English

• 0.   Introduction

  Rational design and synthesis of coordination polymers (CPs) is currently of significant interest notmerely due to the diverse network topology but mainly due to these extended systems playing a significant role in catalysis, chirality, luminescence, magnetism, nonlinear optics, adsorption, and separation^[1-4]. As is well known, applications of CPs highly depend on their structures, so design and construction of the CPs with desired structures and functions are extremely essential. There are many factors that influence the diversity of structures, such as central metal ions, organic ligands, metal-ligand ratio, solvents, temperature, pH value, and other factors^[5-8], which have been validated and summarized. Among them, the critical factor for the construction of CPs is the rational choice of organic building block. In our strategy, multidentate O-or N- donor ligands have been employed in the construction of CPs^[9-11]. So many polycarboxylate ligands are often employed as bridging ligands to construct CPs, due totheir extension ability both in covalent bonding and in supramolecular interactions^[12-14]. In contrast to polycar-boxylate ligands, the flexible etheroxygen dicarboxylate ligands have been employed in the construction of CPs.

  The oxybis(benzoic acid) (H₂oba) is a kind of typical flexible dicarboxylic acid ligands, such as 2, 2'-oxy- bis(benzoic acid), 2, 4-oxybis(benzoic acid) and 4, 4'- oxybis(benzoicacid), hasalsobeenreported^[15-17].Wealso employ 2, 2΄-oxybis(benzoic acid) and N-donor ligands to synthesize some complexes, such as [Zn(2, 2'-oba)(4, 4'-bipy)], [Zn(2, 2'-oba)(bpe)_0.5(H₂O)], [Cu(2, 2'-oba)(phen)], [Ni₂(2, 2'-oba)2(bpe)], [Co(2, 2'-oba)(2, 2'-bipy)(H₂O)₂] [Co(2, 2'-oba) (4, 4'-bimbp)_1.5] ·1.5H₂O^[18-20], among which [Zn(2, 2'-oba)(4, 4'-bipy)] shows a 3D metalorganic framework with a uninodal 4-connected 6⁵.8 dmp network. We continued our investigation and chose 2, 2' - H₂oba as a bridging ligand to react with Zn(Ⅱ)/Pb(Ⅱ) metal ions, and three new complexes were obtained, namely [Zn(2, 2'-oba)(bipy)₂]·2H₂O (1), [Pb(2, 2'-oba)(phen)]_n (2) and {[Pb(2, 2'-oba)(bimbp)]·H₂O}_n(3) (bipy=2, 2'-bipyridine, phen=1, 10-phenanthroline, bimbp=4, 4'- bis(imidazolyl)biphenyl). The syntheses, crystal structures and properties of three complexes are presented and discussed.

  1.   Experimental

  1.1   Materials and chemical analysis

  The ligands 2, 2'-H ₂oba, bipy, bimbp and phen were purchased from Jinan Henghua Sci. & Technol. Co., Ltd.; all other reagents and solvents employed were commercially available and used without further purification. Elemental analyses were performed with a Perkin-Elmer 2400 CHN Elemental analyzer. Infrared spectra on KBr pellets were recorded on a Nicolet 170SX FT -IR spect rophotometer in a range of 400~ 4 000 cm^-1. TG analyses were conducted with a Nietzsch STA 449C micro analyzer underatmosphere at a heating rate of 5 ℃·min^-1. The fluorescence spectra were studied using a Hitachi F-7100 fluorescence spectrophotometer at room temperature. All calcula-tions have been processed in Gaussian 09 package^[21].

  1.2   Synthesis of [Zn(2, 2′-oba)(bipy)₂]·2H₂O (1)

  A mixture of 2, 2'-H₂oba (0.1 mmol, 0.025 8 g), bipy (0.1 mmol, 0.015 6 g), Zn(NO₃)₂·6H₂O (0.1 mmol, 0.029 7 g) and H₂O (10 mL) was stirred evenly and heated in a 23 mL Teflon-lined autoclave at 140 ℃ for 4 days, followed by slow cooling (5 ℃·h^-1) to room temperature. The resulting mixture was washed with H₂O, and colorless block crystals of 1 were collected and dried in air. Yield: 53% (based on Zn). Elemental analysis Calcd. for C₃₄H₂₈N₄O₇Zn(%): C 60.95, H 4.21, N 8.36; Found(%): C 62.56, H 4.32, N 8.76. IR (KBr, cm^-1): 3 384(s), 3 044(m), 1 625(s), 1 548(s), 1 446(w), 1 396(vs), 1 234(m), 1 174(s), 796(s), 737(w), 681(vs).

