The catalytic reduction of nitrobenzene at the [MoVIO2(O2CC(S)(C6H5)2)2]2- complex intercalated in a Zn(II)-Al(III) layered double hydroxide host:: A kinetic model for the molybdenum-pterin binding site in nitrate reductase

The heterogeneous reduction of nitrobenzene by thiophenol catalyzed by the dianionic bis(2-sulfanyl-2,2-diphenylethanoxycarbonyl) dioxomolybdate(VI) complex, [(MoO2)-O-VI(O2CC(S)(C6H5)(2))](2-), intercalated into a Zn(II)-Al(III) layered double hydroxide host [Zn3-xAlx(OH)(6)](x+), has been investigated under anaerobic conditions. Aniline was found to be the only product formed through a reaction consuming six moles of thiophenol for each mol of aniline produced. The kinetics of the system have been analyzed in detail. In excess of thiophenol, all reactions follow First-order kinetics (In([PhNO2]/[PhNO2](0)) = -k(app)t) with the apparent rate constant k(app) being a complex function of both initial nitrobenzene and thiophenol concentrations, as well as linearly dependent on the amount of solid catalyst used. A mechanism for this catalytic reaction consistent with the kinetic experiments as well as the observed properties of the intercalated molybdenum complex has thiophenol inducing the initial coupled proton-electron transfer steps to form an intercalated Mo-IV species, which is oxidized back to the parent Mo-VI complex by nitrobenzene via a two-electron oxygen atom transfer reaction that yields nitrosobenzene. This mechanism is widespread in enzymatic catalysis and in model chemical reactions. The intermediate nitrosobenzene thus formed is reduced directly by excess thiophenol to aniline. The values of rate coefficients indicate that reduction of nitrobenzene proceeds much faster than proton-assisted oxidation of thiophenol. This may account for the observation that the presence of protonic amberlite IR-120(H) increases considerably the rate of the overall reaction catalyzed. Activation parameters in excess of the protonic resin and PhSH were DeltaH(not equal) = 80 kl mol(-1) and DeltaS(not equal) = -70 J mol(-1) K-1. The large negative activation entropy is consistent with an associative transition state. The present system is characterized by a well-defined catalytic cycle with multiple-turnovers reductions of nitrobenzene to aniline without appreciable deactivation. (C) 2001 John Wiley & Sons, Inc.