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Nonheme Oxo-Iron(IV) Intermediates Form an Oxyl Radical Upon Approaching the C–H Bond Activation Transition State

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Nonheme Oxo-Iron(IV) Intermediates Form an Oxyl Radical Upon Approaching the C–H Bond Activation Transition State Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state Shengfa Ye and Frank Neese1 Lehrstuhl für Theoretische Chemie, Universität Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany Edited* by Brian M. Hoffman, Northwestern University, Evanston, IL, and approved December 5, 2010 (received for review June 29, 2010) Oxo-iron(IV) species are implicated as key intermediates in the mately 100 times faster than the corresponding mononuclear IS catalytic cycles of heme and nonheme oxygen activating iron oxo-iron(IV) complex (8). This experimental finding is in accord enzymes that selectively functionalize aliphatic C–H bonds. Ferryl with the theoretical prediction that quintet ferryl species are complexes can exist in either quintet or triplet ground states. more aggressive oxidants than the corresponding triplet counter- Density functional theory calculations predict that the quintet parts (9, 10, 11). However, there is a long-term debate on how to oxo-iron(IV) species is more reactive toward C–H bond activation rationalize the differential reactivity between quintet and triplet than its corresponding triplet partner, however; the available oxo-iron(IV) species. De Visser, Shaik, and coworkers argued experimental data on model complexes suggests that both spin that the enhanced exchange interaction upon approaching the multiplicities display comparable reactivities. To clarify this ambigu- transition state (TS) on the quintet surface flattens the potential ity, a detailed electronic structure analysis of alkane hydroxylation energy surface (PES) and hence lowers the barrier (12, 13). Baer- by an oxo-iron(IV) species on different spin-state potential energy ends, Solomon, and coworkers proposed that the greater degree surfaces is performed. According to our results, the lengthening of exchange stabilization in the HS ferryl reactant relative to the of the Fe–oxo bond in ferryl reactants, which is the part of the IS analogues significantly stabilizes the Fe-dz2 based σ-antibond- reaction coordinate for H-atom abstraction, leads to the formation ing molecular orbital (MO) (14–18). Therefore, this orbital is of oxyl-iron(III) species that then perform actual C–H bond activa- able to serve as the electron acceptor on the quintet surface, tion. The differential reactivity stems from the fact that the two whereas the energy of the corresponding MO in the triplet reac- spin states have different requirements for the optimal angle at tant is too high to be an electron acceptor. Thus, the lower energy þ which the substrate should approach the ðFeOÞ2 core because π-antibonding MOs that mainly consist of the Fe-dxz∕yz and the distinct electron acceptor orbitals are employed on the two sur- O-px∕y fragment orbitals act as electron acceptors on the triplet faces. The H-atom abstraction on the quintet surface favors the surface (17). As a consequence, a σ-pathway is often suggested to “σ-pathway” that requires an essentially linear attack; by contrast be operative on the quintet surface, whereas the triplet reaction a “π-channel” is operative on the triplet surface that leads to an proceeds via a π-channel (18). However, under certain circum- ideal attack angle near 90°. However, the latter is not possible stances quintet π- and triplet σ-pathways were also reported in due to steric crowding; thus, the attenuated orbital interaction the process of C–H bond activation (19, 20). Recent studies and the unavoidably increased Pauli repulsion result in the lower on the reaction of alkane hydroxylation by ferryl model com- reactivity of the triplet oxo-iron(IV) complexes. pounds demonstrated that the reaction can take place through both the σ- and the π-pathways on the quintet and triplet surfaces 5 5 3 3 density functional calculation ∣ nonheme iron ∣ reaction mechanism with barrier heights displaying the order σ > π ≈ π > σ (21). The results indicated that quintet oxo-iron(IV) species are not – xo-iron(IV) intermediates have attracted much interest in always more reactive toward C H bond cleavage than the corre- bioinorganic chemistry because they are implicated as key sponding triplet species and confirmed that of all possible path- O σ intermediates in the catalytic cycles of heme and nonheme oxy- ways the quintet -pathway is the most effective one. More gen activating iron enzymes that selectively functionalize unacti- importantly, a careful analysis of the electronic structure changes vated C–H bonds (1). Detailed experimental and theoretical along the quintet pathway revealed that the reaction requires a studies on the hydroxylation of saturated hydrocarbons by ferryl preparatory stage