title: Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins.
abstract: The hangman motif provides mechanistic insights into the role of pendant proton relays in governing proton-coupled electron transfer (PCET) involved in the hydrogen evolution reaction (HER). We now show improved HER activity of Ni compared with Co hangman porphyrins. Cyclic voltammogram data and simulations, together with computational studies using density functional theory, implicate a shift in electrokinetic zone between Co and Ni hangman porphyrins due to a change in the PCET mechanism. Unlike the Co hangman porphyrin, the Ni hangman porphyrin does not require reduction to the formally metal(0) species before protonation by weak acids in acetonitrile. We conclude that protonation likely occurs at the Ni(I) state followed by reduction, in a stepwise proton transfer-electron transfer pathway. Spectroelectrochemical and computational studies reveal that upon reduction of the Ni(II) compound, the first electron is transferred to a metal-based orbital, whereas the second electron is transferred to a molecular orbital on the porphyrin ring.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: D Kwabena
authors.initials: DK
authors.lastname: Bediako
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors.forename: Brian H
authors.initials: BH
authors.lastname: Solis
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: Dilek K
authors.initials: DK
authors.lastname: Dogutan
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: Manolis M
authors.initials: MM
authors.lastname: Roubelakis
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Maher
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: Chang Hoon
authors.initials: CH
authors.lastname: Lee
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and.
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Chambers
authors.affiliation: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.
authors.forename: Sharon
authors.initials: S
authors.lastname: Hammes-Schiffer
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902; and dnocera@fas.harvard.edu.
authors.forename: Daniel G
authors.initials: DG
authors.lastname: Nocera
keywords: electrocatalysis; renewable; solar fuels