Proposed electrocatalytic proton reduction intermediates of hydrogenase mimics were synthesized, observed, and studied computationally. A new mechanism for H2 generation appears to involve Fe2(CO)6(1,2-S2C6H4) (3), the dianions {[1,2-S2C6H4][Fe(CO)3(μ-CO)Fe(CO)2]2− (32−), the bridging hydride {[1,2-S2C6H4][Fe(CO)3(μ-CO)(μ-H)Fe(CO)2]}−, 3H−(bridging), and the terminal hydride 3H−(term-stag), {[1,2-S2C6H4][HFe(CO)3Fe(CO)3]}−, as intermediates. The dimeric sodium derivative of 32−, {[Na2(THF)(OEt2)3][32−]}2 (4) was isolated from reaction of Fe2(CO)6(1,2-S2C6H4) (3) with excess sodium and was characterized by X-ray crystallography. It possesses a bridging CO and an unsymmetrically bridging dithiolate ligand. Complex 4 reacts with 4 equiv. of triflic or benzoic acid (2 equiv. per Fe center) to generate H2 and 3 in 75% and 60% yields, respectively. Reaction of 4 with 2 equiv. of benzoic acid generated two hydrides in a 1.7 : 1 ratio (by 1H NMR spectroscopy). Chemical shift calculations on geometry optimized structures of possible hydride isomers strongly suggest that the main product, 3H−(bridging), possesses a bridging hydride ligand, while the minor product is a terminal hydride, 3H−(term-stag). Computational studies support a catalytic proton reduction mechanism involving a two-electron reduction of 3 that severs an Fe–S bond to generate a dangling thiolate and an electron rich Fe center. The latter iron center is the initial site of protonation, and this event is followed by protonation at the dangling thiolate to give the thiol thiolate [Fe2H(CO)6(1,2-SHSC6H4)]. This species then undergoes an intramolecular acid–base reaction to form a dihydrogen complex that loses H2 and regenerates 3.
