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• 1, 4, 7-Triazacyclonone (tacn) and its derivatives have been progress a rapid expansion in the area of the coordination chemistry, and a stable and wide range of metal coordination compounds have been prepared as a result of the propensity of tacn for facial coordination with metal ions^[1]. It has been extensively noted that tacn and its N-substituted complexes are suitable ligands forming both mono-and bi-metallic complexes.

  Hence, many of tacn and its N-substituted complexes are found to be both structural and functional models of various metalloenzyme^[2-8], capable of promoting phosphate ester degradation with DNA^[9-13] or RNA^[14-15] cleavage and used in oxidative catalysis^[16]. 1, 4, 7-Tribenzyl-1, 4, 7-triazacyclononane (Bn₃tacn), as one type of their derivative, has been investigated many years^[17-19]. The central role of the binding and activation of dioxygen by copper ions with a wide range of important processes in biology and catalyst areas has led to intense interest in the synthesis, characterization, and examination of the reactivity of [Cu₂O₂] complexes^[20]. Previously, we have reported the complexes with the same ligand^[21] and another N-substituted ligand further compared their structures and spectra^[22-23]. Similar ligands were also reported recently^[24-25]. In this work, we prepared two binuclear copper complexes containing the ligand Bn₃tacn using m-bdc (1, 4-benzenedi-carboxylic) and H₂O as bridged ligand: [Cu₂(Bn₃tacn)₂(m-bdc)(CH₃CN)₃(H₂O)₂]ClO₄ (1) and [Cu₂(Bn₃tacn)₂(OH)₂](ClO₄)₂ (2) (L=Bn₃tacn); the syntheses, structures, spectroscopic properties and DNA binding properties will be described here.

  1   Experimental

  1.1   Materials and instrument

  1, 4, 7-Triazacyclononane (tacn) was prepared according to the previous procedure^[26-27] and the ligand Bn₃tacn was prepared according to the literature^[28]. All starting materials and solvents are analytical reagents.

  Elemental analyses of C, H and N were carried out on Model 240 Perkin-Elemer instrument. IR spectra were measured using KBr disks with Bruker Tensor 27 FTIR spectrophotometer in the 400~4 000 cm^-1 region. The UV-Vis spectra were measured on Jasci V-570 UV-Vis spectrophotometer within 200~2 000 nm region.

  1.2   Syntheses of the complexes

  [Cu₂(Bn₃tacn)₂(m-bdc)(CH₃CN)₃(H₂O)₂]ClO₄ (1). A solution of Cu(ClO₄)₂·6H₂O (0.111 g, 0.3 mmol) in acetonitrile (10 mL) was added to a solution of Bn₃tacn(0.024 g, 0.3 mmol) in acetonitrile (10 mL). The reaction mixture was stirred at room temperature for 30 min until all of the solid dissolved to afford a clear deep-blue solution, and then a solution of m-bdc piperidine hydrochloride(0.043 g, 0.15 mmol) disso-lved in water (5 mL) was slowly added to the previous solution. The reaction mixture was continuously stirred for 2 h, filtered to get rid of any insoluble particles. The light yellow rhombic crystals suitable for X-ray crystallographic analysis were obtained by slow evaporation of the filtrate (Yield: 63%). Anal. Calcd. for C₆₅H₇₆Cl₂Cu₂N₈O₁₂(%): C 57.43, H 5.64, N 8.24; Found(%): C 57.40, H 5.62, N 8.21.

  [Cu₂(Bn₃tacn)₂(OH)₂](ClO₄)₂ (2). A suspension of Cu(ClO₄)₂·6H₂O (0.111 g, 0.3 mmol) and the ligand Bn₃tacn(0.024 g, 0.3 mmol) in acetonitrile (20 mL) was stirred for 30 min to give a blue solution, and then the pH value was adjusted to 8.5 and the solution was filtered. The deep blue crystal was obtained after one week evaporation of the solvent, filtered off, washed with ethanol and ether, and dried under vacuum (Yield: 42%). Anal. Calcd. for C₅₄H₆₆Cl₂Cu₂N₆O₁₀(%): C 56.80, H 5.69, N 7.79; Found(%): C 56.88, H 5.70, N 7.80.

