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Abstract Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a Ni^I/IIIcycle have largely remained elusive. Herein, we report a Ni^IIcomplex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s⁻¹and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and Ni^I/Ni^IIIintermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of Ni^IIIintermediates by the independent synthesis and characterization of organometallic Ni^IIIcomplexes. 

A bioinspired (N2S2)Ni( ii ) electrocatalyst is reported that produces H 2 from CF 3 CO 2 H with a turnover frequency (TOF) of ∼1250 s −1 at low acid concentration (<0.043 M) in MeCN. A mechanism for the H 2 production by this electrocatalyst is proposed and its activity is benchmarked against those of other reported molecular Ni H 2 evolution electrocatalysts. The involvement of a hemilabile pyridyl group of the N2S2 ligand is proposed to mimic the role of a cysteine residue involved in the biological proton reduction performed by [NiFe] hydrogenases.