Copper(II)-Mediated Aromaticortho-Hydroxylation: A Hybrid DFT and Ab Initio Exploration

Elusive Active Intermediates and Reaction Mechanisms of ortho-/ipso-Hydroxylation of Benzoic Acid by Hydrogen Peroxide Mediated by Bioinspired Iron(II) Catalysts

Aromatic hydroxylation of benzoic acids (BzOH) to salicylates and phenolates is fundamentally interesting in industrial chemistry. However, key mechanistic uncertainties and dichotomies remain after decades of effort. Herein, the elusive mechanism of the competitive ortho-/ipso-hydroxylation of BzOH by H2O2 mediated by a nonheme iron(II) catalyst was comprehensively investigated using density functional theory calculations. Results revealed that the long-postulated FeV(O)(anti-BzO) oxidant is an FeIV(O)(anti-BzO•) species 2 (anti- and syn- are defined by the orientation of the carboxyl oxygen of BzO to the oxo), which rules out the noted two-oxidant mechanism proposed previously. We propose a new mechanism in which, following the formation of an FeV(O)(syn-BzO) species (3) and its electromer FeIV(O)(syn-BzO•) (3'), 3/3' either converts to salicylate and phenolate via intramolecular self-hydroxylation (route A) or acts as an oxidant to oxygenate another BzOH to generate the same products (route B). In route A, the rotation of the BzO group along the C-O bond forms 2, in which the BzO group is orientated by π-π stacking interactions. An electrophilic ipso-addition forms a phenolate by concomitant decarboxylation or an ortho-attack forms a cationic complex, which readily undergoes an NIH shift and a BzOH-assisted proton shift to form a salicylate. In route B, 3 oxidizes an additional BzOH molecule directed by hydrogen bonding and π-π stacking interactions. In both routes, selectivity is determined by the chemical property of the BzO ring. These mechanistic findings provide a clear mechanistic scenario and enrich the knowledge of hydroxylation of aromatic acids.

Trifluoroacetic Acid-Mediated Denitrogenative ortho-Hydroxylation of 1,2,3-Benzotriazin-4(3H)-ones: A Metal-Free Approach

An efficient trifluoroacetic acid-mediated denitrogenative hydroxylation of 1,2,3-benzotriazin-4(3H)-ones is described. This metal-free approach is compatible with a wide range of 1,2,3-benzotriazin-4(3H)-ones, affording ortho-hydroxylated benzamides in good to high yields with a short reaction time. The reaction is believed to proceed via a benzene diazonium intermediate. The synthetic utility of the reaction was successfully demonstrated by the preparation of an antimicrobial drug, Riparin C, and benzoxazine-2,4(3H)-diones in good yields.

N2O Formation via Reductive Disproportionation of NO by Mononuclear Copper Complexes: A Mechanistic DFT Study

The mechanism of the copper(I)-mediated reductive disproportionation reaction of NO to form N₂O was investigated for five different 3,5-substituted tris(pyrazolyl)borate copper complexes (CuTp^R₁,R₂) by means of DFT calculations. A thorough search of the potential surface was performed, using the B3LYP functional with the def2-SVP basis set for optimization purposes and def2-TZVP single-point calculations for constructing the potential energy surface for two of these complexes. The results can be condensed into six competing reaction mechanisms, two of which were more closely investigated using full def2-TZVP optimized potential and free energies. The results consistently predict the same mechanism to have the lowest overall barrier. For all five different complexes, this is found to be in good agreement with the experimental reaction barriers. The key intermediate for the transition from the N-bound reactant to the O-bound product contains a stable (NO)₃ unit with one N-Cu and one O-Cu bond, which was not included in the mechanistic considerations reported in the literature. Further analysis of the charge distribution and the spin density demonstrates the formation of a Cu(II)-(N₂O₂^-) intermediate and the electronic influence of the different ligands.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.

Reactivity of the copper(iii)-hydroxide unit with phenols

Kinetic studies of the reactions of two previously characterized copper(iii)-hydroxide complexes (LCuOH and NO2 LCuOH, where L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and NO2 L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of para substituted phenols (XArOH where X = NMe2, OMe, Me, H, Cl, NO2, or CF3) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants (k) were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF3 < NO2 < Cl < H < Me < OMe < NMe2 and LCuOH < NO2 LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO2 and CF3), for which a more complicated mechanism is proposed. For the case of the reactions between NO2 ArOH and LCuOH in particular, competition between a CPET pathway and one involving initial proton transfer followed by electron transfer (PT/ET) is supported by multiwavelength global analysis of the kinetic data, formation of the phenoxide NO2 ArO- as a reaction product, observation of an intermediate [LCu(OH2)]+ species derived from proton transfer from NO2 ArOH to LCuOH, and thermodynamic arguments indicating that initial PT should be competitive with CPET.

