Mixed-valence Copper Research Articles

A computational exploration of CO2 reduction via CO dimerization on mixed-valence copper oxide surface.

The catalytic role of Cu ions in CO2 reduction on oxide-derived Cu has been elusive. In the presence of oxygen vacancy, COCO dimerization is predicted to be thermodynamically favorable with an accessible barrier on Cu4O3(202). The material's mixed valency is responsible for stabilizing the charge-separated (OC)δ+(CO)δ- intermediate.

Syntheses and crystal structures of two heterometallic iodoplumbates containing mixed-valence copper(I/II).

Mixed-valence copper(I/II) atoms have been introduced successfully into a Pb/I skeleton to obtain two heterometallic iodoplumbates, namely poly[bis(tetra-n-butylammonium) [bis(μ3-dimethyldithiocarbamato)dodeca-μ3-iodido-hexa-μ2-iodido-tetracopper(I)copper(II)hexalead(II)]], {(C16H36N)2[Cu4ICuIIPb6(C3H6NS2)2I18]}n, (I), and poly[[μ3-iodido-tri-μ2-iodido-iodido[bis(1,10-phenanthroline)copper(I)]copper(I)copper(II)lead(II)] hemiiodine], {[CuICuIIPbI5(C12H8N2)2]·0.5I2}n, (II), under solution and solvothermal conditions, respectively. Compound (I) contains two-dimensional anionic layers, which are built upon the linkages of CuII(S2CNMe2)2 units and one-dimensional anionic Pb/I/CuI chains. Tetra-n-butylammonium cations are located between the anionic layers and connected to them via C-H...I hydrogen-bonding interactions. Compound (II) exhibits a one-dimensional neutral structure, which is composed of [PbI5] square pyramids, [CuII4] tetrahedra and [CuIIN4I] trigonal bipyramids. Face-to-face aromatic π-π stacking interactions between adjacent 1,10-phenanthroline ligands stabilize the structure and assemble compound (II) into a three-dimensional supramolecular structure. I2 molecules lie in the voids of the structure.

Synthesis and characterization of a multifunctional inorganic–organic hybrid mixed-valence copper(I/II) coordination polymer: {[CuCN][Cu(isonic)2]}n

A new multifunctional mixed-valence copper(I/II) coordination polymer, {[CuCN][Cu(isonic)2]}n(1) (Hisonic = isonicotinic acid), was synthesized by treating isonicotinic acid and 5-amino-tetrazolate (Hatz = 5-amino-tetrazolate) with copper(II) salts under hydrothermal conditions, and characterized by elemental analysis, infrared spectroscopy, and single crystal X-ray diffraction, respectively. The X-ray diffraction analysis reveals that compound exhibit noncentrosymmetric polar packing arrangement. It is three-dimensional (3D) framework with (3,5)-connected ‘seh-3’ topological network constructed from metal organic framework {[Cu(isonic)2]}n and the inorganic linear chain{Cu(CN)}n subunits. A remarkable feature of 1 is the rhombic open channels that are occupied by a linear chain of {Cu(CN)}n. Impressively compound 1 displays not only a second harmonic generation (SHG) response, but also a ferroelectric behavior and magnetic properties.

Cu6 (NGuaS)6 ]2+ and its oxidized and reduced derivatives: Confining electrons on a torus.

The hexanuclear thioguanidine mixed-valent copper complex cation [Cu6 (NGuaS)6 ]+2 (NGuaS = o-SC6 H4 NC(NMe2 )2 ) and its oxidized/reduced states are theoretically analyzed by means of density functional theory (DFT) (TPSSh + D3BJ/def2-TZV (p)). A detailed bonding analysis using overlap populations is performed. We find that a delocalized Cu-based ring orbital serves as an acceptor for donated S p electrons. The formed fully delocalized orbitals give rise to a confined electron cloud within the Cu6 S6 cage which becomes larger on reduction. The resulting strong electrostatic repulsion might prevent the fully reduced state. Experimental UV/Vis spectra are explained using time-dependent density functional theory (TD-DFT) and analyzed with a natural transition orbital analysis. The spectra are dominated by MLCTs within the Cu6 S6 core over a wide range but LMCTs are also found. The experimental redshift of the reduced low energy absorption band can be explained by the clustering of the frontier orbitals. © 2017 Wiley Periodicals, Inc.

