Cu2 O2 Research Articles

Less-Coordinated Atomic Copper-Dimer Boosted Carbon-Carbon Coupling During Electrochemical CO2 Reduction.

This work reports a metal-organic framework (MOF) with less-coordinated copper dimers, which displays excellent electrochemical CO2 reduction (eCO2 RR) performance with an advantageous current density of 0.9 A cm-2 and a high Faradaic efficiency of 71% to C2 products. In comparison with MOF with Cu monomers that are present as Cu1 O4 with a coordination number of 3.8 ± 0.2, Cu dimers exist as O3 Cu1 ···Cu2 O2 with a coordination number of 2.8 ± 0.1. In situ characterizations together with theoretical calculations reveal that two *CO intermediates are stably adsorbed on each site of less-coordinated Cu dimers, which favors later dimerization via a key intermediate of *CH2 CHO. The highly unsaturated dual-atomic Cu provides large-quantity and high-quality actives sites for carbon-carbon coupling, achieving the optimal trade-off between activity and selectivity of eCO2 RR to C2 products.

Comprehensive Theoretical View of the [Cu2 O2 ] Side-on-Peroxo-/Bis-μ-Oxo Equilibria.

Coupled binuclear copper (CBC) sites are employed by many metalloenzymes to catalyze a broad set of biochemical transformations. Typically, the CBC catalytic sites are activated by the O2 molecule to form various [Cu2 O2 ] reactive species. This has also inspired synthesis and development of various biomimetic inorganic complexes featuring the CBC core. From theoretical perspective, the [Cu2 O2 ] reactivity often hinges on the side-on-peroxo-dicopper(II) (P) vs. bis-μ-oxo-dicopper(III) (O) isomerism - an equilibrium that has become almost iconic in theoretical bioinorganic chemistry. Herein, we present a comprehensive calibration and evaluation of the performance of various composite computational protocols available in contemporary computational chemistry, involving coupled-cluster and multireference (relativistic) wave function methods, popular density functionals and solvation models. Starting with the well-studied reference [Cu2 O2 (NH3 )6 ]2+ system, we compared the performance of electronic structure methods and discussed the relativistic effects. This allowed us to select several 'calibrated' DFT functionals that can be conveniently employed to study ten experimentally well-characterized [Cu2 O2 ] inorganic systems. We mostly predicted the lowest-energy structures (P vs. O) of the studied systems correctly. In addition, we present calibration of the used electronic structure methods for prediction of the spectroscopic features of the [Cu2 O2 ] core, mostly provided by the resonance Raman (rR) spectroscopy.

DFT Analysis of Methane C-H Activation and Over-Oxidation by [Cu2 O]2+ and [Cu2 O2 ]2+ Sites in Zeolite Mordenite: Intra- versus Inter-site Over-Oxidation.

Methane over-oxidation by copper-exchanged zeolites prevents realization of high-yield catalytic conversion. However, there has been little description of the mechanism for methane over-oxidation at the copper active sites of these zeolites. Using density functional theory (DFT) computations, we reported that tricopper [Cu3 O3 ]2+ active sites can over-oxidize methane. However, the role of [Cu3 O3 ]2+ sites in methane-to-methanol conversion remains under debate. Here, we examine methane over-oxidation by dicopper [Cu2 O]2+ and [Cu2 O2 ]2+ sites using DFT in zeolite mordenite (MOR). For [Cu2 O2 ]2+ , we considered the μ-(η2 :η2 ) peroxo-, and bis(μ-oxo) motifs. These sites were considered in the eight-membered (8MR) ring of MOR. μ-(η2 :η2 ) peroxo sites are unstable relative to the bis(μ-oxo) motif with a small interconversion barrier. Unlike [Cu2 O]2+ which is active for methane C-H activation, [Cu2 O2 ]2+ has a very large methane C-H activation barrier in the 8MR. Stabilization of methanol and methyl at unreacted dicopper sites however leads to over-oxidation via sequential hydrogen atom abstraction steps. For methanol, these are initiated by abstraction of the CH3 group, followed by OH and can proceed near 200 °C. Thus, for [Cu2 O]2+ and [Cu2 O2 ]2+ species, over-oxidation is an inter-site process. We discuss the implications of these findings for methanol selectivity, especially in comparison to the intra-site process for [Cu3 O3 ]2+ sites and the role of Brønsted acid sites.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores.

Coupled dinuclear copper oxygen cores (Cu2 O2 ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O2 transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (C P) have been proposed as key structures in the early stages of O2 binding in these proteins; their reversible isomerization to other Cu2 O2 cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic C P cores in biological and synthetic systems, the properties and reactivity of C P Cu2 O2 species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (T P) and C P dicopper cores. CaII mediates this process by reversible binding at the Cu2 O2 core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O2 activation in biomimetic systems.

Infrared Photodissociation Spectroscopy of Mass-Selected Cu2O2(CO)n+ Clusters in the Gas Phase.

Stoichiometric Cu2O2(CO)n+ (n = 3-7) clusters were generated via a laser vaporization supersonic cluster source in the gas phase and identified by infrared photodissociation spectroscopy in the C-O stretching region. The infrared spectra were interpreted, and the cluster structures were determined by density functional calculations. The ground states of the Cu2O2(CO)n+ complexes were formed by a dicopper superoxide carbonyl with [(OC)xCuOOCu(CO)y]+ (x + y = n) structures in which the CO ligands coordinate a zigzag Cu(OO)Cu+ core. The structural characterization for the Cu(OO)Cu+ core-based clusters is crucial in order to correctly understand the associated reactions catalyzed by metal clusters.

Valence-to-Core X-ray Emission Spectroscopy as a Probe of O-O Bond Activation in Cu2 O2 Complexes.

Valence-to-Core (VtC) X-ray emission spectroscopy (XES) was used to directly detect the presence of an O-O bond in a complex comprising the [CuII2 (μ-η2 :η2 -O2 )]2+ core relative to its isomer with a cleaved O-O bond having a [CuIII2 (μ-O)2 ]2+ unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [CuII2 (μ-η2 :η2 -O2 )]2+ core corresponds to the copper stabilized in-plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O-O bond activation in μ-η2 :η2 -O2 species and highlight the potential of this method for time-resolved studies of reaction mechanisms.

Theoretical study of aromatic hydroxylation of the [Cu2(H-XYL)O2]2+ complex mediated by a side-on peroxo dicopper core and Cu-ligand effects.

In this work, the aromatic hydroxylation mechanism of the [Cu2(H-XYL)O2]2+ complex mediated by a peroxo dicopper core and Cu-ligand effects are investigated by using hybrid density functional theory (DFT) and the broken symmetry B3LYP method. Based on the calculated free-energy profiles, we proposed two available mechanisms. The first reaction steps of both mechanisms involve concerted O-O bond cleavage and C-O bond formation and the second step involves the Wagner-Meerwein rearrangement of the substrate by a [1,2] H shift (HA shift from CA to CC) or (HA shift from CA to OA) across the phenyl ring to form stable dienone intermediates, and this is followed by the protonation of bridging oxygen atoms to produce the final hydroxylated dicopper(ii) product. The HA shift from CA to CC mechanism is the energetically most favorable, in which the first reaction step is the rate-limiting reaction, with a calculated free-energy barrier of 19.0 kcal mol-1 and a deuterium kinetic isotope effect of 1.0, in agreement with experimental observations. The calculation also shows that the reaction started from the P-type species of [Cu2(H-XYL)O2]2+ which is capable of mediating the direct hydroxylation of aromatic substrates without the intermediacy of an O-type species. Finally, we designed some new complexes with different Cu-ligands and found the complex that computationally possesses a higher activity in mediating the hydroxylation of the ligand based aromatic substrate; here, Cu loses a pyridyl ligand donor by dissociation, compared to the [Cu2(H-XYL)O2]2+ complex.

