Diiron dithiolato carbonyls related to the H_OX^CO state of [FeFe]-hydrogenase

Oxidation of the electron-rich (E_1/2 = -175 vs Ag/AgCl) ethanedithiolato complex Fe₂(S₂C₂H ₄)-(CO)₂(dppv)₂ (1) under a CO atmosphere yielded [Fe₂(S₂C₂H₄)(μ-CO)(CO) ₂(dppv)₂]⁺ ([1(CO)]⁺), a model for the H_OX^CO state of the [FeFe]-hydrogenases. This complex exists as two isomers: a kinetically favored unsymmetrical derivative, unsym-[1(CO)]⁺, and a thermodynamically favored isomer, sym-[1(CO)]⁺, wherein both diphosphines span apical and basal sites. Crystallographic characterization of sym-[1(CO)]⁺ confirmed a C ₂-symmetric structure with a bridging CO ligand and an elongated Fe-Fe bond of 2.7012(14) Å, as predicted previously. Oxidation of sym-[1(CO)]⁺ and unsym-[1(CO)]⁺ again by 1e^- oxidation afforded the respective diamagnetic diferrous derivatives where the relative stabilities of the sym and unsym isomers are reversed. DFT calculations indicate that the stabilities of sym and unsym isomers are affected differently by the oxidation state of the diiron unit: the mutually trans CO ligands in the sym isomer are more destabilizing in the mixed-valence state than in the diferrous state. EPR analysis of mixed-valence complexes revealed that, for [1]⁺, the unpaired spin is localized on a single iron center, whereas for unsym/sym-[1(CO)]⁺, the unpaired spin was delocalized over both iron centers, as indicated by the magnitude of the hyperfine coupling to the phosphine ligands trans to the Fe-Fe vector. Oxidation of 1 by 2 equiv of acetylferrocenium afforded the dication [1]²⁺, which, on the basis of low-temperature IR spectrum, is structurally similar to [1]⁺. Treatment of [1]²⁺ with CO gives unsym-[1(CO)]²⁺.