Characterization of a Dinuclear Copper(II) Complex and Its Fleeting Mixed-Valent Copper(II)/Copper(III) Counterpart.

The synthesis of a dinuclear copper(II) complex, supported by a 1,3-diamino-2-propanol-based tetraamide ligand, is reported. Structural properties in the solid state and in solution, by means of XRD analysis and NMR spectroscopy, respectively, provide evidence of a highly flexible complex that can display several conformations, leading to the image of the wings of a butterfly. The complex was fully characterized and the redox properties were investigated. Room-temperature spectro-electrochemistry was used to monitor the formation of a metastable mono-oxidized product that displayed an absorption band centered at λ=463 nm. EPR investigation of the low-temperature, chemically generated, mono-oxidized product reveals the presence of an intermediate described as a mixed-valent CuII CuIII species, which is a model of the possible highly oxidizing intermediate in particulate methane monooxygenase.

Crystal structures of two mixed-valence copper cyanide complexes with N-methyl-ethylenedi-amine.

The crystal structures of two mixed-valence copper cyanide compounds involving N-methyl-ethylenedi-amine (meen), are described. In compound (I), poly[bis(μ3-cyanido-κ3C:C:N)tris(μ2-cyanido-κ2C:N)bis(N-methylethane-1,2-di-amine-κ2N,N')tricopper(I)copper(II)], [Cu4(CN)5(C3H10N2)2] or Cu4(CN)5meen2, cyanide groups link CuI atoms into a three-dimensional network containing open channels parallel to the b axis. In the network, two tetra-hedrally bound CuI atoms are bonded by the C atoms of two end-on bridging CN groups to form Cu2(CN)6 moieties with the Cu atoms in close contact at 2.560 (1) Å. Other trigonally bound CuI atoms link these units together to form the network. The CuII atoms, coordinated by two meen units, are covalently linked to the network via a cyanide bridge, and project into the open network channels. In the mol-ecular compound (II), [(N-methylethylenediamine-κ2N,N')copper(II)]-μ2-cyanido-κ2C:N-[bis(cyanido-κC)copper(I)] monohydrate, [Cu2(CN)3(C3H10N2)2]·H2O or Cu2(CN)3meen2·H2O, a CN group connects a CuII atom coordinated by two meen groups with a trigonal-planar CuI atom coordinated by CN groups. The mol-ecules are linked into centrosymmetric dimers via hydrogen bonds to two water mol-ecules. In both compounds, the bridging cyanide between the CuII and CuI atoms has the N atom bonded to CuII and the C atom bonded to CuI, and the CuII atoms are in a square-pyramidal coordination.

Mixed valence copper-sulfur clusters of highest nuclearity: a Cu8 wheel and a Cu16 nanoball.

Fully spin delocalized mixed valence copper-sulfur clusters, 1 and 2, supported by μ4-sulfido and NSthiol donor ligands are synthesized and characterized. Wheel shaped 1 consists of Cu2S2 units. The unprecedented nanoball 2 can be described as S@Cu4(tetrahedron)@O6(octahedron)@Cu12S12(cage) consisting of both Cu2S2 and (μ4-S)Cu4 units. The Cu2S2 and (μ4-S)Cu4 units resemble biological CuA and CuZ sites respectively.

Synthesis and Characterization of Copper Complexes with CuICuI, Cu1.5Cu1.5m and CuIICuII Core Structures Supported by a Flexible Dipyridylamide Ligand.

