Reduction of Nitrate to Nitrite by Molybdenum-Mediated Atom Transfer: A Nitrate Reductase Analogue Reaction System

Article abstract of DOI:10.1021/ja00188a026

The kinetics of the oxygen atom transfer reaction Mo^IVO(L-NS2)(DMF) + NO3^-=Mo^VIO2(L-NS2) + NO2^- + DMF was investigated in DMF solution (L-NS2=2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine(2-)).The reaction is quantitative and well-behaved when conducted in the presence of an excess of nitrate and ca. 1.5 equiv. of sulfamic acid, which rapidly scavenges nitrite that otherwise bleaches the Mo chromophores.It is characterized by saturation kinetics in which nitrate reversibly forms a substrate-Mo(IV) complex that generates products in a first-order pathway with k₁=(1.49+/-0.05)*10^-3s^-1 at 295.5 K, ΔH^<*>=23.7+/-0.6 kcal/mol, and ΔS^<*>=8.0+/-2.0 eu.The moderate activation entropy suggests that the ground and transition states are structurally similar.The activation enthalpy is indistinguishable from previously reported values for the reductions of S-oxide and N-oxide substrates, which also follow pseudo-first-order kinetics.Inasmuch as the difference in S-O (Me2SO) and N-O (pyridine N-oxide) bond energies is about 14 kcal/mol, the essentially constant activation enthalpies indicate that the transition state is reached without significant substrate bond weakning.The recently introduced thermodynamic reactivity scale for oxo transfer as applied to substrates with N-O bonds is discussed.This work contributes the only well-documented reduction of nitrate to nitrite mediated at a Mo(IV) atom and demonstrates that reduction of nitrate by atom transfer is plausible (but unproven) pathway in the mechanism of action of nitrate reductases.