DNA photocleavage and anticancer activity of terpyridine copper(II) complexes having phenanthroline bases

Copper(II) complexes [Cu(ph-tpy)(B)](ClO₄) (1–3), where ph-tpy is (4′-phenyl)-2,2′:6′,2″-terpyridine and B is N,N-donor phenanthroline base, viz. 1,10-phenanthroline (phen, 1), dipyridoquinoxaline (dpq, 2), and dipyridophenazine (dppz, 3), were prepared and characterized from analytical and spectral data. Complex 1, characterized by X-ray crystallography, shows a distorted square-pyramidal (4 + 1) CuN₅ coordination geometry having the tridentate ph-tpy ligand at the basal plane and bidentate phen bound to the axial-equatorial sites. The complexes display a d–d band near 650 nm in aqueous DMF. The complexes are avid binders to calf thymus DNA giving the binding order: 3 (dppz) > 2 (dpq) > 1 (phen). The dpq and dppz complexes show photo-induced DNA cleavage activity in red light via photo-redox pathway forming hydroxyl radicals. The cytotoxicity of the dppz complex 3 was studied by MTT assay in HeLa cancer cells. The IC₅₀ values are 3.7 and 12.4 μM in visible light of 400–700 nm and dark, respectively.     

Ferrocene-conjugated copper(II) dipyridophenazine complex as a multifunctional model nuclease showing DNA cleavage in red light

The copper(II) complex [Cu(L)(dppz)](ClO₄)₂ (1) having a tripodal ligand ferrocenylmethylbis(2-pyridylmethylamine) (L) with a pendant ferrocenyl unit and a planar NN-donor dipyrido-[3,2-a:2′,3′-c]-phenazine (dppz) base is prepared and its DNA binding and cleavage properties studied. The complex is redox active showing cyclic voltammetric responses at 0.52 and –0.01 V vs. SCE due to Fe(III)/Fe(II) and Cu(II)/Cu(I) couples, respectively. The complex that binds to the major groove of DNA shows dual chemical nuclease activity involving both the metal centres. The complex displays efficient photo-induced DNA cleavage activity in visible laser light of 458 and 568 nm wavelengths forming cleavage active hydroxyl radicals. Significant DNA cleavage is also observed in red light of 647 nm within the photodynamic therapy (PDT) window.     

DNA-binding and photocleavage, cytotoxicity, apoptosis and antioxidant activity studies of ruthenium(II) complexes

Two ruthenium(II) polypyridyl complexes, namely [Ru(phen)₂(DMDPPZ)](ClO₄)₂ 1 (phen = 1,10-phenanthroline, DMDPPZ = 3,6-dimethyldipyrido[3,2-a:2′,3′-c]phenazine) and [Ru(dmp)₂(DMDPPZ)](ClO₄)₂ 2 (dmp = 2,9-dimethyl-1,10-phenanthroline), have been synthesized and characterized. The DNA-binding properties of the complexes were investigated by spectrophotometric methods, viscosity measurements, and photoactivated cleavage studies. The DNA-binding constants for complexes 1 and 2 have been determined as 8.78 (±0.94) × 10⁵ M⁻¹ (s = 3.02) and 1.26 (±0.35) × 10⁵ M⁻¹ (s = 1.69), respectively. The results suggest that these complexes bind to calf thymus DNA through intercalation. When irradiated at 365 nm, the complexes promote the photocleavage of pBR322 DNA, and complex 1 cleaves DNA more effectively than complex 2 under comparable experimental conditions. The cytotoxicities of complexes 1 and 2 have been evaluated by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) method. Complex 2 shows higher anticancer potency than complex 1 against four tumor cell lines. The apoptosis-inducing activity was assessed by acridine orange/ethidium bromide staining assay, and the antioxidant activities of these complexes against hydroxyl radical were also explored.     

