Syntheses, Crystal Structures and Luminescent Properties of Lead(II) and Cadmium(II) Coordination Polymers Constructed from Biphenyl-2,5,3?-tricarboxylate①

GAO Zhu-Qing LI Hong-Jin M. Kirillov Alexnder GU Jin-Zhong (Shool of Chemil nd Biologil Engineering,Tiyun University of Siene nd Tehnology, Tiyun 030021, Chin)

b (Centro de Química Estrutural, Complexo I, Instituto Superior Técnico,The University of Lisbon, Av. Rovisco Pais, 1049-001, Lisbon (Portugal))

c (College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China)

1 INTRODUCTION

In the last two decades, more and more researchers have paid attention to the construction of transition coordination polymers not only due to their versatile architectures[1–5]but also for their desirable properties such as luminescent, magnetic,catalytic, and gas absorption and separation properties[6–10]. In previous reports, researchers found that many factors may seriously influence the structures of the resulting complexes, such as the ligands, kinds of metal salt, the solvent system, pH value, the metal-to-ligand ratio, reaction temperature and time, and so on[11–15]. A basic design route for this kind of polymers is directed by self-assembly of designed organic ligands and inorganic metal cations.Multicarboxylate ligands have been proven to be good candidates in the construction of coordination polymers due to their diverse coordination modes[1–2,12–14,16]. In order to extend our researches in this field, we have selected H3btc as a functional building block on account of the following considerations: (a) H3btc possesses three carboxyl groups that may be completely or partially deprotonated, depending on the pH; (b) it is a flexible ligand allowing the rotation of two phenyl rings around the C–C single bond; (c) to our knowledge,H3btc has not been adequately explored in the con-struction of coordination polymers. In fact, the search of the Cambridge Structural Database (version 5.34,May 2013) reveals that there is no example of a coordination compound derived from H3btc.

Taking into account these factors, we herein report the synthesis, crystal structures, and luminescent properties of Pb(II) and Cd(II) coordination polymers constructed from H3btc and/or phen ligand.

2 EXPERIMENTAL

2. 1 General procedures

All chemicals and solvents were of A.R. grade and used without further purification. Carbon,hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer. IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. Thermogravimetric analysis (TG) was performed under N2atmosphere with a heating rate of 10 ℃/min on a LINSEIS STA PT1600 thermal analyzer. Excitation and emission spectra were recorded for the solid samples on an Edinburgh FLS920 fluorescence spectrometer at room temperature.

2. 2 Synthesis of compound 1

A mixture of PbCl2(0.083 g, 0.3 mmol), H3btc(0.086 g, 0.3 mmol), phen (0.060 g, 0.3 mmol), and H2O (10 mL) was adjusted to pH = 6.0 with a 0.5 M NaOH solution. The mixture was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160℃ for 3 days, followed by cooling to room temperature at a rate of 10 ℃·h–1. Colourless block-shaped crystals of 1 were isolated manually, and washed with distilled water. Yield: 60% (based on Pb). Anal.Calcd. (%) for C27H16PbN2O6: C, 48.28; H, 2.40; N,4.17. Found (%): C, 48.67; H, 2.05; N, 3.81. IR(KBr, cm–1): 1693s, 1608m, 1539s, 1424m, 1355s,1215s, 1133w, 1097w, 907w, 857m, 798m, 774m,726w, 685w, 633w, 516w. vCOOH1693, vas(CO2)1608 and 1539, vs(CO2) 1424 and 1355.

2. 3 Synthesis of compound 2

A mixture of CdCl2·H2O (0.060 g, 0.3 mmol),H3btc (0.057 g, 0.2 mmol), and H2O (10 mL) was adjusted to pH = 7.0 with a 0.5 M NaOH solution.The mixture was stirred at room temperature for 15 min, and then sealed in a 25 mL Teflon-lined stainless steel vessel, and heated at 160 ℃ for 3 days,followed by cooling to room temperature at a rate of 10 ℃/h. Colourless block-shaped crystals of 2 were isolated manually, and washed with distilled water.Yield 0.075 g, 70% (based on Cd). Anal. found for C30H26Cd3O18(%): C, 35.6; H, 2.6. Calcd. (%): C,35.8; H, 3.0. IR (KBr, cm–1): 3429m, 1609m, 1534s,1430m, 1394s, 1268m, 1168w, 1092w, 1045w, 843m,768m, 699m, 603w, 502w. vOH3429, v as(CO2) 1609 and 1534, v s(CO2) 1430 and 1394.

