COPPER(I) COMPLEXES WITH N-(P-NITROPHENYL)-N′-ALKOXYCARBONYLTHIOUREA: SYNTHESES AND CRYSTAL STRUCTURES OF [Cu(H2net)2CI] · CH2CI2 AND [Cu(H2nmt)2CI]2·(CHCI3)2

Abstract

Two copper(I) complexes, [Cu(H₂net)₂Cl] · CH₂Cl₂ (1) and [Cu(H₂nmt)₂Cl]₂ · (CHCl₃)₂ (2), were synthesized by the reaction of CuCl₂ · 2H₂O with N-(p-nitrophenyl)-N′-(ethoxycarbonyl)-thiourea (H₂net) and N-(p-nitrophenyl)-N′-(methoxycarbonyl)-thiourea(H₂nmt), respectively. Both complexes crystallize in the monoclinic space group C2/c. For complex 1, a = 29.52(2), b=13.920(6), c = 14.873(3)Å; β= 101.75(2)°, V = 5984(4) ų, Z = 8 and R = 0.053; for complex 2, a = 30.68(1), b = 13.369(4), c = 14.226(7) Å, β = 99.52(4)°. V = 5754(4) ų, Z = 4 and R = 0.063. In complex 1, two H₂net molecules are bonded to Cu(I) atom through two S atoms forming a mononuclear complex with trigonal geometry for the Cu(I) ion [Cl(1)-Cu-S(1)=118.54(7), Cl(1)-Cu-S(2)=119.70(7), S(1)-Cu-S(2)=112.17(8)°, Cu-S(1) = 2.251(2), Cu-S(2) = 2.255(2), Cu-Cl(1) = 2.263(2) Å]. Complex 2 is a dimer formed by long Cu-S interactions [Cu-S* = 2.607(3) Å] from adjacent twc H₂nmt molecules; the Cu(I) ion has distorted tetrahedral coordination [Cl(1)-Cu-S(1) = 119.8(1), Cl(1)-Cu-S(2)=120.0(1), S(1)-Cu-S(2)=108.85(9)°] with unequal Cu-S [2.268(2), 2.247(2)Å] and Cu-Cl(1) [2.255(2)Å] bonds.     

On the Crystal Structure of K2Cu5Cl8(OH)4·2H2O

Single crystals of K₂Cu₅Cl₈(OH)₄·2H₂O were grown using hydrothermal techniques. The compound is monoclinic with a = 11.6424(11), b = 6.5639(4), c = 11.7710(10)Å, β = 91.09(1)°, V = 899.4(2)ų, space group P2₁/c, Z = 2. The crystal structure was determined using single crystal X‐ray diffraction data and refined to a residual of R(|F|) = 0.025 for 1208 independent observed reflections with I > 2σ(I). Two out of three crystallographically independent Cu atoms are coordinated to four near hydroxyl groups or chlorine atoms and two more distant Cl atoms, giving an octahedrally Jahn‐Teller distorted (4+2)‐configuration. For the remaining third copper cation a square‐planar coordination can be found. Edge‐sharing of the octahedra results in the formation of kagome‐type sheets parallel to (100). The octahedral layers are decorated on both sides by planar [Cu(OH)₂Cl₂]‐units around the third Cu atom. The K atoms are located between adjacent sheets and are surrounded by six Cl atoms as well as two water molecules. The coordination polyhedra about the K‐atoms can be described as distorted bicapped trigonal prisms. Additional linkage is provided by intra‐ as well as inter‐layer hydrogen bonds (O—H···Cl, O—H···O).     

Synthesis,Crystal Structure and DNA Cleavage Activity of a Ternary Copper(Ⅱ)Complex of Dipyrido[3,2-d:2',3'-f]-quinoxaline and Glycine

A ternary copper(II) complex, [Cu(Dpq)(Gly)(H₂O)]·NO₃·1.5H₂O (Dpq=dipyrido[3,2‐d:2′,3′‐f]quinoxaline, Gly=glycine), has been synthesized and structurally characterized. The complex crystallized in a triclinic system with space group P‐1, a=6.6979(9) Å, b=7.6351(10) Å, c=18.980(2) Å, α=85.815(2) °, β=88.182(2) °, γ=71.203(2) °. The five‐coordinate copper(II) center is a distorted square pyramid. Absorption spectra, fluorescence spectra and CD spectra showed that there were interactions between the copper complex and DNA through a groove binding mode. The complex exhibited efficient DNA cleavage activity at micromolar concentration in the presence of ascorbate with hydroxyl radicals as the active species.     

