Mononuclear copper active-oxygen complexes

Photoredox Oxidation of Alkanes by Monometallic Copper-Oxygen Complexes Using Visible Light Including One Sun Illumination

Oxygenation of hydrocarbons offers versatile catalytic routes to more valuable compounds, such as alcohols, aldehydes, and ketones. Despite the importance of monometallic copper-oxygen species as hydroxylating agents in biology, few synthetic model compounds are known to react with hydrocarbons, owing to high C-H bond dissociation energies. To overcome this challenge, the photoredox chemistry of monometallic copper (pyrazolyl)borate complexes coordinated by chlorate has been explored in the presence of C1-C6 alkanes with BDEs ≥ 93 kcal/mol. Ethane is photooxidized at room temperature under N2 with yields of 15-30%, which increases to 77% for the most oxidizing tris(3,5-trifluoromethyl-pyrazolyl)borate complex (Cu-3). This complex also promotes the photooxidation of methane to methanol in significant yield (38%) when the photoredox reaction is run under aerobic conditions. Ligand modification alters the reaction selectivity by tuning the redox potential. The ability to activate 1° C-H bonds of C1-C6 alkanes using visible light is consistent with the photogeneration of a powerfully oxidizing copper-oxyl, which is supported by photocrystallographic studies of the tris(3,4,5-tribromopyrazolyl)borate chlorate complex. Mechanistic studies are consistent with the hydrogen atom abstraction of the C-H bond by the copper-oxyl intermediate. We demonstrate for Cu-3 with hexane as an exemplar, that the photoredox chemistry may be achieved under solar conditions of one-sun illumination.

Searching for molecular hypoxia sensors among oxygen-dependent enzymes

The ability to sense and respond to changes in cellular oxygen levels is critical for aerobic organisms and requires a molecular oxygen sensor. The prototypical sensor is the oxygen-dependent enzyme PHD: hypoxia inhibits its ability to hydroxylate the transcription factor HIF, causing HIF to accumulate and trigger the classic HIF-dependent hypoxia response. A small handful of other oxygen sensors are known, all of which are oxygen-dependent enzymes. However, hundreds of oxygen-dependent enzymes exist among aerobic organisms, raising the possibility that additional sensors remain to be discovered. This review summarizes known and potential hypoxia sensors among human O2-dependent enzymes and highlights their possible roles in hypoxia-related adaptation and diseases.

Metformin counters oxidative stress and mitigates adverse effects of radiation exposure: An overview

Metformin (1,1-dimethylbiguanidine hydrochloride) (MF) is a drug that has long been in use for the treatment of type 2 diabetes mellitus and recently is coming into use in the radiation therapy of cancer and other conditions. Exposure to ionizing radiation disturbs the redox homeostasis of cells and causes damage to proteins, membranes, and mitochondria, destroying a number of biological processes. After irradiation, MF activates cellular antioxidant and repair systems by signaling to eliminate the harmful consequences of disruption of redox homeostasis. The use of MF in the treatment of the negative effects of irradiation has great potential in medical patients after radiotherapy and in victims of nuclear accidents or radiologic terrorism.

Copper oxidation chemistry using a 19-iminopyridine-bearing steroidal ligand: (i) C5-C6 olefin difunctionalization and (ii) C1β-hydroxylation/C19-peroxidation

The Schönecker oxidation involves the 12beta-hydroxylation of 17-imino pyridine DHEA derivatives using copper and either molecular oxygen or hydrogen peroxide as the oxidant. In this study, a 19-imino pyridine DHEA derivative was synthesized and was treated with copper nitrate and hydrogen peroxide. Our results showed the difunctionalization of an olefin for delta-5 steroid substrates to yield a 5beta-hydroxylated 6alpha-nitrate ester product. In contrast, for 19-imino pyridine precursors with a 5alpha-androstane steroid backbone: a 1beta-hydroxylation and 19-peroxidation occurred to yield a 1beta-hydroxylated 19-imidoperoxoic acid product. In conclusion, new Schönecker oxidation chemistry was discovered (C5-C6 olefin difunctionalization and C1beta-hydroxylation/C19-peroxidation) when a 19-imino pyridine DHEA derivative was used as the substrate.