A series of copper complexes supported by a simple dipyridylamide ligand (H2pcp) were isolated and characterized. Treatment of H2pcp with NaH and copper(I) salts led to the formation of [Cu2(2pcp)2] (1a) and {Na[(Cu2(2pcp)2)2]PF6}n (1b). The X-ray crystal structures of both complexes feature CuICuI cores with close Cu···Cu interactions. Electrochemical studies of 1a showed a reversible one-electron oxidation wave in CH2Cl2. On the basis of the work on 1a, we began studying the mixed-valence copper species supported by this ligand. The reaction of H2pcp with Cu(OAc)2 and CuCl in different stoichiometries yielded [Cu2(2pcp)2Cl] (2) and [Cu3(2pcp)2Cl2] (3). X-ray crystallography and spectroscopic characterization suggested delocalized Cu1.5Cu1.5 core structures of both compounds. These results further inspired us to explore the coordination properties of H2pcp toward CuII ions. The complexes [HNEt3][Cu2(2pcp)3(ClO4)](ClO4) (4a), [Cu2(2pcp)3(NO3)] (4b), and [Cu2(2pcp)3(H2O)]BF4 (4c) featuring dinuclear CuIICuII cores were prepared and characterized by X-ray crystallography and spectroscopic methods. Structural analysis of these complexes implied that the accommodation of CuICuI, Cu1.5Cu1.5, and CuIICuII is attributed to the structural flexibility of the ligand H2pcp. Complexes 1a, 2, 3, and 4a were examined by X-ray photoelectron spectroscopy, which confirmed the oxidation state assignments. Computational studies were also performed to provide insight into the electronic structures of these complexes.

Porous layered and open-framework mixed-valence copper tellurites

|Cu+Cl3|[Cu2+2(TeO3)] (1), |Сu+1.7Cl3.8|[Cu2+4O(TeO3)2] (2) and Tl+2[Cu2+2Te6+Te4+6O18] (3) were obtained by CVT and hydrothermal methods in CuCl-CuCl2-TeO2 and Tl2CO3-CuO-TeO2 systems. 1 demonstrates layered topology with pores (1×0.65nm), whereas 2 has open-framework structural architecture with two-dimensional system of channels (1.16×0.74nm). Channels in open-framework of 3 are occupied by Tl+ cations. ‘Host-guest’ structural organization of 1 and 2 with host Cu2+-tellurite units of different dimensionality formed by oxocentered OCu4 tetrahedra and OCu2Te triangles and guest Cu+-chloride species is the result of formation from gases in CVT reactions. Oxocentered units determine basic topologies of the structures of 1 and 2 and influence their stability and properties. [Te6+Te4+6O18]6− polytellurite-tellurate framework in 3 can be represented as consisting of Kagome-like layers.

Synthesis and structural characterization of a simple Cu(I) and an unexpected mixed-valence copper(I/II) complex with a supporting phosphinoferrocene amine ligand

1′-(Aminomethyl)-1-(diphenylphosphino)ferrocene (1) reacts with [Cu(MeCN)4][BF4] in a 2:1 molar ratio to afford the chelate complex [Cu(1-κ2N,P)2][BF4] (2). A similar reaction of 1 with copper(I) chloride furnished an unexpected mixed-valence Cu(I)/Cu(II) octacopper complex, [Cu4(μ(P,N)-1)2(μ-Cl)5Cl(1H-κP)(H2O)]2 (3), comprising P,N-coordinated 1 and chloride ions as the bridging ligands, and N-protonated phosphinoamine 1H+ as a monodentate phosphine donor. The structures of 2·AcOEt and 3·10CHCl3 were determined by single-crystal X-ray diffraction analysis, and the redox behavior of 2 was examined by voltammetric techniques.

