English

• 1. INTRODUCTION

  Lanthanide coordination polymers have been receiving wide attention for their fascinating structures and potential applications in gas storage/separation, magnetism, and luminescence^[1-5]. To date, a large number of lanthanide coordination polymers have been successfully synthesized with interesting architectures and topologies^[6-11]. For example, Long et al reported several unprecedented seven- and eight-connected lanthanide coordination networks by using the 4, 4'-bipyridine-N, N'-dioxide ligand and La³⁺ ions^[12]. These structures suggest that lanthanide ions can adopt high coordination numbers and can be employed for the construction of highly connected frameworks. Yang and coworkers reported a series of porous lanthanide-organic open frameworks with helical tubes constructed from interweaving triple-helical and double-helical chains using 4, 5-imidazoledicarboxylic ligand^[13]. However, the design of lanthanide contained architectures with predetermined structures is still a great challenge, perhaps due to the flexible coordination environments, high and variable coordination numbers of the lanthanide ions.

  The Zheng group proposed the synthesis of lanthanide clusters via a ligand-controlled hydrolytic approach with the judiciously chosen supporting ligands to limit the degree of lanthanide hydrolysis^{[14, 15]}. At present, the largest reported lanthanide clusters is a wheel shaped Gd₁₄₀. This nano-sized molecular wheel shows 10-fold symmetry and possesses the largest diameter of 6.0 nm, and displays high stability in solution^[16]. Lanthanide clusters can be used to design high-connected coordination polymers for their larger sizes, more coordination sites, and smaller steric hindrance for ligands. For example, Liu and coworkers reported a series of binodal (4, 8)-connected network with cubane-shaped Ln₄ clusters and 4, 8-disulfonyl-2, 6-naphthalene dicarboxylic acid ligand^[17]. In addition, Ln₄ clusters can be linked by organic 3-sulfobenzoate ligand or inorganic SO₄^2- to form the (3, 12)-connected network^[18]. The architecture of Ln₄(OH)₄(SO₄)₄(H₂O)₃ contains helical tubes and channels constructed from double left- and right-handed helical chains^[18b].

  Recently, we have reported a series of lanthanideorganic frameworks constructed by Ln₄(OH)₄ clusters and mixed ligands of 4, 4'-oxybis(benzoate) (oba) and 2-pyrazinecarboxylic acid^[19]. In this work, we use 4, 4'-oxybis(benzoate) and 2, 3-pyridinedicarboxylic acid ligands to continue our search for new lanthanide coordination polymers based on the following considerations: the cooperativity of both ligands can be used to understand the effect of ligands on the lanthanide-organic frameworks and may provide new opportunities for the discovery of interesting structures. Herein, we report the systematic synthesis, structure and topology analysis of a new lanthanide coordination polymer, [Er₃(oba)₄-(na)]_n (1, na = nicotinic acid) under hydrothermal conditions. Compound 1 exhibits a 3D lanthanideorganic framework with an unusual (3, 11)-connected net, which consists of trinuclear lanthanide building units and the mixed ligands of oba^2- and na^-.

  2. EXPERIMENTAL

  2.1 Materials and measurements

  All chemicals were purchased commercially and used without further purification. Powder X-ray diffraction (PXRD) data were obtained by placing the picked crystals onto the flat sample holders using a Philips PW3040/60 diffractometer with CuKα radiation (λ = 1.54056 Å). Elemental analyses were performed on a Vario EL III elemental analyzer. The Fourier transform infrared (FT-IR) spectra (KBr pellets) were recorded on a Nicolet NEXUS670 spectrometer. Thermogravimetric analyses (TGA) was performed on a Netzsch STA 449C analyzer at a heating rate of 10 ℃/min under an air atmosphere.

