Abstract

Templated coordination polymers [Ni(H₂O)₄(bipy)](BTA)_0.5·H₂O (1) and [Co(H₂O)₄(bipy)](BTA)_0.5·H₂O (2) (bipy = 4,4′-Bipyridine, H₄BTA = 1,2,4,5-Benzenetetracarboxylic Acid) were synthesized and characterized by single-crystal X-ray diffraction, powder diffraction, elemental analysis, IR, and thermogravimetric analysis. Both of complexes 1 and 2 are monoclinic crystal system, C2/c space group, and isostructural. The unit of the structure, the metal ion is coordinated by four water molecules and two bipy molecules, is a slightly distorted octahedral configuration, and the carboxyl group from BTA^4- ion doesn't coordinate with the metal ion, the H-bonding interactions further connect the mononuclear molecules to generate a 3D supramolecular complex.

1. Introduction

Coordination polymers are currently of considerable interest and importance because of the scope they offer for the generation by design of new materials with a range of potentially useful properties, such as magnetism, adsorption, ion exchange, and catalysis [1]. Self-assembly by intermolecular H-bonding and/or aromatic stacking interactions is an effective way on constructing functional supramolecular complex [2, 3]. Therefore, we have constructed coordination polymers based on the H₄BTA ligand and transition metal ions by self-assembly method. So far, a mass of coordination polymers constructed by 1,2,4,5-benzenetetracarboxylic acid (H₄BTA) have been reported, but studies on H₄BTA as a guest molecule are less reported [4]. The results of previous research show that auxiliary ligands such as 4,4′-bipyridine, 2,2′-bipyridine play important role in constructing the complexes with unexpected architectures [5–12]. So, in this paper, we have constructed two supramolecular complexes with H₄BTA as a templating agent and bipy as the auxiliary ligand: [Ni(H₂O)₄(bipy)](BTA)_0.5·H₂O (1) and [Co(H₂O)₄(bipy)](BTA)_0.5·H₂O (2), and the phase purity of the crystalline products was confirmed by powder X-ray diffraction (PXRD, shown in  Figures 5s and 6s Supplementary Material available online at http://dx.doi.org/10.1155/2012/291682).

2. Experimental

2.1. Materials and General Methods

All the reagents were commercially available and used without further purification. Distilled water was used throughout. Elemental (C, H, and N) analyses were performed on a CE-440 (Leeman labs) analyzer. Thermal gravimetric analysis (TGA) was performed on a PerkinElmer TGA 7 instrument in the temperature range of 25–700°C at a heating rate of 10°C/min under air atmosphere. IR spectra were recorded as KBr pellets on a FTIR-8400 spectrometer in the range of 4000–400 cm⁻¹. The single-crystal X-ray structures were determined on a Bruker Smart APEX CCD area detector.

2.2. Synthesis of Compounds (1) and (2)

A mixture of Ni(NO₃)₂·6H₂O (0.02 g, 0.069 mmol), H₄BTA (0.0127 g, 0.05 mmol), and bipy (0.0078 g, 0.05 mmol) was dissolved in 6 mL methanol and 6 mL distilled water was stirred for 10 min, the mixture was filtered and placed in a tube, evaporated at room temperature for one week, green crystals of 1 suitable for X-ray analysis were obtained (yield: 65% based on Ni). Anal. Calcd. (%) for C₁₅H₁₉N₂O₉Ni: C, 43.16; N, 5.04; H, 3.99. Found (%): C, 42.86; N, 4.67; H, 3.67. IR (KBr, cm⁻¹): 3389 (s), 3223 (w), 1654 (s), 1563 (s), 1546 (m), 1496 (s), 1360 (s), 1222 (m), 1148 (m), 1073 (m), 815 (w), 633 (w).

The synthesis method of compound 2 is the same as that for 1 except that the metal salt was replaced by Co(NO₃)₂·6H₂O (0.02 g, 0.07 mmol). Yield: 30% (based on Co). Anal. Calcd. (%) for C₁₅H₁₉N₂O₉Co: C, 43.08; N, 5.03;H, 3.98. Found (%): C, 42.73; N, 4.68; H, 3.76. IR (KBr, cm⁻¹): 3381 (s), 3191 (w), 1646 (s), 1596 (s), 1560 (m), 1488 (s), 1355 (s), 1230 (m), 1131 (m), 1064 (m), 824 (w), 633 (w).

2.3. X-Ray Structure Determination

Single-crystal X-ray diffraction was put on a Bruker Smart Apex CCD diffractometer equipped with agraphite-monochromatic MoKα  radiation ( nm) at 293(2) K by using a scan mode. All data were corrected by factors and empirical adsorption. The structures were solved by direct methods using SHELXS-97 program [13–15] and Fourier difference techniques and refined by full-matrix least squares on [16]. All hydrogen atoms were replaced in the located positions. The crystallographic data and collected data for compounds 1, 2 are summarized in Table 1. The selected bond lengths and bond angels are listed in Table 2.

3. Results and Discussion

3.1. Crystal Structures of Compounds 1 and 2

The structures of two complexes were determined by single-crystal X-ray diffraction analyses. Complexes 1 and 2 are isostructural and belong to the monoclinic system with C2/c space group. ORTEP drawing of the asymmetric unit of the complex 1 is shown in Figure 1.

Figure 1 

ORTEP drawing of asymmetric unit of complex 1.

