Characterization of the structure and reactivity of monocopper-oxygen complexes supported by β-diketiminate and anilido-imine ligands

Mechanistic Insights into the N-Hydroxylations Catalyzed by the Binuclear Iron Domain of SznF Enzyme: Key Piece in the Synthesis of Streptozotocin

SznF, a member of the emerging family of heme-oxygenase-like (HO-like) di-iron oxidases and oxygenases, employs two distinct domains to catalyze the conversion of Nω-methyl-L-arginine (L-NMA) into N-nitroso-containing product, which can subsequently be transformed into streptozotocin. Using unrestricted density functional theory (UDFT) with the hybrid functional B3LYP, we have mechanistically investigated the two sequential hydroxylations of L-NMA catalyzed by SznF's binuclear iron central domain. Mechanism B primarily involves the O-O bond dissociation, forming Fe(IV)=O, induced by the H+/e- introduction to the FeA side of μ-1,2-peroxo-Fe2(III/III), the substrate hydrogen abstraction by Fe(IV)=O, and the hydroxyl rebound to the substrate N radical. The stochastic addition of H+/e- to the FeB side (mechanism C) can transition to mechanism B, thereby preventing enzyme deactivation. Two other competing mechanisms, involving the direct O-O bond dissociation (mechanism A) and the addition of H2O as a co-substrate (mechanism D), have been ruled out.

High-Valent Ni and Cu Complexes of a Tetraanionic Bis(amidateanilido) Ligand

High-valent metal species are often invoked as intermediates during enzymatic and synthetic catalytic cycles. Anionic donors are often required to stabilize such high-valent states by forming strong bonds with the Lewis acidic metal centers while decreasing their oxidation potentials. In this report, we discuss the synthesis of two high-valent metal complexes [ML]+ in which the NiIII and CuIII centers are ligated by a new tetradentate, tetraanionic bis(amidateanilido) ligand. [ML]+, obtained via chemical oxidation of ML, exhibits UV-vis-NIR, EPR, and XANES spectra characteristic of square planar, high-valent MIII species, suggesting the locus of oxidation for both [ML]+ is predominantly metal-based. This is supported by theoretical analyses, which also support the observed visible transitions as ligand-to-metal charge transfer transitions characteristic of square planar, high-valent MIII species. Notably, [ML]+ can also be obtained via O2 oxidation of ML due to its remarkably negative oxidation potentials (CuL/[CuL]+: -1.16 V, NiL/[NiL]+: -1.01 V vs Fc/Fc+ in MeCN). This demonstrates the exceptionally strong donating nature of the tetraanionic bis(amidateanilido) ligation and its ability to stabilize high-valent metal centers..

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

In an effort to prepare small molecule mimics of the active site of lytic polysaccharide monooxygenase (LPMO), three monoanionic tridentate N donor ligands comprising a central deprotonated amide group flanked by two neutral donors were prepared, and their coordination chemistry with Cu(I) and Cu(II) was evaluated. With Cu(I), a dimer formed, which was characterized by X-ray crystallography and NMR spectroscopy. A variety of mononuclear and dinuclear Cu(II) species with a range of auxiliary ligands (MeCN, Cl^-, OH^-, OAc^-, OBz^-, CO₃ ^2-) were prepared and characterized by X-ray diffraction and various spectroscopies (UV-vis, EPR). The complexes exhibit structural similarities to the LPMO active site.

A computational mechanistic study of CH hydroxylation with mononuclear copper–oxygen complexes

A computational study of methane hydroxylation by oxygen-bound monocopper complexes.

Cu-promoted intramolecular hydroxylation of CH bonds using directing groups with varying denticity

In this research article, we describe the Cu-promoted intramolecular hydroxylation of sp² and sp³ CH bonds using directing groups with varying denticity (bi-, tri- and tetradentate) and natural oxidants (O₂ and H₂O₂). We found that bidentate directing groups, in combination with Cu and H₂O₂, led to high hydroxylation yields. On the other hand, tetradentate directing groups did not form the hydroxylation products. Our mechanistic investigations suggest that bidentate directing groups allow for generating reactive mononuclear copper(II) hydroperoxide intermediates while tetradentate systems form dinuclear Cu₂O₂ species that do not oxidize CH bonds. Our findings might shed light on the reaction mechanism(s) by which Cu-dependent metalloenzymes such as particulate methane monooxygenase or lytic polysaccharide monooxygenase oxidize strong CH bonds.

