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 MCl (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 -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 -Me-CHD was added to each solution, giving [M(═Npy)Cl] (: M = Ta; : 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)(μ-pyNNpy)] (: M = Ta; : M = Nb), which were isolated by treating MCl with 2,2'-azopyridine and -Me-CHD in a 2:1:1 molar ratio. In and , the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes and with 1 equiv of -Me-CHD afforded complexes and . An anionic doubly μ-imido-bridged ditantalum complex, [BuN][Ta(μ-Npy)Cl] (), was generated upon addition of BuNCl to complex , while addition of BuNCl to niobium complex gave a polymeric terminal imido complex, [BuN][{Nb(═Npy)Cl}(μ-Cl)] (). Complexations of and with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl(bipy) (: M = Ta; : 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 and reacted with [RhCl(cod)] to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl(μ-Npy)RhCl(cod) (: M = Ta; : M = Nb).