[5,10,15,20-Tetrakis(l-methylpyridinium-2-, -3-, and -4-yl)porphinato]iron Complexes As Seen by Electronic Absorption, Magnetic Circular Dichroism, and Electron Paramagnetic Resonance Spectroscopy

Magnetic circular dichroism (MCD) and electronic absorption spectra are reported for [5,10,15,20-tetrakis(l-methylpyridinium-2-and -3-yl)porphinato]iron, FeT2MP and FeT3MP, in the near-UV to near-IR regions. Electron paramagnetic resonance (EPR) spectra are also reported for the Fe^IIIT2MP, Fe^IIIT3MP, and Fe^IIIT4MP ([5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphinatoiron(III)) species. In each FeTMP, three ferric and two ferrous species are sufficient to explain the results at all pH values. Proton equilibria exist between the three Fe^IIIT2MP or Fe^IIIT3MP monomers with pk_a values around 5-6 and 11-12. After electrochemical reduction, an equilibrium is observed between the two Fe^IIT2MP or Fe^IIT3MP monomers with a pK_a of around 11-12. The species are in high-spin states except at high pH; low-spin states exist for Fe^IIIT3MP and Fe^IIIT4MP at pH >13 and for Fe^IIIT2MP and Fe^IITMPs at pH >12. The effective symmetry of iron increases approximately in the sequence of FeT2MP, FeT3MP, and FeT4MP; i.e. the farther the pyridinium group from the porphyrin plane, the more symmetric iron becomes. The electronic structures of the coordination spheres of these structural isomers are discussed in the context of previously studied iron porphyrins and hemes.