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DOI: 10.1039/C6RA05239H (Paper) RSC Adv., 2016, 6, 47196-47202

Synthesis, crystal structure, DNA-binding and magnetism of copper 15-metallacrown-5 complexes based on glycinehydroxamic acid ligand†

Yanxia Menga, Hua Yanga, Dacheng Li*a, Suyuan Zenga, Guifang Chenb, Shengli Lib and Jianmin Dou*a
aShandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng University, 252059 Liaocheng, People's Republic of China. E-mail: jmdou@lcu.edu.cn; Tel: +86-0635-8239298
bDongchang College, Department of Chemistry and Biology, 252000 Liaocheng, People's Republic of China

Received 28th February 2016 , Accepted 3rd May 2016

First published on 4th May 2016

Abstract

Six metallacrown complexes {La(OAc)(H2O)2[15-MCCu(N)glyha-5](H2O)4}·2NO3·5H2O (1), {Gd(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)3}·2NO3·5H2O (2), {Tb(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)4}·2NO3·4H2O (3), {Pr(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)}·2NO3·8H2O (4), {Nd(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·7H2O (5) and {Sm(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·6H2O (6) (glyha2− = dianion glycinehydroxamic acid) have been synthesized and structurally characterized by X-ray single crystal diffraction, IR spectroscopy and elemental analysis. Spectroscopic studies show that these complexes bind to CT-DNA via an intercalative mode and cause a more B-like to a more C-like conformational change. Magnetic measurements indicate that complexes 1, 3, 4, 5 and 6 dominantly show antiferromagnetic interactions between metal centers, whereas complex 2 reveals ferromagnetic interactions.

Introduction

During the past few decades, in reducing cytotoxicity, many attempts have been made to develop and synthesize novel metal complexes as anticancer drugs. DNA is the primary target for most anticancer and antiviral drugs.1 Very recent studies have demonstrated that certain metal complexes can efficiently promote the hydrolytic cleavage of DNA, which was effected by the steric structure, nature of central metal ions and the size of the chelate rings of the complexes.2 Also, DNA binding studies are very important for the development of DNA probes and chemotherapy agents.3 So far the study of the binding mode between metal complex and DNA has been a hot and basic research subject of bioinorganic chemistry.4 Metal complexes interact with DNA in either non-covalent (which includes intercalative, electrostatic and groove binding) or covalent ways. Among them, intercalative binding is an important binding mode.5 A survey reveals that the intercalative capability of metal complexes was often affected by the planarity of the ligand, the type of central metal ion and the coordination geometry.6–9 Such as with the planar ligand, 1-[3-(2-pyridyl)pyrazol-1-ylmethyl]naphthalene, its Cu(II) and Cd(II) complexes can bind to CT-DNA by intercalation mode.6a But with the planar ligand, N,N′′′,N′′,N′′′-tetramethyltetra-3,4-yridinoporphyrazinatozinc(II) complex showed higher intercalation towards CT-DNA with respect to the cobalt(II) complex.6c Additionally, with the planar ligand and metal ion, [Cu3L3(μ3-OH)Cl2] and {[Cu3L3(μ3-OH)(OAc)2]py} (HL = phenyl 2-pyridyl ketoxime; dca = dicyanamide anion),8 as well as [Ni(L)(CH3COOH)2]2 and [Ni2(L)2(CH3OH)] (H2L = N,N-bis(salicylidene)-3,6-dioxa-1,8-diaminooctane),9 present different intercalative capability, which should be related to their different coordination geometry for the central metal ions. Nowadays, the interactions of copper complexes with DNA continue to be an attractive research filed.10 Two reasons are responsible for it. On one hand, copper, as an essential element for human, plays very important roles in several biological processes. On the other hand, copper complexes can bind to DNA through non-covalent ways. Many copper complexes have shown inhibitory effects on various tumor cells.11 Based on the studies of the copper complexes, the biological activitie of aminohydroxamic acid and the metallacrown, we want to synthesize the Ln[15-MCCu(N)glyha-5] (glyha = glycinehydroxamic acid) metallacrown, in order to get some insights into the DNA-binding of this type complex. Compared with the other reported metallacrowns, the Ln[15-MCCu(N)glyha-5] cases have the following advantages: (i) they present planar structures; (ii) they possess copper ions and lanthanide ions within a small molecule, and the Ln ions were capsulated in the ring centrum; (iii) copper atoms have two coordination modes: square-planar and square-pyramidal.

