New cyclopentadienyl rhodium catalysts for electrochemical hydrogen production

The electrocatalytic activity of two new molecular rhodium catalysts was investigated in a hydrogen evolution system in the presence of a proton source using glassy carbon electrodes in acetonitrile and water. Rhodium complexes supported by pbi and pbt ligands, i.e., [Cp*Rh(pbt)Cl](PF₆) (1) and [Cp*Rh(pbi)Cl] (2) (where Cp* is pentamethylcyclopentadienyl, pbt is 2-(2′-pyridyl)benzothiazole, and pbi is 2-(2′-pyridyl)benzimidazole), were observed to electrocatalytically evolve H₂ at potential of −0.90 V vs Ag/AgCl in CH₃CN and CH₃CN/H₂O. Cyclic voltammetry of 1 and 2 in the presence of acid revealed redox waves consistent with the Rh(III)/Rh(I) couple. Bulk electrolysis were used to confirm the catalytic nature of the process for complexes 1 and 2, with turnover numbers in excess of 100 and essentially quantitative faradaic yields for H₂ production. The potentials at which these Rh complexes catalyzed H₂ evolution were close to the thermodynamic potentials for the production of H₂ from protons in CH₃CN and CH₃CN/H₂O, with the small overpotential being 50 mV for 1 as determined by electrochemistry. The complex 1 with more positive Rh(III/I) redox potentials exhibited higher activity for H₂ production.