Syntheses and structural characterization of coordination polymers of Cu(II) and Zn(II) chlorobenzoates and bis(pyridin-4-yl)-substituted species

2.1 Preparation

The reaction of Cu(NO₃)₂·3H₂O with 2-chlorobenzoic acid (HL¹) in the presence of (E)-1,2-di(pyridin-4-yl)ethene (dpe) as auxiliary ligand yields the complex [Cu(L¹)₂(dpe)(H₂O)] (1). When 4-chlorobenzoic acid (HL²) reacts with Zn(NO₃)₂·6H₂O in the presence of 1,2-di(pyridin-4-yl)ethane (dpa) as auxiliary ligand, the complex [Zn(L²)₂(dpa)]·CH₃OH (2) is obtained. Complexes 1 and 2 are stable in air.

2.2 Structural description of [Cu(L¹)₂(dpe)(H₂O)] (1)

Structure analysis shows that complex 1 exhibits a 3D CdS network structure in the monoclinic system, space group C2/c with Z = 4 (Table 1). The HL¹ ligand was deprotonated to the (L¹)^– anion. In the asymmetric unit of 1 there are one centrosymmetric Cu(II) ion with occupancy of 0.5, one (L¹)^– ligand, half of a dpe, and half of a coordinated water molecule. As shown in Fig. 1a, each Cu(II) ion is six-coordinated by two nitrogen atoms from two different dpe units, two oxygen atoms from two water molecules, and two carboxylate oxygen atoms from two different (L¹)^– ligands to furnish a distorted octahedral coordination geometry [CuN₂O₄]. The bond lengths around Cu(II) range from 1.9547(15) to 2.5901(5) Å, and the bond angles around Cu(II) are in the range of 86.10(7) to 180.00(15)° (Table 2). The carboxylate group of the (L¹)^– anion exhibits the terminal monodentate mode. The coordinated water molecules in 1 interconnect the Cu(II) ions as μ₂-bridges to form a metal chain with the nearest Cu···Cu distance of 5.08 Å (Fig. 1b). The dpe ligand also acts as a μ₂-bridge linking different chains to construct a 3D framework (Figs. 1c–e). Being μ₂-bridges, both the dpe and the coordinated water molecules can be considered as connectors in the view of topology; Cu(II) can be treated as a four-connected node. The structure of 1 can be simplified as an uninodal 4-connected 3D CdS (6⁵.8) framework (Fig. 1f) [13].

Fig. 1

(a) The coordination environment of the Cu(II) ion in 1 with ellipsoids drawn at the 30 % probability level. The hydrogen atoms are omitted for clarity, symmetry options: A = #2 = –x, –y, –z, B = 1/2–x, –1/2–y, –z. (b) View of the water-bridging of the Cu²⁺ cations chain in 1. (c) The 3D network structure of 1 along the a axis. (d) The 3D network structure of 1 along the b axis; (e) the 3D network structure of 1 along the c axis. (f) Topological view of the 2D network of 1.

2.3 Structural description of [Zn(L²)₂(dpa)]·CH₃OH (2)

Complex 2 also crystallizes in the monoclinic system, space group C2/c with Z = 4 (Table 1). The asymmetric unit consists of one centrosymmetric Zn(II), one (L²)^–, half a dpa, and half a methanol molecule. The methanol of solvation in 2 is disordered, could not be modeled, and had to be removed from the structure by the routine SQUEEZE in Platon. The suggested number of solvent methanol molecules was determined by elemental and thermogravimetric analyses. Each Zn(II) is four-coordinated by two carboxylate oxygen atoms from two (L²)^– anions and two nitrogen atoms from two dpa molecules in a distorted tetrahedral coordination geometry (Fig. 2a). The bond lengths around Zn(II) are 1.9589(18) for Zn–O and 2.036(2) Å for Zn–N, and the bond angles are in the range of 102.52(11) to 117.79(9)°. The (L²)^– anion adopts the μ₁ - η¹:η⁰-monodentate carboxylate coordination mode and acts as a monodentate ligand. Each dpa molecule acts as a μ₂-bridge to link two Zn(II); each Zn(II) is coordinated by two dpa molecules. This kind of interconnection is repeated infinitely to form a chain structure (Fig. 2a and c).

