Structural Diversity and Magnetic Properties of Two Coordination Polymers Based on 6-(3,5-Dicarboxylphenyl)nicotinic Acid①

ZHAI Li-Jun TONG Ru WANG Ci-Bin SHI Meng-Jie MO Y-Nn CHE Wen-Ki NIU Yu-Ln② HU Tuo-Ping

a (Taiyuan Institute of Technology, Department of Chemistry and Chemical Engineering, Taiyuan 030008, China)

b (Department of Chemistry, College of Science,North Uniνersity of China, Taiyuan 030051, China)

c (Key Laboratory of Adνanced Energy Materials Chemistry (Ministry of Education),College of Chemistry, Nankai Uniνersity, Tianjin 300071, China)

ABSTRACT Two new coordination polymers, namely {[Co8(DCPN)4(1,2-bimb)4(μ3-OH)4]·(CH3CN)0.5(H2O)2}n(1) and {[Co2(DCPN)(1,3-bimb)(μ3-OH)]·H2O}n (2), where H3DCPN = 6-(3,5-dicarboxyl-phenyl)nicotinic acid,1,2-bimb = 1,2-bis(imidazole-1-methyl)benzene and 1,3-bimb = 1,3-bis(imidazole-1-methyl)benzene, have been solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction analysis, elemental analysis (EA), infrared (IR) analysis, powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA).The structural analysis indicated that 1 is a 2D structure based on two different [Co4(DCPN)2(μ3-OH)2]n SBUs, and 2 is a 3D framework based on [Co4(DCPN)2(μ3-OH)2] SBUs. In addition, the magnetic measurements showed that 1 and 2 have antiferromagnetic interactions between adjacent Co(II) ions. The slight difference of magnetic properties between 1 and 2 may be attributed to their different structures formed during the assembly process.

Keywords: coordination polymers, solvothermal synthesis, magnetic properties;

1 INTRODUCTION

As a new class of organic-inorganic hybrid materials, coordination polymers (CPs) with adjustable structure and high specific surface area[1-3]have a wide range of potential applications in gas adsorption and separation[4,5], fluorescence[6,7], catalysis[8,9], magnetism[10-13], etc. Among them,the magnetic properties of CPs have become one of the hot spots, which largely depends on the choice of central metal ions and ligands. According to the literature[14], by the incorporation of magnetic moment carrier (magnetic metal node or connector) into CPs in the self-assembly process, the magnetic CPs with different SBUs can be synthesized, and the existence of different SBUs endows the CPs with good stability and strong antiferromagnetic interaction. Currently,transition metals such as Cu, Mn, Fe, Co and Ni are usually used as magnetic metal centers[15-21]. As for ligands, aromatic polycarboxylic acids are one of the preferred bridging ligands to construct CPs based on the following two aspects: (a) its carboxylate groups can adopt a variety of coordination modes to construct versatile CPs with different SBUs; (b) its rigidity helps to build CPs with stable structure.

Inspired by the above considerations, based on a Ncontaining 6-(3,5-dicarboxylphenyl)nicotinic acid (H3DCPN),1,2-bis(imidazole-1-methyl) benzene (1,2-bimb), 1,3-bis-(imidazole-1-methyl) benzene(1,3-bimb) and cobalt salts, two new CPs, 1 and 2, were successfully obtained under solvothermal conditions. Meanwhile, the phase purity and structural stability of CPs were characterized by powder Xray diffraction (PXRD) and thermogravimetric analysis(TGA), respectively. In addition, the magnetic susceptibilities(χM) of 1 and 2 were studied under the external 1000 Oe field in the temperature range of 2～300 K.

2 EXPERIMENTAL

2. 1 Materials and measurements

All analytical chemicals and reagents were received from commercial sources and used without further purification. C,H, and N analyses were made on a Vario EL analyzer. The crystal structures were performed on a Bruker Apex II CCD diffractometer. The thermogravimetric analyses were determined on a ZCT-A analyzer at a heating rate of 10 °C·min-1under air atmosphere. The IR spectra were recorded on a FTIR-8400s spectrometer with KBr discs in the 4000～400 cm-1region. The magnetic properties of complexes 1 and 2 were measured with the Quantum Design SQUID MPMS XL-7.

