Syntheses, Crystal Structures, and Magnetic Properties of the Cobalt(II) and Nickel(II) Coordination Polymers Constructed from 5-Halonicotinate and 2,2?-Biimidazole①

LI Hong-Jin GAO Zhu-Qing② GU Jin-Zhong

a (School of Chemical and Biological Engineering,Taiyuan University of Science and Technology, Taiyuan 030021, China)

b (College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China)

1 INTRODUCTION

During the past decade, the construction of transition coordination polymers is the current interest in the field of supramolecular chemistry and crystal engineering, and more and more people have paid considerable attention to research it due to its extensive potential applications in catalysis, separation,ion-exchange, magnetism, as well as their fascinating architectures and topological frameworks[1-5]. In literature, to construct the coordination polymers,flexible carboxylate ligands are of special interest because of their varied coordination modes, abundant structural motifs, and flexible molecular backbones[6-10]. They often act as linkers in the networks and result in fascinating configurations in coordination polymers. Apart from the polycarboxylate ligands, various aromatic N-donors also play an important role and frequently serve as ancillary ligands to adjust the coordination mode of carboxylate block and the supramolecular structure of the resulting network[6-9].

In order to extend our researches in this field, we chose 5-ClnicH and H2biim as mixed ligands based on the following considerations: (i) 5-ClnicH possesses one carboxyl group and one N atom available for coordination to a metal center; (ii) Although several examples of transition metal coordination polymers derived from 5-ClnicH have been reported[11], this ligand still remains little explored in the construction of coordination polymers; (iii) H2biim ligand has three kinds of existing forms (neutral,monovalent, and divalent anion) and a variety of coordination modes in the structures of coordination polymers[9]. Furthermore, H2biim can also be considered as different hydrogen-bonding donors upon the deprotonated forms[12].

Hence, in this work by using 5-ClnicH as a functional ligand and changing the reaction metal nitrates, two coordination compounds ([Co(5-Clnic)2(H2biim)]n(1) and {[Ni(5-Clnic)(Hbiim)]·H2O}n(2)) were prepared through the hydrothermal method.Their structural diversities show that the assembly process is metal ion-dependent. Both complexes were characterized by elemental analyses, IR spectroscopy and single-crystal X-ray diffraction analyses. The thermal stabilities and magnetic properties of these two compounds have been investigated.

2 EXPERIMENTAL

2.1 General procedures

All chemicals and solvents were of A.R. 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 at a heating rate of 10 ℃/min on a LINSEIS STA PT1600 thermal analyzer. Magnetic susceptibility data were collected in the 2～300 K temperature range with a Quantum Design SQUID Magnetometer MPMS XL-7 in 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 Co(NO3)2·6H2O (0.044 g, 0.15 mmol), 5-ClnicH (0.047 g, 0.3 mmol), H2biim(0.020 g, 0.15 mmol), and H2O (10 mL) was adjusted to pH = 6.0 with a 0.5 M NaOH solution.The mixture was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 ℃ for 3 days,followed by cooling to room temperature at a rate of 10 ℃·h-1. Pink block-shaped crystals of 1 were isolated manually, and washed with distilled water.Yield: 60% (based on Co). Anal. Calcd. (%) for C18H12Cl2CoN6O4: C, 42.71; H, 2.39; N, 16.60.Found (%): C, 42.47; H, 2.65; N, 16.87. IR (KBr,cm-1): 1605s, 1555m, 1468m, 1370s, 1332w, 1280m,1240w, 1188w, 1124s, 1091w, 1031m, 991m, 926w,898w, 857m, 778m, 750m, 690m, 434m.

2.3 Synthesis of compound 2

The preparation of 2 was similar to that of 1 except using Ni(NO3)2·6H2O instead of Co-(NO3)2·6H2O. After being cooled to room temperature, violet block-shaped crystals of 2 were isolated manually, and washed with distilled water.Yield 60% (based on Ni). Anal. Calcd. (%) for C12H10ClNiN5O3: C, 39.34; H, 2.75; N, 19.11. Found(%): C, 39.68; H, 2.47; N, 19.46. IR (KBr, cm-1):3516w, 1603s, 1552s, 1447m, 1369s, 1336w, 1280m,1242w, 1190w, 1125s, 1097w, 1032m, 993m, 928w,900w, 854w, 778s, 751m, 694m, 439m.