  1.3   Synthesis of [Pb(2, 2′-oba)(phen)]n (2)

  A mixture of Pb(OAc)₂·3H₂O (0.1 mmol, 0.037 9 g), 2, 2'-H₂oba (0.1 mmol, 0.025 8 g), phen (0.1 mmol, 0.019 8 g) and H2O (10 mL) was stirred for 30 min inair. The mixture was transferred to a 23 mL Teflon reactor and kept at 140 ℃ for 5 days under autogenously pressure, and then cooled to room temperature at a rate of 5 ℃·h^-1. Colorless block crystals of 2 were obtained (Yield: 48% based on Pb). Elemental analysis Calcd. for C₂₆H₁₆N₂O₅Pb(%): C 48.52, H 2.51, N 4.35; Found(%): C 48.29, H 2.34, N 4.53. IR data (KBr, cm^-1): 1 646(vs), 1 589(s), 1 443(m), 1 378(s), 1 226(m), 1 137(s), 876(m), 787(m), 664(w).

  1.4   Synthesis of {[Pb(2, 2′-oba)(bimbp)]·H₂O}_n (3)

  Complex 3 was prepared with the method for 2 byusing bimbp (0.1 mmol, 0.028 6 g) instead of phen. Col-orless crystals of 3 were obtained (Yield: 56% based on Pb). Elemental analysis Calcd. for C₃₂H₂₄N₄O₆Pb(%): C 50.06, H 3.15, N 7.30; Found(%): C 50.27, H 3.34, N 7.36. IR (KBr, cm^-1): 3 354(m), 1 607(vs), 1 573(s), 1 506(s), 1378(s), 1237(w), 1187(s), 872(m), 779(m), 668(m).

  1.5   X-ray crystallographic studies

  Diffraction intensities for the three complexes were collected at 293 K on a Bruker SMART 1000 CCD diffractometer employing graphite-monochromated Mo Kα radiation (λ=0.071 073 nm). A semi-empirical absorption correction was applied using the SADABS program^[22]. The structures were solved by direct methods and refined by fullmatrix least-squares on F² using the SHELXS 2014 and SHELXL 2014 programs, respectively^[23-24]. Non-hydrogen atoms were refined an-isotropically, and H atoms bonded to C atoms were placed in calculated positions and water H atoms were refined in a riding mode. The crystallographic data for complexes 1~3 are listed in Table 1, and selected bond lengths and angles are listed in Table S1 (Supporting information).

  CCDC: 1973760, 1; 1973771, 2; 1973772, 3.

  Table 1

  

  Table 1.  Crystal data and structural refinement summary of the complexes 1~3

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  Complex	1	2	3
  Empirical formula	C34H28N4O7Zn	C26H16N2O5Pb	C32H24N4O6Pb
  Formula weight	669.98	643.60	767.74
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	C2/c	P1	P21/n
  a/nm	2.494 4(4)	0.880 98(8)	1.053 23(15)
  b/nm	1.631 1(4)	1.102 64(10)	1.968 0(3)
  c/nm	1.880 0(4)	1.173 01(11)	1.387 1(2)
  α/(°)		70.468 0(10)
  β/(°)	103.331(5)	84.660 0(10)	100.159(2)
  γ/(°)		77.030 0(10)
  V/nm3	7.443(3)	1.0460 31(17)	2.830 0(7)
  Dc/(g•cm-3)	1.421	2.043	1.802
  Z	8	2	4
  μ/mm-1	0.703	8.107	6.015
  Reflection collected, unique	18 354, 6 561 (Rint=0.059 2)	5 232, 3 632 (Rint=0.034 0)	13 789, 5 000 (Rint=0.026 5)
  Data, restraint, parameter	6 561, 4, 411	3 632, 0, 307	5 000, 6, 396
  Goodness-of-fit (GOF) on F2	1.014	1.092	1.039
  Final R indices [I>2σ(I)]	R1=0.056 9, wR2=0.140 9	R1=0.045 0, wR2=0.116 3	R1=0.033 5, wR2=0.089 4
  Largest difference in peak and hole/(e•nm-3)	579 and -569	2 662 and -4 674	1 476 and -943