Insight into selective removal of copper from high-concentration nickel solutions with XPS and DFT: New technique to prepare 5N-nickel with chelating resin

An efficient and profitable separation process was proposed to prepare 5N (the purity of the metal solution reaches 99.999%) high-purity nickel from 3N nickel-solutions using Purolite S984. The adsorption performance of this superior resin, especially its selectivity for metal ions, was explored quantitatively. The maximum adsorption capacity for copper was 2.286mmol/g calculated by the Langmuir model, which was twice as large as that for nickel. In the binary systems, the adsorption capacity for nickel was decreased by 45%, indicating direct competition for the active sites. The infinite separation factor for copper versus nickel exceeded 300, revealing the feasibility of preparing 5N-level high-purity nickel solutions, which was further verified using the 800BV (bed volume) effluent in the column dynamic process. According to the cost-benefit analysis, purification contributed to a profit of approximately 60,000USD per cycle, and the investment return period was less than 1/3years. Density functional theory analysis confirmed that four nitrogen atoms would be involved in the coordination complex and thus a structure involving two five-membered rings could be achieved. The X-ray photoelectron spectra confirmed the involvement of nitrogen atoms, implying a coordination ratio of approximately 1:1.

Ligand-Enabled, Copper-Promoted Regio- and Chemoselective Hydroxylation of Arenes, Aryl Halides, and Aryl Methyl Ethers

We report here a practical method for the ortho C-H hydroxylation of benzamides with inexpensive copper(II) acetate monohydrate and a pyridine ligand. An intra- and intermolecular ligand combination was explored to achieve regio- and chemoselective hydroxylation. Interestingly, typical regiochemical scrambling associated with the C-H activation was further resolved by introducing a ligand-directed ortho hydroxylation of haloarenes and aryl methyl ethers.

Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation

Two new ligand sets, (pipMe)LH2 and (NO2)LH2 ((pipMe)L = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, (NO2)L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)-hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O-H bonds of the corresponding Cu(II)-OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)-hydroxide complexes from substrates with C-H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from -80 to -20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.

Experimental and Theoretical Investigations of the Existence of Cu(II), Cu(III), and Cu(IV) in Copper Corrolato Complexes

The most common oxidation states of copper in stable complexes are +I and +II. Cu(III) complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu(IV) species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu(IV) complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu(IV) species, and describe a redox series where Cu(II), Cu(III), and Cu(IV) are discussed within the same molecular platform.

[CuO](+) and [CuOH](2+) complexes: intermediates in oxidation catalysis?

Characterization of monocopper intermediates in enzymes and other catalysts that attack strong C-H bonds is important for unraveling oxidation catalysis mechanisms and, ultimately, designing new, more efficient catalytic systems. Because initially formed 1:1 Cu/O2 adducts resulting from reactions of Cu(I) sites with O2 react relatively sluggishly with substrates with strong C-H bonds, it has been suggested that reductive O-O bond scission might occur instead to yield more reactive [CuO](+) or protonated [CuOH](2+) cores. Experimental and theoretical studies of [CuO](+) species in the gas phase have provided key insights into the possible reactivity of these species, but detailed information is lacking for discrete complexes with the [CuO](+) or [CuOH](2+) core in solution or the solid state. We describe herein our recent efforts to address this issue through several disparate approaches. In one strategy based on precedent from studies of enzymes and synthetic compounds with iron-α-ketocarboxylate motifs, reactions of O2 with Cu(I)-α-ketocarboxylate complexes were explored, with the aim of identifying reaction pathways that would implicate the intermediacy of a [CuO](+) species. A second approach focused on the reaction of N-oxides with Cu(I) complexes, with the goal being to elicit O-N bond heterolysis to yield [CuO](+) complexes. For both strategies, the course of the reactions depended on the nature of the supporting bidentate N-donor ligand, and indirect evidence in support of the sought-after [CuO](+) intermediates was obtained in some instances. In the final approach discussed herein, strongly electron donating and sterically encumbered pyridine-dicarboxamide ligands (L) enabled the synthesis of [LCu(II)OH](-) complexes, which upon one-electron oxidation formed complexes with the [CuOH](2+) core that were characterized in solution. Rapid hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) was observed, yielding LCu(II)OH2. The O-H bond dissociation enthalpy (BDE) of ∼90 kcal/mol for this complex was determined through evaluation of its pKa (∼19) and the [LCu(II)OH](-)/LCu(III)OH reduction potential (approximately -0.08 V vs Fc/Fc(+)). Thus, the poor oxidizing power of the complex is offset by the high basicity of the hydroxide moiety to yield a strong O-H bond. This high BDE provided a thermodynamic rationale for the rapid HAT rate from DHA and suggested that stronger C-H bonds could be attacked. Indeed, using an inert solvent (1,2-difluorobenzene), substrates with C-H bond strengths as high as 99 kcal/mol were shown to react with the [CuOH](2+) complex, and a linear log k vs C-H BDE plot supported similar HAT pathways across the series. Importantly, these results provided key evidence in favor of the possible intermediacy of this core in oxidation catalysis, and we suggest that because it is a more energetically accessible intermediate than the [CuO](+) moiety, it should be considered as an alternative in proposed mechanisms for oxidations by enzymes and other synthetic systems.

