One- and Two-Electron Reactivity of a Tantalum(V) Complex with a Redox-Active Tris(amido) Ligand

A new redox-active, tris(amido) ligand platform, bis(2-isopropylamino-4-methoxyphenylamine [NNNcat]3−, has been prepared and used in the preparation of tantalum(V) complexes. The ligand was prepared in its protonated form by a three-step procedure from commercially available 4-methoxy-2-nitroaniline and 1-iodo-4-methoxy-2-nitrobenzene. Direct reaction of [NNNcat]H3 with TaCl2Me3 afforded five-coordinate [NNNcat]TaCl2 (1), which accepted the strong σ-donor ligand tBuNC to form the six-coordinate adduct [NNNcat]TaCl2(CNtBu) (2). Complex 1 is formally a d0, TaV complex; however, one- and two-electron reactivity is enabled at the metal center by the redox-activity of the ligand platform. Complex 1 was oxidized by one electron to afford the radical species [NNNsq•]TaCl3 (3), which was characterized by solution EPR spectroscopy. Cyclic voltammetry studies of complex 3 showed clean one-electron oxidation and reduction processes at 0.148 and −0.324 V vs [Cp2Fe]+/0, indicating the accessibility of three oxidation states, [NNNcat]3−, [NNNsq•]2−, and [NNNq]−, for the metallated ligand. Complex 1 also can undergo two-electron reactions, as evidenced by the reaction with nitrene transfer reagents to form tantalum imido species. Thus 1 reacted with organic azides, RN3 (R = Ph, p-C6H4Me, p-C6H4tBu), to form [NNNq]TaCl2(NR) (4). Similarly, the tantalum diphenylmethylidenehydrazido complex, [NNNq]TaCl2(NNCPh2) (5), was formed by reaction of 1 with the diazoalkane, N2CPh2.