Metal alkoxides heterometallic

Metal AlkoxidesandDiketonates, and Double-Metal Alkoxides/ Heterometallic Alkoxides, sections in Gelest Inc. Handbook, Milwaukee, WI (1995)... 

As has been mentioned above, of the many works devoted to the studies of the chemical properties of metal alkoxides, we here can consider only the reactions of partial substitution for the alkoxide groups (Chapter 7), decomposition with formation of oxocomplexes (Chapter 5), and hydrolysis with the formation of homo- and heterometallic hydroxides (Chapters 9 and 10). We discuss here the complex formation products of metal alkoxides — the adducts with neutral ligands, M(OR) mL, and the bimetallic alkoxides, precursors of complex oxides in sol-gel technology. 

Besides direct hydrolysis, heterometallic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO)w 1MOM,OM(OR)k 15 where M is Ti or Al, M is a bivalent metal (such as Mn, Co, Ni, and Zn), n is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocyclic monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been published (571). 

The reaction of metal halides with an alkoxide or aryloxide (most commonly an alkali metal alkoxide), can resnlt in metathetic exchange to give the required complex (equation 6). Such reactions can also result in the formation of heterometallic species (such as NaZr2(OR)4) this can be a problem when homometallic aUcoxides are desired, but it is an important route to heterometallic alkoxides and aryloxides. The nature of the product can be influenced by the alkali metal, the alkoxide ligand, and the relative amounts of the reactantsd ... 

Interestingly, the synthesis of the heterotrimetallic alkoxides of beryllium, which began in 1985 (8), was extended in 1988 to include heterotri- and tetra-metallic alkoxides of copper (25) as well as other 3d metals (26). These last two pioneering publications were included as invited papers in two reputed international journals, leading to almost universal acceptance of the formation of the stable species called heterometallic alkoxides (3, 4, 27-33). These al-... 

In the following periodic table (Table I), the elements for which the homo-metallic and heterometallic alkoxide chemistry have been explored in detail are circled and put within a square, respectively. The metals of later Ad and 5d groups are underlined by a semicircle to indicate that not much is known about their alkoxide chemistry (40, 41) due to their instability, which is brought about by facile decomposition pathways (e.g., /3-hydrogen elimination). The elements whose alkoxides have been utilized as synthons for ceramic and other materials are marked with an asterisk ( ). 

Since the preparation of RBeOR from the reaction of BeR2 with R OH by Coates et al. (6), it was shown that reactions of metal alkoxides with organo-lithium or other main group metal alkyls, such as Grignard reagents or aluminium alkyls, can yield (4a) either substitution (Eq. 59) or addition products (Eqs. 60 and 61) the latter often results in formation of heterometallic alkoxides, for example,... 

The general properties of the significantly covalent behavior of metal alkoxides (both homo- and heterometallic), in spite of the polar character of the M5+—O5- bond, were already dealt with in some detail (6). The effect of steric and inductive factors on the extent of polarization of the M6+—O5- bond as well as the consequent degree of association and volatility can be exemplified by the boiling points (under 1-mm pressure) and the observed degrees of their association (given in parentheses) by the three isomeric butoxides of zirconium Zr(0-n-Bu)4 ( 250°C 3.5) Zr(0-sec-Bu)4 ( 150°C 2.0 Zr(0-/-Bu)4 ( 50°C 1.0). However, the similarities in the molecular association of the neopentyloxides of Ti, Zr, and A1 to the secondary rather than primaiy amy-loxides have been adduced to indicate the higher predominance of steric rather than inductive factors in the above directions. Similarly, the insolubility and... 

As mentioned earlier, fairly successful conjectures (147, 148) about the structural features of homo- as well as heterometallic alkoxides were made earlier on the basis of physicochemical studies in solution. Even as late as 1989, Bradley (72) pointed out the special difficulties in the X-ray structural elucidation of metal alkoxide complexes. Such difficulties were reemphasized by Caul-ton and Hubert-Pfalzgraf (4), particularly the heterometallic alkoxides, which in many cases are obtained in a highly plastic (deformable) state even when crystallizable, which renders them unsuitable for single-crystal X-ray analysis. Therefore, the process of X-ray crystallographic structural characterization was gathering considerable momentum (4, 23, 28, 38, 39) and even with the availability of other more sophisticated techniques, publications were being limited mainly to derivatives characterized crystallographically. 