Fig. 2

(a) The coordination environment of the Zn(II) ion in 2 with ellipsoids drawn at the 30 % probability level. The hydrogen atoms are omitted for clarity, Symmetry options: A = #1 = –x, y, 1/2–z, B = –x, 3–y, –z; (b) view of the 1D structure of 2; (c) view of the 1D structure of 2 along the c axis.

2.4 Thermal stability of complexes 1 and 2

Thermogravimetric analyses (TGA) were carried out for complexes 1 and 2, and the results are shown in Fig. 3. For complex 1, there is a weight loss of 3.4 % from 155 to 185 °C corresponding to the release of water (calcd. 3.1 %). The decomposition of the framework of 1 can be observed at 415 °C. For 2, a continuous weight loss totaling 5.3 % from 130 to 200 °C is attributed to the liberation of the methanol (calcd. 5.4 %). The subsequent collapse of the framework starts at 384 °C.

Fig. 3

TGA curves of complexes 1 and 2.

3.1 Preparation of [Cu(L¹)₂(dpe)(H₂O)] (1)

(E)-1,2-Di(pyridin-4-yl)ethene (91.1 mg, 0.50 mmol) was dissolved in 10 mL methanol, and then 10 mL of an aqueous solution of Cu(NO₃)₂·3H₂O (93.8 mg, 0.50 mmol) was added whilst stirring. To this solution a mixture of 2-chlorobenzoic acid (156 mg, 1.0 mmol) and NaOH (40.0 mg, 1.0 mmol) in water (20 mL) was added and the mixture refluxed for 10 min and filtered. The filtrate was kept at ambient temperature and evaporated for several days. Blue block-shaped crystals of 1 were formed with an approximate yield of 60 % based on HL¹. – C₂₆H₂₀Cl₂N₂O₅Cu (574.88): calcd. C 54.32, H 3.51, N 4.87 %; found C 54.60, H 3.26, N 4.90 %. – IR (KBr pellet, cm^–1): ν = 3417(m), 3166(m), 1588(s), 1535(s), 1408(s), 1389(m), 1282(m), 1228(m), 1166(m), 1025(m), 980(m), 964(m), 940(m), 912(m), 857(m), 826(m), 802 (m), 778(m), 753(m), 718(m), 650(m), 626(m).

3.2 Preparation of [Zn(L²)₂(dpa)]·CH₃OH (2)

Complex 2 was obtained by an analogous experimental procedure as that used for synthesis of 1 except that Zn(NO₃)₂·6H₂O (148.7 mg, 0.5 mmol), 1,2-di(pyridin-4-yl)ethane (92.1 mg, 0.50 mmol) and 4-chlorobenzoic acid (156 mg, 1.0 mmol) were used. Colorless block-shaped crystals of 2 were formed with an approximate yield of 55 % based on HL². – C₂₇H₂₄Cl₂N₂O₅Zn (592.81): calcd. C 54.70, H 4.08, N 4.73 %; found C 54.56, H 4.15, N 4.50 %. – IR (KBr pellet, cm^–1): ν = 3390(m), 3182(m), 1589(m), 1542(s), 1412(s), 1393(s), 1271(m), 1226(m), 1185(w), 1052(w), 1019(w), 993 (m), 962(m), 915(m), 870(m), 827(m), 799(m), 754(m), 711(m), 672(m), 652(w).

3.3 X-ray structure determinations

The crystallographic data collections for complexes 1 and 2 were carried out on a Bruker Smart ApexII CCD area-detector diffractometer using graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) at 293(2) K. The diffraction data were integrated by using the program Saint [14], which was also used for the intensity corrections for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using the program Sadabs [15]. The structures of 1 and 2 were solved by Direct Methods, and all nonhydrogen atoms were refined anisotropically on F² by the full-matrix least-squares techniques using the Shelxl-97 crystallographic software package [16–18]. In 1 and 2, all hydrogen atoms at C atoms were generated geometrically. The hydrogen atoms at O₃ in 1 could be found at reasonable positions in the difference Fourier maps and located there. The methanol of solvation in 2 is badly disordered, could not be modeled, and was removed from the structure by the routine SQUEEZE in Platon [19–21]. The details of crystal parameters, data collection, and refinements are summarized in Table 1; selected bond lengths and angles are listed in Table 2.

CCDC 1018843 and 1018844 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.