ZHAO Xiao-Yang

(Department of Environmental Engineering, Henan Polytechnic Institute, Nanyang 473009, China)

ABSTRACT A novel dinuclear copper(II) coordination compound, [CuII(Hpdc)(phen)Cl][CuII(pdc)(phen)]· 4H2O (1, H2pdc = pyridine-2,6-dicarboxylic acid, phen = 1,10-phenanthroline), has been obtained by an ionothermal method using the ionic liquid 3-butyl-1-methylimidazolium bromide ([Bmim]Br) as solvent, and fully characterized by elemental analyses, energy-dispersive X-ray spectroscopy (EDX), IR and single-crystal X-ray diffraction. 1 belongs to the triclinic system, space group with a = 11.406(4), b = 11.801(5), c = 16.093(6) ?, α = 106.124(6)°, β = 113.480(7)°, γ = 93.853(7)°, Z = 2, V = 1942.3(13) ?3, Dc = 1.570 g?cm-3, F(000) = 928, μ = 1.235 mm-1, the final R = 0.0588, wR = 0.1369 and S = 1.013. The compound exhibits a two-dimensional layered structure constructed by [CuII(Hpdc)(phen)Cl] and [CuII(pdc)(phen)] fragments via the hydrogen bonds and π-π stacking interactions. In addition, its thermal properties and magnetic behaviors were also studied in detail. The magnetic properties revealed that there is a ferromagnetic interaction between Cu···Cu in 1.

Keywords: ionothermal, dinuclear, π-π stacking interactions, thermal properties, magnetic behaviors;

1 INTRODUCTION

Metal coordination compounds are built by various metal cations and organic ligands with rich structural features and different structural dimensions. These compounds always own widely potential applications in magnetic, catalysis, nonlinear optical materials, electronic devices, storage and adsorption of gases, and so on[1,2]. Up to the present, a large number of metal coordination compounds with unique topological structures and different dimensions, such as wheel-shaped, honeycomb-shaped, brick-wall-shaped structures, have been reported and studied[3,4]. During the design and synthesis of such compounds, besides the characteristics of organic ligands and the coordination conformations of metal cations, many factors would affect the final structures of the compounds like the reaction temperature, reaction time, types of solvents, ratio of metal ions and ligands and pH,etc[5-7]. Currently, carboxyl- containing organic ligands and N-containing ligands have been widely used in the synthesis of metal coordination compounds because they often exhibit flexible coordination modes with metal cations, which provide a great opportunity to obtain undesirable compounds with new structure features. In addition, copper metal compounds often exhibit good magnetic behaviors and can be used as potential magnetic materials. Ionothermal synthesis is an effective method for metal coordination compounds. In recent years, some new compounds with unique structural characteristics have been synthesized by this method[8,9]. Herein, we have successfully synthesized a dinuclear copper coordination compound [CuII(Hpdc)(phen)Cl][CuII(pdc)(phen)]·4H2O (1) by using two kinds of organic ligands, 2,6-pyridinedicarboxylic acid and 1,10-phenanthroline under the ionothermal conditions.

2 EXPERIMENTAL

2. 1 Instruments and reagents

The C, H and N elemental analyses were measured on a Perkin-Elmer 240 analyser. Energy-dispersive X-ray spectro- scopy (EDX) analysis was tested on a FEI-Quanta-200 scanning electron microscope. Infrared spectra were performed on a Nicolet 5700 FT-IR spectrometer (400～4000 cm-1) using KBr pellets. TG curve was recorded on a TG-209- F3 thermal analyzer. The variable temperature magnetic properties were measured on an MPMS SQUID VSM magnetometer in an applied magnetic field of 1000 Oe between 1.8 and 300 K. Crystal diffraction data were collected on a Bruker SMART APEX-II CCD diffractometer equipped with a graphite-monochromated Mo-Kαradiation (λ= 0.71073 ?) at 293(2) K using anω-φscan mode. All chemical reagents are purchased directly and used without further purification.

