Tantalum complexes with benzynes

The tantalum-benzyne complex (130) is much less reactive than other early transition-metal aryne complexes. It shows no reaction with acetone, benzophenone, benzaldehyde, acetonitrile, 3-hexyne, or methanol. The lack of reactivity of 130 was attributed to nonlability of the PMe3 ligand. Indeed, no phosphine exchange was observed when 130 was mixed with an excess of PMe3-d9. Refluxing 129 in a mixture of methanol and toluene (3 10 v/v) leads to clean formation of 131. This presumably results from reaction of a 16-electron benzyne complex with the alcohol. 

The phenyl complexes often decompose as metal-alkyl complexes according to the P-H-elimination mechanism. The benzyne intermediate can sometimes be stabilized as shown by Schrock with a tantalum complex ... 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

A tantalum-benzyne complex possessing a mdo-carborane as a ligand (130) has been prepared66 by reaction of 128 with phenylmagnesium bromide followed by refluxing in toluene in the presence of PMe3 (Scheme 16). An X-ray crystal structure was reported for 130. Bond lengths for the benzyne unit are recorded in Table III. 

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