Models of the cytochromes. Redox properties and thermodynamic stabilities of complexes of ''hindered'' iron(III) and iron(II) tetraphenylporphyrinates with substituted pyridines and imidazoles

The Fe-III/Fe-II and Fe-II/Fe-I reduction potentials of a series of model hemes have been measured by cyclic voltammetry in dimethylformamide at 25 degrees 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 pK(a)(PyH(+)) from 9.7 to 1.1. From the electrochemical titration, the equilibrium constants (log beta(2) and some log beta(1)) of each iron porphyrinate with each ligand were determined. The values of log beta(2) 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)Fe-II complexes are usually, but not always, more stable than the corresponding hindered (TPP)Fe-III complexes. For both (TPP)Fe-III and the hindered (TPP)Fe-III complexes, the slope of the correlation of log beta(2)(III) with the pK(a) of the conjugate acid of the pyridine ligand is 1.0. The equilibrium constants for hindered (TPP)Fe-II complexes, however, have virtually no dependence upon the base strength of the axial pyridines, while those for (TPP)Fe-II 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)Fe-II, ((2,6-Cl-2)TPP)Fe-II, and ((2,6-Br-2)(4)(TPP)Fe-II), although it does not with (TPP)Fe-II and its (2,6-F-2)(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.