2. 4 Structure determination

A single crystal of the title compound with dimensions of 0.28mm × 0.26mm × 0.24mm (1) and 0.30mm × 0.26mm × 0.24mm (2) were mounted on a Bruker CCD diffractometer equipped with a graphite-monochromatic Mo Kα (λ = 0.71073 ?)radiation using φ-ω scan mode at 293(2) K in the ranges of 3.15＜θ＜25.50o and 2.89＜θ＜25.50o, respectively. The structures were solved by direct methods with SHELXS-97[17]and refined by fullmatrix least-squares techniques on F2with SHELXL-97[18]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms (except those bound to water molecules) were placed in the calculated positions with fixed isotropic thermal parameters and included in structure factor calculations in the final stage of full-matrix least-squares refinement. The hydrogen atoms of water molecules were located by difference maps and constrained to ride on their parent O atoms. Details of X-ray experiment and crystal data are summarized in Table 1.The selected important bond parameters are given in Tables 2 and 3.

3 RESULTS AND DISCUSSION

3. 1 Synthesis

Both compounds 1 and 2 were obtained by hydrothermal reactions at pH = 6.0 or 7.0 and 160 ℃for three days (Scheme 1).

Scheme 1. Synthetic routes for compounds 1 and 2

Table 1. Crystal Data and Structural Refinements for Compounds 1 and 2

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

Table 3. Selected Bond Lengths (?) and Bond Angles (°) for Compound 2

3. 2 Crystal structure of 1

The asymmetric unit of compound 1 contains one crystallographically unique Pb(II) atom, one Hbtc2–ligand, and one phen moiety. The partial deprotonation of H3btc to give Hbtc2–is also confirmed by the IR spectra data of 1, since a strong -COOH band at 1693 cm-1was observed (see Experimental Section). As depicted in Fig. 1, each Pb(II) atom is six-coordinated and adopts a distorted octahedral geometry formed by two N atoms of phen ligand and four O atoms of two different Hbtc2–moieties. The Pb–O (2.393(12) ～ 2.747(14) ?) and Pb–N(2.566(16)～2.727(16) ?) bond lengths are in good agreement with those observed in some other Pb(II)compounds[19-20]. In 1, the Hbtc2–ligand adopts a μ2-coordination mode, in which two deprotonated carboxylate groups show a η1:η1bidentate mode(Scheme 2). The dihedral angle of two benzene rings in Hbtc2–is 49.93o. The carboxylate groups of Hbtc2–ligands alternately bridge the neighboring Pb(II)ions to form a zigzag chain with the Pb··Pb separation of 10.973(2) ? (Fig. 2). Adjacent chains are connected to a form 2D sheet through O–H··O hydrogen bonding (Table 4). Then a 3D supramolecular framework is assembled by π-π packing interactions (Fig. 3). There are four kinds of π-π stacking interactions observed. One is between adjacent phenly planes of the phen ligands with the centroid-centroid separation of 3.560(2) ?; One is between adjacent pyridyl planes of the phen ligands with the centroid-centroid separation of 3.733(2) ?;The other one is between adjacent phenyl planes of the Hbtc2–ligands with the centroid-centroid separation of 3.706(2) ?; and the last one is between adjacent phenyl planes of the phen ligands and the pyridyl planes of the phen ligands with the centroid-centroid separation of 3.863(2) ?.

Scheme 2. Coordination modes of the Hbtc2–/btc3– ligands in compounds 1 and 2

Fig. 1. Coordination environment of the Pb(II) atom in compound 1.H atoms except those of COOH groups were omitted for clarity. Symmetry code: i: x+1/2, –y+1/2, z–1/2

