Mechanistic studies on the addition of dihydrogen to tantalocene complexes

The mechanism of dihydrogen addition to Cp(2)Ta(CH(2))(H) was examined using parahydrogen-induced polarization (PHIP), (13)C labeling, and comparison to the related complex Cp(2)Ta(CH(2))(CH(3)). The reaction of para-enriched hydrogen with Cp(2)Ta(CH(2))(H) leads to polarized resonances for both Cp(2)Ta(CH(3))(H)(2) and Cp(2)TaH(3), even at the earliest reaction times. Use of the labeled compound Cp(2)Ta((13)CH(2))(H) shows that the polarized resonances of Cp(2)Ta(CH(3))(H)(2) correspond to the two hydride ligands. The results thus support a mechanistic pathway of H(2) addition to an unsaturated Ta(III) intermediate, [Cp(2)Ta(CH(3))], rather than addition directly across the Ta=C bond. In a same sample comparison, the rates of initial H(2) addition and subsequent C-H reductive elimination for both Cp(2)Ta(CH(2))(H) and Cp(2)Ta(C(6)H(4))(H) were examined. The methylene complex exhibits greater reactivity than the benzyne complex, with the major difference due to the C-H coupling step, in which formation of methane is more facile than that of benzene. The reactivity of the related ethylene hydride complex, Cp(2)Ta(C(2)H(4))(H), with hydrogen was also examined. The PHIP study of this system leads to unusual and unexpected polarization, which is found to be due to a minor impurity in the sample.