Reactivity of Compound II: Electronic Structure Analysis of Methane Hydroxylation by Oxoiron(IV) Porphyrin Complexes Angela Rosa* and Giampaolo Ricciardi* Dipartimento di Chimica, Universita ̀ della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza, Italy ABSTRACT: The methane hydroxylation reaction by a Compound II (Cpd II) mimic PorFe IV =O and its hydrosulfide-ligated derivative [Por(SH)Fe IV =O] - is investigated by density functional theory (DFT) calculations on the ground triplet and excited quintet spin-state surfaces. On each spin surface both the σ- and π-channels are explored. H-abstrac- tion is invariably the rate-determining step. In the case of PorFe IV =O the H-abstraction reaction can proceed either through the classic π-channel or through the nonclassical σ-channel on the triplet surface, but only through the classic σ-mechanism on the quintet surface. The barrier on the quintet σ-pathway is much lower than on the triplet channels so the quintet surface cuts through the triplet surfaces and a two state reactivity (TSR) mechanism with crossover from the triplet to the quintet surface becomes a plausible scenario for C-H bond activation by PorFe IV =O. In the case of the hydrosulfide-ligated complex the H-abstraction follows a π-mechanism on the triplet surface: the σ* is too high in energy to make a σ-attack of the substrate favorable. The σ- and π-channels are both feasible on the quintet surface. As the quintet surface lies above the triplet surface in the entrance channel of the oxidative process and is highly destabilized on both the σ- and π-pathways, the reaction can only proceed on the triplet surface. Insights into the electron transfer process accompanying the H-abstraction reaction are achieved through a detailed electronic structure analysis of the transition state species and the reactant complexes en route to the transition state. It is found that the electron transfer from the substrate σ CH into the acceptor orbital of the catalyst, the Fe-O σ* or π*, occurs through a rather complex mechanism that is initiated by a two-orbital four-electron interaction between the σ CH and the low-lying, oxygen-rich Fe-O σ-bonding and/or Fe-O π-bonding orbitals of the catalyst. ■ INTRODUCTION Oxoiron(IV) species are invoked as key oxidizing intermediates in the catalytic cycles of heme 1 and non heme enzymes. 1e,2 In heme iron enzymes, oxoiron(IV) porphyrin π-cation radical (Compound I, Cpd I) and oxoiron(IV) porphyrin (Compound II, Cpd II) species are proposed as reactive intermediates in dioxygen activation and oxygen-atom transfer reactions. 1 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 oxida- tion reactions, such as epoxidations of olefins and hydroxylations of hydrocarbons. 1a,3 The mechanisms of these reactions have been studied experimentally 1e,3c,4 and theoretically. 5 Dissimilar from Cpd I mimics, oxoiron(IV) porphyrins, Cpd II mimics, have been considered to be very poor oxidants 6 and hence have received less attention. However, several instances of exper- imental evidence have accumulated proving that oxoiron(IV) porphyrin complexes are competent oxidants of a variety of sub- strates. Groves and co-workers first reported that an oxoiron(IV) porphyrin complex, (TMP)Fe IV O (TMP = tetramesitylpor- phyrinate) is able to oxidize olefins. 7 Several years later, Nam and co-workers reported that oxoiron(IV) porphyrins are also able to activate C-H bonds of alkanes, thereby yielding alcohol products. 6a These authors demonstrated that an oxoiron(IV) por- phyrin complex bearing an electron-deficient porphyrin ligand, (TPFPP)Fe IV O (TPFPP = meso-tetrakis(pentafluorophenyl)- porphyrinate), is able to conduct two-electron oxidations of olefins to epoxides and of alkanes to alcohols, with high stereoselectivity and reactivities similar to those found for Cpd I mimics. More recently, experimental studies by Nam, Fukuzumi, and co-workers have shown that oxoiron(IV) porphyrin complexes bearing electron-deficient porphyrin ligands are the active oxidants in the oxidations of alkyl aromatics. 8 In contrast with this experimental evidence, but in line with the general credence that the hydrogen-atom abstraction ability of Cpd II is much weaker than that of Cpd I, 6 a quantum mechanics/molecular mechanics (QM/MM) study on the relative hydrogen-abstraction capabilities of Cpd I and Cpd II in a P450 cam model predicted sluggish oxidative properties for the latter, with H-abstraction barriers of about 5 kcal/mol higher than those computed for Cpd I. 5d A density functional theory (DFT) study by de Visser, Nam and co-workers 5c on the AcrH 2 (AcrH 2 = 10-methyl-9,10-dihydro acridine) hydroxylation by a Cpd I mimic, [(Por +• )(Cl)Fe IV O], and its one-electron reduced form, [(Por)(Cl)Fe IV O] - , came to similar con- clusions. Both these complexes proved to be plausible oxidants with the former showing lower (ca. 4 kcal/mol) H-abstraction barrier than the latter. It was also found that Cpd I, but not Cpd II, is able to react via hydride transfer, a route that proved to be Received: June 9, 2012 Published: September 4, 2012 Article pubs.acs.org/IC © 2012 American Chemical Society 9833 dx.doi.org/10.1021/ic301232r | Inorg. Chem. 2012, 51, 9833-9845