Determination of aqueous acid‐dissociation constants of aspartic acid using PCM/DFT method

Determination of acid‐dissociation constants, pK_a, of aspartic acid in aqueous solution, using density functional theory calculations combined with the conductor‐like polarizable continuum model (CPCM) and with integral‐equation‐formalism polarizable continuum model (IEFPCM) based on the UAKS and UAHF radii, was carried out. The computed pK_a values derived from the CPCM and IEFPCM with UAKS cavity model of bare structures of the B3LYP/6‐31+G(d,p)‐optimized tetrahydrated structures of aspartic acid species are mostly close to the experimental pK_a values. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

A new two-dimensional polymeric copper(II) complex bridged both by oxamidate and azide groups: synthesis,structure and DNA binding properties

A new two-dimensional polymeric copper(II) complex, [Cu₂(heae)(N₃)₂] _n , where heae stands for the dianion of N,N′-bis(N-hydroxyethylaminoethyl)oxamide, has been synthesized and characterized by elemental analysis, molar conductivity measurement, IR, electronic spectral studies and single-crystal X-ray diffraction. The compound crystallizes in the monoclinic system, P2₁/c space group with crystallographic data: a = 9.1588(18) Å, b = 6.6238(13) Å, c = 14.602(3) Å and Z = 2. The X-ray analysis reveals a two-dimensional copper(II) polymeric coordination network constructed by bis-tridentate chelated [Cu(trans-heae)Cu]²⁺ building blocks and end-on azido ligands. The environment around the copper(II) atom can be described as a square-based pyramid. The azido bridge is very asymmetric with one Cu–N bond distance short and the other long. The Cu ··· Cu separations through μ-trans-oxamidate and μ-azido bridges are 5.2996(13) Å and 4.2464(7) Å, respectively. The copper(II) complex is a polymer in the solid state, whereas in solution it exists as discrete neutral binuclear copper(II) species. Coordination mode of the azide in solution is proved by electronic spectra. The DNA-binding properties of the binuclear copper(II) species were investigated by emission spectral and electrochemical techniques, indicating the binuclear copper(II) complex binds to HS-DNA via a groove mode.     

Ternary copper(II) complex of 1,10-phenanthroline and L-glycine: crystal structure and interaction with DNA

A ternary copper(II) complex, [Cu(phen)(L-Gly)(H₂O)] ·?NO₃ ·?1.5H₂O (phen =?1,10-phenanthroline, L-Gly =?L-glycine), has been synthesized and structurally characterized. The complex crystallized in a monoclinic system with space group C2/c, a =?20.572(3) Å, b =?6.9987(10) Å, c =?23.561(3) Å, β?= 98.776(5)°. The five-coordinate copper(II) center is a distorted square pyramid. Absorption spectra, fluorescence spectra and viscosity measurements showed interaction between the copper complex and DNA through an intercalative mode. The complex exhibited efficient DNA cleavage activity at micromolar concentration in the presence of ascorbate with hydroxyl radicals as the active species.     

The structure of di-μ-chloro- bis - {chloro( N,N -diethylethane-1,2-diamine-κ 2)copper(II)}

Dichloro(N,N-diethyl-ethane-1,2-diamine)copper(II) has copper(II) ions in square pyramidal coordination. The two nitrogen atoms of the diamine {Cu–N_primary?=?1.979(3), Cu–N_tertiary?=?2.108(2)?Å} and two chloride ions are in the basal plane {Cu–Cl1?=?2.2680(9), Cu–Cl2?=?2.2989(8)?Å}. A centrosymmetrical dimer di-μ-chloro-bis{chloro(N,N-diethylethane-1,2-diamine-κ²)copper(II)}, C₆H1₆Cl₂CuN₂, is formed by axial coordination by Cl2, trans to the tertiary nitrogen, to a second copper(II) ion, with Cu?···?Cuⁱ?=?3.4855(9) and Cl2–Cuⁱ?=?2.7860(8)?Å. The dimer is also linked by H-bond N1–H?···?Cl1ⁱ.     

