Biomimetic model for [FeFe]-hydrogenase: Asymmetrically disubstituted diiron complex with a redox-active 2,2′-bipyridyl ligand

[FeFe]-hydrogenases feature a unique active site in which the primary catalytic unit is directly coordinated via a bridging cysteine thiolate to a secondary, redox active [4Fe4S] unit. The goal of this study was to evaluate the impact of a bidentate, redox non-innocent ligand on the electrocatalytic properties of the (μ-S(CH₂)₃S)Fe₂(CO) ₄L₂ family of [FeFe]-hydrogenase models as a proxy for the iron-sulfur cluster. Reaction of the redox non-innocent ligand 2,2′-bipyridyl (bpy) with (μ-S(CH₂)₃S)Fe ₂(CO)₆ leads to substitution of two carbonyls to form the asymmetric complex (μ-S(CH₂)₃S)Fe₂(CO) ₄(κ²-bpy) which was structurally characterized by single crystal X-ray crystallography. This complex can be protonated by HBF ₄·OEt₂ to form a bridging hydride. Furthermore, electrochemical investigation shows that, at slow scan rates, the complex undergoes a two electron reduction at -2.06 V vs. Fc⁺/Fc that likely involves reduction of both the bpy ligand and the metal. Electrocatalytic reduction of protons is observed in the presence of three distinct acids of varying strengths: HBF₄·OEt₂, AcOH, and p-TsOH. The catalytic mechanism depends on the strength of the acid.