End-On Copper(I) Superoxo and Cu(II) Peroxo and Hydroperoxo Complexes Generated by Cryoreduction/Annealing and Characterized by EPR/ENDOR Spectroscopy

In this report, we investigate the physical and chemical properties of monocopper Cu(I) superoxo and Cu(II) peroxo and hydroperoxo complexes. These are prepared by cryoreduction/annealing of the parent [LCuI(O2)]+ Cu(I) dioxygen adducts with the tripodal, N4-coordinating, tetradentate ligands L = PVtmpa, DMMtmpa, TMG3tren and are best described as [LCuII(O2•-)]+ Cu(II) complexes that possess end-on (η1-O2•-) superoxo coordination. Cryogenic γ-irradiation (77 K) of the EPR-silent parent complexes generates mobile electrons from the solvent that reduce the [LCuII(O2•-)]+ within the frozen matrix, trapping the reduced form fixed in the structure of the parent complex. Cryoannealing, namely progressively raising the temperature of a frozen sample in stages and then cooling back to low temperature at each stage for examination, tracks the reduced product as it relaxes its structure and undergoes chemical transformations. We employ EPR and ENDOR (electron-nuclear double resonance) as powerful spectroscopic tools for examining the properties of the states that form. Surprisingly, the primary products of reduction of the Cu(II) superoxo species are metastable cuprous superoxo [LCuI(O2•-)]+ complexes. During annealing to higher temperatures this state first undergoes internal electron transfer (IET) to form the end-on Cu(II) peroxo state, which is then protonated to form Cu(II)-OOH species. This is the first time these methods, which have been used to determine key details of metalloenzyme catalytic cycles and are a powerful tools for tracking PCET reactions, have been applied to copper coordination compounds.

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

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

Fluorescein-Containing Superoxide Probes with a Modular Copper-Based Trigger

Fluorescein-derived superoxide probes featuring a copper(II) complex that can be activated by superoxide to initiate ether bond cleavage and uncage a fluorescein reporter for imaging in live cells are described. Compared to other superoxide sensing moieties, this bond cleavage strategy can be modularly adapted to fluorescent reporters with different properties without compromising the superoxide reactivity and selectivity. A green-emitting probe and its lysosome-targeting analogue have been successfully developed. Both probes are sensitive with more than 30-fold fluorescence enhancement towards superoxide and are highly selective with no significant response towards other reactive oxygen species. A structure-activity relationship study of the copper-based superoxide trigger showed that the secondary coordination environment of the copper(II) center is important for the superoxide reactivity and selectivity. The probes have been applied in imaging changes in intracellular superoxide level in live HeLa and HEK293T cells upon menadione stimulation and also in a cellular inflammation model in RAW 264.7 cells.

Mechanistic Dichotomy in Proton-Coupled Electron-Transfer Reactions of Phenols with a Copper Superoxide Complex

The kinetics and mechanism(s) of the reactions of [K(Krypt)][LCuO₂] (Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, L = a bis(arylcarboxamido)pyridine ligand) with 2,2,6,6-tetramethylpiperdine- N-hydroxide (TEMPOH) and the para-substituted phenols ^XArOH (X = para substituent NO₂, CF₃, Cl, H, Me, ^tBu, OMe, or NMe₂) at low temperatures were studied. The reaction with TEMPOH occurs rapidly ( k = 35.4 ± 0.3 M^-1 s^-1) by second-order kinetics to yield TEMPO^• and [LCuOOH]^- on the basis of electron paramagnetic resonance spectroscopy, the production of H₂O₂ upon treatment with protic acid, and independent preparation from reaction of [NBu₄][LCuOH] with H₂O₂ ( K_eq = 0.022 ± 0.007 for the reverse reaction). The reactions with ^XArOH also follow second-order kinetics, and analysis of the variation of the k values as a function of phenol properties (Hammett σ parameter, O-H bond dissociation free energy, p K_a, E_1/2) revealed a change in mechanism across the series, from proton transfer/electron transfer for X = NO₂, CF₃, Cl to concerted-proton/electron transfer (or hydrogen-atom transfer) for X = OMe, NMe₂ (data for X = H, Me, ^tBu are intermediate between the extremes). Thermodynamic analysis and comparisons to previous results for LCuOH, a different copper-oxygen intermediate with the same supporting ligand, and literature for other [CuO₂]⁺ complexes reveal significant differences in proton-coupled electron-transfer mechanisms that have implications for understanding oxidation catalysis by copper-containing enzymes and abiological catalysts.

A theoretical investigation into the first-row transition metal–O2 adducts

Effects of both metal center and ligand ring size on the properties of metal–O₂ adducts.

