Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 81–85

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Crystal structure, Hirshfeld surfaces and DNA cleavage investigation of two copper(II) complexes containing polypyridine and salicylide ligands Yang-Hui Luo, Bai-Wang Sun ⇑ School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Two copper complexes derived from

polypyridine and salicylide ligands have been prepared.  Intermolecular interactions in them have been investigated by using Hirshfeld surfaces.  These two complexes can intercalation with DNA.

a r t i c l e

i n f o

Article history: Received 6 October 2013 Received in revised form 21 January 2014 Accepted 27 January 2014 Available online 13 February 2014 Keywords: Copper complex Crystal structure Hirshfeld surfaces DNA cleavage

a b s t r a c t Two copper complexes 1 [Cu2(phen)2(salicylaldehyde)2(ClO4)2] and 2 [Cu2 (2,20 -dipyridyl)2(salicylaldehyde)2(ClO4)2] have been synthesized and characterized by elemental analysis and single-crystal X-ray diffraction. These two complexes were display binuclear structure with CuII ions in distorted octahedral environment but antipodal orientation of the binuclear units between them. Molecular Hirshfeld surfaces revealed that the crystal structures of 1 and 2 were supported mainly by H–H, C–H  p, p  p (C–C), and C–H  O intermolecular interactions. DNA cleavage experiments of complexes 1 and 2 revealed that these complexes can intercalation with DNA. Ó 2014 Elsevier B.V. All rights reserved.

Introduction It has been known for more than five decades that some series of ligands such as polypyridine (dipyridine, 1,10-phenanthroline, etc.), salicylide and salicylic acid, which display markedly biological activity, can improve their biological activities remarkably by means of complexation with transitions-metals, especially with copper [1–8]. The most classical example is the copper complex ⇑ Corresponding author. Tel./fax: +86 25 52090614. E-mail address: [email protected] (B.-W. Sun). http://dx.doi.org/10.1016/j.saa.2014.01.138 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

of acetylsalicylic acid: laboratory synthesized [Cu2(aspirin)4] displayed more effective anti- inflammatory activity than acetylsalicylic acid itself [9]. Some copper–organic systems, such as copper–phenanthroline, copper–dipyridine, and copper–salicylate systems have been shown to cleave isolated DNA and bind RNA by intercalative interaction of the corresponding ligands, thus they possess strong cytotoxicity and antiviral activity [10–13]. DNA is the target of many anti-tumor drugs, so these above copper–organic systems could possibly be explored to be chemotherapeutic agents or anticancer drugs.

