Synthesis , Crystal Structures , and Magnetic Properties of Three Cobalt ( II ) Coordination Polymers Constructed from 3 , 5-Pyridinedicarboxylic Acid or 3 , 4-Pyridinedicarboxylic Acid Ligands

Three 2D new coordination polymers Co2(L1)2(1,10-Phenanthroline)2(DMF)0.5(H2O) (1), (H2L1 = Pyridine-3,5-dicarboxylic acid) Co(L1)(2,2-bipyridine) (2), and Co(L2)(2,2-bipyridine) (DMF) (3) (H2L2 = Pyridine-3,4-dicarboxylic acid) were synthesized through a solvothermal reaction of cobalt nitrate and pyridine carboxylic acid ligand with the auxiliary ligand (1,10-Phenanthroline or 2,2-bipyridine). They were characterized by X-ray diffraction and elemental analysis, infrared spectroscopy, thermogravimetry analysis, and magnetism. Compounds 1–3 featured 2D hexagonal (6,3) networks which linked into 3D supramolecular architectures through π–π interaction. In addition, compounds 1 and 2 showed the antiferromagnetic exchange interactions, and the magnetic property of compound 3 exhibited ferromagnetic exchange interactions.


Introduction
The rational design and synthesis of coordination polymers have been under intensive investigation in the past two decades because of their potential applications in a number of fields, such as in the areas of storage, separation, catalysis, magnetism, and luminescence [1][2][3][4][5][6][7][8][9].However, the reasonable synthesis and design of such target materials are still a great challenge in crystal engineering.There are many factors that can influence the final structures of coordination polymers, such as metal ions, ligands, metal-to-ligand ratio, solvent system, pH, etc., and among these factors, the selection of suitable organic ligands plays a crucial rule in the constructions of fascinating coordination polymers [10][11][12].Just like the reported structures, it is a convenient strategy to construct coordination polymers with a single ligand containing N or O donor atoms [13,14].Nevertheless, for the purpose of getting more coordination polymers with novel structures and excellent properties, single coordination atoms sometimes cannot meet the needs of researchers.Therefore, much attention has been paid to organic ligands containing various coordination atoms, of which pyridylcarboxylates are one variety.Accordingly, many works have been conducted to construct coordination polymers with different structures using pyridine carboxylic acid ligands, and various extended structures have been successfully constructed with different metal ions, for instance, six novel 3D coordination polymers with interesting topologies were synthesized by Zhu with pyridine-3,4-dicarboxylic acid [15], and a series of Ln coordination polymers with photoluminescent properties were constructed based on pyridine-3,5-dicarboxylic acid [16][17][18][19][20][21][22].Taking into account the reported works, in this paper we chose the symmetrical pyridine-3,5-dicarboxylic acid and asymmetrical pyridine-3,4-dicarboxylic acid as the ligands because these two ligands can be deprotonated to form HL − and L 2− anions which have the flexible and versatile coordination modes to bridge metal ions.Furthermore, the π-electric conjugated system of the pyridine dicarboxylic acid and auxiliary ligands (1,10-Phenanthroline and 2,2-bipyridine) are beneficial for the formation of stable supramolecular structures [15,[21][22][23].Meanwhile we chose cobalt nitrate as the metal source, and thus they could form both discrete and consecutive new Co compounds with magnetic behavior, having potential applications in magnetic materials [24,25].

Materials and Physical Measurements
All reagents and solvents used in the experiment were obtained directly from a commercial source, and used without further purification.Inductively coupled plasma (ICP) analysis of Co and elemental analysis were performed on a Perkin-Elmer Optima 3300DV Spectrometer and a Perkin-Elmer 2400 element analyzer, respectively.Infrared (IR) spectra were recorded on a Nicolet Impact 410 FTIR spectrometer using a KBr pellet in the range of 4000-400 cm −1 .Powder X-ray diffraction (PXRD) measurements were executed by using a Rigaku D/max 2550 X-Ray Powder Diffractometer.Thermogravimetric analysis was performed with a TGA Q500 V20.10 Build 36 instrument with a heating rate of 10 • C/min in a flowing N 2 atmosphere.Magnetic susceptibility data were obtained by SQUID magnetometer (Quantum MPMS) in the range of 2-300 K using an applied field of 1000 Oe.

Crystal Structural Determination
Single crystal X-ray diffraction data for compounds 1-3 were obtained using a Rigaku RAXIS-RAPID equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K.The data processing was accomplished with the PROCESS-AUTO processing program.The structures were solved with the direct methods of the SHELXL crystallographic software package and refined on F 2 by full-matrix least square techniques.All non-hydrogen atoms of the three compounds were refined with anisotropic thermal parameters.All hydrogen atoms of the organic molecule were geometrically placed and added to the structure factor calculation.Crystal data and structure refinement details of compounds 1-3 are summarized in Table 1.Crystallographic data for the structures reported in this paper have been deposited in the Cambridge Crystallographic Data Centre (CCDC), and the CCDC numbers of the three compounds are CCDC-1876151 (1), CCDC-1876152 (2), and CCDC-1876153 (3).As for the molecular formula of compounds 1 and 3, it is difficult to obtain the exact solvent molecules in the structure, so we further determined these using Elemental analysis, TGA and Platon program, and the solvent composition (compound 1: 0.5DMF + H 2 O; compound 2: DMF) was calculated from the TGA and elemental analysis that were in agreement with the data from Platon program [26].

