CATALYTIC REDUCTION OF HYDRAZINE TO AMMONIA BY THE VFE3S4 CUBANES - FURTHER EVIDENCE FOR THE DIRECT INVOLVEMENT OF THE HETEROMETAL IN THE REDUCTION OF NITROGENASE SUBSTRATES AND POSSIBLE RELEVANCE TO THE VANADIUM NITROGENASES

The catalytic behavior of synthetic Fe/V/S clusters that structurally resemble the Fe/V/S site of nitrogenase is reported. The [(L)(L')(L'')VFe3S4Cl3](n-) clusters (L, L', L'' = DMF, n = 1; L = PEt(3), L', L'' = DMF, n = 1; L, L' = 2,2'-bipyridyl, L'' = DMF, n = 1) that contain the [VFe3S4](2-) cuboidal core are effective catalysts in the reduction of hydrazine (a nitrogenase substrate) to ammonia in the presence of cobaltocene and 2,6-lutidine hydrochloride as sources of electrons and protons, respectively. Reactivity studies show that V-coordinated terminal ligands have a profound effect on the relative rates of hydrazine reduction. Specifically, as the number of labile solvent molecules coordinated to the V atom decreases, the relative rate of hydrazine reduction decreases. The behavior also is observed with the [(HBpz3)VFe3S4Cl3](2-) cubane (L, L', L'' = hydrotris(pyrazolyl)borate, n = 2), where all coordination sites on the V atom are ''blocked''. The latter shows no catalytic or stoichiometric hydrazine reduction and its structure has been determined. To investigate the role of the Fe sites in the [VFe3S4](2+) cubanes during catalysis, a series of cubanes [(DMF)(3)VFe(3)S(4)X(3)](-) (X = Cl-, Br-, or I-) was synthesized. Relative rates of hydrazine reduction with each catalyst were virtually identical, indicating little or no involvement of the Fe atoms during catalysis. The result's of this study strongly implicate the heterometal (V) as the site directly involved in the binding and activation of hydrazine. Additionally, reduction of phenylhydrazine to ammonia and aniline is observed in these systems. Importantly, the single cubane (Me(4)N)[(PhHNNH(2))(bpy)VFe3S4Cl3] has been synthesized, indicating the ability of a hydrazine-like substrate molecule to interact directly with the V atom. These reactivity studies are compared to those communicated previously for the reduction of hydrazine by the [MoFe3S4](3+) cuboidal core. Additionally, a plausible reaction pathway for the reduction of hydrazine-like substrates by the [VFe3S4](2+) core is presented. Implications regarding the function of the Fe/M/S (M = V, Mo) center in nitrogenase are discussed.