Chelate Control of Diiron(I) Dithiolates Relevant to the [Fe−Fe]- Hydrogenase Active Site

The reaction of Fe2(S2C2H4)(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 PMe3 and cyanide to afford 1(CO)3(PMe3)
and (Et4N)[1(CN)(CO)3], respectively. Kinetic studies show that 1(CO)4 reacts faster with donor ligands than does
its parent Fe2(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 Fe2(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(S2C2H4)(μ-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2.