A Multifunctional Pincer Ligand for Cobalt-Promoted Oxidation by N2 O

The divalent cobalt complex of the diprotic pincer ligand bis-pyrazolylpyridine, (H₂ L)CoCl₂ , is dehydrohalogenated twice by LiN(SiMe₃ )₂ in the presence of PEt₃ to give monomeric S=1/2 LCo(PEt₃ )₂ (1), fully characterized in the solid-state and solution as a square pyramidal monomer with a long axial Co-P bond. This 17-electron species reacts in time of mixing with N₂ O to form L₂ Co₂ (μ-OPEt₃ ) (2)+3 OPEt₃ , the former the first example of phosphine oxide bridging two transition metals. The same products are formed from O₂ , and divalent cobalt persists even in the presence of excess oxidant. Species (2) catalyzes oxygen atom transfer (OAT) for generation of O=PEt₃ from PEt₃ from either N₂ O or O₂ . Bridging and terminal cobalt oxo intermediates are suggested, and the electron donor power, and potential redox activity of the dianionic pincer ligand is emphasized.

A Structurally Characterized CuIII Complex Supported by a Bis(anilido) Ligand and Its Oxidative Catalytic Activity

Three copper(II) complexes of the (R,R)-N,N'-bis(3,5-di-tert-butyl-2-aminobenzylidene)-1,2-diaminocyclohexane ligand, namely [Cu(^N L)], [Cu(^N LH)]⁺ and [Cu(^N LH₂ )]²⁺ , were prepared and structurally characterized. In [Cu(^N LH₂ )]²⁺ the copper ion lies in an octahedral geometry with the aniline groups coordinated in equatorial positions. In [Cu(^N L)] the anilines are deprotonated (anilido moieties) and coordinated to an almost square-planar metal ion. Complex [Cu(^N L)] displays two oxidation waves at E_1/2^ox, 1 =-0.14 V and E_1/2^ox, 2 =0.36 V vs. Fc⁺ /Fc in CH₂ Cl₂ . Complex [Cu(^N LH₂ )]²⁺ displays an irreversible oxidation wave at high potential (1.21 V), but shows a readily accessible and reversible metal-centered reduction at E_1/2^red =-0.67 V (Cu^II /Cu^I redox couple). Oxidation of [Cu(^N L)] by AgSbF₆ produces [Cu(^N L)](SbF₆ ), which was isolated as single crystals. X-ray structure analysis discloses a contraction of the coordination sphere by 0.05 Å upon oxidation, supporting a metal-centered process. Complex [Cu(^N L)](SbF₆ ) displays an intense NIR band at 1260 nm corresponding to an anilido-to-copper(III) charge transfer transition. This compound slowly evolves in CH₂ Cl₂ solution towards [Cu(^N LH)](SbF₆ ), which is a copper(II) complex comprised of both anilido and aniline groups coordinated to the metal center. The copper(III) complex [Cu(^N L)](SbF₆ ) is an efficient catalyst for benzyl alcohol oxidation, with 236 TON in 24 h at 298 K, without additives other than oxygen and a base.

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.

β-Diketiminate complexes of the first row transition metals: applications in catalysis

Although β-diketiminate complexes have been widely explored in stoichiometric studies, their use as catalysts is largely underdeveloped.

Spectroscopic capture and reactivity of a low-spin cobalt(IV)-oxo complex stabilized by binding redox-inactive metal ions

High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt-complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S=1/2) Co(IV)-oxo species in the presence of redox-inactive metal ions, such as Sc(3+), Ce(3+), Y(3+), and Zn(2+), and the investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)Co(III)(O˙)](2-) species to a more stable [(TAML)Co(IV)(O)(M(n+))] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology.

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.

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.

Coordination of peroxide to the Cu(M) center of peptidylglycine α-hydroxylating monooxygenase (PHM): structural and computational study

Many bioactive peptides, such as hormones and neuropeptides, require amidation at the C terminus for their full biological activity. Peptidylglycine α-hydroxylating monooxygenase (PHM) performs the first step of the amidation reaction-the hydroxylation of peptidylglycine substrates at the Cα position of the terminal glycine. The hydroxylation reaction is copper- and O(2)-dependent and requires 2 equiv of exogenous reductant. The proposed mechanism suggests that O(2) is reduced by two electrons, each provided by one of two nonequivalent copper sites in PHM (Cu(H) and Cu(M)). The characteristics of the reduced oxygen species in the PHM reaction and the identity of the reactive intermediate remain uncertain. To further investigate the nature of the key intermediates in the PHM cycle, we determined the structure of the oxidized form of PHM complexed with hydrogen peroxide. In this 1.98-Å-resolution structure (hydro)peroxide binds solely to Cu(M) in a slightly asymmetric side-on mode. The O-O interatomic distance of the copper-bound ligand is 1.5 Å, characteristic of peroxide/hydroperoxide species, and the Cu-O distances are 2.0 and 2.1 Å. Density functional theory calculations using the first coordination sphere of the Cu(M) active site as a model system show that the computed energies of the side-on L(3)Cu(M)(II)-O(2) (2-) species and its isomeric, end-on structure L(3)Cu(M)(I)-O(2) (·-) are similar, suggesting that both these intermediates are significantly populated within the protein environment. This observation has important mechanistic implications. The geometry of the observed side-on coordinated peroxide ligand in L(3)Cu(M)(II)O(2) (2-) is in good agreement with the results of a hybrid quantum mechanical-molecular mechanical optimization of this species.

