The synthesis and characterisation of phenolate complexes of Cu(II) and Ni(II) that are capable of supporting a phenoxyl radical ligand

New Cu^II and Ni^II complexes of potentially tridentate N₂O Schiff base ligands 1 and 2 have been synthesised and characterised. [Cu(2)(OH₂)]⁺ possesses a square planar geometry in the solid state whereas [Ni(1)₂] possesses a distorted octahedral geometry in which the amine donors of 1 coordinate weakly to the Ni^II centre. EPR spectroscopy demonstrates that the N₂O₂ coordination sphere of [Cu(2)(OH₂)]⁺ is retained in CH₂Cl₂ solution. [Cu(2)(OH₂)]⁺ exhibits a reversible one electron oxidation at E_1/2 = 0.54 V versus [Fc]⁺/[Fc], the product of which has been characterised by UV-Vis absorption and EPR spectroscopies. The spectroscopic signature of the oxidised product is consistent with the formation of a stable phenoxyl radical ligand bound to a Cu^II centre. [Ni(1)₂] possesses a reversible metal-based oxidation process at E_1/2 = 0.03 V versus [Fc]⁺/[Fc] and a further oxidation, attributed to the generation of a phenoxyl radical centre, at  = 0.44 V versus [Fc]⁺/[Fc]. UV-Vis absorption and EPR spectroscopic studies indicate that the lower potential process is a formal Ni^III/II couple. In contrast, the pro-ligands 1H and 2H exhibit chemically irreversible oxidation processes at  = 0.42 and 0.40 V versus Fc⁺/Fc, respectively, and do not support the formation of stable phenoxyl radical species.     

Squarato-metal(II) complexes. 1: Structural and magnetic characterization of squarato-bridged dinuclear nickel(II) and copper(II) complexes

A new series of dinuclear squarato-bridged nickel(II) and copper(II) complexes [Ni₂(2,3,2-tet)₂(μ_1,3-C₄O₄)(H₂O)₂](ClO₄)₂ (1), [Ni₂(aepn)₂(μ_1,3-C₄O₄)(H₂O)₂](ClO₄)₂ (2), [Cu₂(pmedien)₂(μ_1,3-C₄O₄)(H₂O)₂](ClO₄)₂.4H₂O (3) and [Cu₂(DPA)₂(μ_1,2-C₄O₄)(H₂O)₂](ClO₄)₂ (4) where is the dianion of 3,4-dihydroxycyclobut-3-en-1,2-dione (squaric acid), 2,3,2-tet = 1,4,8,11-tetraazaundecane, aepn = N-(2-aminoethyl)-1,3-propanediamine, pmedien = N,N,N′,N″,N″-pentamethyldiethylenetriamine and DPA = di(2-pyridylmethyl)amine were synthesized and structurally characterized by X-ray crystallography. The spectral and structural characterization as well as the magnetic behaviour of these complexes is reported. In this series, structures consist of the groups as counter ions and the bridging the two M(II) centers in a μ-1,3- (1-3) and in a μ-1,2-bis(monodentate) (4) bonding fashions. The coordination geometry around the Ni(II) ions in 1 and 2 is six-coordinate with distorted octahedral environment achieved by N atoms of the amines and by one or two oxygen atoms from coordinated water molecules, respectively. In the Cu(II) complexes 3 and 4, a distorted square pyramidal geometry is achieved by the three N-atoms of the aepn or DPA and by an oxygen atom from a coordinated water molecule. The electronic spectra of the complexes in aqueous solutions are in complete agreement with the assigned X-ray geometry around the M(II) centers. The complexes show weak antiferromagnetic coupling with ∣J∣ = 1.8-4.2 cm⁻¹ in the μ-1,3- bridged squarato compounds 1-3, and J = −16.1 cm⁻¹ in the corresponding μ-1,2- bridged squarato complex 4. The magnetic properties are discussed in relation to the structural data.     

Copper(II) complexes of salicylaldehyde hydrazones: synthesis, structure, and DNA interaction

Three hydrazone ligands, H2L1-H2L3, made from salicylaldehyde and ibuprofen- or naproxen-derived hydrazides, were prepared and transformed into the corresponding copper(II) complexes [Cu(II)L1] x H2O, [Cu(II)L2], and [(Cu(II))2(L3)2] x H2O x DMF (Scheme). The X-ray crystal structure of the last-mentioned complex was solved (Fig. 1), showing a square-planar complexation geometry, and the single units were found to form a one-dimensional chain structure (Fig. 2). The interactions of these complexes with CT-DNA were studied by different techniques, indicating that they all bind to DNA by classical and/or non-classical intercalation modes.     

