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

The reaction of [TaCpRX4] (CpR = η5-C5Me5, η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) with SiH3Ph resulted in the formation of the dinuclear hydride tantalum­(IV) compounds [(TaCpRX2)2(μ-H)2], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCpRX2(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta­(η5-C5Me5)­X2}2(μ-H)2] derivatives and the cyclic diazo reagent benzo­[c]­cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta­(η5-C5Me5)­X2}2(μ-NC6H4C6H4N)] along with the release of molecular hydrogen. When the compounds [(TaCpRX2)2(μ-H)2] (CpR = η5-C5H4SiMe3, η5-C5HMe4; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCpRX)2{μ-(η2,η2-NC6H4C6H4N)}] (CpR = η5-C5H4SiMe3, η5-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.