Mimicking of Tunichlorin: Deciphering the Importance of a β-Hydroxyl Substituent on Boosting the Hydrogen Evolution Reaction

Mimicking of tunichlorin is of importance to correlate its biological function to the unusally similar structure to chlorophylls but with a nickel cofactor. Benefiting from the facile derivatization of porpholactones, we herein constructed a tunichlorin mimic 6 carrying a β-hydroxyl group ([meso-tetrakis(pentafluorophenyl)-3-hydroxy-2-oxaisobacteriochlorinato]nickel(II)), which exhibits significant hydrogen evolution reaction (HER) rate acceleration of ca. 56-fold compared to its porphyrin analogues. Importantly, in the presence of water, the TOF of 6 is up to 6.1 × 10⁴ s^-1 with the lowest overpotential, ranking the best among the catalysts described. Coating catalyst 6 on a Ni foam electrode showed good HER performance in a two-electrode alkaline (1 M KOH) electrolyzer (η₂₀ = 540 mV). The functional roles of the β-hydroxyl group on the acceleration of electron transfer and the formation of the hydrogen bond network with water has been suggested in either chemical or electrochemical reductions and H/D kinetic isotope effects (KIEs), combined with DFT calculations. Interstingly, the DFT model suggested that the formation of the hydrogen bond renders more electron density on the Ni center (ρ_Ni = 0.91) in a one-electron reduced intermediate [6_H2O]^-, which helps the stabilization of both one- or two-electron reduced intermediates and dramatically enhances the HER rates.