Nonheme oxo-iron(IV) intermediates form an oxyl radical upon approaching the C–H bond activation transition state

Abstract

Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C-H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C-H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe-oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C-H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the ðFeOÞ 2þ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the "σ-pathway" that requires an essentially linear attack; by contrast a "π-channel" is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes. density functional calculation | nonheme iron | reaction mechanism O xo-iron(IV) intermediates have attracted much interest in bioinorganic chemistry because they are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize unactivated C-H bonds (1). Detailed experimental and theoretical studies on the hydroxylation of saturated hydrocarbons by ferryl species in heme systems, foremost cytochrome P450, have been performed (2). On the other hand a number of nonheme enzymes are able to activate molecular dioxygen to modify alkane or arene substrates as well. So far, nonheme ferryl species have been spectroscopically characterized in four mononuclear iron enzymes and were found to feature high-spin (HS) (S ¼ 2) electronic ground state configurations (3). In parallel, a wide range of synthetic FeðIVÞ¼O complexes were synthesized and characterized (4). In almost all cases they contain intermediate-spin (IS) (S ¼ 1) rather than HS ferryl centers (4). The only exceptions are ½FeðIVÞðOÞðH 2 OÞ 5 2þ (5) and the recently reported model complex ½FeðIVÞðOÞðTMG 3 trenÞ 2þ (TMG 3 tren ¼ N½CH 2 CH 2 N ¼ CðNMe 2 Þ 2 ) (6). Presumably because the ðFeOÞ 2þ core is sheltered by the sterically bulky supporting ligand in ½FeðIVÞðOÞ ðTMG 3 trenÞ 2þ , its reactivity toward C-H bond cleavage is only comparabl...

High-spin Mn–oxo complexes and their relevance to the oxygen-evolving complex within photosystem II

“…5). Similar stretch-induced oxyl formation has also been observed in calculations of Fe IV = O systems and stems from the propensity of a free O 2− ion to lower its energy by ejecting an electron (33,34).…”

The biology and chemistry of high-valent iron–oxo and iron–nitrido complexes

“…The reactivity of non-haem oxoiron(IV) complexes in C-H hydroxylation and oxo-transfer reactions has been considered in depth by theoretical [61][62][63] and experimental methods 44,64,65 . So far, all theoretical studies have led to the common conclusion that the ferryl species are better oxidants on the quintet-state than the corresponding triplet-state.…”

cPCET versus HAT: A Direct Theoretical Method for Distinguishing X–H Bond‐Activation Mechanisms

“…For the HAT reaction of the high‐valent oxoiron(IV) intermediate TauD‐J, we selected a model system studied previously by Ye and Neese 18. This consists of a high‐spin ( S =2) Fe IV =O unit ligated by two imidazoles and one acetate mimicking a 2‐His‐1‐carboxylate facial triad19 and one additional acetate mimicking the coordination of decarboxylated α‐ketoglutarate.…”