Because of the extremely high susceptibility of metal alkoxides to even traces of moisture, stringent precautions for maintenance of anhydrous conditions (solvents, reagents, equipment, etc.) as well as careful protection from atmospheric moisture were generally regarded as essential conditions for the synthesis of simple binary or heterometallic alkoxides in a pure state. 

Both homometallic and heterometallic oxo-alkoxides can be produced by partial hydrolysis of metal alkoxide precursors via dehydroxylation and dealkoxolation of metal hydroxo alkoxides. The crystal structures of a number of polynuclear titanium oxo alkoxides have been elucidated, these structures having evolved during the slow addition of small amounts of water to Ti(OEt)4 47,48,76-79... 

Transition-metal-based heterometallic alkoxide complexes have been prepared. For example, the heterometallic complex Fe(Al(OBut)4)(Ti2(OPr1)9) was reported but not structurally characterized.494 The reaction of (Rh(/i-OH)(COD))2 with TiMe(OBu )3 at low temperatures afforded (Ti(OBu )2)2-(/i-03)(Rh(C0D))4 (50)495 In the same vein, the Ti/Rh complex (Ti(OPr)gRh(COD))2 was obtained.496... 

Alkali metal alkoxides are often involved in the preparation of metal alkoxides [Eq. 5] heterometallic alkoxides can thus be a side product if the stoichiometry of the reaction is poorly controlled (x>n). 

Although metal b-diketonates can be solubilized in the presence of a metal alkoxide and thus form heterometallic species, at least as intermediates, most reactions are more in favor of the formation of redistribution products [Af(OR)n-x(P-dik) c] as the final result [92]. 

Mixed-metal alkoxides are generally covalent [4]. As a result, they are soluble in organic nonpolar solvents (often less in alcohols), generally more than the parent alkoxides. Some can be sublimed without disproportionation, the volatility following the same variation as for the homometallic alkoxides (OtBu > OiPr > OEt > OMe). Mass spectrometry, for instance, established that heterometallic LiNb fragments were retained in the gas phase for LiNb(OEt)6. 

Data on the reactivity of mixed metal alkoxides are limited. Their reactivity remains dominated by the lability of the metal alkoxo bond, but a problem that needs to be addressed is that of the maintaining or not of the heterometallic unit, of the stoichiometry between the different metals, and finally of the ho-... 

Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic. Partial hydrolysis of [LiNb(OEt)6]< leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in which both the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1 1 molar ratio) offers Ba4Tii30i8(0C2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron of BaOs units surimposed on a Ti06(Ti03)i2 core this Tii3042 core is related to that of the aluminum salt [A1i304(0H)24(H20)]2+ [83]. The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur. 

In covalent alkoxides M(OR)z metal atoms usually exhibit a lower coordination than in the oxide MOz/2- Coordination expansion is therefore a general tendency of the sol-gel chemistry of metal alkoxides. It occurs via nucleophilic addition, leading to the formation of oligomers, heterometallic alkoxides or oxoalkoxides. ... 

Many technically important materials prepared by sol-gel processing, such as perovskites, contain more than one kind of metal atom. The first approach is to employ precursor mixtures, which can be either mixtures of metal alkoxides or mixtures of one or more metal alkoxides and metal salts. The co-hydrolysis and co-condensation of different precursors may result in inhomogeneous materials because of their different reaction rates and the fact that the formation of a gel is a kinetically controlled process. The inhomogeneity can be on any length scale from the nanometer range up to macroscopic phase separation. Common methods to solve this problem and to obtain homogeneous (mixed on a nanometer scale) heterometallic gels from precursor... 

CL-substituted metal alkoxide derivatives can, in principle, also be obtained by ligand exchange reactions between different metal complexes. For example, reaction of Y(0CH2CH20Me)3 with Cu(acac)2 resulted in a variety of species, from which Y3(0CH2CH20Me)s(acac)4 was isolated." Depending on the combination of metal complexes, heterometallic compounds may also be formed in such reactions." ... 

The mass spectra do not give any conclusive evidence in the gas phase probably due to the breakdown of the heterometallic alkoxides into simple metal alkoxides, and finally decomposes into simple metal oxides under the high vacuum pressure used in mass speedometer. 

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