A New Pyridine-3,4-dicarboxylic Acid Bridged Zinc(II) Coordination Complex: Crystal Structure,Quantum Chemistry and Luminescence Property①

ZHAN Pei-Ying FENG Wn-Zhong LI Xiu-Mei② LIU Bo

a(Faculty of Chemistry, Tonghua Normal University, Tonghua 134002, China)

b (Faculty of Chemistry, Jilin Normal University, Siping136000, China)

ABSTRACT A new 2D metal coordination polymer, [Zn(pydc)(L)(H2O)]n·5nH2O (1, H2pydc = pyridine-3,4-dicarboxylic acid, L = 3-(2-pyridyl)pyrazole), was synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, powder XRD, FT-IR, thermogravimetric, fluorescence spectrum and elemental analysis techniques. Complex 1 belongs to the monoclinic system, P21/c space group, with a =10.707(5), b = 14.221(5), c = 13.278(5) Å, β = 102.071(5)°, V = 1977.1(14) Å3 and Z = 2. It features a 2D network constructed by pydc2- and L ligand. In addition, the quantum-chemical calculations were accomplished on‘molecular fragments’ extracted from the crystal structure of 1 using the PBE0/LANL2DZ method built in Gaussian 16 Program. The calculation values denoted the distinct covalent interaction between the coordinated atoms and Zn(II) ion.

Keywords: coordination polymers, hydrothermal synthesis, crystal structure, luminescence property,quantum-chemical; DOI: 10.14102/j.cnki.0254–5861.2011–3092

1 INTRODUCTION

Owing to the unique structural tailorability and compositional diversity, metal coordination polymers (MCPs)have received much attention and become a hot topic due to their appealing applications as functional materials in adsorption separation, magnetism, luminescence, electrochemical sensors, and so on[1]. The variety of the structures relies on the presence of suitable metal-ligand interactions and supramolecular contacts, which is directly related to the coordination characteristics of the components, such as the charge and radii of metal ions, the amount of dentate and steric hindrance of the ligands, etc[2]. Although many MCPs with intriguing topologies have been reported, the control of precise structures of MCPs remains a great challenge in crystal engineering[3].

The idea of mixed ligands can indeed obtain a great diversity of MCPs[4]. However, the resulting structures are somewhat unpredictable and the governing principles in this system are less ascertained and remain elusive[5]. The organic pyridine-carboxylates as mixed ligand components are considered as a kind of remarkable building blocks in the construction of MCPs[6]. They have the ability to balance charges, good coordination ability, and stability in acid and base. On the other hand, (pyridyl)pyrazole is a good organic ligand, which not only has various classes but also shows excellent coordination ability and spatial expansion ability in the process of assembling with metal ions[7]. Thus, it is meaningful to investigate the effect of the combination of pyridine-carboxylates and heterocyclic (pyridyl)pyrazole mixed ligands on tuning the architectures of MCPs.

Based on the above consideration, a pyridine-3,4-dicarboxylic acid and a 3-(2-pyridyl)pyrazole were selected to react with Zn(II) ion under hydrothermal conditions. As a result, a 2D complex of [Zn(pydc)(L)(H2O)]n·5nH2O (1), was obtained and structurally characterized by single-crystal X-ray diffraction, powder XRD, FT-IR, thermogravimetric,fluorescence spectrum, elemental analysis techniques and quantum-chemical calculations.

2 EXPERIMENTAL

2. 1 Materials and methods

All starting materials purchased were of reagent quality and used without further purification. IR spectra (KBr pellets) and luminescence spectra were recorded on a Varian 640 FTIR spectrometer within the scope of 400～4000 cm-1and a Hitachi F-7000 fluorescence spectrometer at 296 K,respectively; powder X-ray diffraction and elemental analyses for C, N and H were performed with a D/teX Ultra diffractometer (40 kV and 40 mA, CuKα) and a PE 2400C elemental analyzer, respectively.

