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

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.