LI Yu CHEN Yong-Xun ZHAO Zhen-Yu ZOU Xun-Zhong FENG An-Sheng

a (Guangdong Research Center for Special Building Materials and Its Green Preparation Technology/Foshan Research Center for Special Functional Building Materials and Its Green Preparation Technology, Guangdong Industry Polytechnic, Guangzhou 510300, China)

b (School of Intelligent Manufacturing and Equipment, Shenzhen Institute of Information Technology, Shenzhen 518172, China)

ABSTRACT Two 1D and one 3D coordination polymers, namely [Ni2(μ3-deta)(H2biim)3(H2O)2]n (1), {[Zn2(μ4-deta)(phen)2(H2O)]·H2O}n (2) and [Cd2(μ6-deta)(phen)2(H2O)]n (3), have been constructed hydrothermally using H4deta (H4deta = 2,3,3?,4?-diphenyl ether tetracarboxylic acid), H2biim (H2biim = 2,2?-biimidazole), phen (phen = 1,10-phenanthroline), and nickel, zinc or cadmium chlorides at 160 °C. The products were isolated as stable crystalline solids and characterized by IR spectra, elemental analyses, thermogravimetric analyses (TGA) and single-crystal X-ray diffraction analyses. Single-crystal X-ray diffraction analyses revealed that three compounds crystallize in the triclinic system, space group Compounds 1 and 2 disclose a 1D chain structure. Compound 3 features a 3D framework. The structural diversity of compounds 1～3 is driven by the metal(II) node and the type of supporting ligand. Magnetic studies for compound 1 demonstrate an antiferromagnetic coupling between the adjacent Ni(II) centers. The luminescence behavior of compounds 2 and 3 was also investigated.

Keywords: coordination polymer, tetracarboxylic acid, magnetic properties, luminescent properties;

1 INTRODUCTION

Coordination polymers have received widespread attention over the past decade owing to their modular assembly, structural diversity, fascinating topology, as well as their excellent properties with promising applications in gas storage and separation[1-3], catalysis[4-6], magnetism[7-9]and lumine- scence[10-13]. In the attainment of coordination polymers, many factors can influence the construction progress, such as metal ions, organic ligands, solvents, pH values, reaction tempera- ture, and so on[14-19].

Among many influential factors, the judicious selection of well-designed organic ligands has been proven to be one of the most effective and controllable routes in building a wide variety of functional coordination polymers materials[15,19-23].

Following our interest in the exploration of novel and poorly investigated multicarboxylic acids for the design of coordination polymers[24-26], in the present study we selected 2,3?,4?,5-diphenyl ether tetracarboxylic acid (H4deta) as a main building block. The selection of H4deta has been governed by the following reasons. (1) This ligand contains two phenyl rings that are interconnected by a rotatableO-ether group that can provide a subtle conformational adaptation. (2) H4deta contains two different types of func- tionalities (i.e., -COOH andO-ether) and has nine potential coordination sites, which can result in diverse coordination patterns and high dimensionalities, especially when acting as a multiply bridging spacer. (3) This tetracarboxylic acid block remains poorly used for the generation of coordination polymers.

Herein, we report the synthesis, crystal structures, and luminescent and magnetic properties of Ni(II), Zn(II), and Cd(II) coordination polymers with H4deta ligands.

2 EXPERIMENTAL

2. 1 General procedures

All chemicals and solvents were of AR grade and used without further purification. Carbon, hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer. IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TGA) was performed under N2atmosphere with a heating rate of 10 K/min on a LINSEIS STA PT1600 thermal analyzer. Excitation and emission spectra were recorded on an Edinburgh FLS920 fluorescence spectrometer using the solid samples at room temperature. Magnetic susceptibility data were collected in the 2～300 K temperature range with a Quantum Design SQUID Magnetometer MPMS XL-7 with a field of 0.1 T. A correction was made for the diamagnetic contribution prior to data analysis.

2. 2 Synthesis of compound 1

A mixture of NiCl2?6H2O (0.048 g, 0.2 mmol), H4deta (0.035 g, 0.1 mmol), H2biim (0.027 g, 0.2 mmol), NaOH (0.016 g, 0.4 mmol) and H2O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless-steel vessel, and heated at 433 K for 3 days, followed by cooling to room temperature at a rate of 10 K·h-1. Green block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 35% (based on H4deta). Anal. Calcd. (%) for C34H28Ni2N12O11: C, 45.47; H, 3.14; N, 18.72. Found (%): C, 45.63; H, 3.16; N, 18.61. IR (KBr, cm-1): 3618m, 3266w, 3005w, 1543s, 1481m, 1431m, 1399s, 1370m, 1336m, 1270w, 1231w, 1186w, 1119w, 1075w, 991w, 901w, 862w, 830w, 806w, 763w, 696w, 617w, 550w.vOH3618, 3266 and 3005,vas(CO2) 1543 and 1481,vs(CO2) 1431 and 1399.

