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

Abstract 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 μ-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 μ-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 (p K ) for Fe(III)TMPyP were obtained in pure buffer and only three p K 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.