Synthesis and characterisation of three diiron tetracarbonyl complexes related to the diiron centre of [FeFe]-hydrogenase and their protonating, electrochemical investigations

The synthesis, characterisation of three diiron tetracarbonyl complexes, [Fe2(SCH2)2C(Me)(CH2OR)(PNP)(CO)4] (R = H and PNP = L11: 2; R = H and PNP = L22: 3; R = Ts and PNP = L11: 4) as models for the diiron sub-unit of [FeFe]-hydrogenase are described, where OTs, L11 and L22 are toluenesulfonate, (Ph2P)2NCH2(2-C5H4N) and (Ph2P)2NCH2Ph, respectively. These complexes are fully characterised and the structure of complex 4 is crystallographically determined. Protonation of these complexes with HBF4·Et2O is probed by using infrared and NMR spectroscopies which reveals that no hydride can be formed upon addition of the acid. Instead addition of excess of the acid leads to protonating the N atom of the PNP skeleton, which is a weak base due to participating conjugating interactions with the Fe–Fe centre, as revealed by crystallographic analysis. Electrochemistry of these complexes and their electrocatalytic reduction of protons are also investigated. Our results suggest that the existence of the pendant pyridine group can lower the overpotential for proton reduction but does not seem to enhance electrocatalytic efficiency in our case.