  Caution! Although no problems were encoun-tered in this work, perchlorate complexes containing organic ligands are potentially explosive. They should be prepared in small quantities and handled with care.

  1.3   X-ray crystallography

  Diffraction data for 1 and 2 were collected at 293 K, with a Bruker SMART 1000 CCD diffractometer using Mo Kα radiation (λ=0.071 073 nm) with the φ-ω scan technique. An empirical absorption correction (SADABS) was applied to raw intensities^[29]. The struc-tures were solved by direct methods (SHELXS-97^[30]) and refined by full-matrix least-squares procedures on F² using SHELXL-97^[30], the structure of complex 2 was further solved by SQUEEZE software. According to the elimination electronics, relevant H₂O solvent was eliminated. The hydrogen atoms were added theoretically, and riding on the concerned atoms and refined with fixed thermal factors. Further details about crystal data and structure refinement are summarized in Table 1.

  Table 1.  Crystal data and structure refinements for 1 and 2

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  Complex	1	2
  Empirical formula	C66H76Cl2Cu2N8O12	C54H66Cl2Cu2N6O10
  Formula weight	1 371.32	1 157.10
  Temperature / K	153(2)	294(2)
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	C2/c
  a / nm	1.028 41(16)	2.787 5(16)
  b / nm	3.307 9(3)	1.534 3(7)
  c / nm	1.116 61(18)	2.967 4(13)
  β/(°)	113.351(2)	91.978(13)
  V / nm3	3.487 5(9)	12.684(11)
  Z	2	8
  Dc / (g·cm-3)	1.306	1.218
  μ / mm-1	0.75	0.809
  Crystal size / mm	0.16×0.14×0.10	0.26×0.18×0.16
  θ range / (°)	2.08~27.85	1.52~25.02
  Limiting indices	-13 ≤ h ≤13, -41 ≤ k ≤42, -11 ≤ l ≤ 14	-33≤h≤ 18, -18 ≤ k ≤ 18, -35 ≤ l ≤ 35
  Reflection collected, unique	26 745, 7 536 (Rint=0.018 1)	30 996, 11 052 (Rint=0.136 3)
  F(000)	1 432	4 832
  Goodness-of-fit on F2	1.053	0.946
  Final R indices [I>2σ(I)]	R1=0.040 2, wR2=0.118 7	R1=0.081 4, wR2=0.256 2
  R indices (all data)	R1=0.047 7, wR2=0.115 2	R1=0.175 0, wR2=0.199 2
  (Δρ)max, (Δρ)min / (e ·nm-3)	866, -640	757, -714

  CCDC: 609919, 1; 611963, 2.

  1.4   Procedure for DNA binding experiments

  By the electronic absorption spectral method, the relative binding of the two complexes to calf thymus (CT) DNA was studied in 10 mmol·L^-1 Tris-HCl/NaCl buffer (pH=7.5). The solution of CT-DNA gave a ratio of UV absorbance at 260 nm and 280 nm (A₂₆₀ /A₂₈₀) of 1.8~1.9, indicating that the DNA was sufficiently free of protein^[31]. The stock solution of CT-DNA was prepared in Tris-HCl/NaCl buffer, pH=7.5 (stored at 4 ℃ and used not more than 4 days). The concentration of CT-DNA was determined from its absorption intensity at 260 nm with a molar extinction coefficient of 6 600 L·mol^-1·cm^{-1 [32]}.

  2   Results and discussion

  2.1   Crystal structure description

  Selected bond lengths and angles of 1 and 2 are listed in Table 2. The structure of complex 1 is shown in Fig. 1a. One Cu ion was five coordinated by three N atoms from ligand Bn₃tacn and two O atoms from m-bdc ligand while the other Cu ion was five coordinated by three N atoms from ligand Bn₃tacn, one O atom from m-bdc ligand and the other O atom from H₂O. As shown in Scheme 1, complex 1 adopts an unusual bis(bidentate/monodentate) coordination type due to the steric hindrance. The intermolecular distance of the two Cu(Ⅱ) ions is 1.004 3 nm. Three benzyl groups are flatted opposite the m-bdc ligand, which favor atom N coordinating to copper ion. The length of Cu(1)-N(1) is 0.219 17 nm, which is slightly longer than the length of Cu(1)-N(2) of 0.201 97 nm, Cu(1)-N(3) of 0.203 75 nm, Cu(1)-O(1) of 0.200 63 nm and Cu(1)-O(2) of 0.195 91 nm as show in Fig. 1a.

