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

Abstract

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 absolute value(deltaC(beta)) = 1.13-1.15 A. The dihedral angles between axial ligand planes, delta phi, are far from being either ideal parallel or perpendicular: 30.1 degrees, 57.2 degrees for [(OETPP)Fe(HIm)2]Cl (molecules 1 and 2), 56.8 degrees for [(OETPP)Fe(N-BzIm)(2)]Cl, and 16.0 degrees, 44.6 degrees, 59.6 degrees, and 88.1 degrees for [(OETPP)Fe(N-MeIm)2]Cl, which has disordered axial ligands. Among the complexes of this study, an axial ligand delta phi of 56.8 degrees 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 degrees is found to be the smallest "perpendicular" delta phi (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)Fe(III) complexes. However, in comparison to several proteins with dihedral angles near 60 degrees and Type II EPR spectra, we may conclude that the switch in EPR signal type occurs near 57 degrees +/- 3-5 degrees.