Red-light photosensitized cleavage of DNA by (l-lysine)(phenanthroline base)copper(II) complexes

Ternary copper(II) complexes [Cu(l-lys)B(ClO4)](ClO4)(1-4), where B is a heterocyclic base, viz. 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 3) and dipyrido[3,2-a:2',3'-c]phenazene (dppz, 4), are prepared and their DNA binding and photo-induced DNA cleavage activity studied (l-lys =l-lysine). Complex 2, structurally characterized by X-ray crystallography, shows a square-pyramidal (4 + 1) coordination geometry in which the N,O-donor l-lysine and N,N-donor heterocyclic base bind at the basal plane and the perchlorate ligand is bonded at the elongated axial site. The crystal structure shows the presence of a pendant cationic amine moiety -(CH2)4NH3+ of l-lysine. The one-electron paramagnetic complexes display a d-d band in the range of 598-762 nm in DMF and exhibit cyclic voltammetric response due to Cu(II)/Cu(I) couple in the range of 0.07 to -0.20 V vs. SCE in DMF-Tris-HCl buffer. The complexes having phenanthroline bases display good binding propensity to the calf thymus DNA giving an order: 4 (dppz) > 3 (dpq) > 2 (phen)> 1 (bpy). Control cleavage experiments using pUC19 supercoiled DNA and distamycin suggest major groove binding for the dppz and minor groove binding for the other complexes. Complexes 2-4 show efficient DNA cleavage activity on UV (365 nm) or visible light (694 nm ruby laser) irradiation via a mechanistic pathway involving formation of singlet oxygen as the reactive species. The amino acid l-lysine bound to the metal shows photosensitizing effect at red light, while the heterocyclic bases are primarily DNA groove binders. The dpq and dppz ligands display red light-induced photosensitizing effects in copper-bound form.     

Photocytotoxic and anaerobic DNA cleavage activity of binuclear 3,3′-dithiodipropionic acid cobalt(II) complexes having phenanthroline bases

Dicobalt(II) complexes [{(B)Co^II}₂(μ-dtdp)₂] (1–3) of 3,3′-dithiodipropionic acid (dtdp) and phenanthroline bases (B), viz. 1,10-phenanthroline (phen in 1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq in 2) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz in 3), have been prepared, characterized and their photo-induced anaerobic DNA cleavage activity studied. The elemental analysis and mass spectral data suggest binuclear formulation of the complexes. The redox inactive complexes have magnetically non-interacting dicobalt(II) core showing magnetic moment of ∼3.9 μ_B per cobalt(II) center. The complexes show good binding propensity to calf thymus DNA giving K_b values within 4.3 × 10⁵–4.0 × 10⁶ M⁻¹. Thermal melting and viscosity data predict DNA groove binding and/or partial intercalative nature of the complexes. The complexes show significant anaerobic DNA cleavage activity in green light under argon atmosphere possibly involving radical species generated from the disulfide moiety in a type-I pathway. The DNA cleavage reaction under aerobic medium in green light is found to involve hydroxyl radical species. The dppz complex 3 exhibits significant photocytotoxicity in HeLa cervical cancer cells with an IC₅₀ value of 2.3 μM in UV-A light of 365 nm, while it is essentially non-toxic in dark giving an IC₅₀ value of >200 μM. A significant reduction of the dark toxicity of the organic dppz base (IC₅₀ = 8.3 μM in dark) is observed on binding to the cobalt(II) center while essentially retaining its photocytotoxicity in UV-A light (IC₅₀ = 0.4 μM).     

DNA binding and photocleavage in vitro by new dirhodium(II) dppz complexes: correlation to cytotoxicity and photocytotoxicity

Two new dirhodium(II) complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine), cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(eta(1)-O(2)CCH(3))(CH(3)OH)](+) (1) and cis-[Rh(2)(mu-O(2)CCH(3))(2)(dppz)(2)](2+) (2), were synthesized and characterized as potential agents for photochemotherapy. Various techniques show that 1 binds to DNA through intercalation, although some aggregation of the complex on the DNA surface is also present. In contrast, 2 does not intercalate between the DNA bases; however, strong hypochromic behavior is observed in the presence of DNA, which can be attributed to intermolecular pi-stacking of 2 enhanced by the polyanion. The apparent DNA binding constants determined using optical titrations are compared to those from dialysis experiments. Both complexes photocleave pUC18 plasmid in vitro under irradiation with visible light (lambda(irr) >or= 395 nm, 15 min), resulting in the nicked, circular form. Greater photocleavage is observed for 1 relative to 2, which may be due to the ability of 1 to intercalate between the DNA bases. The cytotoxicity toward human skin cells (Hs-27) measured as the concentration at which 50% cell death is recorded, LC(50), was found to be 135 +/- 8 microM for 2 in the dark (30 min), which is significantly lower than those of 1 (LC(50) = 27 +/- 2 microM) and Rh(2)(O(2)CCH(3))(4) (LC(50) = 15 +/- 2 microM). Irradiation of cell cultures containing 1 and Rh(2)(O(2)CCH(3))(4) with visible light (400-700 nm, 30 min) has little effect on their cytotoxicity, with LC(50) values of 21 +/- 3 and 13 +/- 2 microM, respectively. Interestingly, a 3.4-fold increase in the toxicity of 2 is observed when the cell cultures are irradiated (400-700 nm, 30 min), resulting in LC(50) = 39 +/- 1 microM. The greater toxicity of 1 compared to 2 in the dark may be related to the ability of the former compound to intercalate between the DNA bases. The lower cytotoxicity of 2, together with its significantly greater photocytotoxicity, makes this complex a potential agent for photodynamic therapy (PDT). These results suggest that intercalation or strong DNA binding may not be a desirable property of a potential PDT agent.     

