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

Abstract

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.