The electrochemical hydrogen evolution reaction (HER) is considered to be a sustainable approach for advancing fuel-cell technologies. The [NiFe] hydrogenase performs this reaction at rates rivaling platinum, the state-of-the-art catalyst for commercial HER. Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in hydrogen redox catalyzed by [NiFe] hydrogenase. However, electrocatalytic systems which operate through a NiI/III cycle have remained elusive. Herein, we report a bioinspired NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for HER with high turnover frequencies in the presence of low acid concentration. This electrocatalyst follows a unique HER mechanism via NiI/NiIII intermediates hitherto unobserved in model complexes. EPR studies suggest the formation of a NiI species upon the one-electron reduction of the S = 0 NiII complex. We propose that activation of the Cipso-H bond of the phenyl group in NCHS2 occurs in order to stabilize a NiIII-H intermediate, which undergoes further reduction and protonation to evolve H2. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.

Stabilization of key paramagnetic states relevant to [NiFe]-Hydrogenase by an organometallic (NCHS2)Ni electrocatalyst

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