Spectroscopic study of a water-soluble iron(III) meso-tetrakis(4-N-methylpyridiniumyl) porphyrin in aqueous solution: effects of pH and salt.

The equilibrium behavior of cationic iron(III) meso-tetrakis(4-N-methyl-pyridiniumyl) porphyrin, Fe(III)TMPyP, in aqueous solution was studied as a function of pH by optical absorption, EPR and (1)H NMR spectroscopies. The presence of several Fe(III)TMPyP species in solution was unequivocally demonstrated: monomeric porphyrin species (a monoaqueous five-coordinated complex, a diaaqueous six-coordinated complex and a monoaqueous-hydroxo six-coordinated complex), a micro-oxo dimer and a bis-hydroxo complex. The addition of salt to the porphyrin solution leads to a simplification of the equilibrium as a function of pH. In this case, only three species were observed in solution: a monomeric porphyrin species, a micro-oxo dimer and a bis-hydroxo complex. Optical absorption, EPR and (1)H NMR spectra contributed to the characterization of these species. Four critical pH values (pK) for Fe(III)TMPyP were obtained in pure buffer and only three pK values were observed in the presence of NaCl. The addition of salt favors the presence of the dimeric species in solution and simplifies the equilibrium in the acidic pH range.