High-valence CuIICuIII species in action: demonstration of aliphatic C–H bond activation at room temperature

The electrochemically generated Cu^IICu^III mixed-valence species promotes activation of strong aliphatic C–H bonds (i.e. toluene), turning from stoichiometric to catalytic upon addition of a base.

α-Amidated Peptides: Approaches for Analysis

α-Amidation is a terminal modification in peptide biosynthesis that can itself be rate limiting in the overall production of bioactive α-amidated peptides. More than half of the known neural and endocrine peptides are α-amidated and in most cases this structural feature is essential for receptor recognition, signal transduction, and thus biologic function. This chapter describes methods for developing and using analytical tools to study the biology of α-amidated peptides. The principal analytical method used to quantify α-amidated peptides is the radioimmunoassay (RIA). Detailed protocols are provided for (1) primary antibody production and characterization; (2) radiolabeling of RIA peptides; (3) sample preparation; and (4) performance of the RIA itself. Techniques are also described for the identification and verification of α-amidated peptides. Lastly, in vivo models used for studying the biology of α-amidation are discussed.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Oxygen Activation by Cu LPMOs in Recalcitrant Carbohydrate Polysaccharide Conversion to Monomer Sugars

Natural carbohydrate polymers such as starch, cellulose, and chitin provide renewable alternatives to fossil fuels as a source for fuels and materials. As such, there is considerable interest in their conversion for industrial purposes, which is evidenced by the established and emerging markets for products derived from these natural polymers. In many cases, this is achieved via industrial processes that use enzymes to break down carbohydrates to monomer sugars. One of the major challenges facing large-scale industrial applications utilizing natural carbohydrate polymers is rooted in the fact that naturally occurring forms of starch, cellulose, and chitin can have tightly packed organizations of polymer chains with low hydration levels, giving rise to crystalline structures that are highly recalcitrant to enzymatic degradation. The topic of this review is oxidative cleavage of carbohydrate polymers by lytic polysaccharide mono-oxygenases (LPMOs). LPMOs are copper-dependent enzymes (EC 1.14.99.53-56) that, with glycoside hydrolases, participate in the degradation of recalcitrant carbohydrate polymers. Their activity and structural underpinnings provide insights into biological mechanisms of polysaccharide degradation.

Spin crossover dynamics studies on the thermally activated molecular oxygen binding mechanism on a model copper complex

The theoretical description of the primary dioxygen (O₂) binding and activation step in many copper or iron enzymes, suffers from the instrinsically electronic non-adiabaticity of the spin flip events of the triplet dioxygen molecule (³O₂), mediated by spin–orbit couplings.

Formally Copper(III)-Alkylperoxo Complexes as Models of Possible Intermediates in Monooxygenase Enzymes

Reaction of [NBu₄][LCu^IIOH] with excess ROOH (R = cumyl or tBu) yielded [NBu₄][LCu^IIOOR], the reversible one-electron oxidation of which generated novel species with [CuOOR]²⁺ cores (formally Cu^IIIOOR), identified by spectroscopy and theory for the case R = cumyl. This species reacts with weak O-H bonds in TEMPO-H and 4-dimethylaminophenol (^NMe2PhOH), the latter yielding LCu(OPh^NMe2), which was also prepared independently. With the identification of [CuOOR]²⁺ complexes, the first precedent for this core in enzymes is provided, with implications for copper monooxygenase mechanisms.

Substrate and Lewis Acid Coordination Promote O-O Bond Cleavage of an Unreactive L2CuII2(O22-) Species to Form L2CuIII2(O)2 Cores with Enhanced Oxidative Reactivity

