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 t he [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 ter minal ligands have a profound effect on the relative rates of hydrazin e reduction. Specifically, as the number of labile solvent molecules c oordinated to the V atom decreases, the relative rate of hydrazine red uction decreases. The behavior also is observed with the [(HBpz3)VFe3S 4Cl3](2-) cubane (L, L', L'' = hydrotris(pyrazolyl)borate, n = 2), whe re all coordination sites on the V atom are ''blocked''. The latter sh ows no catalytic or stoichiometric hydrazine reduction and its structu re has been determined. To investigate the role of the Fe sites in the [VFe3S4](2+) cubanes during catalysis, a series of cubanes [(DMF)(3)V Fe(3)S(4)X(3)](-) (X = Cl-, Br-, or I-) was synthesized. Relative rate s 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 t he site directly involved in the binding and activation of hydrazine. Additionally, reduction of phenylhydrazine to ammonia and aniline is o bserved in these systems. Importantly, the single cubane (Me(4)N)[(PhH NNH(2))(bpy)VFe3S4Cl3] has been synthesized, indicating the ability of a hydrazine-like substrate molecule to interact directly with the V a tom. These reactivity studies are compared to those communicated previ ously for the reduction of hydrazine by the [MoFe3S4](3+) cuboidal cor e. Additionally, a plausible reaction pathway for the reduction of hyd razine-like substrates by the [VFe3S4](2+) core is presented. Implicat ions regarding the function of the Fe/M/S (M = V, Mo) center in nitrog enase are discussed.