Amine oxidative N-dealkylation via cupric hydroperoxide Cu-OOH homolytic cleavage followed by site-specific fenton chemistry

Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu(II)-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu(II)-OOH species were generated by addition of H2O2/base in acetone at -90 °C. These convert (t1/2 ≈ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu(II)-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu(II)-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a Cu(I)OOH species undergoes O-O homolytic cleavage to yield a hydroxyl radical and Cu(II)OH rather than heterolytic cleavage to yield water and a Cu(II)-O(•-) species.

Mechanistic insight into the aerobic oxidation of benzyl alcohol catalyzed by the CuII–TEMPO catalyst in alkaline water solution

The benzyl alcohol oxidation to benzaldehyde in the alkaline water solution catalyzed by Cu^II/L–TEMPO catalyst is investigated by density functional method. The favorable mechanisms and the role for the water ​have also been predicted.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

DFT studies on the mechanism of alcohol oxidation by the (bpy)CuI-TEMPO/NMI catalytic system

The oxidation of alcohol to acetaldehyde catalyzed by (bpy)Cu^I-TEMPO/NMI is investigated by the density functional method. On the basis of the studied catalytic cycle, a possible mechanism is presented to explain the experimental observations.

Excitation wavelength dependent O2 release from copper(II)-superoxide compounds: laser flash-photolysis experiments and theoretical studies

Irradiation of the copper(II)-superoxide synthetic complexes [(TMG3tren)Cu(II)(O2)](+) (1) and [(PV-TMPA)Cu(II)(O2)](+) (2) with visible light resulted in direct photogeneration of O2 gas at low temperature (from -40 °C to -70 °C for 1 and from -125 to -135 °C for 2) in 2-methyltetrahydrofuran (MeTHF) solvent. The yield of O2 release was wavelength dependent: λexc = 436 nm, ϕ = 0.29 (for 1), ϕ = 0.11 (for 2), and λexc = 683 nm, ϕ = 0.035 (for 1), ϕ = 0.078 (for 2), which was followed by fast O2-recombination with [(TMG3tren)Cu(I)](+) (3) and [(PV-TMPA)Cu(I)](+) (4). Enthalpic barriers for O2 rebinding to the copper(I) center (∼10 kJ mol(-1)) and for O2 dissociation from the superoxide compound 1 (45 kJ mol(-1)) were determined. TD-DFT studies, carried out for 1, support the experimental results confirming the dissociative character of the excited states formed upon blue- or red-light laser excitation.