Extending spin-symmetry projected coupled-cluster to large model spaces using an iterative null-space projection technique.

Recently, we introduced an orbital-invariant approximate coupled-cluster (CC) method in the spin-projection manifold. The multi-determinantal property of spin-projection means that the parametrization in the spin-extended CC (ECC) ansatz is nonorthogonal and overcomplete. Therefore, the linear dependencies must be removed by an orthogonalization procedure to obtain meaningful solutions. Multi-reference methods often achieve this by diagonalizing a metric of the equation system, but this is not feasible with ECC because of the enormous size of the metric, a consequence of the incomplete active space of the spin-projected Hartree-Fock reference. As a result, the applicability of ECC has been limited to small benchmark systems, for which the ansatz was shown to be superior to the configuration interaction and linearized approximations. In this article, we provide a solution to this problem that completely avoids the metric diagonalization by iteratively projecting out its null-space from the working equations. As the additional computational cost required for this iterative projection is only marginal, it greatly expands the application range of ECC. We demonstrate the potential of approximate ECC by studying the complete basis set limit of F2 and transition metal complexes such as NiO, Mn2 , and [Cu2 O2 ]2+ , which have all been hindered by the prohibitively large metric size. We also identify the potential inadequacy of the molecular orbitals given by spin-projected Hartree-Fock in some cases, and propose possible solutions. © 2018 Wiley Periodicals, Inc.

Aerobic Aliphatic Hydroxylation Reactions by Copper Complexes: A Simple Clip-and-Cleave Concept.

A simple imine clip-and-cleave concept has been developed for the selective hydroxylation of non-activated CH bonds in aliphatic aldehydes with dioxygen through a copper complex. The synthetic protocol involves reaction of a substrate aldehyde with N,N-diethyl-ethylendiamine to give the corresponding imine, which is used as a bidentate donor ligand forming a copper(I) complex with [Cu(CH3 CN)4 ][CF3 SO3 ]. After exposure of the reaction mixture to dioxygen acidic cleavage and aqueous workup liberates the corresponding β-hydroxylated aldehyde. The concept for the hydroxylation of trimethylacetaldehyde as well as adamantane and diamantane 1-carbaldehydes was investigated and the corresponding β-hydroxy aldehydes were obtained with high selectivities. The results of low temperature stopped-flow measurements indicate the formation of a bis(μ-oxido)dicopper complex as reactive intermediate. According to density functional theory (DFT, RI-BLYP-D3/def2-TZVP(SDD)/ COSMO(CH2 Cl2 )//RI-PBE-D3/def2-TZVP(SDD)) computations CH bonds suitably predisposed to the [Cu2 O2 ]2+ core undergo hydroxylation in a concerted step with particularly low activation barriers, which explains the experimentally observed regioselectivities.

Ion size effect on chemical bonds of the RBa2Cu2.9Zn0.1Oy system

Single-phase polycrystalline samples of RBa2Cu2.9Zn0.1Oy (R = Y, Nd, Gd, Er, and Tm) (ZnR123) were synthesized using the standard solid-state reaction method. They were characterized by X-ray powder diffraction (XRD) and scanning electron microscope. XRD shows that all samples consist essentially of a single phase and retain the orthorhombic structure. The structure of the samples was refined by the Rietveld method with the help of the bond valence sum method. The variation of the lattice parameters and some meaningful bond angles and lengths with the ionic radius are discussed. In these compounds, the variations of the buckling angles Cu2–O(2,3)–Cu2 and Cu2–Cu2–O(2,3) are unique: the bond angles Cu2–O3–Cu2 and Cu2–Cu2–O2 increase, whereas the bond angles Cu2–O2–Cu2 and Cu2–Cu2–O3 decrease. The variation of these bond angles brings about a strong curvature of the Cu2O plane. Furthermore, we have found tree fixed triangles formed by the Cu2, O2, and O3 atoms in addition to another fixed triangle O1–Ba–O1 observed for the first time. BVS of Cu2 atom shows a specific and unique variation compared with other compounds.

