A Luminescent Cd(II) Coordination Polymer Constructed from Isophthalic Acid and 1,3-Bis-(4-pyridyl)propane①

XU BoLI Zhi-Wang JIANG Yang LI Cun-Cheng

(Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China)

A Luminescent Cd(II) Coordination Polymer Constructed from Isophthalic Acid and 1,3-Bis-(4-pyridyl)propane①

XU Bo②LI Zhi-Wang JIANG Yang LI Cun-Cheng②

(Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China)

A new coordination complex [Cd2(ip)2(bpp)]n(1, H2ip = isophthalic acid, bpp = 1,3-bis-(4-pyridyl) propane) has been synthesized and structurally characterized.Crystal data for the title complex are as follows: triclinic system, space group P1 with a = 9.6724(6), b = 10.2243(6), c = 14.8205(9) ?, α = 88.927(5), β = 71.664(6), γ = 75.767(5)°, V = 1345.79(14) ?3, Mr= 751.29, Z = 2, F(000) = 740, Dc= 1.854 g/cm3, μ(MoKα) = 1.636 mm-1, R = 0.0259 and wR = 0.0623 for 4754 observed reflections with I > 2σ(I).X-ray structure analysis reveals that complex 1 holds a 3D architecture based on α-Po primitive cubic units with the [Cd4(COO)8] units treated as 6-connected nodes.Solid-state properties for 1, such as infrared spectroscopy, elemental analyses, thermal stability and luminescent property, have also been investigated.

coordination polymer, crystal structure, luminescence, mixed-ligand approach;

1 INTRODUCTION

Considerable experimental and theoretical efforts have been focused on the design and synthesis of coordination polymers (CPs) not only because of their intriguing variety of crystal structures and topologies[1-3], but also for their potential applications in gas storage, luminescence, molecular recognition and catalysis, magnetism and sensing[4-7].However, precise control synthesis of CPs with desired structure and property is still a great challenge because too many factors can affect the formation and topology of CPs.To design and synthesize CPs with a unique structural diversity, it would be highly desirable to choose organic ligands with ideal rigidity, length and coordination modes[8-10].In particular, rigid organic aromatic multicarboxylate ligands such as 1,4-benzenedicarboxylate, 5-nitroisophthalate, 5-methylisophthalate and 1,3,5-benzenetricarboxylic acid are very good candidates for the construction of coordination polymers with peculiar structures and intriguing functionalities[11-14].On the other hand, bidentate pyridyl-functionalized an- cillary ligands with flexible spacer in the middle are also very effective building blocks because the flexible nature of the spacers allows the ligands to bend and rotate.Bearing these in mind, we have reported a series of CPs with interesting structures and properties based on mixed aromatic multicarboxylate and bidentate pyridyl-functionalized ancillary ligand system[15-17].As a continuation of our previous work, we report herein the synthesis,structure and luminescent property of a new Cd(II) complex [Cd2(ip)2(bpp)]n(1) with isophthalic acid and the flexible 1,3-bis-(4-pyridyl) propane ligand as the raw materials.

2 EXPERIMENTAL

2.1 Materials and methods

All commercially available reagents and starting materials were of reagent-grade quality and used without further purification.Elemental analyses (C, H, N) were carried out on an Elementar Vario EL III analyzer.Infrared (IR) spectra were recorded on PerkinElmer Spectrum One as KBr pellets in the range of 4000～400 cm-1.The thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 449C unit at a heating rate of 10 ℃/min under a nitrogen atmosphere.Fluorescence spectroscopy of the compound was performed on an Edinburgh Analytical instrument FLS920.This instrument is equipped with an Edinburgh Xe900 xenon arc lamp as exciting light source.X-ray powdered diffraction (XRPD) pattern of the sample was recorded by an X-ray diffractometer (Bruker D8 FOCUS) with a graphite monochromator and CuKα radiation at room temperature while the voltage and electric current are held at 40 kV and 20 mA.

2.2 Synthesis of the complex

A mixture of Cd(NO3)2·4H2O (0.25 mmol), H2ip (0.3 mmol), bpp (0.3 mmol) and NaOH (0.4 mmol) in deionized water (8 mL) and methanol (3 mL) was stirred under air atmosphere for half an hour.It was then sealed in a 25 mL Teflon-lined stainless steel autoclave, and heated at 130 ℃ for three days.After slowly cooling to room temperature within twenty hours, colorless block crystals of complex 1 were obtained in 43% yield based on H2ip.Anal.Calcd.for C29H22Cd2N2O8(751.32): C, 46.36; H, 2.95; N, 3.73%.Found: C, 46.42; H, 2.91; N, 3.77%.IR (KBr, cm-1): 3406(s), 3110(w), 1632(s), 1387(s), 1134(m), 1058(w), 918(w), 832(w), 788(w), 610(w).