Coordination chemistry and reactivity of a cupric hydroperoxide species featuring a proximal H-bonding substituent

At -90 °C in acetone, a stable hydroperoxo complex [(BA)Cu(II)OOH](+) (2) (BA, a tetradentate N(4) ligand possessing a pendant -N(H)CH(2)C(6)H(5) group) is generated by reacting [(BA)Cu(II)(CH(3)COCH(3))](2+) with only 1 equiv of H(2)O(2)/Et(3)N. The exceptional stability of 2 is ascribed to internal H-bonding. Species 2 is also generated in a manner not previously known in copper chemistry, by adding 1.5 equiv of H(2)O(2) (no base) to the cuprous complex [(BA)Cu(I)](+). The broad implications for this finding are discussed. Species 2 slowly converts to a μ-1,2-peroxodicopper(II) analogue (3) characterized by UV-vis and resonance Raman spectroscopies. Unlike a close analogue not possessing internal H-bonding, 2 affords no oxidative reactivity with internal or external substrates. However, 2 can be protonated to release H(2)O(2), but only with HClO(4), while 1 equiv Et(3)N restores 2.

Rapid C-H bond activation by a monocopper(III)-hydroxide complex

One-electron oxidation of the tetragonal Cu(II) complex [Bu(4)N][LCuOH] at -80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH(2)). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M(-1) s(-1) at -80 °C, ΔH(‡) = 5.4(2) kcal mol(-1), ΔS(‡) = -30(2) eu) and with very large kinetic isotope effects (cf. k(H)/k(D) = 44 at -70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.

Spectroscopic and computational characterization of CuII-OOR (R = H or cumyl) complexes bearing a Me6-tren ligand

A copper(II)-hydroperoxo complex, [Cu(Me(6)-tren)(OOH)](+) (2), and a copper(ii)-cumylperoxo complex, [Cu(Me(6)-tren)(OOC(CH(3))(2)Ph)](+) (3), were synthesized by reacting [Cu(Me(6)-tren)(CH(3)CN)](2+) (1) with H(2)O(2) and cumyl-OOH, respectively, in the presence of triethylamine. These intermediates, 2 and 3, were successfully characterized by various physicochemical methods such as UV-vis, ESI-MS, resonance Raman and EPR spectroscopies, leading us to propose structures of the Cu(II)-OOR species with a trigonal-bipyramidal geometry. Density functional theory (DFT) calculations provided geometric and electronic configurations of 2 and 3, showing trigonal bipyramidal copper(II)-OOR geometries. These copper(II)-hydroperoxo and -cumylperoxo complexes were inactive in electrophilic and nucleophilic oxidation reactions.

Spectroscopic and computational studies of an end-on bound superoxo-Cu(II) complex: geometric and electronic factors that determine the ground state

A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (η(1)) bound superoxo-Cu(II) complex [TMG(3)trenCuO(2)](+) (where TMG(3)tren is 1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG(3)trenCuO(2)](+) are assigned, including the first definitive assignment of a superoxo intraligand transition in a metal-superoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG(3)trenCuO(2)](+) eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo π*(σ) character of the copper dz(2) orbital leads to its ferromagnetically coupled, triplet, ground state.

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.

A peroxynitrite complex of copper: formation from a copper-nitrosyl complex, transformation to nitrite and exogenous phenol oxidative coupling or nitration

Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)-(*NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO(-))-Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO(2) (-)) complex and 0.5 mol equiv O(2). In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper-nitrosyl and copper-peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

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.