Development of 3,5-Di-tert-butylphenol as a Model Substrate for Biomimetic Aerobic Copper Catalysis

We develop 3,5-di-tert­butylphenol as a strategic substrate for the evaluation of biomimetic Cu2–O2 complexes intended to mimic the activity of tyrosinase. We describe a practical and scalable synthesis and validate its use in an aerobic ortho-oxygenation catalyzed by N,N′-di-tert-butylethylenediamine and [Cu(CH3CN)4]PF6.

Oxygen Delivery as a Limiting Factor in Modelling Dicopper(II) Oxidase Reactivity.

Deprotonation of ligand-appended alkoxyl groups in mononuclear copper(II) complexes of N,O ligands L1 and L2 , gave dinuclear complexes sharing symmetrical Cu2 O2 cores. Molecular structures of these mono- and binuclear complexes have been characterized by XRD, and their electronic structures by UV/Vis, 1 H NMR, EPR and DFT; moreover, catalytic performance as models of catechol oxidase was studied. The binuclear complexes with anti-ferromagnetically coupled copper(II) centers are moderately active in quinone formation from 3,5-di-tert-butyl-catechol under the established conditions of oxygen saturation, but are strongly activated when additional dioxygen is administered during catalytic turnover. This unforeseen and unprecedented effect is attributed to increased maximum reaction rates vmax , whereas the substrate affinity KM remains unaffected. Oxygen administration is capable of (partially) removing limitations to turnover caused by product inhibition. Because product inhibition is generally accepted to be a major limitation of catechol oxidase models, we think that our observations will be applicable more widely.

Simplest Monodentate Imidazole Stabilization of the oxy-Tyrosinase Cu2 O2 Core: Phenolate Hydroxylation through a Cu(III) Intermediate.

Tyrosinases are ubiquitous binuclear copper enzymes that oxygenate to Cu(II) 2 O2 cores bonded by three histidine Nτ-imidazoles per Cu center. Synthetic monodentate imidazole-bonded Cu(II) 2 O2 species self-assemble in a near quantitative manner at -125 °C, but Nπ-ligation has been required. Herein, we disclose the syntheses and reactivity of three Nτ-imidazole bonded Cu(II) 2 O2 species at solution temperatures of -145 °C, which was achieved using a eutectic mixture of THF and 2-MeTHF. The addition of anionic phenolates affords a Cu(III) 2 O2 species, where the bonded phenolates hydroxylate to catecholates in high yields. Similar Cu(III) 2 O2 intermediates are not observed using Nπ-bonded Cu(II) 2 O2 species, hinting that Nτ-imidazole ligation, conserved in all characterized Ty, has functional advantage beyond active-site flexibility. Substrate accessibility to the oxygenated Cu2 O2 core and stabilization of a high oxidation state of the copper centers are suggested from these minimalistic models.

Observation of a Cu(II)(2) (μ-1,2-peroxo)/Cu(III)(2) (μ-oxo)(2) equilibrium and its implications for copper-dioxygen reactivity.

Synthesis of small-molecule Cu2 O2 adducts has provided insight into the related biological systems and their reactivity patterns including the interconversion of the Cu(II) 2 (μ-η(2) :η(2) -peroxo) and Cu(III) 2 (μ-oxo)2 isomers. In this study, absorption spectroscopy, kinetics, and resonance Raman data show that the oxygenated product of [(BQPA)Cu(I) ](+) initially yields an "end-on peroxo" species, that subsequently converts to the thermodynamically more stable "bis-μ-oxo" isomer (Keq =3.2 at -90 °C). Calibration of density functional theory calculations to these experimental data suggest that the electrophilic reactivity previously ascribed to end-on peroxo species is in fact a result of an accessible bis-μ-oxo isomer, an electrophilic Cu2 O2 isomer in contrast to the nucleophilic reactivity of binuclear Cu(II) end-on peroxo species. This study is the first report of the interconversion of an end-on peroxo to bis-μ-oxo species in transition metal-dioxygen chemistry.