  2.   Results and discussion

  2.1   Results and discussion

  2.1.1   Crystal structure of [Zn(2, 2'-oba)(bipy)₂]·2H₂O(1)

  Single crystal X-ray diffraction analysis suggests that compound 1 consists of one Zn(Ⅱ) ion, one 2, 2'-obaanions, two bipy molecules and two free water molecules. Each Zn(Ⅱ) center is six-coordinated by four pyridyl nitrogen donors from two bipy ligands and two oxy-gen atoms coming from one 2, 2'-oba ligand (Zn-N/O 0.210 7(3)~0.224 3(2) nm), forming a distorted ZnN₄O₂ octahedral geometry (Fig. 1). The O/N-Zn-O/N bond angles are in a range of 59.27(10)°~173.81(14)°. One carboxylate group of 2, 2'-oba ligand is not coordinated, and another one adopts μ₁-η¹-η¹ chelating mode to link one Zn(Ⅱ)ion (mode A in Scheme S1). Two phenyl rings are severely bent with a dihedral angle of 77.53°.The 2, 2'-oba ligand and two bipy ligands chelate with Zn(Ⅱ) to form a 0D structural unit. Through aromatic π-π stacking interactions between two bipy ligands (centroid-to-centroid distance: 0.372 0 nm), the adjacent units are bridged to form a 1D chain running along a-axis (Fig. 2). Due to the strong intermolecular O-H···O hydrogen bonds (between free water molecule and carboxylate O atoms of two 2, 2'-oba anions with the O6-H6A/H6B·O4/O4 distance of 0.283 9 nm/ 0.270 6 nm), the adjacent 1D chains are further extended to produce a 2D supramolecular framework (Fig. 3).

  Figure 1

  

  Figure 1.  Coordination environment of Zn(Ⅱ) ion of compound 1

  Hydrogen atoms and the free H₂O molecules are omitted for clarity; Displacement ellipsoids are drawn at 40% probability level

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  Figure 2

  

  Figure 2.  One-dimensional chain of 1

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  Figure 3

  

  Figure 3.  Two-dimensional supramolecular structure of 1

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  2.1.2   Crystal structures of [Pb(2, 2'-oba)(phen)]_n (2)

  Single-crystal X-ray analysis reveals that com-pound 2 shows an infinite 1D chain structure. Com-pound 2 is made up of the Pb(Ⅱ) ion, 2, 2'-oba ligand and phen ligand, and each Pb(Ⅱ) ion is coordinated to five oxygen atoms of three 2, 2'-oba ligands and two nitrogen atoms of one phen ligand, forming a distorted pentagonal biconical geometry, as shown in Fig. 4. The Pb-N/O bond lengths are in a range of 0.258 6(5)~0.284 1(6) nm, and the O/N-Pb-O/N bond angles are in a range of 63.2(2)°~145.2(2)°. As compared to compound 1, the carboxylic groups of 2, 2'-oba ligand have two coordination modes: one carboxylate group adopts μ₂-η¹-η² bridging mode to link two Pb(Ⅱ) ions, and another one adopts μ₂-η²-η⁰ bridging mode to link two Pb(Ⅱ) ion (mode B in Scheme S1), resulting in a 1D chain structure (Fig. 5). All phen ligands bristle out from the two sides of the chain, and further through aromatic π-π stacking interactions between two phen ligands (centroid-to-centroid distance: 0.356 6 nm), the adjacent chains are expanded to a 2D layer struc-ture, as shown in Fig. 6.

  Figure 4

  

  Figure 4.  Coordination environment of Pb(Ⅱ) ion incompound 2

  All hydrogen atoms are omitted for clarity; Symmetry codes: #1: x-1, y, z; #2: -x+1, -y, -z+2

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  Figure 5

  

  Figure 5.  One-dimensional chain of 2

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  Figure 6

  

  Figure 6.  Two-dimensional layer structure of 2

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  2.1.3   Crystal structures of {[Pb(2, 2'-oba)(bimbp)]·H₂O}_n (3)