in which an oxyl-iron(III) species is formed en species in heme systems, foremost cytochrome P450, have been route to the TS that then functions as the active species in the – performed (2). On the other hand a number of nonheme enzymes actual C H bond activation process (22). Thus far all interpreta- – are able to activate molecular dioxygen to modify alkane or tions about which spin multiplicity is more reactive toward C H “ ” arene substrates as well. So far, nonheme ferryl species have bond activation have been based on the notion that the real been spectroscopically characterized in four mononuclear iron oxidant is oxo-iron(IV) rather than oxyl-iron(III). Therefore, enzymes and were found to feature high-spin (HS) (S ¼ 2) elec- to address this question one needs to understand how the real tronic ground state configurations (3). In parallel, a wide range of oxyl-ferric reactant is formed and how it then reacts with the sub- synthetic FeðIVÞ¼O complexes were synthesized and character- strate on the quintet and triplet surfaces, respectively. ized (4). In almost all cases they contain intermediate-spin (IS) In the present contribution we address the above formulated (S ¼ 1) rather than HS ferryl centers (4). The only exceptions are problem through an analysis of the electronic structure changes 2þ that occur upon alkane C–H bond hydroxylation by the taurine: ½FeðIVÞðOÞðH2OÞ5 (5) and the recently reported model com- 2þ plex ½FeðIVÞðOÞðTMG3trenÞ (TMG3tren ¼ N½CH2CH2N ¼ 2þ CðNMe2Þ2) (6). Presumably because the ðFeOÞ core is shel- Author contributions: S.Y. and F.N. designed research; S.Y. performed research; S.Y. and tered by the sterically bulky supporting ligand in ½FeðIVÞðOÞ F.N. analyzed data; and S.Y. and F.N. wrote the paper. 2þ ðTMG3trenÞ , its reactivity toward C–H bond cleavage is only The authors declare no conflict of interest. comparable with triplet ferryl analogues (7). Recently, an un- *This Direct Submission article had a prearranged editor. masked HS FeðIVÞ¼O unit was identified in a valence-localized 1To whom correspondence should be addressed. E-mail: [email protected]. ð Þ∕ ð Þ open core diiron (Fe III Fe IV ) complex (8). The reaction of This article contains supporting information online at www.pnas.org/lookup/suppl/ the C–H bond cleavage by this complex turned out to be approxi- doi:10.1073/pnas.1008411108/-/DCSupplemental. 1228–1233 ∣ PNAS ∣ January 25, 2011 ∣ vol. 108 ∣ no. 4 www.pnas.org/cgi/doi/10.1073/pnas.1008411108 Downloaded by guest on September 26, 2021 α-ketoglutarate dioxygenase (TauD) oxo-iron(IV) intermediate Table 1. The relative enthalpy, entropy, and free energy for (23) along the septet (hypothetical), quintet, and triplet reaction the key local minima and transition states on the septet, channels. This allows us to gain electronic structure insight into quintet, and triplet surface the differential reactivity of the distinct spin states and draw con- ΔH (kcal∕mol) −TΔS kcal∕mol) ΔG (kcal∕mol) clusions that might have a wider range of applicability. 5R0 0 0 Results 5TS1 14.0 10.6 24.6 5 The energy profile for H-atom abstraction by the TauD oxo-iron IN 0.9 9.0 9.8 7R16.3 −1.3 15.1 (IV) intermediate is shown in Fig. 1. The reaction follows the 7 – TS1 19.1 10.0 29.0 consensus mechanism for the activation of aliphatic C H bonds 7IN 2.7 7.8 10.5 by oxo-iron(IV) compounds (24).The reaction is initiated by the 3R 6.8 2.4 9.2 rate-determining H-atom abstraction reaction via TS1 to yield a 3TS1 28.5 13.7 42.2 hydroxo-iron(III) complex with a nearby alkyl radical intermedi- 3IN 14.1 8.9 23.0 ate (IN). This step is followed by an OH-rebound process leading to the final product ethanol. Interestingly, for H-atom transfer 7 1 the hypothetical septet reaction pathway is calculated to show the The geometry of TS exhibits a slight contraction of the – 7 1 lowest barrier (Fig. 1 and Table 1). However, because the singlet Fe Ooxo bond relative to the reactant (1.89 Å in TS (Table 2) 7 1 product ethanol cannot be formed on the septet surface, the vs. 1.91 Å in the reactant). The electronic structure of TS is best 7 S ¼ 5∕2 reaction on the septet surface stops at intermediate IN. In interpreted as featuring a HS ferric ion ( Fe ) interacting – σ S ¼ 1∕2 addition, the reaction cannot start on the septet surface, because with a three-center C H-O -radical ( CHO ) in a ferromag- the septet reactant is much higher in energy than the quintet oxo- netic fashion (Fig. 2, Right). In comparison to the electronic struc- iron(IV) reactant. In fact, a low-lying septet state is only possible ture of the reactant, one may envision that during the approach of 7ð Þ2þ for a mononuclear 3d transition metal center if at least one ligand the substrate toward the FeO core, the singly occupied O-px – σ carries an unpaired electron. orbital interacts with the C H -bonding orbital in the substrate σ σà to form a pair of three-centered MOs ( CHO and CHO).
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