USE OF [MOFE3S4](3- IDENTIFICATION OF MOLYBDENUM AND IRON ATOMS AS CATALYTIC SITES DURING SUBSTRATE REDUCTION AND IMPLICATIONS FOR NITROGENASE ACTION() SINGLE CUBANES IN THE CATALYTIC REDUCTION OF ACETYLENE TOETHYLENE AND ETHANE )

The reactivity modeling of the nitrogenase enzyme is currently being p ursued in our laboratory using various single cubanes possessing the [ MFe(3)S(4)](3+) core as catalysts for the reduction of enzymatically r elevant substrates (M = Mo, n = 3; M = V, n = 2). One such substrate, acetylene, is catalytically reduced by (NEt(4))(2)[(Cl-4-cat)(CH3-CN)M oFe3S4Cl3] (I: Cl(4-)cat = tetrachlorocatecholate dianion) to ethylene and small amounts of ethane in the presence of added protons (lutidin e hydrochloride) and reducing equivalents (cobaltocene). Gas chromatog raphy was employed to monitor the progress of the reaction. Catalysis in excess of 15 turnovers has been demonstrated over a period of 24 h. A kinetics study reveals saturation kinetics to be operating at high substrate concentrations; however, at lower, optimum substrate levels initial reaction velocities upsilon(0) may be obtained and further use d in a double reciprocal plot, upsilon(0)(-1) vs [C2H2](-1), to determ ine K-m approximate to 17.9 mM and V-max approximate to 1.1 x 10(-4) M /min. A study of this reaction at five temperatures indicates a modera te activation energy (E(ast) = 9(1) kcal mel(-1)) but a large entropy of activation (Delta S+ = -32(2) cal K-1 mel(-1)) which extrapolates t o a significant Gibbs free energy (Delta G(+) = 19(1) kcal mel(-1)). T he large negative Delta S+ is consistent with an ordered transition st ate. Considerable evidence has been amassed which directly implicates the Mo atom of I as the primary catalytic site. Replacement of the Mo bound solvent molecule of I with non-labile ligands acts to suppress t he observed rate of reaction. In addition, the Fe sites on the catalys t have been found to effect substrate reduction albeit at a markedly r educed rate compared to the heterometal. Catalyst integrity has also b een demonstrated by a variety of techniques, primarily EPR spectroscop y which identifies the characteristic S = 3/2 Signal of cubane I after at least 18 h of reaction time. The geometry of addition across the s ubstrate triple bond has been shown to be cis by the use of C2D2 which results in the formation of cis-1,2-C2H2D2 as the predominant isomer. Implications for the action of the nitrogenase enzyme are also addres sed.