Two NiII/CuII Coordination Polymers Based on Pyridyl-carboxylate Ligand: Synthesis, Crystal Structures and Magnetic Properties①

YE Xio DNOG Wen-Wen FANG Yi-Ming ZHAO Jun, b LI Dong-Sheng

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Two NiII/CuIICoordination Polymers Based on Pyridyl-carboxylate Ligand: Synthesis, Crystal Structures and Magnetic Properties①

YE XiaoaDNOG Wen-Wena②FANG Yi-MingaZHAO Juna, bLI Dong-Shenga

a(443002)b(438000)

Under hydrothermal conditions, the reactions of NiII/CuIIions with 3-(6-aminpyri- dinium-3-yl) benzoate (HL) afford two compounds [NiL2]n(1) and [CuL2(H2O)]n(2). On the basis of X-ray diffraction analysis of the two compounds, the results show that compound 1 features one-dimensional (1) double-strand coordination arrays, while 2 presents the 63-layers. Both compounds are further constructed into a 3supramolecular structure with the aid of weak secondary interactions. Thermal stabilities and magnetic properties of compounds 1and 2were also investigated.

coordination polymers, crystal structure, NiII/CuIIcompounds, magnetic property;

1 INTRODUCTION

Over the past couple of decades, the design and synthesis of novel functional coordination polymers (CPs) have attracted much attention because of their diverse structural topologies and potential applica- tions in catalysis[1-4], gas storage and separation[5-8], luminescence[9-12], biomedical imaging[13-15]and magnetism[16-18]. At present, although a lot of novel CPs have been designed and synthesized[19-22], it is still a great challenge to rationally prepare and predict their exact structures because so many factors can affect the final structure formation. Among all of these factors, the organic ligands play a key role. Generally, multidentate bridge ligands containing functional groups such as the familiar pyridyl and/or carboxylate groups are most effective building units due to their rich coordination modes. On the other hand, the NH or N atom of multiple N-containing groups as well as the OH or O atom of carboxylate groups can act as hydrogen bonding donor or ace- ptor[23-26]. In the assembly of CPs, hydrogen bonding,stacking interaction, besides the metal-ligand coordination, are also usually utilized higher-dimen- sional networks.

In this context, we have further investigated the coordination chemistry of 3-(6-aminpyridinium-3-yl) benzoate (HL), which could be introduced as an attractive building block. In the case of HL, although it belongs to the rigid ligand category, the C–C bond between pyridyl and benzyl groups can turn over/rotate to different extents in order to meet the=steric requirement upon metal complexation (see Scheme 1). On the other hand, due to the presence of aromatic rings and -NH2groups in the assembled system, the delicatestacking interactions and hydrogen bonding are available to play a decisive role in regulating the resulting supramolecular networks.In our previous work[27], we have synthe- sized two novel ZnII/CdIICPs with isomeric ligands 3-(6-aminpyridinium-3-yl) benzoate and 4-(6-amin- pyridinium-3-yl) benzoate, and discussed their fluorescent properties. Herein, we report the cons- truction of two CPs NiL2(1) and CuL2(H2O) (2) obtained by hydrothermal reactions of HL, and further studied their crystal structures,thermal stabilities, and magnetic properties.

2 EXPERIMENTAL

2. 1 Materials and general methods

All solvents and reagents for syntheses were commercially available and used as received. The infrared spectra (400～4000 cm-1) were recorded as KBr pellets on a FTIR Nexus spectrophotometer. Elemental analyses were performed on a Perkin- Elmer 2400 Series II analyzer. Powder X-ray diffraction (PXRD) patterns for microcrystalline samples were taken on a Rigaku Ultima IV diffract- tometer (Curadiation,= 1.5406 ?) with a scan speed of 8 °/min and a step size of 0.02o in 2. The simulated PXRD patterns were calculated using Mercury 3.9. Thermogravimetric (TGA) curves were recorded on a NETZSCH STA 449C microanalyzer in air atmosphere at a heating rate of 10 ℃/min. Variable-temperature magnetic susceptibility mea- surements (2～300 K) were carried out on a Quan- tum Design MPMS3 SQUID magnetometer in a magnetic field of 1 KOe, and the diamagnetic corrections were evaluated by using Pascal’s constants.