2. 2 Syntheses of coordination polymers

2. 2. 1 Synthesis of {[Co8(DCPN)4(1,2-bimb)4(μ3-OH)4]·(CH3CN)0.5(H2O)2}n (1)

A mixture of H3DCPN (0.01 mmol, 2.9 mg), 1,2-bimb(0.02 mmol, 4.8 mg), CoCl2·6H2O (0.02 mmol, 4.8 mg) and a mixed solution of DMA and H2O (1 mL, V/V = 1/2) was sealed to a glass tube, which was heated at 120 ℃ for 72 h,then cooled to room temperature with a cooling rate of 1.4 ℃/h. The prune crystals of 1 were obtained with the yield of 48% (based on H3DCPN). Anal. Calcd. (%) for C113H89.5Co8N20.5O30: C, 50.48; H, 3.33; N, 10.69. Found (%):C, 50.51; H, 3.28; N, 10.73. IR (KBr pellet, cm-1): 3458 (s),1627 (s), 1606 (m), 1529 (m), 1440 (m), 1379 (w), 1059 (m),861 (m), 773 (m), 691 (w), 492 (w) (Fig. S1).

2. 2. 2 Synthesis of {[Co2(DCPN)(1,3-bimb)(μ3-OH)]·H2O}n (2)

The synthesis procedure of 2 is the same as that of 1 except that the 1,2-bimb was replaced by 1,3-bimb. The pink crystals of 2 were collected by washing with distilled water, filtering and drying in the air, with a yield of 55% (based on H3DCPN).Anal. Calcd. (%) for C28H23Co2N5O8: C, 49.75; H, 3.41; N,10.36. Found (%): C, 49.78; H, 3.36; N, 10.32. IR (KBr pellet,cm-1): 3457 (s), 1621 (m), 1590 (m), 1533 (m), 1436 (m), 1376(m), 1043 (w), 833 (m), 765 (m), 644 (w), 544 (w) (Fig. S2).

2. 3 X-ray crystallography

The crystal data were gained with a Siemens SMART diffractometer using Mo-Kαradiation (λ= 0.71073 Å) at 25 °C .The structures of 1 and 2 were solved by direct methods with the non-hydrogen atoms refined anisotropically using the OLEX withF2value-based full-matrix least-squares method[22].All the hydrogen atoms except those for water molecules were generated geometrically with fixed isotropic thermal parameters, and are included in the calculation of structure factors. Relevant crystallographic data, selected bond lengths and bond angles are shown in Table 1 and Table 2, respectively.Topological analysis was carried out by TOPOs program[23,24].

Table 1. Crystallographic Data for 1 and 2

Table 2. Selected Bond Lengths (Å) and Bond Angles (°) of 1 and 2

3 RESULTS AND DISCUSSION

3. 1 Crystal structure descriptions

3. 1. 1 Crystal structure of {[Co8(DCPN)4(1,2-bimb)4(μ3-OH)4]·(CH3CN)0.5(H2O)2}n (1)

Single-crystal X-ray diffraction analysis shows that 1 crystallizes in the triclinic system ofP1 space group. The asymmetric unit includes eight Co(II) ions, four DCPN3-ligands, four 1,2-bimb ligands, four coordinatedμ3-OH ions, a half of lattice acetonitrile molecule, and two lattice water molecules. As exhibited in Fig. 1a, Co(1)(II) is hexacoordinated and encircled by three oxygen atoms (O(9A),O(10) and O(13A)) from two different DCPN3-ligands, two oxygen atoms (O(14) and O(14B)) from twoμ3-OH ions, and one nitrogen atom (N(2)) from one 1,2-bimb ligand,presenting a distorted octahedral geometry, while Co(2)(II) is penta-coordinated and connected by three oxygen atoms(O(8C), O(11) and O(13C)) from three different DCPN3-ligands, one oxygen atom (O(14)) from oneμ3-OH ion and one nitrogen atom (N(3)) from one 1,2-bimb ligand, giving a slightly distorted pyramidal geometry. As can be seen from Fig. 1b, two Co(1)(II) and two Co(2)(II) ions are connected by carboxylate oxygen atoms andμ3-OH ions to form a tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU with the shape of a parallelogram. The distances between the cobalt ions are 3.4384(8) and 3.0858(6) Å, respectively, and the bond angle of Co(2)-Co(1)-Co(2B) is 121.893(19)°.

