| Abstract
| - The mechanism of dihydrogen addition to Cp*2Ta(CH2)(H) was examined using parahydrogen-induced polarization (PHIP), 13C labeling, and comparison to the related complex Cp*2Ta(CH2)(CH3). Thereaction of para-enriched hydrogen with Cp*2Ta(CH2)(H) leads to polarized resonances for both Cp*2Ta(CH3)(H)2 and Cp*2TaH3, even at the earliest reaction times. Use of the labeled compound Cp*2Ta(13CH2)(H) shows that the polarized resonances of Cp*2Ta(CH3)(H)2 correspond to the two hydride ligands. Theresults thus support a mechanistic pathway of H2 addition to an unsaturated Ta(III) intermediate, [Cp*2Ta(CH3)], rather than addition directly across the TaC bond. In a same sample comparison, the rates ofinitial H2 addition and subsequent C−H reductive elimination for both Cp*2Ta(CH2)(H) and Cp*2Ta(C6H4)(H) were examined. The methylene complex exhibits greater reactivity than the benzyne complex, with themajor difference due to the C−H coupling step, in which formation of methane is more facile than that ofbenzene. The reactivity of the related ethylene hydride complex, Cp*2Ta(C2H4)(H), with hydrogen was alsoexamined. The PHIP study of this system leads to unusual and unexpected polarization, which is found tobe due to a minor impurity in the sample.
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