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 FeIIIT2MP, FeIIIT3MP, and FeIIIT4MP ([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 FeIIIT2MP or FeIIIT3MP monomers with pka values around 5-6 and 11-12. After electrochemical reduction, an equilibrium is observed between the two FeIIT2MP or FeIIT3MP monomers with a pKa of around 11-12. The species are in high-spin states except at high pH; low-spin states exist for FeIIIT3MP and FeIIIT4MP at pH >13 and for FeIIIT2MP and FeIITMPs 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry