The Copper(II)‐Catalyzed Oxidation of Glutathione

The kinetics and mechanisms of the copper(II)‐catalyzed GSH (glutathione) oxidation are examined in the light of its biological importance and in the use of blood and/or saliva samples for GSH monitoring. The rates of the free thiol consumption were measured spectrophotometrically by reaction with DTNB (5,5′‐dithiobis‐(2‐nitrobenzoic acid)), showing that GSH is not auto‐oxidized by oxygen in the absence of a catalyst. In the presence of Cu²⁺, reactions with two timescales were observed. The first step (short timescale) involves the fast formation of a copper–glutathione complex by the cysteine thiol. The second step (longer timescale) is the overall oxidation of GSH to GSSG (glutathione disulfide) catalyzed by copper(II). When the initial concentrations of GSH are at least threefold in excess of Cu²⁺, the rate law is deduced to be ?d[thiol]/dt=k[copper–glutathione complex][O₂]^0.5[H₂O₂]^?0.5. The 0.5^th reaction order with respect to O₂ reveals a pre‐equilibrium prior to the rate‐determining step of the GSSG formation. In contrast to [Cu²⁺] and [O₂], the rate of the reactions decreases with increasing concentrations of GSH. This inverse relationship is proposed to be a result of the competing formation of an inactive form of the copper–glutathione complex (binding to glutamic and/or glycine moieties).     

Photocorrosion of Cuprous Oxide in Hydrogen Production: Rationalising Self‐Oxidation or Self‐Reduction

Cuprous Oxide (Cu₂O) is a photocatalyst with severe photocorrosion issues. Theoretically, it can undergo both self‐oxidation (to form copper oxide (CuO)) and self‐reduction (to form metallic copper (Cu)) upon illumination with the aid of photoexcited charges. There is, however, limited experimental understanding of the “dominant” photocorrosion pathway. Both photocorrosion modes can be regulated by tailoring the conditions of the photocatalytic reactions. Photooxidation of Cu₂O (in the form of a suspension system), accompanied by corroded morphology, is kinetically favourable and is the prevailing deactivation pathway. With knowledge of the dominant deactivation mode of Cu₂O, suppression of self‐photooxidation together with enhancement in its overall photocatalytic performance can be achieved after a careful selection of sacrificial hole (h⁺) scavenger. In this way, stable hydrogen (H₂) production can be attained without the need for deposition of secondary components.     

Deciphering Ligands’ Interaction with Cu and Cu2O Nanocrystal Surfaces by NMR Solution Tools

The hydrogenolysis of [Cu₂{(iPrN)₂(CCH₃)}₂] in the presence of hexadecylamine (HDA) or tetradecylphosphonic acid (TDPA) in toluene leads to 6–9 nm copper nanocrystals. Solution NMR spectroscopy has been used to describe the nanoparticle surface chemistry during the dynamic phenomenon of air oxidation. The ligands are organized as multilayered shells around the nanoparticles. The shell of ligands is controlled by both their intermolecular interactions and their bonding strength on the nanocrystals. Under ambient atmosphere, the oxidation rate of colloidal copper nanocrystals closely relies on the chemical nature of the employed ligands (base or acid). Primary amine molecules behave as soft ligands for Cu atoms, but are even more strongly coordinated on surface Cu^I sites, thus allowing a very efficient corrosion protection of the copper core. On the contrary, the TDPA ligands lead to a rapid oxidation rate of Cu nanoparticles and eventually to the re‐dissolution of Cu^II species at the expense of the nanocrystals.     

Mechanistic features of reduction of copper chromite and state of absorbed hydrogen in the structure of reduced copper chromite

The review discusses the experimental data on the unusual mechanism of the reduction of copper cations from the copper chromite, CuCr₂O₄, structure. Treatment of copper chromite in hydrogen at 180–370°C is not accompanied by water formation but leads to absorption of hydrogen by the oxide structure with simultaneous formation of metallic copper as small flat particles which are epitaxially bound to the oxide. This process is due to the redox reaction Cu²⁺ + H₂ → Cu⁰ + 2H⁺; the protons are stabilized in the oxide phase, which is confirmed by neutron diffraction studies. The reduced copper chromite which contains absorbed hydrogen in its oxidized state and the metallic copper particles epitaxially bound to the oxide phase structure exhibit catalytic activity in hydrogenation reactions.     

