WU Gng QING Cheng-Song WANG Xio-Fengb② (College of Mteril nd Chemicl Engineering,Chuzhou University, Chuzhou, Anhui 239012, Chin)

b (College of Environmental Science, Xiaozhuang University, Nanjing 211171, China)

1 INTRODUCTION

In last decades, the metal-organic frameworks(MOFs) have attracted more and more attention not only for their fascinating architectures and topologies, but also for their potential applications, such as luminescence, magnetism, gas storage and catalysis,and so on[1-4]. Carboxylic acid, owing to its multifarious species and various coordination modes, has been widely introduced to construct the metal organic framework (MOFs)[5-7]. A large number of metal aromatic carboxylates have been reported as potential porous functional materials as catalysis,separation, gas storage, etc[8-12]. Among them, 1,3,5-benzenetricarboxylate is a versatile building block for the synthesis of functional metal organic frameworks due to its polytopic carboxylate groups, high symmetry and planar geometry[13-17].

On the other hand, alkali-earth-metal-containing coordination compounds have received considerable interest recently because of their ability to form bonds with oxygen donors, their large radii, high coordination numbers, various coordination modes,and potential applications of complexes in several possible applications in luminescent and electronic devices[18]. In this article, a barium complex of 1,3,5-benzenetricarboxylic acid was synthesized and its structure was characterized. The fluorescent and thermal stable properties of the complex were also investigated.

2 EXPERIMENTAL

2. 1 General procedures

All reagents commercially available were of reagent grade and used without further purification.Solvents were purified according to the standard methods. The luminescent spectra for the solid samples were recorded at room temperature on CaryEclipse 300 spectrophotometer with a xenon arc lamp as the light source. In the measurements of the emission and excitation spectra, the pass width was 5.0 nm. C and H elemental analyses were carried out on a Perkin-Elmer 240C elemental analyzer. IR spectra were recorded on a Nicolet 6700 FT-IR spectrophotometer by using KBr pellet in the range of 4000～ 400 cm–1. Powder X-ray diffraction patterns were performed with a Bruker D8 ADVANCE X-ray diffractometer with Cu-Kα radiation at 40 kV and 40 mA. Thermogravimetric analyses (TGA) were carried out with a SDT Q600 instrument under 100.0 mL/min flowing nitrogen from room temperature to 600 ℃ at a heating rate of 20.00 ℃/min.

2. 2 Preparation of [Ba3(btc)2(H 2O)8]·2H2O (1)

A mixture of 1,3,5-benzenetricarboxylate (21.0 mg, 0.1 mmol) and Na2CO3(2.7 mg, 0.025 mmol)was stirred for 10 min in 8 mL water at room temperature. Then BaCl2(24.4 mg, 0.1 mmol) and 2 mL DMF were added into this mixture. Finally,1,10-phenanthroline (10.0 mg, 0.05 mmol) was added into this clear solution. This suspended mixture was sealed in a 25 mL Teflon-lined reactor.The reactor was heated in an oven to 160 ℃ for 72 h,and then cooled to room temperature. The clear solution was stood still in air for about two weeks.Colorless block-shaped crystals of 1 were obtained by filtration, and dried in air. Anal. Calcd. for C18H26Ba3O22(%): C, 21.48; H, 2.60. Found (%): C,21.44; H, 2.63. FT-IR (KBr pellet, cm-1): 3389(bs),2790(m), 1625(s), 1550(s), 1430(m), 1368(s),1102(w), 1024(w), 844(w), 764(s), 722(s).

