Proton Relay in Iron Porphyrins for Hydrogen Evolution Reaction

• Published in: Inorganic chemistry 2021-09, Vol.60 (18), p.13876-13887
• Main Authors: Bhunia, Sarmistha, Rana, Atanu, Hematian, Shabnam, Karlin, Kenneth D, Dey, Abhishek
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.1c01079

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 pK a 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 pK a of the acid source used suggests that the rate of the HER becomes almost independent of pK a 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.