Effects of electron-deficient beta-diketiminate and formazan supporting ligands on copper(I)-mediated dioxygen activation

Copper(I) complexes of a diketiminate featuring CF(3) groups on the backbone and dimethylphenyl substituents (4) and a nitroformazan (5) were synthesized and shown by spectroscopy, X-ray crystallography, cyclic voltammetry, and theory to contain copper(I) sites electron-deficient relative to those supported by previously studied diketiminate complexes comprising alkyl or aryl backbone substituents. Despite their electron-poor nature, oxygenation of LCu(CH(3)CN) (L = 4 or 5) at room temperature yielded bis(hydroxo)dicopper(II) compounds and at -80 degrees C yielded bis(mu-oxo)dicopper complexes that were identified on the basis of UV-vis and resonance Raman spectroscopy, spectrophotometric titration results (2:1 Cu/O(2) ratio), electron paramagnetic resonance spectroscopy (silent), and density functional theory calculations. The bis(mu-oxo)dicopper complex supported by 5 exhibited unusual spectroscopic properties and decayed via a novel intermediate proposed to be a metallaverdazyl radical complex, findings that highlight the potential for the formazan ligand to exhibit "noninnocent" behavior.

Probing the mechanism of O2 activation by a copper(I) biomimetic complex of a C-H hydroxylating copper monooxygenase

In this paper, we report, for the first time, a plausible full reaction pathway for the activation of O(2) by a tetraazamacrocyclic monocopper(I) complex and for the subsequent intramolecular alkylic hydroxylation to yield the alkoxide product. This theoretical insight offers remarkable support to the fundamental hypothesis in the field that a hydroperoxo complex of the type Cu(II)OOH intermediate is the key intermediate in this class of reactions. Overall, we give insight into an intramolecular alkylic C-H bond activation due to the O(2) binding to copper(I) with an end-on eta(1)-O(2) ligation. The loss of a water molecule involves the final substrate oxygenation. The complex we consider is a biomimetic of several systems of biological relevance, such as amine oxidases, peptidylglycine-alpha-hydroxylating monooxygenase, and dopamine-beta monooxygenases.

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.

Theoretical study of the hydroxylation of phenolates by the Cu(2)O (2)(N,N'-dimethylethylenediamine) (2) (2+) complex

Tyrosinase catalyzes the ortho hydroxylation of monophenols and the subsequent oxidation of the diphenolic products to the resulting quinones. In efforts to create biomimetic copper complexes that can oxidize C-H bonds, Stack and coworkers recently reported a synthetic mu-eta(2):eta(2)-peroxodicopper(II)(DBED)(2) complex (DBED is N,N'-di-tert-butylethylenediamine), which rapidly hydroxylates phenolates. A reactive intermediate consistent with a bis-mu-oxo-dicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative mechanism to the concerted or late-stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase. In this work, the reaction mechanism of this peroxodicopper(II) complex was studied with hybrid density functional methods by replacing DBED in the mu-eta(2):eta(2)-peroxodicopper(II)(DBED)(2) complex by N,N'-dimethylethylenediamine ligands to reduce the computational costs. The reaction mechanism obtained is compared with the existing proposals for the catalytic ortho hydroxylation of monophenol and the subsequent oxidation of the diphenolic product to the resulting quinone with the aim of gaining some understanding about the copper-promoted oxidation processes mediated by 2:1 Cu(I)O(2)-derived species.

Heterobimetallic dioxygen activation: synthesis and reactivity of mixed Cu-Pd and Cu-Pt bis(mu-oxo) complexes

Heterobimetallic CuPd and CuPt bis(mu-oxo) complexes have been prepared by the reaction of (PPh3)2MO2 (M=Pd, Pt) with LCu(I) precursors (L=beta-diketiminate and di- and triamine ligands) and characterized by low-temperature UV-vis, resonance Raman, and 1H and 31P[1H] NMR spectroscopy in conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh3, and in one case this was shown to occur by an intramolecular process through crossover experiments using double-labeling (oxo and phosphine). The reactivity of one of the complexes, LMe2Cu(mu-O)2Pt(PPh3)2 (LMe2 = beta-diketiminate), with a variety of reagents including CO2, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate, [NH4][PF6], and dihydroanthracene, was compared to that of homometallic Pt2 and Cu2 counterparts. Unlike typical [Cu2(mu-O)2]2+ cores which have electrophilic oxo groups, the oxo groups in the [Cu(mu-O)2Pt]+ core behave as bases and nucleophiles, similar to previously described Pt2 compounds. In addition, however, the [Cu(mu-O)2Pt]+ core is capable of oxidatively coupling 2,4-di-tert-butylphenol and 2,4-di-tert-butylphenolate. Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics of the Cu2 and CuM (M=Pd, Pt) cores showed that the latter are more basic and form stronger O-H bonds.