Electrochemical behavior and antioxidant activity of tetradentate Schiff bases and their copper(II) complexes

This study describes the effects of the substituents on electrochemical behavior and antioxidant activity of the six tetradentate Schiff bases, containing ethane-1,2-diamine or propane-1,2-diamine as the amine part and pentane-2,4-dione and/or 1-phenylbutane-1,3-dione as ??-diketone, and corresponding copper(II) complexes. Cyclic voltammograms of these compounds were recorded in dimethylsulfoxide and 0.1?M sodium perchlorate as supporting electrolyte with glassy carbon as working electrode at different scan rates. The voltammograms of Schiff bases alone showed only one irreversible peak. Voltammograms recorded for complexes showed the presence of quasi-reversible processes taking place at the metal center and reversible process at the ligand part. Both steric and inductive effects of substituents and structure of imine bridge of Schiff base ligands as well as complexes were discussed. These effects appear relevant for the antioxidant activity. Antioxidant activity of the investigated compounds expressed as Trolox equivalent antioxidant capacity is also discussed. The electrochemical behavior showed a high correlation with the antioxidant activity for investigated compounds.     

Synthesis,characterization, DNA interaction and cleavage activity of new mixed ligand copper(II) complexes with heterocyclic bases

Mixed ligand complexes having the formulae Cu(RPO)₂Py₂, Cu(RPO)₂Im₂ and Cu(DBO)₂Py₂ [RPO = resacetophenone oxime, DBO = 2,4-dihydroxybenzophenone oxime, Py = pyridine and Im = imidazole] have been synthesized and characterized by UV–Vis, IR, ESR, cyclic voltammetry and magnetic susceptibility methods. Absorption studies revealed that each of these octahedral complexes is an avid binder of calf thymus DNA. The apparent binding constants for mixed ligand complexes are in order of 10⁴–10⁵ M⁻¹. Based on the data obtained in the DNA binding studies a partial intercalative mode of binding is suggested for these complexes. The nucleolytic cleavage activity of the adducts was carried out on double stranded pBR322 circular plasmid DNA by using a gel electrophoresis experiment in the presence and absence of oxidant (H₂O₂). All the metal complexes cleaved supercoiled DNA by hydrolytic and oxidative paths. The oxidative path dominates the hydrolytic cleavage. The hydrolytic cleavage of DNA is evidenced from the control experiments showing discernable cleavage inhibition in the presence of the hydroxyl radical inhibitor DMSO or the singlet oxygen quencher azide ion.     

Metal-assisted red light-induced DNA cleavage by ternary L-methionine copper(II) complexes of planar heterocyclic bases

Ternary copper(II) complexes [Cu(l-met)B(Solv)](ClO4) (1-4), where B is a N,N-donor heterocyclic base like 2,2'-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq, 3) and dipyrido[3,2-a:2'],3'-c]phenazene (dppz, 4), are prepared and their DNA binding and photo-induced DNA cleavage activity studied (L-Hmet =L-methionine). Complex 2, structurally characterized by X-ray crystallography, shows a square pyramidal (4 + 1) coordination geometry in which the N,O-donor L-methionine and N,N-donor heterocyclic base bind at the basal plane and a solvent molecule is coordinated at the axial site. The complexes display a d-d band at approximately 600 nm in DMF and exhibit a cyclic voltammetric response due to the Cu(II)/Cu(I) couple near -0.1 V in DMF-Tris-HCl buffer. The complexes display significant binding propensity to the calf thymus DNA in the order: 4(dppz) > 3(dpq) > 2(phen> 1(bpy). Control cleavage experiments using pUC19 supercoiled DNA and distamycin suggest major groove binding for the dppz and minor groove binding for the other complexes. Complexes 2-4 show efficient DNA cleavage activity on UV (365 nm) or red light (632.8 nm) irradiation via a mechanistic pathway involving formation of singlet oxygen as the reactive species. The DNA cleavage activity of the dpq complex is found to be significantly more than its dppz and phen analogues.     