Quantum mechanical calculations suggest that lytic polysaccharide monooxygenases use a copper-oxyl, oxygen-rebound mechanism

Lytic polysaccharide monooxygenases (LPMOs) exhibit a mononuclear copper-containing active site and use dioxygen and a reducing agent to oxidatively cleave glycosidic linkages in polysaccharides. LPMOs represent a unique paradigm in carbohydrate turnover and exhibit synergy with hydrolytic enzymes in biomass depolymerization. To date, several features of copper binding to LPMOs have been elucidated, but the identity of the reactive oxygen species and the key steps in the oxidative mechanism have not been elucidated. Here, density functional theory calculations are used with an enzyme active site model to identify the reactive oxygen species and compare two hypothesized reaction pathways in LPMOs for hydrogen abstraction and polysaccharide hydroxylation; namely, a mechanism that employs a η(1)-superoxo intermediate, which abstracts a substrate hydrogen and a hydroperoxo species is responsible for substrate hydroxylation, and a mechanism wherein a copper-oxyl radical abstracts a hydrogen and subsequently hydroxylates the substrate via an oxygen-rebound mechanism. The results predict that oxygen binds end-on (η(1)) to copper, and that a copper-oxyl-mediated, oxygen-rebound mechanism is energetically preferred. The N-terminal histidine methylation is also examined, which is thought to modify the structure and reactivity of the enzyme. Density functional theory calculations suggest that this posttranslational modification has only a minor effect on the LPMO active site structure or reactivity for the examined steps. Overall, this study suggests the steps in the LPMO mechanism for oxidative cleavage of glycosidic bonds.

Exploring the possibility of high-valent copper in models of copper proteins with a three-histidine copper-binding motif

An important function of many copper-containing proteins is activation of O2 and subsequent substrate oxidation. The Cu (III) oxidation state is generally considered to be less accessible because of the highly positive Cu (III)/Cu (II) redox potentials with typical amino acid ligands. Here, we employ density functional (DFT) calculations to explore to what extent copper (III) may be accessed in a biologically-relevant coordination environment around a mononuclear copper center, by breaking the oxygen-oxygen bond in a copper-(hydro) peroxide complex. In agreement with previous findings by Solomon and co-workers on copper models with related coordination patterns, the formally high-valent copper complex produced by O-O bond cleavage appears to harbor both oxidizing equivalents on the ligands. The potential energy surface for such a reaction reveals that with the three-histidine binding motif at the copper, O-O bond cleavage is not impossible, but rather disfavored thermodynamically.

Cupric superoxo-mediated intermolecular C-H activation chemistry

The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.

Mechanistic insight into the alcohol oxidation mediated by an efficient green [CuBr(2)(2,2'-bipy)]-TEMPO catalyst by density functional method

Density functional theory (DFT) calculations have been performed to investigate the alcohol oxidation to acetaldehyde catalyzed by [CuBr(2)(2,2'-bipy)]-TEMPO (TEMPO stands for 2,2,6,6-tetramethylpiperidinyloxy; bipy stands for bipyridine). The total charge for the studied catalytic system is +1. The catalytic cycle consists of two parts, namely, alcohol oxidation and TEMPO regeneration. In alcohol oxidation, the reaction follows the Sheldon's mechanism for the proposed two mechanisms, i.e., Semmelhack's mechanism and Sheldon's mechanism. The water participation plays minor role in the H atom abstraction step. In TEMPO regeneration, the proposed three paths are competitive in energy. By comparing with experimental observation, it is found that the path, in which alcohol provides the proton to TEMPO(-) to produce TEMPOH followed by the oxidation of TEMPOH directly to TEMPO by O(2), is favored. In TEMPO regeneration, CH(3)CN acts as the ligand to stabilize the Cu(I) species during the catalytic cycle.

Intermediates in reactions of copper(I) complexes with N-oxides: from the formation of stable adducts to oxo transfer

Reactions of copper(I) complexes of bidentate N-donor supporting ligands with pyridine- and trimethylamine-N-oxides or PhIO were explored. Key results include the identification of novel copper(I) N-oxide adducts, aryl substituent hydroxylation, and bis(mu-oxo)dicopper complex formation via a route involving oxo transfer.

Copper/alpha-ketocarboxylate chemistry with supporting peralkylated diamines: reactivity of copper(I) complexes and dicopper-oxygen intermediates

To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N'-di-tert-butyl-N,N'-dimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by X-ray crystallography and spectroscopy in solution, and their reactivity with O(2) at -80 degrees C examined. Oxidative decarboxylation of the alpha-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene, cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates involving displacement of the alpha-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I) moiety to yield copper-oxygen species that subsequently induce decarboxylation. To test these hypotheses, spectroscopic and kinetic studies of the reactions of Bu(4)NBF with preformed mu-eta(2):eta(2)-peroxodicopper(II) and/or bis(mu-oxo)dicopper(III) complexes supported by L or N,N,N',N'-tetramethylpropylenediamine were performed. In an illustration of a new mode of reactivity for such dicopper-oxygen cores, decarboxylation of the added alpha-ketocarboxylate was observed and the intermediacy of a carboxylate-bridged mu-eta(2):eta(2)-peroxodicopper(II) complex was implicated.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Copper-dioxygen complex mediated C-H bond oxygenation: relevance for particulate methane monooxygenase (pMMO)