Influence of different copper(II) salts on the oxidation and doping reactions of emeraldine base polyaniline

A spectroscopic investigation of the products formed in the reaction of emeraldine base (EB-PANI) with copper(II) ions in dimethylacetamide (DMA) is presented. It is well known that metal cations can dope emeraldine base polyaniline (EB-PANI) through a pseudo-protonation reaction. Resonance Raman, UV–vis-NIR, and EPR data, obtained for Cu²⁺/EB-PANI solutions prepared using CuCl₂·2 H₂O, Cu(NO₃)₂· 3 H₂O or Cu(CH₃COO)₂·H₂O as Cu²⁺ sources, showed that the species formed in reactions of EB-PANI and Cu²⁺ ions are dependent on the anions of the copper salt employed. EPR spectra pointed out that the environments of Cu²⁺ ions with acetate, chloride or nitrate as anions in DMA solution are distinct. Resonance Raman and UV–vis-NIR data demonstrated that the main reactions are the oxidation of EB-PANI to pernigraniline base (PB-PANI) and doping of EB-PANI to ES-PANI (emeraldine salt) when a direct coordination of Cu²⁺ ions to PANI exists. With nitrate as very weak coordinating anion, ES-PANI is formed preferentially. When copper chloride is used, both oxidation and doping of EB-PANI are verified. Conversely with acetate, the dimeric cage structure of this copper salt is preserved in solution, and oxidation of EB-PANI to PB-PANI is the only observed reaction. These results demonstrate the possibility of modulating the products of reaction between Cu²⁺ ions and EB-PANI in DMA solution by changing the counter ion of the Cu²⁺ source.     

Effect of Ligands on Formation of Photosensitive Oxide Layers on Copper Electrode*

Photosensitive oxide layers are found to develop on copper electrode exposed to solutions containing Cu(II), different ligands, and K₂SO₄ as a supporting electrolyte. Two mechanisms of Cu₂O formation are discussed: corrosion of copper in naturally aerated Cu(II)-free solutions, and interaction between Cu and Cu²⁺ yielding intermediate Cu⁺ ions. Oxide layers formed in the supporting electrolyte at pH 5 and 7 exhibit n-type conduction; the n–p transition is observed at pH 10. An addition of ligands suppresses the oxide formation. The correlation between the photoelectrochemical effects and the stability of Cu(II) complexes is revealed: the higher the complexation degree, the lower the level of photoresponse. A model of nonuniform Cu₂O-containing layer with predominant n- and p-type properties at copper/oxide and oxide/solution interfaces, respectively, is discussed.     

Catalytic CO Oxidation by O2 Mediated by Noble‐Metal‐Free Cluster Anions Cu2VO3–5−

Catalytic CO oxidation by molecular O₂ is an important model reaction in both the condensed phase and gas‐phase studies. Available gas‐phase studies indicate that noble metal is indispensable in catalytic CO oxidation by O₂ under thermal collision conditions. Herein, we identified the first example of noble‐metal‐free heteronuclear oxide cluster catalysts, the copper–vanadium bimetallic oxide clusters Cu₂VO_3–5^? for CO oxidation by O₂. The reactions were characterized by mass spectrometry, photoelectron spectroscopy, and density functional calculations. The dynamic nature of the Cu?Cu unit in terms of the electron storage and release is the driving force to promote CO oxidation and O₂ activation during the catalysis.     

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

Operando X‐ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu‐exchanged SSZ‐13. Catalysts prepared to contain only isolated, exchanged Cu^II ions evidence both Cu^II and Cu^I ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH₃ together are necessary for Cu^II reduction to Cu^I. DFT calculations show that NO‐assisted NH₃ dissociation is both energetically favorable and accounts for the observed Cu^II reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu^I upon reduction, which we quantify in separate titration experiments. Both NO and O₂ are necessary for oxidation of Cu^I to Cu^II, which DFT suggests to occur by a NO₂ intermediate. Reaction of Cu‐bound NO₂ with proximal NH₄⁺ completes the catalytic cycle. N₂ is produced in both reduction and oxidation half‐cycles.     

