Models of the iron-only hydrogenase enzyme: structure, electrochemistry and catalytic activity of Fe2(CO)3(μ-dithiolate)(μ,κ1,κ2-triphos)

Abstract

A series of diiron bis(2-diphenylphosphinoethyl)phenylphosphine (triphos) complexes Fe₂(CO)₃(μ-dithiolate)(μ,κ¹,κ²-triphos) (1–4) [dithiolate = 1 pdt; 2 edt; 3 adt (R = Bz), 4 (SMe)₂] have been prepared and investigated as biomimics of the diiron site of [FeFe]-hydrogenases. The triphos ligand bridges the diiron vector whilst also chelating to one iron and 1–3 exist as a mixture of basal–basal–apical (bba) and basal–basal–basal (bbb) isomers which differ in the mode of chelation. In solution the bba and bbb forms do not interconvert on the NMR time scale, but the bba isomers are fluxional, and at low temperature four forms of 1bba are seen as the conformations for the pdt ring and triphos methylene groups are frozen. Crystallographic studies have established bba (pdt) and bbb (adt) ground state conformations and in both there is a significant deviation away from the expected eclipsed conformation (L_ap–Fe–Fe–L_ap torsion angle 0°) by 49.4 and 24.9° respectively, suggesting that introduction of triphos leads to significant strain and DFT calculations have been used to understand the relative energies of isomers. The electron rich nature of the diiron centre in 1–4 would suggest rapid protonation, but while bridging hydride complexes such as [Fe₂(CO)₃(μ-pdt)(μ,κ¹,κ²-triphos)(μ-H)][BF₄] (1H⁺) can be formed the process is slow. This behavior is likely a result of the high energy barrier in forming the initial (not observed) terminal hydride which requires a significant conformational change in triphos coordination. CV studies show that all starting compounds oxidize at low potentials and the addition of [Cp₂Fe][PF₆] to 1 affords [Fe₂(CO)₃(μ-pdt)(μ,κ¹,κ²-triphos)][PF₆] (1⁺) which has been characterised by IR spectroscopy. DFT studies suggest a ground state for 1⁺ with a partially rotated Fe(CO)₂P moiety that yields a weak semi-bridging carbonyl with the adjacent Fe(CO)P₂ group. No reduction peaks are seen for 1–4 within the solvent window but 1H⁺ undergoes reduction at −1.7 V. All complexes act as proton-reduction catalysts in the presence of HBF₄·Et₂O. For 1, three separate processes are observed and their dependence on acid concentration has been probed, and a mechanistic scheme is proposed based on formation via a CECE process of 1(μ-H)H which can either slowly release H₂ or undergo further reduction. Relative contributions of the three processes to the total current were found to be highly dependent upon the background electrolyte, being attributed to their relative abilities to facilitate proton transfer processes. While 2 and 4 show similar proton reduction behaviour, the adt complex 3 is quite different being attributed to facile protonation of nitrogen which is followed by addition of a second proton at the diiron centre.