2. 3 Synthesis of compound 2

A mixture of ZnCl2(0.026 g, 0.2 mmol), H4deta (0.035 g, 0.1 mmol), phen (0.040 g, 0.2 mmol), NaOH (0.016 g, 0.4 mmol) and H2O (10 mL) was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless-steel vessel, and heated at 433 K for 3 days, followed by cooling to room temperature at a rate of 10 K·h-1. Colorless block- shaped crystals of 2 were isolated manually, and washed with distilled water. Yield: 55% (based on H4deta). Anal. Calcd. (%) for C40H26Zn2N4O11: C, 55.26; H, 3.01; N, 6.44. Found (%): C, 55.07; H, 3.03; N, 6.41. IR (KBr, cm-1): 3434w, 3061w, 1593s, 1560s, 1515m, 1431s, 1371s, 1252w, 1225m, 1147w, 1086w, 963w, 847w, 802w, 774w, 724m, 662w, 640w, 545w.vOH3434 and 3061,vas(CO2) 1593 and 1560,vs(CO2) 1431 and 1371.

2. 4 Synthesis of compound 3

Synthesis of 3 was similar to 2 except using CdCl2?H2O (0.040 g, 0.2 mmol) instead of ZnCl2. Colourless block- shaped crystals of 3 were isolated manually, and washed with distilled water. Yield: 60% (based on H4deta). Anal. Calcd. (%) for C40H24Cd2N4O10: C, 50.81; H, 2.56; N, 5.93. Found (%): C, 51.02; H, 2.55; N, 5.90. IR (KBr, cm-1): 3466w, 3072w, 1590s, 1576s, 1537s, 1509m, 1431m, 1392s, 1364s, 1260m, 1231w, 1136w, 1080w, 974w, 879w, 847m, 823w, 778w, 728m, 634w, 521w.vOH3466 and 3072,vas(CO2) 1590, 1576 and 1537,vs(CO2) 1431 and 1392.

2. 4 Structure determination

Two single crystals of the title compounds were mounted on a Bruker CCD diffractometer equipped with a graphite- monochromatic MoKα(λ= 0.71073 ?) radiation using aφ-ωscan mode at 293(2) K. The structures were solved by direct methods with SHELXS-97[27]and refined by full-matrix least-squares techniques onF2with SHELXL-97[28]. All non-hydrogen atoms were refined anisotropically. All hy- drogen atoms (except those bound to water molecules) were placed in the calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hy- drogen atoms of water molecules were located by difference Fourier maps and constrained to ride on their parent O atoms. Some lattice solvent molecules in 2 are highly disordered and were removed using the SQUEEZE routine in PLATON[29]. The number of solvent H2O molecules was obtained on the basis of elemental and thermogravimetric analyses. Detailed crystallographic data and structural refinements of compounds 1～3 are listed in Table 1. The selected important bond parameters are given in Table 2. The hydrogen bonds andπ???πinteractions in crystal packing of compounds 1～3 are listed in Table 3.

Table 1. Crystal Data and structure Refinement for 1～3

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for 1～3

Symmetry codes: i: -x+1, -y+2, -z; ii: x+1, y+1, z; iii: -x+1, -y+1, -z+1; iv: x+1, y, z

Table 3. Geometrical Parameters of Hydrogen Bonds and π???π Interactions for 1～3

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

X-ray crystallography analysis reveals that compound 1 crystallizes in triclinic space group. As shown in Fig. 1, the asymmetric unit of 1 bears two crystallographically unique Ni(II) atoms (Ni(1) and Ni(2)), oneμ3-deta4-block, three H2biim moieties, and two H2O ligands. The six-coor- dinate Ni(1) atom exhibits a distorted octahedral {NiN4O2} environment, which is occupied by two carboxylate O donors from twoμ3-deta4-blocks and four N donors from two H2biim moieties. The Ni(2) center is also six-coordinated and forms a distorted octahedral {NiN2O4} geometry. It is completed by two carboxylate O atoms from oneμ3-deta4-block, two O atoms from two H2O ligands, and two N atoms from the H2biim moiety. The Ni-O and Ni-N bond distances are 2.036(2)～2.315(2) and 2.036(3)～2.108(3) ?, respectively; these are within the normal ranges observed in related Ni(II) compounds[17,20,30]. In 1, the deta4-ligand adopts the coordination mode I (Scheme 1) with carboxylate groups being monodentate or bidentate. In the deta4-ligand, a dihedral angle (between two aromatic rings) and a C-Oether-C angle are 80.68 and 118.65°, respectively. Theμ-deta4-ligands connect Ni(1) and Ni(2) atoms to give a 1Dchain (Fig. 2). The neighboring chains are assembled into a 2Dsheet through the O-H???O hydrogen bonds (Table 3 and Fig. 3).

