Crystal Structure and Thermal Behavior of Two New Supramolecular Complexes Templated with 1 , 2 , 4 , 5-Benzenetetracarboxylic Acid

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 BTA4− ion doesn’t coordinate with the metal ion, the H-bonding interactions further connect the mononuclear molecules to generate a 3D supramolecular complex.


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 π • • • π stacking interactions is an effective way on constructing functional supramolecular complex [2,3].Therefore, we have constructed coordination polymers based on the H 4 BTA ligand and transition metal ions by self-assembly method.So far, a mass of coordination polymers constructed by 1,2,4,5-benzenetetracarboxylic acid (H 4 BTA) have been reported, but studies on H 4 BTA 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][6][7][8][9][10][11][12].So, in this paper, we have constructed two supramolecular complexes with H 4 BTA as a templating agent and bipy as the auxiliary ligand: [Ni(H 2 O) 4 (bipy)](BTA) 0.

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.

Synthesis of Compounds (1) and (2).
A mixture of Ni(NO 3 ) 2 •6H 2 O (0.02 g, 0.069 mmol), H 4 BTA (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

X-Ray Structure Determination.
Single-crystal X-ray diffraction was put on a Bruker Smart Apex CCD diffractometer equipped with agraphite-monochromatic MoKα radiation (λ = 0.071073 nm) at 293(2) K by using a ω-2θ scan mode.All data were corrected by Lp factors and empirical adsorption.The structures were solved by direct methods using SHELXS-97 program [13][14][15] and Fourier difference techniques and refined by full-matrix least squares on F 2 [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.

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.
In the coordination environment, as one hydrogen-bond donor and a double hydrogen-bond acceptor, O9 from lattice water with two O atoms of BTA 4− and coordinated water, respectively, form intermolecular H-bonding (O9-H9A 4).The 3D supramolecular network was constructed by H-bonding interaction among the crossed 1D chains, BTA 4− and lattice water, seen in Figures 5(a 3. 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. 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

Infrared Spectra. IR spectrum (Figures
180.00 ( 9 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 ν as (COO) and symmetric ν s (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].

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     Table 3: Hydrogen bond lengths ( Å) and bond angles ( • ) for compounds 1 and 2.

D-H•
from single-crystal diffraction data.The PXRD pattern of the removal BTA 4− 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 BTA 4− ions.

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 BTA 4− 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].

Figure 2 :
Figure 2: The coordination environment of Ni atom in complex 1.

Figure 3 :
Figure 3: the cross-structure of one-dimensional chain.

Figure 4 :
Figure 4: Complex 1 constructed by interchain hydrogen bonds (red dashed lines), in which the water molecules are omitted for clarity.

Figure 5 :
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.