5-(Pyridylmethylidene)-substituted 2-thiohydantoins and their complexes with CuII, NiII, and CoII: Synthesis, electrochemical study, and adsorption on the cystamine-modified gold surface

A series of Cu^II, Ni^II, and Co^II complexes with 5-(pyridylmethylidene)-substituted 2-thiohydantoins (L) were synthesized by the reactions of the corresponding organic ligands with MCl₂·nH₂O. The resulting complexes have the composition LMCl₂ (M = Cu or Ni) or L₂MCl₂ (M = Co). The reactions with N(3)-unsubstituted thiohydantoins afford complexes containing four-membered metallacycles, in which the metal ion is coordinated by the S and N(3) atoms of the thiohydantoin ligand. The reactions of N(3)-substituted thiohydantoins give complexes in which the S and N(1) atoms are involved in coordination. Study by IR spectroscopy demonstrated that the pyridine nitrogen atom is not involved in coordination. Based on the results of electrochemical study of the ligands and complexes by cyclic voltammetry and calculation of their frontier orbitals by the PM3(tm) method, the mechanism of oxidation and reduction of these compounds was proposed. In the first reduction and oxidation steps, the metal atom in the copper and nickel complexes remains, apparently, intact, and these processes occur with the involvement of the ligand fragments, viz., the coordinated thiohydantoin ligand and chloride anion, respectively. In the cobalt complexes, the first reduction step occurs at the ligand; the first oxidation state, at the metal atom. Measurements of the contact angle of aqueous wetting and electrochemical study demonstrated that carboxy-containing 2-thiohydantoins and their complexes can be adsorbed on the cystamine-modified gold surface. The structures of the complexes on the surface differ from the structures of these complexes in solution. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 978–990, June, 2006.     

Preparing graphene oxide–copper composite material from spent lithium ion batteries and catalytic performance analysis

The wide use of lithium ion batteries (LIBs) has created much waste, which has become a global issue. It is vital to recycle waste LIBs considering their environmental risks and resource characteristics. Anode graphite from spent LIBs still possess a complete layer structure and contain some oxygen-containing groups between layers, which can be reused to prepare high value-added products. Given the intrinsic defect structure of anode graphite, copper foils in LIB anode electrodes, and excellent properties of graphene, graphene oxide–copper composite material was prepared in this work. Anode graphite was firstly purified to remove organic impurities by calcination and remove lithium. Purified graphite was used to prepare graphene oxide–copper composite material after oxidation to graphite oxide, ultrasonic exfoliation to graphene oxide (GO), and Cu²⁺ adsorption. Compared with natural graphite, preparing graphite oxide using anode graphite consumed 40% less concentrated H₂SO₄ and 28.6% less KMnO₄. Cu²⁺ was well adsorbed by 1.0 mg L^?1 stable GO suspension at pH 5.3 for 120 min. Graphene oxide–copper composite material could be successfully obtained after 6 h absorption, 3 h bonding between GO and Cu²⁺ with 3/100 of GO/CuSO₄ mass ratio. Compared to CuO, graphene oxide–copper composite material had better catalytic photodegradation performance on methylene blue, and the electric field further improved the photodegradation efficiency of the composite material.     

Solid-state reactions in the system Cu—Sb—O; Formation of a new copper(I) antimony oxide

The solid-state reactions in the system Cu—Sb—O were investigated by thermogravimetry and X-ray diffraction. Equimolar mixtures of CuO and Sb₂O₃ form Cu(II)Sb₂O₆ when slowly heated in air up to 1000°C. The firt step in this reaction is the oxidation of Sb₂O₃ to Sb₂O₄ at 380–500°C, followed by further oxidation of Sb₂O₄ and the formation of CuSb₂O₆ at 500–1000°C. Thermal decomposition of CuSb₂O₆ in a flowing nitrogen atmosphere occurs in three stages; the first, with an activation energy of 356 kJ mole^?1, results in the formation of a new copper(I) antimony oxide, with a composition of Cu₄SbO_4.5, as determined by atomic absorption analysis and X-ray fluoresecence. Confirmation of predominantly monovalent copper and pentavalent antimony in the new compound was by ESR and ESCA, respectively. Two forms of Cu₄SbO_4.5 have been distinguished; one of these (form II) has a structure of lower symmetry, and decomposes when heated in air at 600°C to a mixture of CuO and another new copper antimony oxide, as yet uncharacterized. On further heating to 1100°C in air, Cu₄SbO_4.5 (form I) gradually reforms. Details of these reactions are summarized and X-ray powder data presented for Cu₄SbO_4.5.     

Cu(II)-catalyzed oscillatory chemical reactions

The role of copper ions in the copper-catalyzed chemical reactions is discussed. It is pointed out that copper ions can induce oscillatory behavior in many systems for the following reasons: (1) Copper cations can exist in three oxidation states (+1, +2 and +3); (2) Copper cations can form precipitates and stable complexes with a large number of reactants and intermediates; (3) Copper ions can participate in both oxidation and reduction processes, due to the surprisingly large range of redox potentials exhibited by the Cu²⁺/Cu⁺ and Cu³⁺/Cu²⁺ couples (known redox potentials span from 0.1 to 1.8 V, depending on the counter-ion or ligand present).     

