Models of the Cytochromes b. 5. EPR Studies of Low-Spin Iron(III) Tetraphenylporphyrins

Article abstract of DOI:10.1021/ja00335a003

The EPR spectra of a wide range of tetraphenylporphyrin complexes of Fe(III) have been investigated as a function of solvent, ligand type, ligand basicity, porphyrin substituents, covalent attachment of axial ligands, and mixed axial ligand coordination.The results show the following: (1) EPR parameters of low-spin bis-axial ligand complexes of Fe(III) porphyrins depend not only on ligand basicity but also on ligand type.Of the three major classes studied, bis(imidazole) and -(aminopyridine) complexes all have similar values of g_x, g_y, and g_z which are nearly independent of ligand basicity, while bis(pyrazole) (and bis(indazole)) complexes have g_x, g_y, and g_z values which tend to converge as ligand basicity increases. (2) The effect of the electron-donating or -withdrawing nature of phenyl substituents on the EPR parameters of a large series of phenyl-substituted (TPP)Fe(N-MeIm)2⁺ derivatives is very small: The rhombicity V/Δ=0.64+/-0.01 for all complexes, while the tetragonality Δ/λ ranges from 2.97 for electron-donating substituents to 3.33 for electron-withdrawing substituents, the opposite trend from that expected for increasing axial ligand donor strength.No difference was observed in the EPR parameters of unsymmetrically as compared to symmetrically substituted TPP derivatives. (3) Covalent attachment of axial ligands or steric crowding of externally supplied axial ligands in the hope of seeing variation in the EPR parameters with relative axial ligand plane orientation (parallel vs. perpendicular) was not successful in producing pure isomers, and thus no effects on EPR parameters with axial ligand plane orientation were detected. (4) A covalently attached (N-alkylimidazole-TPP)Fe(III) derivative was utilized to allow formation of mixed-ligand low-spin Fe(III) complexes.The alkylimidazole-imidazolate ligand combination was only very slightly more tetragonal than its protonated imidazole counterpart, while the alkylimidazole-pyrazole, 3-aminopyrazole, 1,2,4-triazole, and 2-methylimidazole mixed ligand complexes all had EPR parameters uniquely different from those of the parent bis-ligand complexes.Discussion of these results in light of the g values of membrane-bound cytochromes b, c, and a₃ bound to cyanide is also included.