Mixed-ligand copper(II)-phenanthroline-dipeptide complexes: synthesis, characterization, and DNA-cleavage properties

The mixed-ligand complexes [Cu(II)(HisLeu)(phen)](+) (1) and [Cu(II)(HisSer)(phen)](+) (2; phen=1,10-phenanthroline) were synthesized and characterized. The intercalative interaction of the Cu(II) complexes with calf-thymus DNA (CT-DNA) was probed by UV/VIS and fluorescence titration, as well as by thermal-denaturation experiments, and the intrinsic binding constants (K(b)) for the complexes with 1 and 2 were 4.2x10(3) and 4.9x10(3) M(-1), resp. Both complexes were found to be efficient catalysts for the hydrolytic cleavage of plasmid pUC19 DNA, as tested by gel electrophoresis, converting the DNA from the supercoiled to the nicked-circular form at rate constants of 1.32 and 1.40 h(-1) for 1 and 2, resp.     

Copper(II) complexes as superoxide dismutase mimics: Synthesis, characterization, crystal structure and bioactivity of copper(II) complexes

Two new copper(II) complexes of the type [Cu(L)X₂), where L = (E)-N-phenyl-2-[phenyl (pyridine-2-yl)methylene]hydrazinecarboxamide X = Cl/Br have been synthesized and characterized by elemental analyses, FAB (fast atomic bombardment) magnetic measurements, electronic absorption, conductivity measurements cyclic voltammetry (CV) and Electron paramagnetic resonance (epr) spectroscopy. The structures of these complexes determined by single crystal X-ray crystallography show a distorted square based pyramidal (DSBP) geometry around copper(II) metal center. The distorted CuN₂OX (X = Cl/Br) basal plane in them is comprised of two nitrogen and one oxygen atoms of the meridionally coordinated ligand and a chloride or bromide ion and axial position is occupied by other halide ion. The epr spectra of these complexes in frozen solutions of DMSO showed a signal at g ca. 2. The trend in g-value (g_|| > g_⊥ > 2.00) suggest that the unpaired electron on copper(II) has d_x²-y² character. Biological activities in terms of superoxide dismutase (SOD) and antimicrobial properties of copper(II) complexes have also been measured. The superoxide dismutase activity reveals that these two complexes catalyze the fast disproportionation of superoxide in DMSO solution.     

Copper(II) and palladium(II) complexes of a terdentate pyrrolidine diester ligand

The synthesis and characterisation of three copper(II) complexes, [CuLCl] (4), [CuLBr] (5) and [CuL(NCS)] (6), and a palladium(II) complex, [PdLCl] (7), of the monoanionic terdentate ligand L⁻ [L⁻ is the deprotonated form of diethyl 2,2′-(pyrrolidine-2,5-diylidene)diacetate (HL) (3)] are reported. The X-ray crystal structure determinations show that both 4 and 6 feature square planar copper(II) centres. The cis bond angles are close to 90° (85-93°) thanks in large part to the introduction of an additional carbon atom between the pyrrole ring and each of the ester moieties. These results indicate that the failure of the 2,5-diiminopyrrole head unit to coordinate in a terdentate manner to first row transition metal ions can, in principle, be resolved by introducing an extra carbon atom between the pyrrole ring and each of the imine moieties. A comparison of structural parameters between the two structurally characterised complexes (4 and 6) and the non-deprotonated free ligand HL (3) indicates that there is an increase in delocalisation of the electron density throughout the π system of the deprotonated ligand on coordination, as the CC and CO double bonds are longer and C-C and C-O single bonds are shorter than the corresponding bond distances in 3.     

