DFT Mechanistic Insight into the Dioxygenase-like Reactivity of a CoIII-peroxo Complex: O–O Bond Cleavage via a [1,3]-Sigmatropic Rearrangement-like Mechanism (original) (raw)
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Metal-oxo-mediated O-O bond formation reactions in chemistry and biology
BioInorganic Reaction Mechanisms, 2012
O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photosystem II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are controversially discussed -although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.
Probing metal-mediated O2 activation in chemical and biological systems
Journal of Molecular Catalysis A: Chemical, 2006
The use of oxygen ( 18 O) isotope fractionation as a mechanistic probe of chemical and biological oxidation reactions, particularly those which involve metal-O 2 adducts, is currently being explored. Summarized here are reactions of enzymes and inorganic compounds for which competitive isotope effect measurements have been performed using natural abundance molecular oxygen and isotope ratio mass spectrometry. The derived 18 O equilibrium isotope effects (EIEs) and kinetic isotope effects (KIEs) reflect the ground state and transition state structures, respectively, for reactions of 16 O-16 O and 18 O-16 O. Normal isotope effects (>1) characterize the binding of O 2 to transition metal centers. The magnitudes, which are primarily determined by the decrease in the O-O force constant accompanying formal electron transfer from the metal to O 2 , suggest that metal superoxo complexes can be distinguished from metal peroxo complexes. Because 18 O isotope effects can be measured during catalytic turnover, they complement existing approaches to elucidating the structures of activated oxygen intermediates based on low-temperature spectroscopy and crystallographic analysis of inorganic model compounds.
Inorganic Chemistry, 2016
Mononuclear nonheme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially-coordinating tris(4,5-diphenyl-1methylimidazol-2-yl)phosphine (Ph2 TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate L-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substratebound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(Ph2 TIP)(L S,N)]BPh 4 complexes, where L S,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O 2 to yield the corresponding sulfinic acid (RSO 2 H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O 2-bound intermediates. Treatment of the TDO models with nitric oxide (NO)-a wellestablished surrogate of O 2-led to a mixture of high-spin and low-spin {FeNO} 7 species at low temperature (−70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O 2 /NO reactivity studies were carried out using previously-reported TDO models that feature an anionic hydrotris(3-phenyl-5-methylpyrazolyl)borate (Ph,Me Tp −) ligand. Though the O 2 reactivities of the Ph2 TIP-and Ph,Me Tp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.
Substrate-Assisted O2 Activation in a Cofactor-Independent Dioxygenase
Chemistry & Biology, 2014
In contrast to the majority of O 2 -activating enzymes, which depend on an organic cofactor or a metal ion for catalysis, a particular group of structurally unrelated oxygenases is functional without any cofactor. In this study, we characterized the mechanism of O 2 activation in the reaction pathway of a cofactorindependent dioxygenase with an a/b-hydrolase fold, which catalyzes the oxygenolytic cleavage of 2-alkyl-3-hydroxy-4(1H)-quinolones. Chemical analysis and electron paramagnetic resonance spectroscopic data revealed that O 2 activation in the enzyme's active site is substrate-assisted, relying on single electron transfer from the bound substrate anion to O 2 to form a radical pair, which recombines to a C2-peroxide intermediate. Thus, an oxygenase can function without a cofactor, if the organic substrate itself, after activation to a (carb)anion by an active-site base, is intrinsically reactive toward molecular oxygen.
Activating a Peroxo Ligand for C-O Bond Formation
Angewandte Chemie International Edition
Dioxygen activation for effective C−O bond formation in the coordination sphere of a metal is a long standing challenge in chemistry for which the design of catalysts for oxygenations is slowed down by the complicated −sometimes poorly understood− mechanistic panorama. In this context, olefin-peroxide complexes could be valuable models for the study of such reactions. Herein, we showcase the isolation of rare 'Ir(cod)(peroxide)' complexes (cod =1,5-cyclooctadiene) from reactions with oxygen, and then the activation of the peroxide ligand for O−O bond cleavage and C−O bond formation by transfer of a hydrogen atom through PT/ET reactions to give 2-iradaoxetane complexes and water. 2,4,6-Trimethylphenol, 1,4-hydroquinone, and 1,4-cyclohexadiene were used as hydrogen atom donors. These reactions can be key steps in the oxy-functionalization of olefins with oxygen and they constitute a novel mechanistic pathway disclosed for iridium whose full reaction profile is supported by DFT-calculations.
