Electrochemical Proton Reductions Catalyzed by the Simpler Hexacoordinate Iron Compounds as Functional Mimics of the Active Site in [FeFe] Hydrogenase

Ligand-substitution reaction of [( μ -S 2 C 4 N 2 H 2 )Fe 2 (CO) 6 ] ( 1 ) generated a normal mono-substituted diiron dithiolate derivative [( μ -S 2 C 4 N 2 H 2 )Fe 2 (CO) 5 (PMe 3 )] ( 1P ) and a simpler hexacoordinate mononuclear compound [( μ -S 2 C 4 N 2 H 2 )Fe([CO) 2 (PMe 3 ) 2 ] ( 2 ) via distinct reactivity pathways under identical conditions. 1P and 2 could act as the electrochemically functional mimics of the [2Fe] sub-cluster and the distal Fe moiety of the active site of [FeFe] hydrogenase. The electrochemical investigations showed that 2 catalyzed the production of hydrogen from weak acid (acetic acid, HOAc) via two catalytic processes with an initial metal-orbital based reduction. In contrast, the electrocatalytic reaction of 2 with stronger acid (trifluoroacetic acid, TFA) occurred via an initial ligand protonation, and proceeded through different pathways that involved distinct oxidation states of the catalyst. The most striking result obtained in this study was that the hydrogen formation by 2 from TFA occurred at a relatively low overpotential as small as −0.28 V. It could be rationalized by the exclusive employment of iron(II) and iron(I) redox levels in the catalytic cycle, which was consistent with the enzymatic process. The observations might shed light on some aspects of ways by which the model compounds catalyzed the reduction of protons. Graphical Abstract