Synthesis and crystal structure of a new copper(II) binuclear complex bridged by the reduced derivative of a nitronyl nitroxide biradical

A new copper(II) binuclear complex, [Cu(hfac)₂]₂(IMH)₂Ph(OMe)₂ ((IMH)₂Ph(OMe)₂?=?2,5-dimethoxyl-1,4-bis(4′,4′,5′,5′-tetramethylimidazoline-3′-oxide)benzene, hfac?=?hexafluoroacetonate), bridged by a reduced derivative of a nitronyl nitroxide biradical has been synthesized and characterized by X-ray crystallography. The complex crystallizes in the monoclinic space group P2₁/n, with a?=?10.002(14), b?=?19.950(3), c?= 14.504(2) Å, β?=?108.842(3)° and Z?=?2. The structure refined to a final R value of 0.0644. The complex contains two copper(II) ions bridged by a reduced derivative of a nitronyl nitroxide biradical with a Cu?···?Cu separation of 8.430(2) Å.     

Crystal Structure and Thermal Behaviour of K2[CrF5·H2O]

K₂[CrF₅·H₂O] is monoclinic: a = 9.6835(3) Å, b = 7.7359(2) Å, c = 7.9564(3) Å, β = 95.94(1)°, Z = 4, space group C2/c (n^o 15). Its crystal structure was solved from its X‐ray powder pattern recorded on a powder diffractometer, using for the refinement the Rietveld method. It is built up from isolated octahedral [CrF₅·OH₂]^2? anions separated by potassium cations. The dehydration of K₂[CrF₅·H₂O] leads to anhydrous orthorhombic K₂CrF₅: a = 7.334(2) Å, b = 12.804(4) Å, c = 20.151(5) Å, Z = 16, space group Pbcn (n^o 60), isostructural with K₂FeF₅.     

Synthesis,DFT analysis and DNA studies,cytotoxicity and luminescence properties of a dinuclear copper(II) complex with 1,10-phenanthroline and 4-aminobenzoate

The dinuclear copper(II) complex, [Cu₂(phen)₂(4-aminobenzoate)₂(H₂O)₂](NO₃)₂·2(4-aminobenzoic acid)·3H₂O (phen = 1,10-phenanthroline), has been synthesized and structurally characterized by elemental analyses, IR, EPR, UV-visible and single-crystal X-ray crystallography. The complex crystallized in a monoclinic system with space group C2/c, a?=?26.0022(10) Å, b?=?10.2524(4) Å, c?=?20.9983(7) Å, α?=?90°, β?=?106.9550(10)° and γ?=?90°. The Cu(II) ion adopts a distorted square-pyramidal geometry formed by two N atoms from the phen ligand and two O atoms of the two 4-aminobenzoic acid ligands and one water O atom. The Cu…Cu separation is 3.0570(5) Å. A twofold axis passes through the midpoint of the Cu-Cu vector. The complex has intraligand (π–π*) fluorescence properties. The binding of this dinuclear copper(II) complex with calf thymus DNA (CT-DNA) was investigated by UV-vis absorption, fluorescence spectroscopic, cyclic voltammetric and viscosity techniques. Also, the cleavage of pBR322 DNA with dinuclear copper(II) complex was studied using gel electrophoresis method. The exhibited potent cytotoxic effects against human cell line (HepG2) and it was found to have good antimicrobial activities. The primary coordination sphere of dinuclear copper(II) complex is optimized, structural parameters are calculated and energy gaps of frontier orbital (HOMO-LUMO) have been calculated with B3LYP/6-31G/LANL2DZ level of theory in the gaseous phase. The calculated geometric and spectral results reproduced the experimental data with well agreement. Theoretical calculated molecular orbitals (HOMO-LUMO) and their energies have been calculated that suggest charge transfer occurs within the complex.     

