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

Abstract

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 .

Unsaturated, Mixed‐Valence Diiron Dithiolate Model for the Hox State of the [FeFe] Hydrogenase

“…The "proximal" {Fe(CO) 2 (PMe 3 )} moiety remains relatively unperturbed, and overall the bond lengths match those in 1 within 0.02 Å. [7] The unique CO ligand is only partially bridging, with a relatively long Fe prox -C separation of 2.62 Å compared to the Fe dist -CO bond length of 1.781(3) Å (∢(Fe dist -C-O) = 170.2(3)°; Figure 1). We have recently described [Fe 2 -(S 2 C 2 H 4 )(CO)(μ-COAX 3 )(dppv) 2 ] (AX 3 = AlBr 3 , B(C 6 F 5 ) 3 ), which is not mixed-valence but also adopts the rotated structure.…”

“…[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) The purple-colored compound 1-BF 4 is stable in solution for several minutes at room temperature and is sufficiently stable that we were able to grow single crystals for X-ray diffraction (see below). The X-band EPR spectrum of polycrystalline 1-BF 4 (15 K) displays g values at 2.14, 2.03, 2.01 as well as a signal for an apparent impurity at g =2.05; this rhombic pattern is similar to that seen for H ox from D. desulfuricans (g =2.10, 2.04, 1.99).…”

“…[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+ .…”

Unsaturated, Mixed‐Valence Diiron Dithiolate Model for the Hox State of the [FeFe] Hydrogenase

“…[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 The purple-colored compound 1-BF 4 is stable in solution for several minutes at room temperature and is sufficiently stable that we were able to grow single crystals for X-ray diffraction (see below). The X-band EPR spectrum of polycrystalline 1-BF 4 (15 K) displays g values at 2.14, 2.03, 2.01 as well as a signal for an apparent impurity at g = 2.05; this rhombic pattern is similar to that seen for H ox from D. desulfuricans (g = 2.10, 2.04, 1.99).…”

“…The "proximal" {Fe(CO) 2 (PMe 3 )} moiety remains relatively unperturbed, and overall the bond lengths match those in 1 within 0.02 . [7] The unique CO ligand is only partially bridging, with a relatively long Fe prox À C separation of 2.62 compared to the Fe dist À CO bond length of 1.781(3) (a(Fe dist -C-O) = 170.2(3)8; Figure 1). We have recently described [Fe 2 -(S 2 C 2 H 4 )(CO)(m-COAX 3 )(dppv) 2 ] (AX 3 = AlBr 3 , B(C 6 F 5 ) 3 ), which is not mixed-valence but also adopts the rotated structure.…”