  Table 2.  Selected bond lengths (nm) and angles (°) for 1 and 2

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  1
  Cu(1)-O(2)	0.195 9(11)	Cu(1)-O(4)	0.204 7(10)	Cu(1)-O(1)	0.200 6(3)
  Cu(1)-N(1)	0.219 17(19)	Cu(1)-N(2)	0.201 96(19)	Cu(1)-N(3)	0.203 74(19)
  O(2)-Cu(1)-O(1)	70.69(18)	N(2)-Cu(1)-N(1)	86.22(7)	O(2)-Cu(1)-N(2)	101.67(17)
  O(2)-Cu(1)-C(28)	33.9(2)	O(1)-Cu(1)-N(2)	168.78(11)	O(1)-Cu(1)-C(28)	36.75(15)
  O(2)-Cu(1)-N(3)	167.2(2)	N(2)-Cu(1)-C(28)	135.09(14)	O(1)-Cu(1)-N(3)	99.35(11)
  N(3)-Cu(1)-C(28)	135.56(13)	N(2)-Cu(1)-N(3)	86.9O(7)	O(4)-Cu(1)-C(28)	47.5(2)
  O(2)-Cu(1)-O(4)	13.6(2)	N(3)-Cu(1)-C(28)	46.8(2)	O(1)-Cu(1)-O(4)	84.2(2)
  N(1)-Cu(1)-C(28)	106.76(13)	N(2)-Cu(1)-O(4)	88.48(19)	N(2)-Cu(1)-N(1)	86.22(7)
  N(3)-Cu(1)-O(4)	172.1(3)	N(3)-Cu(1)-N(1)	86.33(7)	O(2)-Cu(1)-N(1)	103.6(3)
  O(4)-Cu(1)-N(1)	99.7(3)	O(1)-Cu(1)-N(1)	103.38(11)
  2
  Cu(1)-O(2)	0.196 5(5)	Cu(1)-Cu(2)	0.295 7O(15)	Cu(1)-O(1)	0.202 1(5)
  Cu(2)-O(2)	0.195 7(5)	Cu(1)-N(3)	0.207 9(6)	Cu(2)-O(1)	0.199 3(5)
  Cu(1)-N(1)	0.211 9(6)	Cu(2)-N(4)	0.209 1(6)	Cu(1)-N(2)	0.227 5(6)
  Cu(2)-N(6)	0.208 8(6)	Cu(2)-N(5)	0.230 3(6)
  O(2)-Cu(2)-O(1)	81.3(2)	O(2)-Cu(1)-N(3)	168.3(2)	O(2)-Cu(2)-N(4)	96.4(2)
  O(1)-Cu(1)-N(3)	95.O(2)	O(1)-Cu(2)-N(4)	174.2(2)	O(2)-Cu(1)-N(1)	98.2(2)
  O(2)-Cu(2)-N(6)	168.O(2)	O(1)-Cu(1)-N(1)	169.2(2)	O(1)-Cu(2)-N(6)	96.O(2)
  N(3)-Cu(1)-N(1)	84.4(2)	N(4)-Cu(2)-N(6)	85.2(2)	O(2)-Cu(1)-N(2)	108.O(2)
  O(2)-Cu(2)-N(5)	108.5(2)	O(1)-Cu(1)-N(2)	107.8(2)	O(1)-Cu(2)-N(5)	102.9(2)
  N(3)-Cu(1)-N(2)	83.7(2)	N(4)-Cu(2)-N(5)	82.9(2)	N(1)-Cu(1)-N(2)	82.5(2)
  N(6)-Cu(2)-N(5)	83.5(2)	O(2)-Cu(1)-Cu(2)	40.97(15)	O(2)-Cu(2)-Cu(1)	41.18(14)
  O(1)-Cu(1)-Cu(2)	42.2O(15)	O(1)-Cu(2)-Cu(1)	42.91(16)	N(3)-Cu(1)-Cu(2)	130.78(16)
  N(4)-Cu(2)-Cu(1)	133.11(16)	N(1)-Cu(1)-Cu(2)	132.71(16)	N(6)-Cu(2)-Cu(1)	131.84(17)