Theoretical study on the degree of delocalization of unpaired spin in two mixed valence copper(II/I) complexes with isomeric chelating diamines and iodide

Two new mixed valence copper(II)–copper(I) complexes, [Cu(L1)2CuI3] (1) and [Cu(L2)2]2 [Cu(L2)2(Cu2I5)2]·2CH3CN (2) where L1=N,N-dimethylethane-1,2-diamine and L2=N-ethylethane-1,2-diamine, have been synthesized using isomeric chelating diamines and iodide. Complexes are characterized by elemental analyses, IR and UV–Vis spectroscopy. Single crystal X-ray diffraction studies confirm their structures. The degree of delocalization of the unpaired spin has been investigated by computing the spin density using the crystallographic coordinates. Interestingly, it can be observed from that the unpaired electron is delocalized between both copper centers in complex 1, whereas complex 2 is a completely localized mixed valence complex.

Demonstrating the Presence of Cyanide in Bitter Seeds while Helping Students Visualize Metal–Cyanide Reduction and Formation in a Copper Complex Reaction

An activity for hydrogen cyanide detection in bitter seeds is presented. The procedure is based on the absorption of hydrogen cyanide (HCN) from a mixture of chopped seeds into a ninhydrin or tetramminecopper(II) complex solution followed by their reaction. A qualitative experiment is proposed in order to demonstrate the presence of HCN in bitter seeds and to visualize metal–cyanide reduction and formation of an interesting mixed valence copper amino–cyanide complex: Cu3(NH3)4(CN)4.

Mixed-valence copper(I,II) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines: from ionic structures to coordination polymers.

Two pyrimidine-based ligands, 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-(morpholino)pyrimidine () and 4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-phenoxypyrimidine (), and a series of mixed-valence copper(i,ii) halide complexes, [Cu(L(2))2Br]2[Cu2Br4] (), [Cu(L(2))2Cl][CuCl2] (), and [Cu2L(3)Br3]n (), have been synthesized. The complex [Cu(L(2))2Br]2[Cu2Br4] was prepared by the reaction of with CuBr2 in a 1 : 1 molar ratio in MeCN. Its chlorido-analogue, the complex [Cu(L(2))2Cl][CuCl2], was synthesized by the reaction between , CuCl2 and CuCl in a 2 : 1 : 1 molar ratio in MeCN. The ligand acts as a chelating one. In the structures of the complexes [Cu(L(2))2Br]2[Cu2Br4] and [Cu(L(2))2Cl][CuCl2] the Cu(2+) ion is in the cationic part of the complex whereas the Cu(+) ion is located in the anionic part. The best way to synthesize the mixed-valence 1D coordination polymer [Cu2L(3)Br3]n is to react CuBr2 with in a 2 : 1 molar ratio in the MeCN/CHCl3 mixture on heating. In the structure of [Cu2L(3)Br3]n the ligand shows chelating/bridging tridentate coordination. This is the first example of the tridentate coordination of 4-(1H-pyrazol-1-yl)-6-R-pyrimidines. The striking difference between the coordination behavior of and (chelating bidentate vs. chelating/bridging coordination) is related with the possibility of rotation of the 6-phenoxy group around the C-O bond which makes the N(1) pyrimidine atom less sterically hindered, enabling it to participate in metal ion binding. Importantly, all copper ions in [Cu2L(3)Br3]n show similar tetrahedral environments, CuNBr3 and CuN2Br2, which is extremely rare for mixed-valence copper(i,ii) compounds. The ligands and show blue emission which is quenched upon their coordination to copper ions. The 1D coordination polymer [Cu2L(3)Br3]n shows high thermal stability and unusual solvent-occlusion properties. The role of the substituents favoring the formation of the mixed-valence copper(i,ii) complexes with 4-(1H-pyrazol-1-yl)-6-R-pyrimidines is discussed.

