REDOX CHEMISTRY OF WATER-SOLUBLE IRON, MANGANESE, AND CHROMIUM METALLOPORPHYRINS AND ACID-BASE BEHAVIOR OF THEIR LYATE AXIAL LIGANDS IN AQUEOUS-SOLUTION - INFLUENCE OF ELECTRONIC EFFECTS

Electrochemical oxidation and reduction of water-soluble and non-mu-oxo dimer-forming (5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrinato]iron(III) {(2)Fe(III)(X)2}, -manganese(III) {(2)Mn(III)(X)2}, and-chromium(III) {(2)Cr(III)(X)2} (where X = H2O or HO-) hydrates have been investigated in aqueous solutions as a function of pH. The midpoint potentials for the stepwise 1e- oxidations and reductions of (2) Fe(III)(X)2 shift to more positive values compared to those of [5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)porphyrinato]iron(III) {(1)Fe(III)(X)2}, and the pK(a) values (4.1 and 7.8) for the acid dissociation of the H2O molecules axially ligated to (2)Fe(III)(H2O)2 have lower values than those (6.55 and 10.55) for (1)Fe(III)(H2O)2 due to the inductive effects of the 2,6-dichloro substituents of (2)Fe(III)(H2O)2 compared to the 2,6-dimethyl substituents of (1)Fe(III)(H2O)2. Values of formal potentials for the interconversion of the iron(III) and iron(IV) porphyrin hydrates are e- + (2)Fe(IV)(H2O)2 reversible (2)Fe(III)(H2O)2, 1.20 V; e- + (2)Fe(IV)(H2O)(OH) reversible (2)Fe(III)(H2O)(OH), 1.10 V; e- + (2(Fe(IV)(OH)2 reversible (2)Fe(III)(OH)2, 1.06 V. Spectroelectrochemical investigations of the controlled-potential 1e- oxidation of(2)Fe(III)(X)2 show that the various iron(IV) species, formed at different pHs, have the same spectral characteristics as seen previously in the oxidation of (2)Fe(III)(X)2 by tert-butyl hydroperoxide in aqueous solution. The half-wave potential (1.34 V) for the 1e- oxidation of (2)Fe(IV)(X)2 to (2.+)Fe(IV)(X)2 is pH independent, due to the equality of the pK(a) values of water ligated to the iron(IV) porphyrin and iron(IV) porphyrin pi-cation radical. The pK(a1) (4.4) of (2)Mn(III)(H2O)2 is smaller than the pK(a1), (5.8) of (1)Mn(III)(H2O)2, and the formal potentials for 1e- oxidation of (2)Mn(III)(X)2 are 1.20 V for (2)Mn(III)(H2O)2 and 1.05 V for (2)Mn(III)(H2O)(OH). Likewise, the pK(a) values (8.0 and 10.4) of (2)Cr(III)H2O)2 are smaller than those (9.4 and 12.4) for (1) Cr(III)(H2O)2, and the formal potentials for 1e- oxidation of (2)Cr(III)(X)2 are 0.71 V for (2)Cr(III)(H2O)2, 0.63 V for (2)Cr(III)(H2O)(OH), and 0.56 V for (2)Cr(III)(OH)2. The differences in pK(a) values and formal potentials between (2)M(II)(X)2 (M = Fe, Mn, or Cr) and (1) M(III)(X)2 are discussed in view of the inductive effects rather than the field effects of phenyl substituents of metallo-5,10,15,20-tetrakis(phenyl)porphyrins on the metalloporphyrin moiety.