Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = 1/2 FeIII−O−FeIV═O Complexes. Superexchange-Mediated Spin Transition at the FeIV═O Site

Hont, Raymond F. De; Xue, Genqiang; Hendrich, Michael P.; Que, Lawrence; Bominaar, Emile L.; Münck, Eckard

doi:10.1021/ic100870v

Cited by 25 publications

(72 citation statements)

References 57 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…37 The EPR spectrum of this solution shows an isotropic S = ½ signal with g = 2.00 and exhibits 57 Fe hyperfine broadening (Figure 8, inset), properties like those of 2 -OH. 37,54 The striking spectroscopic similarity strongly suggests that coordination of methoxide to 2 generates a complex with a [CH 3 O–Fe III –O–Fe IV =O] core structure ( 2 -OCH 3 ). The spectroscopic similarities also suggest that the terminal oxoiron(IV) moiety of 2 -OCH 3 is high-spin ( S = 2), which should be a potent oxidant for C–H bond cleavage.…”

Section: Resultsmentioning

confidence: 98%

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

2011

Self Cite

View full text Add to dashboard Cite

An [FeIV2(μ-O)2] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C–H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Due to the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe2(μ-O)2] cores. We report here that water or alcohol substrates can activate synthetic [FeIIIFeIV(μ-O)2] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C–H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [FeIIIFeIV(μ-O)2] core, resulting in the formation of a more reactive species with a [X–FeIII–O–FeIV=O] ring-opened structure (1-X, 2-X, X = OH− or OR−). Treatment of 2 with methoxide at −80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 FeIII/S = 2 FeIV] pair. Even at this low temperature, the complex undergoes facile intramolecular C–H bond cleavage to generate formaldehyde, showing that the terminal high-spin FeIV=O unit is capable of oxidizing a C–H bond as strong as 96 kcal mol−1. This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C–H bond (DC-H 81 kcal mol−1). The activation of the [FeIIIFeIV(μ-O)2] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second order rate constant that is 3.6 x 107-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 FeIV=O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [FeIV2(μ-O)2] diamond core of sMMO-Q to selectively attack the strong C–H bond of methane in the presence of weaker C–H bonds of amino acid residues that define the diiron active site pocket.

show abstract

Section: Resultsmentioning

confidence: 98%

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

2011

Self Cite

View full text Add to dashboard Cite

show abstract

“…This interaction decreases the Fe III –O–Fe IV angle to 130.0°, from the value, 175.0° calculated for 4 (F) (and by extension to 4 -OCH 3 ), complexes for which such H-bonding and formation of a closed cycle are precluded. 11 Likewise, the hydrogen bonding interaction reduces the dihedral angle in the DFT-optimized structure, O x =Fe IV -Fe III -O H to 32.0°, whereas it is 180.0° for 4 (F). 11 The position of the hydroxyl proton, 1 H A ( E ), determined by ENDOR measurements of 1 in frozen solution is the same as that of the proton in the DFT-generated structure, 1 H A ( D ), within the accuracy of measurements/analysis, and thus the ENDOR measurement confirms the presence in frozen solution of the Fe IV =O···H–O-Fe III hydrogen bond predicted by DFT to drive Fe-O-Fe bending in 1 required to generate the cyclic structure.…”

mentioning

confidence: 97%

“…7,8–11 Complex 1 can be obtained by nucleophilic attack of hydroxide ion on the [Fe III Fe IV (μ-O) 2 ] diamond core of 2 , generating the open HO–Fe III –O–Fe IV =O core of 1 . The closely related 4 (OCH 3 ) can be generated by a similar reaction with methoxide, and exhibits similar spectroscopic features.…”

mentioning

confidence: 99%

“…Its 35 GHz EPR spectrum is described by the g -tensor, g = [2.008, 2.003, 1.992], comparable to that of X . 11,13 The spectrum shows a small variability in g ⊥ that is assigned to solvent/freezing effects (Fig S1). 10,14–16 …”

mentioning

confidence: 99%

“…23 One-electron reduction of 3 thus occurs without an appreciable change in the net Fe–Fe distance. 10,11 …”

mentioning

confidence: 99%

See 2 more Smart Citations

¹H-ENDOR Evidence for a Hydrogen-Bonding Interaction That Modulates the Reactivity of a Nonheme Fe^IV═O Unit

Shanmugam¹,

Xue

Que

et al. 2012

Inorg. Chem.

