Tantalum pentaethoxides

The reactions are exothermic at RT, but successive insertions into polyalkoxides may become more difficult e.g., three of the five ethoxy groups of tantalum pentaethoxide react exothermically at RT with phenyl isocyanate, but the remaining two react only after 5 h at 80°C . The importance of nucleophilic attack of the alkoxy oxygen on the isocyanate is illustrated by reaction of the mixed isopropoxy 2,2,2-trifluoroethoxy vanadium compound, where insertion occurs into the V-OPr-i bond d ... 

Whitleyshowed that Ta2(OEt)io reacted with excess benzoylacetone (bzac) to give Ta(OEt)2(bzac)3 which may contain eight-coordinated tantalum. Mehroira and co-workers have also investigated the reactions of niobium and tantalum pentaethoxides with acetylacetone, benzoylacetone, dibenzoylmethane, methyl and ethyl acetoacetates and ethylbenzoylacetate and only observed partial replacement in these cases. It has been shown that the above reactions proceeded quite smoothly up to the formation of the bis-derivative, and slowed down considerably beyond that stage, but could be forced to trisubstitution for some jS-diketone or ketoester molecules similar results have been reported in the reactions of U(OMe)5 with S-diketones ... 

Figure 27.9 Ta(V) sols prepared from (a) tantalum pentaethoxide, with high rates of hydrolysis and condensation reactions, which lead to the formation of predpitate in the sol and/or gelation, and (b) tantalum...

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