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Models of the Cytochromes:  Crystal Structures and EPR Spectral Characterization of Low-Spin Bis-Imidazole Complexes of (OETPP)FeIII Having Intermediate Ligand Plane Dihedral Angles

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journal contribution
posted on 10.07.2006, 00:00 by Liliya A. Yatsunyk, Alice Dawson, Michael D. Carducci, Gary S. Nichol, F. Ann Walker
The preparation, EPR spectra, and crystal structures of octaethyltetraphenylporphyrinatoiron(III) having two imidazole, N-benzylimidazole, and N-methylimidazole axial ligands are reported, [(OETPP)Fe(HIm)2]Cl, [(OETPP)Fe(N-BzIm)2]Cl, and [(OETPP)Fe(N-MeIm)2]Cl. Despite large variation in axial ligand size, the unit cell parameters for all complexes are very similar; each structure has the same basic motif, with large voids formed by the extended porphyrin framework (filled by ordered or disordered axial ligands and disordered solvent), which allows differently sized ligands to fit within the same cell dimensions. Each porphyrin core adopts a saddled conformation with |ΔCβ| = 1.13−1.15 Å. The dihedral angles between axial ligand planes, Δφ, are far from being either ideal parallel or perpendicular:  30.1°, 57.2° for [(OETPP)Fe(HIm)2]Cl (molecules 1 and 2), 56.8° for [(OETPP)Fe(N-BzIm)2]Cl, and 16.0°, 44.6°, 59.6°, and 88.1° for [(OETPP)Fe(N-MeIm)2]Cl, which has disordered axial ligands. Among the complexes of this study, an axial ligand Δφ of 56.8° is found to be the largest “parallel” angle (as defined by the observation of a normal rhombic or Type II EPR signal (N-BzIm, g = 3.08, 2.19, 1.31)), while 57.2° is found to be the smallest “perpendicular” Δφ (as defined by the observation of a “large gmax” or Type I EPR signal (HIm, gmax = 3.24)). From the results of this study, it is clear that the size of the largest g for Types I and II complexes varies continuously, with no break between the two. While the switch in EPR signal type, from Type II to Type I, appears to be very sharp in this study, this may be somewhat artificial based upon limited numbers of examples and the required saddle distortion of the (OETPP)FeIII complexes. However, in comparison to several proteins with dihedral angles near 60° and Type II EPR spectra, we may conclude that the switch in EPR signal type occurs near 57° ± 3−5°.