  Single-crystal X-ray analysis reveals that the asymmetric unit of 3 contains one Pb(Ⅱ) ion, one 2, 2'-oba dianion, one bimbp ligand and one free water molecule. Each Pb(Ⅱ) ion is coordinated to three oxygen atoms of two 2, 2'-oba ligands and two nitrogen atoms oftwo bimbp ligands, forming a distorted triangular biconical geometry, as shown in Fig. 7. The Pb-N/O bondlengths are in a range of 0.229 1(4)~0.287 5(5) nm, andthe O/N-Pb-O/N bond angles are in a range of 78.84(15)°~163.59(18)°. In complex 3, the coordination mode of2, 2'-oba ligand is different from those of 1 and 2, (μ₁-η¹-η⁰ and μ₁-η¹-η¹ bridging mode, mode C in Scheme S1).The neighboring Pb(Ⅱ) ions are linked by 2, 2'-oba ligands to generate a loop shaped unit, and throughbimbp ligand bridging, the adjacent units are connected to produce a 1D double chain structure (Fig. 8). Fur-thermore, through strong intermolecular O-H·O hy-drogen bonds (between free water molecule and carbox-ylate O atoms of two 2, 2'-oba anions with the O6-H6A/H6B·O2/O5 distance of 0.284 6 nm/0.285 1 nm), theadjacent 1D chains are further extended to produce a 2D supramolecular framework (Fig. 9).

  Figure 7

  

  Figure 7.  Coordination environment of Pb(Ⅱ) ion in compound 3

  Hydrogen atoms are omitted for clarity; Symmetry codes: #1: -x+ 1, -y+2, -z+1; #2: x+1, y, z-1

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  Figure 8

  

  Figure 8.  One-dimensional double chain structure of 3

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  Figure 9

  

  Figure 9.  Two-dimensional layer structure of 3

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  2.2   Thermogravimetric analysis

  To study the thermal stability of 1~3, thermogravi-metric (TG) analyses were performed on polycrystalline samples under a nitrogen atmosphere with a heating rate of 10 ℃ ·min^-1 (Fig.S1~S3). The TG curve of 1 showed two weight loss steps. The first weight loss in a range of 70~100 ℃ (Obsd. 3.4%, Calcd. 5.37%) is assignable to the loss of free water. The second weight loss of 87.8% in a temperature range of 220~560 ℃ corresponds to the decomposition of the 2, 2'-oba and bipy ligands (Calcd. 84.87%). The TG curve of 2 suggested that no weight loss were observed until 230 ℃, above which, significant weight loss (Obsd. 68.2%) occurred and ended at about 580 ℃, indicating the complete decomposition of 2, 2'-oba and phen ligands (Calcd. 67.81%). TG curve of 3 showed the first weight loss in a range of 70~90 ℃ (Obsd. 2.6%, Calcd. 2.35%) is assignable to the loss of free water. The second weight loss of 71.5% in a temperature range of 210~540 ℃ corresponds to the decomposition of 2, 2'-oba and bimbp ligands (Calcd. 70.67%).

  2.3   Photoluminescence properties

  The luminescent emission spectra of 1~3 were examined in the solid state at room temperature and are shown in Fig. 10. The main emission peaks of free H₂oba and bimbp ligand appeared at 378 nm (λ_ex=334 nm) and 401 nm (λ_ex=340 nm), which can be assigned to the intra-ligand π-π* transitions^[25]. The bipy ligand exhibited two emission bands at 418 and 442 nm upon excitation at 389 nm. Complex 1 showed the main emission peak at 442 nm (λ_ex=380 nm), which is similar to that of bipy ligand, probably owing to bipy ligand-based charge transfer^[26-27]. Complex 2 had an intense emission peak at 350 nm (λ_ex=324 nm), showing a blue shift in comparison with those of free H₂oba and phen ligand (λ_em=384 nm, λ_ex=350 nm), which is attributed to ligand to metal charge transfer (LMCT)^[28-29]. Complex 3 exhibited one weaker emission peak at 385 nm upon excitation at 334 nm, having a red shift of ca. 7 nm relative to that of H₂oba ligand, which is attributed to H2oba ligand-based charge transfer. By comparing the emission spectra of 1~3 and ligands, we can conclude that the enhancement of luminescence in 1~3 may be attributed to the ligation of ligand to the metal center, which effectively increases the rigidity and reduces the loss of energy by radiationless decay^[30].

  Figure 10

  

  Figure 10.  Solid state luminescent emission spectra of complexes 1~3 and ligands

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  Figure 11

  

  Figure 11.  HOMO and LUMO of compound 1

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  2.4   TDDFT calculations of compound 1

