Electrocatalytic reduction of oxo anions by aqueous molybdenum-catechol complexes

Abstract

The electrocatalytic reduction of nitrite, chlorate and bromate ions by monomeric molybdenum-catechol complexes has been investigated by dc polarography and cyclic voltammetry in pH 8–10 catechol buffers. The Mo(V) and Mo(IV) complexes. MoO(Hcat)(cat) 2 2− and Mo(cat) 3 2− , produced by one- and two-electron reduction of MoO 2 (cat) 2 2− are catalytically active, but the Mo(III) complex, Mo(cat) 3 3− , is not. This sequence of reactivities among molybdenum oxidation states leads to peak-shaped electrochemical responses characteristic of catalytic chemical reactions coupled between consecutive charge transfers. Apparent second-order rate constants in 1 M KCl, 0.15 M catechol, pH 9.4 are 11.1±0.6, 1.19±0.06 and 460±45 M −1 s −1 for reactions of MoO(Hcat)(cat) 2 2− and 85±10, 30±2 and 2700±300 M −1 s −1 for reactions of Mo(cat) 3 2− with NO 2 − , ClO 3 − and BrO 3 − , respectively. A mechanism is proposed for oxo anion reduction by MoO(Hcat)(cat) 2 2− . Substrate first binds to the Mo(V) center at a coordination site created by dissociation of monodentate catechol (Hcat − ). The unshared electron pair on the central atom of the substrate is implicated in this process because anions which lack this structural feature (NO 3 − and ClO 4 − ) are catalytically inactive. We propose that the N-, Cl- or Br-bonded substrate then rearranges to an O-bonded form which undergoes rapid oxygen atom/electron transfer to produce MoO 2 (cat) 2 2− and reduced product. Similar mechanistic features are indicated for catalysis by Mo(cat) 3 2− . Mo(cat) 3 3− apparently is a kinetically inert d 3 transition metal complex which does not readily undergo the ligand dissociation and substrate coordination reactions that are prerequisite to electron transfer. The significance of these findings is discussed in light of previous observations of Mo-catalyzed oxo anion reductions.