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

The reaction of [(TaCpX4)-X-R] (Cp-R = eta(5)-C5Me5, eta(5)-C5H4SiMe3; X = Cl, Br) with SiH3Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [((TaCpX2)-X-R)(2)(mu-H)(2)], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [(TaCpX2)-X-R(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(eta(5)-C5Me5)X-2}(2)(mu-H)(2)] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(eta(5)-C5Me5)X-2}(2)(mu-NC6H4C6H4N)] along with the release of molecular hydrogen. When the compounds [((TaCpX2)-X-R)(2) (mu-H)(2)]( )(Cp-R = eta(5)-C5H4SiMe4, eta(5)-C5HMe4; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCp(R)x)(2){mu-(eta(2)-eta(2)-NC6H4C6H4N)}] (CpR = eta(5)-C5H4SiMe3, eta(5)-C5HMe4; 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.