EPR/ENDOR, Mössbauer, and quantum-chemical investigations of diiron complexes mimicking the active oxidized state of [FeFe]hydrogenase

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe₂(S₂C₂H₄NH)(CN) ₂(CO)₂(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe₂(S₂X)(CO)₃(dppv) (PMe₃)]⁺ [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S₂X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe ^IFe^II state analogous to the active oxidized "H _ox" state of the native H-cluster. For both complexes, the ³¹P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the ⁵⁷Fe parameters were measured by electron spin-echo envelope modulation and Mössbauer spectroscopy, while for 1adt, ¹⁴N and selected ¹H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two ¹⁴N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.