Impact of metal on the DNA photocleavage activity and cytotoxicity of ferrocenyl terpyridine 3d metal complexes

Ferrocenyl terpyridine 3d metal complexes and their analogues, viz. [M(Fc-tpy)(2)](ClO(4))(2) (1-4), [Zn(Ph-tpy)(2)](ClO(4))(2) (5) and [Zn(Fc-dpa)(2)]X(2) (X = ClO(4), 6; PF(6), 6a), where M = Fe(II) in 1, Co(II) in 2, Cu(II) in 3 and Zn(II) in 4, Fc-tpy is 4'-ferrocenyl-2,2':6',2'-terpyridine, Ph-tpy is 4'-phenyl-2,2':6',2'-terpyridine and Fc-dpa is ferrocenyl-N,N-dipicolylmethanamine, are prepared and their DNA binding and photocleavage activity in visible light studied. Complexes 2, 4, 5 and 6a that are structurally characterized by X-ray crystallography show distorted octahedral geometry with the terpyridyl ligands binding to the metal in a meridional fashion, with Fc-dpa in 6a showing a facial binding mode. The Fc-tpy complexes display a charge transfer band in the visible region. The ferrocenyl (Fc) complexes show a quasi-reversible Fc(+)-Fc redox couple within 0.48 to 0.66 V vs. SCE in DMF-0.1 M TBAP. The DNA binding constants of the complexes are ～10(4) M(-1). Thermal denaturation and viscometric data suggest DNA surface binding through electrostatic interaction by the positively charged complexes. Barring the Cu(II) complex 3, the complexes do not show any chemical nuclease activity in the presence of glutathione. Complexes 1-4 exhibit significant plasmid DNA photocleavage activity in visible light via a photoredox pathway. Complex 5, without the Fc moiety, does not show any DNA photocleavage activity. The Zn(II) complex 4 shows a significant PDT effect in HeLa cancer cells giving an IC(50) value of 7.5 μM in visible light, while being less toxic in the dark (IC(50) = 49 μM).     

DNA binding and oxidative cleavage activity of ternary (l-proline)copper(II) complexes of heterocyclic bases

Ternary copper(II) complexes [Cu(l-pro)(B)(H₂O)](NO₃) (1, 2) where l-pro = l-proline, B is a N,N-donor heterocyclic base, viz. 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), are synthesized, characterized, and their DNA binding and cleavage activity studied. The bpy complex (1) is structurally characterized by single-crystal X-ray crystallography. The complexes show the presence of a distorted square-pyramidal (4 + 1) CuN₃O₂ coordination geometry. Complex [Cu(l-pro)(bpy)(H₂O)](NO₃) (1) crystallizes in the triclinic space group P1 with unit cell parameters: a = 7.082(3) Å, b = 10.483(5) Å, c = 11.581(5) Å, α = 89.700(7)°, β = 83.488(8)°, γ = 84.109(8)° and V = 849.7(7) Å³. The one-electron paramagnetic complexes display a d–d band near 600 nm in water and show a cyclic voltammetric response due to Cu(II)/Cu(I) couple near 0.1 V (versus SCE) in Tris–HCl buffer–0.1 M KCl. Binding interactions of the complexes with calf thymus (CT) DNA have been investigated by emission, absorption, viscosity and DNA thermal denaturation studies. The phen complex displays significant binding propensity to the CT DNA giving an order: 2 (phen)  1 (bpy). The bpy complex does not show any apparent binding to the DNA and hence poor cleavage efficiency. Complex 2 shows efficient oxidative cleavage of SC-DNA in the presence of 3-mercaptopropionic acid (MPA) involving hydroxyl radical species as evidenced from the control data showing inhibition of DNA cleavage in the presence of DMSO and catalase.     

Preparation and characterization of copper(II) and nickel(II) complexes with novel tetraaza-macrocyclic schiff bases

Novel tetraaza-macrocycles with N-substituted carbamoyl groups were prepared by the reaction of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,5,7,12-tetraene with isocyanates (RNCO, R = Ch₃ and C₆H₅). Their copper(II) and nickel(II) complexes were characterized by magnetic susceptibilities, electronic absorption spectra, and electrochemical properties. The complexing abilities and extractabilities of the ligands for the metal ions were investigated.     

