ELECTROCHEMISTRY AND THE DEPENDENCE OF POTENTIALS ON PH OF IRON AND MANGANESE TETRAPHENYLPORPHYRINS IN AQUEOUS-SOLUTION

Microelectrode electrochemical studies have been carried out with aqueous solutions (mu = 0.2 with NaNO3) of the water soluble and non-mu-oxo dimer forming 5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphinatoiron(III) and - manganese(III) hydrates [(1)Fe(III)(X2) and (1)Mn(III)(X)2, X = H2O, HO-, and O2-]. The pH dependence of the potentials (SCE) for the stepwise le- oxidations of (1.-)Fe(II)(X)2 --> (1)Fe(II)(X)2 --> (1)Fe(III)(X)2 --> (1)Fe(IV)(X)2 --> (1.+)Fe(IV)(X)2 and (1)Mn(III)(X)2 --> (1)Mn(IV)(X)2 --> (1)Mn(V)(X)2 were determined from Nernst-Clark plots of E(m) values (determined at constant pH) vs pH. Also obtained from the Nernst-Clark plots are the pK(a) values for the acid dissociation of the water molecules ligated to [(1)Fe(III)]+, [(1)Fe(IV)]2+, [(1.+)Fe(IV)]3+ and to [(1)Mn(III)+, [(1)Mn(IV)]2+, and [(1)Mn(V)]3+. The midpoint potentials for the le- oxidations (1.-)Fe(II)(X)2 --> (1)Fe(II)(X)2 and (1)Fe(IV)(X)2 --> (1.+)Fe(IV)(X)2 are pH independent, due to the equality of the pK(a) values of water ligated to reduced and oxidized species, and equal -1.15 and 1.17 V (SCE), respectively. Representative values of E-degrees' for the interconversion of the various iron(III) and iron(IV) porphyrin hydrates are e- + (1)Fe(IV)(H2O)2 --> (1)Fe(III)(H2O)2, 0.91 V; e- + (1)Fe(IV)(H2O)(OH) --> (1)Fe(III)(H2O)(OH), 0.86 V; and e- + (1)Fe(IV)(OH)2 --> (1)Fe(III)(OH)2, 0.77 V. The best first and second pK(a) values at mu = 0.2 for the dissociation of protons from the ligated water molecules are (1.-)Fe(II)(H2O)2 and (1)Fe(II)(H2O)2, pK(a1) 9.7; (1)Fe(III)(H2O)2, PK(a1) 6.55 and pK(a2) 10.55; (1)Fe(IV)(H2O)2 and (1.+)Fe(IV)(H2O)2, pK(a1) 5.7 and pK(a2) 9.0. Spectroelectrochemical studies of the controlled potential oxidation of (1)Fe(III)(X)2 --> (1)Fe(IV)(X)2 show that the various iron(IV) species, formed a different pH values, possess the same spectral characteristics as seen previously in the le- oxidation of (1)Fe(III)(X)2 by tert-butyl hydroperoxide in water. Comparison of plots of E(m) vs pH in H2O to E(m) vs pD plots in D2O for the oxidation of (1)Fe(III)(X)2 --> (1)Fe(IV)(X)2 shows that there is no (within experimental error) deuterium isotope effects upon potentials. The deuterium isotope effects upon the acidity of ligated H(D)2O (K(a)H/K(a)D) are (L = H or D) (1)Fe(III)(L2O)2 --> (1)Fe(III)(LO)(L2O) + L+, 5.0; (1)Fe(III)(LO)(L2O) --> (1)Fe(III)(LO)2 + L+, 2.8; (1)Fe(IV)(L2O)2 --> (l)-Fe(IV)(LO)(L2O) + L+, 7.9; (1)Fe(IV)(LO)(L2O) --> (1)Fe(IV)(LO)2 + L+, 3.55. Representative values of E-degrees' for the interconversion of the various manganese(III) and manganese(IV) porphyrin hydrates are e- + (1)Mn(IV)(H2O)2 --> (1)Mn(III)(H2O)2, 1.04 V; e- + (1)Mn(IV)(H2O)(OH) --> (1)Mn(III)(H2O)(OH), 0.93 V; e- + (1)Mn(IV)(HO)2 --> (1)Mn(III)(HO)2, 0.79 V. First and second pK(a) values for the dissociation of manganese ligated water are (1)Mn(III)(H2O)2, PK(a1) 5.8 and pK(a2) 12.2; (1)Mn(IV)(H2O)2, pK(a1) 4.1 and pK(a2) 9.9; and for (1)Mn(V)(H2O)2, pK(a1) 3.7. Our thermodynamic data (both E(m) and pK(a) values) supports an iron(IV) porphyrin pi-cation radical [(1.+)Fe(IV)(X)2] rather than an iron (V) porphyrin [(1) Fe(V)(X)2] as the product of 2e- oxidation of (1)Fe(III)(X)2. On the other hand, our data support a manganese(V) porphyrin [(1)Mn(V)(X)2] rather than a manganese(IV) porphyrin pi-cation radical. Of considerable importance is the finding, in water, that H2O and HO- serve as the axial ligands for [(porph)M]2+ and [(porph)M]3+ species and that oxo ligation [as in (1.+)Fe(IV)(= O)] cannot be important except at high pH [where structures as (1.+)Fe(IV)(OH)2 and (1.+)Fe(IV)(= O)(H2O) have the same proton inventory].