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 H-2 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 H-2 production. The potentials at which these Rh complexes catalyzed H-2 evolution were close to the thermodynamic potentials for the production of H-2 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 H-2 production.