DNA binding, nuclease activity and cytotoxicity studies of Cu(II) complexes of tridentate ligands

Cu(II) complexes of three tridentate ligands, L(1), L(2) and L(3), [L(1), N-((1H-imidazole-2-yl)methyl)-2-(pyridine-2-yl)ethanamine; L(2), N-((1-methyl-1H-imidazole-2-yl)methyl)-2-(pyridine-2-yl)ethanamine; L(3), 2-(pyridine-2-yl)-N-((pyridine-2-yl)methyl)ethanamine] respectively, were synthesized and characterized. The single crystal X-ray structure of complex 1 reveals the pseudo octahedral coordination geometry around the copper center. Absorption and fluorescence experimental evidence show good DNA binding propensity (in the order of 10(5) M(-1)) of the complexes. Thermal denaturation and circular dichroism (CD) analyses reveal minor structural changes of calf thymus (CT) DNA in presence of complexes and groove and/or surface binding of the complexes to CT-DNA. Kinetic DNA cleavage assay shows pseudo-first-order kinetic reaction between the complex and supercoiled (SC) DNA. In addition, mechanistic SC DNA cleavage results show higher DNA cleavage activity in presence of reducing agent, due to the presence of hydroxyl radicals. In vitro cytotoxicity assay of the complexes demonstrate that the complexes have low toxicity for different cancer cell lines and IC(50) values were between 37 and 156 μM.     

Syntheses and copper(II)-dependent DNA photocleavage by acridine and anthracene 1,10-phenanthroline conjugate systems

We report the syntheses and characterization of a series of compounds based on 1,10-phenanthroline covalently tethered, at the 2 and 9 positions, to either two benzene, naphthalene, acridine or anthracene chromophores. The acridine and anthracene derivatives are shown to efficiently cleave pUC19 plasmid DNA upon irradiation with ultraviolet light (pH = 7.0, 22 degrees C, 350 nm). Furthermore, photocleavage levels are markedly increased by the addition of Cu2+ to the DNA photolysis reactions. Interestingly, when the concentrations of the anthracene compounds are lowered from 35 microM to 0.25 microM, the reverse trend is observed. DNA photocleavage is markedly reduced in the presence of copper(II).     

Synthesis,characterization, antimicrobial,SOD mimic and DNA interaction behavior of copper(II) complexes with pefloxacin and phenanthroline derivatives

Square pyramidal copper(II) complexes with second‐generation fluoroquinolone, pefloxacin and phenanthroline derivatives were prepared and characterized. They were also screened for in–vitro activity against two Gram^(+ve) and three Gram^(−ve) microorganisms. SOD mimic behavior was actively sought for clinical and mechanistic purposes under a nonenzymatic system (NBT–NADH–PMS), and was found to have good antioxidant activity. The complexes are avid DNA binders having binding constant (K_b) in the power of 4. Viscosity measurement data confirmed that complexes bind through classical intercalative mode of binding. The cleavage ability on pUC19 DNA was determined using gel electrophoresis technique. Copyright © 2011 John Wiley & Sons, Ltd.     

Selective cytotoxic Pt(II) complexes of phenanthroline derivatives

A new dichloroplatinum(II) complex with 5-methyl-1,10-phenanthroline (1) has been obtained and characterized by X-ray diffraction. MTT assay was used for in vitro cytotoxicity evaluation of this complex, along with a similar complex with 4,7-diphenyl-1,10-phenanthroline (2) against normal and cancerous cell lines. Interestingly, the IC₅₀ values of the new complexes were higher for normal cells and in the case of complex 2, lower against all studied human cancer cells, in comparison with cisplatin.     

Construction of chlorido-bridged dinuclear copper(II) complexes with similar tridentate Schiff bases

