Metal Alkoxides and Dialkylamides

D. C. Bradley, Metal Alkoxides and Dialkylamides, in Advances in Inorganic Chemistry and Radiochemistry , eds. H. J. Emeleus and A. G. Sharpe, Academic Press, New York,... 

Metal alkoxides and dialkylamides. (D. C. Bradley, Adv. Inorg. Chem. Radiochem., 1972,15, 259). 

Besides binary compounds (MLa.) containing the uninegative ligand L = XRa-, a number of ternary (MLa-L y) and quaternary (ML L L" ) complexes were also isolated by use of supplementary ligands U and L", and there is scope for further exploitation of the field. In this account, we restrict the discussion to metal alkoxides and metal dialkylamides since the metal alkyls are the subject of papers by Lappert et al, (6, 7). 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

This method is particularly useful for the synthesis of the alkoxide and phenoxide derivatives of the earlier transition elements. The method is extremely convenient in view of the high volatility of the generated dialkylamines, which are readily removed in vacuum. One major drawback is the synthetic availability of the corresponding metal dialkylamide complex. In some cases the method represents not only the most convenient but also the only synthetic route to an alkoxide derivative. Hence, zirconium tetra-t-butoxide is formed in excellent yield from Zr(NEt2)4 and Bu OH, and the Vlv and CrIV r-butoxides are also readily obtained via this pathway (equation 11).74... 

The individual dialkylamides were first applied for the synthesis of metal alkoxides by Gilman etal. who used them forthe preparation of U(OR)4 [856]. The universal character of this method was demonstarted later by Bradley and Thomas [196], who reported the synthesis of Cr(OBu )4, V(OR)4, Sn(OR)4, and Nb(OBu ) usingthis route. The advantage of this technique lies in almost quantitative yields. It also can be applied in the preparation of the derivatives of any alcohols including M(OR )n. Formation of only gaseous side products (i.e., HNR2) that can be easily removed by the evacuation can be considered to be another advantage of the technique. However, in some cases the formation of... 

Carbon dioxide readily undergoes insertion reactions probably via initial C02 complexing, with metal alkoxides, hydroxides, oxides, dialkylamides, and metal hydrides and alkyls 103... 

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) ... 

In addition to the above, alcoholysis or transesterification reactions of metal alkoxides themselves have been widely used for obtaining the targeted homo- and heteroleptic alkoxide derivatives of the same metal. Since the 1960s, the replacement reactions of metal dialkylamides with alcohols has provided a highly convenient and versatile route (Section 2.9) for the synthesis of homoleptic alkoxides of a number of metals, particularly in their lower valency states. 

General reviews of transition metal complexes of alkoxides, dialkylamides and nitrides showed that the main focus of attention was on the earlier transition metal elements, in particular, the Ti, V and Cr triads. 

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