Ligand-oriented assembly of a porous metal–organic framework by [CuI4I4] clusters and paddle-wheel [CuII2(COO)4(H2O)2] subunits

A novel porous mixed-valence copper–organic framework based on CuI4I4 clusters and paddle-wheel [CuII2(COO)4(H2O)2] moieties was synthesized through a ligand-oriented assembly. The mixed-valence Cu(I)–Cu(II) complex features the coexistence of micro-sized cages and 1D channels and its sorption behaviour has been investigated.

A rare polyniobotungstate-based framework and its structural transformation in a single-crystal-to-single-crystal process induced by iodide ions

A rare polyniobotungstate-based framework constructed from Lindqvist [Nb3W3O19]5− polyanions and mixed-valent copper ions has been made, which can undergo a solid–liquid reaction with KI solution to form a copper halide via a single-crystal-to-single-crystal process.

Cyanide-bridged mixed-valence copper(II/I) coordination polymers: Unique 7-connected sev-type 3D network versus anionic 2D host network encapsulated with cationic complex

Reactions of CuCl2, K2[Ni(CN)4]/K3[Co(CN)6], and 5-amino-1H-tetrazole led to cyanide-bridged mixed-valence Cu(II/I) complexes, [Cu(H2O)3Cu3(CN)5] (1) and [Cu(H2O)4][Cu4(CN)6] (2).The uncommon semi-closed {Cu2(CN)3} dimmers in 1 are bridged by μ3-C,C,N cyanide ligands to afford rare 1D centipedal-like chain motifs, which are further bridged by μ2-C,N and metalloligand “CN–Cu(4)–CN” to finish sophisticated zeolite-like 3-D structure. Topologically, 1 is the first example of uninodal 7-connected sev-like cyanide-bridged network. Compound 2 consists of two-dimensional honeycomb-related anionic layers [Cu4(CN)6]2−, within which the [Cu(H2O)4]2+ cations are intercalated into alternate interlamellar space. Compound 1 shows antiferromagnetic couple arising from pure ground-state configuration of Cu(II) mediated by diamagnetic bridges –CN–CuI–CN−. The inclusion compound 2 shows blue photoluminescence originated from Cu(I) center to the unoccupied π* orbital of the cyanide ligand (MLCT) charge transfer.

Active Sites in Copper-Based Metal–Organic Frameworks: Understanding Substrate Dynamics, Redox Processes, and Valence-Band Structure

We have developed an integrated approach that combines synthesis, X-ray photoelectron spectroscopy (XPS) studies, and theoretical calculations for the investigation of active unsaturated metal sites (UMS) in copper-based metal–organic frameworks (MOFs). Specifically, extensive reduction of Cu+2 to Cu+1 at the MOF metal nodes was achieved. Introduction of mixed valence copper sites resulted in significant changes in the valence band structure and an increased density of states near the Fermi edge, thereby altering the electronic properties of the copper-based framework. The development of mixed-valence MOFs also allowed tuning of selective adsorbate binding as a function of the UMS oxidation state. The presented studies could significantly impact the use of MOFs for heterogeneous catalysis and gas purification as well as foreshadow a new avenue for controlling the conductivity of typically insulating MOF materials.

Di- and Trinuclear Mixed-Valence Copper Amidinate Complexes from Reduction of Iodine.

Molecular examples of mixed-valence copper complexes through chemical oxidation are rare but invoked in the mechanism of substrate activation, especially oxygen, in copper-containing enzymes. To examine the cooperative chemistry between two metals in close proximity to each other we began studying the reactivity of a dinuclear Cu(I) amidinate complex. The reaction of [(2,6-Me2C6H3N)2C(H)]2Cu2, 1, with I2 in tetrahydrofuran (THF), CH3CN, and toluene affords three new mixed-valence copper complexes [(2,6-Me2C6H3N)2C(H)]2Cu2(μ2-I3)(THF)2, 2, [(2,6-Me2C6H3N)2C(H)]2Cu2(μ2-I) (NCMe)2, 3, and [(2,6-Me2C6H3N)2C(H)]3Cu3(μ3-I)2, 4, respectively. The first two compounds were characterized by UV-vis and electron paramagnetic resonance spectroscopies, and their molecular structure was determined by X-ray crystallography. Both di- and trinuclear mixed-valence intermediates were characterized for the reaction of compound 1 to compound 4, and the molecular structure of 4 was determined by X-ray crystallography. The electronic structure of each of these complexes was also investigated using density functional theory.