  In order to reveal the photoluminescence emission of compound 1, we chose the structural unit from its single crystal X-ray diffraction data set and carried out its theoretical cal-culation. Based on the optimized geometries (with no constraints during the optimizations), time-dependent density functional (TDDFT) calculations were performed at the B3LYP level with 6-31G(d) basis set for C, H, N and O atoms, and effective core potentials basis set LanL2DZ for Zn atoms, em-ploying the Gaussian 09 suite of programs package.The characteristics of HOMO and LUMO of compound 1 are shown in Fig. 11. Based on the TDDFT energy level and MO analyses, S₀-S₁ transitions of compound 1 is mainly associated with the transitions from the corre- sponding HOMO to LUMO, inwhichtheelectron-density distribution of HOMO is totally resided at the coordi-nating π-orbital of one bipy ligand with the energy of -0.149 61 Hartree; however, the electron-density popu-lation of LUMO locates at π-orbital of other bipy ligand and the energy of LUMO was calculated to be -0.106 77 Hartree. Complexation of ligands with Znions usually reduces significantly the energy gaps between LUMOs and HOMOs^[31]. The energy difference of compound 1 between LUMO and HOMO was 0.042 84 Hartree (1.165 eV), and this is small enough to allow the charge transfer from HOMO to LUMO. According to this observation, it is proposed that the essence of the photoluminescence of compound 1 could be-signed to the ligand to ligand charge transfer. This cal-culation result is in good agreement with the experi-mental observations.

  3.   Conclusions

  In summary, three complexes have been synthe-sized by the self-assembly of M(Ⅱ) (M=Zn and Pb) salts with 2, 2'-H₂oba and auxiliary N-donor Ligands. Assem-blies of these complexes generate two types of diverse frameworks: one 0D structure and two 1D chains. More⁃over, the solid-state fluorescent properties of 1~3 have also been investigated. According to the crystal struc-tures, the TDDFT/6 -31G(d) approach was applied to study the photoluminescence emission of 1, the calcu-lated results are in good agreement with the experimental results.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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• 

  Figure 1  Coordination environment of Zn(Ⅱ) ion of compound 1

  Hydrogen atoms and the free H₂O molecules are omitted for clarity; Displacement ellipsoids are drawn at 40% probability level

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  Figure 2  One-dimensional chain of 1

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  Figure 3  Two-dimensional supramolecular structure of 1

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  Figure 4  Coordination environment of Pb(Ⅱ) ion incompound 2

  All hydrogen atoms are omitted for clarity; Symmetry codes: #1: x-1, y, z; #2: -x+1, -y, -z+2

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  Figure 5  One-dimensional chain of 2

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  Figure 6  Two-dimensional layer structure of 2

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  Figure 7  Coordination environment of Pb(Ⅱ) ion in compound 3

  Hydrogen atoms are omitted for clarity; Symmetry codes: #1: -x+ 1, -y+2, -z+1; #2: x+1, y, z-1

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  Figure 8  One-dimensional double chain structure of 3

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  Figure 9  Two-dimensional layer structure of 3

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  Figure 10  Solid state luminescent emission spectra of complexes 1~3 and ligands

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  Figure 11  HOMO and LUMO of compound 1

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  Table 1.  Crystal data and structural refinement summary of the complexes 1~3

  Complex	1	2	3
  Empirical formula	C34H28N4O7Zn	C26H16N2O5Pb	C32H24N4O6Pb
  Formula weight	669.98	643.60	767.74
  Crystal system	Monoclinic	Triclinic	Monoclinic
  Space group	C2/c	P1	P21/n
  a/nm	2.494 4(4)	0.880 98(8)	1.053 23(15)
  b/nm	1.631 1(4)	1.102 64(10)	1.968 0(3)
  c/nm	1.880 0(4)	1.173 01(11)	1.387 1(2)
  α/(°)		70.468 0(10)
  β/(°)	103.331(5)	84.660 0(10)	100.159(2)
  γ/(°)		77.030 0(10)
  V/nm3	7.443(3)	1.0460 31(17)	2.830 0(7)
  Dc/(g•cm-3)	1.421	2.043	1.802
  Z	8	2	4
  μ/mm-1	0.703	8.107	6.015
  Reflection collected, unique	18 354, 6 561 (Rint=0.059 2)	5 232, 3 632 (Rint=0.034 0)	13 789, 5 000 (Rint=0.026 5)
  Data, restraint, parameter	6 561, 4, 411	3 632, 0, 307	5 000, 6, 396
  Goodness-of-fit (GOF) on F2	1.014	1.092	1.039
  Final R indices [I>2σ(I)]	R1=0.056 9, wR2=0.140 9	R1=0.045 0, wR2=0.116 3	R1=0.033 5, wR2=0.089 4
  Largest difference in peak and hole/(e•nm-3)	579 and -569	2 662 and -4 674	1 476 and -943

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