Since the first glycinehydroximate metallacrown Eu(NO3)3[15-MCCu(N)glyha-5] was reported,12 much attention has been focused on the Ln[15-MCCu(N)glyha-5] metallacrown. The second complex, Gd(NO3)[15-MCCu(N)glyha-5](NO3)2, was synthesized by Binnemans et al. in aqueous solution by a two-step method.13 After five years, the isostructural complexes Ln(SO4)[15-MCCu(N)glyha-5](SO4)2 were synthesized by Addison et al.14 with Ln referring to Pr, Nd, Sm, Eu, Gd, Dy and Ho. Recently, Eu(OAc)[15-MCCu(N)glyha-5](NO3)2 metallacrown with acetate anion coordinating to the central lanthanide ion was reported by Marina et al.15 In synthetic course, one-pot strategy involving calcium compounds was used. In the former reports, the studies mainly focused on the syntheses and structures of Ln[15-MCCu(N)glyha-5] systems. Herein, we report the syntheses and structures. And ​the DNA-binding with [15-MCCu(N)glyha-5] complexes has been involved. In addition, based on the interesting magnetic properties of 3d–4f heterometallic compounds, we also investigated magnetic properties of Ln[15-MCCu(N)glyha-5] complexes.

In this paper, we synthesized six metallacrown complexes with glycinehydroxamic acid ligand, namely, {La(OAc)(H2O)2[15-MCCu(N)glyha-5](H2O)4}·2NO3·5H2O (1), {Gd(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)3}·2NO3·5H2O (2), {Tb(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)4}·2NO3·4H2O (3), {Pr(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)}·2NO3·8H2O (4), {Nd(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·7H2O (5) and {Sm(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·6H2O (6). DNA-binding properties of the six complexes with calf thymus DNA (CT-DNA) were investigated by fluorescence spectroscopic of the EB–DNA system and circular dichroism (CD) studies. Direct current (dc) magnetic susceptibility studies were also examined.

Experimental section

Materials and physical methods

Glycinehydroxamic acid (H2glyha) was synthesized as previously reported.14 Ethidium bromide (EB) and calf thymus DNA (CT-DNA) were purchased from Sigma. Tris–HCl/NaCl buffer solution was prepared using double-distilled water. All other reagents were analytical grade as purchased from commercial sources and used without further purification.

Melting points were measured through a Kofler micro-melting point apparatus. Elemental analyses of C, H and N were determined using an Elemental Vario EL analyzer. IR spectra (400–4000 cm−1) were recorded on a Perkin-Elmer spectrophotometer with simples prepared as KBr disks. UV-vis spectra were performed on a UV-2550 ultraviolet spectrophotometer. Fluorescence spectra were obtained on an F-7000 FL Spectrophotometer. Direct current (dc) magnetic susceptibility studies were carried out on polycrystalline samples with the Quantum Design SQUID magnetometer MPMSXL in the temperature range 2–300 K under an applied field of 1 kOe.

X-ray crystallography

X-ray diffraction single-crystal data for the complexes were obtained on a Bruker Smart 1000 CCD diffractometer (graphite monochromized Mo or Cu Kα radiation, λMo = 0.71073 Å, λCu = 1.54184 Å) at 293(2) K or 298(2) K. A semiempirical absorption correction was applied to the data. All of the structures were solved by direct methods using SHELXS-97 and refined against F2 by full-matrix least-squares using SHELXS-97.16 Hydrogen atoms were added theoretically, riding on the concerned atoms and refined with fixed thermal factors. Crystallographic data for complexes 1–6 have been deposited to the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers: CCDC-1423973 (1), 1423977 (2), 1411162 (3), 1423974 (4), 1423975 (5) and 1423976 (6).