Mechanism of microwave-induced carbothermic reduction and catalytic performance of Cu/activated carbon catalysts in the oxidative carbonylation of methanol

Activated carbon–supported copper catalysts (Cu/AC) were prepared by impregnation of AC support with aqueous copper (II) nitrate solutions and subsequent microwave carbothermic reduction under different irradiation conditions. This was done to investigate the effect of the valence of copper on the vapor-phase oxidative carbonylation of methanol to dimethyl carbonate (DMC). Thermogravimetric analysis–mass spectrometry were carried out to explore the mechanism of carbothermic reduction in catalyst preparation. X-ray diffractometry, hydrogen temperature-programmed reduction, X-ray photoelectron spectroscopy, and scanning electron microscopy were employed to examine the bulk and surface properties of the Cu/AC catalysts. Microwave irradiation of the catalyst precursors rapidly induced a series of successive reactions, namely decomposition of Cu2(OH)3NO3 into CuO, reduction in CuO to Cu2O, and carbothermic reduction in Cu2O to Cu0. These reactions resulted in mixed-valence copper species, CuO, Cu2O, and Cu0, in the Cu/AC catalysts. CuO, Cu2O, and Cu0 were active in DMC synthesis, but the catalytic performance of Cu/AC was highly dependent on the Cu0 concentration. High irradiation temperatures resulted in higher Cu0 concentrations because of enhanced carbothermic reduction, and fast heating rates improved the degree of dispersion of Cu2O and Cu0. The catalyst irradiated at 540 °C at a heating rate of 48 °C min−1 showed the highest catalytic activity in the oxidative carbonylation of methanol.

Ferromagnetic copper(II)–copper(II) interaction in a single chlorido and dicyanamido bridged mixed–valence copper(I/II) 2-D polymer generated by in situ partial reduction of copper(II)

A mixed-valence 2-D Cu(I/II) complex, [{Cu(II)(dmen)(μ-Cl)(μ1,5-dca)}{Cu(I)(μ1,5-dca)}]n (1) (dmen = N,N-dimethylethylenediamine, dca = dicyanamide = [N(CN)2]−), has been synthesized by in situ partial reduction of Cu(II) to Cu(I) using benzoin (2-hydroxy-1,2-di(phenyl)ethanone) as reductant. Complex 1 was characterized by spectroscopic techniques, single crystal X-ray diffraction, and low temperature magnetic measurements. Structural investigation reveals that 1 represents a mixed-valence 2-D coordination polymer formed by parallel 1-D [Cu(II)(dmen)(Cl)Cu(I)(μ1,5-dca)2]n chains running along the b axis, where chloride bridges Cu(II) ions of adjacent polymers through long connections (2.8401(1) Å) to form a 2-D network. The metal centers have two different geometrical environments (distorted square pyramidal and distorted trigonal planar geometries for Cu(II) and Cu(I), respectively). The Cu(II) ions in [Cu(II)(dmen)(μ-Cl)(dca)]n are interconnected through single chloride bridges while within the [Cu(I)(μ1,5-dca)]n units, the dca connects adjacent Cu(I) ions through μ1,5-dca bridges. Magnetic susceptibility measurements reveal weak ferromagnetic interactions (J = +0.3 cm−1) within the chlorido-bridged Cu(II) regular chain present in 1. Simultaneous presence of μ1,5-dca and single chlorido bridges with ferromagnetic coupling is believed to be unique in mixed-valence Cu(I)/Cu(II) complexes.