2. 2 Synthesis of NiL2 (1)

A mixture of HL (0.0214 g, 0.1 mmol), Ni(ClO4)2.6H2O (0.0366 g, 0.1 mmol), NaOH (0.0040 g, 0.1 mmol) and 8 mL deionized water solution was sealed in a Teflon-lined stainless vessel (25 mL) and heated at 120 ℃for 96 h, and then the vessel was cooled slowly to room temperature at 2℃/h. The blue needle shaped crystalline products were obtained in 62% yield (based on ligand). Elemental analysis calcd. (%): C, 59.51; H, 3.84; N, 11.63. Found (%): C, 59.33; H, 3.66; N, 11.48. IR(KBr, cm-1): 3370(w), 1630(s), 1567(m), 1531(s), 1510(s), 1486(m), 1421(s), 1385(s), 914(w), 887(m), 775(s), 760(s), 750(m), 694(m), 664(s).

2. 3 Synthesis of CuL2(H2O) (2)

It was prepared in a similar way to 1, but using Cu(ClO4)2.6H2O (0.0370 g, 0.1 mmol) instead of Ni(ClO4)2.6H2O (0.0370 g, 0.1 mmol). The yield was 59% (based on ligand). Elemental analysis calcd. (%): C, 56.91; H, 4.02; N, 11.12. Found (%): C, 56.74; H, 3.88; N, 10.96. IR (KBr, cm-1): 3389(w), 1672(m), 1632(m), 1554(m), 1511(m), 1381(s), 1259(m), 772(s), 758(s), 702(m), 672(m), 652(s).

2. 4 Crystal structure determination

Single-crystal X-ray diffraction analyses of 1 and 2 were carried out with a Rigaku XtaLAB mini diffractometer equipped with graphite-monochro- mated Moradiation (= 0.71073 ?) by using a-scan mode at 296(2) K. The collected data were reduced using the program CrystalClear[28]and an empirical absorption correction was applied. The structures were solved by direct methods using SHELXS-97and refined on2by full-matrix least- squares methods using SHELXL-97[29]. All non- hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms attached to carbon and oxygen were placed in geometrically idealized positions and refined using a riding model.Selected bond lengths and bond angles are listed in Table 1.

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

Symmetry codes for 1: a:–1, –1, –2;b: –,1/2, –3/2; c:–,

–1/2, –3/2;for2:a: –1,–1/2, –1/2;b:–1,1/2, –1/2

3 RESULTS AND DISCUSSION

3. 1 Structure description of NiL2 (1)

Single-crystal X-ray diffraction analysis reveals that compound 1 assembles a H-bonded 2network with a 1double helical NiL2chain units. It crystallizes in monoclinic with space group21/, and the smallest asymmetric unit of 1 contains one NiIIion and two L ligands. As show in Fig. 1a, each independent NiIIion is six-coordinated by two pyridine nitrogen atoms from two L ligands, respectively and four coordinated oxygen atoms from the other two chelate carboxylic-acid oxygen atoms of two L ligands, respectively. In this structure, the L ligands show the same coordination modes,2-uni- dentate (Npy)/bidentate chelating (OCOO-) (2-N,O,O shown in Scheme 1, left). The dihedral angles amount to 25° and 26.5° between pyridine and phenyl in L ligands. And further, the adjacent NiIIcenters with the separation of 8.247(2) ? are linked by paired L bridges to generate an infinite 1double-strand chain along theaxis (see Fig. 1b). Each coordination polymer chain is thus linked to four adjacent chainssuch interchain H-bonds with N(4)–H(4B)···O(1) (the bond length is 3.009(4) ? and the bond angle 158.5°) (see Table 2 for details), affording a 2H-bonded structure (see Fig. 1c).