Particulate methane monooxygenase (pMMO), an integral membrane protein found in methanotrophic bacteria, catalyzes the oxidation of methane to methanol. Expression and greater activity of the enzyme in the presence of copper ion suggest that pMMO is a cuprous metalloenzyme. Recent advances - especially the first crystal structures of pMMO - have energized the field, but the nature of the active site(s) and the mechanism of methane oxidation remain poorly understood-yet hotly contested. Herein the authors briefly review the current understanding of the pMMO metal sites and discuss advances in small molecule Cu-O(2) chemistry that may contribute to an understanding of copper-ion mediated hydrocarbon oxidation chemistry.

Generating Cu(II)-oxyl/Cu(III)-oxo species from Cu(I)-alpha-ketocarboxylate complexes and O2: in silico studies on ligand effects and C-H-activation reactivity

A mechanism for the oxygenation of Cu(I) complexes with alpha-ketocarboxylate ligands that is based on a combination of density functional theory and multireference second-order perturbation theory (CASSCF/CASPT2) calculations is elaborated. The reaction proceeds in a manner largely analogous to those of similar Fe(II)-alpha-ketocarboxylate systems, that is, by initial attack of a coordinated oxygen molecule on a ketocarboxylate ligand with concomitant decarboxylation. Subsequently, two reactive intermediates may be generated, a Cu-peracid structure and a [CuO](+) species, both of which are capable of oxidizing a phenyl ring component of the supporting ligand. Hydroxylation by the [CuO](+) species is predicted to proceed with a smaller activation free energy. The effects of electronic and steric variations on the oxygenation mechanisms were studied by introducing substituents at several positions of the ligand backbone and by investigating various N-donor ligands. In general, more electron donation by the N-donor ligand leads to increased stabilization of the more Cu(II)/Cu(III)-like intermediates (oxygen adducts and [CuO](+) species) relative to the more Cu(I)-like peracid intermediate. For all ligands investigated, the [CuO](+) intermediates are best described as Cu(II)-O(*-) species with triplet ground states. The reactivity of these compounds in C-H abstraction reactions decreases with more electron-donating N-donor ligands, which also increase the Cu-O bond strength, although the Cu-O bond is generally predicted to be rather weak (with a bond order of about 0.5). A comparison of several methods to obtain singlet energies for the reaction intermediates indicates that multireference second-order perturbation theory is likely more accurate for the initial oxygen adducts, but not necessarily for subsequent reaction intermediates.

Copper-hydroperoxo-mediated N-debenzylation chemistry mimicking aspects of copper monooxygenases

A substantial oxidative N-debenzylation reaction along with PhCHO formation occurs from a hydroperoxo-copper(II) complex that has a dibenzylamino substrate (N(CH 2Ph) 2 appended as a substituent on one pyridyl group of its tripodal tetradentate TMPA (also TPA, (2-pyridylmethyl)amine)) ligand framework. During the course of the (L (N(CH 2 ) (Ph) 2 ))Cu (II)( (-)OOH) reactivity, the formation of a substrate and a (-)OOH-derived (an oxygen atom) alkoxo Cu (II)( (-)OR) complex occurs. The observation that the same Cu (II)( (-)OR) species occurs from Cu (Iota)/PhIO chemistry suggests the possibility that a copper-oxo (cupryl) reactive intermediate forms during the alkoxo species formation; new ESI-MS data provide further support for this high-valent intermediate. A net H atom abstraction chemistry is proposed on the basis of the kinetic isotope effect studies provided here and the previously published study for a closely related Cu (II)( (-)OOH) species incorporating dimethylamine (N(CH 3) 2) as the internal substrate; the Cu (Iota)/PhIO reactivity with similar isotope effect results provides further support. The reactivity of these chemical systems closely resembles the proposed oxidative N-dealkylation mechanisms that are effected by the copper monooxygenases, dopamine beta-monooxygenase (DbetaM) and peptidylglycine- alpha-hydroxylating monooxygenase (PHM).