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 i, 2, and 4 are as follows: 1 . 3H(2)O crystallizes in the orthorhombic space group Pca2(1) with n = 19.800(4) Angstrom, b = 18.450(4) Angstrom, c = 14.800(3) Angstrom, and Z = 8. 2 .1/2EtOH .1/2H(2)O crystallizes in the monoclinic space group C-c with a = 24.792(4) at b = 14.364(3) Angstrom, c = 17.527(3) Angstrom, beta = 124.91(2)degrees, and Z = 8. 4 crystallizes in the triclinic space group P (1) over bar with a = 8.0152(6) Angstrom, b = 10.0221(8) Angstrom, c = 11.8384(10) Angstrom, alpha = 73.360(3)degrees, beta = 71.451(5)degrees, gamma = 72.856(4)degrees 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 Angstrom 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 similar to 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 (similar to -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.