Synthesis, structure, and magnetic spectroscopies of non-planar hemes as models of the cytochrome B heme centers

Abstract

Models of the heme b centers in the cytochrome bc₁ complex based on octaalkyltetraphenyliron(III) porphyrins (OETPP, OMTPP, and TC₆TPP) with various axial ligands (4-Me₂NPy, 4-CNPy, 1-MeIm, 2-McImH, CN- and t-BuNC) have been synthesized and explored via NMR and EPR spectroscopy and X-ray crystallography. The electron spin distributions and the electronic ground states of the complexes were determined based on the NMR and EPR results. The type of ground state of Fe(III) is defined by the number and nature of the axial ligands and peripheral substituents. Five-coordinate complexes with Cl- are high-spin (HS, S = 5/2) with ∼10% of intermediate spin (IS, S = 3/2) mixed in. Bis-(4-CNPy) porphyrins at relatively high temperatures, as well as five-coordinate perchlorates, have the IS state. Finally, six-coordinate complexes with pyridines, imidazoles, cyanides and isocyanides are all low-spin (LS, S = 1/2), with the (dxy)²(d xz,dyz)³ ground state in the case of strongly and moderately basic axial ligands, and the (dxz,dyz)⁴(dxy)¹ ground state when bound to weak sigma-donors and/or strong pi-acceptors like t-BuNC or 4-CNPy (at low temperatures only). Crystal structures of more than 15 complexes have been obtained in order to establish structure-property relationships in the systems studied. All of them adopt a saddled conformation with different degrees of ruffling and non-planarity and a wide range of dihedral angles between the axial ligands. Non-planar distortion of the macrocycle induced by the steric interaction between the peripheral substituents increases in the order TC₆TPP < OMTPP < OETPP. The rate of ring inversion of the porphyrin cores decreases in the same order, as shown by NMR spectroscopy. While all bis-ligated iron(III) porphyrinate of this study with pyridine or hindered imidazole ligands have close to perpendicular axial ligand orientations, bis-(1-MeIm) complexes have perpendicular as well as close to parallel ligand arrangements both in crystals and in solutions as observed by EPR and Mössbauer spectroscopy and X-ray crystallography. Preliminary Mössbauer studies of four of the complexes of this study have been carried out in the laboratory of Professor A. X. Trautwein, Institute of Physics, University of Lubeck, Germany. The results of the work presented in this dissertation suggest that octaalkyltetraphenylporphyrinatoiron(III) complexes are promising models for the heme b centers in the cytochrome bc₁ complex.