Magnetostructural correlation in tetracopper(II) cubanes

The tetradendate Schiff base ligand N-2-hydroxy-3-methoxybenzaldehyde-α-amino-iso-butyrate (H23-MeOSAIB) has been complexed with copper(II) giving rise to a tetranuclear complex [Cu4(3-MeOSAIB)4]·H2O with cubane structure classified, according to the number of Cu⋯Cu contacts and bridging angles Cu–O–Cu, as 4+2 (type II) cubane. Magnetic data is reconstructed with two exchange coupling constants. The open angle Cu1–O1–Cu2 (α=114°) existing four times facilitates a strong antiferromagnetic coupling that is in competition with ferromagnetic interactions through the path Cu1–O2–Cu2 (α∗=88°) occurring four times; the resulting coupling constant is J4/hc=−80.5cm−1 (4×). The second exchange coupling constant occurring twice is J2/hc=+6.2cm−1 (2×) as a result of weak ferromagnetic interaction via bridges Cu1–O1–Cu1 and Cu2–O2–Cu2 (β1=92 and β2=94°). The analysis of available experimental data reveals a transparent magnetostructural correlation of J2 versus β (mean value), according to a straight line with negative slope; J4 weakly correlates with the angle α. A deep chemometric analysis has been applied in order to unhide latent magnetostructural correlations.

Structure and Charge-Transfer Mechanism in Y1−x Ca x Ba2Cu3O7−δ Through Direct Doping

Here we study the effect of Ca doping on the charge-transfer mechanism of polycrystalline YBa2Cu3O7−δ compound. The samples of composition Y1−xCaxBa2Cu3O7−δ (x=0.00,0.05,0.10,0.20 and 0.30) are synthesized through standard solid-state reaction route. Carrier doping is controlled by annealing of samples in oxygen and subsequently in reducing atmosphere. Samples are investigated using resistivity, dc magnetization (M–T) and magnetization with field (M–H) measurements. With increase of Ca the transition temperature (Tc) decreases in oxygenated samples, whereas the same increases in reduced samples. Further reduction of samples at higher temperatures (>600 °C) though results in non-superconducting nature up to Ca concentration of x=0.20, the x=0.30 sample is superconducting below 30 K. This provides a remarkably simple and effective way to study the relationship between structure, superconductivity, and associated electronic properties. Variation in Cu1–O4, Cu2–O4 and Cu2–O2 bond lengths with oxygen content, is seen through the structural refinement of XRD pattern. The effective coordination of Cu2 atom with oxygen changes with the change in these bond lengths and hence the holes in the CuO2 planes. The charge-transfer mechanism from CuOx chains to CuO2 planes and thus effective hole doping is discussed in the context of the results observed.

Comparative study of MSr 2RECu 2O 7+ δ compounds with M = Al, Nb, Fe, Ru, Ga and Co and RE = Eu, Y

Comparative study on various systems originated from MSr 2RECu 2O z (M-1212; 0 ≤ δ < 1) compounds (M = Al, Nb, Fe, Ru, Ga and Co and RE = Eu, Y) has been made. The Rietveld refinement is done for all samples on their respective X-ray diffraction (XRD) patterns, which showed that all compounds are crystallized in single phase. While, Nb-, Fe-, Ru- and Al-1212 possess tetragonal P4/ mmm space group structure, the Ga-1212 and Co-1212 are crystallized in orthorhombic Ima2 space group. The change of space group from P4/ mmm to Ima2 resulted in doubling of unit cell. The buckling angle [Cu2–O2–Cu2 angle] determined, showed that most of the studied samples are under doped and hence they lack superconductivity. Thermogravimetric analysis (TGA) showed the M-1212 compounds to be more stable than widely studied 90 K superconductor Cu-1212 (RE-123). The magnetization behavior of Ga-, Nb- and Al-1212 follows a Curie–Weiss like behavior due to paramagnetic contribution of Cu. In case of Co-, Fe-1212 short magnetic correlations/spin-glass like features is seen below 100 K. Different surface morphologies have been observed from the scanning electron microscopy (SEM) study of all the samples. Efforts are underway to dope mobile carriers in studied single phase under doped M-1212 (M = Ga, Nb, Fe, Al and Co) compounds and to introduce superconductivity in them.

