero (bi- and tri-) metallic alkoxide derivatives The halide heterobimetallic ethoxide [C1Mg{Ti2 (OEt)sC1}]2 (eq. 49) obtained by the dissolution of MgC12 in Ti(OEt)4 is the only well-characterised derivative that represent an alternative synthetic route to halide heterometallic alkoxides. The molecule is observed as a centrosymmetric dimer in the solid state and the overall structure is similar to that of [IBa{Ti2(OPr%}]2 derivative (Fig. 32). The central atom, magnesium, coordinated by the three oxygen and one chlorine atom of the modified face-sharing bioctahedral unit [Yi2(OEt)8C1} and bearing a "terminal' chloride ligand dimerises (Mg(It-Cl)2Mg) to achieve the M3X~ form. A new chloro bismuth-vanadium heterobimetallic oxoalkoxide [BiC13OV(OC2H4OCHd3]2 [53] is obtained (eq. 50) by the reaction of BiC13 with OV (OPri)3 in 2-methoxyethanol. The X-ray structural study reveals the vanadium atom in a distorted octahedral environment whereas, bismuth is present in a distorted capped octahedron of chlorine and oxygen atoms. The OC2H4OCH3 group binds to vanadium in a bidentate fashion which contrasts the monodentate mode observed in homometallic 2-methoxy ethoxide derivatives of Pb" [63] and Bi m [64]. The single crystal X-ray diffraction analysis per-

1027

formed on (CsHs)Sn(/~-OBu')2Ge(OBu') [51] (eq. 47) reveals (Fig. 39a) a nearly planar four-membered SnO2Ge ring terminated at Sn" by the cyclopentadienyl ring and at Ge II by the OBu' group. The two terminal ligands occupy cis positions with respect to the plane of the ring. The sum of the angles around Ge" and Sn H indicate a stereochemically active lone pair giving the two bivalent metal atoms a trigonal pyramidal configuration. Interestingly, a similar reaction between (C5Hs)SnC1 and KPb(OBu')3 afford an unanticipated product (CsHs)Pb(/~-OBu')2Sn(OBu') [50] which involves a transfer of cyclopentadienyl group from tin to lead. However, the overall molecular framework remains the same. Owing to the presence of two N M R active metal nuclei, the solution spectral data were indicative of a transfer of the CsHs ligand from tin to lead. The most interesting feature in the solid state structures of the two derivatives is the different hapticity of the C5H5 [igands toward the tin and lead atoms. The tin germanate shows a peripherally 0l I/3) bonded cyclopentadienyl ring (Fig. 39a) possessing a diene character whereas, a centrally bonded 01S) cyclopentadienyl ring (Fig. 39b) is observed in the tin plumbate. Out of the [(CsHs)Sn{M2(OPr%}] (M = Zr, HI)

C(17) C(13)

~(~

(a)

C(lOa) C ( 8 ~ c(9a) $ "~ I # - C(9)

(b)

~ c(t) Fig. 39. Molecular structures of (a) (CsHs)Sn(#-OBu')2Ge(OBu') [51] and (b) (CsHs)Pb(#-OBu')2Sn(OBu') [50].

M. Veith

1028

2.2.3

rJ ii

et al.

ii

Reactions

The halo-functionalised heterometal alkoxides are interesting synthons and the substitution of anionic halide ligand by an appropriate alkoxo, aryloxo, siloxo or cyclopentadienyl ligand results in a variety of heteroleptic heterometal alkoxy derivatives. Some typical reactions examined (eq. 51) are shown below. XM{M'2 (OR)9} + M"L ~ LM {MI(OR)9} + M"X M

=

Snll,Cd n ;

M' = Ti, Zr, Hf;

(51)

M" = Na, K

L = OPr i, OBu', OSiMe3, OSiPH3, C5H5 ;

C Fig. 40. Molecular structure of [(CsHs)Sn.[Zr2(OPr%}] [50]. The unfilled line shows the partially occupied tin and cyclopentadienyl sites.