2. 2 Synthesis

A mixture of Zn(NO3)2·6H2O (0.2 mmol, 0.0595 g),H2pydc (0.20 mmol, 0.0334 g) and L (0.20 mmol, 0.029 g)was dissolved in 15 mL H2O. Suitable amount of triethylamine was added to this solution to adjust the pH value to 7.03, followed by stirring at room temperature for 0.5 h until a homogeneous solution was obtained. Then it was sealed in a Parr Teflon-lined stainless-steel vessel (25 mL)under autogenous pressure at 170 ℃ for 5 days. After cooling to room temperature, pale yellow block crystals of 1 suitable for X-ray diffraction were gained. Yield: 17% based on L.Elem. anal. calcd. for C30H20N8O19Zn2: C, 38.86; H, 2.17; N,12.08%. Found: C, 38.11; H, 1.98; N, 11.91%. IR (KBr, cm−1):3448(w), 3101(w), 1618(m), 1587(s), 1557(w), 1526(w),1493(w), 1471(w), 1431(w), 1403(s), 1355(w), 1328(w),1164(w), 1127(w), 1055(w), 1024(w), 982(w), 861(m),840(m), 809(w), 780(w), 767(s), 718(m), 687(m), 650(w),486(w), 462(w).

2. 3 Determination of the crystal structures

A suitable single crystal of complex 1 was carefully selected under an optical microscope and data collection was performed on a Bruker D8 QUEST CMOS diffractometer with graphite-monochromatized MoKαradiation (λ= 0.71073 Å) using theω-scan mode at room temperature. The raw data frames were integrated into SHELX-format reflection files and corrected using the SAINT program. Absorption corrections based on multi-scan were obtained using the SADABS program. All the structures were solved by direct methods and refined with full-matrix least-squares onF2using SHEXL-97[8,9]. Hydrogen atoms were located by geometric calculations and their positions and thermal parameters were fixed during the structure refinement. A total of 8836 reflections were collected in the range of 3.09≤θ≤25.00°, of which 3412 were independent (Rint= 0.1162). The finalR= 0.0849 andwR= 0.1646 for observed reflections withI> 2σ(I), andR= 0.1726 andwR= 0.2319 for all data with (Δρ)max= 1.131 and (Δρ)min= –1.130 e·Å-3. Selected bond length and angle parameters are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3. 1 Structural description

The single-crystal X-ray study reveals that complex 1,[Zn(pydc)(L)(H2O)]n·5nH2O, consists of divalent Zn2+ion,one pydc2-anion, one L ligand, one coordination water molecule and five crystalline water molecules, as shown in Fig.1, which also shows the coordination environments of Zn(II) ions and the ligands. Each Zn(1) ion in 1 is six-coordinated by two N atoms from L chelating ligands, one N atom and two oxygen atoms from three bridging pydc2-anions, and one coordination water molecule, in which the Zn(1)–O bond lengths are in the range of 2.026(11)～2.120(14) Å and those of Zn(1)–N are from 2.103(16) to 2.269(18) Å, which fall in the normal ranges and the coordination angles around the Zn ion vary from 76.8(6) to 173.4(6)o[10].

Fig.1. Coordination environment of Zn(II) ion in 1. Symmetry codes: (A) –x+1, –y, z+1; (B) x, –y+1/2, z–1/2

In complex 1, each pydc2-ligand adopts theμ3coordination mode, acting as aμ3-bridge to link four Zn(1) atoms, thus forming a two-dimensional network. It is different from our previous reported complex, [Ni(PDB)(bix)(H2O)]n[6(b)], which was obtained by using the same pyridine-carboxylate and different nitrogenous ligands. Meanwhile, each L ligand adopts theμ2chelating coordination mode. Additionally, there are N–H···O and C–H···O hydrogen bonds derived from the pyrazol, pyridine and carboxylic groups. The hydrogenbonding parameters of 1 are listed in Table 2. Moreover, there existπ-πinteractions between the pyridine ring of pzdc2-ligand and the imidazole ring of L ligand. The centroid distance is 3.623(11) Å for N(4)C(1)C(2)C(3)C(4)C(5) and N(2)N(3)C(15)C(14)C(13) rings (symmetry code: 1–x, –y,1–z), with the vertical distance to be 3.197(8) Å and the dihedral angle of 10°, indicating the existence ofπ-πinteraction. The complex extends into a three-dimensional supramolecule (Fig.2), so that it is more stable. From the view of topology, it takes a 2D net with a point symbol of{4462} (Fig.3).

Fig.2. View of the three-dimensional supramolecular structure along the aaxis

Fig.3. 2D topological structure of 1

3. 2 IR analysis

IR spectra of 1 are shown in the frequency range of 400～4000 cm-1(Fig.S1). The feature peak of -OH groups from water molecules of 1 is observed at about 3448 cm-1[11]. The strong bands at about 1618 cm-1for 1 are identified forνC–Ovibration of the pydc2-groups[12]. The peak located at 1403 cm-1for 1 is attributed to the stretching vibration of carboxylic groups[13]. The strong bands in the region of 687～718 cm-1can be assigned to theνC–Nvibration of the N-heterocyclic rings of the ligands[14].

