Electron-transfer kinetics for generation of organoiron(IV) porphyrins and the iron(IV) porphyrin π radical cations

Shunichi Fukuzumi, Ikuo Nakanishi, Keiko Tanaka, Tomoyoshi Suenobu, Alain Tabard, Roger Guilard, Eric Van Caemelbecke, Karl M. Kadish

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Homogeneous electron-transfer kinetics for the oxidation of seven different iron(III) porphyrins using three different oxidants were examined in deaerated acetonitrile, and the resulting data were evaluated in light of the Marcus theory of electron transfer to determine reorganization energies of the rate-determining oxidation of iron(III) to iron(IV). The investigated compounds are represented as (P)Fe(R), where P = the dianion of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP) and R = C6H5, 3,5-C6F2H3, 2,4,6-C6F3H2, or C6F5 or P = the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and R = C6H5, 2,4,6-C6F3H2, or 2,3,5,6-C6F4H. The first one-electron transfer from (P)Fe(R) to [Ru(bpy)3]3+ (bpy = 2,2'-bipyridine) leads to an Fe(IV) σ-bonded complex, [(P)Fe(IV)(R)]+, and occurs at a rate which is much slower than the second one-electron transfer from [(P)Fe(IV)(R)]+ tO [Ru(bpy)3]3+ to give [(P)Fe(IV)(R)]·2+. The one- or two-electron oxidation of each (OETPP)Fe(R) or (OEP)Fe(R) derivative was also attained by using [Fe(phen)3]3+ (phen = 1,10-phenanthroline) or [Fe(4,7-Me2phen)3]3+ (Me2phen = 4,7-dimethyl- 1,10-phenanthroline) as an electron-transfer oxidant. The reorganization energies (kcal mol-1) for the metal-centered oxidation of (P)Fe(III)(R) to [CP)Fe(IV)(R)]+ increase in the order (OEP)Fe(R) (83 ± 4) << (OETPP)Fe(C6F5) (99 ± 2) < (OETPP)Fe(2,4,6-C6F3H2) (107 ± 2) < (OETPP)Fe(3,5-C6F2H3) (109 ± 3) < (OETPP)Fe(C6H5) (113 ± 3). Each value is significantly larger than the reorganization energies determined for the porphyrin-centered oxidations involving the same two series of compounds, i.e., the second electron transfer of (P)Fe(R). In each case, the first metal-centered oxidation is the rate-determining step for generation of the iron(IV) porphyrin π radical cation. Coordination of pyridine to (OETPP)Fe(C6F5) as a sixth axial ligand enhances significantly the rate of electron-transfer oxidation.

Original languageEnglish
Pages (from-to)785-790
Number of pages6
JournalJournal of the American Chemical Society
Issue number4
StatePublished - 3 Feb 1999


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