A new dinuclear unsymmetric copper(II) complex as model for the active site of catechol oxidase

The crystal structure, magnetic, redox and spectroscopic properties of a novel unsymmetrical dinuclear copper(II) complex, prepared by the reaction between copper(II) perchlorate, sodium acetate and the unsymmetrical, binucleating ligand HTPPNOL, where HTPPNOL is N,N,N′-tris-(2-pyridylmethyl)-1,3-diaminopropan-2-ol, is reported. HTPPNOL (1 equiv.) reacted with 1 equiv. of copper(II) ion, in methanol, and produced the mononuclear copper complex [Cu(TPPNOL)](ClO₄)(BPh₄) (1). On the other hand, the reaction of 1 equiv. of HTPPNOL with 2 equiv. each of copper (II) ion and acetate, in methanol, produced the dinuclear complex [Cu₂(TPPNOL)(OOCCH₃)](ClO₄)₂ (2), whose structure has been determined by X-ray diffraction. In complex 2, as a result of the inherent asymmetry of the ligand HTPPNOL, one copper ion is five-coordinated (distorted trigonal-bipyramidal) while the other copper is four-coordinated (distorted square-planar). Then, as a result of the presence of distinct geometries for the metal centres, complex 2 exhibits a ferromagnetic coupling (J=+25.41 cm⁻¹). Titration experiments carried out on the dinuclear complex suggest a pK_a=8.0, which was related to the aquo/hydroxo equilibrium. Complex 2 is able to oxidise 3,5-di-tert-butylcatechol to the respective o-quinone. The oxidation reaction was studied by following the appearance of the quinone spectrophotometrically, at pH 8.0 and 25 °C.     

Synthesis and characterisation of copper(II), zinc(II) and cobalt(II) complexes of salicylglycine, a metabolite of aspirin

Copper(II), nickel(II) and cobalt(II) complexes of the aspirin metabolite salicylglycine (H₂L), of stoichiometry M(HL)₂·solvate, have been prepared and characterised. In these complexes salicylglycinate is coordinated to the metal via its carboxylato group and possibly also its amide oxygen in the copper(II) complex. Under basic conditions copper(II) forms the complex Cu(LH₋₁)·2H₂O·MeOH, in which the ligand is coordinated to the metal via its carboxylate and phenolate oxygen atoms and the deprotonated peptide nitrogen atom.     

Metal-directed assembly of large dinuclear copper(II) dithiocarbamate macrocyclic complexes

The synthesis of a range of dinuclear Cu(II) dithiocarbamate (dtc)-based macrocycles and their characterisation are described. By carefully tuning the size of the aromatic spacer, cavities of different dimensions can be designed. The length and flexibility of the chosen spacer group dictates the intermetallic distance and hence the degree of communication between the two metal centres as evidenced by electrochemical and EPR experiments. This is illustrated by crystallographic evidence that show the macrocycles can host guests (such as CH₂Cl₂) or can fold and form unexpected Cu(I) dtc clusters.     

Long-distance magnetic coupling in dinuclear copper(II) complexes with oligo-para-phenylenediamine bridging ligands

Two novel dinuclear copper(II) complexes of formulae [Cu₂(tren)₂(bpda)](ClO₄)₄ (2) and [Cu₂(tren)₂(tpda)](ClO₄)₄ (3) containing the tripodal tris(2-aminoethyl)amine (tren) terminal ligand and the 4,4′-biphenylenediamine (bpda) and 4,4″-p-terphenylenediamine (tpda) bridging ligands have been synthesized and structurally, spectroscopically, and magnetically characterized. Their experimentally available electronic spectroscopic and magnetic properties have been reasonably reproduced by DFT and TDDFT calculations. Single crystal X-ray diffraction analysis of 2 shows the presence of dicopper(II) cations where the bpda bridging ligand adopts a bismonodentate coordination mode toward two [Cu(tren)]²⁺ units with an overall non-planar, orthogonal anti configuration of the N-Cu-N threefold axis of the trigonal bipyramidal Cu^II ions and the biphenylene group. The electronic absorption spectra of 2 and 3 in acetonitrile reveal the presence of four moderately weak d-d transitions characteristic of a slightly distorted trigonal bipyramid stereochemistry of the Cu^II ions. TDDFT calculations on 2 identify these transitions as those taking place between the four lower-lying, doubly occupied a₂ (d_yz)², b₂ (d_xz)², b₁ (d_xy)², and a₁ (d_x²-y²)² orbitals and the upper, singly occupied a₁ (d_z²)¹ orbital of each trigonal bipyramidal Cu^II ion. Variable-temperature magnetic susceptibility measurements of 2 and 3 show the occurrence of moderate (J = −8.5 cm⁻¹) to weak intramolecular antiferromagnetic couplings (J = −2.0 cm^-1) [H = −JS₁·S₂ with S₁ = S₂ = S_Cu = ½] inspite of the relatively large copper-copper separation across the para-substituted biphenylene- (r = 12.3 Å) and terphenylenediamine (r = 16.4 Å) bridges, respectively. DFT calculations on 2 and 3 support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the d_z² orbital of each trigonal bipyramidal Cu^II ion through the predominantly π-type orbital pathway of the oligo-p-phenylenediamine bridges, as reported earlier for the parent compound [Cu₂(tren)₂(ppda)](ClO₄)₄·2H₂O (1) with the 1,4-phenylenediamine (ppda) bridging ligand. Finally, a rather slow exponential decay of the antiferromagnetic coupling (-J) with the number of phenylene repeat units, -(C₆H₄)_n- (n = 1-3), has been found both experimentally and theoretically along this series of oligo-p-phenylenediamine-bridged dicopper(II) complexes. These results further support the ability of linear π-conjugated oligo-p-phenylene spacers to transmit the exchange interaction between the unpaired electrons of the two Cu^II centers with intermetallic distances in the range of 7.5-16.4 Å.     

