Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction.

The efficiency of the hydrogen evolution reaction (HER) can be facilitated by the presence of proton-transfer groups in the vicinity of the catalyst. A systematic investigation of the nature of the proton-transfer groups present and their interplay with bulk proton sources is warranted. The HERs electrocatalyzed by a series of iron porphyrins that vary in the nature and number of pendant amine groups are investigated using proton sources whose pKa values vary from ∼9 to 15 in acetonitrile. Electrochemical data indicate that a simple iron porphyrin (FeTPP) can catalyze the HER at this FeI state where the rate-determining step is the intermolecular protonation of a FeIII-H- species produced upon protonation of the iron(I) porphyrin and does not need to be reduced to its formal Fe0 state. A linear free-energy correlation of the observed rate with pKa of the acid source used suggests that the rate of the HER becomes almost independent of pKa of the external acid used in the presence of the protonated distal residues. Protonation to the FeIII-H- species during the HER changes from intermolecular in FeTPP to intramolecular in FeTPP derivatives with pendant basic groups. However, the inclusion of too many pendant groups leads to a decrease in HER activity because the higher proton binding affinity of these residues slows proton transfer for the HER. These results enrich the existing understanding of how second-sphere proton-transfer residues alter both the kinetics and thermodynamics of transition-metal-catalyzed HER.