[50] derivatives (eq. 48), the zirconium compound characterized by X-ray crystallographic study (Fig. 40) features a (CsHs)Sn + fragment disordered with a 80:20 occupancy between the doubly bridging arms of the Zr2 (OPri)9 unit. An interesting aspect of these heteroleptic ( C s H ; - / O R ) derivatives is their nuclearity with respect to that of the precursors. The substitution of chloride in dimeric [C1Sn{Mz(OPr%}]2 (M = Zr, Hf) complexes by a ligand ( C p ) of higher bulk and denticity renders the product monomeric. This synthetic strategy can be used for obtaining more tractable heterometallic species as in contrast to the extraordinary bridging tendency of OR ligands, a cyclopentadienyl bridging would be thermodynamically disfavoured and can result in less associated heterometallic systems. A recent study reports the formation of heteroleptic (CsHMe2/OMe) titanium(III)-magnesium (II) derivative [(CsHMe4)2Ti(#-OMe)2]zMg [65] by the methanolysis of the hydride complex [(CsHMea)2Ti (/~-H2)]2Mg. The molecule (Fig. 41) contains two nonequivalent octamethyl titanocene moieties each bonded through two bridging methoxy groups to the central magnesium atom.

X = CI, I A controlled alcoholysis of CpSn(OBu')2Ge(OBu') gives a product devoid of cycopentadienyl group and shows only one single environment in the 119Sn N M R spectrum which differs from the H9Sn N M R chemical shift of [Sn(OBut)2]2 and suggests the formation of a tin-germanium heterobimetallic compound with 1 : 1 metal stoichiometry. However, no such product is formed on reacting [Sn(OBu')2]2 and [Ge(OBu')2]2 in equimolar ratio [18b]. Mixed ligand heterometal systems are of considerable interest as they provide access to metal stoichiometries not achievable by pure alkoxide constituent. Efforts to obtain definitive structural information about this compound by single crystal Xray crystallography are currently underway. (C5 Hs)Sn(/~-OBu')2Ge(OBu') + BurOH --* SnGe(OBu~)4 '+CsH6

In analogy to the heterobimetallic derivatives based on main group elements with n s 2 configuration, the novel cyclopentadienyl containing heterobimetallic alkoxides function as soft ligands too and have been used to synthesise elusive examples of heterotermetallic derivatives illustrated below (eq. 53) [66]. (CsHs)Sn(#-OBu')2Ge(OBu t) + Mo(CO)6 (CsHs)Sn(OBu')3Ge-Mo(CO)5 + C O

Me4-~'~.

~-Me OR~ _ _ Ti . . ~

j.Mg

(52)

~OR_ ~

4

/~.~"~

Ti

(n = M0 Fig. 41. Representation of the molecular structure of [(CsHMe4)2Ti(#-OMe)2]2Mg[65].

(53)

Hetero (bi- and tri-) metallic alkoxide derivatives

O{5a} /0(4) .

o(5} %

1029

ination and Lewis acid-base reactions (eqs 54-56) [6,13,14].

Mo

~

O3 (a)

(i) Salt elimination reactions

ICd{M2 (OPf)9} + KM'(OPr~)~ --, [{Cd(OPr')3}M'{M2(OPr%}]2 + K I

O-N4o, ,

M = S n , Ti, Zr, Hf;

(54)

M ' = B a , Sr, Ca

1/2[IBa{Zr2 (OPf) 9}]2 + KHf2 (OPf)9 {Hf2(OPr99}Ba{Zr2(OPf)9} + K I Fig. 42. Molecular structure of [(CsHs)Sn(OBu')3Ge-Mo (CO15] [66].

I/2[IBa{M2 (OPf)9}]2 + KM'(OPf)6 --* {M'(OPr')6}Ba{M2(OPr99} + K I M = Sn, Ti, Zr, Hf;

The X-ray structural study shows (Fig. 42) the selective coordination of metal carbonyl fragment at the Ge" centre, protracted attempts to attach another Mo(CO)5 unit at Sn" were not successful. This observation is of considerable interest when compared with the high reactivity of Sn" centre found in the heterobimetallic alkoxides of the type Sn(/~-OBu')3M and [Sn(/~-OBut)3]2M ' (see Section 2.1).

3.1 Synthesis We have synthesised a large variety of heterotermetallic alkoxides employing both salt elim-

M' = Nb, Ta

The halide-free heterometallic alkoxides can also undergo Lewis acid base reactions with another alkoxide to give novel heterotermetallic systems as illustrated in the following examples (eqs 57-60) [13]. ' [BaTi~ (OPf)~ 0] +~, M~ (OPr'), 0 -

~

ALKOXIDES

Until recently the heterotermetallic nature of the alkoxide clusters based on three different metals was considered ambiguous and the identity of the examples reported [67] mainly on the basis of elemental analysis and low resolution ~H N M R data, in a few cases, was considered to be unreliable. This scepticism was supported by the fact that despite extensive investigations on metal alkoxide chemistry by various research groups, the existence of heterotermetallic alkoxides as discrete molecular species in solution and the solid state could not be unequivocally established. The reactive halide group in halo-functionalised heterobimetallic alkoxides, described in Section 2, can be replaced by a number of anionic alkoxometallate ligands to obtain new heterometal species containing three different metals. The elimination of salt (KC1, KI, etc.) as an insoluble product provides a thermodynamic assist and the yields are nearly quantitative in these reactions. The isolation and characterisation of novel heterotermetallic alkoxides demonstrate that salt elimination is not merely an ionexchange phenomenon and if appropriate combinations of halide precursors and alkali reagents are chosen, it can prove to be an efficient strategy for the directed synthesis of heterometal alkoxide systems.