2.4 Structure determination

Two single crystals of the title compounds with dimensions of 0.26mm × 0.26mm × 0.25mm (1)and 0.28mm × 0.26mm × 0.25mm (2) were mounted on a Bruker CCD diffractometer equipped with a graphite-monochromatic MoKα (λ = 0.71073 ?) radiation using a φ-ω scan mode at 293(2) K in the ranges of 2.94<θ<25.05o and 3.29<θ<25.05o,respectively. The structures were solved by direct methods with SHELXS-97[13]and refined by fullmatrix least-squares techniques on F2with SHELXL-97[14]. All non-hydrogen atoms were refined anisotropically. All hydrogen 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 hydrogen atoms of water molecules were located by difference maps and constrained to ride on their parent O atoms. The final R = 0.0489 in 1 for 281 parameters and 3494 observed reflections with I > 2σ(I) and wR = 0.1022 (w = 1/[σ2(Fo2)+ (0.0346P)2+ 2.0865P], where P = (Fo2+ 2Fc2)/3)for all 12409 reflections. S = 1.068, (Δρ)max= 0.475,(Δρ)min= –0.580 e/?3and (Δ/σ)max= 0.001. And in 2,the final R = 0.0415 for 199 parameters and 2608 observed reflections with I > 2σ(I) and wR = 0.0948(w = 1/[σ2(Fo2) + (0.0418P)2+ 10.9793P], where P =(Fo2+ 2Fc2)/3) for all 8370 reflections. S = 1.024,(Δρ)max= 0.627, (Δρ)min= –0.453 e/?3and (Δ/σ)max= 0.001. The selected important bond parameters are given in Tables 1 and 2.

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

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

3 RESULTS AND DISCUSSION

3.1 Crystal structure of 1

The asymmetric unit of compound 1 contains one crystallographically unique Co(II) atom, two 5-Clnic–ligands, and one H2biim moiety. As depicted in Fig. 1, each Co(II) atom is six-coordinated and adopts a distorted octahedral geometry formed by two carboxylate O atoms of one 5-Clnic–ligand, two N atoms of two different 5-Clnic–ligands and two N atoms of one H2biim moiety. The Co–O (2.177(3)～2.189(3) ?) and Co–N (2.078(4)～2.161(3) ?) bond lengths are in good agreement with those observed in some other Co(II) compounds[6,7,15]. In 1, the 5-Clnic–ligands adopt two different μ1- and μ2-coordination modes (Scheme 1, modes I and II). The H2biim ligand takes a bidentate chelating mode(Scheme 1, mode III). The dihedral angle of two imidazole groups is 1.87o. The carboxylates and N atoms of the 5-Clnic–ligands bridge alternately the neighboring Co(II) ions to form a chain with the Co···Co separation of 7.796(2) ? (Fig. 2). The similar chain has been observed in [Zn(5-Brnic)2(H2biim)]n[16]. Adjacent chains are connected to form a 3D supramolecular architecture via (Fig. 3)N–H···O hydrogen bonds (Table 3) and π-π packing interactions (the centroid-centroid separation of adjacent imidazole planes of the H2biim ligands is 3.719(2) ?).

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (o) of Compound 1

Scheme 1. Coordination modes of the Clnic–/H2biim/Hbiim– ligands in compounds 1 and 2

Fig. 1. Coordination environment of the Co(II) atom in compound 1.H atoms were omitted for clarity. Symmetry code: i: x+1, y, z

Fig. 2. A perspective view of the 1D chain along the ab plane in compound 1

3.3 Crystal structure of 2

Compound 2 crystallizes in the trigonal space group R3. In the asymmetric unit, there are one crystallographically unique Ni(II) atom, one 5-Clnic–ligand, one Hbiim–moiety and one lattice water molecule. As shown in Fig. 4, the Ni(1) atom is six-coordinated by two O atoms of one 5-Clnic–ligand, three N atoms of two Hbiim–moieties and one N atom of one 5-Clnic–ligand, constructing a distorted octahedron. The lengths of Ni–O bonds are in the range of 2.112(2)～2.279(3) ?, and those of the Ni–N are 2.036(3)～2.099(3) ?, which are comparable to those of other Ni(II) compounds[7,17].In 2, the 5-Clnic–ligand adopts the coordination mode I (Scheme 1). The Hbiim–ligand takes the bridging mode IV (Scheme 1). The dihedral angle of two imidazole rings in the Hbiim–ligand is 13.64o.Six Hbiim–ligands link six Ni(II) ions to form a[Ni6(Hbiim–)6] hexagonal ring, with the Ni···Ni distance of 5.903(2) ? (Fig. 5). The similar trinuclear metallocycle was reported in[Cd3(bta)(Hbiim)2(H2biim)]n[9]. The hexagonal ring units are connected by 5-Clnic–ligands to form a 1D chain (Fig. 6), which are held together into a 3D supramolecular framework (Fig. 7) via N–H···O and O–H···O hydrogen bonding interactions (Table 4).

