Low-Spin Ferriheme Models of the Cytochromes: Correlation of Molecular Structure with EPR Spectral Type

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Abstract
The preparation and characterization of the following bis-imidazole and bis-pyridine complexes of octamethyltetraphenylporphyrinatoiron(III), Fe(III)OMTPP, octaethyltetraphenylporphyrinatoiron(III), Fe(III)OETPP, and tetra-beta,beta'-tetramethylenetetraphenylporphyrinatoiron(III), Fe(III)TC6TPP, are reported: paral[FeOMTPP(1 -Melm)(2)]Cl, perp-[FeOMTPP(l -MeIM)(2)]Cl, [FeOETPP(1 -Melm)(2)]Cl, [FeTC6TPP(1 -Melm)(2)]Cl, [FeOMTPP(4-Me2NPY)(2)]Cl, and [FeOMTPP(2-MeHlm)(2)]Cl. Crystal structure analysis shows that paral[FeOMTPP(1 -Melm)(2)]Cl has its axial ligands in close to parallel orientation (the actual dihedral angle between the planes of the imidazole ligands is 19.5degrees), while perp-[FeOMTPP(1-Melm)(2)]Cl has the axial imidazole ligand planes oriented at 90degrees to each other and 29degrees away from the closest N-P-Fe-N-P axis. [FeOETPP-(1 -Melm)(2)]Cl has its axial ligands close to perpendicular orientation (the actual dihedral angle between the planes of the imidazole ligands is 73.1degrees). In all three cases the porphyrin core adopts relatively purely saddled geometry. The [FeTC6TPIP(1-Melm)(2)]Cl complex is the most planar and has the highest contribution of a ruffled component in the overall saddled structure compared to all other complexes in this study. The estimated numerical contribution of saddled and ruffled components is 0.68:0.32, respectively. Axial ligand planes are perpendicular to each other and 15.3degrees away from the closest N-P-Fe-N-P axis. The Fe-N-P bond is the longest in the series of octaalkyltetraphenylporphyrinatoiron(III) complexes due to [FeTC6TPP-(1 -Melm)(2)]Cl having the least distorted porphyrin core. In addition to these three complexes, two crystalline forms each of [FeOMTPP(4-Me2NPY)(2)]Cl and [FeOMTPP(2-MeHlm)(2)]Cl were obtained. In all four of these cases the axial planes are in nearly perpendicular planes in spite of quite different geometries of the porphyrin cores (from purely saddled to saddled with 30% ruffling). The EPR spectral type correlates with the geometry of the OMTPP, OETPP and TC6TPP complexes, For the patal-[FeOMTPP(1-Melm)(2)]Cl, a rhombic signal with g(1) = 1.54, g(2) = 2.51, and g(3) = 2.71 is consistent with nearly parallel axial ligand orientation. For all other complexes of this study, "large g(max)" Signals are observed (g(max) = 3.61 - 3.27), as are observed for nearly perpendicular ligand plane arrangement. On the basis of this and previous work, the change from "large g(max)" to normal rhombic EPR signal occurs between axial ligand plane dihedral angles of 70degrees and 30degrees.

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