A Synthetic High‐Spin Oxoiron(IV) Complex: Generation, Spectroscopic Characterization, and Reactivity

England, Jason; Martinho, Marlène; Farquhar, Erik R.; Frisch, Jonathan R.; Bominaar, Emile L.; Münck, Eckard; Que, Lawrence

doi:10.1002/ange.200900863

Cited by 64 publications

(52 citation statements)

References 36 publications

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“…Due to the effective C 4v symmetry of the complex, the lowest-energy septet state of the ðFeOÞ 2þ core is 7 (Fig. 2, Left).…”

Section: Resultsmentioning

confidence: 99%

“…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 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).…”

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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.

245

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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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“…Due to the effective C 4v symmetry of the complex, the lowest-energy septet state of the ðFeOÞ 2þ core is 7 (Fig. 2, Left).…”

Section: Resultsmentioning

confidence: 99%

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.

245

232

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“…3). The S = 0 ground state 85,87 59,60 with an {(x 2 − y 2 ) 1 (xy) 1 (xz) 1 (yz) 1 (z 2 ) 0 } electronic configuration (Fig. 3).…”

Section: Iron-nitrido Model Complexesmentioning

confidence: 99%

“…Moreover, direct experimental evidence for the higher reactivity of the S = 2 state is lacking in the literature. Presumably, because the oxoiron(IV) core is protected by the sterically bulky chelator in the recently reported S = 2 [(TMG 3 tren)Fe IV (O)] 2 + complex 59,60,67 , its reactivity towards C-H bond cleavage is only comparable with triplet ferryl analogues. Indirect evidence of the higher reactivity of the quintate state is, however, provided by Seo et al 64 65 .…”

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confidence: 99%

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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“…A series of intermediate-spin S ¼ 1 Fe IV ═O model complexes, supported by amine, pyridine, and deprotonated amido ligands, has become available for structural, reactivity, and spectroscopic investigations (7)(8)(9)(10)(11)(12)(13). Recently, high-spin S ¼ 2 Fe IV ═O complexes have also become available, with two being structurally defined (14)(15)(16)(17)(18)(19)(20), but notably their reactivity does not surpass that of the most reactive S ¼ 1 complexes (15,18,21).…”

Section: S ¼ 2 Fementioning

confidence: 99%

π-Frontier molecular orbitals in S = 2 ferryl species and elucidation of their contributions to reactivity

Srnec

Wong

England

et al. 2012

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

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155

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S ¼ 2 FeIV ═O species are key intermediates in the catalysis of most nonheme iron enzymes. This article presents detailed spectroscopic and high-level computational studies on a structurallydefined S ¼ 2 Fe IV ═O species that define its frontier molecular orbitals, which allow its high reactivity. Importantly, there are both π-and σ-channels for reaction, and both are highly reactive because they develop dominant oxyl character at the transition state. These π-and σ-channels have different orientation dependences defining how the same substrate can undergo different reactions (H-atom abstraction vs. electrophilic aromatic attack) with Fe IV ═O sites in different enzymes, and how different substrates can undergo different reactions (hydroxylation vs. halogenation) with an Fe IV ═O species in the same enzyme.excited-state potential energy surfaces | reaction coordinates | magnetic circular dichroism | density functional calculations | multiconfigurational calculations

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A Synthetic High‐Spin Oxoiron(IV) Complex: Generation, Spectroscopic Characterization, and Reactivity

Cited by 64 publications

References 36 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

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

π-Frontier molecular orbitals in S = 2 ferryl species and elucidation of their contributions to reactivity

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