DNA-binding experiments

DNA-binding studies were performed by fluorescence spectroscopic using Tris–HCl (pH = 7.2) buffer solution as solvent. The DNA stock solution was prepared by dissolving the commercially purchased calf thymus DNA in Tris–HCl buffer solution. The CT-DNA concentration was determined by UV-vis absorption spectroscopy at 260 nm using a molar absorption coefficient of 6600 L mol−1 cm−1. The EB was also dissolved in Tris–HCl buffer solution and the concentration was calculated. All complexes were dissolved in double-distilled water at the concentration of 0.25 mmol L−1. A certain volume of each complex starting solution was gradually added into the DNA–EB solution and the DNA–EB solution containing CT-DNA (25 μmol L−1) and EB (3 μmol L−1). Fluorescence emission quenching spectra were recorded at 550–700 nm with all complexes excited at 258 nm and scan speed of 300 nm min−1.

CD spectra of DNA in the absence or presence of complexes were recorded on a Jasco J-810 spectropolarimeter with a scanning speed of 100 nm min−1 with scope of 220–400 nm at room temperature. The concentration of DNA and complexes were 0.1 mmol L−1 and 0.2 μmol L−1.

Syntheses

{La(OAc)(H2O)2[15-MCCu(N)glyha-5](H2O)4}·2NO3·5H2O (1). H2glyha (45 mg, 0.5 mmol) was dissolved in H2O (15 mL), followed by the addition of Cu(OAc)2·H2O (200 mg, 1.0 mmol). After 30 min, La(NO3)3·6H2O (88 mg, 0.2 mmol) was added, and the dark green solution turned blue. Then the reaction solution was stirred for about 5 h at room temperature. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 45.0%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C12H45Cu5LaN12O29: C, 9.71%; H, 3.27%; N, 11.48%. Found (%): C, 9.36%; H, 3.24%; N, 11.41%. IR (KBr, cm−1): 3416.3 (vs.), 2960.9 (m), 1590.1 (m), 1437.5 (m), 1385.1 (m), 1312.4 (m), 1129.9 (m), 1032.8 (m), 583.0 (w), 545.1 (m), 479.4 (m).
{Gd(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)3}·2NO3·5H2O (2). Synthesis of complex 2 was similar to complex 1 with Gd(NO3)3·6H2O (91 mg, 0.2 mmol) substituting La(NO3)3·6H2O. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 37.3%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C12H41Cu5GdN12O27: C, 11.43%; H, 3.28%; N, 13.33%. Found (%): C, 11.26%; H, 3.09%; N, 13.11%. IR (KBr, cm−1): 3426.7 (vs.), 2934.1 (m), 1594.4 (m), 1435.5 (m), 1385.0 (m), 1312.4 (m), 1147.0 (m), 1033.0 (m), 584.3 (w), 548.1 (m), 481.1 (m).
{Tb(OAc)(H2O)[15-MCCu(N)glyha-5](H2O)4}·2NO3·4H2O (3). Synthesis of complex 3 was similar to complex 1 with Tb(NO3)3·6H2O (91 mg, 0.2 mmol) substituting La(NO3)3·6H2O. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 39.6%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C12H41Cu5TbN12O27: C, 11.42%; H, 3.27%; N, 13.32%. Found (%): C, 11.23%; H, 3.11%; N, 13.07%. IR (Kr, cm−1): 3424.4 (vs.), 2930.0 (m), 1600.6 (m), 1447.8 (m), 1385.1 (m), 1385.2 (m), 1316.6 (m), 1143.6 (m), 1035.5 (m), 594.6 (w), 547.6 (m), 480.3 (m).
{Pr(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)}·2NO3·8H2O (4). Synthesis of complex 4 was similar to complex 1 with Pr(NO3)3·6H2O (44 mg, 0.1 mmol) substituting La(NO3)3·6H2O. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 56.9%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C10H42Cu5PrN13O30: C, 9.36%; H, 3.30%; N, 14.19%. Found (%): C, 9.12%; H, 3.13%; N, 14.02%. IR (KBr, cm−1): 3426.4 (vs.), 2961.8 (m), 1589.3 (m), 1441.1 (m), 1384.7 (m), 1324.9 (m), 1141.6 (m), 1031.5 (m), 593.2 (w), 547.1 (m), 480.7 (m).
{Nd(NO3)(H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·7H2O (5). Synthesis of complex 5 was similar to complex 1 with Nd(NO3)3·6H2O (44 mg, 0.1 mmol) substituting La(NO3)3·6H2O. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 63%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C10H42Cu5NdN13O30: C, 9.34%; H, 3.29%; N, 14.15%. Found (%): C, 9.10%; H, 3.12%; N, 13.96%. IR (KBr, cm−1): 3430.5 (vs.), 2963.6 (m), 1597.5 (m), 1442.3 (m), 1385.3 (m), 1362.2 (m), 1034.1 (m), 594.9 (w), 547.4 (m), 478.8 (m).
{Sm(NO3) (H2O)2[15-MCCu(N)glyha-5](H2O)2}·2NO3·6H2O (6). Synthesis of complex 6 was similar to complex 1 with Sm(NO3)3·6H2O (44 mg, 0.1 mmol) substituting La(NO3)3·6H2O. The crystals were obtained in the solutions during slow evaporation of the solvent with the yield 23.5%. Decomposition point > 300 °C. Elem. anal. calcd (%) for C10H40Cu5SmN13O29: C, 9.42%; H, 3.16%; N, 14.29%. Found (%): C, 9.20%; H, 3.01%; N, 14.07%. IR (KBr, cm−1): 3419.0 (vs.), 2949.6 (m), 1598.8 (m), 1407.8 (m), 1385.2 (m), 1159.0 (m), 1032.6 (m), 593.5 (w), 547.0 (m), 482.9 (m).

