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

Abstract

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 x 10(-4) M(-1) s(-1). The activation parameters for this substitution reaction, DeltaH = 5.8(5) kcal/mol and DeltaS = -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)(mu-CO)(CO)2(dppv)(PMe3)(NCMe)](PF6)2. Reaction of this compound with PMe3 yielded [Fe2(S2C2H4)(mu-CO)(CO)(dppv)(PMe3)2(NCMe)](PF6)2.

Figure 1

Active site of the [Fe–Fe]⁻hydrogenase in the reduced and oxidized forms.

Figure 2

Structure of 1(CO)₄, with thermal ellipsoids set at 35%. Phenyl ellipsoids and phenyl hydrogen atoms have been omitted. Selected bond lengths (Å) and angles (deg): Fe(1)–Fe(2), 2.5249(9); Fe(1)– S(1), 2.2451-(13); Fe(1)–S(2), 2.2575(13); Fe(1)–P(1), 2.1743(13); Fe(1)–P(2), 2.2070-(13); Fe(1)–C(4), 1.754(5); Fe(2)–C(1), 1.783(5); Fe(2)–C(2), 1.779(5); Fe(2)–C(3), 1.780(5); Fe(2)–Fe(1)–P(1), 154.05(5); Fe(2)–Fe(1)–P(2), 109.23(4); Fe(2)–Fe(1)–C(4), 105.11(15); P(1)–Fe(1)–P(2), 87.83(5); P(1)–Fe(1)–C(4), 94.32(15); P(2)–Fe(1)–C(4), 88.88(15).

Figure 3

³¹P NMR spectra of Fe₂[(SCH₂)₂NH](CO)₄(dppv) in a CD₂Cl₂ solution at temperatures of +20 (top), −40, and −60 °C (bottom).

Figure 4

Axial–basal (left) and dibasal (right) isomers of Fe₂(edt)(CO)₄(dppv) as obtained by DFT calculations (distances in angstroms and computed energy differences in kilocalories per mole relative to the more stable isomer).

Figure 5

Axial–basal (top) and dibasal (bottom) isomers of Fe₂(adt)(CO)₄(dppv), as obtained by DFT calculations (distances in angstroms and computed energy differences in kilocalories per mole relative to the most stable isomer).

Figure 6

Left: structure of 1(CO)₃(PMe₃) with thermal ellipsoids set at 35%. Phenyl ellipsoids and phenyl hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe(1)–Fe(2), 2.5344(7); Fe(1)–S(1), 2.2633(11); Fe(1)–S(2), 2.2690(10); Fe(1)–P(1), 2.2006(11); Fe(1)–P(2), 2.1918(10); Fe(1)–C(1), 1.755(4); Fe(2)–P(3), 2.2312(11); Fe(2)–C(2), 1.767(4); Fe(2)–C(3), 1.758(4); Fe(2)–Fe(1)–P(1), 108.38(3); Fe(2)–Fe-(1)–P(2), 157.03(4); Fe(2)–Fe(1)–C(1), 101.08(12); P(2)–Fe(1)–P(1), 88.19(4); P(2)–Fe(1)–C(1), 93.97(12); C(1)–Fe(1)–P(1), 91.82(13). Right: structure of the anion in Et₄N[1(CN)(CO)₃], with thermal ellipsoids set at 35%. Phenyl ellipsoids, phenyl hydrogen atoms, and the Et₄N⁺ groups are not shown. Selected bond lengths (Å) and angles (deg): Fe(1)–Fe(2), 2.5416(16); Fe(1)–S(1), 2.2453(12); Fe(1)–S(2), 2.2512(12); Fe(1)–P(1), 2.1828(13); Fe(1)–P(2), 2.1763(12); Fe(1)–C(4), 1.733(3); Fe(2)–C(1), 1.916(3); Fe(2)–C(2), 1.766(3); Fe(2)–C(3), 1.751(3); Fe(2)–Fe(1)–P(1), 112.81(5); Fe(2)–Fe(1)–P(2), 154.49(3); Fe(2)–Fe(1)–C(4), 101.07(10); P(2)–Fe(1)–P(1), 87.54(6); P(2)–Fe(1)–C(4), 94.50(10); P(1)–Fe(1)–C(4), 88.25(9).

Figure 7

IR spectra for the reaction of Fe₂(S₂C₂H₄)(CO)₄(dppv) (0.014 M) with 20 equiv of PMe₃ in a CH₂Cl₂ solution at 20 °C.

Figure 8

DFT-optimized reactant, transition state, and product for the reaction 1(CO)₄ + KCN (distances in angstroms).

Figure 9

DFT-optimized structures of axial–basal and dibasal isomers of [Fe₂(S₂C₂H₄)(μ-CO)(CO)₂(dppv)(PMe₃)(NCMe)]²⁺ (distances in angstroms and computed energy differences in kilocalories per mole relative to the most stable isomer). For the sake of brevity, only the four most relevant (and most stable) isomers are shown.

Scheme 1

Scheme 2