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The prediction and computation of molecular crystal structure through the aspect of intermolecular interactions through Hirshfeld surfaces also have attracted attention in recently years [14–18]. Hirshfeld surfaces serves as a powerful tool for elucidating molecular crystal structures, it is a space partitioning construct that summarizes the crystal packing into a single 3D surface, and the surface can reduced to a 2D fingerprint plot, which summarize the complex information on intermolecular interactions present in molecular crystals [15]. The principles of Hirshfeld surface and fingerprint plot are reported in other literature [14]. In order to in-depth investigate the interactions of copper–organic systems with DNA for the purpose of develop more excellent chemotherapeutic agents or better anticancer drugs, in this work, we synthesized two copper(II) complexes (1 [Cu2(phen)2(salicylaldehyde)2(ClO4)2] and 2 [Cu2 (2,20 -dipyridyl)2(salicylaldehyde)2 (ClO4)2]) by using Cu(ClO4)26H2O, salicylide with 1,10-phenanthroline and 2,20 -dipyridine, respectively, and we characterized their structures by single-crystal X-ray diffraction and elemental analyses. We further compared these two crystal structures by Hirshfeld surfaces. The DNA cleaves interaction investigated of these two complexes were performed by using calf thymus DNA (CT-DNA) through the measurement of emission spectroscopy. Experimental section Materials and methods All reagents are obtained commercially, and used as received without further purification, Cu(ClO4)26H2O, salicylide 1,10phenanthroline and 2,20 -dipyridine were purchased from Alfa Aesar; (CT-DNA), ethidium bromide (EB) and pUC19 were purchased from Sigma Chemical Co. Physical measurements Elemental analyses were performed by a Vario-EL III elemental analyzer for carbon, hydrogen, and nitrogen of these compounds. Emission spectroscopy were recorded by using a Fluoromax-4 spectrofluorometer from Horiba with kex = 360 nm and slit width of 8. X-ray crystallographic study The single-crystal X-ray diffraction data of complexes (1) and (2) were collected at 293 K with graphite–monochromated Mo Ka radiation (k = 0.071073 nm), and used a Rigaku SCXmini diffractometer with the x-scan technique [19]. The lattice parameters were integrated using vector analysis and refined from the diffraction matrix, the absorption correction was carried out by using Bruker SADABS program with multi-scan method. A summary of crystallographic data, data collection, and refinement parameters for (1) and (2) were summarized in Table 1. The structures were solved by full-matrix least-squares methods on all F2 data, and the structure solution and structure refinement were performed by SHELXS-97 and SHELXL-97 programs [20], respectively. Reliability factors were defined as R1 = R(|Fo|  |Fc|)/R|Fo| and the hP i1=2 . All nonwðF 2o  F 2c Þ2=wðF o Þ4 function minimized was Rw ¼ hydrogen atoms were refined anisotropically, and hydrogen atoms were inserted at their calculated positions and fixed at their positions [21]. The molecular graphics were prepared by using the DIAMOND program [22] and mercury [23]. Synthesis and characterization Complex 1 [Cu2(phen)2(salicylaldehyde)2(ClO4)2] was obtained by adding Cu(ClO4)26H2O, salicylide and 1,10-phenanthroline at

Table 1 Crystal data and structure refinement for complexes (1) and (2). Complex

1

2

Formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg m3) T (K) l (mm1) Cryst. dimensions No. of reflns collected No. of unique reflns No. of params Goodness-of-fit on F2 R1 ,wR2 ((I > 2r(I)) R1 ,wR2 (all data) CCDC NO.

C38H24Cl2Cu2N4O12 926.59 Triclinic P-1 9.1476(18) 9.3021(19) 11.713(2) 96.17(3) 111.07(3) 97.09(3) 910.4(3) 1 1.69 293(2) 1.388 0.2  0.12  0.1 mm 4154 3221 262 1.013 0.0454, 0.0992 0.0645, 0.1072 888752

C34H26Cl2Cu2N4O12 880.57 Triclinic P-1 8.4799(19) 9.420(2) 11.858(3) 78.042(2) 69.604(2) 82.910(2) 867.2(3) 1 1.686 293(2) 1.452 0.3  0.2  0.15 mm 3004 2510 244 1.05 0.0334, 0.0930 0.0416, 0.1003 888754