(56)

(ii) Lewis acid-base reactions

2

3. H E T E R O T E R M E T A L L I C

(55)

-

{M(OPr96 } Ba{Ti2 (OPrl)9 }

(57)

M = Nb. Ta [BaTi2 (OPr'), 0] + 2/n[Zr(OPf)4], Ba2Zr~Ti2(OPrl)20

(58)

( P r ' O ) f d {HL (OPri) 9} + Ti(OPf )4 {Ti(OPrl)5 }Cd{Hf2 (OPf)9 }

(59)

(PRO) Cd {H f2 (OPf)9 } + ~ [Ta(OPf)5]: --, [Ta(OPf)~}fd{Hf2 (OPf)9 }

(60)

3.2 Structural and spectral features Recently, we reported on the first synthesis and structural characterization of a series of novel heterotermetallic isopropoxides of general formula [{Cd(OPff)3}M'{M~(OPr')9}]2 ( M ' = B a , Sr, Ca; M = Sn, Ti, Zr, Hf) [6,13,14] (eq. 54). The most remarkable phenomenon encountered in the formation of these heterotermetallic derivatives is the exchange of the central atoms (Cd and Ba) between the precursors ICd{M2(OPrS)9} and KM'(OPri)3 . Astonishingly, no catastrophic breakdown or change in the structural features of the precursor complexes occurs and the M2(OR)9 sub-structure in the obtained heterotermetallic compounds binds to the alkaline earth metal atom rather than to the cadmium as anticipated in the targeted molecule. The rearrange-

M. Veith et al.

1030

ment of metallic elements in the formation of heterometallic framework is an unprecedented phenomenon in metal alkoxide chemistry and emphasises the need of single-crystal X-ray crystallographic studies in the complete characterization of heterometal alkoxides. The generality of the reaction has been examined for various M2(OR)~ (M = Sn 'v, Ti 'v, Zr W, Hf W) structures and alkaline earth metals (Ba, Sr, Ca). The mechanism for the observed switching of metals is not clear, however, it is possibly induced by the greater oxophilicity and tendency of larger alkaline earth metal cations to achieve higher coordination numbers. Another interesting aspect of this study is the role of multidentate Me(OPr')~ unit in the formation of heterotermetallic systems, which exhibit extraordinary versatility in binding the different metal atoms (Cd in the precursor and an alkaline earth element in the resulting complex} leading to the formation of a thermodynamically favoured heterotermetallic framework. The [{Cd(Oer93}M'[M'(OPr')9112 ( M ' = Ba, Sr, Ca ; M = Sn, Ti, Zr, HI) derivatives are isostructural and the heterotermetallic framework (Fig. 43) can be viewed as a combination of two triangular BaM2(l~3-OPri)2(l~e-OPri)3(OPri)4 units which are joined together by a [(PriO)eCd(it:-OPr92Cd (OPri)2] 2 unit. Alternatively, the structures can be described as a spirocyclic linking of two Ba(OPr%L (L = M2(OPri)9) units to a four membered CddOpri), ring. The novel potassium alkoxometallates KM'(OPFF)3 act as a source to generate coordinatively unsaturated "M'(OPr~)3 ' species which owing to the propensity of M '-'+ to command higher coordination number initiates a rearrangement of metal atoms to form a stable heterotermetallic system that optimally fulfills the requirements of different metal atoms. In contrast to the deceptively simply spectroscopic pattern generally exhibited by the heterometal derivatives, the solution ~H and I~C N M R spectra of [{Cd(OPr~)3}M'{M'_,(OPr~)~,}]2 [6,13,14] compounds are structurally informative. The retention of the dimeric structure (Fig. 43) in solution is indicated by the cryoscopic molecular weight measurements and is corroborated by the similar ~3Cd NMR chemical