Fig. 2. A perspective of 1D chain along the bc plane

Fig. 3. A perspective of 3D supramolecular structure along the bc plane

3. 3 Crystal structure of 2

Compound 2 crystallizes in the triclinic space group P. In the asymmetric unit, there are two crystallographically unique Cd(II) atoms, one μ6-btc3–ligand, two coordinated and one lattice water molecules. As shown in Fig. 4, the Cd(1) atom is sevencoordinated by four O atoms of four different btc3–ligands, and one O atom of coordinated water molecule, constructing a distorted pentagonal bipyramid. The Cd(2) atom is six-coordinated by four carboxylate O atoms of four independent btc3–ligands, and two O atoms of two coordinated water molecules, forming a distorted octahedron. The Cd–O bond lengths are in the range of 2.193(3)～2.452(3) ?, which are comparable to those of other Cd(II) compounds[6,11,19]. The btc3–ligands adopt aμ6-coordination mode (Scheme 2), in which the carboxylate groups show μ2-η1:η1bidentate and μ2-η1:η2tridentate modes. The dihedral angle of two rings in the btc3–moieties is 55.59o. The three neighboring Cd(II) ions are bridged by means of four carboxylate groups from the four different btc3–ligands, giving rise to a centrosymmetric linear trinuclear Cd(II) subunit (Fig. 5). In this tricadmium(II) unit, the Cd··Cd distance is 3.833(2) ?.The adjacent Cd3subunits are further linked by the btc3–blocks into a 3D open framework (Fig. 6). The 3D framework is stabilized by O–H··O hydrogen bonding (Table 5) and π-π packing interactions.There are two kinds of π-π stacking interactions observed between adjacent phenyl planes of the btc3–ligands with the centroid-centroid separations of 3.585(2) and 3.760(2) ?. The resulting network features channels (11.34? × 10.01? measured by atom-to-atom distances) which are filled with guest water molecules. If viewed down the c axis, the framework of 2 also displays channels with size ca.4.54? × 11.14? measured by atom-to-atom distances. Upon removal of guest water molecules, we computed by the PLATON an effective free volume that is 7.5% of the crystal volume[21]. However, after eliminating both coordinated and guest water molecules, the effective free volume attains 19.2%of the crystal volume of 2.

Table 4. Hydrogen Bond Lengths (?) and Bond Angles (o) of Compound 1

Table 5. Hydrogen Bond Lengths (?) and Bond Angles (o) of Compound 2

To get further insight into the structure of 2, a topological analysis of its 3D framework was performed using the TOPOS software[22]. We have generated an underlying network by omitting the terminal H2O ligands and reducing the μ6-btc3–moieties to their centroids. This network (Fig. 7) is built from the 4-connected Cd(1)/Cd(2) (topologically different) and 6-connected μ6-btc3–nodes.From the topological viewpoint[22], it is classified as a trinodal 4,4,6-connected net with the 4,4,6T24 topology defined by the point symbol of(44·62)3(46·69)2, wherein the (44·62) and (46·69)2indices are those of the Cd(1)/Cd(2) and μ6-L nodes,respectively. This topological type is rather rare and has only been observed in five other metal-organic frameworks[22–26].

Fig. 6. View of the 3D supramolecular framework of compound 2 along the ac plane

3. 4 Thermal analysis

To study the stability of compounds 1 and 2,thermal gravimertric analyses (TGA) were performed. As shown in Fig. 8, the TGA curve of compound 1 indicates that it is stable up to 285 ℃.Further heating leads to its decomposition. Compound 2 undergoes a mass loss of 10.56% between 66 and 176 ℃, which corresponds to the loss of four coordinated and two lattice water molecules(calcd. 10.67%). Above 373 ℃, the framework is destroyed gradually.

Fig. 7. Topological representation (arbitrary view) of the underlying trinodal 4,4,6-connected 3D network in 2 with the 4,4,6T24 topology and the point symbol of (44·62)3(46·69)2.Colour codes: 4-connected Cd(1)/Cd(2) nodes (cyan), centroids of 6-connected μ6-L nodes (gray)

Fig. 8. TGA curves of compounds 1 and 2

Fig. 9. Solid state emission spectra of H3btc and compounds 1 and 2

3. 5 Luminescent properties

The emission spectra of H3btc and compounds 1 and 2 were measured in the solid state at room temperature, as depicted in Fig. 9. The “free” H3btc ligand displays a weak photoluminescence with two emission peaks at 418 and 525 nm if excited at 358 nm, which may be ascribed to intraligand π*→n or π*→π transitions[16]. For the two compounds, the significantly more intense emission bands are observed with maximum at 512 nm (λex= 352 nm)for 1 and 430 nm (λex= 358 nm) for 2. All bands can be assigned to an intraligand (π*→n or π*→π)emission[27]. The enhancement of luminescence of the compounds can be attributed to the binding of ligands to the metal centers, which effectively increases the rigidity of the ligand and reduces the loss of energy by radiationless decay[28].

4 CONCLUSION

In this work, by adjusting the reaction pH and/or adding the auxiliary ligand, two coordination polymers driven by the biphenyl-2,5,3?-tricarboxylate blocks were synthesized by hydrothermal method. The structures of the obtained compounds vary from 1D chains (1) to 3D metal-organic frameworks (2) with distinct architectures. Their structural diversities demonstrate that the pH value of the reaction system, the nature of the metal ion,and auxiliary ligand play a crucial role in the assembly of structurally distinct coordination polymers. This work shows that biphenyl-2,5,3?-tricarboxylic acid is an excellent bridging ligand for the construction of coordination polymers.

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