stoichiometric ratio of 1:1:1 to a certain amount of methanol at temperature of 25 °C under continuous stirring. After stirring for 30 min, the solution which results from the mixture was filtered off and allowed to evaporate at room temperature. Single crystals of 1 were grown from the solution by slow evaporation at room temperature within 5 days. Yield 80%, based on Cu. Elemental analysis for 1, Anal. Calcd. (%): C, 49.25; N, 6.05; H, 2.61. Found: C, 49.13; N, 6.12; H, 2.56. Complex 2 [Cu2(2,20 -dipyridyl)2(salicylaldehyde)2(ClO4)2] was prepared following the identical procedure as 1 but 2,20 -dipyridine instead of 1,10-phenanthroline. Yield 82%, based on Cu. Elemental analysis for 2, Anal. Calcd. (%): C, 49.37; N, 6.36; H, 2.97. Found: C, 49.31; N, 6.52; H, 2.86. DNA cleavage experiments DNA cleavage experiments were performed in Tris–HCl buffer (5 mM Tris/50 mM NaCl, pH 7.2). CT-DNA in Tris–HCl buffer solutions was sufficiently free of protein, and the concentration of it was determined spectrophotometrically using the molar absorption coefficient 6600 M1 cm1 at 260 nm. The stock solution were stored at 4 °C and used for 2 days. In the fluorescence quenching experiments, the ethidium bromide (EB)-DNA (1:1, 1  105 M) solution was prepared, and stored in the dark for 12 h. Complexes 1 and 2 were titrated into the EB-DNA mixture, resulting the different concentration ratios of the complex to DNA. The solution was incubated at room temperature for 2 h in the dark. Viscosity measurements were performed at 25 °C using a fixed concentration of DNA solution (1  104 M) and increasing the concentration of complexes. Each sample was measured in triplicate and the average flow time was calculated with a digital stopwatch. Hirshfeld surfaces calculations Molecular Hirshfeld surfaces calculations were performed by using the CrystalExplorer [24] program. When the cif files of 1 and 2 were read into the CrystalExplorer program, all bond lengths to hydrogen were automatically modified to typical standard neu0 0 tron values (C–H = 1.083 Å A and N–H = 1.009 Å A). The molecular Hirshfeld surfaces were generated using a standard (high) surface resolution with the 3D dnorm surfaces are mapped over a fixed color

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Fig. 1. Molecular structures and numbering scheme of complexes 1 and 2, hydrogen atoms were omitted for clarity (Symmetry code: A: x, y, z). 0

scale of 0.42 (red) to 1.6 Å A (blue), the 2D fingerprint plots 0 displayed by using the standard 0.6–2.6 Å A view with the de and di distance scales displayed on the graph axes. Results and discussion Crystal structure Complex 1 crystallizes as blue block crystals in the monoclinic P-1 space group with Z = 1, the asymmetric unit (ASU) consisting of a binuclear copper complex which completed by two CuII ions, two 1,10-phenanthroline molecules, two salicylide anions, and  two ClO4 anions (Fig. 1(1)). In the ASU, the two CuII ions are crystallographically independent and display an almost identical environment: each CuII ion presents a distorted octahedral geometry formed by one 1,10-phenanthroline chelate via N,N0 atoms 0  (average bond length 2.0015 (2) Å A), one ClO4 anions chelate via an oxygen atom, and two salicylide anions bidentate via the 0 carbonyl and hydroxyl groups (average bond length 1.925 (2) Å A), moreover, this hydroxyl 0group is bonded to the second CuII ion (bond length 2624 (2) Å A). Thus, the salicylide anion acts as a bridging ligand between the CuII ions with bond angle Cu1–O1– Cu1A of 95.13 (10)° (the main bond lengths and angles in complexes 1 and 2 were summarized in Table 2). For the distorted octahedral geometry, the trans axial positions are occupied by O4 and O1A with the O4–Cu1–O1A bond angle of 175.11 (10)°, which is almost perpendicular to the equatorial mean plane. The equatorial mean plane is slightly distorted (Rms

deviation of 0.0548 Å) with CuII ion displaced by 0.0193 Å towards O4. Complex 2 shows an almost identical crystal structure as 1. The PXRD profiles of them were shown in Fig. 2, the characteristic peaks of 1 and 2 almost have identical 2 theta degree except a peak around 10°, where the characteristic peak for 1 is located at 10.56°, while at 11.16° for 2. The complexes 1 and 2 also shown identical TGA profiles (Fig. S1), they all undergo three steps of mass  loss attribute to sublimation of ClO4 anions, salicylaldehyde and 1,10-phenanthroline/2,20 -dipyridine. So, we no longer describe the crystal structures of 2 detailed. In the binuclear copper complex unit, p  p interactions between 1,10-phenanthroline and salicylide anion (plane separation of 3.599 Å) for 1, and between 2,20 -dipyridyl and salicylide anion (plane separation of 3.575 Å) for 2 were observed (Fig. S2), which make a significant contribution to the stabilization of the binuclear copper complex units. The different binuclear copper complex units of 1 and 2 then stacked paralleled with each other through non-classical C–H  O  hydrogen bonding interactions between ClO4 anions and the ligands into 3D intricate structure along a axis (Fig. S3). It is interesting that the binuclear copper complex units display antipodal orientation for 1 and 2, and the C–H  O hydrogen bonding interactions are more complicated in 2 than in 1. Molecular Hirshfeld surfaces 3-D dnorm surface is used for identification of very close intermolecular interactions. The value of dnorm is negative or positive when

