Million-fold activation of the [Fe2(µ-O)2] diamond core for C–H bond cleavage

Xue, Genqiang; Hont, Raymond F. De; Münck, Eckard; Que, Lawrence

doi:10.1038/nchem.586

Cited by 206 publications

(255 citation statements)

References 50 publications

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“…Recently, an unmasked HS FeðIVÞ ¼ O unit was identified in a valence-localized open core diiron (FeðIIIÞ∕FeðIVÞ) complex (8). The reaction of the C-H bond cleavage by this complex turned out to be approximately 100 times faster than the corresponding mononuclear IS oxo-iron(IV) complex (8). This experimental finding is in accord with the theoretical prediction that quintet ferryl species are more aggressive oxidants than the corresponding triplet counterparts (9,10,11).…”

supporting

confidence: 79%

“…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 comparable with triplet ferryl analogues (7). Recently, an unmasked HS FeðIVÞ ¼ O unit was identified in a valence-localized open core diiron (FeðIIIÞ∕FeðIVÞ) complex (8). The reaction of the C-H bond cleavage by this complex turned out to be approximately 100 times faster than the corresponding mononuclear IS oxo-iron(IV) complex (8).…”

mentioning

confidence: 99%

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

Neese

2011

Proc. Natl. Acad. Sci. U.S.A.

244

232

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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...

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supporting

confidence: 79%

mentioning

confidence: 99%

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

Neese

2011

Proc. Natl. Acad. Sci. U.S.A.

244

232

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“…This is in agreement with the "diamond core" intermediate QS of methane monooxygenase, which has one short (ϳ1.77 Å) and one long (ϳ2.05 Å) Fe--O bond (42). QS can also be viewed as a headto-tail dimer of Fe IV ϭO units (Fe , for which the terminal oxo group is more reactive than the bridging oxo group (43,44). Therefore, we suggest that the terminal oxo group attacks the C2 atom of the phenyl ring.…”

Section: -O-o-fe2supporting

confidence: 61%

Structure and Mechanism of the Diiron Benzoyl-Coenzyme A Epoxidase BoxB

Rather

Weinert

Demmer

et al. 2011

Journal of Biological Chemistry

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The coenzyme A (CoA)-dependent aerobic benzoate metabolic pathway uses an unprecedented chemical strategy to overcome the high aromatic resonance energy by forming the nonaromatic 2,3-epoxybenzoyl-CoA. The crucial dearomatizing reaction is catalyzed by three enzymes, BoxABC, where BoxA is an NADPH-dependent reductase, BoxB is a benzoyl-CoA 2,3-epoxidase, and BoxC is an epoxide ring hydrolase. We characterized the key enzyme BoxB from Azoarcus evansii by structural and Mössbauer spectroscopic methods as a new member of class I diiron enzymes. Several family members were structurally studied with respect to the diiron center architecture, but no structure of an intact diiron enzyme with its natural substrate has been reported. X-ray structures between 1.9 and 2.5 Å resolution were determined for BoxB in the diferric state and with bound substrate benzoyl-CoA in the reduced state. The substrate-bound reduced state is distinguished from the diferric state by increased iron-ligand distances and the absence of directly bridging groups between them. The position of benzoylCoA inside a 20 Å long channel and the position of the phenyl ring relative to the diiron center are accurately defined. The C2 and C3 atoms of the phenyl ring are closer to one of the irons. Therefore, one oxygen of activated O 2 must be ligated predominantly to this proximate iron to be in a geometrically suitable position to attack the phenyl ring. Consistent with the observed iron/phenyl geometry, BoxB stereoselectively should form the 2S,3R-epoxide. We postulate a reaction cycle that allows a charge delocalization because of the phenyl ring and the electron-withdrawing CoA thioester.The metabolism of aromatic compounds has attracted broad attention due to its importance in the biogeochemical carbon cycle (includes 10 -20% of the biomass) and because of the chemistry necessary to overcome the high resonance energy of the conjugated cyclic ring. A central intermediate of the aromatic metabolism is benzoate, which can be degraded by microorganisms using three different strategies. The first strategy in the presence of oxygen uses mono-and dioxygenases to hydroxylate benzoate to catechol or protocatechuate. Central ring-cleaving dioxygenases with mononuclear iron centers subsequently cleave the aromatic ring either between the two hydroxyl groups (ortho cleavage, ␤-ketoadipate pathway) or next to one of the hydroxyl groups (meta cleavage) (1). The second strategy under anoxic conditions involves benzoylCoA, which is reduced by two electrons to a non-aromatic cyclic diene followed by hydrolytic ring opening. Benzoyl-CoA reduction is accomplished by two completely different enzyme systems that may catalyze a Birch-like reduction (2).The third, semi-aerobic strategy to metabolize benzoate shares characteristic features of both of these options (3). Oxygen is still required for attacking the ring, as in the aerobic metabolism, whereas all metabolites are activated to CoA thioesters and ring cleavage is performed hydrolytically, as in the anaerobic pathwa...

