Mimicking of Tunichlorin: Deciphering the Importance of a β-Hydroxyl Substituent on Boosting the Hydrogen Evolution Reaction
Zhuoyan Wu, Hao‐Zong Xue, Teng Wang, Yanru Guo, Yin‐Shan Meng, Xingguo Li, Jie Zheng, Christian Brückner, Guodong Rao, R. David Britt, Jun‐Long Zhang
Abstract
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 (ρNi = 0.91) in a one-electron reduced intermediate [6H2O]−, which helps the stabilization of both one- or two-electron reduced intermediates and dramatically enhances the HER rates.