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

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. Electron Paramagnetic Resonance (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(S2CnH2n)(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 density-functional theory (DFT) calculations, suggest 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 [FeI2(S2C2H4)(CO)3(dppv)](μ-CN)[FeII2(S2C2H4)(μ-CO)(CO)2(CN)(dppv)], which was characterized spectroscopically.