Copper-dependent metalloenzymes are widespread throughout metabolic pathways, coupling the reduction of O2 with the oxidation of organic substrates. Small-molecule synthetic analogs are useful platforms to generate L/Cu/O2 species that reproduce the structural, spectroscopic, and reactive properties of some copper-/O2-dependent enzymes. Landmark studies have shown that the conversion between dicopper(II)-peroxo species (L2CuII2(O22-) either side-on peroxo, SP, or end-on trans-peroxo, TP) and dicopper(III)-bis(μ-oxo) (L2CuIII2(O2-)2: O) can be controlled through ligand design, reaction conditions (temperature, solvent, and counteranion), or substrate coordination. We recently published ( J. Am. Chem. Soc. 2012 , 134 , 8513 , DOI: 10.1021/ja300674m ) the crystal structure of an unusual SP species [(MeAN)2CuII2(O22-)]2+ (SPMeAN, MeAN: N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) that featured an elongated O-O bond but did not lead to O-O cleavage or reactivity toward external substrates. Herein, we report that SPMeAN can be activated to generate OMeAN and perform the oxidation of external substrates by two complementary strategies: (i) coordination of substituted sodium phenolates to form the substrate-bound OMeAN-RPhO- species that leads to ortho-hydroxylation in a tyrosinase-like fashion and (ii) addition of stoichiometric amounts (1 or 2 equiv) of Lewis acids (LA's) to form an unprecedented series of O-type species (OMeAN-LA) able to oxidize C-H and O-H bonds. Spectroscopic, computational, and mechanistic studies emphasize the unique plasticity of the SPMeAN core, which combines the assembly of exogenous reagents in the primary (phenolates) and secondary (Lewis acids association to the MeAN ligand) coordination spheres with O-O cleavage. These findings are reminiscent of the strategy followed by several metalloproteins and highlight the possible implication of O-type species in copper-/dioxygen-dependent enzymes such as tyrosinase (Ty) and particulate methane monooxygenase (pMMO).

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.

Lytic polysaccharide monooxygenases: a crystallographer's view on a new class of biomass-degrading enzymes

Lytic polysaccharide monooxygenases (LPMOs) are a new class of microbial copper enzymes involved in the degradation of recalcitrant polysaccharides. They have only been discovered and characterized in the last 5-10 years and have stimulated strong interest both in biotechnology and in bioinorganic chemistry. In biotechnology, the hope is that these enzymes will finally help to make enzymatic biomass conversion, especially of lignocellulosic plant waste, economically attractive. Here, the role of LPMOs is likely to be in attacking bonds that are not accessible to other enzymes. LPMOs have attracted enormous interest since their discovery. The emphasis in this review is on the past and present contribution of crystallographic studies as a guide to functional understanding, with a final look towards the future.

Peroxo and Superoxo Moieties Bound to Copper Ion: Electron-Transfer Equilibrium with a Small Reorganization Energy

Oxygenation of [Cu2(UN-O(-))(DMF)](2+) (1), a structurally characterized dicopper Robin-Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O(-) as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [Cu(II)2(UN-O(-))(O2(•-))](2+) (2). The formation kinetics provide that kon = 9 × 10(-2) M(-1) s(-1) (-80 °C), ΔH(‡) = 31.1 kJ mol(-1) and ΔS(‡) = -99.4 J K(-1) mol(-1) (from -60 to -90 °C data). Complex 2 can be reversibly reduced to the peroxide species [Cu(II)2(UN-O(-))(O2(2-))](+) (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with Ket = 5.3 (-80 °C); a standard reduction potential for the superoxo-peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (2(1,2)) and μ-1,1- (2(1,1)) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [Cu(II)2(UN-O(-))(μ-1,2-O2(•-))](2+) (2(1,2)) and μ-1,2-peroxo structures [Cu(II)2(UN-O(-))(O2(2-))](+) (3) having a small bond reorganization energy of 0.4 eV (λin). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 2(1,2) and 3 calculated in the context of Marcus theory.

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.

Tuning the Reactivity of Chromium(III)-Superoxo Species by Coordinating Axial Ligands

Metal-superoxo species have attracted much attention recently as key intermediates in enzymatic and biomimetic oxidation reactions. The effect(s) of axial ligands on the chemical properties of metal-superoxo complexes has never been explored previously. In this study, we synthesized and characterized chromium(III)-superoxo complexes bearing TMC derivatives with pendant pyridine and imidazole donors, such as [Cr(III)(O2)(TMC-Py)](2+) (1, TMC-Py = 4,8,11-trimethyl-1-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and [Cr(III)(O2)(TMC-Im)](2+) (2, TMC-Im = 4,8,11-trimethyl-1-(2-methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane). The reactivity of chromium(III)-superoxo complexes binding different axial ligands, such as 1, 2, and [Cr(III)(O2)(TMC)(Cl)](+) (3, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was then investigated in C-H bond activation and oxygen atom transfer reactions. Kinetic studies revealed that the reactivity of the Cr(III)-superoxo complexes depends on the axial ligands, showing the reactivity order of 1 > 2 > 3 in those electrophilic oxidation reactions. It was also shown that there is a good correlation between the reactivity of the chromium(III)-superoxo complexes and their redox potentials, in which the redox potentials of the chromium(III)-superoxo complexes are in the order 1 > 2 > 3. DFT calculations reproduced the reactivity order between 1 and 3 in both C-H bond activation and oxygen atom transfer reactions, and the latter reaction is described using orbital interactions. The calculations are also in agreement with the experimentally obtained redox potentials. The present results provide the first example showing that the reactivity of metal-superoxo species can be tuned by the electron-donating ability of axial ligands bound trans to the metal-superoxo moiety.

