Hydrogen evolution by cobalt hangman porphyrins under operating conditions studied by vibrational spectro-electrochemistry

Abstract

Cobalt hangman complexes are promising catalysts for dihydrogen production, yet their electrocatalytic performance in aqueous environment is still a topic of dispute. Surface-enhanced resonance Raman (SERR) spectro-electrochemistry has a great potential to give insight into the reaction mechanism of such molecular catalysts attached to electrodes under turnover conditions. However, the intrinsic catalytic activity of plasmonic supports and photoinduced side-reactions make the in situ analysis of their structures very challenging. In this work, the structure of hangman complexes attached to electrodes via dip-coating was investigated during catalytic turnover by electrochemistry and SERR spectroscopy. In order to explore the relevance of the hanging group for proton supply, complexes bearing a carboxylic acid and an ester hanging group were compared. For the former, SERR spectra recorded under turnover conditions indicate the reductive formation of a Co^(III)–H species, followed by laser-induced translocation of a proton to the carboxylic hanging group and the associated formation of the Co^I state. Due to the lack of a proton accepting group, hangman complexes with an ester group could not be trapped in the Co^I intermediate state and as a consequence SERR spectra solely reflected the (photo-enriched) Co^(II) resting state under turnover conditions. These results represent the first Raman spectroscopic insights into intermediates of dihydrogen evolution catalysed by cobalt hangman complexes on electrodes and support the direct involvement of the hanging group as a proton shuttle.

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