Two new chlorido-bridged dinuclear copper(II) complexes [Cu₂Cl₂(L¹)₂] (1) and [Cu₂Cl₂(L²)₂] (2), where L¹ and L² are the deprotonated form of Schiff bases 2-[1-(2-morpholin-4-ylethylimino)ethyl]phenol (HL¹) and 2-[1-(2-piperidin-1-ylethylimino)ethyl]phenol respectively, are prepared and structurally characterized by elemental analysis, IR spectra, and single crystal X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2₁/c with unit cell dimensions a = 8.0816(2) Å, b = 19.1780(3) Å, c = 9.6757(3) Å, β = 106.465(2)°, V = 1438.13(6) ų, Z = 2, R ₁ = 0.0409, and wR ₂ = 0.1085. Complex 2 crystallizes in the monoclinic space group P2₁/c with unit cell dimensions a = 7.7640(10) Å, b = 19.930(3) Å, c = 9.628(2) Å, β = 103.890(3)°, V = 1446.2(4) ų, Z = 2, R ₁ = 0.0634, and wR ₂ = 0.1316. Each Cu atom in the complexes is coordinated by three donor atoms of the Schiff bases and by two bridging Cl atoms, forming square pyramidal geometry. The Cl anions are preferred bridging groups for the construction of dinuclear copper complexes with tridentate Schiff bases.     

Synthesis, structural characterization and antibacterial activity of 2,6-diacetylpyridine bis(benzenesulfonohydrazide) Schiff bases and their copper(II) complexes

2,6-Diacetylpyridine bis(benzenesulfonohydrazide) Schiff bases (L¹, L² and L³) and their Cu(II) complexes of the general formula [CuL·H₂O] were synthesized and characterized by various spectroscopic techniques. The crystal structure of [CuL³·(py)]·py was investigated by single crystal X-ray structure analysis. The Cu(II) cation has near square pyramidal, penta-coordinate geometry. The binegatively charged tetradentate Schiff base is asymmetrically coordinated to the Cu(II) ion via the pyridine N atom, the azomethine N atom, the sulfonyl O atom and the deprotonated hydrazine N atom. There is a pyridine molecule apically coordinated to the Cu(II) ion. All the Schiff bases and their copper(II) complexes were screened by the disc diffusion method against multi-drug resistant (MDR) gram-negative and gram-positive bacteria. The minimum inhibitory concentration (MIC) values were also determined. These results show that the antibacterial activity of the Schiff bases against Methicillin-resistant Staphylococcus aureus (MRSA) is enhanced when they are chelated with the copper(II) ion.     

Synthesis and crystal structures of two azido-bridged copper(II) complexes with schiff bases

Two new end-on azido-bridged dinuclear copper(II) complexes with the formula [Cu(μ_1,1-N₃)₂(IEP)₂] (I) and [Cu(μ_1,1-N₃)₂(EMP)₂] (II), where IEP and EMP are the deprotonated forms of 2-[1-(2-isopropylaminoethylimino)ethyl]phenol and 2-ethoxy-6-[(2-methylaminoeth-ylimino)methyl]phenol, respectively, were synthesized. Both complexes were characterized by elemental analyses and IR spectra. The crystal structures were determined by the X-ray diffraction. The crystal of I is monoclinic: space group P2₁/c, a = 9.662(2), b = 15.282(3), c = 10.639(2) Å, β = 115.418(10)°, V = 1418.9(5) ų, Z = 2. The crystal of II is monoclinic: space group P2₁/n, a = 12.588(2), b = 7.705(1), c = 14.481(2), β = 91.736(5)°, V = 1403.9(2) ų, Z = 2. The two Cu atoms in each complex are bridged by two end-on azide groups. Each Cu atom is in a square pyramidal coordination. The Cu…Cu distances are 3.230(2) Å in I and 3.150(2) Å in II.     

Syntheses and crystal structures of two tetranuclear copper(II) complexes with Schiff bases

Two similar tetranuclear copper(II) complexes with the formulae [Cu₄(L1)₂(μ_1,1-N₃)₄(μ₂-CH₃COO)₂] and [Cu₄(L2)₂(μ_1,1-N₃)₂(μ-Br)₂Br₂(CH₃OH)₂], where L1 and L2 are the deprotonated forms of 2-[(2-ethylaminoethylimino)methyl]-5-methoxyphenol and 5-methoxy-2-[(2-piperidin-1-ylethylimino)methyl]phenol, respectively, have been synthesized and characterized by spectroscopic methods and single-crystal X-ray diffraction. Both complexes are centrosymmetric tetranuclear copper(II) compounds. The bridging groups in [Cu₄(L1)₂(μ_1,1-N₃)₄(μ₂-CH₃COO)₂] are μ_1,1-azide ligands and μ₂-acetate ligands, and those in [Cu₄(L2)₂(μ_1,1-N₃)₂(μ-Br)₂Br₂(CH₃OH)₂] are μ_1,1-azide ligands and μ-bromide ligands. Each Cu atom in the complexes is in a square pyramidal geometry.