Cyclic voltammetric and e.s.r. studies of a tetraaza 14-membered macrocyclic copper(II) complex derived from 3-salicylideneacetylacetone ando-phenylenediamine: stabilization and activation of unusual oxidation states

Summary A 14-membered macrocyclic Schiff base derived from 3-salicylideneacetylacetone ando-phenylenediamine acts as a tetradentate and strongly conjugated ligand to form a cationic solid complex with CuCl₂. U.v.-vis. and e.s.r. spectral data reveal a strong ligand to metal -interaction in the square planar complex. C.v. data reveal that the title ligand is able to stabilize the copper(III) oxidation state more effectively than comparable saturated or partially unsaturated macrocyclic ligands and confers a weaker tendency for reduction of copper(II) to copper(I) and copper(0). While the inclusion of a PPh₃ ligand suppresses the Cu⁰ Cu^I Cu^II oxidation, imidazole and pyridine strongly enhance the Cu^II Cu^III oxidation of the complex.     

一种大环化合物的晶体结构及其双核铜配合物的酪氨酸酶活性研究

A ring-contracted form macrocycle, 29,30-dioxo-3,6,9,17,20,23,29,30-octaazapentacyclo[23,3,1,1^11,15,0^2,6,0^16,20]-triacontaneocta- 1 (28),9,11 (12), 13,15(30),23,25(29),26-ene (L) was synthesized by condensation of diethyltriamine with pyridine-1-oxide-2,6-dicarboxaldehyde. A porous three-dimensional layer structure in its crystal was formed by self-assembly through hydrogen bonds and π-π interaction. Its dinuclear copper(I) complex [Cu2L(MeOH)2]-(BF4)2*2H2O and dinuclear-copper(II) complex [Cu2L(MeOH)2](ClO4)4*2H2O were obtained and could oxidize catalytically four phenolic substrates hydroquinone, 2-methyl-hydroquinone, 2,6-di-tert-butylphenol and 2,6-dimethylphenol, in a mixture of methanol and acetonitrile (V : V, 4 : 1). The copper(I) complex reacted with dioxygen to form an oxygenated species as an initial active intermediate for oxidation of the phenols. Oxidation of the substrates by the copper(II) complex produced a copper(I) complex and the oxidation products of the substrates.     

Experimental Evidence for the Involvement of Dinuclear Alkynylcopper(I) Complexes in Alkyne–Azide Chemistry

Dinuclear alkynylcopper(I) ladderane complexes are prepared by a robust and simple protocol involving the reduction of Cu₂(OH)₃OAc or Cu(OAc)₂ by easily oxidised alcohols in the presence of terminal alkynes; they function as efficient catalysts in copper‐catalysed alkyne–azide cycloaddition reactions as predicted by the Ahlquist–Fokin calculations. The same copper(I) catalysts are formed during reactions by using the Sharpless–Fokin protocol. The experimental results also provide evidence that sodium ascorbate functions as a base to deprotonate terminal alkynes and additionally give a convincing alternative explanation for the fact that the Cu^I‐catalysed reactions of certain 1,3‐diazides with phenylacetylene give bis(triazoles) as the major products. The same dinuclear alkynylcopper(I) complexes also function as catalysts in cycloaddition reactions of azides with 1‐iodoalkynes.     

A thermoanalytical study of the oxidation of chalcocite

A sample of chalcocite of particle size 45–75 μm, has been oxidised in a TG-DTA apparatus at a heating rate of 10 deg·min⁻¹ and the products at various temperatures characterised by XRD, SEM, FTIR and EPMA. This has enabled the events in the TG-DTA record to be assigned to specific chemical reactions, as well as the development of a full reaction scheme for the oxidation of chalcocite. Only minor reactions occurred up to 430°C, but above this temperature there was significant oxidation which resulted in an exotherm and mass gain. These events were due primarily to the oxidation of sulfide to copper(I) oxide, and the formation of copper(II) sulfate. The reaction then slowed, but melting commenced at 490°C which permitted further oxidation to take place with the appearance of a second exotherm and mass gain. By 570°C, sulfide oxidation was complete, but solid-solid reactions took place between Cu₂O and CuSO₄ to produce CuO·CuSO₄. Some conversion of Cu₂O had occurred. By 775°C, CuO and CuO·CuSO₄ were the only phases detected. Above this temperature the latter phase was unstable and decomposed to the end product CuO. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

