Probing the Influence of Local Coordination Environment on the Properties of Fe-Type Nitrile Hydratase Model Complexes

A series of four structurally related cis-dithiolate-ligated Fe(III) complexes, [Fe^III(DITpy)₂]Cl (1), [Fe^III(DITIm)₂]Cl (2), [Fe^III(ADIT)₂]Cl (3), and [Fe^III(AMIT)₂]Cl (4), are described. The structural characterization of 3 as well as the spectroscopic properties of 3 and 4 has been previously reported. Crystal data for 1, 2, and 4 are as follows:  1·3H₂O crystallizes in the orthorhombic space group Pca2₁ with a = 19.800(4) Å, b = 18.450(4) Å, c = 14.800(3) Å, and Z = 8. 2·¹/₂EtOH·¹/₂H₂O crystallizes in the monoclinic space group C_c with a = 24.792(4) Å, b = 14.364(3) Å, c = 17.527(3) Å, β = 124.91(2)°, and Z = 8. 4 crystallizes in the triclinic space group P1̄ with a = 8.0152(6) Å, b = 10.0221(8) Å, c = 11.8384(10) Å, α = 73.460(3)°, β = 71.451(5) °, γ = 72.856(4)°, and Z = 2. Complexes 1−4 share a common S₂N₄ coordination environment that consists of two cis-thiolates, two trans-imines, and two cis-terminal nitrogen donors:  N_term = pyridine (1), imidazole (2), and primary amine (3 and 4). The crystallographically determined mean Fe−S bond distances in 1−4 range from 2.196 to 2.232 Å and are characteristic of low-spin Fe(III)−thiolate complexes. The low-spin S = ¹/₂ ground state was confirmed by both EPR and magnetic susceptibility measurements. The electronic spectra of these complexes are characterized by broad absorption bands centered near ∼700 nm that are consistent with ligand-to-metal charge-transfer (CT) bands. The complexes were further characterized by cyclic voltammetry measurements, and all possess highly negative Fe(III)/Fe(II) redox couples (∼ −1 V vs SCE, saturated calomel electrode) indicating that alkyl thiolate donors are effective at stabilizing Fe(III) centers. Both the redox couple and the 700 nm band in the visible spectra show solvent-dependent shifts that are dependent upon the H-bonding ability of the solvent. The implications of these results with respect to the active site of the iron-containing nitrile hydratases are also discussed.