CHARACTERIZATION OF IRON TETRAKIS(N-METHYL-2-PYRIDYL)PORPHINE IN AQUEOUS-MEDIA - ELECTROCHEMICAL GENERATION OF STABLE IRON(I), IRON(II), IRON(III) AND IRON(IV) PORPHYRINS AT ROOM-TEMPERATURE

Iron tetrakis(N-methyl-2-pyridyl)porphine (Fe(2-TMPyP)), which has four strong electron-withdrawing groups at the meso positions, was studied as a function of the pH. For the first time, stable FeI, FeII, FeIII and FeIV forms of the iron porphyrin could be generated electrochemically at room temperature and monitored by their UV-visible spectra in the same aqueous media. FeIII (2-TMPyP) exhibits pKa at 5.1 and pKa at 10.8 in 0.1 M Na2SO4 solution. The value of pKa depends on the anions and anion concentration whereas pKa2 is constant. A monomer-dimer equilibrium exists in alkaline solutions with a dimerization constant of 2 × 102 M-1. In acidic solution, FeIII(2-TMPyP) undergoes a one-electron reduction at the central metal ion with a formal potential (Eo') of +0.11 V (vs. Ag/AgCl). The pKa value for FeII(2-TMPyP) is 11.2. FeII(2-TMPyP) could be further reduced to FeI(2-TMPyP), which is stable in weakly acidic and alkaline solutions. The reduction product of FeI(2-TMPyP), presumably FeI(2-TMPyP)-, shows reversible redox waves only at fast scan rates. The oxidation of FeIII(2-TMPyP) in alkaline solution generates FeIV(2-TMPyP), which has a pKa value of 10.0. The species have been confirmed by resonance Raman spectroscopy. © 1990.