3. 3 Powder X-ray diffraction (PXRD),and thermal stability analyses

The phase purity of the title complex was confirmed by comparison of its experimental powder X-ray diffraction(PXRD) pattern with the reference powder diffractogram(calculated from single-crystal X-ray diffraction data) (Fig.4).The as-synthesized pattern agrees well with the corresponding simulated one, indicating that the phase purity of the sample is better. The difference in strength is due to the preferred orientation of the powder sample.

Fig.4. PXRD analysis of 1: bottom-simulated, top-experimental

Fig.5. Solid-state emission spectrum of 1 at room temperature

The thermal stability of complex 1 was investigated by thermogravimetric method using bulk materials under N2atmosphere of 20.0 mL·min−1and 10 °C·min−1. As shown in Fig.S2, the weight loss of 20.08% (calcd. 18.98%) for 1 from 87 to 181 °C is attributed to the release of water molecules,and 65.02% (calcd. 63.90%) from 181 to 505 °C belongs to the departure of L and pydc2-ligands. The title complex can be stable up to 475 °C, following the decomposition of the framework.

3. 4 Photoluminescent properties

Fig.5 is a solid fluorescence diagram of complex 1 at room temperature. As can be seen from the diagram, upon excitation at 380 nm, complex 1 shows a broad emission peak at 459 nm. The strongest emission peak of H2pydc is around 370 nm, mainly due to theπ*→n transition of ligand[15].Therefore, the emission peak of complex 1 can not be caused byπ*→n transition of H2pydc. And the 3-(2-pyridyl)pyrazole molecule has a strong emission peak at about 443 nm at room temperature. By consulting literature[16,17], the fluorescence of complex 1 is neither produced by the charge transition from metal to ligand, nor the charge transition from ligand to metal,but generated electronic transitions within the 3-(2-pyridyl)pyrazole ligand. Compared with the fluorescence of 3-(2-pyridyl)pyrazole molecules, the emission spectra of complex 1 exhibit red shifts of ca. 16 and enhanced fluorescence intensity, which is mainly due to the formation of complex. The metal-ligand coordination increases the rigidity of the system, reduces the energy loss due to vibration and enhances the fluorescence intensity.

4 QUANTUM CHEMISTRY CALCULATIONS

All calculations in this manuscript were performed with the Gaussian16 program[18]. The parameters of the molecular fragments for calculation were extracted from the crystal structure of the title complex. Natural bond orbital (NBO)analysis was performed by density functional theory (DFT)[19]with the PBE0[20-23]hybrid functional and the LANL2DZ basis set[24].

The selected atom net charges, electron configuration,Wiberg bond and NBO bond orders (a.u) of the title complex are listed in Table 2. The calculation results indicate that the electronic configuration of Zn(II) ion is 4s0.293d9.984p0.30, and those of N and O atoms are 2s1.32～1.332p4.00～4.20and 2s1.66～1.702p5.10～5.29. Based on the above values, the Zn(II) ion coordination with N and O atoms is mainly on 3d, 4sand 4porbitals. N atoms form coordination bonds with Zn(II) ion using 2sand 2porbitals. All O atoms supply electrons of 2sand 2pto Zn(II) ion and form the coordination bonds.Therefore, the Zn(II) ion obtained some electrons from three N atoms of L and three O atoms of pydc2-ligand[25]. Thus,according to valence-bond theory, the atomic net charge distribution and the NBO bond orders of the title complex(Table 3) show obvious covalent interaction between the coordinated atoms and Zn(II) ion. The differences of the NBO bond orders for Zn–O and Zn–N make their bond lengths different[26], which is in good agreement with the X-ray crystal structural data of the title complex.

Table 2. Hydrogen Bonds for Complex 1

Table 3. Selected Atom Net Charges, Electron Configurations, Wiberg Bond Indexes and NBO Bond Orders of 1

As can be seen from Fig.6, the LUMO (the lowest unoccupied molecular orbital) and HOMO (the highest occupied molecular orbital) electron clouds are mainly located at the L ligand. Therefore, ILCT may be inferred from some contours of molecular orbital of the title complex.