Chelate control of diiron(I) dithiolates relevant to the [Fe-Fe]-hydrogenase active site

The reaction of Fe₂(S₂C₂H ₄)(CO)₆ with cis-Ph₂PCH=CHPPh₂ (dppv) yields Fe₂(S₂C₂H₄)(CO) ₄(dppv), 1(CO)₄, wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO)₄, the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH₂ of the larger dithiolates. The Fe(CO) ₃ subunit in 1(CO)₄ undergoes substitution with PMe ₃ and cyanide to afford 1(CO)₃(PMe₃) and (EttN)[1(CN)(CO)₃], respectively. Kinetic studies show that 1(CO)₄ reacts faster with donor ligands than does its parent Fe ₂(S₂C₂H₄)(CO)₆. The rate of reaction of 1(CO)₄ with PMe₃ was first order in each reactant, k = 3.1 × 10^-4 M^-1 s^-1. The activation parameters for this substitution reaction, ΔH‡ = 5.8(5) kcal/mol and ΔS‡ = -48(2) cal/deg·mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe ₂(S₂C₂H₄)(CO)₆, the enhanced electrophilicity of 1(CO)₄ arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO) ₃(PMe₃) with Cp₂FePF₆ yielded [Fe₂(S₂C₂H₄)(μ-CO)(CO) ₂(dppv)(PMe₃)(NCMe)](PF₆)₂. Reaction of this compound with PMe₃ yielded [Fe₂(S ₂C₂H₄)-(μ-CO)(CO)(dppv)(PMe ₃)₂(NCMe)](PF₆)₂. © 2007 American Chemical Society.