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“…This has been attributed to strong electron donation from the axial thiolate ligand and explains the high reactivity of the natural thiolate-bound P450-I. Another important finding is the recently demonstrated 73,74 increased reactivity of the linear [(O)Fe IV -O-Fe III (OH 2 )] 2 + model complex, as compared with the ring-like [Fe IV 2 (µ-O) 2 ] 2 + core, that provides evidence for a comparable, more ring-opened form of Q with a terminal Fe IV = O unit as the active species in the reactivity of MMO. Additionally, although direct evidence for the involvement of oxoiron(V) intermediates in water oxidation is lacking in the literature, Kundu et al 97 , has recently demonstrated a O-O bond formation reaction mediated by polynuclear oxoiron(IV) intermediates.…”

Section: Conclusion and Future Challengesmentioning

confidence: 88%

“…Xue et al 71 (Fig. 5c) showed that the terminal iron(IV) oxo units are at least three orders of magnitude more reactive than the ones with diamond cores 74 . The most potent C-H activator is the complex with an S = 2 quintet ground state of the Fe = O moiety.…”

Section: Dinuclear Bis(µ-oxo)diiron(iv) Model Complexesmentioning

confidence: 97%

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

2012

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selective functionalization of unactivated c-H bonds and ammonia production are extremely important industrial processes. a range of metalloenyzmes achieve these challenging tasks in biology by activating dioxygen and dinitrogen using cheap and abundant transition metals, such as iron, copper and manganese. High-valent iron-oxo and -nitrido complexes act as active intermediates in many of these processes. the generation of well-described model compounds can provide vital insights into the mechanism of such enzymatic reactions. advances in the chemistry of model high-valent iron-oxo and -nitrido systems can be related to our understanding of the biological systems.H igh-valent oxoiron(IV) and formally oxoiron(V) species have been spectroscopically identified as active intermediates in the catalytic cycles of a number of enzymatic systems 1-10 . Haem and non-haem proteins use these reactive intermediates to couple the activation of dioxygen to the oxidation of their substrates. In most cases, an oxygen atom is inserted into an unactivated C-H bond of the substrate; for example, in hydroxylation reactions [1][2][3][4][5][6][7][8][9][10] . However, many other reactions, including halogenation, desaturation, cyclization, epoxidation and decarboxylation, are also known to involve oxoiron species 1,3 . Superoxidized iron complexes with (valence) isoelectronic imido and nitrido ligands, as well as 'surface nitrides' , have also been implicated as key intermediates in the nitrogen atom transfer reactions 11 , the biological synthesis of ammonia by the nitrogenase enzyme 12-16 and the industrial Haber-Bosch process 17 .The generation of well-described model compounds can provide vital insights into the mechanism of such enzymatic reactions. Consequently, considerable effort has been made by synthetic chemists to prepare viable models for the putative reaction intermediates in the catalytic cycles of O 2 and N 2 activating enzymes. In this review, we provide an overview of all high-valent oxoiron and nitridoiron species that have been either identified or proposed as reactive intermediates in biology. Subsequently, we summarize some of the recent advances in bioinorganic chemistry that have led to the identification and isolation of iron complexes in unusually high formal oxidation states, containing iron-oxygen or iron-nitrogen multiple bonds. The spectroscopic characterization and the reactivity studies of these model complexes provide vital insights into the mechanism that nature uses to induce the reductive cleavage of dioxygen or dinitrogen in carrying out a number of important biochemical oxidative transformations. Moreover, the comparative review of the electronic structures of the isoelectronic oxoiron and nitridoiron functionalities reveals that the Fe-N bonds are intrinsically more covalent than the Fe-O bonds.

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Million-fold activation of the [Fe2(µ-O)2] diamond core for C–H bond cleavage

Cited by 206 publications

References 50 publications

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

Structure and Mechanism of the Diiron Benzoyl-Coenzyme A Epoxidase BoxB

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

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