Dramatic increase of paramagnetism before superconductivity quenches in Y 1−XPr XBa 2Cu 3O 7−δ

The samples of Y 1− x Pr x Ba 2Cu 3O y − δ were synthesized with 0 ⩽ X ⩽ 1.0 and characterized by DC magnetization measurement and X-ray diffraction. The structure of the samples was refined by Rietveld method. The paramagnetism in the samples increases with the increase of Pr dopant. We separate magnetization–temperature curves into two parts, paramagnetic and diamagnetic parts, trying to get more quantifiable analysis. After X = 0.4, the percent of paramagnetic part increases dramatically and finally destroys superconductivity. We attribute this dramatic increase of the paramagnetism to structural modulation, especially changes of the chemical bonds (such as: O2–Cu2–O2, O2–Cu2–O4 and O2–O4–O2). The results show that the origin of paramagnetism is strongly related to Cu2 in CuO plane. The relationship between the paramagnetism and superconductivity is discussed based on the experimental results.

Hydroxylation of Phenolic Compounds by a Peroxodicopper(II) Complex: Further Insight into the Mechanism of Tyrosinase

The dicopper(I) complex [Cu2(MeL66)]2+ (where MeL66 is the hexadentate ligand 3,5-bis-{bis-[2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino}-meth ylbenzene) reacts reversibly with dioxygen at low temperature to form a mu-peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from -80 to -55 degrees C yielded the activation parameters DeltaH1(not equal) = 40.4 +/- 2.2 kJ mol(-1), DeltaS1)(not equal) = -41.4 +/- 10.8 J K(-1) mol(-1) and DeltaH(-1)(not equal) = 72.5 +/- 2.4 kJ mol(-1), DeltaS(-1)(not equal) = 46.7 +/- 11.1 J K(-1) mol(-1) for the forward and reverse reaction, respectively, and the binding parameters of O2 DeltaH degrees = -32.2 +/- 2.2 kJ mol(-1) and DeltaS degrees = -88.1 +/- 10.7 J K(-1) mol(-1). The hydroxylation of a series of p-substituted phenolate salts by [Cu2(MeL66)O2]2+ studied in acetone at -55 degrees C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant rho = -1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 degrees C for a range of sodium p-cyanophenolate concentrations. The rate plots were hyperbolic and enabled to derive the activation parameters for the monophenolase reaction DeltaH(not equal)ox = 29.1 +/- 3.0 kJ mol(-1), DeltaS(not equal)ox = -115 +/- 15 J K(-1) mol(-1), and the binding parameters of the phenolate to the mu-peroxo species DeltaH degrees(b) = -8.1 +/- 1.2 kJ mol(-1) and DeltaS degrees(b) = -8.9 +/- 6.2 J K(-1) mol(-1). Thus, the complete set of kinetic and thermodynamic parameters for the two separate steps of O2 binding and phenol hydroxylation have been obtained for [Cu2(MeL66)]2+.

Dioxygen mediated oxo-transfer to an amine and oxidative N-dealkylation chemistry with a dinuclear copper complex

Reaction of dioxygen with a dinuclear copper(I) complex of a new binucleating ligand is described, wherein a peroxo–dicopper(II) (Cu2–O2) intermediate leads to an oxo-transfer reaction to give an N-oxide of an N-benzyl internal ligand substrate; additionally observed regioselective oxidative N-dealkylation chemistry occurs.