Complexes of copper(II) dihalide with 2,2′-dipyridylamine. X-ray diffraction structures of the [dibromo-bis(dipyam)copper(II)]–water (1:2) and di[chloro-bis(dipyam)copper(II)]diiodide–acetonitrile (1:2) complexes

The formation of complexes between copper(II) halides and 2,2′-dipyridylamine (dipyam) has been studied systematically. Only complexes with a 1:1 and 1:2 metal-to-ligand ratio are formed. Some mixed chloro–iodide and halide–PF₆ compounds have also been isolated. The X-ray diffraction structures of the [Cu(dipyam)₂Br₂] · 2H₂O (I) and the [Cu(dipyam)₂Cl]₂I₂ · 2CH₃CN (II) complexes are reported. I is a rare example of an octahedral coordination among the copper(II) halide complexes of dipyam. The two bromo atoms, which occupy the apical positions, are H-bonded to the water molecules of crystallization. II is a dimer, where each copper forms a cationic chloro-complex of approximately trigonal bipyramidal geometry, the dimerization being due to hydrogen bonds formed by the NH group of one of the two dipyams coordinated to each metal atom with the chlorine atom of the centrosymmetric cationic complex. The iodide anions are hydrogen-bonded to the NH groups of the dipyams not involved in the dimerization.     

Copper(II) nitrato and chloro complexes with sterically hindered tridentate ligands: Influence of ligand framework and charge on their structure and physicochemical properties

Copper(II) coordination complexes of the neutral ligand, tris(3-tert-butyl-5-methyl-1-pyrazolyl)methane (L2′), i.e. the copper(II) nitrato complexes [Cu(L2′)(NO₃)][Cu(NO₃)₄]_1/2 (1) and [Cu(L2′)(NO₃)](ClO₄) (2) and the copper(II) chloro complex [Cu(L2′)(Cl)](ClO₄) (3), and its anionic borate analogue, hydrotris(3-tert-butyl-5-methyl-1-pyrazolyl)borate (L2⁻), i.e. the copper(II) nitrato complex [Cu(L2)(NO₃)] (4) and the copper(II) chloro complex [Cu(L2)(Cl)] (5), were synthesized in order to investigate the influence of ligand framework and charge on their structure and physicochemical properties. While X-ray crystallography did not show any definitive trends in terms of copper(II) atom geometry in four-coordinate copper(II) chloro complexes 3 and 5, different structural trends were observed in five-coordinate copper(II) nitrato complexes 1, 2, and 4. These complexes were also characterized by spectroscopic techniques, namely, UV-Vis, ESR, IR/far-IR, and X-ray absorption spectroscopy.     

Ligand chirality-controlled selective formation of mono- and dinuclear copper complexes

Reaction of copper(II) acetate with the (S)-enantiomer of a tridentate binaphthyl Schiff base ligand, 2-(3,5-dichloro-2-hydroxybenzylideneamino)-2′-hydroxy-1,1′-binaphthyl (H₂L), in methanol afforded mononuclear copper(II) complex [Cu^II(HL)₂] ((S,S)-1) in 52% isolated yield. The same reaction gave dinuclear copper(II) complex [Cu^II₂(L)₂] ((R,S)-2) in 73% isolated yield when racemic-H₂L was used instead of (S)-H₂L. Both complexes (S,S)-1 and (R,S)-2 were characterized by elemental analysis, mass spectrometry, and X-ray crystallography. The present work highlights the functioning of ligand chirality as a ‘switch’ for selective formation of mono- and dinuclear metal complexes.     