Generation, Characterization, and Reactivity of a Cu(II)-Alkylperoxide/Anilino Radical Complex: Insight into the O-O Bond Cleavage Mechanism

The reaction of [Cu(I)(TIPT3tren) (CH3CN)]ClO4 (1) and cumene hydroperoxide (C6H5C(CH3)2OOH, ROOH) at -60 °C in CH2Cl2 gave a Cu(II)-alkylperoxide/anilino radical complex 2, the formation of which was confirmed by UV-vis, resonance Raman, EPR, and CSI-mass spectroscopy. The mechanism of formation of 2, as well as its reactivity, has been explored.

Developing mononuclear copper-active-oxygen complexes relevant to reactive intermediates of biological oxidation reactions

Active-oxygen species generated on a copper complex play vital roles in several biological and chemical oxidation reactions. Recent attention has been focused on the reactive intermediates generated at the mononuclear copper active sites of copper monooxygenases such as dopamine β-monooxygenase (DβM), tyramine β-monooxygenase (TβM), peptidylglycine-α-hydroxylating monooxygenase (PHM), and polysaccharide monooxygenases (PMO). In a simple model system, reaction of O2 and a reduced copper(I) complex affords a mononuclear copper(II)-superoxide complex or a copper(III)-peroxide complex, and subsequent H(•) or e(-)/H(+) transfer, which gives a copper(II)-hydroperoxide complex. A more reactive species such as a copper(II)-oxyl radical type species could be generated via O-O bond cleavage of the peroxide complex. However, little had been explored about the chemical properties and reactivity of the mononuclear copper-active-oxygen complexes due to the lack of appropriate model compounds. Thus, a great deal of effort has recently been made to develop efficient ligands that can stabilize such reactive active-oxygen complexes in synthetic modeling studies. In this Account, I describe our recent achievements of the development of a mononuclear copper(II)-(end-on)superoxide complex using a simple tridentate ligand consisting of an eight-membered cyclic diamine with a pyridylethyl donor group. The superoxide complex exhibits a similar structure (four-coordinate tetrahedral geometry) and reactivity (aliphatic hydroxylation) to those of a proposed reactive intermediate of copper monooxygenases. Systematic studies based on the crystal structures of copper(I) and copper(II) complexes of the related tridentate supporting ligands have indicated that the rigid eight-membered cyclic diamine framework is crucial for controlling the geometry and the redox potential, which are prerequisites for the generation of such a unique mononuclear copper(II)-(end-on)superoxide complex. Reactivity of a mononuclear copper(II)-alkylperoxide complex has also been examined to get insights into the intrinsic reactivity of copper(II)-peroxide species, which is usually considered as a sluggish oxidant or just a precursor of copper-oxyl radical type reactive species. However, our studies have unambiguously demonstrated that copper(II)-alkylperoxide complex can be a direct oxidant for C-H bond activation of organic substrates, when the C-H bond activation is coupled with O-O bond cleavage (concerted mechanism). The reactivity studies of these mononuclear copper(II) active-oxygen species (superoxide and alkylperoxide) will provide significantly important insights into the catalytic mechanism of copper monooxygenases as well as copper-catalyzed oxidation reactions in synthetic organic chemistry.

Elaboration of copper-oxygen mediated C-H activation chemistry in consideration of future fuel and feedstock generation

To contribute solutions to current energy concerns, improvements in the efficiency of dioxygen mediated C-H bond cleavage chemistry, for example, selective oxidation of methane to methanol, could minimize losses in natural gas usage or produce feedstocks for fuels. Oxidative C-H activation is also a component of polysaccharide degradation, potentially affording alternative biofuels from abundant biomass. Thus, an understanding of active-site chemistry in copper monooxygenases, those activating strong C-H bonds is briefly reviewed. Then, recent advances in the synthesis-generation and study of various copper-oxygen intermediates are highlighted. Of special interest are cupric-superoxide, Cu-hydroperoxo and Cu-oxy complexes. Such investigations can contribute to an enhanced future application of C-H oxidation or oxygenation processes using air, as concerning societal energy goals.

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.