Electronic Structure Analysis of Methane Hydroxylation by Electron-deficient oxoiron(IV) porphyrins

In heme iron enzymes, oxoiron(IV) porphyrin π-cation radical (Cpd I) and oxoiron(IV) porphyrin (Cpd II) species are proposed as reactive intermediates in dioxygen activation and oxygen-atom transfer reactions. Because of its biological significance, the reactivity of Cpd I has been widely investigated with in situ-generated oxoiron(IV) porphyrin π-cation radical complexes in various types of oxidation reactions. Dissimilar from Cpd I mimics, Cpd II mimics have been considered to be very poor oxidants and hence have received less attention. Several experimental evidences have now accumulated proving that oxoiron(IV) porphyrins are competent oxidants of a variety of substrates. The relationship between the nature of the porphyrin ring and the reactivity of Cpd II mimics is, however, still unclear. As a matter of fact, the experimental data reported for (TPFPP)Fe(IV)=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphyrinate) seem to indicate that electron-deficient porphyrin rings greatly enhance the reactivity of Cpd II mimics.1 On the other hand, a Cpd II mimic also bearing an electron-deficient porphyrin ligand, [(4-TMPyP)Fe(IV)=O]4+ (4-TMPyP = 5,10,15,20-tetrakis(N-methyl-4-pyridinium) porphyrinate), has been recently found to be almost unreactive toward C–H hydroxylation.2 With the aim to understand why synthetic Cpd II mimics have such a great variability in their reactivity, we have theoretically investigated by DFT (Density Functional Theory) methods the methane hydroxylation reaction by the electron-deficient Cpd II mimics above mentioned, (TPFPP)Fe(IV)=O and [(4-TMPyP)Fe(IV)=O]4+. The hydroxylation reaction has been studied on the ground triplet and excited quintet spin-state surfaces within the rebound mechanism scheme. On each spin surface both the σ- and π-channels have been explored. It is found that crossover from the ground state triplet surface to the highly reactive quintet state surface is a plausible scenario for C–H bond activation by these complexes. The efficiency of this TSR (two state reactivity) mechanism depends on the nature of the macrocycle, the presence and nature of an axial ligand and the solvent. A detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state reveals that, just as found in the unsubstituted PorFeIV=O Cpd II model,3 the electron transfer from the substrate σCH into the acceptor orbital of the catalyst, the Fe–O σ* or π*, occurs through a complex mechanism that is initiated by a two-orbital four-electron interaction between the σCH and the low-lying, oxygen-rich Fe–O σ-bonding and/or Fe–O π-bonding orbitals of the catalyst. REFERENCES 1. Fukuzumi, S.; Kotani, H.; Lee, Y.-M.; Nam, W. J. Am. Chem. Soc. 2008, 130, 15134–15142. 2. Bell, S. R.; Groves, J. T. J. Am. Chem. Soc. 2009, 131, 9640–9641. 3. Rosa, A.; Ricciardi, G. Inorg. Chem. 2012, 51, 9833–9845.