Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(μ-edt)2] and Phosphine Derivatives [Fe3(CO)7–x(PPh3)x(μ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] as Proton Reduction Catalysts

The mixed-valence triiron complexes [Fe3(CO)7–x(PPh3)x(μ-edt)2] (x = 0–2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)–Fe(II)–Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at −0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between −1.47 V for [Fe3(CO)7(μ-edt)2] and around −1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(μ-edt)], [Fe3(CO)7(μ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron–iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

The oxidation behavior of the complexes was studied in detail over scan rates of 0.01 -10 V s -1 , as shown for 2 in Figure ESI 1 a and b. At all scan rates one oxidation peak occurs, which shifts to higher potentials with increasing scan rate thus indicating quasi-reversibility. The corresponding reduction peak consists of two overlapping processes taking place at a similar potential, which at scan rates above 1 V s -1 can be resolved into two components. The first is centered at 0.05 V with a position independent of scan rate and the second, at -0.15 V (at 1 V s -1 ), shifts to more negative potentials at faster scan rates indicating quasi-reversibility. This would suggest that after the first oxidation the product undergoes chemical or structural change allowing further loss of an electron at a similar potential.
Figure ESI 1 c shows a plot of peak current normalized by dividing by square root of scan rate vs. square root of scan rate, which reveals that twice as many electrons are transferred at low scan rates than high; thus we suggest a one electron transfer at fast scan rates that tends to two electrons as the electrochemical timescale is increased. This would suggest that after the first oxidation the product undergoes chemical or structural change allowing further loss of an electron at a similar potential. The rate of this chemical step is likely to be different for complexes with different degrees of substitution and may explain why fewer electrons are involved in oxidation of 3 than the other two complexes. Figure S2: CV at 0.1 Vs -1 of 0.5 mM 1 in 0.1 M TBAPF 6 / dichloromethane in Ar-saturated (black) and CO-saturated (brown) solution.

b. Cyclic voltammetry of 1 in CO-saturated solution
The CV of 1 was carried out in a CO saturated solution, where the first oxidation and reduction peaks were unchanged in position and magnitude. Additional reduction peaks were observed at -1.87 V and -2.1 V, along with oxidation peaks at -1.7 V (associated with the reduction at -1.87 V) -0.85 V and 0.82 V. Reduction of [Fe 2 (CO) 6 (µ-edt)] under the same experimental conditions occurs reversibly at -1.9 V vs. Fc/Fc + [20,22] We therefore tentatively assign the reduction response at -1.87 V to this diiron complex, formed as a decomposition product after the reduction of 1. A common decomposition process for these complexes after reduction is the loss of a CO ligand and rapid dimerisation of the remaining products [20,49], which is suppressed in excess CO. The observation that reversibility of the reduction of 1 is not improved in a CO-saturated solution suggests that CO ligand loss is not a major decomposition route in this case and cleavage of iron-sulfur bond(s) and fragmentation of the cluster into di-and mono-iron complexes is most likely.
In acetonitrile addition of one molar equivalent of acid to 4 results in two new reduction peaks at E p = -1.63 V and E p = -2.26 V, in addition to the reduction peaks at E p = -1.89 V and E p = -2.70 V associated with 4 in the absence of acid. All the peak currents increase with acid concentration. The shift in reduction to less negative potentials on addition of acid is indicative of protonation of neutral 4 taking place before reduction, i.e. a CE mechanism.
Asymmetric diphosphine substitution of these complexes results in sufficient basicity for protonation to take place in MeCN, allowing the electrocatalytic reduction of protons to take place at ca. 0.2 V more positive than the reduction potential of the complex.  Figure S5: Simulated CVs for 2 (black) and 1 (grey) obtained from DigiSim using the mechanism and parameters given in the text and in ref [23] from main document.
The electrocatalytic response of 1 has been reported previously [23] and rate constants for the protonation steps obtained from simulated results for a very simple ECEC mechanism. A similar mechanism is presented for 2 below (Red text indicates parameters that differ between 1 and 2). and simulated CVs of this mechanism are shown above. Neutral 2 undergoes reduction at ca. -1.7 V and the resulting 2species can either undergo further reaction to undefined 'products' or protonate to give 2H. The rate and equilibrium constants for the non-catalytic route to 'products' were obtained by simulating voltammograms for 2 obtained in the absence of acid and comparing their scan rate dependence with experimental CVs. The equilibrium constant K(1) = 10 10 for protonation of 2reflects the difference in pKa of 2and the strong acid HBF 4 . 2H then undergoes further reduction to form 2Hfollowed by a simultaneous proton addition / hydrogen elimination step. The equilibrium constant K(2) for this step again reflects the difference in pKa of the acid and 2Hspecies. The two rate constants k(1) and k (2) for the protonation steps have been varied in the simulation until a fit to the experimental peak currents was obtained. The simulated peak currents are plotted against experimental peak currents for 1 and 2 in Figure 6 of the main document.