N=N Bond Cleavage by Tantalum Hydride Complexes: Mechanistic Insights and Reactivity

[Image: see text] The reaction of [TaCp(R)X(4)] (Cp(R) = η(5)-C(5)Me(5), η(5)-C(5)H(4)SiMe(3), η(5)-C(5)HMe(4); X = Cl, Br) with SiH(3)Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCp(R)X(2))(2)(μ-H)(2)], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCp(R)X(2)(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η(5)-C(5)Me(5))X(2)}(2)(μ-H)(2)] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η(5)-C(5)Me(5))X(2)}(2)(μ-NC(6)H(4)C(6)H(4)N)] along with the release of molecular hydrogen. When the compounds [(TaCp(R)X(2))(2)(μ-H)(2)] (Cp(R) = η(5)-C(5)H(4)SiMe(3), η(5)-C(5)HMe(4); X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCp(R)X)(2){μ-(η(2),η(2)-NC(6)H(4)C(6)H(4)N)}] (Cp(R) = η(5)-C(5)H(4)SiMe(3), η(5)-C(5)HMe(4); X = Cl, Br) as intermediates in the N=N bond cleavage process. DFT calculations provide insights into the N=N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N=N bond at two different metal–metal bond splitting stages.