Redox and Structural Properties of Mixed-Valence Models for the Active Site of the FeFe-Hydrogenase: Progress and Challenges

摘要

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the Hox state of the [FeFe]-hydrogenases. The propanedithiolates Fe2(S2C3H6)(CO)3(L)(dppv) (L = CO, PMe3, Pi-Pr3) oxidize at potentials ~180 mV milder than the related ethanedithiolates (Angew. Chem. Int. Ed. >2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. EPR spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe2(S2C3H6)(CO)4(dppv)]BF4 and [Fe2(S2C₃H₆)(CO)₃(PMe₃)(dppv)]BF₄, as seen previously for the ethanedithiolate [Fe₂(S₂C₂H₄)(CO)₃(PMe₃)(dppv)]BF₄. For [Fe₂(S₂C_nH_2n)(CO)₃(Pi-Pr₃)(dppv)]BF₄, however, the spin is localized on the Fe(CO)₂(Pi-Pr₃) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and DFT-calculations, suggest, however, that the Fe(CO)₂(Pi-Pr₃) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe₂(S₂C₂H₄)(CN)(CO)₃(dppv)]⁻ resulted in 2e oxidation of 0.5 equiv to give the μ-cyano derivative [Fe^I₂(S₂C₂H₄)(CO)₃(dppv)](μ-CN)[Fe^II₂(S₂C₂H₄)(μ-CO)(CO)₂(CN)(dppv)], which was characterized spectroscopically.