Mechanism of the Electrocatalytic Reduction of Protons with Diaryldithiolene Cobalt Complexes

A series
of dimeric cobalt-diaryldithiolene complexes [Co­(S2C2Ar2)2]2, possessing
various aryl para substituents (OMe, F, Cl, and Br),
were studied as electrocatalysts for proton reduction in nonaqueous
media, in an effort to correlate dithiolene donor strength with catalyst
activity. Cyclic voltammetry data acquired for the cobalt-diaryldithiolene
dimers guided the isolation of chemically reduced monoanionic ([Co­(S2C2Ar2)2]−) and dianionic ([Co­(S2C2Ar2)2]2–) monomers. The potassium and tetrabutylammonium
salts of dianionic cobalt-diaryldithiolene complexes have been characterized
by single crystal X-ray crystallography. Treatment of the dianionic
species with stoichiometric quantities of a weak acid afforded H2 and the monoanionic cobalt-diaryldithiolene species. Density
functional theory (BP86) suggests that hydrogen elimination proceeds
through a diprotonated intermediate with a Co–H bond and a
protonated S center. A transition state for transfer of the S–H
proton to the metal center was located with a computed free energy
of 5.9 kcal/mol, in solution (DMF via C-PCM approach).