Results and discussion

Crystal structures of the complexes

The single X-ray crystallography studies revealed that complexes 1–6 were isostructural and crystallized in the triclinic space group P[1 with combining macron] with Z = 2. Based on the difference of the binding anion to the central lanthanide ion, these complexes are divided into two categories: one category is the acetate anion coordinating the central lanthanide ion (for complexes 1–3), the other is nitrate anion coordinating the central lanthanide ion (for complexes 4–6). The six complexes were synthesized in aqueous solution using the same two-step method.13 The difference is that the amount of copper acetate and lanthanide nitrate has been doubled in the syntheses of complexes 1–3. As previously reported, the coordinating nitrate anion to the central Ln ion can effectively compete with the acetate anion.15b

Crystallographic and structure refinement data were summarized in Table 1. The selected bond distances and angles were shown in Table S1 (see ESI†). Herein, complex 1 and complex 4 are described as representatives in detail. The hydroxamate nitrogen and oxygen atoms of each glyha2− ligand link two Cu(II) ions to form the Cu–N–O–Cu linkage, which further construct a ring with the bold outline in Fig. 1. The metallacrown ring is formally neutral since the positive charges of the Cu2+ are balanced by the negative charges of the glyha2−. A La(III) ion is encapsulated at the center of the ring with one coordinated acetate anion and two unbound nitrate anions balancing its positive charge. The La(III) ion is nine-coordinate with the coordinating atoms from five oxime oxygen atoms of metallacrown ring, two carbonyl oxygen atoms and two water oxygen atoms. The average distances of La(III) ion with carbonyl oxygen is 2.605 Å, noticeably longer than that of La–O(oxime) 2.509 Å and La–O(water) 2.520 Å. The La(III) ion is displaced 0.516 Å out of the best least-squares oxygen plane towards the bidentate acetate anion. Each copper ion is bound into two five-membered chelate rings. The Cu–O and Cu–N distances range between 1.940 Å and 1.957 Å. These distances are comparable with those in similar complex.15b The copper ions Cu1 and Cu4 have square-pyramidal environments by forming weak axial bonds on the same face of the metallacrown ring to water molecules, with bond lengths of 2.399 Å and 2.428 Å, respectively. Cu3 and Cu5 have square-pyramidal environments by forming weak axial bonds on the opposite face of the metallacrown ring to water molecules, with bond lengths of 2.366 Å and 2.498 Å, respectively. Cu2 has a square-planar environment. The Gd1 atom in complex 2 and Tb1 atom in complex 3 have eight-coordinate environments and deviate from the best least-squares oxygen plane by 0.261 Å and 0.245 Å, respectively (Fig. S1 and S2 ESI†).

