Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of...

We previously reported that the [Rh^III(dmbpy)₂Cl₂]⁺ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H₂-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [Ru^II(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [Rh^I(dmbpy)₂]⁺, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H₂ following different pathways. To improve the current knowledge of these key intermediate species for H₂ production, we performed herein a detailed electrochemical investigation of the [Rh^III(dmbpy)₂Cl₂]⁺ and [Rh^III(dtBubpy)₂Cl₂]⁺ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH₃CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [Rh^I(Rbpy)₂]⁺ and, more importantly, the hydride [Rh^III(Rbpy)₂(H)Cl]⁺ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF₃CO₃H), owing to the fast reaction of the electrogenerated [Rh^I(Rbpy)₂]⁺ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H₂ is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H₂ evolution process.