Similar to the coordination mode of Co(1)(II) ion, Co(3)(II)ion also adopts a six-coordinate mode and is surrounded by three oxygen atoms (O(1), O(3A) and O(5)) from two different DCPN3-ligands, two oxygen atoms (O(4) and O(4D))from twoμ3-OH ions and one nitrogen atom (N(6)) from one 1,2-bimb ligand, exhibiting a slightly distorted octahedral geometry, while Co(4) and Co(2) ions have similar coordination patterns (Fig. 1a). The Co(4)(II) ion displays a slightly distorted tetrahedral geometry, and is surrounded by three oxygen atoms (O(3A), O(6) and O(7D)) from three different DCPN3-ligands, one oxygen atom (O(4)) from oneμ3-OH ion and one nitrogen atom (N(8D)) from one 1,2-bimb ligand.Similarly, both carboxylate oxygen atoms andμ3-OH ions connect with two Co(3)(II) and two Co(4)(II) ions to form another tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU (Fig. 1c).The lengths of the adjacent cobalt ions are 3.3893(8) and 3.0838(7) Å, respectively, and the Co(4)-Co(3)-Co(4D) bond angle is 120.372(19)°. The bond lengths of Co-O and Co-N around all Co(II) ions fall in the 1.968(2)～2.266(2) Å and 2.065(3)～2.135(3) Å regions, respectively.

Fig. 1. (a) Coordination environment of 1; (b) Tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU formed by Co(1) and Co(2) ions;(c) Tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU formed by Co(3) and Co(4) ions.Symmetry codes: A: -1+x, y, z; B: -1-x, 1-y, 2-z; C: -x, 1-y, 2-z; D: -x, 2-y, 1-z; E: 1-x, 2-y, 1-z

Three carboxylate groups of DCPN3-ligands are all deprotonated and adopted two different coordination modes:bridging bidentate (μ2-η1:η1) and bridging unidentate (μ2-η2),which are also consistent with results of IR spectra (Fig. S1).Furthermore, the adjacent different tetranuclear [Co4(DCPN)2(μ3-OH)2] SBUs are linked by the DCPN3-ligands to construct a 2D structure (Fig. 2a), in which the imidazole-N in 1,2-bimb ligands also coordinates with the Co(II) ion to further strengthen the 2D structure, as shown in Fig. 2b. The layered structure forms a 3D supramolecular framework through hydrogen bonding andπ-πinteractions (Fig. S3).

By TOPOs software, the framework of 1 is simplified as a(3,6)-cnetwork with the point symbol of (43)2(46.66.83), when the DCPN3-ligands and tetranuclear [Co4(DCPN)2(μ3-OH)2]SBUs are served as 3-c and 6-c nodes, respectively (Fig. 2c).

Fig. 2. (a) 2D structure of 1 based on Co(II) ions and DCPN3- ligands;(b) 3D framework of 1; (c) Topology of 1

3. 1. 2 Crystal structure of{[Co2(DCPN)(1,3-bimb)(μ3-OH)]·H2O}n (2)