2. 3 Structure determination and refinement

A single crystal with dimensions of 0.32mm ×0.30mm × 0.27mm was put on a Bruker Smart Apex CCD diffractometer equipped with a graphite-monochromatized Mo Kα radiation (λ = 0.71073 ?) by using a φ-ω scan mode at 296(2) K. In the range of 1.61≤θ≤27.45°, a total of 10647 reflections were collected and 3050 were independent with Rint=0.0944, of which 2803 were observed with I > 2σ(I).The structure was solved by direct methods using the SHELXS-97 program package and refined against F2by full-matrix least-squares with SHELXL-97[19,20]. All non-hydrogen atoms were refined anisotropically and hydrogen atoms located and refined isotropically. Complex 1 crystallizes in monoclinic, space group P21/c with a = 12.6263(6),b = 16.0391(8), c = 6.8240(3) ?, β = 91.136(2)o, Z =2, C18H26Ba3O22, Mr= 1006.41, Dc= 2.419 g/cm3,F(000) = 956, μ = 4.326 mm-1, the final R = 0.0422 and wR = 0.1066 (w = 1/[σ2(Fo2) + (0.0682P)2+1.2752P], where P = (Fo2+ 2Fc2)/3) for all data, and S = 1.044. The selected bond lengths and bond angles are listed in Table 1.

3 RESULTS AND DISCUSSION

3. 1 IR spectrum of 1

The infrared spectra of the complex were recorded in the range of 4000～400 cm-1with KBr pallets.The absence of peaks at ～1700 cm-1suggests that the carboxyl groups are all deprotonated, which agrees with the results of X-ray analysis. The absorption peaks at 1625, 1550, 1430 and 1368 cm-1are observed, indicating that the carboxylate groups have coordinated with the Ba(II) cations. The peaks at 1625 and 1550 cm-1should be ascribed to the asymmetrical stretching vibration and those at 1430 and 1368 cm-1to symmetrical stretching vibration of-COO- groups[21]. The separation of νasand νs(Δν are 195, 183 and 120 cm-1) implies the coordination of carboxyl group with Ba(II) in bridging modes[22-25], which is in agreement with the single-crystal X-ray diffraction study.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for Complex 1

3. 2 Crystal structure of 1

Complex 1 crystallizes in the monoclinic space group P21/c and possesses a 3D coordination framework. The ORTEP view of coordination environment of the Ba(II) atom for compound 1 is shown in Fig. 1. In complex 1, there are two kinds of crystallographically independent Ba(II) centers. It is seen that the Ba(1) center is coordinated by eight O atoms from four btc3-anions and three water molecules, two taking monodentate terminal coordination model and another two adopting μ2-bridging coordination model (Fig. 1). The Ba(1) center is located in a distorted square antiprismatic coordination geometry with Ba(1)–O distances ranging from 2.728(3) to 2.873(3) ?. The Ba(1)–Ow (water,Ba(1)–O(2), Ba(1)–O(11)) distances are 2.857(4)and 2.873(3) ?, respectively. The O–Ba(1)–O bond angles range from 63.91(10) to 155.89(11)o(Table 1).

Each Ba(2) ion is ten-coordinated by O atoms from four btc3-ligands and four μ2-bridging coordination water molecules in a distorted dicapped square prism (Fig. 1). The Ba(2)–O(carboxylate)bond lengths vary from 2.760(3) to 3.013(4) ?, and the Ba(2)–Ow(water, Ba(2)–O(8), Ba(2)–O(9))distances are 2.977(3) and 3.013(4) ?, respectively.The average distance of Ba(2)–O is longer than that of Ba(1)–O. The O–Ba(2)–O bond angles range from 45.22(9) to 180.00(12)o(Table 1). The distan-ces of Ba–O are in the normal ranges of those observed in the reported Ba(II) compound[26-28].

Fig. 1. ORTEP view of coordination environment of Ba(II) atom in 1 with 50%probability displacement. The hydrogen atoms are omitted for clarity

There is only one crystallographically unique anion of btc3-in the structure. In complex 1, three carboxylate groups of the anion btc3-take two different coordination modes (Scheme 1). Two carboxylate groups take a μ2-η1:η1bidentate bridging mode. The third carboxylate group takes a μ2-η2:η1coordination mode. Therefore, the whole btc3-anion acts as a μ6-bridge connecting six different Ba(II)ions (Scheme 1).