Fig. 3. A perspective view of the 3D supramolecular structure along the ab plane

Fig. 4. Coordination environment of the Ni(II) atom in compound 2 (H atoms were omitted for clarity). Symmetry codes: i: x–y +2/3, x+1/3, –z+7/3; ii: x, y, z+1

Fig. 5. A hexanuclear Ni(II) macrocycle in compound 2

Fig. 6. A perspective view of the 1D chain along the ab plane in compound 2

Table 4. Hydrogen Bond Lengths (?) and Bond Angles (o) of Compound 2

3.4 Thermal analysis

To study the stability of compounds 1 and 2,thermal gravimertric analyses (TGA) were performed. As shown in Fig. 8, the TGA curve of compound 1 indicates that it is stable up to 294 ℃.

Fig. 7. A perspective of the 3D supramolecular structure along the ab plane in compound 2

3.5 Magnetic properties

Fig. 8. TGA curves of compounds 1 and 2

The temperature-dependent magnetic properties of 1 are shown in Fig. 9 in the form of χMT versus T Further heating leads to its decomposition. Compound 2 undergoes a mass loss of 4.62% between 38 and 92 ℃, which corresponds to the loss of one lattice water molecule (calcd. 4.91%). Above 298 ℃,the framework is destroyed gradually.curve. The χMT value of 2.94 cm3·mol-1·K at 300 K is much larger than the value (1.87 cm3·mol-1·K)expected for one magnetically insulated high-spin Co(II) ion (S = 3/2, g = 2.0). This is a common phenomenon for Co(II) ions due to their strong spin-orbital coupling interactions[6]. The χMT values steadily decrease with decreasing the temperature to reach the minimum values of 1.61 cm3·mol-1·K at 1.99 K. Between 100 and 300 K, the magnetic susceptibilities can be fitted to the Curie-Weiss law with CM= 3.10 cm3·mol-1·K and θ = –15.80 K. These results indicate an antiferromagnetic interaction between the nearest Co(II) ions.

We tried to fit the magnetic data of 1 using the following expression for a 1D Co(II) chain[18]:

Using this rough model, the susceptibilities for 1 were simulated, leading to J = –5.72 cm-1, g = 2.48,and the agreement factor R = 5.36 × 10-5.

For 2, the χMT value at 300 K is 1.10 cm3·mol-1·K,which is higher than the spin only value of 1.00 cm3·mol-1·K for one magnetically isolated Ni(II)center (SNi= 1, g = 2.0). Upon cooling, the χMT value drops down very slowly from 1.10 cm3·mol-1·K at 300 K to 1.06 cm3·mol-1·K at 20 K and then decreases steeply to 0.49 cm3·mol-1·K at 2 K (Fig. 10). The plot of χM-1vs. T for compound 2 in 2～300 K obeys the Curie-Weiss law with a Weiss constant θ of -1.08 K and a Curie constant C of 1.10 cm3·mol-1·K, suggesting a weak antiferromagnetic interaction between the Ni(II) ions.

Fig. 9. Temperature dependence of χMT (О) and 1/χMvs. T for compound 1. The blue line represents the best fit to the equations in the text. The red line shows the Curie-Weiss fitting

Fig. 10. Temperature dependence of χMT (О) and 1/χMvs. T for compound 2. The blue line represents the best fit to the equations in the text. The red line shows the Curie-Weiss fitting

An empirical (Wang’s) formula has been reported in the literature to analyze 1D systems with S = 1,using numerical procedures[18,19];

Using this method, the best-fit parameters for 2 were g = 2.10, J = –0.65 cm-1and R = 5.8 × 10-5,where

The J value of –0.65 cm-1for complex 2 indicates that the coupling between the Ni(II) centers is weakly antiferromagnetic, which can be attributed to the large Ni···Ni separations (5.903(2) and 7.796(2)?) in complex 2.

4 CONCLUSION

In this work, two new coordination polymers derived from 5-halonicotinic acid and 2,2?-biimidazole ancillary ligand were synthesized and fully characterized. Their structural diversities demonstrate that the metal cations play a crucial structuredriven role in the generation of these two compounds. This work shows that 5-halonicotinic acid is an excellent bridging ligand for the construction of coordination polymers.

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