Insights into the mechanism of photosynthetic H2 evolution catalyzed by a heptacoordinate cobalt complex

Abstract

H₂ evolution catalyzed by a heptacoordinate complex under both electro- (1) and photo-chemical (2) conditions is analyzed in detail in order to gain insights into the mechanism of the processes. The complex shows high catalytic activity for hydrogen production in acetonitrile, in the presence of trifluoroacetic acid and triethylammonium tetrafluoroborate as proton sources. Foot-of-the-wave analysis (FOWA) and Tafel plot analysis have been applied to elucidate the kinetics of the hydrogen evolution mechanism and to benchmark the catalytic performances, respectively, with remarkably high rates being obtained at the expense of high overpotentials. Transient absorption spectroscopy measurements have been performed to characterize the kinetics and relevant intermediates formed under photocatalytic conditions. A computational investigation, based on density functional theory (DFT) and time-dependent DFT (TD-DFT), has also been carried out to characterize the intermediate species and support the experimental results. A combination of both experimental and theoretical data suggests the formation of catalytic intermediates displaying dangling pyridine groups in both the one- and two-electron reduced species, possibly acting as proton-transfer relays to enable efficient H–H bond formation.