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

Abstract

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 H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2(S 2C 3H 6)(CO) 3(L)(dppv) (L = CO, PMe 3, P i-Pr 3) oxidize at potentials approximately 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 [Fe 2(S 2C 3H 6)(CO) 4(dppv)]BF 4 and [Fe 2(S 2C 3H 6)(CO) 3(PMe 3)(dppv)]BF 4, as seen previously for the ethanedithiolate [Fe 2(S 2C 2H 4)(CO) 3(PMe 3)(dppv)]BF 4. For [Fe 2(S 2C n H 2 n )(CO) 3(P i-Pr 3)(dppv)]BF 4; however, the spin is localized on the Fe(CO) 2(P i-Pr 3) 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(P i-Pr 3) 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 [Fe 2(S 2C 2H 4)(CN)(CO) 3(dppv)] (-) resulted in 2e oxidation of 0.5 equiv to give the mu-cyano derivative [Fe (I) 2(S 2C 2H 4)(CO) 3(dppv)](mu-CN)[Fe (II) 2(S 2C 2H 4)(mu-CO)(CO) 2(CN)(dppv)], which was characterized spectroscopically.