Contrasting <i>cis</i> and <i>trans</i> Effects on the Reactivity of Nonheme Oxoiron(IV) Complexes

Zhou, Yuming; Shan, Xuechuan; Mas‐Ballesté, Rubén; Bukowski, Michael R.; Stubna, Audria; Chakrabarti, Mrinmoy; Slominski, Luke M.; Halfen, Jason A.; Münck, Eckard; Que, Lawrence

doi:10.1002/anie.200704228

Cited by 43 publications

(30 citation statements)

References 38 publications

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“…We followed the decay of 2 by UV/Vis spectrophotometry and observed the formation of a green species 4 with l max = 740 nm ( Figure S19 in the Supporting Information). This near-IR chromophore closely resembles low-spin L-Fe IV =O complexes generated by O À O bond homolysis of L-Fe III -A C H T U N G T R E N N U N G (OOH) intermediates supported by several polyamine-or aminopyridine ligands (L) analogous to bpmen, [33] suggesting that 4 can also be formulated as an Fe IV =O intermediate. This formulation is consistent with the EPR-silent nature of the green intermediate 4: it was found that 2 decayed over 70 s (k % 0.01 s À1 , RT) at room temperature without formation of any new EPR-active species ( Figure S20 in the Supporting Information).…”

Section: Fe III a C H T U N G T R E N N U N G (Ooh) Reactivity Pathwamentioning

confidence: 77%

Aromatic Hydroxylation at a Non‐Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species

Makhlynets

Rybak‐Akimova

2010

Chemistry A European J

125

145

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Mechanism of substrate oxidations with hydrogen peroxide in the presence of a highly reactive, biomimetic, iron aminopyridine complex, [Fe(II)(bpmen)(CH(3)CN)(2)][ClO(4)](2) (1; bpmen=N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine), is elucidated. Complex 1 has been shown to be an excellent catalyst for epoxidation and functional-group-directed aromatic hydroxylation using H(2)O(2), although its mechanism of action remains largely unknown. Efficient intermolecular hydroxylation of unfunctionalized benzene and substituted benzenes with H(2)O(2) in the presence of 1 is found in the present work. Detailed mechanistic studies of the formation of iron(III)-phenolate products are reported. We have identified, generated in high yield, and experimentally characterized the key Fe(III)(OOH) intermediate (λ(max)=560 nm, rhombic EPR signal with g=2.21, 2.14, 1.96) formed by 1 and H(2)O(2). Stopped-flow kinetic studies showed that Fe(III)(OOH) does not directly hydroxylate the aromatic rings, but undergoes rate-limiting self-decomposition producing transient reactive oxidant. The formation of the reactive species is facilitated by acid-assisted cleavage of the O-O bond in the iron-hydroperoxide intermediate. Acid-assisted benzene hydroxylation with 1 and a mechanistic probe, 2-Methyl-1-phenyl-2-propyl hydroperoxide (MPPH), correlates with O-O bond heterolysis. Independently generated Fe(IV)=O species, which may originate from O-O bond homolysis in Fe(III)(OOH), proved to be inactive toward aromatic substrates. The reactive oxidant derived from 1 exchanges its oxygen atom with water and electrophilically attacks the aromatic ring (giving rise to an inverse H/D kinetic isotope effect of 0.8). These results have revealed a detailed experimental mechanistic picture of the oxidation reactions catalyzed by 1, based on direct characterization of the intermediates and products, and kinetic analysis of the individual reaction steps. Our detailed understanding of the mechanism of this reaction revealed both similarities and differences between synthetic and enzymatic aromatic hydroxylation reactions.

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Section: Fe III a C H T U N G T R E N N U N G (Ooh) Reactivity Pathwamentioning

confidence: 77%

Aromatic Hydroxylation at a Non‐Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species

Makhlynets

Rybak‐Akimova

2010

Chemistry A European J

125

145

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“…It has been found that ligand donor moieties in cis-position to the oxo unit (equatorial donors) exert less influence on the reactivity of Fe IV O (S = 1) octahedral complexes as compared to those ligated trans (axial) 48 as "trans-ligands can interact with both σ-and π-orbitals involved in the FeO bond but cis-ligands cannot". 53 However, in the present case, the effect of replacing equatorial pyridyl donors with (Nmethyl)benzimidazolyl units is quite significant. The (number and positions of) equatorial donors in octahedral complexes can modulate the spin state and reactivity of the oxo unit by altering the energies, and thus the occupancies, of the iron d xy and d x 2 −y 2 orbitals.…”

Section: ■ Discussionmentioning

confidence: 94%

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

et al. 2015

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Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L(1)) and 2.5 h (L = L(2)). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe(IV)(O)(N4Py)](2+)), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe(IV)(O)(L(2))](2+) exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

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“…20 This is partly due to the high stability of this complex, but also because ligands trans to the oxo unit have been found to exert a greater influence than those ligated cis . 21 This difference was attributed to the fact that trans ligands can interact with both σ- and π-type orbitals involved in the Fe=O bond but cis ligands cannot. Interestingly, whereas the complexes [Fe IV (O)(TMC)(X)] + ( 1-X , where X = monoanion) were found to display increased reactivity in O-atom transfer (OAT) with increasing electrophilicity, the reverse trend was found in hydrogen atom transfer (HAT).…”

Section: Introductionmentioning

confidence: 99%

“…pyridine N-oxide 21 ). This approach failed in all cases, presumably due to a combination of steric hindrance around the binding site and mass action (CH 3 CN was used as the solvent).…”

Section: Introductionmentioning

confidence: 99%

An ultra-stable oxoiron(iv) complex and its blue conjugate base

et al. 2014

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Treatment of [FeII(L)](OTf)2 (4), (where L = 1,4,8-Me3cyclam-11-CH2C(O)NMe2) with iodosylbenzene yielded the corresponding S = 1 oxoiron(IV) complex [FeIV(O(L)](OTf)2 (5) in nearly quantitative yield. The remarkably high stability of 5 (t1/2 ≈ 5 days at 25 °C) facilitated its characterization by X-ray crystallography and a raft of spectroscopic techniques. Treatment of 5 with strong base was found to generate a distinct, significantly less stable S = 1 oxoiron(IV) complex, 6 (t1/2 ~ 1.5 hrs. at 0 °C), which could be converted back to 5 by addition of a strong acid; these observations indicate that 5 and 6 represent a conjugate acid-base pair. That 6 can be formulated as [FeIV(O)(L-H)](OTf) was further supported by ESI mass spectrometry, spectroscopic and electrochemical studies, and DFT calculations. The close structural similarity of 5 and 6 provided a unique opportunity to probe the influence of the donor trans to the FeIV=O unit upon its reactivity in H-atom transfer (HAT) and O-atom transfer (OAT), and 5 was found to display greater reactivity than 6 in both OAT and HAT. While the greater OAT reactivity of 5 is expected on the basis of its higher redox potential, its higher HAT reactivity does not follow the anti-electrophilic trend reported for a series of [FeIV(O)(TMC)(X)] complexes (TMC = tetramethylcyclam) and thus appears to be inconsistent with the Two-State Reactivity rationale that is the prevailing explanation for the relative facility of oxoiron(IV) complexes to undergo HAT.

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Contrasting cis and trans Effects on the Reactivity of Nonheme Oxoiron(IV) Complexes

Cited by 43 publications

References 38 publications

Aromatic Hydroxylation at a Non‐Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species

Aromatic Hydroxylation at a Non‐Heme Iron Center: Observed Intermediates and Insights into the Nature of the Active Species

Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

An ultra-stable oxoiron(iv) complex and its blue conjugate base

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