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 H-2 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).