  图1 (a) Molecular structure of complex 1; (b) Molecular structure of complex 2 with thermal ellipsoids at 30% probability level Figure1. (a) Molecular structure of complex 1; (b) Molecular structure of complex 2 with thermal ellipsoids at 30% probability level

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  图Scheme 1 Eight coordination mode of m-bdc Scheme1. Eight coordination mode of m-bdc

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  As shown in Fig. 1b, the crystal structure is made up of [Cu₂(Bn₃tacn)₂(μ-OH)₂]²⁺ cation and ClO₄^- anions. The complex crystallizes in the monoclinic C2/c space group. An interesting feature of complex 2 is that the complex has a central [Cu₂O₂] coordination core where copper and hydroxo oxygen atom are alternate to each other, constituting a cyclic four-membered ring. Noteworthily, the Cu1-O1-Cu2-O2 bridging unit is not strictly coplanar, whereas a roof shape core as the complex have been reported^[15] because of the hydrogen in H₂O was adopted as syn-syn type. Cu1-O1-Cu2 angle is 94.91°, while the angle of Cu1-O2-Cu2 is 97.85°. The geometry around both Cu atoms can best be described as an axially elongated octahedron, with the atoms N1, N2, N3, O2 constituting the basal plane and the atom O1 occupying the axial position. The essential similarity of the two independent bridging Cu-O bond lengths of 0.195 7 and 0.199 3 nm demon-strates that the oxygen atoms are asymmetrically bound to the two copper atoms. The length of Cu-Cu is 0.295 7 nm.

  2.2   IR and UV-Vis spectrum

  Both the two complexes have the similar IR spectra. The broad peak at 3 600~3 300 cm^-1 should be attributed to the ν(O-H) stretching frequency of water molecule in the complexes. The ν(C-H) stretching frequency of Bn₃tacn is indicated by a shoulder band involving a split sharp peak at 2 980 and 2 850 cm^-1. Two moderate absorption, at 1 593 and 1 548 cm^-1, are ascribed to the characteristic absorption of C=O in the m-bdc. The peak at 1 121 and 625 cm^-1 should be the absorption of ClO₄^-. Additionally, the abundant of absorption peaks in the region of 670~840 cm^-1 strongly argues in favor of the presence of phenyl. All of these results are quite consistent with the crystal structures of complexes 1 and 2.

  The UV-Vis spectra of complexes 1 and 2 were tested in dissolving DMSO solvent at room temperature. The wide absorption band in 656 nm was attributed to the ²B₁→²B₂ electronic transformation of Cu²⁺ in the tetragonal pyramid C₄ confirmation of complex 1^[33]. The absorption of 300 nm was attributed to the charges transfer of ligand Bn₃tacn in complex 2, while the absorption at 656 nm was also observed due to ²B₁→²B₂ electronic transformation of Cu²⁺ (3d⁹) with symmetric C₄ coordination environments^[34].

  2.3   DNA binding

  In order to investigate whether DNA was the biological target of the complex, its interactions with calf thymus DNA (CT-DNA) were tested by UV-Vis, Fluorescence spectroscopy. The absorption spectra of the complex 1 in the absence and presence of CT-DNA at various concentrations are given in Fig. 2. Free Bn₃tacn does not have any absorption band in the ultraviolet region because of the high energy gap between LUMO and HOMO. However, copper binding lowered the gap and thus caused the transition occurring at 256 nm. The potential CT-DNA binding ability of complexes was studied by UV spectroscopy by following the intensity changes of the intraligand π-π^* transition band at 256 nm. Upon addition of an increasing amount of CT-DNA (from 1×10^-5 to 1×10^-4 mol·L^-1) to the complex 1 (1×10^-5 mol·L^-1), a 20% hypochromism and a slight red shift (7~12 nm) were observed, which indicate strong binding of the complex to DNA.