Table 2 Selected bond length () and bond angles for complexes 1 and 2. Complex 1 Cu1–O1 Cu1–O1A Cu1–O2 Cu1–N1 Cu1–N2 Cu1–O4

1.899(2) 2.624(2) 1.952(2) 2.004(3) 1.999(2) 2.494(2)

O1–Cu1–O2 O1–Cu1–N2 O2–Cu1–N2 O1–Cu1–N1 O2–Cu1–N1 N1–Cu1–N2

93.39(9) 92.41(10) 173.92(9) 173.05(9) 92.10(10) 82.26(11)

O4–Cu1–O1 O4–Cu1–O2 O4–Cu1–N1 O4–Cu1–N2 O4–Cu1–O1A Cu1–O1–Cu1A O1–Cu1–O1A

97.93(10) 82.51(9) 87.01(9) 94.89(10) 175.11(10) 95.13(10) 84.87(11)

Complex 2 Cu1–O1 Cu1–O1A Cu1–O2 Cu1–N1 Cu1–N2 Cu1–O5

1.9075(19) 2.666(2) 1.955(2) 1.988(2) 1.991(2) 2.559(2)

O1–Cu1–O2 O1–Cu1–N2 O2–Cu1–N2 O1–Cu1–N1 O2–Cu1–N1 N1–Cu1–N2

93.22(8) 93.79(9) 172.78(9) 173.81(8) 91.55(9) 81.58(10)

O5–Cu1–O1 O5–Cu1–O2 O5–Cu1–N1 O5–Cu1–N2 O5–Cu1–O1A Cu1–O1–Cu1A O1–Cu1–O1A

89.23(10) 81.65(9) 95.39(9) 96.72(10) 175.11(10) 96.35(10) 83.65(11)

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interactions in 1 and 2 are H–H, C–H  p, p  p (C–C), and C–H  O intermolecular interactions (Fig. 4). H–H interactions, which are reflected in the middle of scattered points in the 2-D fingerprint plots, have a most significant contribution to the total Hirshfeld surfaces (36.5% and 33.2% for 1 and 2, respectively). Then followed by the C–H  O (indicated by the points in the upper left region of the 2-D fingerprint plot), C–H  p (indicated by the ‘‘wings’’ in the upper left and lower right of the 2-D fingerprint plot), and p  p (C–C) interactions (indicated by the ‘‘backbone’’ of the 2-D fingerprint plot). Apart from those detailed above, the presence of O–O, O–N, O–C, N–C, and N–H interactions are observed, and they were all summarized in Table 3. DNA cleavage investigation

Fig. 2. PXRD profiles of complexes 1 and 2.

Table 3 Summary of the various contact contributions to the Hirshfeld surface area in complexes 1 and 2.

1 2

H–H

O–H

C–H

C–C

O–O

O–N

O–C

N–C

N–H

36.5 33.2

21.5 23.6

16.9 22.9

11.1 7.4

1.3 1.3

2.4 2.7

3.5 2.4

1.6 1.7

1.7 1.2

intermolecular contacts are shorter or longer than rvdW (van der Waals (vdW) radii), respectively. The dnorm values are mapped onto the Hirshfeld surface by using a red–blue–white color scheme: red regions represent closer contacts and negative dnorm value; blue regions represent longer contacts and positive dnorm value; and white regions represent the distance of contacts is exactly the vdW separation and with a dnorm value of zero. The 3-D dnorm surface of complexes 1 and 2 were shown in Fig. S4, the red points on them are correspond to the significant C–H  p and C–H  O intermolecular interactions, and it is clear that the number of red points in 2 is more than that in 1. Actually, the C–H  p and C–H  O intermolecular interactions contribute 16.9% and 21.5% for 1, 22.9% and 23.6% for 2 to the total Hirsheld surfaces (Table 3). While the white regions on the dnorm surfaces in 1 are larger than in 2. The 2-D fingerprint plots, which analyses all of the intermolecular contacts at the same time, reveal that the main intermolecular