2. 2 Synthesis of [CuII(Hpdc)(phen)Cl]- [CuII(pdc)(phen)]·4H2O (1)

Compound 1 was obtained under mixed-solvothermal conditions using the ethylene glycol and water as solvents. A mixture of CuCl2·2H2O (0.22 g, 1.30 mmol), 2,6-pyridinedi- carboxylic acid (0.12 g, 0.72 mmol), 1,10-phenanthroline (phen) (0.08 g, 0.60 mmol), H2O (1.0 mL, 55.56 mmol) and 3-butyl-1-methylimidazolium bromide ([Bmim]Br) (2.0 g, 0.90 mmol) was placed into a Teflon-lined stainless-steel autoclave (30 mL) and heated at 423 K for 120 h. After the mixture has been cooled slowly to room temperature at the rate of 5 K/h, green block crystals of 1 were obtained. The product was filtered off and washed with distilled water (yield: 53%, based on CuCl2·2H2O). If the 3-butyl-1-methylimi- dazolium bromide was not added into the synthesis system, compound 1 could not be obtained.

2. 3 X-ray crystallographic determination

The single cystal of 1 (0.26mm × 0.22mm × 0.18mm) was selected for structure determination. Its crystal diffraction data were performed on a Bruker SMART APEX-II CCD diffractometer equipped with a graphite-monochromated Mo-Kαradiation (λ= 0.71073 ?) at 296(2) K using anω-φscan mode. In the range of 2.63≤θ≤25.00o with -11≤h≤13, -11≤k≤14 and -19≤l≤19, a total of 9930 reflections were collected, among which 6735 were unique (Rint= 0.0254). Absorption correction was used by using the SADABS[10]. The structure was solved by direct methods using the SHELXS-97 software and refined by full-matrix least-squares techniques onF2with SHELXL-97 software[11]. All non- hydrogen atoms were refined anisotropically and hydrogen atoms isotropically by full-matrix least-squares refinement. The organic hydrogen atoms from 2,6-pyridinedicarboxylato and 1,10-phenanthroline were added geometrically. The isolated water molecules (O1W, O2W, O3W and O4W) were slightly disordered in the structure. The H atoms bound to isolated water molecules were not added. The selected bond lengths and bond angles of 1 are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1

3 RESULTS AND DISCUSSION

3. 1 EDX and elemental analyses

The energy-dispersive X-ray spectroscopy (EDX) results for 1 (Fig. S1) show that the structure contains the elements of Cu, Cl, C, N and O[12,13]. The C, H and N elemental analyses were also measured on a Perkin-Elmer 240 analyser with the results listed as follows: analysis calculated: C, 49.28; H, 3.37; N, 9.07%. Found: C, 48.83; H, 3.64; N, 9.49%. These results are consistent with the single-crystal X-ray structural analysis.

3. 2 Infrared spectroscopy

IR spectrum of 1 was studied on a Nicolet 5700 FT-IR spectrometer using KBr pellets in a range of 4000～400 cm-1(Fig. S2). The peak at 3423 cm-1can be attributed to water molecules, and the peaks at 1627, 1267 and 854 cm-1should be assigned to the -COO-groups. In the high-frequency region of the IR spectrum, absorption band observed at 3083 cm-1can be attributed to the C-H vibration of the aromatic groups, while in the low frequency region, the absorptions in the scope of 1577～1023 cm-1(1577, 1516, 1426, 1382, 1311, 1267, 1174, 1147, 1085 and 1023 cm-1) should be assigned to the pdc2-and phen ligands[14].

3. 3 Crystal structure

Single-crystal X-ray diffraction determination reveals that 1 crystallizes in the triclinic space group. The molecular structure of 1 (Fig. 1) consists of one neutral dinuclear [CuII(Hpdc)(phen)Cl][CuII(pdc)(phen)] fragment containing two types of organic ligands and four uncoordinated water molecules. Within the dinuclear [CuII(Hpdc)(phen)Cl]- [CuII(pdc)(phen)] fragment, both of Cu atoms (Cu(1) and Cu(2)) are six-coordinated: Cu(1) atom is octahedrally coordinated by one tridentate pdc2-ligand (one N atom and two O), one phen ligand (two N atoms) and one bridging carboxylate O atom from an adjacent Hpdc-ligand. The dihedral angle between the planar phen ligand and pdc2-is 88.342(70)°. The Cu(2) atom adopts one tridentate Hpdc-ligand (one N atom and two O), one phen ligand (two N atoms) and one terminal Cl atom, forming a distorted {CuO2N3Cl} octahedron. The dihedral angle between the planar phen ligand and the pdc2-is 80.910(114)°. The Cu(1) and Cu(2) atoms are bridged by one carboxylato group, resulting in a dinuclear structure with the Cu···Cu separation of 5.4232(19) ?. It is noteworthy that the pdc2-and Hpdc-ligands in 1 adopt two kinds of coordination modes:μ3- andμ4-coordination modes. The bond length of Cu-Cl is 2.2557(18) ?, and those of Cu-O and Cu-N are fall in the ranges of 1.988(4)～2.6305(47) and 1.993(4)～2.057(4) ?, respectively, well within normal ranges and consistent with the previously reported dinuclear CuIIcoordination com- pounds[5,6,15]. On the basis of bond strength calculations, the bond valence sums (BVS) for Cu(1)and Cu(2) atoms are close to their normal valences of +2 (Cu(1): 1.717 (0.434, 0.355, 0.329, 0.299, 0.181 and 0.119) and Cu(2): 1.763 (0.333, 0.314, 0.303, 0.236, 0.076 and 0.501).

