Heterometallic alkoxides formation

The sol gel chemistry of sihcon alkoxides is much simpler (see Silicon Inorganic Chemistry)P Si is fourfold coordinated (N = z = 4,) in the precursor as well as in the oxide so that coordination expansion does not occur. The electronegativity of Si is rather high compared to transition metals. Silicon alkoxides are therefore not very sensitive toward hydrolysis. Their reactivity decreases when the size of the alkoxy groups increases. This is mainly due to steric hindrance, which prevents the formation of hypervalent sihcon intermediates (see Hypervalent Compounds). Silicon alkoxides, Si(OR)4, are always monomeric. Heterometallic alkoxides have never been obtained via the reaction of a sihcon alkoxide with another alkoxide. Silicon alkoxides have to be prehydrolyzed before Si T M bonds can be formed. 

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-... 

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,... 

Preparation of heterometallic alkoxides between pairs of metals as similar to each other as aluminum and gallium (137) or even niobium and tantalum (138) were described. However, the formation constants of the latter derivative were found to be rather low, precluding its isolation in view of the dynamism of the equilibrium. 

Similarly, in addition to the H NMR and molecular weight measurements in solution, mass spectral confirmation (8) of the stability of alkoxide species such as [(i-Pr-0)3Zr(pi-0-i-Pr)2Be(jtt-0-/-Pr)2Al(0-/-Pr)2] and [O -Pr—0)4Nb(/i-0-/-Pr)3Be(/i-0-/-Pr)2Al(0-i-Pr)2], provided the first evidence for the formation of heterometallic alkoxides containing more than two different metals in the same molecular species. This result opened up a new dimension in heterometallic alkoxide chemistry. 

This strategy was adopted to alleviate tedious techniques of mixing multicomponent precursors with different properties. In view of their technological demands, tin(II) heterometallic alkoxides are the most studied class of compounds and their properties can be modulated by chelating ligands like /3-diketonates, carboxylates, and functionalized alkoxides for specific purposes. These compounds also facilitate the formation of oxoalkoxides, either by hydrolysis or by some side reactions. 

Formation of heterometallic alkoxides by simple mixing, often at room temperature and in nonpolar solvents, is one of the most general routes and corresponds to the complexation step in the sol-gel process. This general reaction [Eq. (8)] applies to nearly all alkoxides, with the exception of the silicon Si(OR)4 ... 

The formation of heterometallic alkoxides is sometimes driven by that of a volatile by-product, mainly a dialkyl ether. Typical examples are obtaining the oxoalkoxides Ba4Ti404(OiPr)i6(iPrOH) i and Pb6Nb404(0Et)24 ... 

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. ... 

V. C. Kessler, S. Cohil and S. Parola, Interaction of some divalent metal acetylacetonate with Al, Ti, Nb and Ta isoproxides. Factors influencing the formation and stability of heterometallic alkoxide complexes, Dalton Trans., vol. 4, pp. 544-550, 2003. 

Further, the chemistry of formation of a variety of heterometallic alkoxides with the above and other coordinating figands has been described in another review article entitled New perspectives in the tailoring of the heterometallic alkoxide derivatives . 

Heterometallic alkoxides, therefore, constitute a novel class of heterometal coordination systems, stabilized by alkoxide bridges without the support of any auxiliary ligands (like CO) or metal-metal bonds. The factors responsible for this extraordinary stability of heterometallic alkoxides are not yet fully understood, but the formation of... 

Heterometallic Alkoxides formed via Formation of Heteronuclear Metal—Metal Bonds or Isomorphous Substitution... 

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. 

A modification of this route is a simultaneous reaction of two metals with alcohol that can be considered as the in situ reaction of the alkoxides formed. Thus the interaction of Mg and A1 with ethanol (leading to the formation of the perfectly soluble MgAl2(OEt)8) occurs very rapidly and does not need activation, in contrast with the reactions of individual metals with alcohol, leading to polymeric Mg(OEt)2 andAl(OEt)3 [1101, 887] (see Section 2.1). The preparation of the heterometallic derivatives has been achieved even via anodic oxidation of one metal in solution of the other (the complex solutions obtained have been used for the deposition of films in sol-gel technique) [1777]. 

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]. 

As a general observation, it is important to note that reaction between alkoxides and other metal derivatives, such as acetates, can be strongly dependent on the solvent and/or the addition order of the reactants [84]. Thus, no reaction is observed between anhydrous Pb(OAc)2 and Nb(OEt)s in THE at room temperature, but the acetate dissolves readily in toluene at room temperature in the presence of the niobium alkoxide with formation of a Pb-Nb species [80]. Another general feature is that in heterometallic acetatoalkoxides obtained by this route, the metal introduced as an alkoxide appears predominant. More condensed species are obtained if the reaction is achieved at higher temperatures [93] ... 

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... 

The first type of transformations is characteristic of both homo- and heterometallic derivatives and leads to formation of stable fully substituted complexes with chelating ligands from the mixed-ligand derivatives. It has been first observed and described in detail by Wengrovius (1986) for the alkoxide )8-diketonate derivatives of aluminium and then was studied also by Errington (1998) for the same type of titanium derivatives ... 

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