A Two-Dimensional Zinc Coordination Polymer Based on a Pyridyl-Carboxylate Linking Ligand Containing an Intervening Amide Group: [ZnCl(L)]_∞ (HL = 6-(nicotinamido)-2-naphthoic acid)

Experimental

All solid chemicals were purified by recrystallization, and all solvents were distilled. Infrared (IR) samples were prepared as KBr pellets, and their spectra were obtained in the range 400–4000 cm−1 on a Vertex 70 FTIR spectrophotometer. Elemental analyses were carried out with an Elementar Vario EL cube at the Cooperative Center for Research Facilities (CCRF) in Sungkyunkwan University. Thermogravimetric analysis (TGA) was performed on a TA4000/SDT 2960 instrument (CCRF). The ligand (HL = 6-(nicotinamido)-2-naphthoic acid) was prepared by the literature methods.31

Synthesis of [ZnCl(L)]∞ (1). An aqueous solution containing of anhydrous ZnCl2 (14 mg, 0.1 mmol), HL (58 mg, 0.1 mmol), H2O (15 mL), and 1 N NaOH (0.2 mL, 0.1 mmol) was heated in a 23-mL Teflon-lined reaction vessel at 150 ℃ for 3 days, and then air-cooled slowly to room temperature. The resulting pink crystals were filtered, washed with dimethyl sulfoxide (DMSO, 5 mL × 5), H2O (5 mL × 3), and ethanol (5 mL × 3), and then vacuum-dried to give the product (12 mg, 0.031 mmol, 31％ yield). mp 435–437 ℃. IR (KBr, cm−1): 3729 (w), 3335 (w), 2981 (w), 2897 (w), 1690 (s), 1613 (m), 1549 (m), 1475 (m), 1422 (m), 1302 (m), 1262 (m), 1216 (m), 1118 (m), 1058 (w), 910 (w), 835 (w), 755 (w), 628 (w), 462 (w). Anal. Calc. for C17H11ClN2O3Zn: C 52.07; H 2.83; N 7.14; O 12.24. Found: C 53.12; H 2.14; N 7.01; O 12.84.

X-ray Structure Determination. All X-ray data were collected with a Bruker Smart APEX2 diffractometer equipped with a Mo X-ray tube (CCRF). Collected data were corrected for absorption with SADABS based upon the Laue symmetry by using equivalent reflections.38 All calculations were carried out with SHELXTL programs.39

A pink crystal of polymer 1, shaped as a block of approximate dimensions 0.40 × 0.36 × 0.12 mm, was used for crystal-and intensity-data collection. The structure was solved by direct methods. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were generated in idealized positions and refined in a riding mode. Details on crystal data, intensity collection, and refinement details are given in Table 1. Selected bond lengths and bond angles are presented in Table 2.

Table 1.aR =∑[|Fo| − |Fc|]/∑|Fo|], bwR2 = ∑[w(Fo2−Fc2)2]/∑[w(Fo2)2]1/2

Table 2.Symmetry transformations used to generate equivalent atoms: #1 = −x + 1, −y, −z; #2 = −x + 2, y + 1/2, −z + 3/2; #3 = −x + 2, y − 1/2, −z + 3/2.

 

Results and Discussion

Preparation. Polymer 1 was prepared from anhydrous ZnCl2, HL, and NaOH in the mole ratio of 1:1:1, under hydrothermal conditions (eq. 1). The base was added to deprotonate the free ligand (HL) to its deprotonated form (L−), and the unreacted ligand could be removed with DMSO during the work-up process. The product was characterized by elemental analysis, IR spectroscopy, TGA, and singlecrystal X-ray crystallography.

The IR spectrum of the free ligand displays a characteristic N–H stretching band at 3328 cm−1 and C=O stretching band at 1621 cm−1.31 On the other hand, the IR spectrum of the polymer 1 shows the corresponding N–H and C=O stretching bands at 3335 and 1613 cm−1, respectively

Crystal Structure. Figure 1 shows an asymmetric unit that consists of a Zn2+ ion, an L− ligand, and a Cl− ligand. All non-hydrogen atoms occupy general positions. The local coordination environment of the Zn2+ ion in polymer 1 is given in Figure 2, in which two Zn2+ ions are joined by two bridging carboxylate groups. The Zn2+ ion is coordinated to one nitrogen and two oxygen atoms from three ligands, in addition to the chloro ligand. The amide group does not coordinate to the metal. The amide N–H bond forms a weak intermolecular hydrogen bond with the Cl− ligand [N2−HN2 = 0.86 Å, N2…Cl1 (x + 1, y, z + 1) = 3.555(2) Å, Cl1…HN2 = 2.71 Å, N2−HN2…Cl1 = 168°]. As mentioned in Introduction, the ligand HL was previously employed to produce a two-dimensional copper polymer, [CuL2(H2O)]·(H2O)2]∞, in which the Cu2+ ion has a distorted square-pyramidal coordination sphere.28 On the other hand, the Zn2+ ion in polymer 1 has a distorted tetrahedral coordination sphere.

Figure 1.The asymmetric unit of polymer 1. Displacement ellipsoids for non-hydrogen atoms exhibit 40％ probability level.

Figure 2.Local coordination environment around the Zn2+ ion.

Figure 3 shows a repeat unit in polymer 1, which consists of two subunits: (1) two Zn2+ ions and two carboxylate groups (subunit 1, an 8-membered ring) and (2) four Zn2+ ions and four ligands (subunit 2, a 60-membered ring). The Zn…Zn separations in subunits 1 and 2 are 3.5735(4) and 16.2769(4) Å, respectively. These two subunits are linked to form a two-dimensional layer structure in the direction (Figure 4), in which the Cl− ligands lie nearly perpendicular to this layer.

Figure 3.Repeat unit consisting of two subunits.

Figure 4.Packing diagram showing a part of a two-dimensional layer.

To the best of our knowledge, only the two ligands in Scheme 1 and 5-(nicotinamido)isophthalic acid (H2NAIP),24,28,40 all of which are basically pyridine–carboxylate ligands and possess an intervening amide group in common, have been employed so far to prepare CPs. For instance, the hydrothermal reactions involving the H2NAIP ligand, a pyridyl–dicarboxylate ligand, produced 1-D and 3-D polymers: {[M(NAIP)(H2O)4]·2(H2O)}∞ (M = Co, Mn), {[Zn(NAIP)]·0.5(H2O)}∞, and {Cd(NAIP)(H2O)2]·1.5(H2O)}∞. Hence, polymer 1 is another example of a coordination polymer constructed from the pyridine–carboxylate-type linking ligand with an intervening amide group.

In order to examine the thermal behavior of polymer 1, the thermogravimetric analysis was performed. The TGA curve displays a single well-defined weight loss. This polymer is stable up to 436 ℃, which clearly demonstrates its high thermal stability (Figure 5). The abrupt weight loss occurs from 436 to 480 ℃, above which the gradual decomposition ensues.

Figure 5.TGA curve for polymer 1.

In summary, a two-dimensional zinc coordination polymer, [ZnCl(L)]∞ (1), was prepared from ZnCl2, 6-(nicotinamido)-2-naphthoic acid (HL), and NaOH, under hydrothermal conditions. Polymer 1 contains a pyridyl–carboxylate-type linking ligand with an intervening amide group. This polymer is constructed on the basis of a repeat unit consisting of two subunits: an 8-membered ring and a 60-membered ring, and its framework appears to have a very high thermal stability that is retained up to 436 ℃.

Supporting Information. CCDC 985715 contains the supplementary crystallographic data for polymer 1. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.