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

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.