Through-Space Electrostatic Effects of Positively Charged Substituents on the Hydrogen Evolution Reaction.

Elucidating the effects of various structural components on energy-related small molecule activation is of fundamental and practical significance. Herein we report on the inhibition effect of positively charged substituents on the hydrogen evolution reaction (HER). With the use of Cu porphyrins 1-5 containing different numbers and locations of positively charged substituents, we demonstrated that their electrocatalytic HER activities significantly decrease when more cationic units are located close to the Cu ion: the icat/ip (icat is the catalytic peak current, ip is the one-electron reduction peak current) value decreases from 38 with zero cationic unit to 15 with four closely located cationic units. Inspired by this result, we designed Cu porphyrin 6 with four meso-phenyl groups each bearing a negatively charged para-sulfonic substituent. With these anionic units, 6 outperformed the other Cu porphyrins for electrocatalytic HER under the same conditions.