PAN Y-Ru LI Xiu-Mei② LIU Bo ZHOU Shi

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

b (Faculty of Chemistry, Jilin Normal University, Siping 136000, China)

ABSTRACT A new Pb(II) coordination polymer, [Pb(adip)(L)2]n (1, H2adip = adipic acid, L = 2-(4-N,N΄-dimethylphenyl)-1-H-imidazo[4,5-f][1,10]phenanthroline), 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 crystallizes in triclinic system, space group P1 with a = 11.406(3), b= 12.036(4), c = 17.520(5) Å, V = 2228.3(11) Å3, Mr = 1028.09, Dc = 1.532 g·cm-3, F(000) = 1024, μ(MoKα) =3.842, Rint = 0.0274 and Z = 2. The final R = 0.1055 and wR = 0.3277 for 6954 observed reflections with I > 2σ(I). It features a one-dimensional chain linked by adip2- ligand. Moreover, complex 1 exhibits strong green luminescence with emission peak of 514 nm. 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 Pb(II)ion.

Keywords: coordination polymer, hydrothermal synthesis, crystal structure, luminescence property,quantum-chemical; DOI: 10.14102/j.cnki.0254-5861.2011-3195

1 INTRODUCTION

Lead(II) coordination polymers (CPs) are an active research topic in functional materials and have received much attention,not only because of their unique structural tailoring abilities and compositional diversity but also of their appealing applications in adsorption separation and luminescence[1-5].The variety of their structures and properties relies on their multitudinous components and diverse assembly methods;these are directly related to the coordination characteristics of the components, such as organic ligands, solvent systems,temperature, charge, number of dentates, positions and types of substituents, and steric hindrance of the ligands[6,7].Although many CPs with intriguing topologies have been reported, the control of precise structures of CPs remains a great challenge in the crystal engineering field[8,9].

The strategy of mixed ligands can indeed afford excellent new CPs with promising properties[10,11]. However, the resulting structures are somewhat unpredictable, and their directed synthesis is still a challenge[12,13]. Organic carboxylates are remarkable building blocks in the construction of CPs; they are considered to be good mixed ligand components because of their ability to balance charges,their good coordination ability and their stability in acid and base conditions[14-16]. On the other hand, phenanthroline derivatives are regarded as excellent organic building blocks[17,18]. First, they not only have various types but also show excellent coordination ability in the process of assembly with metal ions. Secondly, they have variable conformations and good supporting capacity because of their rigid organic skeletons. Thus, it is meaningful to investigate the effects of the combination of carboxylates and phenanthroline derivative ligands on tuning the architectures of CPs.

Based on the above considerations, flexible adipic acid and rigid 2-(4-N,N΄-dimethylphenyl)-1-H-imidazo[4,5-f][1,10]phenanthroline mixed ligands were selected to react with Pb(II) cations. As a result, a new one-dimensional complex of[Pb(adip)(L)2]n(1) was obtained under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction, powder XRD, FT-IR, fluorescence spectrum,elemental analysis techniques and quantumchemical calculations. Complex 1 exhibits green photolu-minescence with an emission maximum at ca. 514 nm upon excitation at 320 nm.Quantum chemical calculations indicate significant covalent interaction between the coordination atoms and Pb(II) ion.

2 EXPERIMENTAL

2. 1 Materials and methods

2-(4-N,N΄-dimethylphenyl)-1-H-imidazo[4,5-f][1,10]phenanthroline (L) was prepared according to a reported method[19]. All the other chemicals purchased were of reagent grade and used without further purification. IR spectra (KBr pellets) and powder X-ray diffraction were performed with a Varian-640 spectrometer and an Ultima IV diffractometer (40 kV and 40 mA, CuKα), respectively. The luminescence spectra and elemental analyses for C, N and H were obtained with a Hitachi F-7000 fluorescence/phosphorescence spectrometer and a PE 2400C elemental analyzer, respectively.

2. 2 Preparation of the complex

A mixture of Pb(NO3)2(0.2 mmol, 0.0662 g), H2adip (0.20 mmol, 0.0292 g), L (0.20 mmol, 0.0676 g), NaOH (0.4 mmol,0.016 g), and H2O (8 mL) was sealed in a 25 mL Teflon-lined autoclave under autogenous pressure at 120 °C for 5 days.After cooling to room temperature at a rate of 5 ºC per hour,reddish-brown block crystals of 1 suitable for X-ray diffraction were obtained in 24.0% yield based on Pb(II).Anal. Calcd. (%) for C48H40N10O4Pb: C, 56.08; H, 3.92; N 13.62. Found (%): C, 55.88; H, 3.08; N, 13.03. IR (KBr,cm−1): 3445(w), 3150(w), 1602(s), 1587(m), 1558(w),1526(w), 1471(w), 1451(w), 1403(m), 1315(w), 1295(s),1197(w), 1165(w), 1126(m), 1024(w), 856(w), 829(w),810(w), 755(w), 735(w), 703(w), 686(w), 640(w), 519(w),465(w).