  2.2 Synthesis

  Synthesis of [Er₃(oba)₄(na)]_n (1). A mixture of Er₂O₃ (0.5 mmol, 0.191 g), H₂oba (0.5 mmol, 0.129 g), 2, 3-pyridinedicarboxylic acid (1 mmol, 0.167 g), HNO₃ (0.5 mmol), and H₂O (10 mL) was sealed in a 23 mL Teflon-lined bomb at 170 ^oC for 6 days and then slowly cooled to room temperature. The pink crystals of 1 were obtained (yield: 51% based on H₂oba). Anal. Calcd. for C₆₂H₃₆Er₃NO₂₂ (1): C, 45.17; H, 2.20; N, 0.85%. Found: C, 44.86; H, 2.43; N, 0.98%. IR bands (cm^-1) for 1: 3430 (vs), 2976 (w), 1591 (s), 1518 (s), 1406 (vs), 1240 (s), 1156 (m), 1090 (w), 1044 (w), 877 (m), 774 (m), 700 (w), 635 (w), 570 (w), 505 (w).

  2.3 Crystal structure determination

  The intensity data were collected on a Bruker APEX II with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All absorption corrections were performed using the multi-scan program. The structure was solved by direct methods and refined by full-matrix leastsquares on F² with the SHELXS-97 and SHELXL-97 programs^[20]. The hydrogen atoms of ligands were calculated with the riding model. All atoms except H atoms were refined anisotropically. Selected bond lengths of compounds 1 are listed in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) for 1

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  Bond	Dist.		Bond	Dist.
  Er(1)–O(4A)	2.229(5)		Er(2)–O(18)	2.319(5)
  Er(1)–O(16B)	2.231(6)		Er(2)–O(6)	2.325(5)
  Er(1)–O(22C)	2.239(4)		Er(2)–O(7)	2.634(4)
  Er(1)–O(11D)	2.245(4)		Er(2)–O(9)	2.637(5)
  Er(1)–O(1)	2.336(5)		Er(3)–O(21B)	2.196(4)
  Er(1)–O(7)	2.340(4)		Er(3)–O(17E)	2.218(5)
  Er(1)–O(2)	2.671(5)		Er(3)–O(12F)	2.219(5)
  Er(2)–O(5A)	2.213(5)		Er(3)–O(9)	2.276(5)
  Er(2)–O(13)	2.219(5)		Er(3)–O(14)	2.318(5)
  Er(2)–O(2)	2.279(5)		Er(3)–O(19)	2.334(6)
  Er(2)–O(8)	2.303(5)		Er(3)–O(18)	2.592(6)
  Symmetry codes for 1: (A) –x, –y + 1, z – 1/2; (B) x + 1/2, –y + 1/2, z; (C) –x, –y, z – 1/2; (D) –x + 1, –y + 1, z –1/2; (E) –x, –y + 1, z + 1/2; (F) x – 1/2, –y + 3/2, z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of the title complex

  Structure of [Er₃(oba)₄(na)]_n (1). Single-crystal X-ray analysis reveals that compound 1 crystallizes in the noncentrosymmetric orthorhombic space group Pna2₁^[21]. The asymmetric unit of 1 contains three Er³⁺ ions, four oba^2- ligands and one na^- ligand. Interestingly, decarboxylation occurred in the ortho position of 2, 3-pdc (2, 3-pdc = 2, 3-pyridinedicarboxylic acid) which was transformed into na^- under hydrothermal conditions^[22]. The Er(1) and Er(3) ions are seven-coordinated with a capped trigonalantiprism and a capped trigonal-prism coordination environment, respectively (seven carboxylate oxygen atoms (O_COO) from four oba^2- ligands and one na ligand for Er(1); seven O_COO from six oba^2- ligands for Er(3)). The Er(2) ion is eight-coordinated with a bicapped trigonal-prism coordination environment: eight O_COO from five oba^2- ligands and one na^- ligand. The oba^2- ligands show two types of coordination modes (Fig. S1). The Er−O distances range from 2.196(4) to 2.671(5) Å (Table 1), which are similar to the reported results^{[19, 22]}. Three crystallographically unique Er³⁺ ions are linked by six O_COO to give a tri-nuclear building block [Er₃(COO)₆] {Er₃} (Fig. 1a). The adjacent {Er₃} units are further linked via O_COO groups of oba^2- ligands to form 1D chains along the c axis (Fig. 1b). Each chain connects with neighboring others through oba²⁻ ligands to give rise to the final framework (Fig. 1c). In order to better identify the nature of the intricate framework, suitable connectors can be defined by using a topological approach. As shown in Fig. 1a and 2a, each tri-nuclear {Er₃} is surrounded by eleven oba^2- ligands and one terminal na^- ligand, and acts as an 11-connected node (Fig. 1a); while oba^2- ligands bridge three neighboring Er₃(CO₂)₆ units and serve as a 3-connected node (Fig. 2a). In this way, the network of 1 can be simplified as a (3, 11)-connected net (Fig. 2b).