Synthesis,crystal structure,and magnetic properties of a salt containing [Cu2Cl7]3− and 4-nitrobenzyl-4′-dimethylaminopyridinium

A new salt, [NO₂BzDMAP]₃[Cu₂Cl₇]?·?H₂O (1), has been synthesized, where [NO₂BzDMAP]⁺ is 1-(4′-nitrobenzyl)-4-dimethylaminopyridinium. Herein, the synthesis, spectral and structural characterization, and magnetic behavior of 1 are reported. It is orthorhombic, with space group Pca2₁, and a?=?26.639(2)?Å, b?=?9.638(1)?Å, and c?=?20.011(2)?Å with V?=?5137.5(7)?ų for Z?=?4. The anion shows a chloride-bridged binuclear structure with Cu?···?Cu distance of 3.872?Å; the two Cu(II) ions have a tetrahedral geometry. The cations stack through p?···?π and π?···?π interactions, and a complicated hydrogen-bonding network structure is formed through C–H?···?Cl and C–H?···?O hydrogen bonds. The variable temperature magnetic susceptibility measurements reveal that 1 exhibits strong antiferromagnetic interaction with J?=??193.0?cm^?1.     

π‐Complexes of Copper(I) with Terminal Alkynes. Synthesis and Crystal Structure of [(HC≡CCH2NH3)(Cu2Br3)] π‐Complex

Crystals of the zwitterionic copper(I) π‐complex [(HC≡CCH₂NH₃)Cu₂Br₃] have been synthesized by interaction of CuBr with [HC≡CCH₂NH₃]Br in aqueous solution (pH < 1) and X‐ray studied. The crystals are monoclinic: space group P2₁/n, a = 6.722(4), b = 12.818(8), c = 9.907(3) Å, β = 100.25(4)°, V = 840.0(8) ų, Z = 4, R = 0.0592 for 3015 reflections. The crystal structure of the π‐complex contains isolated [(HC≡CCH₂NH₃)⁺(Cu₂Br₃)^?]₂ units which are incorporated into a framework by strong hydrogen N–H···Br and C≡C–H···Br bonds. The length of π‐coordinated propargylammonium C≡C bond is equal 1.216(8) Å and Cu(I)–(C≡C) distance equals 1.958(5) Å.     

The salt–cocrystal spectrum in salicylic acid–adenine: the influence of crystal structure on proton‐transfer balance

At one extreme of the proton‐transfer spectrum in cocrystals, proton transfer is absent, whilst at the opposite extreme, in salts, the proton‐transfer process is complete. However, for acid–base pairs with a small ΔpK_a (pK_a of base ? pK_a of acid), prediction of the extent of proton transfer is not possible as there is a continuum between the salt and cocrystal ends. In this context, we attempt to illustrate that in these systems, in addition to ΔpK_a, the crystalline environment could change the extent of proton transfer. To this end, two compounds of salicylic acid (SaH) and adenine (Ad) have been prepared. Despite the same small ΔpK_a value (≈1.2), different ionization states are found. Both crystals, namely adeninium salicylate monohydrate, C₅H₆N₅⁺·C₇H₅O₃^?·H₂O, I , and adeninium salicylate–adenine–salicylic acid–water (1/2/1/2), C₅H₆N₅⁺·C₇H₅O₃^?·2C₅H₅N₅·C₇H₆O₃·2H₂O, II , have been characterized by single‐crystal X‐ray diffraction, IR spectroscopy and elemental analysis (C, H and N) techniques. In addition, the intermolecular hydrogen‐bonding interactions of compounds I and II have been investigated and quantified in detail on the basis of Hirshfeld surface analysis and fingerprint plots. Throughout the study, we use crystal engineering, which is based on modifications of the intermolecular interactions, thus offering a more comprehensive screening of the salt–cocrystal continuum in comparison with pure pK_a analysis.     

Dinuclear copper(II) complex and 1-D copper(II) complex with taurine Schiff base: synthesis,crystal structure,and properties

