Hydroxypyridinate-bridged paddlewheel-type dirhodium complex as a catalyst for photochemical and electrochemical hydrogen evolution Available to Purchase

Electrochemical and photochemical hydrogen (H₂) evolution activities of a 6-fluoro-2-hydroxypyridinate (fhp⁻)-bridged paddlewheel-type dirhodium (Rh₂) complex, [Rh₂(fhp)₄], were investigated through experimental and theoretical approaches. In DMF, the [Rh₂(fhp)₄] underwent a one-electron reduction (assigned to Rh₂^4+/3+) at −1.31 V vs SCE in the cathodic region. Adding trifluoroacetic acid as a proton source to the electrochemical cell containing [Rh₂(fhp)₄], the significant catalytic current, i.e., electrochemical H₂ evolution, was observed; the turnover frequency and overpotential of electrochemical H₂ evolution were 18 244 s⁻¹ and 732 mV, respectively. The reaction mechanism of electrochemical H₂ evolution catalyzed by [Rh₂(fhp)₄] in DMF was examined in detail by theoretically predicting the redox potentials and pKa values of the reaction intermediates using density functional theory calculations. The calculations revealed that (i) the formation of a one-electron reduced species, [Rh₂(fhp)₄]⁻, triggered for H₂ evolution and (ii) the protonation and reduction processes of [Rh₂(fhp)₄]⁻ to further reduced hydride intermediates proceeded directly via a concerted proton–electron transfer mechanism. Moreover, [Rh₂(fhp)₄] was shown to be a highly efficient H₂ evolution catalyst (HEC) for photochemical proton reduction reactions when combined with an artificial photosynthetic (AP) system containing [Ir(ppy)₂(dtbbpy)]PF₆ and triethylamine, which served as a photosensitizer and a sacrificial electron donor, respectively. Under visible light irradiation, the total amount of H₂ evolved and its turnover number (per Rh ion) were 1361.0 µmol and 13 610, respectively, which are superior to those of previously reported AP systems with rhodium complexes as HEC.