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(IV)OL(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(IV)OL(x) yields the corresponding bis(dithiolene)dioxomolybdenum(vi) complexes in basic solution, where the conversion of Mo(IV)OL(dmed) supported by a smaller electron donating dithiolene ligand (1,2-dicarbomethoxyethylene-1,2-dithiolate, L(dmed)) to Mo(VI)O(2)L(dmed) is faster than that of Mo(IV)OL(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(IV)OL(x). In the conversion process of Mo(IV)OL(x) to Mo(VI)O(2)L(x), the five-coordinate bis(dithiolene)monooxomolybdenum(v) complex (Mo(V)OL(x)) being a one-electron oxidized species of Mo(IV)OL(x) is suggested to react with OH(-). Mo(V)OL(x) reacts with OH(-) in CH(3)CN or C(2)H(5)CN in a 2 : 2 ratio to give one equivalent Mo(IV)OL(x) and one equivalent Mo(VI)O(2)L(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(V)OL(x) with OH(-). The kinetic study for the reaction of Mo(V)OL(dmed) with OH(-) suggests that Mo(V)OL(dmed) reacts with OH(-) to give a six-coordinate oxo-hydroxo-molybdenum(v) species, Mo(V)O(OH), and, then, the resulting species undergoes successive deprotonation by another OH(-) and oxidation by a remaining Mo(V)OL(dmed) to yield the final products Mo(IV)OL(dmed) and Mo(VI)O(2)L(dmed) complexes in a 1 : 1 ratio. In this case, the Mo(V)O(2) species are involved as an intermediate in the reaction. On the other hand, in the reaction of Mo(V)OL(bdt) with OH(-), coordination of OH(-) to the Mo(V) centre to give a six-coordinate Mo(V)O(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.

Nitrate and periplasmic nitrate reductases

“…154 The involvement of a proton in the reaction has also been demonstrated with model compounds. [173][174][175][176][177] During the two-electron substrate transformation, the metal center changes its redox state by two units, and the catalytically competent center is regenerated in two one-electron steps. In most cases, including nitrate reductase, the intermediate +5 oxidation state of molybdenum has been characterized by electron paramagnetic (EPR) spectroscopy.…”

“…154,189 Furthermore, such OAT reactivity has been observed in discrete inorganic molecules, 144,161,173,[190][191][192][193][194] as well as the proton coupled electron transfer reactions. [173][174][175]177,[195][196][197] Interestingly, stoichiometric nitrate reduction can be achieved by inorganic molybdenum and tungsten compounds with 148,190,198,199 or without 200 dithiolene coordination.…”

Bis‐dioxomolybdenum (VI) oxalyldihydrazone complexes: Synthesis, characterization, DFT studies, catalytic epoxidation potential, molecular modeling and biological evaluations

“…Particularly, the catalytic reactivity of MoO 2 L2 in the (ep)oxidation reactions was little inhibited with t BuOOH. The miscibility of the high polar catalyst with t BuOOH could diminish its reactivity towards the formation of oxo−/peroxomolybdenum complex intermediate catalyst …”

Structural and functional models in molybdenum and tungsten bioinorganic chemistry: description of selected model complexes, present scenario and possible future scopes

“…A recent report on the study of electron transfer oxidation reactions involving proton-coupled electron transfer for several bis(dithiolene)monooxomolybdenum(IV) complexes as a model of the oxidative-half reaction of arsenite oxidase is also available. 99…”