2. 3 Determination of the crystal structures

The X-ray intensity data for 1 were collected on a Bruker D8 QUEST CMOS diffractometer, and the related crystal structure was solved by direct methods using the SHELXS and SHELXL programs of the SHELXTL crystallographic software package and refined onF2by the full-matrix least-squares method[20,21]. The non-hydrogen atoms were refined with anisotropic temperature parameters and the hydrogen atoms of organic ligands were placed in calculated positions and treated as riding atoms with isotropic displacement parameters set at 1.2 to 1.5 times theUeqvalues of the parent atoms. Selected bond lengths and bond angles are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3. 1 Structural description

X-ray diffraction study indicates that 1 belongs to the triclinic space groupP1 and the asymmetric unit contains one four-coordinated Pb(II) center, one adipate anion and one L ligand, as shown in Fig. 1. Each Pb(II) enter exhibits a slightly distorted tetrahedral coordinated environment completed by two nitrogen atoms from two symmetry-related L ligands and two oxygen atoms from adipate group. The distances of Pb–O are in the range of 2.448(14)～2.535(17) Å,and Pb(1)–N bond lengths are from 2.680(16) to 2.784(17) Å.The bond angles around the Pb(II) cation are in the range of 71.3(5)～143.0(5)°, all of which fall in the normal range of those observed in the reported Pb(II) complexes[22,23].

In complex 1, adipic acids are fully deprotonated, as proved to be no stretching vibration around 1700 cm-1and serves as a monodentate ligand through one carboxylate oxygen atom coordinating to Pb(II). It bridges two adjacent Pb(II) forming a 1D chain (Fig. 2). The Pb(II)–Pb(II) separation across adip is 10.955 and 10.649 Å, respectively. The structure is different with the reported Pb(II) complex, which is a double-line structure because of different carboxylates[24].Meanwhile, each L ligand also adopts a monodentate coordination mode, and it sits up and down the chain.Additionally, there are C–H···N and C–H···O hydrogen bonds derived from the pyridine, benzene, imidazloe of L and carboxylic groups of adip2-ligand. The hydrogen-bonding parameters of 1 are listed in Table 2. Moreover,π-πinteractions exist between the imidazole ring and the pyridine ring, the imidazole ring and the benzene ring of L ligand, as listed in Table 3. As a result, the complex extends into a three-dimensional supramolecule (Fig. 3) through hydrogen bonds andπ-πstackinginteractions, so that it is more stable.

Fig. 1. Coordination environment of Pb(II) ion in 1

Fig. 2. View of the one-dimensional chain

Fig. 3. View of the 3D supramolecular structure of complex 1

Table 2. Hydrogen Bonds for Complex 1

Table 3. Parameters between the Planes in 1

3. 2 IR analysis

IR spectra of 1 with the frequency range of 400～4000 cm-1are shown in Fig. S1. The absorption observed at 3150 cm−1in 1 can be attributed to theνN−Hstretching band of L ligand[25]. The weak absorption peaks of the -CH3group of L in 1 is observed at 2918 cm−1[26]. The peaks observed at 1602 cm−1for the complex is assigned to the stretching bands ofνas(COO−), whereas those at about 1390 cm−1to the stretching bands ofνs(COO−). The skeletal vibrations of phenyl and pyridyl rings vary from 1587 to 1471 cm−1. The strong bands in the range of 686～735 cm−1can be attributed to theν(C−N)stretching of the N-heterocyclic rings of the L ligand[27-29].

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

To confirm that the crystal structures are truly representative of the bulk materials, PXRD experiment was carried out for 1. As shown in Fig. 4, the experimental powder X-ray diffraction (PXRD) measured and simulated patterns of complex 1 show that the synthesized bulk material is the same as single crystal[30-33]. In order to characterize the thermal stability of 1, its thermal decomposition behavior was investigated at 30～800 °C under N2atmosphere using the thermal gravimetric analysis technique. As illustrated in Fig.S2, the weight loss appears in the range of 400～610 °C with a very significant loss, which may be corresponding to the decomposition of L ligands and adipate anions. Finally, a yellow residue of PbO (observed 21.16%, calculated 21.71%)is remained. TG result indicates that complex 1 possesses great thermal stability.

3. 4 Photoluminescent properties

The fluorescence properties of the L ligand and the solid phase of complex 1 were determined at room temperature (Fig. 5).The ligand of L has a relatively wide emission peak at 476 nm,upon excitation at 320 nm. And complex 1 also has a relatively wide emission peak at 514 nm, which is upon excitation at 320 nm, too. There is a red shift relative to the emission peak of the L ligand, which is caused by the inductive effect in the structure of L ligand.

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

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

4 QUANTUM CHEMISTRY CALCULATIONS

Quantum chemistry calculations in this manuscript were carried out with the Gaussian16 program[34]. The parameters of the molecular structure for calculation were all from the experimental data of the complex. Natural bond orbital (NBO)study was performed by density functional theory (DFT)[35]with the PBE0[36,37]hybrid functional and the LANL2DZ basis set[38-40].

The selected natural atomic charges, natural electron configuration, Wiberg bond indices and NBO bond orders (a.u)for 1 are listed in Table 5 with the electronic configurations of Pb(II) ion, nitrogen, and oxygen atoms being 6s1.956p0.63,2s1.34～1.362p4.08～4.13and 2s1.702p5.06～5.007, respectively. We infer that the Pb(II) ion coordination with N and O atoms is mainly on 6sand 6porbitals. Both N and O atoms form coordination bonds with Pb(II) ion using 2sand 2porbitals and Pb(II) ion obtained some electrons from two N atoms of different L ligands and two O atoms of two adip2-ligands[39,40]. Thus,according to valence-bond theory, the atomic net charge distribution and the NBO bond orders of complex 1 (See Table 4) show obvious covalent interaction between the coordinated atoms and Pb(II) ion. The differences of the NBO bond orders for Pb–O and Pb–N make their bond lengths different[40], which is in good agreement with the X-ray crystal structural data of 1.

Table 4. Selected Atom Net Charges, Electron Configurations, Wiberg Bond Indexes and NBO Bond Orders of 1

As can be seen from Fig. 6, the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) are mainly composed of L ligands.Thereby, ILCT may be inferred from some contours of the molecular orbital of 1.

Fig. 6. Frontier molecular orbitals of 1