Table 1 Crystal data and refinement information for complexes 1-6
Complex 1 2 3 4 5 6
Empirical C12H45Cu5LaN12O29 C12H41Cu5GdN12O27 C12H41Cu5TbN12O27 C10H42Cu5PrN13O30 C10H42Cu5NdN13O30 C10H40Cu5SmN13O29
M 1278.21 1260.52 1262.19 1283.18 1286.51 1292.62
Crystal system Triclinic Triclinic Triclinic Triclinic Triclinic Triclinic
Space group P[1 with combining macron] P[1 with combining macron] P[1 with combining macron] P[1 with combining macron] P[1 with combining macron] P[1 with combining macron]
a/Å 10.4509(5) 10.5669(8) 10.5766(9) 10.7585(9) 10.7503(8) 10.7100(8)
b/Å 12.5437(6) 12.3139(9) 12.3448(8) 11.9424(7) 11.5848(11) 11.5449(9)
c/Å 16.3152(8) 16.1763(10) 16.1597(12) 16.8500(10) 16.9532(14) 16.8941(13)
α/° 71.343(4) 101.321(6) 101.337(6) 72.440(5) 72.7020(10) 72.927(2)
β/° 76.532(4) 107.610(6) 107.519(7) 87.722(6) 87.426(2) 87.173(2)
γ/° 76.328(4) 106.655(7) 106.783(7) 73.844(6) 74.9780(10) 74.987(3)
V/Å 1940.05(16) 1827.4(2) 5926.9(9) 1905.6 (2) 1945.8(3) 1927.9(3)
Z 2 2 2 2 2 2
Dcalcd/g cm−3 2.188 2.291 2.289 2.236 2.196 2.227
μ/mm−1 3.881 4.758 4.869 13.733 4.108 4.323
F(000) 1270 1244 1246 1274 1276 1280
Crystal size/mm3 0.42 × 0.36 × 0.16 0.42 × 0.31 × 0.16 0.42 × 0.31 × 0.16 0.17 × 0.15 × 0.12 0.45 × 0.42 × 0.36 0.43 × 0.41 × 0.32
Reflections collected/unique 12[thin space (1/6-em)]812/6850[R(int) = 0.0668] 11[thin space (1/6-em)]689/6459[R(int) = 0.0708] 11[thin space (1/6-em)]881/6468[R(int) = 0.0772] 11[thin space (1/6-em)]839/6831[R(int) = 0.0763] 9754/6709[R(int) = 0.0335] 9835/6685[R(int) = 0.0330]
Goodness-of-fit on F2 1.047 1.108 1.083 1.021 1.024 1.049
R1 [I > 2σ(I)] 0.0684 0.0759 0.0803 0.0784 0.0480 0.0411
wR2 (all data) 0.1982 0.2053 0.2055 0.2204 0.1318 0.988


image file: c6ra05239h-f1.tif
Fig. 1 (a) The molecular structure (left) and the ORTEP diagram of complex 1 (right). Thermal ellipsoids are drawn at 30% probability level. (b) The two-dimensional supramolecular network of complex 1 (all hydrogen atoms and non-coordinating solvent molecules and two unbound nitrate anions are omitted for clarity).

The molecular structure of complex 4 is presented in Fig. 2. A Pr(III) ion is encapsulated in the center of the ring with one coordinated nitrate anion and two unbound nitrate anions balancing its positive charge. The Pr(III) ion is also nine-coordinate and is coordinated by five oxygen atoms from the [15-MCCu(N)glyha-5] ring as well as two oxygen atoms of nitrate anion and two oxygen atoms of water molecule. The average distances of Pr–O(nitrate anion), Pr–O(oxime) and Pr–O(water) are 2.721 Å, 2.490 Å and 2.463 Å, respectively. The Pr(III) ion is displaced 0.429 Å out of the best least-squares oxygen plane toward the bidentate nitrate anion. Four of the five Cu(II) ions are four-coordinate, only Cu5 is five-coordinate, square-pyramidal and the distance of Cu5–O13 is 2.354 Å. The Cu–O and Cu–N distances range between 1.941 Å and 1.960 Å. The Nd1 atom in complex 5 and Sm1 atom in complex 6 have nine-coordinate environment and displacements from the best least-squares oxygen planes toward the bidentate nitrate anions are 0.432 Å and 0.420 Å, respectively (Fig. S3 and S4 ESI†). As we all know, lanthanide ionic radius decreases with increasing atomic number. Compared the deviations of LnIII cations from the best least-squares oxygen plane we found that decrease consecutively from LaIII to TbIII, which indicates the smaller lanthanide cations fit the cavity than the lager ones.11


image file: c6ra05239h-f2.tif
Fig. 2 (a) The molecular structure (left) and the ORTEP diagram of complex 4 (right). Thermal ellipsoids are drawn at 30% probability level. (b) The two-dimensional supramolecular network of complex 4 (all hydrogen atoms and non-coordinating solvent molecules and two unbound nitrate anions are omitted for clarity).

