LU Wei-Hong XU Ting YANG Fan SUN Jun-Bin CHEN Man-Sheng② YAN Qun-Xuan

a (Key Laboratory of Functional Metal-organic Compounds of Hunan Province & Key Laboratory of Functional Organometallic Materials of Hunan Province College, Hengyang 421008, China)

b (Department of Chemistry and Materials Science, Hengyang Normal University, Hengyang 421008, China)

c (Hunan Jinkai Recycling Technology Co. LTD, Hengyang 421008, China)

ABSTRACT Two new bearing Bi(III) supramolecular compounds [Bi(pydcH)3(Bi(pydcH)(pydcH2)(pydc)(Bi(pydcH)(pydcH2)(pydc)] (1) and {[Bi(pydcH)3]·(H2O)5(Dmap)}2 (2) have been successfully synthesized via hydrothermal reaction (pydcH2 = 2,6-pyridine dicarboxylate, Dmap = 4-dimethylamiopyridine). They have been structurally determined by single-crystal X-ray analysis, and characterized by fluorescence analysis and thermogravimetric analysis (TG). Both compounds 1 and 2 are of the same triclinic system with different structures.The decomposition mechanisms of compounds 1 and 2 are discussed. Compared to 1, lots of small molecules were possibly removed from compound 2 below 250 ℃. In addition, the photoluminescence properties were also investigated.

Keywords: Bi(III) supramolecular compounds, crystal structure, fluorescence, thermal stability;

1 INTRODUCTION

Compared to pure organic or inorganic compounds, metal organic framework compound (MOF) possesses special properties, which show a certain degree of adjustability,modifiablity and multifunctional features[1-6], such as energy storage, magnetism properties, luminescence, ion exchange and so on[7-10]. Furthermore, the design and synthesis of functional complexes with special structure and expected performance have attracted attention of researchers[11-13]. As we know, organic ligands[14-16]have always been the focus of research in coordination chemistry, especially nitrogen-containing heterocyclic and carboxylic acids. It is not only because these organic ligands have many kinds of coordination modes with different metals, but they possess strong coordination ability of chelated interaction from the neighbouring nitrogen/oxygen atoms. Especially, pyridine-2,6-dicarboxylic acid, pydcH2, is a versatile chelating ligand,with a recognized biological function in the body metabolisms.Furthermore, some bismuth-based compounds with different structures have been reported in recent years, which have many potential applications in the fields of photoluminescence, medicine, catalysis and high dielectric constant materials[17-19].

On the other hand, for supramolecular chemistry, there are several impressive demonstrations of high affinity binding of small molecule guests by cyclodextrin, cucurbituril, or cyclophane hosts[20-24]. These hosts have undoubted practical and scientific values, and the spatial separation of hydrophobic and polar regions restricts the structural range of host-guest pairs that can be designed for selective recognition.Smith groups[25]described a new water-soluble organic host-guest pair in which polar and hydrophobic interactions combine to give nanomolar dissociation constants. However,bismuth MOFs containing the guest molecules have been very rarely reported to date.

In this work, two novel supramolecular compounds[Bi(pydcH)3(Bi(pydcH)(pydcH2)(pydc)(Bi(pydcH)(pydcH2)-(pydc)] (1) and {[Bi(pydcH)3]·(H2O)5(Dmap)}2(2) were successfully synthesized. The removal of guest molecules in 2 was studied by thermogravimetric analyses (TG), which has not been reported in the bismuth-pydc complexes.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All commercially available chemicals and solvents were of reagent grade and used as received without further purification. Thermogravimetric analyses were performed on a simultaneous SPRT-2 pyris1 thermal analyzer from room temperature to 600 ℃ at a heating rate of 20 K/min in the nitrogen atmosphere. Photoluminescence analyses were performed on an Edinburgh Instrument FLS920 fluorescence spectrometer. In the measurements of the emission and excitation spectra, the pass width was 5.0 nm.

2. 2 Synthesis of the title compounds Compound [Bi(pydcH)3(Bi(pydcH)-(pydcH2)(pydc)(Bi(pydcH)(pydcH2)(pydc)] (1)

Compound 1 was synthesized by mixing Bi2O3(1 mmol),2,6-pyridine dicarboxylate (6 mmol) and 30 mL distilled water into a 50 mL Teflon-lined stainless-steel autoclave. This autoclave was heated to 170 ℃ in an oven, kept for three days,and then powered off. When cooled down to room temperature, colorless block crystals of 1 suitable for X-ray diffraction analysis were obtained in 75% yield based on Bi2O3by slow evaporation for 7 days.