  Figure 1

  

  Figure 1.  (a) Tri-nuclear lanthanide building block in 1. (b) 1D chain in 1. (c) 3D framework along the b-axis in 1

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  Figure 2

  

  Figure 2.  (a) View of the 3-connected oba^2- ligand. (b) Schematic representation of the (3, 11)-connected net

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  3.2 Physical characterization

  The experimental powder X-ray diffraction (PXRD) patterns of compound 1 match well with the simulated PXRD pattern. The difference in reflection intensities between the simulated and experimental patterns was due to the variation in the preferred orientation of the powder sample during the collection of experimental PXRD data (Fig. 3). The thermal behavior of 1 was examined by Thermogravimetric analyses (TGA) in a dry air atmosphere from 30 to 800 ℃. Compound 1 has a one step of weight loss. Above 500 ℃, the weight loss is due to the decomposition of ligands (calcd./found, 65.2/64.0%) (Fig. S2). The strong and broad absorption bands around 3400 cm⁻¹ in the IR spectra of 1 are assigned as characteristic peak of OH vibration. The strong vibrations appearing around 1592 and 1406 cm⁻¹ are corresponding to the asymmetric and symmetric stretching vibrations of the carboxylate group, respectively (Fig. S3). The second-harmonic-generation (SHG) measurements were tested for compound 1's non-centrosymmetric space group Pna2₁. Compound 1 shows a very weak SHG response of ∼0.03 KDP (KH₂PO₄) via nonlinear-optical (NLO) measurement under a 1064 nm laser beam (Fig. S4).

  Figure 3

  

  Figure 3.  Experimental and simulated PXRD patterns of 1

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  4. CONCLUSION

  In summary, we have successfully constructed a 3D lanthanide coordination polymer comprising tri-nuclear lanthanide building units and mixed ligands of oba^2- and na^- under hydrothermal conditions. The framework of 1 exhibits a (3, 11)-connected net, and displays weak second-harmonic generation response for its non-centrosymmetric space group. Our studies indicate that using different mixed-ligands is an effective approach to preparing novel versatile lanthanide-containing coordination frameworks. Further work on this subject is in progress.

  
  
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• 

  Figure 1  (a) Tri-nuclear lanthanide building block in 1. (b) 1D chain in 1. (c) 3D framework along the b-axis in 1

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  Figure 2  (a) View of the 3-connected oba^2- ligand. (b) Schematic representation of the (3, 11)-connected net

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  Figure 3  Experimental and simulated PXRD patterns of 1

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  Table 1.  Selected Bond Lengths (Å) for 1

  Bond	Dist.		Bond	Dist.
  Er(1)–O(4A)	2.229(5)		Er(2)–O(18)	2.319(5)
  Er(1)–O(16B)	2.231(6)		Er(2)–O(6)	2.325(5)
  Er(1)–O(22C)	2.239(4)		Er(2)–O(7)	2.634(4)
  Er(1)–O(11D)	2.245(4)		Er(2)–O(9)	2.637(5)
  Er(1)–O(1)	2.336(5)		Er(3)–O(21B)	2.196(4)
  Er(1)–O(7)	2.340(4)		Er(3)–O(17E)	2.218(5)
  Er(1)–O(2)	2.671(5)		Er(3)–O(12F)	2.219(5)
  Er(2)–O(5A)	2.213(5)		Er(3)–O(9)	2.276(5)
  Er(2)–O(13)	2.219(5)		Er(3)–O(14)	2.318(5)
  Er(2)–O(2)	2.279(5)		Er(3)–O(19)	2.334(6)
  Er(2)–O(8)	2.303(5)		Er(3)–O(18)	2.592(6)
  Symmetry codes for 1: (A) –x, –y + 1, z – 1/2; (B) x + 1/2, –y + 1/2, z; (C) –x, –y, z – 1/2; (D) –x + 1, –y + 1, z –1/2; (E) –x, –y + 1, z + 1/2; (F) x – 1/2, –y + 3/2, z

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