Alkoxides of molybdenum and

Reactions of Metal-Metal Bonded Alkoxides of Molybdenum and Tungsten... 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

In the preceding paper, Malcolm Chisholm (1) has presented a cogent case for the modeling by metal alkoxides of certain aspects of the structural chemistry and reactivities of metal oxides. The focus of this work has been the dinuclear and polynuclear alkoxides of molybdenum and tungsten, an area of research which has also attracted our interest 02-4) and upon which I would now like to take this opportunity to comment. 

The reaction must involve an alkoxide or aryloxide of a highly electropositive metal, i.e. the M—O bond must have high ionic character. Alkoxides of molybdenum and tungsten of formula M2 (OR)6, which have strong M—O a- and re-bonds will not, for example, react with alkyl halides to yield ether.261... 

In spite of the difficulties involved (98) in the synthesis of the dialkylamide and bis(trimethylsilyl) amide reagents, the procedure was increasingly employed for the synthesis of metal alkoxides, particularly in their lower oxidation states [e.g., Cr" and Mn11 (99)]. This method proved to be specially useful for the preparation of alkoxides of molybdenum and tungsten in their 3- and 4-oxidation states (7) and more recently of U2(0-f-Bu)8-(r-Bu0H) (100) ... 

The remarkable utility of the amide route has been established by the synthesis of alkoxides of molybdenum and tungsten (M) in their lower oxidation states (+3 or +4), e.g. [M(0R)3] and [M(OR)4] . Derivatives of the empirical formula M(OR)3 are of special interest as, in these, association occurs primarily through the formation of metal-metal triple (M=M) bonds rather than through the normal alkoxo bridges (Eq. 2.113)... 

A convenient method for the stepwise substitution of halide groups by alkoxide ligands involves the use of alkoxysilanes. By this method, Handy and coworkers have synthesized and studied a large number of mixed alkoxy halides of molybdenum and tungsten (equation 39).131-134 The method has also been used for the preparation of phenoxy and substituted phenoxy derivatives.133... 

The compounds of these two elements belong apparently to the most studied and most fully characterized alkoxoderivatives of metals. The particular features of their synthesis and physicochemical properties are discussed in detail in a large number of review publications authored by Chisholm [370] who made tremendous contributions to the development of the chemistry of these substances. The synthetic routes to the bimetallic alkoxide derivatives of molybdenum and tungsten (VI) along with the discussion of their physicochemical properties and application prospects are also reviewed in [1638, 908],... 

The ether elimination is also observed as the first step in thermal decomposition of these alkoxides. The same reaction appears also to be responsible for the observed difference in the hydrolytic behavior of molybdenum and tungsten alkoxides the hydrolysis of the W alkoxides leads as for the majority of other metal alkoxides to the formation of hydroxospecies, forming sols and gels on polycondensation, while in the case of Mo alkoxides the added water, being a stronger acid than alcohols, acts as a catalyst for the ether elimination reaction and causes the formation of individual isopolyanions (independently of the nature of the alkyl radical or quantity of water added) [1774] ... 

Complex formation of molybdenum and tungsten alkoxides with the alkoxides of other metals formation of bimetallic alkoxides... 

The complex formation of tungsten alkoxides with the alkoxides of other metals than the alkali, bismuth, and rhenium (see Section 12.21) has not been investigated in detail, and therefore the major attention below will be paid to the description of the bimetallic alkoxides of molybdenum. 

Metal-metal bonds and metal-carbon bonds in the chemistry of molybdenum and tungsten alkoxides. ... 

In 1998, the anodic oxidation of molybdenum and tungsten in alcohols in the presence of LiCl (as electroconductive additive) was found to yield a variety of interesting oxo-metal alkoxide complexes, some of which have been authenticated by singlecrystal X-ray crystaUograpy. 

The fragmentation pattern for tantalum penta-alkoxides Ta(OR)5 (R = Me, Ft, Pr, CH2CH20Me), in their mass spectra is similar to that for alkoxides of zirconium and hafnium, niobium, molybdenum, and tungsten. For example, at first the loss of OR groups occurs, followed by elimination of the molecules of unsaturated hydrocarbons (usually, with the same number of carbon atoms as R). However, the decomposition of Ta(OMe)s differs from that of other homologues by the loss of HCHO molecules and H atoms. Elimination of ethers, R2O, which is accompanied by the formation of the metal oxoalkoxide ions of the above mentioned alkoxides is typical. In the absence of X-ray crystallographic data, considerable effort had been directed earlier to throw light on the molecular complexities of aluminium trialkoxides in different (solid, liquid, or vapour) states. 

High-Valence Metal Clusters. Structural properties of selected hexanuclear high-valence cluster complexes are described in Table 2.5. A series of molybdenum and tantalum derivatives of type [(M6Y8)X6] and [(M6Ys)L8] (M = Mo or W X = halide or alkoxide Y = halide or other monovalent anion and L = neutral Lewis-base) are known. The structures of these species are like that of the anion [(Mo6Cl8)Cl6] illustrated in Fig. 2.19 in which the molybdenum atoms are in the vertex of an octahedron. Metal-metal distances of about... 

Johansson A., Kessler V.G. Synthesis and X-ray single crystal study of the bi- and trimetal-lic alkoxides of molybdenum(VI) and tantalum, Mo2Ta408(OMe)i6, Mo4Ta40i6(0 Pr)i2 and LiMo4Ta30i4(0 Pr)9(OC2H40Me)3. Polyhedron 2000 19 1791... 

Johansson A., Roman M., Seisenbaeva GA., Kloo L., Szabo Z., Kessler V.G. The solution thermolysis approach to molybdenum(V) alkoxides synthesis, soUd state and solution structures of the bimetallic alkoxides of molybdenum(V) and niobium(V), tantalum(V) and tungsten(VI). J. Chem. Soc., Dalton Trans. 2000 387... 

---