Synthesis, Crystal Structure and Spectroscopic Analysis of a New Sodium Coordination Polymer

Abstract A new sodium coordination polymer, [Na2(L)2(H2O)4]n (1), was synthesized by reflux reaction of p-aminobenzene sulfonic acid, p-hydroxy benzaldehyde and NaOH in ethanol/water solution and characterized by elemental analysis, UV-vis, IR and X-ray single crystal diffraction analysis. The crystal of 1 belongs to monoclinic, space group P21/c with a = 17.765(4) Å, b =10.181(2) Å, c = 8.0450(16) Å, β = 91.67(3)°, V = 1454.5(5) Å3, Z = 4, Dc = 1.531 mg·, μ = 0.28 mm-1, F(000) = 696, and final R1 = 0.0327, ωR2 = 0.0854. The molecules of 1 are extended to a 2D layer structure along ab plane. The 2D layers form 3D framework structure by the interaction of π-π stacking and hydrogen-bond interaction.


Introduction
In recent years, design and synthesis of novel coordination polymers have received much more attention than ever before [1][2][3][4][5][6][7]. The coordination polymers have potential applications in many aspects, such as gas storage, fluorescent, catalysis, separation, biological activity and so on [8][9][10][11]. Many coordination polymers with various structures and properties have been synthesized and reported [12][13][14][15]. Metal ions and ligands play an important role in the construction of coordination polymers. As an important kind of ligand, aromatic sulfonic acid derivates have been extensively used in the synthesis of coordination polymer materials due to their various coordination modes and thermal stability [16][17][18].
In this paper, a new sodium coordination polymer, [Na 2 (L) 2 (H 2 O) 4 ] n (1), has been synthesized with p-aminobenzene sulfonic acid, p-hydroxy benzaldehyde and NaOH as raw material. Furthermore, the sodium coordination polymer has also been characterized by elemental analysis, IR spectrum, UV-vis spectrum, and single crystal X-ray crystallography.

Materials and Instrumentation
p-Aminobenzene sulfonic acid, 4-benzoylbenzoic acid, NaOH and solvents were purchased commercially and used without further purification. The IR spectrum was recorded in the range 4000-400 cm −1 on an infrared spectrophotometer (Beijing Purkinje General Instrument, Beijing, MA, China). UV-vis spectrum was obtained from UV-visible spectrophotometer (Beijing Purkinje General Instrument, Beijing, MA, China). Single crystal data of the sodium coordination polymer were collected by a Bruker smart CCD diffractometer (Bruker, Billerica, MA, USA).

Crystal Structure Determination
A colorless single crystal of sodium coordination polymer 1 with dimensions of 0.21 mm × 0.18 mm × 0.17 mm was selected and mounted on a glass fiber for data collection. The X-ray diffraction data were measured at 293(2) K on a Bruker Smart CCD diffractometer with a graphitemonochromatized MoKα (λ = 0.71073 Å) radiation. The structure was solved by direct methods with  and refined on F 2 by full-matrix least-squares procedures with SHELXTL-97 [20]. The crystal data of 1 are given in Table 1.

Description of 1
The result of X-ray diffraction reveals that the sodium coordination polymer 1 crystallizes in monoclinic P2 1 /c space group. The coordination environment of sodium ion of 1 is shown in Figure 1. The connection mode of sodium ion is shown in Figure 2. Figure 3 is the 2D layer of sodium coordination polymer 1 along bc plane. Figure 4 is the 3D network structure of the sodium coordination polymer 1. From Figure 1, the asymmetrical unit contains one sodium ion, two new ligands and four coordinated H 2 O molecules. In 1, sodium ion is six-coordination and resides in a octahedral environment defined by three oxygen atoms (O2, O4) from two new ligands, and other four oxygen atoms (O1W, O1WA, O2W, O2WA) from four coordinated H 2 O molecules, and two coordinated H 2 O (O1W, O1WA) were shared with another sodium ion (Na1b), the remaining oxygen atoms (O2W, O2WA) were shared with other two sodium ions (Na1c, Na1d), respectively. Four oxygen atoms (O2W, O2WA, O1W, O2) are at the equatorial plane of this octahedral, and the rest two oxygen atoms (O1W, O4) are at the axle of the octahedral, the angle O4-Na1-O1W is 164.313(54)°. The sodium ions in 1 are linked with the neighboring sodium ions through the bridging effect of oxygen atoms, and forming four-member rings and six-member rings (Figure 2). The sodium coordination polymer 1 is extended to a 2D layer structure along the ab plane (Figure 3). The 2D layers form 3D framework structure by the interaction of π-π stacking and hydrogenbond interaction. The oxygen atom of phenolic hydroxyl does not coordinate to Na(I), but phenolic hydroxyl group plays an important role in the stability of the structure by hydrogen bonds (Figure 5). The main bond lengths (Å) and angles (°) for 1 are given in Table 2. Figure 6 shows the IR spectrum of the sodium coordination polymer. From Fig.6, it can be seen that the asymmetrical stretching vibration of C-H in methylene and O-H stretching of inner hydroxyl groups are at 2902 cm -1 and 3442 cm -1 , respectively, indicating that the coordination polymer contains water molecules. The asymmetrical stretching of C=C in benzene is at 1493 cm -1 , the symmetrical stretching vibration of C-N and symmetrical deformation vibration of N-H in secondary amine are at 1230 cm -1 and 1567 cm -1 , respectively. The symmetrical stretching vibration and asymmetrical stretching vibration of sulfonate are at 1041 cm -1 and 1156 cm -1 [21,22]. The stretching vibration of C-O in phenol is at 1243 cm -1 .       Figure 7 is the UV-vis spectrum of the sodium coordination polymer. The absorption spectrum shows that the maximum absorption peak of the sodium coordination polymer was located at 312 nm, which can be assigned to intraligand π-π* transitions [23].