Electrochemical and Spectral Studies of Sterically Hindered Water-Soluble Iron Tetrakis(Sulfonatomesityl)Porphine

Sterically hindered water-soluble iron(III) tetrakis(sulfonatomesityl)porphine (FeTSMP) exists exclusively in its monomer form in neutral and basic aqueous solutions. The acid dissociation constant (pKa) for Fe^IIITSMP is 6.6. The Fe^III/IITSMP couple has a formal reduction potential at ?0.22 V (vs. Ag/AgCl) in an acidic buffer solution. The E°-pH diagram shows that E° is independent of pH until the pH is larger than the pKa. The acid dissociation constant for Fe^IITSMP is 11.7. The redox potential of Fe^III/IITSMP shifts positively when imidazole is present, indicating the ligation of imidazole to Fe^IITSMP. At pH 4.0 and 10.2 buffer solutions, two imidazoles are found to ligate at the iron(III) center with β₂ = 10^4.7 and 10^4.1, respectively. The E°-pH diagram indicates that the pKa of (ImH)₂Fe^IIITSMP is 11.3 at 0.03 M imidazole solution. The oxidation of Fe^IIITSMP could involve two one-electron transfer processes, namely, one electron oxidation at the iron center, while another at the porphyrin ring. The two one-electron oxidations are found to overlap at pH = 5.5 and separate as the pH increases by shifting of the potential at the iron center, which is pH dependent. The radical cation of porphyrin ring is not stable and decomposes rapidly in water.