Compound {[Bi(pydcH)3]·(H2O)5(Dmap)}2(2)

Compound 2 was synthesized by the same procedure for compound 1 except that 4-dimethylamiopyridine (Dmap) (1 mmol) was taken into the mixture. The auto clave was heated to 140 ℃ in an oven and kept for 3 days, then powered off.When the autoclave was cooled down to room temperature,colorless crystals of 2 suitable for X-ray diffraction analysis were obtained by slow evaporation for 10 days.

2. 3 X-ray structural determination

The single crystals were carefully selected for data collection performed for compound 1 by the SuperNova CCD X-ray diffractometer equipped with graphite-monochromated MoKαradiation withλ= 0.71073 ? at 296 K, while by Bruker SMART CCD area detector at 144 K for compound 2.The structures of compounds 1 and 2 were solved by direct methods, and then the non-hydrogen atoms were refined anisotropically with SHELXS-97 by applying a full-matrix least-squares procedure onF2after being located from the trial structure. Moreover, the hydrogen atoms were fixed geometrically at the calculated distances, and allowed to ride on the parent atoms. The molecular structures and packing diagrams of compounds 1 and 2 are illustrated in Figs. 1 and 2,their selected bond lengths and bond angles in Tables 1 and 2,and their hydrogen bonds in Tables 3 and 4, respectively.

Fig. 1. Molecular structures of compounds (1) and (2) with the ellipsoids at 30%probability level. Hydrogen atoms were omitted for clarity

Fig. 2. Packing diagrams of compounds (1) and (2)

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for Compound (2)

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compound 1

Table 4. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compound 2

Symmetry transformation: a: x, y, z; b: –x, 2–y, –1–z; c: 1–x, 3–y, –z; d: 2–x, 2–y, –z; e: 1–x, 2–y, –z; f: 1+x, y, z;g: 1–x, 3–y, 1–z; h: 2–x, 3–y, 1–z; i: 1+x, y, z

3 RESULTS AND DISCUSSION

3. 1 Crystal structures of the compounds

As shown in Fig. 1a, the asymmetric unit of 1 consists of three Bi(III) cations, five pydcH anions, two pydc2-and two pydcH2molecules. The Bi(1) is six-coordinated by three oxygen and three nitrogen atoms from three different pydcH anions, and the Bi(2) is eight-coordinated by five oxygen and three nitrogen atoms. Only one pydcH anion, one pydc2-and one pydcH2are in the coordinated environment, where the Bi(3) is seven-coordinated by four oxygen and three nitrogen atoms with the same three ligands as Bi(2). The Bi(1)N3O3,Bi(2)N3O4and Bi(3)N3O5polyhedra are holodirected with a stereochemically inactive lone pair of electrons. From Table 1,for Bi(1), the Bi–O bond lengths are in the range of 2.333(13)～2.359(13) ?, and the Bi–N bond lengths vary from 2.481(17) to 2.579(17) ?. For Bi(2), the Bi–O and Bi–N bond distances change from 2.302(14) to 2.579(16) ?. The Bi–O and Bi–N bond distances fall in the ranges of 2.308(14)～2.732(16) ? and 2.518(16)～2.614(16) ?,respectively for Bi(3) in compound 1. The bond distances of Bi(2)–O(19) (2.737(13) ?), Bi(2)–O(23) (2.831(15) ?) and Bi(3)–O(25) (2.732(16) ?) are significantly long. However,they are shorter than the maximum of 3.249 ? reported for Bi–O bond distances retrieved from the Cambridge Structural Database (CSD)[17,18,26].

It is noted that the carboxylate groups and pyridyl nitrogen in pydcH anions chelate in bidentate or tridentate manner with the same Bi3+cation to form [Bi(pydcH)3] with a six-coordinated octahedral structure, which is different from any reported complexes. The bond angles are as follows: O–Bi–N 58.1(4) ～150.2(5)o, N–Bi–N 109.8(5) ～128.6(6)o and O–Bi–O 71.0(5)～153.2(4)o, as shown in Table 1. From Table 3 we can see extensive intermolecular O–H···O hydrogen bonds like O(4)–H(4)···O(6) (2.52(2) ?, 168o), O(18)–H(18)···O(16)(2.54(2) ?, 163o) and O(30)–H(30)···O(28) (2.52(2) ?, 164o)which further led to the stability of the crystal structure. Each coordination bismuth unit connects two other such units through the pydcH H-bonding, forming an infinite threedimensional network structure.