A dinuclear copper(II) compound, [Cu(btssb)(H₂O)]₂ · 4(H₂O) (1), and a 1-D chain copper(II) compound, [Cu(ctssb)(H₂O)] _n (2) [where H₂btssb is 2-[(5-bromo-2-hydroxy-benzylidene)-amino]-ethanesulfonic acid and H₂ctssb is 2-[(3,5-dichloro-2-hydroxy-benzylidene)-amino]-ethanesulfonic acid], were prepared and characterized. Compound 1 crystallizes in the monoclinic space group P2₁/c, with a = 10.109(2) Å, b = 20.473(4) Å, c = 6.803(1) Å, β = 100.32(3)°, V = 1385.1(5) ų, and Z = 2; R ₁ for 1796 observed reflections [I > 2σ(I)] was 0.0357. The geometry around each copper(II) can be described as slightly distorted square pyramidal. The Cu^II ··· Cu^II distance is 5.471(1) Å. Compound 1 formed a 1-D network through O–H ··· O hydrogen bonds and 1-D water chains exist. The 1-D chain complex 2 crystallizes in the triclinic space group P 1, with a = 5.030(2) Å, b = 7.725(2) Å, c = 17.011(5) Å, α = 92.706(4)°, β = 97.131(4)°, γ = 102.452(3)°, V = 638.6(3) ų, and Z = 2; R ₁ for 1897 observed reflections [I > 2σ(I)] was 0.0171. In 2, Cu(II) was also a slightly distorted square pyramid formed by two oxygens and one nitrogen from ctssb, one oxygen from another ctssb, and one water molecule. The complex formed a 1-D chain through O–S–O bridge of ctssb ligand. The 1-D chain further constructed a double chain through O?H ··· O hydrogen bonds.     

Estimation of pKa for organic oxyacids using calculated atomic charges

A method for the estimation of pK_a from empirically calculated atomic charges has been developed and tested on a diverse set of organic oxyacids. The approach involves a comparison of the atomic charges calculated for both the acid and the negative ion that is formed after loss of the acidic proton. These charges have been used in conjunction with the familiar concepts of induction and resonance to develop an accurate formula to predict pK_a. Results for a set of 135 compounds, including alcohols, phenols, and carboxylic acids, yielded a fit of pK_a with r = 0.993 and an rms error of 0.455. © John Wiley & Sons, Inc.     

Two‐dimensional square‐grid frameworks formed by self‐associating copper(II) complexes with 1‐(3‐pyridyl)‐ and 1‐(4‐pyridyl)‐substituted butane‐1,3‐diones

In bis­[1‐(3‐pyridyl)butane‐1,3‐dionato]copper(II) (the Cu atom occupies a centre of inversion), [Cu(C₉H₈NO₂)₂], (I), and bis­[1‐(4‐pyridyl)butane‐1,3‐dionato]copper(II) methanol solvate, [Cu(C₉H₈NO₂)₂]·CH₃OH, (II), the O,O′‐chelating diketonate ligands support square‐planar coordination of the metal ions [Cu—O = 1.948 (1)–1.965 (1) Å]. Weaker Cu⋯N inter­actions [2.405 (2)–2.499 (2) Å], at both axial sides, occur between symmetry‐related bis­(1‐pyridylbutane‐1,3‐dion­ato)copper(II) mol­ecules. This causes their self‐organization into two‐dimensional square‐grid frameworks, with uniform [6.48 Å for (I)] or alternating [4.72 and 6.66 Å for (II)] inter­layer separations. Guest methanol mol­ecules in (II) reside between the distal layers and form weak hydrogen bonds to coordinated O atoms [O⋯O = 3.018 (4) Å].     

Gas-phase structure and conformational properties of copper (I) pivalate dimer (CuC5H9O2)2

The molecular structure and conformational properties of gaseous dimer of copper (I) pivalate, Cu₂piv₂, have been studied by gas electron diffraction (GED) at 413(5) K and quantum chemical calculations (DFT and MP2). The molecule possesses a planar eight-membered skeleton. Two conformers, “staggered” of C _2h symmetry and “eclipsed” of C _2v symmetry, were found for Cu₂piv₂ in the gas phase. The following geometric parameters of the skeleton ring and the tert-butyl groups have been determined from the GED experiment for the “staggered” form: r_g(Cu···Cu) = 2.520(8) Å, r_g(Cu–O)_ave = 1.871(4) Å, r_g(C–O)_ave = 1.273(3) Å, r_g(C–C)_ring-tert = 1.531(4) Å, r_g(C–C)_{tert-out-of-plane-ring} = 1.536(4) Å, r_g(C–C)_{tert-in-the-plane-ring} = 1.527(4) Å, r_g(C–H)_ave = 1.087(5) Å, (O–Cu–O) = 172.12°(3). Computations predict the internal rotation of the tert-butyl groups to be independent. The value of calculated Wiberg bond index for Cu···Cu testifies the existence of weak bonding between two copper atoms.