  图2 UV-Vis absorption spectra of the complex in mixed solution of CH₃CN and buffer with increasing concentration of CT-DNA Figure2. UV-Vis absorption spectra of the complex in mixed solution of CH₃CN and buffer with increasing concentration of CT-DNA

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  The DNA-EB system was used to further probe the DNA binding mode of the complex 1. Ethidium bromide (EB) is an intercalator that gives a significant increase in fluorescence emission when bound to DNA and its displacement from DNA results in decrease in fluorescence intensity^[35]. Fluorescence titration spectra are shown in Fig. 3. The emission intensity decreased with the increase of the concentration of the complex, which suggests that the complex can replace EB from CT-DNA and intercalate into the DNA double helix.

  图3 Fluorescence emission spectra of the EB-DNA system in the absence (dotted line) and presence (solid line) of 1 Figure3. Fluorescence emission spectra of the EB-DNA system in the absence (dotted line) and presence (solid line) of 1

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  2.4   Cyclic voltammetry

  As shown in Fig. 4, the cyclic voltammogram of the ternary complex in absolute chromatographic DMSO solution containing 0.1 mol·L^-1 [(n-Bu)₄N]ClO₄ (Scan speed: 100 mV·s^-1) shows the ternary complex is redox-active. The cathodic peak potential (E_pc) and the anodic peak potential (E_pa) are 1 080 and 186 mV, respectively. The separation of the anodic and cathodic peak potentials (ΔE_p) is 894 mV.

  图4 Cyclic voltammogram diagram of the complex 2 in DMSO Figure4. Cyclic voltammogram diagram of the complex 2 in DMSO

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• 

  Figure 1  (a) Molecular structure of complex 1; (b) Molecular structure of complex 2 with thermal ellipsoids at 30% probability level

  Hydrogen atoms have been omitted for clarity in (a); Hydrogen atoms, ClO₄^- and H₂O have been omitted for clarity in (b)

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  Scheme 1  Eight coordination mode of m-bdc

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  Figure 2  UV-Vis absorption spectra of the complex in mixed solution of CH₃CN and buffer with increasing concentration of CT-DNA

  Buffer=10 mmol·L^-1 tris, 1 mmol·L^-1 Na₂EDTA, pH=7.50; V_CH3CN:V_buffer=1:3; c_complex=2.0 ×10^-5 mol·L^-1, r=c_CT-DNA/c_complex= 0~0.6

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  Figure 3  Fluorescence emission spectra of the EB-DNA system in the absence (dotted line) and presence (solid line) of 1

  λ_ex=526 nm; c_EB=2.5×10^-5 mol·L^-1, c_DNA=1×10^-4 mol·L^-1; c₁=1×10^-4 mol·L^-1, 10 μL per scan

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  Figure 4  Cyclic voltammogram diagram of the complex 2 in DMSO

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  Table 1.  Crystal data and structure refinements for 1 and 2

  Complex	1	2
  Empirical formula	C66H76Cl2Cu2N8O12	C54H66Cl2Cu2N6O10
  Formula weight	1 371.32	1 157.10
  Temperature / K	153(2)	294(2)
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	C2/c
  a / nm	1.028 41(16)	2.787 5(16)
  b / nm	3.307 9(3)	1.534 3(7)
  c / nm	1.116 61(18)	2.967 4(13)
  β/(°)	113.351(2)	91.978(13)
  V / nm3	3.487 5(9)	12.684(11)
  Z	2	8
  Dc / (g·cm-3)	1.306	1.218
  μ / mm-1	0.75	0.809
  Crystal size / mm	0.16×0.14×0.10	0.26×0.18×0.16
  θ range / (°)	2.08~27.85	1.52~25.02
  Limiting indices	-13 ≤ h ≤13, -41 ≤ k ≤42, -11 ≤ l ≤ 14	-33≤h≤ 18, -18 ≤ k ≤ 18, -35 ≤ l ≤ 35
  Reflection collected, unique	26 745, 7 536 (Rint=0.018 1)	30 996, 11 052 (Rint=0.136 3)
  F(000)	1 432	4 832
  Goodness-of-fit on F2	1.053	0.946
  Final R indices [I>2σ(I)]	R1=0.040 2, wR2=0.118 7	R1=0.081 4, wR2=0.256 2
  R indices (all data)	R1=0.047 7, wR2=0.115 2	R1=0.175 0, wR2=0.199 2
  (Δρ)max, (Δρ)min / (e ·nm-3)	866, -640	757, -714