When the neutral ligand Dmap was introduced into the reaction of Bi2O3and 2,6-pyridine dicarboxylate, compound 2 was isolated. Single-crystal X-ray diffraction analysis exhibits that the asymmetric unit of 2 contains two independent Bi(III)ions, six 2,6-pyridine dicarboxylate (pydcH-) ligands, two 4-dimethylamiopyridine molecules and ten water molecules,as shown in Fig. 1b. Each Bi3+cation is nine-coordinated in a tricapped trigonal-prism coordination environment by six oxygen and three nitrogen atoms from three pydcH anions. As shown in Table 2, the Bi–O bond lengths are in the range of 2.330(5)～2.728(4) ? and Bi–N vary from 2.435(5) to 2.638(6) ?, similar to those reported[27]. The O–Bi–O bond angles range from 74.74(5) to 153.50(16)o, and O–Bi–N and N–Bi–N are in the ranges of 59.60(15)～143.07(16)o and 110.93(17)～125.75(18)o, respectively.

In fact, from Table 4, all the components in the crystal structure of compound 2 are connected through O–H···O and N–H···O hydrogen bonds, resulting in an interesting 3D supramolecular structure. Namely, O–H···O hydrogen bonds are mainly between the water molecules and O atoms of pydcH–, while N–H···O hydrogen bonds take place between DMAP and pydcH–, such as N(7)–H(7)···O(18) (173°) and N(9)–H(9)···O(6) (171°). It is no doubt that these strong hydrogen bonds make significant contributions to stabilize the molecular structure of 2.

3. 2 TG & DTG analysis

The thermogravimetric analysis (Fig. 3a) demonstrated that in nitrogen atmosphere compound 1 lose weight mainly in two stages from room temperature to 600 °C. The first stage took place from 305 to 350 °C, with the peaks located at 332 and 390 °C, respectively. The first weight loss at 332 °C is attributed to the removal of hydroxyl groups of the ligands.On the other hand, the second DTG peak at 390 °C results from the decomposition of the other oxygen groups. As shown in Fig. 3b, from 100 to 350 °C, the weight loss and DTG peak of compound 2 are different from those for 1. There are eight stronger DTG peaks at 111, 130, 148, 217, 291, 306, 320 and 333 °C in 2, with the former three attributed to the removal of crystal water molecules, the fourth to the departure of Dmap(B.P: 190 °C), and the latter four to the loss of crystal water and hydroxyl groups. The DTG peak at 375 °C of 2 was similar to that at 390 °C of 1. The results of TG and DTG indicate that the two compounds have high stability.

Fig. 3. TG-DTG curves of compounds 1 and 2

3. 3 Fluorescent property

As can be seen from Fig. 4, compound 1 displays a wide emission at 439 nm with the 2.47 μs decay lifetime under 260 nm excitation, while compound 2 exhibits two emission peaks at 450 and 373 nm with the 1.84 μs decay lifetime under the same excitation. Compared to 1, a new emission peak appears in 2, and the peak at 450 nm has a little red shift. The change could be contributed to the existing guest molecule of 4-dimethylamiopyridine. The blue photoluminescence of 1 and 2 is similar to that of the recently reported Bi-organic coordination polymers[17-19]. On the other hand, they have shorter decay lifetime than the reported [Bi(2,6-pdc)(H2btc)(H2O)2]nand [Bi2(2,6-pdc)2(H2pyr)(H2O)2]n.Therefore, these results show that the different Bi-based structures play a crucial role in the fluorescent properties.

Fig. 4. Fluorescent properties of compounds 1 and 2

4 CONCLUSION

In summary, two new supramolecular compounds containing bismuth and pydridine-2,6 dicarboxylate (pydcH2)have been synthesized and structurally characterized. Four different coordination modes of metal-dipicolinate exist in the two compounds, and the DMAP molecule plays an important role in the final structure. According to TG & DTG analyses,the main frameworks of these two compounds feature high stability. Moreover, the successful obtainment of two compounds demonstrates that the hydrothermal approach adjusted by slow solvent evaporation is effective to synthesize versatile bismuth-based coordination frameworks with different structures and properties.