     

Synthesis,Crystal Structure and Spectroscopic Properties of [2,13‐Bis(1‐naphthalenylmethyl)‐5,16‐dimethyl‐2,6,13,17‐tetraazatricyclo(14,4,01.18,07.12)docosane]copper(II) Diperchlorate Acetonitrile Disolvate

The N‐functionalized macrocyclic ligand 2,13‐bis(1‐naphthalenylmethyl)‐5,16‐dimethyl‐2,6,13,17‐tetraazatricyclo(14,4,0^1.18,0^7.12)docosane (L³) and its copper(II) complex were prepared. The crystal structure of [Cu(L³)](ClO₄)₂·2CH₃CN was determined by single‐crystal X‐ray diffraction at 150 K. The copper atom, which lies on an inversion centre, has a square planar arrangement and the complex adopts a stable trans‐III configuration. The longer distance [2.081(2) Å] for Cu–N(tertiary) compared to 2.030(3) Å for Cu–N(secondary) may be due to the steric effect of the attached naphthalenylmethyl group on the tertiary nitrogen atom. Two perchlorate ions are weakly attached to copper in axial sites and are further connected to the ligand of the cation through NH ··· O hydrogen bonds [N ··· O 3.098 Å]. IR and UV/Vis spectroscopic properties are also described.     

Examining the impact of ancillary ligand basicity on copper(I)–ethylene binding interactions: a DFT study

A theoretical investigation at the density functional theory level (B3LYP) has been conducted to elucidate the impact of ligand basicity on the binding interactions between ethylene and copper(I) ions in [Cu(η ²-C₂H₄)]⁺ and a series of [Cu(L)(η ²-C₂H₄)]⁺ complexes, where L = substituted 1,10-phenanthroline ligands. Molecular orbital analysis shows that binding in [Cu(η ²-C₂H₄)]⁺ primarily involves interaction between the filled ethylene π-bonding orbital and the empty Cu(4s) and Cu(4p) orbitals, with less interaction observed between the low energy Cu(3d) orbitals and the empty ethylene π*-orbital. The presence of electron-donating ligands in the [Cu(L)(η ²-C₂H₄)]⁺ complexes destabilizes the predominantly Cu(3d)-character filled frontier orbital of the [Cu(L)]⁺ fragment, promoting better overlap with the vacant ethylene π*-orbital and increasing Cu → ethylene π-backbonding. Moreover, the energy of the filled [Cu(L)]⁺ frontier orbital and mixing with the ethylene π*-orbital increase with increasing pK _a of the 1,10-phenanthroline ligand. Natural bond orbital analysis reveals an increase in Cu → ethylene electron donation with addition of ligands to [Cu(η ²-C₂H₄)]⁺ and an increase in backbonding with increasing ligand pK _a in the [Cu(L)(η ²-C₂H₄)]⁺ complexes. Energy decomposition analysis (ALMO-EDA) calculations show that, while Cu → ethylene charge transfer (CT) increases with more basic ligands, ethylene → Cu CT and non-CT frozen density and polarization effects become less favorable, yielding little change in copper(I)–ethylene binding energy with ligand pK _a. ALMO-EDA calculations on related [Cu(L)(NCCH₃)]⁺ complexes and calculated free energy changes for the displacement of acetonitrile by ethylene reveal a direct correlation between increasing ligand pK _a and the favorability of ethylene binding, consistent with experimental observations.     

Crystal structure and properties of a Cu(II) complex with the tridentate Schiff-base ligand,paeonol-(2-aminoethylpiperazine)

A new copper Schiff-base complex [Cu(C₁₅H₂₃N₃O₂)(NO₃)]NO₃ was synthesized and characterized by spectroscopic (IR), thermal (TG/DTA) and electrochemical methods. Single crystal X-ray diffraction revealed that the complex is monoclinic, space group P2(1)/c, with a = 23.549(2), b = 6.583(12), c = 12.765(15) Å, β = 102.823(2)°, V = 1929.4(5) ų, Z = 4, D _calcd = 1.601 Mg m^?3, F(000) = 964, goodness-of-fit = 1.034 and R ₁ = 0.0407. The Cu(II) is four-coordinate with distorted square planar geometry. The structure consists of isolated [Cu(C₁₅H₂₃N₃O₂)(NO₃)]⁺ and a nitrate, which are further linked to a 3D network by significant N–H ··· O, N–H ··· N hydrogen bonds and weak O ··· Cu interaction.