Structural analysis shows that 2 crystallizes in the monoclinic space groupP21/n. There are two crystallographically independent Co(II) ions, one DCPN3-ligand, one 1,3-bimb ligand, one coordinatedμ3-OH ion, and one lattice H2O molecule in the asymmetric unit. As illustrated in Fig. 3,Co(1)(II) ion is penta-coordinated by three oxygen atoms(O(1), O(4A) and O(6B)) from three bridging unidentate carboxylate groups, one oxygen atom (O(7)) from oneμ3-OH ion, and one nitrogen atom (N(5C)) from one 1,3-bimb, while Co(2)(II) ion is hexa-coordinated and surrounded by one oxygen atom (O(2)) from one bridging unidentate carboxylate group, two oxygen atoms (O(4F) and O(5E)) from one chelating bidentate carboxylate group, two oxygen atoms(O(7) and O(7D)) from twoμ3-OH ions and one nitrogen atom (N(2)) from one 1,3-bimb. The distances of Co(1)···Co(2)and Co(2)···Co(1D) are 3.3925(7) and 3.0983(6) Å,respectively, and the bond angle of Co(1)-Co(2)-Co(1D) is 122.445(15)°. The bond lengths and bond angles around Co(1)(II) and Co(2)(II) ions are 1.9632(16)～2.2526(17) Å and 79.82(6)～177.80(8)°, respectively.

Fig. 3. Coordination environment of 2. Symmetry codes: A: 1-x, 1-y, -z; B: 1/2+x, 1/2-y, -1/2+z;C: 3/2-x, 1/2+y, 1/2-z; D: 1-x, -y, -z; E: 1/2-x, -1/2+y, 1/2-z; F: x, -1+y, z; G: -1/2+x, -1/2-y, -1/2+z

The coordination modes of carboxylate groups in DCPN3-ligand of 2 are the same as that in the DCPN3-ligand of 1. As shown in Fig. 4a, two DCPN3-ligands and twoμ3-OH ions connect four Co(II) ions to form tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU with the Co···Co separation of 3.39257(7) and 3.0983(6) Å, respectively, which are further expanded by DCPN3-ligands into a 2D layer structure (Fig. 4b).Then, the 2D layers are further connected with 1,3-bimb linkers to build a 3D network (Fig. 4c).

From the viewpoint of topology, if 1,3-bimb and DCPN3-ligands are respectively regarded as 2-c and 3-c nodes, and the tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU is regarded as 10-c nodes, the 3D framework of 2 is simplified as a 3-nodal(2,3,10)-c 3D network with the point symbol of{43}2{46.616.821.102}{6}2(Fig. 4d).

Fig. 4. (a) Tetranuclear [Co4(DCPN)2(μ3-OH)2] SBU formed by Co(II) ions;(b) 2D structure of 2 by Co(II) ions and DCPN3- ligands; (c) 3D framework of 2; (d) Topology of 2

3. 2 IR analyses, powder X-ray diffraction analyses and TGA analyses

As the IR spectra of 1 and 2 are similar (Fig. S1 and S2), 1 is taken as an example for detailed discussion. According to the IR spectra of 1, the characteristic absorption peak at 3458 cm-1is attributed to the stretching vibration peak of O-H bonds in the coordination OH-ion and water molecule. The characteristic peak at 1529 cm-1is attributed to the stretching vibration peak of C=N bonds in the H3DCPN ligand and 1,2-bimb ligand. The two peaks of 1627 and 1440 cm-1are attributed to antisymmetric stretching vibration and symmetric stretching vibration of the carboxylate groups H3DCPN ligand, respectively, which has a separation of 187 cm-1(＜ 200), indicating that the coordination mode of the carboxylate group in the formation of 1 is bidentate coordination[25]. Similarly, the corresponding data of the other group are 1606 and 1379 cm-1, with a difference of 227 cm-1(＞ 200), indicating monodentate coordination of the carboxylate group to the Co(II) ion. In conclusion, the carboxylate group in 1 adopts a mixed coordination mode, which is consistent with the analytical results of single crystal structure.

The phase purity of CPs was evaluated by X-ray powder diffraction, and the results showed that the main peaks of the synthesized samples are in good agreement with the simulated ones (Figs. S4 and S5), which indicates that the synthesized CPs have good phase purity. The difference in strength may be caused by the different orientation of the crystal powder.