The fact that polypyridine aromatic rings could insert into the base pairs of the isolated DNA double helix and lead to the cleavage of DNA double helix has been reported in other literature [25]. The DNA cleavage investigation of complexes 1 and 2 were performed by using EB (ethidium bromide) bound to CT-DNA, through the measurement of the quenching extent of the fluorescence of EB bound to DNA for the determination of the cleavage of DNA, which attribute to the binding of the second molecule with DNA. Binding of the complexes 1 and 2 with DNA can result in the cleavage of the DNA-bound EB molecule, which then leads to a reduction of the emission intensity [26]. The results of the DNA cleavage investigation of complexes 1 and 2 were shown in Fig. 3, which reveal the cleavage of DNA duo to the intercalation of 1 and 2 in it. The results in Fig. 3 are in good agreement with the linear Stern–Volmer equation, and the Stern–Volmer quenching constants Ksv are found to be 0.901 and 0.905 for complexes 1 and 2, respectively. The result suggested that complexes 1 and 2 could intercalate into DNA, but with weak affinity. To further elucidate the interactions of 1 and 2 with CT-DNA, viscosity measurements were carried out. Hydrodynamic measurements that are sensitive to length change are regarded as the least ambiguous and most critical test of binding mode in the absence of crystallographic structural data. [27] A classical intercalation model between ligand and DNA helix often causes lengthening of the DNA helix, thus leading to the increase in DNA viscosity. [27] Fig. 4 shows the changes in relative viscosity of CT-DNA in the presence of complexes 1 and 2. The result shows that the both 1 and 2 induce increase in the density, but the extent of increase is a small, indicating weak intercalative binding between them and CT-DNA. The viscosity data are presented as (g  g0)1=3 versus

Fig. 3. Emission spectra of EB bound to DNA in the absence and presence of complexes 1 and 2, respectively; [complex] = 0–1  105 M. The arrow shows the intensity changes upon increasing complex concentration.

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[11] [12] [13] Fig. 4. The relative viscosity of DNA in the presence of complexes 1 and 2. [14] [15]

([complex]/[DNA]), where g is the viscosity of DNA in the presence of complexes 1 and 2, and g0 is the viscosity of DNA.

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Conclusions In conclusion, we synthesized and characterized two new binuclear copper complexes 1 [Cu2(phen)2(salicylaldehyde)2(ClO4)2] and 2 [Cu2(2,20 -dipyridyl)2(salicylaldehyde)2(ClO4)2]. Complexes 1 and 2 display almost identical structure. Hirshfeld surfaces analysis revealed that the intermolecular interactions in these two complexes are dominated by H–H, C–H  p, p  p (C–C), and C–H  O contacts. We also investigated the DNA cleavage of complexes 1 and 2, and found that these complexes can intercalation with DNA.

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Acknowledgements This work has been supported by the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1340), Fundamental Research Funds for the Central Universities (CXZZ12_0119), Natural Science Foundation of China (21371031), Prospective Joint Research Project of Jiangsu province (BY2012193) and Funds for the Ministry of Science and Technology (21241009). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.01.138.

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Crystal structure, Hirshfeld surfaces and DNA cleavage investigation of two copper(II) complexes containing polypyridine and salicylide ligands.

Two copper complexes 1 [Cu2(phen)2(salicylaldehyde)2(ClO4)2] and 2 [Cu2 (2,2'-dipyridyl)2(salicylaldehyde)2(ClO4)2] have been synthesized and characte...
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