Fig. 1. (a)Coordination environment of the Ni(II) center in compound 1 (Hydrogen atoms were omitted for clarity. Symmetry code: #1 –+1,–1/2, –+1/2); (b) 1double-strand chain structure; (c) 2layer indirection via N(4)–H(4B)···O(1) hydrogen bonds (the dotted lines)

Scheme 1. Coordination mode of the HL ligand shown in compounds 1 and 2

Table 2. Hydrogen Bonding Geometry (?, o) for 1 and 2

Fig. 2. (a)Coordination environment of the Cu(II) center in compound 2(Hydrogen atoms were omitted for clarity). Symmetry codes for the generatedatoms: #1: –+1, –+1, –+2; #2: –,+1/2, –+3/2; (b) 263layer structure;(c) 3supramolecular framework formed by N4–H4B···O2 and O5–H5A···O4 hydrogen bonds

3. 3 PXRD and thermogravimetric analysis

In order to confirm whether the crystal structures are truly representative of the bulk materials, the PXRD patterns of coordination polymers 1 and 2 are recorded (Fig. 3). Comparing with the corresponding simulated single-crystal diffraction data, all the peaks present in the measured patterns closely match in the simulated patterns generated from single crystal diffraction data, which indicates compounds 1 and 2 are in pure phase. The thermal stability of com- pounds 1 and 2 was performed on single-phase polycrystalline samples from 20 to 900 ℃. The TGA curves (Fig. 4) show that compound 1 is thermally stable up to 400 ℃, and thereafter significant weight loss occurs resulting in complete decomposition of the compound. The final residual is in good agreement of NiO (calcd. 15.4% and exp. 15.6%). For compound 2, the sample shows two-step weight loss at 200～520 ℃, which loses a water molecule in the first step at 200～300 ℃ (calcd. 3.54% and exp. 3.52%), and final remaining is likely attributed to CuO (calcd. 15.7% and exp. 15.8%).

Fig. 3. PXRD patterns of compounds 1 (a) and 2 (b)

Fig. 4. TG curves of compounds 1 and 2

3. 4 Magnetic properties

Magnetic susceptibility data were collected for crushed crystalline samples of compounds 1 and 2. For compound 1, as shown in Fig. 5, the experi- mentalχT value at 300 K is 1.26 cm3×K/mol per NiIIion. Upon cooling the sample to 20 K, theχT value increases continuously to a maximum of 1.40 cm3×K/mol, indicating the presence of ferromagnetic coupling in 1. Upon further cooling 1 below 20 K, theχT value decreases to 0.76 cm3×K/mol at 1.9 K, which could be due to the interchain antiferro- magnetic interactions and/or the zero-field splitting of the ground state[30]. The thermal evolution ofχ-1obeys the Curie-Weiss law over the whole tempera- ture. Fitting the curves of 1 gives parameters= 1.274 cm3×K/moland= 1.373 K. The small positive value also indicates weak ferromagnetic interactions in 1. For compound 2, theχT value at 300 K is 0.399 cm3×K/mol (see Fig. 6), which is larger than the expected value of 0.375 cm3×K/mol for one magnetically isolated spin-only CuII. Upon cooling the sample, theχT value monotonically decreases to a minimum of 0.364 cm3×K/molat 2 K. This behavior indicates weak antiferromagnetic interactions between the CuIIions in the structure of 2. The fitting ofχ-1obeys the Curie-Weiss law and gives= –0.663 K and= 0.388 cm3×K/mol.

Fig. 5. Magnetic behavior of compound 1 in the form ofmT andmversus T.Insert: temperature dependence ofm-1. The solid red line representsthe best fit of Curie-Weiss lawm=C/(T –)

Fig. 6. Magnetic behavior of compound 2 in the form ofmT andmversus T. Insert: temperature dependence ofm-1. The solid red line represents the best fit of Curie-Weiss lawm= C/(T –)

4 CONCLUSION

In summary, two new NiII/CuIIcoordination com- pounds with L ligand have been obtained.Compound 1 features classical 1double-strand coordination arrays, and compound 2 shows a 2layer structure. For both compounds, hydrogen-bonding interactions play an important role in the formation of supramo- lecular architecture. Interestingly, different coordina- tion modes and conformation of L ligand result in different crystal packing of the compounds, which exhibit various magnetic properties. Compound 1 exhibits weak ferromagnetic interactions, while antiferromagnetic interaction has been found in 2.

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10 July 2017;

8 October 2017 (CCDC 1552054 for 1 and 1552053 for 2)

10.14102/j.cnki.0254-5861.2011-1779

①This project was supported by the National Natural Science Foundation of China (Nos.21671119, 21301106 and 21673127)

②. E-mail: dongww1@126.com