Mössbauer studies on cytochrome b models: bis ligated complexes of iron(III) protoporphyrinate IX with imidazole and substituted imidazoles

Abstract

Iron-57 Mössbauer spectra at 80 K have been recorded for a series of bis ligated complexes of iron(III) protoporphyrinate IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionate)[FeL]⁺ with imidazole (Him), 1-methylimidazole (1Me-im), and 2-methylimidazole (2Me-im). The spectra were recorded in frozen solutions of dimethyl sulphoxide (dmso), acetone, and 50%(v/v) ethanol–water and also in the presence of hydrogen-bonding agents such as trifluoroethanol or 1,10-phenanthroline in acetone. This is the first report of Mössbauer spectroscopic studies on the solvent dependence of a series of substituted imidazole complexes of [FeLCl]. Significant new results are obtained which are interpreted in the light of reported e.s.r., n.m.r., and electronic absorption data. A value of 2.43 mm s^–1 for the quadrupole splitting (ΔE_Q) of the imidazolate complex [FeL(Him)(im)] is assigned to a parallel orientation of the planar axial ligands, whereas a value of ΔE_Q= 1.87 mm s^–1 for the [FeL(2Me-im)₂]⁺ is assigned to a perpendicular orientation of the imidazole planes. Large linewidths of about 0.6–0.98 mm s^–1 are attributed to the presence of a statistical distribution of imidazole planes around the thermodynamically stable arrangement. The influences of hydrogen bonding on the Mössbauer parameters are discussed. The 2Me-im complex and the 1Me-im complex in dmso show slow spin–lattice relaxation at 80 K.