Generation of bis(dithiolene)dioxomolybdenum(vi) complexes from bis(dithiolene)monooxomolybdenum(iv) complexes by proton-coupled electron transfer in aqueous media

Abstract

Electron transfer oxidation reaction of bis(dithiolene)monooxomolybdenum(IV) (Mo^IVOL^x) complexes is studied as a model of oxidative-half reaction of arsenite oxidase molybdenum enzymes. The reactions are revealed to involve proton-coupled electron transfer. Electrochemical oxidation of Mo^IVOL^x yields the corresponding bis(dithiolene)dioxomolybdenum(VI) complexes in basic solution, where the conversion of Mo^IVOL^dmed supported by a smaller electron donating dithiolene ligand (1,2-dicarbomethoxyethylene-1,2-dithiolate, L^dmed) to Mo^VIO₂L^dmed is faster than that of Mo^IVOL^bdt with a larger electron donating dithiolene ligand (1,2-benzenedithiolate, L^bdt) under the same conditions. Titration experiments for the electrochemical oxidation reveal that the reaction involves two-electron oxidation and two equivalents of OH⁻ consumption per Mo^IVOL^x. In the conversion process of Mo^IVOL^x to Mo^VIO₂L^x, the five-coordinate bis(dithiolene)monooxomolybdenum(V) complex (Mo^VOL^x) being a one-electron oxidized species of Mo^IVOL^x is suggested to react with OH⁻. Mo^VOL^x reacts with OH⁻ in CH₃CN or C₂H₅CN in a 2 : 2 ratio to give one equivalent Mo^IVOL^x and one equivalent Mo^VIO₂L^x, which is confirmed by the UV–vis and IR spectroscopies. The low temperature stopped-flow analysis allows investigations of the mechanism for the reaction of Mo^VOL^x with OH⁻. The kinetic study for the reaction of Mo^VOL^dmed with OH⁻ suggests that Mo^VOL^dmed reacts with OH⁻ to give a six-coordinate oxo-hydroxo-molybdenum(V) species, Mo^VO(OH), and, then, the resulting species undergoes successive deprotonation by another OH⁻ and oxidation by a remaining Mo^VOL^dmed to yield the final products Mo^IVOL^dmed and Mo^VIO₂L^dmed complexes in a 1 : 1 ratio. In this case, the Mo^VO₂ species are involved as an intermediate in the reaction. On the other hand, in the reaction of Mo^VOL^bdt with OH⁻, coordination of OH⁻ to the Mo^V centre to give a six-coordinate Mo^VO(OH)L^bdt species becomes the rate limiting step and other intermediates are not suggested. On the basis of these results, the ligand effects of the dithiolene ligands on the reactivity of the bis(dithiolene)molybdenum complexes are discussed.