Biomimetic Oxygenation Reactions in Metal Cryptates
2015
Oxygenase metallo-enzymes are an inspiration for the development of one-step C-H bond hydroxylations reactions. Presented here are two synthetic models (cryptands LTEA and LTTA), that are inspired by the second-coordination sphere features of such enzymes. The reactivity of copper(II)- and iron(III)-hydroperoxo species with these cryptands were studied, as they are key intermediates proposed in the catalytic cycles of C-H bond hydroxylation performed by oxygenases. Ultimately, this work was developed to further our understanding of oxygenation reactions by guiding the reactivity of copper(II)- and iron(III)-hydroperoxo intermediates with second coordination sphere features. The structure and the reactivity of copper(II) complexes of LTEA was influenced by the second coordination sphere. Reaction of the complexes with basic hydrogen peroxide in methanol led to the formation of copper(II)-hydroperoxo intermediates. The mechanism of the reaction was studied by low-temperature mass spec...
Inorganic Chemistry, 2004
Density functional calculations using the B3LYP functional have been used to study the reaction mechanism of [Fe(Tp Ph2)BF] (Tp Ph2) hydrotris(3,5-diphenylpyrazol-1-yl)borate; BF) benzoylformate) with dioxygen. This mononuclear non-heme iron(II) complex was recently synthesized, and it proved to be the first biomimetic complex reproducing the dioxygenase activity of R-ketoglutarate-dependent enzymes. Moreover, the enthalpy and entropy of activation for this biologically interesting process were derived from kinetic experiments offering a unique possibility for direct comparison of theoretical and experimental data. The results reported here support a mechanism in which oxidative decarboxylation of the keto acid is the rate-limiting step. This oxygen activation process proceeds on the septet potential energy surface through a transition state for a concerted O−O and C−C bond cleavage. In the next step, a high-valent iron−oxo species performs electrophilic attack on the phenyl ring of the Tp Ph2 ligand leading to an iron(III)−radical σ-complex. Subsequent proton-coupled electron-transfer yields an iron(II)−phenol intermediate, which can bind dioxygen and reduce it to a superoxide radical. Finally, the protonated superoxide radical leaves the first coordination sphere of the iron(III)−phenolate complex and dismutates to dioxygen and hydrogen peroxide. The calculated activation barrier (enthalpy and entropy) and the overall reaction energy profile agree well with experimental data. A comparison to the enzymatic process, which is suggested to occur on the quintet surface, has been made.
Journal of the American Chemical Society, 2017
This study evaluates the reaction of a biomimetic heme-peroxo-copper complex {[(DCHIm)(F8)FeIII]-(O22-)-[CuII(AN)]}+ (1) with a phenolic substrate, involving a net H-atom abstraction to cleave the bridging peroxo O-O bond that produces FeIV=O, CuII-OH and phenoxyl radical moieties, analogous to the chemistry carried out in heme-copper oxidases (HCOs). A 3D potential energy surface generated for this reaction reveals two possible reaction pathways: one involves nearly complete proton transfer from the phenol to the peroxo ligand before the barrier; the other involves O-O homolysis where the phenol remains H-bonding to the peroxo O(Cu) in the transition state, and transfers the proton after the barrier. In both mechanisms, electron transfer from phenol occurs after the proton transfer (and after the barrier), therefore only the interaction with the proton is involved in lowering the O-O cleavage barrier. The relative barriers depend on covalency, and therefore vary with DFT functional...
Dioxygen activation and bond cleavage by mixed-valence cytochrome c oxidase
Proceedings of the …, 1998
Elucidating the structures of intermediates in the reduction of O 2 to water by cytochrome c oxidase is crucial to understanding both oxygen activation and proton pumping by the enzyme. In the work here, the reaction of O 2 with the mixed-valence enzyme, in which only heme a 3 and Cu B in the binuclear center are reduced, has been followed by time-resolved resonance Raman spectroscopy. The results show that OAO bond cleavage occurs within the first 200 s after reaction initiation; the presence of a uniquely stable FeOOOO(H) peroxy species is not detected. The product of this rapid reaction is a heme a 3 oxoferryl (Fe IV AO) species, which requires that an electron donor in addition to heme a 3 and Cu B must be involved. The available evidence suggests that the additional donor is an amino acid side chain.