[FeFe]-hydrogenase biomimics incorporating redox-active ligands: tuning the oxidation chemistry of ferrocene–dichalcogenolate-bridged centres via phosphine substitution

Ferrocene–dichalcogenolate-bridged complexes, [Fe₂(CO)₆{μ-E(η⁵-C₅H₄)Fe(η⁵-C₅H₄)E}] (E = S, Se), are [FeFe]-hydrogenase biomimics, in which the two redox-active centres lie in close proximity. Here we report the syntheses and electrochemical studies of phosphine-substituted derivatives, which allows tuning of the oxidation chemistry of the Fe₂ centre, while (effectively) leaving that of the ferrocene centre unchanged. Mono-substituted [Fe₂(CO)₅{μ-Se(η⁵-C₅H₄)Fe(η⁵-C₅H₄)Se}(Ph₂P-p-tolyl)], chelated [Fe₂(CO)₄{μ-E(η⁵-C₅H₄)Fe(η⁵-C₅H₄)E}(κ²-dppv)] and bridged [Fe₂(CO)₄{μ-E(η⁵-C₅H₄)Fe(η⁵-C₅H₄)E}(μ-dppf)] complexes have been prepared under carefully controlled reaction conditions, the selenium derivative giving higher yields. Crystal structures of [Fe₂(CO)₄{μ-Se(η⁵-C₅H₄)Fe(η⁵-C₅H₄)Se}(κ²-dppv)] and [Fe₂(CO)₄{μ-Se(η⁵-C₅H₄)Fe(η⁵-C₅H₄)Se}(μ-dppf)] have been determined, which contain three closely located iron redox centres. Cyclic voltammetry (CV) and IR spectroelectrochemistry (IR SEC) have been used to understand changes occurring upon oxidation. Upon successive replacement of carbonyl(s), the oxidation potential of the Fe₂ centre is lowered and in the dppv and dppf complexes it occurs prior to oxidation of the remote Fe(ii) centre in the ferrocene–dithiolate bridge, as confirmed by IR SEC experiments. Dppf complexes contain three different iron oxidation centres, and three separate oxidation waves are identified in the CV of [Fe₂(CO)₄{μ-Se(η⁵-C₅H₄)Fe(η⁵-C₅H₄)Se}(μ-dppf)]. DFT calculations have been used to better understand the likely structures of the oxidised species. They suggest that oxidation of the Fe2 centre results in a structural rearrangement to give a semi-bridging carbonyl, but no such species are observed by IR SEC, possibly due to the high rearrangement energies. IR SEC studies also suggest that for the dppf complexes, oxidation may be delocalised over several sites.