Scheme 1. Coordination model of btc3-

In complex 1, the three carboxylate groups are not in the same plane with the central benzene ring owing to the rotation of carboxylate groups. The dihedral angles between central ring and the carboxylate groups taking μ2-η1:η1bidentate bridging mode are 2.1 and 2.6o, respectively, which are very close to each other. The dihedral angles between central ring and the carboxylate group adopting μ2-η2:η1tridentate bridging mode is 9.8o, suggesting that there may be relation between the configuration and coordination model of the ligand.

The one-dimensional chains are formed through carboxylate groups, taking μ2-η1:η1bidentate bridging mode, μ2-η2:η1coordination mode, and μ2-H2O(Fig. 2). Then the bridged btc3-ligands link these 1D chains to generate a 2D network through carboxylate groups taking μ2-η1:η1bidentate bridging mode (Fig. 3).The 2D networks are further linked together by the μ2-η2:η1tridentate bridging carboxylate groups to produce a new 3D porous coordination framework(Fig. 4).

Fig. 2. One-dimensional structure in 1

Fig. 3. Two-dimensional structure in 1

Fig. 4. Crystal packing diagram of 1

The lattice water molecules locate in the voids and are connected to the 3D framework through O–H··O hydrogen bonds (Table 2, Fig. 4), formed between lattice water and coordinated water molecules, with the O··O distances in the range from 2.792(5) to 2.981(5) ?. In complex 1, there is face-to-face π-π interaction between adjacent benzene rings because the distance of two centroids is 3.517 ? with a dihedral angle of 0o, which are within the normal distances required for π-π interactions.

Table 2. Distances (?) and Angles (o) of Hydrogen Bonds for the Complex

3. 3 Luminescent property of complex 1

The photoluminescent property of complex 1 and H3btc ligand were studied in the solid state at room temperature. No clear luminescence was detected for the H3btc ligand under experimental conditions. In contrast to the H3btc ligand, the Ba(II) (1) complexes show photoluminescence under the same conditions (Fig. 5). Complex 1 exhibits photoluminescence with an emission maximum at ca. 403 nm upon excitation at 320 nm. The luminescence of the complex may be ascribed to transition of organic ligand[28,29].

3. 4 Thermogravimetric analyses

To examine the thermal stability of this compound,thermal gravimetric (TG) analyses were carried out(Fig. 6) under 100.0 mL/min flowing nitrogen,ramping the temperature at a rate of 20.00 ℃/min from room temperature to 600 ℃. The first-step weight loss of 17.12% (calcd. 17.90%) from 30 to 395 ℃ corresponds to the removal of ten water molecules per formula unit. The second weight loss between 395 to 595 ℃ of ca. 14.74% corresponds to the loss of benzene (calcd. 15.52 %), as shown in Fig. 6. The solid residue formed at ca. 595 ℃ is suggested to be BaCO3, which was confirmed by powder X-ray diffraction (Fig. 7), with some further evaporation occurring at higher temperature.

Fig. 5. Luminescence property of complex 1

Fig. 6. TG curve of 1

Fig. 7. PXRD pattern of BaCO3 decomposed from complex 1

4 CONCLUSION

An alkaline earth complex [Ba3(btc)2(H2O)8]·2H2O(1) has been synthesized and characterized. The Ba(1) center is located in a distorted square antiprismatic coordination geometry, while the coordination geometry of Ba(2) is distorted ten-coordinated dicapped square prism. In complex 1, three carboxyl groups of the anion btc3-take μ2-η1:η1and μ2-η2:η1two different coordination modes. The whole btc3-anion acts as a μ6-bridge connecting six different Ba(II) ions to result in a three-dimensional network.We also investigated the thermal stability and luminescent properties of complex 1.

