Models of the Cytochromes. Redox Properties and Thermodynamic Stabilities of Complexes of “Hindered” Iron(III) and Iron(II) Tetraphenylporphyrinates with Substituted Pyridines and Imidazoles

Abstract

The FeIII/FeII and FeII/FeI reduction potentials of a series of model hemes have been measured by cyclic voltammetry in dimethylformamide at 25 °C as a function of the concentration of added axial ligands. The six porphyrinate ligands utilized were tetraphenylporphyrin (TPP), tetramesitylporphyrin (TMP), and a series of “hindered” tetraphenylporphyrins having substituents (OCH3, F, Cl, Br) on both ortho positions of each of the four phenyl rings. The perchlorato salts of the iron(III) porphyrinates were utilized for the titrations. The axial ligands utilized were N-methylimidazole, 2-methylimidazole, and pyridines of basicities ranging in pKa(PyH+) from 9.7 to 1.1. From the electrochemical titration, the equilibrium constants (log β2 and some log β1) of each iron porphyrinate with each ligand were determined. The values of log β2 for the iron(III) complexes decrease in the order Br > Cl > CH3 > OCH3 > H > F, while those for iron(II) decrease in the slightly modified order Br > Cl > CH3 > F > OCH3 > H. Rather than electronegativity or electron-donating/withdrawing characteristics, these results appear to follow primarily the physical size of the ortho substituents: The iron porphyrinates with the largest ortho substituents have the largest equilibrium constants. The hindered (TPP)FeII complexes are usually, but not always, more stable than the corresponding hindered (TPP)FeIII complexes. For both (TPP)FeIII and the hindered (TPP)FeIII complexes, the slope of the correlation of log β2III with the pKa of the conjugate acid of the pyridine ligand is 1.0. The equilibrium constants for hindered (TPP)FeII complexes, however, have virtually no dependence upon the base strength of the axial pyridines, while those for (TPP)FeII has a slight sensitivity, with a slope of 0.15. In contrast to common belief, 2-methylimidazole readily forms bis complexes with some iron(II) tetraphenylporphyrinates ((TMP)FeII, ((2,6-Cl2)TPP)FeII, and ((2,6-Br2)4(TPP)FeII), although it does not with (TPP)FeII and its (2,6-F2)4- and (2,6-(OCH3)2)4- counterparts. N-Methylimidazole is unique among the axial ligands of this study, in that the equilibrium constants for binding to both Fe(III) and Fe(II) are virtually identical for all porphyrinates studied.