Electrocatalytic H2 Evolution by Proton-Gated Hangman Iron Porphyrins

The ability to control proton translocation is essential for optimizing electrocatalytic reductions in acidic solutions. We have synthesized a series of new hangman iron porphyrins with hanging groups of differing proton-donating abilities and evaluated their electrocatalytic hydrogen-evolving ability using foot-of-the-wave analysis. In the presence of excess triphenylphosphine, iron porphyrins initiate proton reduction electrocatalysis upon reduction to Fe-I. By changing the proton-donating ability of the hanging group, we can affect the rate of catalysis by nearly 3 orders of magnitude. The presence of an acid/base moiety in the second coordination sphere results in a marked increase in turnover frequency when extrapolated to zero overpotential.