M(2}I

shifts observed in both solution and the solid state. In view of the wide dispersion and extreme sensitivity of the ~3Cd chemical shifts to the local coordination environments, the presence of cadmium in the heterobimetallic precursor and the resulting heterotermetallic species has proved a valuable tool in studying the structural behaviour of these derivatives. Following the eq. 54, we have recently been able to characterise a novel heterotermetallic alkoxide of composition Ba:ZreTi2(OPri):0 [13] which represents the first example of a structurally characterised heterotrimetallic alkoxide based on 1 : 1 : 1 metal stoichiometry. The molecule is highly symmetrical and exhibits a 1 : 1 metal disorder of tetravalent elements [27]. Figure 44 shows one of the possible modes of disordering of Ti w and Zr ~v. The structural features are closely related to that of [1Ba [Mz(OPr%~ ]2 derivatives. 4. APPLICATIONS The versatility of metal alkoxides as convenient precursors to advanced ceramic materials ria sol-gel and MOCVD processes, is widely acclaimed, however, the use of well-characterised ~single source' precursors for these purposes remain extremely limited primarily due to the non-suitability of metal ratios present in the available molecules and that required in the desired ceramics. Among the useful heterometallic precursors,

°

Fig. 44. Molecular structure of [Ba,ZL,Ti2(OPr%0] [13].

~

)M(~I,)

Fig. 43. Representatmn of the metal-oxygen skeleton observed in [{Cd(OPr%}Ba [MdOPr%}]2 derivatives (M = Sn, Ti, Zr, HI) [6,13].

Hetero (bi- and tri-) metallic alkoxide derivatives [LiNb(OEt)6],, [17] is a well-known example. Owing to the presence of Li/Nb atomic ratio corresponding to lithium niobate ceramic, useful for its ferro- and piezo-electric properties, [LiNb(OEt)6],, has been used in numerous studies to obtain high quality LiNbO3 films and powders [68]. Although volatile heterometal alkoxides are attractive precursors for the deposition of mixed-metal oxides by chemical vapour deposition technique, their use in obtaining multimetallic ceramics has not been pursued and lack of systematic studies make it a thrust area for research. In an attempt to develop new and effective routes to advanced ceramics, we have examined a number of homo- and hetero-metallic alkoxides to deposit new materials on a variety of substrates (quartz, silicon, copper, aluminium) by the MOCVD process. The tert-butoxides of bivalent Ge, Sn and Pb as well as their heterometal derivatives M'M_,(OBu')6 (M = Ge, M ' = Sr, Ba; M = Sn, M ' = Ca, Sr, Ba; M = Pb, M ' = Ca, Ba) when subjected to a single source chemical vapour deposition process produced biphasic metal/metal-oxide composites of the type M/MO: or M/M'MO~ [69]. In all the cases, the uniform dispersion of metallic particles in a ceramic matrix (MO_, or M ' M O 0 has been confirmed by Xray diffraction, electron spectroscopy, energy dispersion X-ray analysis (EDX) and photoelectron spectroscopy (XPS). The bivalent metal M" (M = Ge, Sn, Pb) in the precursors undergo disproportionation (from + 2 oxidation state) to give an elemental (M °) phase and the phase containing formal M Iv cation. The composites consist of ball-shaped particles on which smaller particles are placed in a fractal manner. In the case of the composite Sn.BaSnO3, the core of the ball-shaped particles has been analysed as pure elemental tin. The fundamental novel feature observed in the preparation of these composites is the chemically driven synthesis of two distinct phases with a regular dispersion of one (metallic phase) into another (ceramic phase). Further, on a hot substrate, all components are unified at the shortest distance possible which makes the adjustment of the physical parameters less important as the decomposition is predominantly chemical in nature. Magnesium aluminium spinel (MgAI204) is a promising electronic material [70] and thin fihns of MgAI204 using single molecular precursors have been deposited ria both sol gel and MOCVD techniques. However, the higher carbon content in the resulting material or higher decomposition temperature for the precursor molecule are the intrinsic problems that persist. To design precursors with reduced organic part and enhanced volatility, we have synthesised new MgA1 hydrido heterobimctallic alkoxides of the type [Mg.{AI(OBu')3H}2] [25] and [Mg{AI(OBu')2H2}2 [25]. The single crystal diffraction study performed in the case of mono-hydrido derivative reveals a molecular structure similar to that of [Mg[Al(OBut)4}2] (Fig. 20) where one of the terminal OBu' groups on each of the AI atoms has been replaced by a hydride

1031

ligand. The modified heterometallic derivatives have indeed proved to be extraordinary precursors for the deposition of MgAI204 thin films, on a variety of substrates, with significantly low decomposition temperatures and minimum carbon impurities [25].