(1) Song, X. Z.; Song, S. Y.; Qin, C.; Su, S. Q.; Zhao, S. N.; Zhu, M.; Hao, Z. M.; Zhang, H. J. Syntheses, structures, and photoluminescent properties of coordination polymers based on 1,4-bis(imidazol-l-yl-methyl)benzene and various aromatic dicarboxylic acids. Cryst. Growth Des. 2012, 12,253-263.

(2) Chen, X.; Zhang, M. X.; Huang, K. L.; Xiao, F.; Zhang, X. P. A 3-connected 3D microporous metal-organic framework with intersected channels and rare DEH topology. Chin. J. Struct. Chem. 2014, 33, 1831-1835.

(3) Zhang, X. M.; Wang, Y. Q.; Wang, K.; Gao, E. Q.; Liu, C. M. Metamagnetism and slow magnetic dynamics in an antiferromagnet composed of cobalt(II) chains with mixed azide-carboxylate bridges. Chem. Commun. 2011, 47, 1815-1817.

(4) Lu, Z. Z.; Zhang, R.; Li, Y. Z.; Guo, Z. J.; Zheng, H. G. Solvatochromic behavior of a nanotubular metal-organic framework for sensing small molecules. J. Am. Chem. Soc. 2011, 133, 4172-4174.

(5) Zhu, H. L.; Qi, J. L.; Lin, J. L.; Xu, W.; Wu, J.; Zheng, Y. Q. Novel topological supramolecular architectures based on partially protonated butane-1,2,3,4-tetracarboxylato complexes: synthesis, structures and magnetic properties. Inorg. Chim. Acta 2013, 404, 49-57.

(6) Wu, G.; Wang, X. F.; Guo, L.; Li, H. H.; Liu, G. X. Synthesis, structure and properties of a two-dimensional samarium(III) complex of benzenedicarboxylic acid and 1,10-phenanthrolin. Chin. J. Struct. Chem. 2013, 32, 564-570.

(7) Wu, G.; Zhang, S. Q.; Guo, L. Synthesis, structure, and properties of a new calcium(II) complex of 1,10-phenanthroline,1,2,4,5-benzenetetracarboxylic acid, and a new precursor to produce pure phase micro-crystalline calcite particles. Z. Anorg. Allg. Chem. 2013, 639,804-809.

(8) Kitagawa, S.; Kitaura, R.; Noro, S. Functional porous coordination polymers. Angew. Chem. Int. Ed. 2004, 43, 2334-2375.

(9) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Metal carboxylates with open architectures. Angew. Chem. Int. Ed. 2004, 43, 1466-1496.

(10) Dinca, M.; Long, J. R. Hydrogen storage in microporous metal-organic frameworks with exposed metal sites. Angew. Chem. Int. Ed. 2008, 47,6766-6779.

(11) Morris, R. E.; Wheatley, P. S. Gas storage in nanoporous materials. Angew. Chem. Int. Ed. 2008, 47, 4966-4981.

(12) Ferey, G. Hybrid porous solids: past, present, future. Chem. Soc. Rev. 2008, 37, 191-214.

(13) Dai, J. C.; Wu, X. T.; Fu, Z. Y.; Cui, C. P.; Hu, S. M.; Du, W. X.; Wu, L. M.; Zhang, H. H.; Sun, R. O. Synthesis, structure, and fluorescence of the novel cadmium(II)-trimesate coordination polymers with different coordination architectures. Inorg. Chem. 2002, 41, 1391-1396.

(14) Wang, Z. Q.; Kravtsov, V. C.; Zaworotko, M. J. Ternary nets formed by self-assembly of triangles, squares, and tetrahedra. Angew. Chem. Int. Ed.2005, 44, 2877-2880.

(15) Lin, Z. Z.; Jiang, F. L.; Chen, L.; Yuan, D. Q.; Hong, M. C. New 3-D chiral framework of indium with 1,3,5-benzenetricarboxylate. Inorg. Chem.2005, 44, 73-76.

