Abstract
We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl(5) (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)Cl(3)](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, [(MCl(4))(2)(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl(5) 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, [(n)Bu(4)N][Ta(2)(μ-Npy)(2)Cl(7)] (3a), was generated upon addition of (n)Bu(4)NCl to complex 1a, while addition of (n)Bu(4)NCl to niobium complex 1b gave a polymeric terminal imido complex, [(n)Bu(4)N](n/2)[{Nb(═Npy)Cl(3)}(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)Cl(3)(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)MCl(3)(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).