Chlorido-bridged polymeric and dinuclear copper(II) complexes with tridentate Schiff base: Synthesis, crystal structure and magnetic properties

Two copper(II) complexes, [Cu(qsal)Cl](DMF) (1) and [Cu₂(qsalBr)₂Cl₂](DMF) (2), with tridentate Schiff base ligands, 8-(salicylideneamino)quinoline (Hqsal) and 8-(5-bromo-salicylideneamino)quinoline (HqsalBr), respectively, were synthesised and structurally characterized. Each copper(II) ion in the two complexes is in a distorted square pyramidal N₂OCl₂ environment. Complex 1 exists as a polymeric species via equatorial-apical chloride bridges, whereas 2 is a di-chlorido-bridged dinuclear complex, where each bridging chloride simultaneously occupies an in-plane coordination site on one copper(II) ion and an apical site on the other copper(II) ion. Variable-temperature magnetical susceptibility measurements on the two complexes in the temperature range 2-300 K indicate the occurrence of intrachain ferromagnetic (J = +6.58 cm⁻¹) and intramolecular antiferromagnetical (J = −6.91 cm⁻¹) interactions.     

DNA binding and nuclease activity of copper(II) complexes of tridentate ligands

Two copper(II) complexes, 1 and 2 with L₁ and L₂ [L₁ = 2-hydroxybenzyl(2-(pyridin-2-yl)ethylamine); L₂ = 2-hydroxybenzyl(2-(pyridin-2-yl)methylamine)] ligands, respectively, have been synthesized and characterized. The interaction of both the complexes with DNA has been studied to explore their potential biological activity. The DNA binding properties of the complexes with calf thymus (CT) DNA were studied by spectroscopic titration. The complexes show binding affinity to CT DNA with binding constant (K_b) values in the order of 10⁵ M⁻¹. Thermal denaturation and circular dichroism studies suggest groove binding of the complexes to CT DNA. Complexes also exhibit strong DNA cleavage activity in presence of reducing agents like 3-mercaptopropionic acid and β-mercaptoethanol. Mechanistic studies reveal the involvement of reactive hydroxyl radicals for their DNA cleavage activity.     

Self-assembly of square-lattice copper sheets displaying intra-ferromagnetism

A homoleptic copper(II) complex supported by methyliminodiacetate [ = MIDA] has been prepared and characterized by elemental, infra-red, X-ray diffraction and magnetic methods. The complex assembles as a two-dimensional lattice in which the copper centers are arranged in a square grid, bridged by the carboxylate moieties of the ligand. [Cu^II₄MIDA₄]_∞ crystallizes in the tetragonal system P-4 2₁ c (a = 9.8943(5), b = 9.8943(5), c = 14.4687(7) Å, Z = 8, R = 0.0495). Each Cu^II atom displays severely distorted square pyramidal geometry (τ = 0.44) and is bridged to four additional copper centers by a syn-anti arrangement of the carboxylate groups. Within the sheet, the copper centers are separated by approximately 4.96 Å. The sheets are layered by arranging the copper squares directly on top of one another, resulting in parallel channels that extend throughout the material. Variable temperature magnetic susceptibility measurements reveal the presence of ferromagnetic exchange between the spin carriers within each two-dimensional sheet and antiferromagnetic exchange across the layers.     

Synthesis, crystal structure, and DNA-cleaving behavior of 5-substituted benzene-1,3-bis(methylene)-spaced dinuclear copper(II) complexes

Three meta-dicopper complexes, 1-3, based on 5-substituted 1,3-xylylene spacer have been synthesized. These complexes are capable of inducing the transformation of supercoiled DNA (pUC19) to its nicked and linear DNA form in the presence of ascorbate, and their DNA nicking efficiency can be correlated to their DNA-binding ability. The cleavage mechanism is similar to that of the non-substituted meta-dicopper complex A. Amongst the three complexes, 5-(aminomethyl)-substituted complex 3 displayed a higher DNA-binding ability and nicking efficiency than unsubstituted complex A. The CD-spectroscopic study and structural analysis imply that the different CuCu distances and DNA binding modes induced by different 5-substituents on benzene-1,3-bis(methylene) spacer may be responsible for the different DNA cleaving behavior of meta-dicopper complexes.     