5. CONCLUSIONS AND O U T L O O K The present review outlines the recent research performed on heterometal alkoxides and emphasises the potential of halide heterobimetallic alkoxides as attractive synthons to heterotermetallic alkoxides and cyclopentadienyl containing heterobimetallic systems. The successful characterization of various stable and soluble heterotermetallic alkoxides is an exciting development which provides optimism, for the first time, for the synthesis of single source precursors to important ternary ceramics e.g., PZT [71] and PNM [72]. The stepwise chelation of a central metal atom by different alkoxometallate units, opens up new possibilities in the rational construction of heteropolymetallic ensemble, following a building block approach. However, the use of presently known alkoxometallate ligands towards various metal halides impose a limitation of fixed metal stoichiometries and the heterometal systems resulting from charge neutralisation principle, although of considerable academic interest can not serve as "molecules of utility'. The synthesis of single source precursors with desired combination of metallic elements require novel alkoxometallate anions based on metals present in unusual oxidation states or with inherent versatility in displaying different coordination numbers. The possibilities of modifying the heterometal alkoxides by introducing hetero-ligands such as a cyclopentadienyl, hydride, siloxide etc. can be advantageous in achieving new metal ratios and new type of reactivity. The sub-valent state of Group 14 elements in M'M2(OBu')6 (Section 2) play a key role in the chemical vapour decomposition process (Section 4) since the use of [Ga(OBu')3]: as precursor in a CVD process gave only Ga203 and no elemental gallium was detected. However in cases where variable oxidation states of the metallic elements are not stable, a similar phenomenon could be induced by using hydride containing alkoxides [73]. The hydride modified alkoxides represent a promising class of heteroleptic alkoxide derivatives and it is relevant to add that using aluminium and gallium hydrido alkoxides of the type [HM(OBu')2]2 and [H,M(OBut)]2 (M = AI, Ga), we have obtained nanoscaled A1/A120~ and Ga/Ga20, composites, respectively [73]. We have recently shown that hydride containing Mg-A1 heterobimetallic alkoxides are excellent precursors to pure MgAI204 ceramic. The emerging chemistry of polynuclear metal hydrido alkoxides and the potential for the development of new types of reactivity has recently been reviewed by Chisholm [74]. The hydrido alkoxides are highly reactive species and their reac-

M. Veith et al.

1032

tions with hydroxy reagents of the type (C6Hs)3SiOH and (C6Hs)zSi(OH)2 has led to novel molecular alumopolysiloxanes [75]. Also compounds involving siloxide ligands are attractive precursors for the dispersion of metallic elements in SiO_, or AI~O3matrices. We have reported on the new heterosiloxanes of bivalent Group 14 elements of the type [Ph3SiOM(/~OBu')]2 (M = Ge, Sn, Pb) [76]. This approach can be extended to mixed-metal systems. The reactivity studies on heterometal alkoxides are limited to a few reports. Ligand exchange or controlled hydrolysis reactions are of great importance in the context of sol-gel process where the precursor is often dissolved in an alcohol/protic reagent (of different pK~ and steric demands) used as solvent and the isolation and characterization of partially modified 'intermediate' molecules can throw light on the chemical transformation process taking place at molecular level. In view of the inherent difficulties (e.g. twinning, plasticity, loss ofcrystallinity at ambient temperature) associated with the deceptively simple spectral patterns commonly observed in metal alkoxide chemistry, the structure determination by X-ray crystallography is often not straightforward and the unambiguous characterization of heterometal derivatives, in the absence of X-ray crystallographic data, requires an arsenal of complementary analytical techniques for instance multinuclear solution and solid state N M R studies, mass spectroscopy. The variable temperature N M R studies can provide useful information about the degree of aggregation of the complex, the coordination number at metal centres and the number of unique ligand environments present in the molecule. The use of N M R active nuclei can be of immense help in probing the solution structure and other associative-dissociative processes (monomer~timer equilibrium) occurring in the solution especially in those cases where the single-crystal subjected to the X-ray diffraction analysis is not representative of the bulk material. Also, the molecular weight studies assume special significance in view of the tendency of heterometal alkoxides to dissociate in the solution.

Acknowledgements--Thecontinuing support of the Deutsche Forschungsgemeinschaft (SFB-277) and Fonds der Chemischen Industrie, in accomplishing this work, is gratefully acknowledged. SM is thankful to the Alexander yon Humboldt Foundation, Germany for a research fellowship.

2. 3. 4. 5.

6.

7.

8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18.

19.

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