CATALYTIC AND STOICHIOMETRIC MULTIELECTRON REDUCTION OF HYDRAZINE TO AMMONIA AND ACETYLENE TO ETHYLENE WITH CLUSTERS THAT CONTAIN THE MFE(3)S(4) CORES (M=MO, V) - RELEVANCE TO THE FUNCTION OF NITROGENASE

A functional model for nitrogenase is currently sought in our laborato ry in reactivity studies using various single cubane clusters that pos sess the [MFe(3)S(4)](n+) core, (M=Mo, n=3; M=V, n=2). These clusters are used as catalysts for the reduction of enzymatically relevant subs trates. Substrates such as hydrazine and acetylene are catalytically r educed by (NEt(4))(2)[(Cl-4-cat cat) (CH3CN)MoFe3S4Cl3], I, to ammonia and ethylene respectively, in the presence of added protons and reduc ing equivalents. Hydrazine also is catalytically reduced by the (NEt(4 )) [(DMF)(3)VFe3S4Cl3] cubane under similar conditions. Gas chromatogr aphy was employed to monitor the reduction of acetylene to ethylene an d a trace of ethane. Catalysis in excess of 100 turnovers (for hydrazi ne reduction) and in excess of 15 turnovers (in acetylene reduction) h as been demonstrated over a period of 24 h. A study of the acetylene r eduction reveals saturation kinetics to be operating at high substrate concentrations. A variable temperature kinetic study of acetylene red uction shows a moderate activation energy (E(act)=9(1) kcal mol(-1)) b ut a large entropy of activation (Delta S-double dagger=-32(2) cal K-1 mol(-1)) which extrapolates to a significant Gibbs free energy (Delta G(double dagger)=19(1) kcal mol(-1)). These results are consistent wi th an ordered transition state. Considerable evidence has been amassed which directly implicates the Mo and V atoms as the primary catalytic sites. Replacement of the Mo or V bound, kinetically labile, solvent molecules with non-labile ligands acts to suppress the observed rates of reaction. The Fe sites on I are totally inactive in the reduction o f hydrazine, however they have been found to effect acetylene reductio n albeit at a markedly reduced rate compared to the Mo site. Catalyst integrity has also been demonstrated by a variety of techniques, prima rily EPR spectroscopy which identifies the characteristic S=3/2 signal s of the Mo and V cubanes after at least 18 h reaction time.