X-Ray Power Diffraction Analysis and Thermal Analysis
The diffraction peaks of compounds 1-3 were confirmed with good agreement between the experimental (black line) and simulated PXRD patterns (red line) (Figures S10-S12), and the phenomenon indicates that the synthesized compounds 1-3 have good phase purity.
The thermal stability of compounds 1-3 were examined by TG analysis in the range of 25−800 °C , and the results are given in Figure 4.For 1, this led to weight loss from 135 to 378 °C as uncoordinated water and DMF molecules were separated from the framework (Obsd.6.26%, Calcd.6.3%), and further loss resulted from the decomposition of organic components.Finally, a residue of Co−O was left.For 2, the framework started to decompose from 338 to 800 °C, and Co−O was the final residue.In terms of 3, weight loss occurred at 141−400 °C because of the removal of uncoordinated

X-Ray Power Diffraction Analysis and Thermal Analysis
The diffraction peaks of compounds 1-3 were confirmed with good agreement between the experimental (black line) and simulated PXRD patterns (red line) (Figures S10-S12), and the phenomenon indicates that the synthesized compounds 1-3 have good phase purity.
The thermal stability of compounds 1-3 were examined by TG analysis in the range of 25−800 • C, and the results are given in Figure 4.For 1, this led to weight loss from 135 to 378 • C as uncoordinated water and DMF molecules were separated from the framework (Obsd.6.26%, Calcd.6.3%), and further loss resulted from the decomposition of organic components.Finally, a residue of Co−O was left.For 2, the framework started to decompose from 338 to 800 • C, and Co−O was the final residue.In terms of 3, weight loss occurred at 141−400 • C because of the removal of uncoordinated DMF molecules in the framework (Obsd.16.3%, Calcd.16.14%), and the decomposition of the framework occurred at 220 • C, so a residue of CoO remained.
The diffraction peaks of compounds 1-3 were confirmed with good agreement between the experimental (black line) and simulated PXRD patterns (red line) (Figures S10-S12), and the phenomenon indicates that the synthesized compounds 1-3 have good phase purity.
The thermal stability of compounds 1-3 were examined by TG analysis in the range of 25−800 °C , and the results are given in Figure 4.For 1, this led to weight loss from 135 to 378 °C as uncoordinated water and DMF molecules were separated from the framework (Obsd.6.26%, Calcd.6.3%), and further loss resulted from the decomposition of organic components.Finally, a residue of Co−O was left.For 2, the framework started to decompose from 338 to 800 °C, and Co−O was the final residue.In terms of 3, weight loss occurred at 141−400 °C because of the removal of uncoordinated DMF molecules in the framework (Obsd.16.3%, Calcd.16.14%), and the decomposition of the framework occurred at 220 °C , so a residue of CoO remained.

Magnetic Properties
When the magnetic field was 1000 Oe and the temperature was in the range of 2-300 K, the variable temperature magnetic susceptibility of compounds 1-3 was measured.The curves of χMT and M -1 vs. T of compounds 1-3 are presented in Figures 5, 6, and 7, respectively.The χMT value of

Magnetic Properties
When the magnetic field was 1000 Oe and the temperature was in the range of 2-300 K, the variable temperature magnetic susceptibility of compounds 1-3 was measured.The curves of χ M T and χ M −1 vs. T of compounds 1-3 are presented in Figure 5, Figure 6, and Figure 7, respectively.The χ M T value of compound 1 at 300 K was 5.60 emu•K mol −1 for a Co 2 unit, where the value was much higher than the theoretical χ M T value of the spin-only one for two isolated Co 2+ ions (3.75 emu•K mol −1 and S = 3/2), which is mainly due to the spin-orbit coupling of the high-spin Co 2+ ions.Upon the temperature cooling to 50 K, the χ M T value was kept roughly constant.Nevertheless, it decreased suddenly and gave the χ M T value 3.07 emu•K mol −1 at 2 K.The sudden decrease below 50 K was due to the antiferromagnetic coupling between the paramagnetic centers and the zero-field splitting (Figure 5).The χ M T value of compound 2 at 300 K was 2.79 emu•K mol −1 for a Co ion, where the value was much higher than the theoretical χ M T value of the spin-only one for one isolated Co 2+ ion (1.875 emu•K mol −1 and S = 3/2), but it still fell within the usual range for octahedral Co 2+ ions in the 4T2g state.Upon the temperature cooling to 50 K, the χ M T value was kept roughly constant.The phenomenon is reminiscent of antiferromagnetic behavior (Figure 6).The χ M T value of compound 3 at 300 K was 2.23 emu•K mol −1 for a Co ion, where the value was much higher than the theoretical χ M T value for one isolated Co 2+ ion (1.875 emu•K mol −1 and S = 3/2), though it fell within the usual range for octahedral Co 2+ ions in the 4T2g state.The χ M T value was kept roughly constant until the temperature decreased to 50 K, suggesting a ferromagnetic interaction between Co ions (Figure 7).Curie−Weiss law was used to fit the χ M −1 data from 2-300 K, acquiring θ = −1.77K, C = 5.67 emu•K mol −1 for compound 1.The negative θ value and χ M T trend with temperature fully demonstrated an antiferromagnetic effect of compound 1.In compound 2, the parameters C = 1.0 emu•K mol −1 and θ = 0 K indicated an antiferromagnetic effect of compound 2, while in compound 3, giving C = 2.2 emu•K mol −1 and θ = 6.5 K, a ferromagnetic effect was suggested between the Co 2+ ions (Table 2) [24,25].
The negative θ value and χMT trend with temperature fully demonstrated an antiferromagnetic effect of compound 1.In compound 2, the parameters C = 1.0 emu•K mol −1 and θ = 0 K indicated an antiferromagnetic effect of compound 2, while in compound 3, giving C = 2.2 emu•K mol −1 and θ = 6.5 K, a ferromagnetic effect was suggested between the Co 2+ ions (Table 2) [24,25].

Conclusions
Three 2D coordination polymers were prepared based on cobalt nitrate and H2L1, H2L2 ligands.