Synthesis, characterization and crystal structures of Cu(II) and Pd(II) complexes of the pentadentate mixed donor macrocycle, [17]aneNO2S2

An aza-oxa-thia macrocycle, 5,14-dioxa-2,17-dithia[6](1,2)benzeno[6](2,6)pyridinophane, L1, the related smaller macrocycle 2,14-dithia-11-oxa-[3](1,2)benzeno[6](2,6)pyridinophane, L2, and the complexes with Pd(II) and Cu(II) of the macrocycle, L1, have been synthesized. The crystal structure of L2 and those of the two metal complexes have been determined. In the complexes, the metal ions adopt exclusively square planar geometry in which the pyridine nitrogen, two sulfurs and one chlorine atom are coordinated and there is no appreciable interaction with the oxygen donors. Thus, the `hard-soft acid-base' principle is illustrated by the behaviour of L1. The structures of both complexes are compared with the previously reported mixed aza-thia macrocycle, 2,5,14,17-tetrathia[6](1,2)benzeno[6](2,6)pyridinophane. The crystal structure of the smaller macrocycle, L2, is also discussed and due to the nature of its smaller cavity, attempts to make complexes with it have not been successful.     

Synthesis, spectroscopic properties and X-ray crystal structures of two dinuclear alkoxo-bridged copper(II) compounds with the ligand bis(1-methyl-2-benzimidazolyl) propane. A unique alkoxo-bridged Cu(II) dinuclear compound with an additional bidentate bridging triflate anion

Based on the new ligand bis(1-methyl-2-benzimidazolyl) propane (abbreviated as mtbz) several new copper(II) coordination compounds have been prepared and characterized structurally and spectroscopically. Two representative compounds, i.e. [Cu₂(mtbz)₂(CH₃)₂- (CF₃SO₃)](CF₃SO₃) (1) and [Cu₂(mtbz)₂(CH₃O)₂](ClO₄)₂ (4) were characterized structurally by X-ray diffraction. Crystal data for 1: monoclinic, space group P2₁/c, a=13.6585(5), B=39.981(3), C=20.919(1) Å, β=125.98(1)°, Z=8. Crystal data for 4: monoclinic, space group P2₁/c, a=13.115(2), B=9.523(2), C=17.908(4) Å, β=111.71(1)°, Z=2. Structures 1 and 4 each consist of a dinuclear unit with bridging methoxo groups and one ligand linked to each copper via an N atom. Structure 1 (which consists of two dinuclear, crystallographically independent, but chemically identical units) has the two copper atoms bridged by a triflate anion, providing each copper atom a square-pyramidal coordination, while the copper atoms in structure 4 have an almost a square-planar geometry. The Cu---Cu distances (Å) within the dinuclear units are: 1, 2.9775(13), 2.9751(13); 4, 2.9872(16); the Cu---O---Cu bridging angles (°) are: 1, 101.7(3), 101.7(3), 100.9(3), 102.1(3); 4, 103.2(2). The mid-IR section focused on the vibrations of the triflate anion reveals interesting results concerning the assignments of that anion related to the v_as(S---O) band. Characteristic Cu---O vibrations in the far-IR section were found at 386 and 230 cm⁻¹ for the methoxo-bridged and 454 and 332 cm⁻¹ for the ethoxo-bridged compounds. These dinuclear species are EPR silent, and only a weak signal of monomeric impurities is observed. They also show a diamagnetic behavior below room temperature.     

Synthesis and structural analysis of copper(II) cysteine complexes

This report describes the synthesis and structural analysis of stable copper(II) cysteine complexes. Pale pink copper(II) cysteine complexes were synthesized in mole ratios of 1:2, 1:4, and 1:6 of copper(II):cysteine in ethanol. Infrared spectroscopy and X-ray absorption spectroscopy confirmed that copper(II) binding occurred via the thiol ligand of cysteine. XANES analysis showed that the oxidation state of copper remained as copper(II) and the local atomic geometry was similar in all of the cysteine complexes. The EXAFS data indicate that the copper(II) cysteine complexes are forming ring type structures with sulfur ligands from the cysteines acting as bridging ligands. X-ray diffraction revealed that the copper(II) cysteine complexes formed monoclinic cells with maximum crystallinity found in the 1:4 copper(II):cysteine complex.