Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N-N Bond of 2,2′-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N-N bond cleavage of 2,2′-azopyridine. Reaction of MCl₅ (M = Ta and Nb) with 2,2′-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(-Npy)Cl₃]_n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N-N bond of 2,2′-azopyridine. The initial products of the above reactions were determined to be 2,2′-azopyridine-bridged dinuclear complexes, [(MCl₄)₂(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl₅ with 2,2′-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N-N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [ⁿBu₄N][Ta₂(μ-Npy)₂Cl₇] (3a), was generated upon addition of ⁿBu₄NCl to complex 1a, while addition of ⁿBu₄NCl to niobium complex 1b gave a polymeric terminal imido complex, [ⁿBu₄N]_n/2[{Nb(-Npy)Cl₃}₂(μ-Cl)]_n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2′-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(-Npy)Cl₃(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2′-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]₂ to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl₃(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).