(16) Loiseau, T.; Lecroq, L.; Volkringer, C.; Marrot, J.; Ferey, G.; Haouas, M.; Taulelle, F.; Bourrelly, S.; Llewellyn, P. L.; Latroche, M. MIL-96, A porous aluminum trimesate 3D structure constructed from a hexagonal network of 18-membered rings and μ3-oxo-centered trinuclear units. J. Am.Chem. Soc. 2006, 128, 10223-10230.

(17) Ferey, G.; Serre, C.; Mellot-Draznieks, C.; Millange, F.; Surblé, S.; Dutour, J.; Margiolaki, I. A hybrid solid with giant pores prepared by a combination of targeted chemistry, simulation, and powder diffraction. Angew. Chem. Int. Ed. 2004, 43, 6296–6301.

(18) Fabio, M.; Claudio, P.; Riccardo, P.; Augusto, C.; Roberto, G.; Michele, R. C.; Andrei, D.; Sergey, I. T. The imidazole role in strontium β-diketonate complexes formation. Inorg. Chem. 2006, 45, 3074-3085.

(19) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal Structures. University of G?ttingen, Germany 1997.

(20) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures. University of G?ttingen, Germany 1997.

(21) Colak, A. T.; Yesilel, O. Z.; H?kelek, T.; Sahin, E. Synthesis, spectral and thermal properties, and crystal structure of catena-poly-μethylenediamine(dipicolinato)zinc(II) trihydrate, {[Zn(dipic)(μ-en)]·3H2O}n. Struct. Chem. 2008, 19, 285-290.

(22) Zheng, Y.; Xu, D. M.; Liu, S. X. Three mononuclear copper complexes of 9-hydroxy-9H-fluorene-9-carboxylic acid. Inorg. Chim. Acta 1999, 294,163-169.

(23) Wang, X. L.; Qin, C.; Wang, E. B.; Li, Y. G.; Hao, N.; Hu, C. W.; Xu, L. Syntheses, structures, and photoluminescence of a novel class of d10metal complexes constructed from pyridine-3,4-dicarboxylic acid with different coordination architectures. Inorg. Chem. 2004, 43, 1850-1856.

(24) Ma, Y.; He, Y. K.; Zhang, L. T.; Wang, X. F.; Gao, J. Q.; Han, Z. B. Synthesis, crystal structure and luminescent properties of a new 3D coordination polymer constructed by Cd(II) with 4,4?-oxybis(benzoate) and 4,4?-bipyridine. Struct. Chem. 2007, 18, 1005–1009.

(25) Wang, H. S.; Li, X. F.; Xia, J. Synthesis, structure and near-infrared photoluminescent property of a tetranuclear Er(III) complex. Chin. J. Struct.Chem. 2015, 34, 87-94.

(26) Yu, J. O.; C?té, A. P.; Enright, G. D.; Shimizu, G. K. H. The first nonlayered metal sulfonate structure: a 1-D Ba2+network incorporating channels.Inorg. Chem. 2001, 40, 582-583.

(27) C?té, A. P.; Shimizu, G. K. H. Coordination solids via assembly of adaptable components: systematic structural variation in alkaline earth organosulfonate networks. Chem. Eur. J. 2003, 9, 5361-5370.

(28) Yan, W. H.; Yang, L. B.; Shen, M. L.; Ji, E. Y. A three-dimensional Ba(II) coordination polymer based on H4AQTC(anthraquinone-1,4,5,8-tetracarboxylic acid): quinone oxygen atoms participate in coordination. Chin. J. Struct. Chem. 2015, 34, 133-139.

(29) Meng, Q. G.; Zhang M. H. A 3D Ba(II) inorganic-organic hybrid framework based on 1,3,5-benzenetricarboxylic acid ligand: synthesis and characterization. Chin. J. Struct. Chem. 2014, 33, 1560-1565.