Fig. 1. Coordination environments of the Ni(II) atoms in compound 1. The hydrogen atoms are omitted for clarity except those bound to N atoms (Symmetry code: i: x, y-1, z)

Fig. 2. Perspective view of the 1D chain along the bc plane in 1. The H2biim moieties are omitted for clarity (Symmetry codes: i: x, y-1, z; ii: x, y-2, z; iii: x, y+1, z)

Fig. 3. Perspective view of the 2D sheet along the bc plane in 1 (Blue dashed lines present H-bonds)

3. 2 Crystal structure of 2

The asymmetric unit of compound 2 contains two crystallographically unique Zn(II) atoms, oneμ4-deta4-block, two phen moieties, one H2O ligand, and one lattice water molecule. As depicted in Fig. 4, six-coordinate Zn(1) atom features a distorted octahedral {ZnN2O4} environment, which is filled by four carboxylate O atoms of two deta4-blocks and two N atoms of one phen moiety. The Zn(2) center is five-coordinated and displays a distorted trigonal bipyramidal {ZnN2O3} geometry. It is taken by two carboxylate O atoms from twoμ4-deta4-blocks and one O donor from the H2O ligand, and two N atoms of one phen moiety. The bond lengths of Zn-O are in the 1.987(2)～2.266(3) ? range, while the Zn-N bonds are 2.089(3)～2.144(3) ?, being comparable to those found in some reported Zn(II) compounds[5,24,30]. In 2, the deta4-block acts as aμ4-linker (mode II, Scheme 1), in which four carboxylate groups adopt monodentate or bidentate modes. Besides,μ4-deta4-ligand is considerably bent showing a dihedral angle of 62.05° (between two aromatic rings) and the C-Oether-C angle of 119.91°. Theμ4-deta4-ligands link Zn centers to furnish a 1Dchain (Fig. 5). The adjacent chains are assembled into a 2Dsheet throughπ???πinteractions (Table 3 and Fig. 6).

Scheme 1. Coordination modes of the deta4- ligands in compounds 1～3

Fig. 4. Coordination environments of the Zn(II) atoms in compound 2. Hydrogen atoms are omitted for clarity (Symmetry codes: i: x-1, y, z; ii: -x+1, -y, -z+1)

Fig. 5. View of 1D metal-organic chain along the ac plane in 2. The phen moieties are omitted for clarity

Fig. 6. View of 2D metal-organic sheet along the bc plane in 2 (Green dashed lines present π???π interactions)

3. 3 Crystal structure of 3

The asymmetric unit of compound 3 comprises two distinct Cd(II) atoms, oneμ6-deta4-block, two phen moieties, and one H2O ligand (Fig. 7). The Cd(1) atom is seven-coordinated and has a distorted pentagonal bipyramidal {ZnN2O5} environ- ment. It is completed by five carboxylate O atoms from threeμ6-deta4-blocks and two Nphendonors. Cd(2) atom is six-coordinate and features a distorted octahedral {CdN2O4} geometry, which is filled by three carboxylate O atoms of three different deta4-blocks, one O atom from the H2O ligand, and a pair of N atoms of one phen moiety. The Cd-O (2.205(3)～2.610(3) ?) and Cd-N (2.351(3)～2.408(3) ?) distances are comparable to those in Cd(II) derivatives[15,24,30]. In 3, the deta4-block behaves as aμ6-linker (mode III, Scheme 1), in which four COO-groups adopt monodentate, bidentate,μ-bridging bidentate, or tridentate modes. In theμ6-deta4-ligand, the dihedral angle between two aromatic rings is 80.35°, while the C-Oether-C angle is 117.62°. The adjacent Cd(II) centers are bridged by means of two carboxylate O atoms and two COO-groups from threeμ6-deta4-blocks, generating two types of dicadmium(II) subunits (Fig. 8). These Cd2subunits are further interlinked by the remaining carboxylate functionalities of theμ6-deta4-blocks to give a 3Dframework (Fig. 9). Three compounds disclose 1Dchain and 3Dframework structures, which can be attributed to the different metal(II) nodes and the types of supporting ligands.

