Structure and Reactivity of Fe(II)-SAr Complexes: Relevance to the Active Site of Isopenicillin N Synthase

A series of (thiophenolato)iron(II) complexes, [Fe(TPA)(SAr)]+ (1, Ar = C₆H₂-2,4,6-Me₃; 2, Ar = C₆H₄-4-Me; 3, Ar = C₆H₅; 4, Ar = C₆H₄-4-Cl), where TPA is tris(2-pyridylmethyl)amine, have been synthesized as models for the isopenicillin N synthase-substrate complex, and [Fe(TPA)(SC₆H₂-2,4,6-Me₃)]⁺ (1) has been crystallographically characterized. Complex 1 crystallizes in the monoclinic system, space group P2_1/a, with cell dimensions a = 14.454(6) Å, b = 9.403(6) Å, c = 21.046(3) Å, β = 105.43(2)°, and Z = 4. The iron(II) center is in a trigonal bipyramidal coordination environment with three pyridine nitrogens forming the equatorial plane and the amine nitrogen and S from the thiophenol occupying the apical positions. The average Fe-N_py distance is 2.126 Å, Fe-N_amine is 2.250(3) Å, and Fe-S is 2.345(1) Å. Complexes 1–4 react with NO at low temperature to form NO adducts. These NO complexes exhibit S-to-Fe charge transfer bands in the visible spectra which are not present in the visible spectrum of [Fe(TPA)(O₂CCH₃)(NO)]⁺. They also exhibit S = ³/₂ EPR spectra that have higher rhombicity (E/D = 0.041) than that of the acetate analogue (E/D = 0). These effects are attributed to the presence of the thiolate ligand and are analogous to those observed for the enzyme system. Exposure of 1–4 to air at low temperature generates metastable green species corresponding to the one-electron-oxidized iron(III) derivatives. The iron(III) complexes decompose to (μ-oxo)diiron(III) complexes and disulfides under air upon warming to room temperature.