Axial Ligand and Spin‐State Influence on the Formation and Reactivity of Hydroperoxo–Iron(III) Porphyrin Complexes

Abstract

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.

Transition Metal Complexes and the Activation of Dioxygen

“…C [62]. In contrast to their meso-substituted counterparts, β-substituted Fe(IV)-oxo π* cation-radicals (24a-e) ( Figure 9) have been characterized as a 1u porphyrin radicals weakly coupled with the Fe(IV) triplet state [65][66][67].…”

“…Invoking a thermodynamic argument, the authors reasoned that 22a, being in the high-spin state, favors the homolytic pathway which proceeds via a high-spin transition state to yield a high-spin Fe(IV)-oxo product, minimizing the energetic cost associated with changes in spin. Similarly, 22b is low-spin and favors the heterolytic pathway yielding an Fe(IV)-oxo π*-cation radical species as the final product [62]. …”

Mechanistic studies on versatile metal-assisted hydrogen peroxide activation processes for biomedical and environmental incentives

“…Moreover, our studies indicated an accelerating effect of the N-methylimidazole axial ligand in a mono-substituted iron(III) porphyrin on the formation of the Compound II model [24]. The influence of an axial ligand on O-O bond scission was also discussed by Franke et al [141]. The observed change in the O-O bond scission mechanism from homolytic for (TPFPP)Fe III (OOH) to heterolytic for (TPFPP)Fe III (OH)(OOH) was ascribed to the significant role of the axial ligand as well as the spin state of the iron(III) center [141].…”

“…Since H2O2 is a very poor ligand, it was proposed that its coordination had to be preceded by its deprotonation, which depending on the reaction conditions and solvent basicity, may occur via a proton transfer either toward an OH − ligand coordinated to the iron(III) center or proton acceptors in solution [24,141].…”

Biomimetic Conversion of Glucose to Organic Acid Facilitated by Metalloporphyrin under Mild Conditions

“…Under alkaline conditions, the iron(III)-peroxo species, acknowledged as nucleophilic oxidant intermediates, are subjected to production by the reaction of H2O2 with iron porphyrins and non-heme iron complexes (Cho et al 2011;Franke et al 2012). Thus, it is reasonable to assume that the metalloporphyrins in glucose oxidation might be responsible for the formation of the metalo-peroxo species, (TSPPMO2 -), by the addition of H2O2 generated in situ.…”