MODELS OF THE CYTOCHROMES-B - CONTROL OF AXIAL LIGAND ORIENTATION WITH A HINDERED PORPHYRIN SYSTEM

The development of an iron porphyrinate/axial ligand system that leads to control of the relative and absolute axial ligand orientations is described. The system consists of the iron(III) complex of the hindered porphyrin meso-tetramesitylporphyrin and pyridine ligands. The preparation and characterization of the low-spin [Fe(TMP)(4-NMe2Py)2]ClO4 (4-NMe2Py = 4-(dimethylamino)pyridine) derivative is described. The crystal structure determination shows that the complex has the desired relative perpendicular orientation of the two axial pyridines. The two pyridine planes are close to eclipsing adjacent Fe-C(m) vectors. The porphinato core is strongly S4-ruffled; the Fe-N(p) bonds show a commensurate shortening from the values expected for planar low-spin species. The EPR spectrum of the complex has a large g(max) feature at 3.48 and no other discernible features. An analysis of the Mossbauer spectrum in a 6-T field allows estimation of a range of possible crystal field parameters (DELTA/lambda = 1.9-3.6, V/lambda = 0.7-0.89). The complex displays the usual quadrupole doublet in zero field with an isomer shift of 0.20 and a quadrupole splitting of 1.74 mm/s. As discussed in the text, relatively low values of the quadrupole splitting constant appear characteristic of perpendicular orientations of the axial ligands in low-spin iron(III). Two ''control'' complexes have also been prepared and characterized. The crystal structure of the bis[4-(dimethylamino)pyridine] derivative of (octaethylporphinato) iron(III) shows that it has the normal parallel orientation of axial ligands and a normal low-spin rhombic EPR spectrum with g(z) = 2.818, g(y) = 2.275, and g(x) = 1.630. Crystal field parameters for this complex have also been obtained from an analysis of the Mossbauer spectrum in a 6-T field. This complex also displays a quadrupole doublet in zero field with an isomer shift of 0.26 and quadrupole splitting of 2.14 mm/s. Bis(1-methylimidazole)(meso-tetramesitylporphinato)iron(III) has also been characterized; as expected, relative parallel orientations of axial ligands are found in the crystal structure as well as a planar porphyrin core. Mossbauer parameters are delta = 0.28 mm/s and DELTA-E(q) = 2.28 mm/s. Crystal data: [Fe(TMP)(4-NMe2Py)2]ClO4.2C6H5Cl: a = 15.240 (3) angstrom, b = 25.800 (6) angstrom, c = 18.181 (4) angstrom, beta = 97.72 (2)-degrees, monoclinic, space group P2(1)/n, V = 7083.7 angstrom3, Z = 4, number of observed data = 6935, R1 = 0.062, R2 = 0.071. [Fe(OEP)(4-NMe2Py)2]ClO4.CH2Cl2: a = 10.480 (6) angstrom, b = 10.692 (6) angstrom, c = 11.210 (4) angstrom, alpha = 84.04 (4)-degrees, beta = 85.88 (4)-degrees, gamma = 78.43 (6)-degrees, triclinic, space group P1BAR = 1222.2 angstrom3, Z = 1, number of observed data = 4105, R1 = 0.067, R2 = 0.086. [Fe(TMP)(1-MeIm)2]ClO4: a = 13.929 (2) angstrom, b = 14.430 (2) angstrom, c = 15.106 (3) angstrom, alpha = 94.02 (1)-degrees, beta = 106.97 (1)-degrees, gamma = 96.24 (1)-degrees, triclinic, space group P1BAR, V = 2870.3 angstrom3, Z = 2, number of observed data = 6276, R1 = 0.046, R2= 0.054.