New cyclopentadienyl rhodium catalysts for electrochemical hydrogen production

- Rhodium complexes supported by pbi and pbt ligands to electrocatalytically evolve H2.
- Rh complexes with turnover numbers over 100 and quantitative faradaic yields for H2.
- The pbi complex with more positive Rh(III/I) redox potential for higher H2 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.

Two new (pentamethylcyclopentadienyl)Rh(III) complexes containing pyridyl-benzothiazole and pyridyl-benzimidazole were prepared and a systematic study of proton electroreduction catalyzed by the two new rhodium catalysts was performed at glassy carbon electrode. The Rh complex of pyridyl-benzothiazole was more efficient in the electrocatalytic hydrogen production in the presence of trifluoroacetic acid.107