Electrochemical proton reductions in varying acidic media by a simple synthetic hydrogenase mimic

Abstract A simple diiron dithiolate [( μ -S 2 C 4 N 2 H 2 )Fe 2 (CO) 6 ] ( M2 ) as mimic of the active site of [FeFe] hydrogenase catalyzed the reduction of protons to dihydrogen (H 2 ). The strength of the acid played an important role in controlling the mechanism of electrocatalytic proton reduction. The catalysis of weak acid (acetic acid) occurred around −2 V vs. Fc/Fc + , at which M2 did not have a counterpart in the absence of acid. The catalyst was actually the reduced form of M2 and an (E)ECEC mechanism was proposed. With a moderately strong acid (trifluoroacetic acid), two significant catalyses operated. Reduction of the 2e –  + 2H + intermediate ([ M2 IM−H 2 ]) catalyzed the proton reduction at ca. −1.4 V, whereas a competed catalysis at ca. −1.6 V dominated when the protonation of the reduced form of [ M2 IM−H 2 ] exceeded H 2 elimination at higher acid concentrations. Upon addition of a strong acid (triflic acid), one of the ring nitrogen atoms could be protonated. After two successive one-electron reductions, the formed species proceeded a catalytic cycle via a CECE process at a comparatively mild potential of −0.65 V vs. NHE.