Fig. 1. Molecular structure of compound 1. Displacement ellipsoids are drawn at the 30% probability level

Fig. 2 shows the crystal packing of 1 along the [001] direction. The hydrogen bonds of O-H···O, C-H···O and C-H···Cl (Table 2) and three types ofπ-πstacking inter- actions (Table 3) between the pyridine rings from the pdc2-and phen ligands are also observed in the packing structure of 1, which further stabilize its crystal structure. The compound displays a two-dimensional layered structure constructed by [CuII(Hpdc)(phen)Cl] and [CuII(pdc)(phen)] fragmentsviathese hydrogen bonds of C-H···O, C-H···Cl andπ-πstacking interactions.

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Table 3. Information of π-π Stacking (Angles (°) and Distances (?))

Fig. 2. A view of the packing structure of 1 along the [001] direction

3. 4 Thermal properties

The thermal properties of 1 were recorded under N2flowing in the range of 30～800 °C (heating rate of 10 K·min-1). Its TG curve exhibits two steps of weight loss (Fig. 3). The first one is 7.61% (Δmcalcd/m= 7.93%) between 107 and 268 °C due to the release of uncoordinated four water molecules per molecular unit. The second one between 317 and 706 °C is 74.74% (Δmcalcd/m= 75.32%), which is attributed to the departure of two types of organic ligands. Depending on the experimental results, we concluded that the final residue may be CuO. The results are well supported by the single-crystal structural analysis.

3. 5 Magnetic properties

Considering compound 1 belongs to di-nuclear structure (the distance of Cu···Cu: 5.4232(19) ?), we plan to study its magnetic properties. The variable temperature magnetic properties of 1 were performed on an MPMS SQUID VSM magnetometer in an applied magnetic field of 1000 Oe and the temperature between 1.8 and 300 K. Its measured curve ofχMTversusTis shown in Fig. 4. The experimental result indicates that theχMTvalue of 1 is 0.78 cm3·K·mol-1at 300 K, close to the expected calculated value of 0.75 cm3·K·mol-1for two uncoupled Cu2+cations (S= 1/2,g= 2). TheχMTvalue decreases with the increase of temperature, which reveals that 1 owns a ferromagnetic interaction between Cu···Cu. Meanwhile, in order to verify the presence of the ferromagnetic interaction in 1, the Curie-Weiss lawχM=C/(T-θ) is applied. The curve ofχMTversusTin the range of 80～300 K was well fitted (the Curie and Weiss constants are as follows:C: 0.76 cm3·K·mol-1,θ: 10.20 K). The positive value Weiss constant proves a ferromagnetic interaction between Cu2+cations[16].

4 CONCLUSION

In summary, the new copper(II) coordination compound containing two kinds of organic ligands has been synthesized under ionothermal conditions. The compound owns a two-dimensional layered structure constructed by [CuII- (Hpdc)(phen)Cl] and [CuII(pdc)(phen)] fragmentsviathese hydrogen bonds of C-H···O, C-H···Cl andπ-πstacking interactions. The thermal properties and magnetic behaviors were also well studied. The thermal properties illustrated that the structure of the compound remains stable below 107 °C. The magnetic properties showed that there is a ferromagnetic interaction between Cu···Cu.

Fig. 4. Magnetic properties of 1