The reaction of Fe 2 (S 2 C 2 H 4)(CO) 6 with cis-Ph 2 PCH=CHPPh 2 (dppv) yields Fe 2 (S 2 C 2 H 4) (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 CH 2 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 (PMe 3) and (Et 4 N)[1(CN)(CO) 3 ], respectively. Kinetic studies show that 1(CO) 4 reacts faster with donor ligands than does its parent Fe 2 (S 2 C 2 H 4)(CO) 6. The rate of reaction of 1(CO) 4 with PMe 3 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 (S 2 C 2 H 4)(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 (PMe 3) with Cp 2 FePF 6 yielded [Fe 2 (S 2 C 2 H 4)(μ-CO)(CO) 2 (dppv)(PMe 3)(NCMe)](PF 6) 2. Reaction of this compound with PMe 3 yielded [Fe 2 (S 2 C 2 H 4)(μ-CO)(CO)(dppv)(PMe 3) 2 (NCMe)](PF 6) 2 .

The [FeFe] hydrogenase enzymes are the most efficient catalysts known for the reduction of protons to H 2 . [1] The active site exists in two functional states (Scheme 1), H red , which is S =0, and H ox , which is S = 1/2. [2] Research in this area is aimed at elucidating the mechanism of the enzymatic catalysis and at using this information to develop protein-free bioinspired synthetic catalysts. [3] A specific research goal is the preparation of molecules that resemble the functional states of the active site with the expectation that function will follow form. Most studies on diiron dithiolato carbonyl complexes rely on organic ligands (e.g. phosphanes) in place of the naturally occurring cyanide and μ-SR[Fe 4 S 4 ] ligands, [4] which have complicated acid-base behavior that is often difficult to control outside of the protein. Another barrier to modeling has been the rarity of mixed-valence diiron dithiolate compounds with the appropriate structures, stability, and reactivity.The first evidence for mixed valency in diiron dithiolate models was obtained in the oneelectron oxidation of [Fe 2 {(SCH 2 ) 2 CMeCH 2 SMe}(CN) 2 (CO) 4 ] 2− , which afforded a thermally sensitive mixed-valence derivative with IR and EPR spectroscopy signatures resembling those for the CO-inhibited enzyme. [5] In very recent work, the oxidation of [Fe 2 (S 2 C 3 H 6 )(CO) 4 (PMe 3 )L 1 ] (L 1 =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) was shown to give a mixed-valence derivative with structural and spectroscopic features resembling H ox . [6] Our recently reported species [Fe 2 (S 2 C 2 H 4 )(CO) 3 (PMe 3 )-(dppv)] (1, dppv =cis-1,2-C 2 H 2 (PPh 2 ) 2 ) is attractive, because, like the active site, this diiron framework bears three donor ligands and three CO ligands. [7] We previously showed that oxidation of 1 in MeCN gives [1 (NCMe)] 2+ . In MeCN solution, one equivalent of oxidizing agent gives an approximately 1:1 mixture of unreacted starting material and [1(NCMe)] 2+ . Oxidation of 1 in the weakly coordinating solvent CH 2 Cl 2 , however, proceeds quite differently.Addition of one equivalent of FcBF 4 (Fc + =[Fe(C 5 H 5 ) 2 ] + ) to a CH 2 Cl 2 solution of 1 at −45 °C resulted in complete consumption of the diiron complex, as indicated by in situ IR spectroscopy. We probed the oxidation of 1 by cyclic voltammetry on a solution of 1 in 0.1m

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 lowmolecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2 (S 2 C 3 H 6 )(CO) 3 (L)(dppv) (L = CO, PMe 3 , Pi-Pr 3 ) 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. 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 2 C 3 H 6 )(CO) 4 (dppv)]BF 4 and [Fe 2 (S 2 C 3 H 6 ) (CO) 3 (PMe 3 )(dppv)]BF 4 , as seen previously for the ethanedithiolate [Fe 2 (S 2 C 2 H 4 )(CO) 3 (PMe 3 ) (dppv)]BF 4 . For [Fe 2 (S 2 C n H 2n )(CO) 3 (Pi-Pr 3 )(dppv)]BF 4 , however, the spin is localized on the Fe (CO) 2 (Pi-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 DFT-calculations, suggest, however, that the Fe(CO) 2 (PiPr 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 2 C 2 H 4 )(CN) (CO) 3 (dppv)] − resulted in 2e oxidation of 0.5 equiv to give the μ-cyano derivative [Fe I 2 (S 2 C 2 H 4 ) (CO) 3 (dppv)](μ-CN)[Fe II 2 (S 2 C 2 H 4 )(μ-CO)(CO) 2 (CN)(dppv)], which was characterized spectroscopically.