In the asymmetric unit of complex 1, two crystallographically independent nickel atoms (Ni1 and Ni2) are present, the asymmetric unit is comprised of two Ni(bipy)_0.5(H₂O)₂, half of free BTA⁴⁻ ion and one lattice water molecule. In the coordination environment of complex 1, the Ni²⁺ ion is six coordinated by four oxygen atoms from water molecules and two nitrogen atoms from two different bipy, O3, O4, O3#2, O4#2 locate at the equatorial plane and N2, N3 occupy the axial positions, the bond angles of O3–Ni2–O4#2, O3#2–Ni2–O4#2, O3#2–Ni2–O4, O3–Ni2–O4 are 89.09(6)°, 90.71(6)°, 89.09(6)°, and 90.71(6)°, respectively, N2#2–Ni2–N3 bond angles is 180.00(1)°, forming a slightly distorted octahedral configuration (Figure 2). The bipy ligands bridge the Ni ion to form 1D Ni-bipy chains with the cross-structure, seen in Figure 3. But the deprotonation BTA⁴⁻ ligands do not coordinate with the metal atoms, serving as templated molecules to charge balance. Templated BTA⁴⁻ ion of this size can generated large voids in the three-dimensional structure of the coordination polymer; however, these voids are filled with the BTA⁴⁻, seen in Figure 5(a). PXRD confirmed that the structure is collapsed upon removal of the templated BTA⁴⁻ ions. Hence, the BTA⁴⁻ ions act as the templated molecules for supporting the three-dimensional structure via hydrogen bonding.

Figure 2 

The coordination environment of Ni atom in complex 1.

Figure 3 

the cross-structure of one-dimensional chain.

In the coordination environment, as one hydrogen-bond donor and a double hydrogen-bond acceptor, O9 from lattice water with two O atoms of BTA⁴⁻ and coordinated water, respectively, form intermolecular H-bonding (, ). The crossed 1D chains are linked by , , ) and () intermolecular H-bonding (Figure 4). The 3D supramolecular network was constructed by H-bonding interaction among the crossed 1D chains, BTA⁴⁻ and lattice water, seen in Figures 5(a), 5(b), and Figure  1s. The hydrogen bond lengths and bond angles are shown in Table 3.

Figure 4 

Complex 1 constructed by interchain hydrogen bonds (red dashed lines), in which the water molecules are omitted for clarity.

(a)

(b)

(a)
(b)

Figure 5 

(a) The 3D structure of 1 constructed by interchain hydrogen bonds (red dashed lines) along the c axis. (b) The 3D structure of 1 constructed by interchain hydrogen bonds (red dashed lines) along the b axis.

The structure of complex 2 is the same as 1. The Co–O bond lengths and O–Co–O bond angles are in the ranges of 2.0448(11)–2.1543(14) Å and 89.75(6)–92.3°, respectively, which are close to complex 1.

3.2. Infrared Spectra

IR spectrum (Figures 2s and 3s) shows that: the absorption bonds arising from the skeletal vibration of aromatic rings in the 1450–1650 cm⁻¹ range. A wide band of strong intensity at 3389 cm⁻¹ for 1 and 3381 for 2 indicates the O–H stretching of water molecules. And the bands in the 1596–1222 cm⁻¹ range are due to the characteristic vibration of bipy groups [17]. The asymmetric (COO) and symmetric (COO) stretching bands are in the 1560–1360 cm⁻¹ range. And the characteristic peaks appear in 1563 and 1360 cm⁻¹ for 1, 1560 and 1355 cm⁻¹ for 2, respectively, which indicate the complete deprotonation of carboxyl in two complexes [18, 19].

3.3. Thermogravimetric Analyses

Thermal gravimetric (TG) analysis has been measured for complexes 1 and 2 seen as Figure  4s, TGA curves of compounds 1 and 2 are almost the same, so only 1 was analyzed. The TG curve of 1 shows the first weight loss of 15.74% from 36°C to 184°C corresponding to losing one uncoordinated water molecules and four coordinated water molecules (calcd: 16.16%). With temperature raised, the residual complex decomposes with a sharp weight loss (32.68%) that ends at 350°C, which corresponds to losing a BTA⁴⁻ ion (calc: 29.07%). Upon further heating to 391°C, no weight loss is observed. The resulting residue is the M^II (M = Ni, Co) oxide.

The PXRD was used to check the purity of compounds 1 and 2. All the peaks displayed in the measured PXRD pattern (Figures  5s(b) and 6s(b)) closely match with those in the simulated PXRD pattern (Figures  5s(a) and 6s(a)) generated from single-crystal diffraction data. The PXRD pattern of the removal BTA⁴⁻ ions sample (Figures  5s(c) and 6s(c)) has serious transformation of the peaks. PXRD showed that the original structure was collapsed upon removal of the BTA⁴⁻ ions.

4. Conclusion

In summary, two novel supramolecular complexes of 1 and 2 have been synthesized and characterized. Compounds 1 and 2 are isostructural. In the coordination environment each metal atom, is coordinated by four water molecules and two bipy molecules is a slightly distorted octahedral configuration, the deprotonation BTA⁴⁻ ligands don’t coordinate with the metal atom, which act as the templated molecules for supporting the three-dimensional structure via hydrogen bonding. At present, constructing 3D porous metal-organic frameworks with these mixed organic ligands is our focus of future research [20].

Acknowledgment

This work was supported by the International Scientific and Technological Cooperation Projects of Shanxi Province (no: 2011081022).

Supplementary Materials