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  Table 2.  Selected bond lengths (nm) and angles (°) for 1 and 2

  1
  Cu(1)-O(2)	0.195 9(11)	Cu(1)-O(4)	0.204 7(10)	Cu(1)-O(1)	0.200 6(3)
  Cu(1)-N(1)	0.219 17(19)	Cu(1)-N(2)	0.201 96(19)	Cu(1)-N(3)	0.203 74(19)
  O(2)-Cu(1)-O(1)	70.69(18)	N(2)-Cu(1)-N(1)	86.22(7)	O(2)-Cu(1)-N(2)	101.67(17)
  O(2)-Cu(1)-C(28)	33.9(2)	O(1)-Cu(1)-N(2)	168.78(11)	O(1)-Cu(1)-C(28)	36.75(15)
  O(2)-Cu(1)-N(3)	167.2(2)	N(2)-Cu(1)-C(28)	135.09(14)	O(1)-Cu(1)-N(3)	99.35(11)
  N(3)-Cu(1)-C(28)	135.56(13)	N(2)-Cu(1)-N(3)	86.9O(7)	O(4)-Cu(1)-C(28)	47.5(2)
  O(2)-Cu(1)-O(4)	13.6(2)	N(3)-Cu(1)-C(28)	46.8(2)	O(1)-Cu(1)-O(4)	84.2(2)
  N(1)-Cu(1)-C(28)	106.76(13)	N(2)-Cu(1)-O(4)	88.48(19)	N(2)-Cu(1)-N(1)	86.22(7)
  N(3)-Cu(1)-O(4)	172.1(3)	N(3)-Cu(1)-N(1)	86.33(7)	O(2)-Cu(1)-N(1)	103.6(3)
  O(4)-Cu(1)-N(1)	99.7(3)	O(1)-Cu(1)-N(1)	103.38(11)
  2
  Cu(1)-O(2)	0.196 5(5)	Cu(1)-Cu(2)	0.295 7O(15)	Cu(1)-O(1)	0.202 1(5)
  Cu(2)-O(2)	0.195 7(5)	Cu(1)-N(3)	0.207 9(6)	Cu(2)-O(1)	0.199 3(5)
  Cu(1)-N(1)	0.211 9(6)	Cu(2)-N(4)	0.209 1(6)	Cu(1)-N(2)	0.227 5(6)
  Cu(2)-N(6)	0.208 8(6)	Cu(2)-N(5)	0.230 3(6)
  O(2)-Cu(2)-O(1)	81.3(2)	O(2)-Cu(1)-N(3)	168.3(2)	O(2)-Cu(2)-N(4)	96.4(2)
  O(1)-Cu(1)-N(3)	95.O(2)	O(1)-Cu(2)-N(4)	174.2(2)	O(2)-Cu(1)-N(1)	98.2(2)
  O(2)-Cu(2)-N(6)	168.O(2)	O(1)-Cu(1)-N(1)	169.2(2)	O(1)-Cu(2)-N(6)	96.O(2)
  N(3)-Cu(1)-N(1)	84.4(2)	N(4)-Cu(2)-N(6)	85.2(2)	O(2)-Cu(1)-N(2)	108.O(2)
  O(2)-Cu(2)-N(5)	108.5(2)	O(1)-Cu(1)-N(2)	107.8(2)	O(1)-Cu(2)-N(5)	102.9(2)
  N(3)-Cu(1)-N(2)	83.7(2)	N(4)-Cu(2)-N(5)	82.9(2)	N(1)-Cu(1)-N(2)	82.5(2)
  N(6)-Cu(2)-N(5)	83.5(2)	O(2)-Cu(1)-Cu(2)	40.97(15)	O(2)-Cu(2)-Cu(1)	41.18(14)
  O(1)-Cu(1)-Cu(2)	42.2O(15)	O(1)-Cu(2)-Cu(1)	42.91(16)	N(3)-Cu(1)-Cu(2)	130.78(16)
  N(4)-Cu(2)-Cu(1)	133.11(16)	N(1)-Cu(1)-Cu(2)	132.71(16)	N(6)-Cu(2)-Cu(1)	131.84(17)

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