Self Cite

View full text Add to dashboard Cite

We report that a novel use of 35 GHz 1H-ENDOR spectroscopy establishes the presence in 1 of an FeIV=O···H–O–FeIII hydrogen bond predicted by DFT computations to generate a six-membered-ring core for 1. The H-bond rationalizes the difference in C–H bond cleavage reactivity between 1 and 4(OCH3) (where a CH3O group has replaced the HO on the FeIII site). This result substantiates the seemingly paradoxical conclusion that the non-heme FeIV=O unit of 1 not only has the electrophilic character required for H-atom abstraction, but nonetheless retains sufficient nucleophilic character to accept a hydrogen bond from the FeIII-OH unit.

show abstract

Interplay of Electronic Cooperativity and Exchange Coupling in Regulating the Reactivity of Diiron(IV)‐oxo Complexes towards C−H and O−H Bond Activation

Ansari

Singha

et al. 2017

Chemistry A European J

View full text Add to dashboard Cite

Activation of inert C-H bonds such as those of methane are extremely challenging for chemists but in nature, the soluble methane monooxygenase (sMMO) enzyme readily oxidizes methane to methanol by using a diiron(IV) species. This has prompted chemists to look for similar model systems. Recently, a (μ-oxo)bis(μ-carboxamido)diiron(IV) ([Fe O(L) ] L=N,N-bis-(3',5'-dimethyl-4'-methoxypyridyl-2'-methyl)-N'-acetyl-1,2-diaminoethane) complex has been generated by bulk electrolysis and this species activates inert C-H bonds almost 1000 times faster than mononuclear Fe =O species and at the same time selectively activates O-H bonds of alcohols. The very high reactivity and selectivity of this species is puzzling and herein we use extensive DFT calculations to shed light on this aspect. We have studied the electronic and spectral features of diiron {Fe -μ(O)-Fe } (complex I), {Fe -μ(O)-Fe } (II), and {Fe -μ(O)-Fe } (III) complexes. Strong antiferromagnetic coupling between the Fe centers leads to spin-coupled S=0, S=3/2, and S=0 ground state for species I-III respectively. The mechanistic study of the C-H and O-H bond activation reveals a multistate reactivity scenario where C-H bond activation is found to occur through the S=4 spin-coupled state corresponding to the high-spin state of individual Fe centers. The O-H bond activation on the other hand, occurs through the S=2 spin-coupled state corresponding to an intermediate state of individual Fe centers. Molecular orbital analysis reveals σ-π/π-π channels for the reactivity. The nature of the magnetic exchange interaction is found to be switched during the course of the reaction and this offers lower energy pathways. Significant electronic cooperativity between two metal centers during the course of the reaction has been witnessed and this uncovers the reason behind the efficiency and selectivity observed. The catalyst is found to prudently choose the desired spin states based on the nature of the substrate to effect the catalytic transformations. These findings suggest that the presence of such factors play a role in the reactivity of dinuclear metalloenzymes such as sMMO.

show abstract

Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = ¹/₂ Fe^III−O−Fe^IV═O Complexes. Superexchange-Mediated Spin Transition at the Fe^IV═O Site

Abstract: Previously we have characterized two high-valent complexes [LFe IV (µ-O) 2

Cited by 25 publications

References 57 publications

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

¹H-ENDOR Evidence for a Hydrogen-Bonding Interaction That Modulates the Reactivity of a Nonheme Fe^IV═O Unit

Interplay of Electronic Cooperativity and Exchange Coupling in Regulating the Reactivity of Diiron(IV)‐oxo Complexes towards C−H and O−H Bond Activation

Contact Info

Product

Resources

About

Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = 1/2 FeIII−O−FeIV═O Complexes. Superexchange-Mediated Spin Transition at the FeIV═O Site

Abstract: Previously we have characterized two high-valent complexes [LFe IV (µ-O) 2

Cited by 25 publications

References 57 publications

Substrate-Triggered Activation of a Synthetic [Fe2(μ-O)2] Diamond Core for C–H Bond Cleavage

Substrate-Triggered Activation of a Synthetic [Fe2(μ-O)2] Diamond Core for C–H Bond Cleavage

1H-ENDOR Evidence for a Hydrogen-Bonding Interaction That Modulates the Reactivity of a Nonheme FeIV═O Unit

Interplay of Electronic Cooperativity and Exchange Coupling in Regulating the Reactivity of Diiron(IV)‐oxo Complexes towards C−H and O−H Bond Activation

Contact Info

Product

Resources

About

Mössbauer, Electron Paramagnetic Resonance, and Density Functional Theory Studies of Synthetic S = ¹/₂ Fe^III−O−Fe^IV═O Complexes. Superexchange-Mediated Spin Transition at the Fe^IV═O Site

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

Substrate-Triggered Activation of a Synthetic [Fe₂(μ-O)₂] Diamond Core for C–H Bond Cleavage

¹H-ENDOR Evidence for a Hydrogen-Bonding Interaction That Modulates the Reactivity of a Nonheme Fe^IV═O Unit