Fluorescence spectra

The studies showed that EB did not exhibit any appreciable emission in buffer solution. However, when DNA was added, EB can emit intense fluorescent light. May be this was deriven from the strong intercalation of DNA base pairs.17 The quenching of DNA–EB emission occurred after the addition of metal complexes intercalating into the DNA base pairs, which proved the intercalation of metal complexes to DNA.18 In this studies, Ln[15-MCCu(N)glyha-5] complexes solutions (0–1.2 μmol L−1) were added to DNA pretreated with EB and then measured the intensity of emission. The quenching curves of EB–DNA system of complexes 1 and 4 is shown in Fig. 3 and 4 (the quenching curves of complexes 2, 3, 5 and 6 are shown in Fig. S5–S8 ESI†). The addition of the complexes to DNA caused appreciable decrease in emission intensity, indicating the complexes bind to DNA at the sites occupied by EB. The data are analyzed through Stern–Volmer equation I0/I = 1 + Kspr, where I0 and I represent fluorescence intensities in the absence and presence of the sample, respectively, and r is the concentration ratio of the sample to DNA. Ksq is the linear Stern–Volmer constant.19 The Ksq value can be obtained from the slope of the linear plot of I0/I versus r. The values of Ksq for complexes 1–6 were calculated as 15.61, 15.85, 11.20, 15.47, 15.44 and 15.33, respectively, suggesting a strong interaction, compared to other complexes.20 Such quenching constants suggest that the interaction of the complexes with DNA should be intercalation.21 Compared with the Ksp values, these complexes show the similar DNA-binding ability except for complex 3, which should be connected with their common Ln[15-MCCu(N)glyha-5] structure. In addition, the reducing trend of lanthanide ionic radius with increasing atomic number will cause the smallest radius TbIII ion being more close to the crown plane. Thus, among the reported six metallacrowns, TbIII complex 3 present higher rigidity and bigger steric hindrance,22 and it is disadvantage for the intercalation between complex and DNA.
image file: c6ra05239h-f3.tif
Fig. 3 Effects of complex 1 on the fluorescence spectra of EB–DNA system. [DNA] = 25 μmol L−1, [EB] = 3 μmol L−1, from 1 to 7 [complex]/[DNA] = 0, 0.016, 0.024, 0.032, 0.04, 0.048, respectively; inset plot of I0/I versus r (r = [complex]/[DNA]). λex = 258 nm.

image file: c6ra05239h-f4.tif
Fig. 4 Effects of complex 4 on the fluorescence spectra of EB–DNA system. [DNA] = 25 μmol L−1, [EB] = 3 μmol L−1, from 1 to 7 [complex]/[DNA] = 0, 0.016, 0.024, 0.032, 0.04, 0.048, respectively; inset plot of I0/I versus r (r = [complex]/[DNA]). λex = 258 nm.

CD spectra

CD spectroscopy is a powerful technique to give useful information on changes in DNA morphology during complex-DNA interactions, this because CD signals are quite sensitive to the mode of DNA interactions with small molecules. The circular dichroism spectrum of CT-DNA exhibits a positive band at 275 nm due to base stacking, and a negative band at 245 nm due to right-handed helicity of B-DNA.19 As shown in Fig. 5, the addition of complexes to DNA solutions, perturbations in ellipticity of the two bands on the CD spectra of DNA are observed. The changes suggested that the DNA conformation is disturbed by binding of complexes. The decreased intensities of both the positive and negative ellipticity bands mean that the intercalation interaction may cause a more B-like to a more C-like conformational change.23
image file: c6ra05239h-f5.tif
Fig. 5 CD spectra of 100 μmol L−1 CT-DNA in the absence and presence of 0.2 μmol L−1 complexes 1–6.