2.3 Structure determination

A single crystal with dimensions of 0.46mm × 0.34mm × 0.28mm for compound 1 was mounted on an Xcalibur, Eos, Gemini diffractometer equipped with graphite-monochromated Mo-Kα (λ = 0.71073 ?) radiation at room temperature.The structure was solved by direct methods with the program SHELXS-97[18]and refined by full-matrix leastsquares on F2using the SHELXTL-97 program[19].All non-hydrogen atoms were refined with anisotropic displacement parameters.The positions of hydrogen atoms attached to carbon atoms were generated geometrically and refined with isotropic thermal parameters.The final R = 0.0329 and wR = 0.0660, S = 1.111, (Δρ)max= 0.42 and (Δρ)min= –0.88 e/?3.The selected bond lengths and bond angles for the complex are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Structure description of 1

Single-crystal X-ray diffraction study on complex 1 reveals that it crystallizes in triclinic space group P1 and exhibits a 3D α-Po framework.As shown in Fig.1a, the asymmetric unit of complex 1 consists of two crystallographically independent Cd(II) ions, one bpp ligand and two ip2-molecules.Both the two Cd(II) centers are six-coordinated in distorted octahedral geometry.Carboxylate groups of the two H2ip ligands are completely deprotonated but show different coordination modes, as shown in Scheme 1.The Cd–O bond length lies in the range of 2.2072(19)～2.4404(19) ?, and the Cd–N distance ranges from 2.257(2) to 2.305(2) ?, which are all consistent with those in literatures[20,21].The minimum and maximum bond angles for the Cd(1) center are 56.59(7) and 153.08(8)°, while the minimum and maximum bond angles for the Cd(2) center are 87.66(8) and 155.87(8)°, respectively, which slightly deviate from the angles of a perfect octahedron.Further insight into the symmetry of the complex shows that the asymmetric units of complex 1 reside on the inversion centers.The operation of the symmetry generates a Cd4cluster bridged by carboxylate group, as shown in Fig.1b.Then these [Cd4(COO)8] units are further linked by ip2-molecules, forming a 2D 44-sql layer as shown in Fig.1c.If the [Cd4(COO)8] units are considered as 6-connected nodes, the bpp and ip2-molecules considered as a linker, and the structure of complex 1 can be simplified into a 3D architecture based on α-Po primitive cubic lattice with a Schl?fli symbol of (41263), as shown in Fig.1d.

Scheme 1.Coordination modes of the H2ip ligand in 1

Fig.1.(a) Coordination environment of the Cd(II) center in complex 1.(b) View of the [Cd4(COO)8] unit.(c) 2D 44-sql layer in 1, d) Schematic illustration of the α-Po network in 1

3.2 IR spectrum analysis of complex 1

As shown in Fig.2, the solid-state IR spectrum of complex 1 was performed and the result is consistent with the single-crystal structure.The broad peak at 3406 cm-1corresponds to the vibrations of water molecules.The peaks at 1387 and 1134 cm-1can be attributed to the C=N stretching vibrations of bpp ligand.The asymmetric and symmetric vibrations of carboxylate groups of the ip2-ligand were observed at 1632 and 918 cm-1, respectively.Absorption peaks in the range of 840～4600 cm-1are attributed to the variations of -CH2- groups in the bpp ligand.

3.3 Thermogravimetric analysis and powder X-ray diffraction of 1

To evaluate the thermal stability of complex 1, thermogravimetric analysis was performed under N2atmosphere in the temperature range of 30～850 ℃with a heating rate of 10 ℃×min-1, as shown in Fig.3.The TGA plot shows a plateau in the temperature range of 30～350 ℃ without any weight loss because there is no coordinated or guest solvent molecule in the structure of complex 1.From about 350 to 685 ℃, there is only one step of weight loss step corresponding to the decomposition of two kinds of ligands in complex 1.The final residue of 33.96% is close to the calculated value of 34.18% based on CdO.To confirm the phase purity of bulk materials of complex 1, PXRD experiment has been carried out.As shown in Fig.4, by comparison, the experimental PXRD patterns of 1 correspond well to the simulated ones, indicating the phrase purity.

Fig.2.IR spectrum of complex 1

Fig.3.TG curve of complex 1

3.4 Luminescent property of 1

It is known that coordination polymers based on d10metals show promising luminescent properties and have potential applications as multi-functional luminescent materials.In this work, the luminescent property of complex 1 was carried out in solid state at room temperature.As shown in Fig.5, complex 1 shows an emission centered at about 457 nm under excitation of 330 nm.According to previous reports, the free H2ip ligand exhibits an emission maximum at 408 nm upon 348 nm excitation[22], while the bpp ligand shows no emission band in the range of 400～500 nm upon excitation at 330 nm[15].The emission of complex 1 shows a red shift compared with that of the H2ip ligand and should be best ascribed to metal-to-ligand charge transfer according to literatures[17,18].

4 CONCLUSION

A new coordination complex based on isophthalic acid and 1,3-bis-(4-pyridyl) propane has been synthesized and characterized.The structure of complex 1 exhibits a 3D coordination network based on α-Po primitive cubic units.Solid state luminescent behavior of complex 1 was also investigated.

Fig.4.PXRD pattern of complex 1

Fig.5.Solid-state emission spectrum of complex 1

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10.14102/j.cnki.0254-5861.2011-0638

13 January 2015; accepted 1 April 2015 (CCDC 1037950)

① Supported by the National Natural Science Foundation of China (No.21301069) and the Natural Science Foundation of Shandong Province (No.ZR2012BQ004)

②Corresponding author.Majoring in coordination chemistry.E-mail: chm_xub@ujn.edu.cn or licc0713@163.com