Electron-Transfer Kinetics for Generation of Organoiron(IV) Porphyrins and the Iron(IV) Porphyrin π Radical Cations

Abstract

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)FeIV(R)]+, and occurs at a rate which is much slower than the second one-electron transfer from [(P)FeIV(R)]+ to [Ru(bpy)3]3+ to give [(P)FeIV(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)FeIII(R) to [(P)FeIV(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.

Contrasting Effects of Axial Ligands on Electron‐Transfer Versus Proton‐Coupled Electron‐Transfer Reactions of Nonheme Oxoiron(IV) Complexes

“…Such a decrease in the reorganization energy in electron-transfer reactions has previously been reported in the electron-transfer oxidation of ironA C H T U N G T R E N N U N G (III) porphyrins in the presence of a strong axial ligand, which causes significant acceleration of the rate of the electron-transfer oxidation. [27] In the case of the electron-transfer reduction of the oxoiron(IV) complexes, the strong binding of the axial ligand makes the electron transfer more difficult thermodynamically due to the negative shift of the one-electron reduction potential, but the intrinsic kinetic barrier of electron transfer is lowered by the decrease in the reorganization energy of electron transfer. …”

Base Control of Electron-Transfer Reactions of Manganese(III) Porphyrins

“…[1,2,24,25] The addition of pyridine to an MeCN solution of (DPPH 20 )MnCl results in a significant change in the UV/ vis spectrum, as shown in Figure 3.…”

“…[1] In this context, we have recently reported that the rates of metal-centered electron-transfer oxidation of (OETPP)Fe III (R) (OETPP ϭ the dianion of octaethyltetraphenylporphyrin and R ϭ σ-bonded axial ligand such as Ph), which has a saddle-shaped nonplanar porphyrin macrocycle, are much slower than those of (OEP)Fe III (R) rate constants derived from the electron-transfer rate constants decrease with an increasing degree of nonplanarity of the porphyrin macrocycle and follow the order: (TPP)MnCl (OEP ϭ the dianion of octaethylporphyrin), which has a planar porphyrin macrocycle, and this can be associated with the large reorganization energies upon electron-transfer oxidation of the metal center of the nonplanar porphyrins. [2] It has also been shown that the slow electron transfer of a nonplanar iron(III) porphyrin is significantly accelerated by axial coordination of pyridine due to the decreased reorganization energy upon electron-transfer oxidation. [2] Such nonplanar conformations of porphyrins have been suggested as being related to their functions in biological systems.…”

“…[2] It has also been shown that the slow electron transfer of a nonplanar iron(III) porphyrin is significantly accelerated by axial coordination of pyridine due to the decreased reorganization energy upon electron-transfer oxidation. [2] Such nonplanar conformations of porphyrins have been suggested as being related to their functions in biological systems. [3Ϫ8] We report herein the effects of conformational distortions of the porphyrin ring on the electron-transfer kinetics for reduction of (P)MnCl, where P represents either the dianion of tetraphenylporphyrin (TPP), which is planar, or the dianion of dodecaphenylporphyrins (DPPX, X ϭ H 20 , Cl 12 H 8 , F 20 ) which are known to adopt a nonplanar conformation.…”

A paradigm shift in rate determining step from single electron transfer between phenylsulfinylacetic acids and iron(III) polypyridyl complexes to nucleophilic attack of water to the produced sulfoxide radical cation: a non‐linear Hammett

“…[6] For clear understanding of these ET reactions, porphyrin and polypyridyl complexes of Fe(III) have been synthesized as model compounds and used as electron acceptors. [9][10][11] Recently iron polypyridyl complexes have been identified as potential anticancer drug by inhibiting the cancer cell proliferation. [12] The iron(III) polypyridyl complexes are well-known one electron acceptors, [13] and several interactions of these complexes with inorganic and organic reductants have been reported.…”