Reduction of protons assisted by a hexanuclear nickel thiolate metallacrown: protonation and electrocatalytic dihydrogen evolution.

The hexanuclear metallacrown [Ni6L12] (L is mu-S-CH2-CH2-S-C6H4-Cl) has been demonstrated to functionally resemble the [NiFe] hydrogenases. Protonation of the [Ni6L12] cluster was studied employing 1H NMR spectroscopy by the sequential additions of dichloroacetic acid or p-toluenesulfonic acid monohydrate into solutions of [Ni6L12] in CD2Cl2 and DMF-d7, respectively; protonation takes place on the thioether sulfurs available in the metallacrown. Electrochemical properties of both the parent and protonated [Ni6L12] species have been studied using cyclic voltammetry. Protonated [Ni6L12] shows an interesting electrocatalytic property as it catalyzes the reduction of protons to molecular hydrogen in the presence of protic acids, such as dichloroacetic acid and chloroacetic acid at -1.5 and -1.6 V vs. Ag/AgCl in DMF, respectively. A catalytic cycle has been proposed based on the observations from the NMR spectroscopic and electrochemical studies of the metallacrown. The behavior of this electrocatalyst was further studied by its immobilization on the surface of a pyrolytic graphite electrode; reduction of a dichloroacetic acid solution in acetonitrile on the surface of the modified electrode occurs at 220 mV more positive potential compared to the unmodified electrode.