Role of the Metal Ion in Bio-Inspired Hydrogenase Models: Investigation of a Homodinuclear FeFe Complex vs Its Heterodinuclear NiFe Analogue

In nature, dihydrogen is catalytically produced or split by the [FeFe] and [NiFe] hydrogenases. Despite common structural features in their dinuclear active site, i.e., a thiolate-rich coordination sphere and CO/CN- ligation, the synergetic way, in which the two metal sites act during catalysis, is specific for each enzyme. With the aim of understanding the role of the nature of the metal (Fe vs Ni), we report on a homodinuclear FeFe complex, a parent of a previously reported NiFe complex, to compare their electrocatalytic activity for H-2 production. The di-iron complex [(CO)-(LFeII)-Fe-N2S2-Fe-II (CO)Cpr (with L-N2S2 = 2,2'-(2,2'-bipyridine-6,6'-diyObis(1,1-diphenylethanethiolate and Cp = cyclopentadienyl) has been synthesized and fully characterized. In the solid state, it contains two CO ligands: one bound to the {FeCp} moiety in a semibridging manner and one terminally bound to the {FeLN2S2} moiety. This dinuclear iron complex is thus not isostructural to [(LNiFeII)-Ni-N2S2-Fe-II(CO)Cp](+), which contains a single CO ligand terminally bound to the Fe site. However, at low concentrations in MeCN solutions, the CO ligand coordinated to the {FeLN2S2} moiety is removed and the CO ligand bound to the {FeCp} moiety becomes fully bridging between the two Fe sites. Under such conditions, the di-iron complex displays similar catalytic performances to the parent NiFe complex (a comparable overpotential, i = 730 and 690 mV, and TON = 15 and 16, respectively). Cyclic voltammetry data give direct experimental evidence for an E[ECEC] mechanism, which was also previously proposed for the NiFe complex. However, the structure of the one-electron reduced species, the entry point of the catalytic cycle, slightly differs for the two systems: in [(LNiI)-Ni-N2S2 (CO)(FeCp)-Cp-II], this is valence localized species on the site Ni and the CO ligand bridges the two metal ions, while in [(CO)L(N2S2F)eFeCp], this is a type II-III mixed-valence species with the CO terminally bound to the {FeLN2S2} unit.