Fig. 7. Coordination environments of the Cd(II) atoms in compound 3. The hydrogen atoms are omitted for clarity (Symmetry codes: i: -x+1, -y+2, -z; ii: x+1, y+1, z; iii: -x+1, -y+1, -z+1; iv: x+1, y, z)

Fig. 8. Dicadmium(II) subunits (Symmetry codes: i: -x+1, -y+2, -z; ii: -x+1, -y+1, -z+1)

Fig. 9. View of 3D metal-organic framework along the ac plane in 3. The phen moieties are omitted for clarity

3. 4 Thermal analysis

To determine the thermal stability of polymers 1～3, their thermal behaviors were investigated under nitrogen atmosphere by thermogravimetric analysis (TGA). As shown in Fig. 10, compound 1 loses its two coordinated water molecules in 359 ～479 K (exptl, 4.0%; calcd. 4.0%), followed by the decomposition at 578 K. For 2, two weight loss steps (exptl, 3.9%; calcd. 4.1%) in the 308～435 K range corresponds to a removal of one lattice water molecule and one H2O ligand; decomposition of the sample occurs only at 603 K. The TGA curve of 3 reveals that one H2O ligand is released between 419～472 K (exptl, 2.0%; calcd. 1.9%), and the dehydrated solid begins to decompose at 596 K.

Fig. 10. TGA curves of compounds 1～3

3. 5 Luminescent properties

The excitation and emission spectra of 2,3,3?,4?-diphenyl ether tetracarboxylic acid (H4deta) and polymers 2 and 3 were measured in the solid state at room temperature (Figs. 11 and 12). The uncoordinated H4deta shows a weak photolumine- scence with an emission maximum at 416 nm (λex= 300 nm). In contrast, compounds 2 and 3 display significantly more intense emission bands with the maxima at 437 and 401 nm (λex= 300 nm), respectively. All bands can be assigned to the intraligand (π*→n orπ*→π) emission[13,15]. The lumine- scence enhancement in the coordination compounds can be attributed to the binding of ligands to the metal centers, which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay[17,24,30]. Two coordination compounds exhibit different emission peaks, which can be attributed to their different metal centers and structures[13,15,24].

Fig. 11. Solid-state excitation spectra of H4deta, and compounds 2 and 3 at room temperature

Fig. 12. Solid-state emission spectra of H4deta, and compounds 2 and 3 at room temperature

3. 6 Magnetic properties

Variable-temperature magnetic susceptibility studies were carried out on powder samples of Ni(II) derivative 1 in the 2～300 K temperature range. As shown in Fig. 13, theχMTvalue of 2.11 cm3?mol-1?K at room temperature is close to the expected one (2.00 cm3?mol-1?K) for two magnetically isolated Ni(II) ion (S= 1,g= 2.0). Upon cooling, theχMTvalue decreases very slowly from 2.11 cm3?mol-1?K at 300 K to 2.02 cm3?mol-1?K at 35 K, and then decreases steeply to 1.02 cm3?mol-1?K at 2 K. In the 2～300 K interval, theχM-1vs.Tplot for 1 obeys the Curie-Weiss law with a Weiss constantθof -5.74 K and a Curie constantCof 2.05 cm3?mol-1?K. An empirical (Weng's) formula can be applied to analyze the 1Dsystems withS= 1, using numerical procedures[31,32];

Fig. 13. Temperature dependence of χMT (o) and 1/χM() vs. T for compound 1. The red curve represents the best fit to the equations in the text. The blue straight line shows the Curie-Weiss fitting

Using this method, the best-fit parameters for 1 were obtained:g= 2.04,J= -1.25 cm-1andR= 3.6 × 10-5, whereR= ∑(obsT-calcT)2/∑(obsT)2. TheJvalue of -1.25 cm-1indicates that the coupling between the Ni(II) centers is antiferro- magnetic.

4 CONCLUSION

In summary, we have synthesized three Ni(II), Zn(II), and Cd(II) coordination polymers based on an unexplored tetracarboxylate ligand. Compounds 1 and 2 possess two different 1Dchain structures. Compound 3 features a 3Dframework. The structural diversity of compounds 1～3 is driven by the metal(II) node and the type of supporting ligand. The magnetic (for 1) and luminescent (for 2 and 3) properties were also investigated and discus