In order to test the thermal stability of the two CPs,thermogravimetric analysis was performed from ambient temperature to 800 °C (Figs. S6 and S7). By analyzing the thermal weight loss curve of CP 1, the weight loss of 2.23%below 300 °C is consistent with the lost two lattice water molecules and half acetonitrile molecules (Found: 2.11%).The structure of 1 began to collapse at about 340 °C. It can be seen from the thermal weight loss curve of CP 2 that the weight loss rate of 2.51% below 105 °C may correspond to the loss of one lattice water molecule (Found: 2.67%). The structure of 2 began to collapse above 350 °C. The results showed that the total collapse temperature of the two CPs structures is over 340 °C , showing both CPs have high thermal stability, which is very important for the practical application of CPs as functional materials.

3. 3 Magnetic properties

Fig. 5. Temperature dependence of magnetic susceptibilities of 1(a) and 2(b) under a static field of 1000 Oe

In order to study the magnetic properties of CPs, the temperature dependent magnetic susceptibilitiesχMof 1 and 2 under a constant magnetic field of 1000 Oe were measured in temperature range from 2 to 300 K. As exhibited in Fig. S8 and S9, CP 1 has no obvious magnetic phase transition, while CP 2 has a magnetic phase transition in the range of 20～40 K.The curves ofχMTandχM-1νsTare shown in Fig. 5. For 1,theχMT experimental value of 9.89 cm3·K·mol-1at room temperature is higher than that of four spin-only Co(II) ions(7.50 cm3·K·mol-1, S = 3/2, g = 2.0), which may be attributed to the formation of unquenched orbital moment caused by the spin-orbit coupling in the distorted octahedral coordination of Co(II) ion[26,27]. As shown in Fig. 5a, theχMTvalue decreased monotonously with the decrease of temperature, finally reached the minimum value of 0.21 at 2.0 K. The result at high temperature region reveals the antiferromagnetic interaction between neighboring Co(II) ions in CP 1[26]. The decrease of theχMTvalue at low temperature can be due to the contribution of the orbital momentum of the single Co(II) ion,intermolecular antiferromagnetic coupling interaction, and intramolecular antiferromagnetic coupling interaction between neighboring Co(II) ions[28-30]. In addition, the curve ofχM-1νsT is not completely linear. In the range of 40～300 K,the magnetic susceptibility is fitted according to the Curie-Weiss law (χM= C/(T-θ)), with the Curie constant C of 17.61 cm3·K·mol-1and the Weis constantθof -241.95 K, which is confirmed again the antiferromagnetic interaction exhibited in 1.

As for 2, theχMTvalue of 7.57 cm3·K·mol-1at 300 K is close to the calculated value of 7.50 cm3·K·mol-1for four isolated high-spin Co(II) ions. As shown in Fig. 5b, theχMTvalue decreased monotonously with the continuous decrease of temperature and reached the minimum value of 0.19 cm3·K·mol-1at 2 K. Similar to 1, the magnetic susceptibilities obey the Curie-Weiss law in the temperature range above 50 K for 2, giving a negative Weiss constantθ= -152.25 K and Curie constant C = 11.34 cm3·K·mol-1for 2, respectively.According to the variation trend ofχMTvalue and the negative Weiss constant, the antiferromagnetic interaction also exists between adjacent Co(II) ions in 2.

The above magnetic difference between the two CPs is related to their structures. In these two CPs, H3DCPN ligands have very similar coordination patterns. Although two different tetranuclear Co4(DCPN)2(μ3-OH)2] SBUs are formed in 1, they both form 2D layered structures when connected to the Co(II) ions. Interestingly, the introduction of imidazole ligands causes structural changes. Specifically, ligand 1,2-bimb is directly connected to Co(II) ions in the same SUB, so that 1 remains a 2D structure, while ligand 1,3-bimb molecules are connected to different SUB, so that different layered structures are connected to form 3D structures, thus resulting in their different properties.

4 CONCLUSION

In summary, two novel Co-based CPs have been solvothermally synthesized by the reaction of H3DCPN, 1,2-bimb, 1,3-bimb and cobalt salts. Meanwhile, 1 and 2 have good phase purities and high thermal stability, while 1,2-bimb and 1,3-bimb bridging linker help form different structures in the self-assembly process of CPs. In addition, magnetic studies indicated that 1 and 2 both have antiferromagnetic interactions between the adjacent Co(II) ions.