On the Ligand‐to‐Metal Charge‐Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

In contrast to the UV‐photoinduced ligand photoionization of the flavonoid complexes of Fe^III, redox reactions initiated in ligand‐to‐metal charge‐transfer excited states were observed on irradiation of the quercetin ( 1 ) and rutin ( 2 ) complexes of Cu^II. Solutions of complexes with stoichiometries [Cu^IIL₂] (L=quercetin, rutin) and [Cu^II₂L_n] (n=1, L=quercetin; n=3, L=rutin) were flash‐irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one‐electron oxidation of L and the reduction of Cu^II to Cu^I. The radical ligands remained coordinated to the Cu^I centers, and the substitution reactions replacing them by solvent occurred with lifetimes τ<350 ns. These are lifetimes shorter than the known lifetimes (τ>1 ms) of the quercetin and rutin radical's decay.     

Selective Alcohol Oxidation by a Copper TEMPO Catalyst: Mechanistic Insights by Simultaneously Coupled Operando EPR/UV‐Vis/ATR‐IR Spectroscopy

The first coupled operando EPR/UV‐Vis/ATR‐IR spectroscopy setup for mechanistic studies of gas‐liquid phase reactions is presented and exemplarily applied to the well‐known copper/TEMPO‐catalyzed (TEMPO=(2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl) oxidation of benzyl alcohol. In contrast to previous proposals, no direct redox reaction between TEMPO and Cu^I/Cu^II has been detected. Instead, the role of TEMPO is postulated to be the stabilization of a (bpy)(NMI)Cu^II‐O₂^??‐TEMPO (bpy=2,2′‐bipyridine, NMI=N‐methylimidazole) intermediate formed by electron transfer from Cu^I to molecular O₂.     

Synthesis and characterization of copper(II) complexes of sulfadiazine with amino acids: catalytic activity toward oxidation of phenol and catechol

New copper complexes of DL-methioninoylsulfadiazine (MTS) and L-cystinoylsulfadiazine (CYS) were prepared and characterized using elemental analysis, IR, electronic spectroscopy, EPR spectroscopy, and thermal analysis. The mode of binding indicates that copper binds to MTS through carbonyl oxygen with the amino group nitrogen while for Cu^II–CYS the copper binds through carbonyl oxygen and SH with removal of its proton. The proposed structures were supported by conformational analysis which showed predominance of the trans form of copper(II)-L-cystinoylsulfadiazine. The two complexes enhanced oxidation of phenol and catechol in the presence of H₂O₂ under mild conditions. The catalyst shows proficiency toward oxidation of phenol and catechol compared to the auto-catalytic oxidation. Cu^II–MTS exhibited higher catalytic activity than Cu^II–CYS. The phenol and catechol oxidation is inhibited by Kojic acid.     

Copper Versus Thioether‐Centered Oxidation: Mechanistic Insights into the Non‐Innocent Redox Behavior of Tripodal Benzimidazolylaminothioether Ligands

A series of Cu⁺ complexes with ligands that feature varying numbers of benzimidazole/thioether donors and methylene or ethylene linkers between the central nitrogen atom and the thioether sulfur atoms have been spectroscopically and electrochemically characterized. Cyclic voltammetry measurements indicated that the highest Cu²⁺/Cu⁺ redox potentials correspond to sulfur‐rich coordination environments, with values decreasing as the thioether donors are replaced by nitrogen‐donating benzimidazoles. Both Cu²⁺ and Cu⁺ complexes were studied by DFT. Their electronic properties were determined by analyzing their frontier orbitals, relative energies, and the contributions to the orbitals involved in redox processes, which revealed that the HOMOs of the more sulfur‐rich copper complexes, particularly those with methylene linkers (? N? CH₂? S? ), show significant aromatic thioether character. Thus, the theoretically predicted initial oxidation at the sulfur atom of the methylene‐bridged ligands agrees with the experimentally determined oxidation waves in the voltammograms of the NS₃‐ and N₂S₂‐type ligands as being ligand‐based, as opposed to the copper‐based processes of the ethylene‐bridged Cu⁺ complexes. The electrochemical and theoretical results are consistent with our previously reported mechanistic proposal for Cu²⁺‐promoted oxidative C? S bond cleavage, which in this work resulted in the isolation and complete characterization (including by X‐ray crystallography) of the decomposition products of two ligands employed, further supporting the novel reactivity pathway invoked. The combined results raise the possibility that the reactions of copper–thioether complexes in chemical and biochemical systems occur with redox participation of the sulfur atom.