Electrochemistry and Spectral Characterization of Oxidized and Reduced (TPPBr_x)FeCl Where TPPBr_xIs the Dianion of β-Brominated-Pyrrole Tetraphenylporphyrin andxVaries from 0 to 8

Abstract

The electrochemistry and spectroelectrochemistry of (TPPBr(x)())FeCl (TPPBr(x)() is the dianion of beta-brominated-pyrrole tetraphenylporphyrin and x = 0-8) were examined in PhCN containing tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. Each compound undergoes two reversible to quasireversible one-electron oxidations and either three or four reductions within the potential limits of the solvent. The two oxidations occur at the conjugated porphyrin pi ring system, and DeltaE(1/2) between these two electrode reactions increases as the molecule becomes more distorted. The overall reduction of each compound involves the stepwise electrogeneration omicronf an iron(II), iron(I), and iron(I) pi anion radical. An equilibrium between chloride-bound and chloride-free iron(II) forms of the porphyrin is observed with association of the anionic ligand being favored for compounds with x > 5. Singly reduced (TPPBr(x)())FeCl (x = 0 to x = 6) forms both mono- and bis-CO adducts in CH(2)Cl(2). Only the mono-CO adduct is observed for (TPPBr(7))FeCl, and there is no binding at all of CO to (TPPBr(8))FeCl. The nu(CO) of both the mono- and bis-adducts increases with increase in the number of Br groups, but in a nonlinear fashion which is explained in terms of two competing effects. One is the electron-withdrawing affinity of the Br substitutents and the other the nonplanarity of the macrocycle.

Bromination of porphyrins (Review)

“…These data confirm the result obtained in [32]. With N-bromosuccinimide it is possible to obtain a full range of substituted porphyrins containing various numbers of bromine atoms -from one to eight [59,64,71,91]. Hexabrominated porphyrin was isolated as a side product during the production of the octabrominated porphyrin [56].…”

“…Previously proposed procedures were used for the production of brominated tetraphenylporphyrins with one, two, three, four, six, seven, and eight bromine atoms [64]. For the production of tetraphenylporphyrin containing five bromine atoms (yield 20%) a solution of zinc porphyrinate was boiled for 4 h with 7 equiv.…”

Towards the mechanisms involved in the antioxidant action of MnIII [meso-tetrakis(4-N-methyl pyridinium) porphyrin] in mitochondria

“…The ortho isomer exhibited the most efficient SOD-like activity assigned to an appropriated combination of adequate redox potential and electrostatic facilitation. (Takeuchi et al 1994;Sen and Krishnan 1997;Giraudeau et al 1979;Autret et al 1996;Tagliatesta et al 1996;Binstead et al 1991;Gali et al 1996). However, other studies consider the rate constant for the reduction of Mn , while that for the oxidation of Mn…”

Partially and fully β-brominated Mn-porphyrins in P450 biomimetic systems: Effects of the degree of bromination on electrochemical and catalytic properties

“…The introduction of more than four halogens into these positions on the ring, for example, results in a pronounced decrease in the porphyrin basicity (associated with the electron-withdrawing effect of the b-halogens) and causes severe distortions on the macrocycle (steric effect) [1,[24][25][26][27][28][29][30]. The combined electronic and steric effects decrease the HOMO-LUMO (highest occupied molecular orbitallowest unoccupied molecular orbital) energy gap and consequently alter the reduction potential of the porphyrin [20,22,[28][29][30][31][32][33][34][35][36][37][38].…”