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°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 (OCH 3 , 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 2 and some log 1 ) of each iron porphyrinate with each ligand were determined. The values of log 2 for the iron(III) complexes decrease in the order Br > Cl > CH 3 > OCH 3 > H > F, while those for iron(II) decrease in the slightly modified order Br > Cl > CH 3 > F > OCH 3 > 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 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-(OCH 3 ) 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.

The synthesis and characterization of seven bis-pyridine and bis-imidazole complexes of iron(II) tetramesitylporphyrinate are reported. X-ray crystal structures of three of the complexes, [Fe(TMP)(4-CNPy) 2 ], [Fe(TMP)(3-CNPy) 2 ], and [Fe(TMP)(4-MePy) 2 ], have been solved and all show parallel axial ligand orientations with nearly planar porphyrinato cores. The Mo ¨ssbauer spectra of six of the complexes, having pyridine ligands with pK a (PyH + ) ranging from ∼1.1 (4-CNPy) to 9.7 (4-NMe 2 Py), have been determined. The Mo ¨ssbauer isomer shifts at 120 K are in the range of 0.36-0.45 mm/s, and the quadrupole splittings (∆E Q ) are in the range of 1.11-1.27 mm/s. Thus, unlike the corresponding Fe(III) complexes, the X-ray structures and Mo ¨ssbauer spectroscopic parameters of these (tetramesitylporphyrinato)iron(II)-bis(pyridine) complexes are shown to be essentially independent of the basicity and π donor/acceptor properties of the axial pyridine ligands. These solid-state structural and spectroscopic properties are compared to the thermodynamic properties of the same series of complexes in solution (

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraphenylporphyrin ligands, where the 2,6-phenyl substituents were -H, -F, -Cl, -Br, or -OMe, as well as two 2,4,6-phenyl-substituted complexes, where the substituents were -Me and -OMe, have been investigated as a function of temperature by 1H NMR spectroscopy. Curvature in the 1/T dependence was evident in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include negative slopes with very large positive 1/T intercepts (Cl approximately Br > Me > H) to negative slope with near zero intercept (tri-OMe) to positive slope with very large negative intercept (F, di-OMe). The NMR results were combined with EPR spectroscopic data and curve-fitting procedures based on an expanded Curie law to arrive at a consistent overview of the variety of temperature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (or more) thermally accessible excited state(s) are populated to varying degrees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state being a largely intermediate-spin state (S = 3/2) for the majority of the complexes but a largely high-spin state (S = 5/2) for ((2,6-F2)4TPP)FeOClO3 and ((2,6-(OMe)2)4TPP)FeOClO3.