Studies of poly(3,4-ethylenedioxythiophene) (PEDOT) films containing cationic Mn porphyrins. A loading-dependent demetalation of Mn(III)TPP in PEDOT (Mn(III)TPP=5,10,15,20-tetraphenylporphyrinato manganese(III))

Thin films of vapor-phase polymerized PEDOT incorporating various cationic Mn porphyrins were assessed for water oxidation catalysis under light illumination. Only Mn(III)TPP/PEDOT displayed a notable photocurrent and this was, counter-intuitively, greatest at the lowest loading levels examined. Studies revealed that a proportion of the Mn(III)TPP within the PEDOT became demetalated during polymerization, leaving free and protonated TPP. Despite the presence of an excess of chemical oxidant during the polymerization step, the Mn(III) ion was reduced — likely under the influence of light — to Mn(II), which was labilized out of the film. Whereas PEDOT films loaded with anionic Mn porphyrins may be active and selective water oxidation photocatalysts, their analogs containing cationic Mn porphyrins, like Mn(III)TPP, are catalytically inert.

Differential coordination demands in Fe versus Mn water-soluble cationic metalloporphyrins translate into remarkably different aqueous redox chemistry and biology

The different biological behavior of cationic Fe and Mn pyridylporphyrins in Escherichia coli and mouse studies prompted us to revisit and compare their chemistry. For that purpose, the series of ortho and meta isomers of Fe(III) meso-tetrakis-N-alkylpyridylporphyrins, alkyl being methyl to n-octyl, were synthesized and characterized by elemental analysis, UV/vis spectroscopy, mass spectrometry, lipophilicity, protonation equilibria of axial waters, metal-centered reduction potential, E(1/2) for M(III)P/M(II)P redox couple (M = Fe, Mn, P = porphyrin), kcat for the catalysis of O2(•-) dismutation, stability toward peroxide-driven porphyrin oxidative degradation (produced in the catalysis of ascorbate oxidation by MP), ability to affect growth of SOD-deficient E. coli, and toxicity to mice. Electron-deficiency of the metal site is modulated by the porphyrin ligand, which renders Fe(III) porphyrins ≥5 orders of magnitude more acidic than the analogous Mn(III) porphyrins, as revealed by the pKa1 of axially coordinated waters. The 5 log units difference in the acidity between the Mn and Fe sites in porphyrin translates into the predominance of tetracationic (OH)(H2O)FeP complexes relative to pentacationic (H2O)2MnP species at pH ∼7.8. This is additionally evidenced in large differences in the E(1/2) values of M(III)P/M(II)P redox couples. The presence of hydroxo ligand labilizes trans-axial water which results in higher reactivity of Fe relative to Mn center. The differences in the catalysis of O2(•-) dismutation (log kcat) between Fe and Mn porphyrins is modest, 2.5-5-fold, due to predominantly outer-sphere, with partial inner-sphere character of two reaction steps. However, the rate constant for the inner-sphere H2O2-based porphyrin oxidative degradation is 18-fold larger for (OH)(H2O)FeP than for (H2O)2MnP. The in vivo consequences of the differences between the Fe and Mn porphyrins were best demonstrated in SOD-deficient E. coli growth. On the basis of fairly similar log kcat(O2(•-)) values, a very similar effect on the growth of SOD-deficient E. coli was anticipated by both metalloporphyrins. Yet, while (H2O)2MnTE-2-PyP(5+) was fully efficacious at ≥20 μM, the Fe analogue (OH)(H2O)FeTE-2-PyP(4+) supported SOD-deficient E. coli growth at as much as 200-fold lower doses in the range of 0.1-1 μM. Moreover the pattern of SOD-deficient E. coli growth was different with Mn and Fe porphyrins. Such results suggested a different mode of action of these metalloporphyrins. Further exploration demonstrated that (1) 0.1 μM (OH)(H2O)FeTE-2-PyP(4+) provided similar growth stimulation as the 0.1 μM Fe salt, while the 20 μM Mn salt provides no protection to E. coli; and (2) 1 μM Fe porphyrin is fully degraded by 12 h in E. coli cytosol and growth medium, while Mn porphyrin is not. Stimulation of the aerobic growth of SOD-deficient E. coli by the Fe porphyrin is therefore due to iron acquisition. Our data suggest that in vivo, redox-driven degradation of Fe porphyrins resulting in Fe release plays a major role in their biological action. Possibly, iron reconstitutes enzymes bearing [4Fe-4S] clusters as active sites. Under the same experimental conditions, (OH)(H2O)FePs do not cause mouse arterial hypotension, whereas (H2O)2MnPs do, which greatly limits the application of Mn porphyrins in vivo.

Solvent, anion, and structural effects on the redox potentials and UV-visible spectral properties of mononuclear manganese corroles

A series of manganese(III) corroles were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous solvents. Up to three oxidations and one reduction were obtained for each complex depending on the solvents. The main compound discussed in this paper is the meso-substituted manganese corrole, (Mes 2PhCor)Mn, and the main points are how changes in axially coordinated anion and solvent will affect the redox potentials and UV-vis spectra of each electrogenerated species in oxidation states of Mn(III), Mn(IV), or Mn(II). The anions OAc (-), Cl (-), CN (-), and SCN (-) were found to form five-coordinate complexes with the neutral Mn(III) corrole while two OH (-) or F (-) anions were shown to bind axially in a stepwise addition to give the five- and six-coordinate complexes in nonaqueous media. In each case, complexation with one or two anionic axial ligands led to an easier oxidation and a harder reduction as compared to the uncomplexed four-coordinate species.