Probing the Influence of Local Coordination Environment on the Properties of Fe-Type Nitrile Hydratase Model Complexes
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A series of four structurally related cis-dithiolate-ligated Fe(III) complexes, [FeIII(DITpy)2]Cl (1), [FeIII(DITIm)2]Cl (2), [FeIII(ADIT)2]Cl (3), and [FeIII(AMIT)2]Cl (4), are described. The structural characterization of 3 as wellas the spectroscopic properties of 3 and 4 has been previously reported. Crystal data for 1, 2, and 4 are asfollows: 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 Cc with a = 24.792(4)Å, b = 14.364(3) Å, c = 17.527(3) Å, ddle"> = 124.91(2)deg.gif">, and Z = 8. 4 crystallizes in the triclinic space group Pwith a = 8.0152(6) Å, b = 10.0221(8) Å, c = 11.8384(10) Å, = 73.460(3)deg.gif">, ddle"> = 71.451(5) deg.gif">, = 72.856(4)deg.gif">,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: Nterm = 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 Åand are characteristic of low-spin Fe(III)-thiolate complexes. The low-spin S = 1/2 ground state was confirmedby both EPR and magnetic susceptibility measurements. The electronic spectra of these complexes are characterizedby 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 highlynegative Fe(III)/Fe(II) redox couples (~ -1 V vs SCE, saturated calomel electrode) indicating that alkyl thiolatedonors are effective at stabilizing Fe(III) centers. Both the redox couple and the 700 nm band in the visiblespectra show solvent-dependent shifts that are dependent upon the H-bonding ability of the solvent. The implicationsof these results with respect to the active site of the iron-containing nitrile hydratases are also discussed.

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