Magnetic properties

Direct current magnetic susceptibility measurements were performed on polycrystalline samples of 1–6 from 2 to 300 K in an applied field of 1 kOe. The results are plotted as χMT versus T in Fig. 6. At room temperature, the χMT values of complexes 1–6 are 1.65, 9.53, 14.2, 3.32, 3.01 and 1.87 cm3 mol−1 K, respectively. Upon cooling, the χMT products decreased to 0.33 (1), 9.18 (3), 0.74 (4), 0.49 (5) and 0.28 cm3 mol−1 K (6) at 2 K. The decrease of χMT values indicates dominate antiferromagnetic coupling in the overall intramolecular exchange interactions. For complex 1, the value of χMT at 300 K is 1.65 cm3 mol−1 K, which is lower than the spin only value of five uncoupled Cu(II) ions (1.875 cm3 mol−1 K for g = 2). The shape of the curve indicates that antiferromagnetic interactions are emerged between adjacent Cu(II) ions. The shape of χMT–T curve of complex 2 is different from other five complexes. The room temperature value of χMT for complex 2 is 9.53 cm3 mol−1 K, which is close to the expected spin-only value 9.75 cm3 mol−1 K. Upon cooling, the χMT value gradually decreases to a minimum of 8.80 cm3 K mol−1 at 42 K; as the temperature further decreased, a maximum value of 10.24 cm3 K mol−1 appeared at 2 K. Pecoraro and co-workers thought the 15-MC-5 structures with ring centra encapsulating a Gd(III) ion presented similar magnetic behavior. Namely, Cu⋯Cu couplings show antiferromagnetic interactions and Cu⋯Gd coupling displays ferromagnetic interaction.12 Therefore, in complex 2, when T < 42 K the χMT value increases as the temperature decreases, possibly indicated a weak ferromagnetic exchange interaction between Cu(II) ions and the Gd(III) ion or the presence of a small ZFS of the Gd(III) ion.12 Moreover, the previous findings also confirmed that it was possible to determine ferro- or antiferromagnetic interactions through the magnetic exchange of Gd⋯Cu pairs.24 Because a number of complexes reports showed the Gd⋯Cu distances and the corresponding J exchange constant influenced magneto-structural correlation. An exponential function is shown as −J = Aexp[BdGd⋯Cu] (where A = 6.5 × 104 and B = −2.833, with J in cm−1 and dGd⋯Cu in Å). This correlation indicates that the nearer the Cu and Gd approach each other, the larger the exchange. It also sets an upper limit for this exchange interaction of around 10 cm−1, as the Gd⋯Cu separation is never likely to be less than 3.1 Å. In this text, the Cu⋯Gd distance is range from 3.845–3.897 Å, and the ferromagnetic interaction of Gd⋯Cu should be strong, leading to complex 2 revealing ferromagnetic interactions. Maybe, the f orbital overlap of the Ln ions with the orbitals on the bridging ligands was ineffective.25 Thus, the magnetic exchanges of Ln⋯Cu are typically weak in other complexes. Therefore, the rest of complexes are antiferromagnetic.
image file: c6ra05239h-f6.tif
Fig. 6 The plots of χMT vs. T for complexes 1–6 with an applied field of 1 kOe.

Conclusion

In summary, six Ln[15-MCCu(N)glyha-5] complexes have been synthesized and characterized. Crystal structures showed that acetate anion and nitrate anion bound to the central Ln ion through changing the amount of copper acetate and lanthanide nitrate. DNA-binding studies indicated that the interactions of complexes and CT-DNA were finished through intercalative mode and the intercalation interaction may be cause a more B-like to a more C-like conformational change. The magnetic behaviour of complexes 1, 3, 4, 5 and 6 show the dominant antiferromagnetic interactions between metal centers, whereas complex 2 reveals ferromagnetic interactions under low temperature.

Acknowledgements

We are greatly acknowledged financial supports from the National Natural Science Foundation of China (grant 21271097) and Liaocheng University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1423973 1423977 1411162 1423974 1423975 and 1423976. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra05239h
This journal is © The Royal Society of Chemistry 2016
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