English

• 1. INTRODUCTION

  Coordination polymers formed from metal ions and functional organic ligands have attracted considerable attentions for their potential applications in gas storage and separation^{[1, 2]}, luminescence^[3], magnetism^[4], catalysis^[5], proton conduction^[6], and second-order nonlinear optics^[7]. For coordination polymers, the functionality is introduced from a cooperative influence of both inorganic components of metal ions and organic components of bridging ligands. Thus the organic component as a linking molecule plays an essential role in the formation of polymeric structures. The carboxylate ligands are the extensive studied ligands due to their diverse coordination modes, which result in a large amount of coordination polymers showing interesting structural topologies and desirable physical properties^{[8, 9]}. It has been documented that the introduction of other functional groups into the carboxylate ligands will afford novel coordination polymers with interesting physical properties. For example, the introduction of hydrophilic sulfonate groups into the carboxylate ligands has resulted in high proton conductivity for the lanthanide compounds^{[10, 11]}. The highly-thermostable and hydrophobic coordination polymers can be achieved by introduing trifluoromethyl into the carboxylate ligands^[12-14]. In this paper, the 3, 3΄, 5, 5΄-tetrafluo- robiphenyl-4, 4΄-dicarboxylic acid (H₂-TFBPDC) ligand was used to construct the fluorinated coordination polymer. In the presence of adenine ligand, a novel compound of [Cd₂(TFBPDC)₂(ad)₂(CH₃OH)₂]_n (1) was obtained. The utili- zation of the adenine ligand is based on the fact that it can serve as a bridging ligand and involve in the hydrogen bonds with its amine group. In addition, Cd(II) was selected because it has a 4d¹⁰ outer electron configuration and a relatively larger ionic radius, which leads to interesting coordination polyhedra. On the other hand, the intrinsic electronic properties of Cd(II) ion make them particularly attractive for the preparation of luminescent coordination compounds. Herein, the synthesis, FT-IR spectrum, powder-X-ray diffraction, crystal structure, and luminescent properties of compound 1 are presented.

  2. EXPERIMENTAL

  2.1 Materials and instruments

  FT-IR spectrum (KBr pellet) was recorded on the PerkinElmer Spectrum One. The thermogravimetric measure- ment was performed with a Netzsch STA449C apparatus in the Al₂O₃ containers with a heating rate of 10 ℃/min from 30 to 800 ℃. Fluorescence spectra were measured with an Edinburgh FLS980 fluorescence spectrophotometer. Powder X-ray diffraction pattern was performed on a Rigaku Miniflex 600 powder diffractometer (Cu-Kα radiation, λ = 1.5418 Å).

  2.2 Synthesis of [Cd₂(TFBDC)₂(ad)₂(CH₃OH)₂]_n (1)

  A mixture of Cd(NO₃)₂·4H₂O (0.0062 g, 0.02 mmol), adenine (0.0008 g, 0.006 mmol) and 3, 3΄, 5, 5΄-tetrafluoro- biphenyl-4, 4΄-dicarboxylic acid (0.0031 g, 0.01 mmol) in 1.8 mL methanol was sealed in a 25 mL Parr Teflon-lined stainless-steel vessel. The vessel was sealed and heated to 65 ℃. This temperature was kept for 3 days and then the mixture was cooled naturally to form colorless crystals of 1. The crystalline product was dried at ambient temperature (yield: 53.1% on the basis of 3, 3΄, 5, 5΄-tetrafluoro- biphenyl-4, 4΄-dicarboxylic acid). IR (KBr pellet, v, cm^-1): 3534 (m), 3433 (m), 3320 (m), 3125 (m), 1697 (m), 1661 (m), 1626 (s), 1589 (w), 1515 (w), 1428 (w), 1382 (s), 1299 (w), 1234 (w), 1190 (w), 1139 (m), 1030 (s), 948 (w), 837 (s), 727 (m), 681 (w), 588 (m), 550 (w).

  2.3 Single-crystal structure determination

  Single-crystal X-ray diffraction experiment was carried on a Rigaku Oxford SuperNova Single Source diffractometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) using the ω-φ scan mode (3.432 < θ < 26.371º). A single crystal of 1 with dimensions of 0.16mm × 0.12mm × 0.10mm was selected and mounted on a glass fiber. A total of 12601 reflections were collected and 4158 were independent (R_int = 0.0284), of which 3791 were observed with I > 2σ(I). CrysAlisPro software was used for collecting the frames of data, indexing the reflections, and determining the lattice constants, absorption correction and data reduction^[15]. The structure was solved by direct methods and successive Fourier difference syntheses (SHELXT-2015)^[16], and refined by the full-matrix least-squares method on F² (SHELXTL- 2015)^[17]. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were assigned to the calculated positions. The R values are defined as R = Σ||F_o| – |F_c||/Σ|F_o| and wR = {Σ[w(F_o² – F_c²)²]/ Σ[w(F_o²)²]}^1/2. The final R = 0.0284 and wR = 0.0635 (w = 1/[σ²(F_o²) + (0.0319P)² + 0.1908P], where P = (F_o² + 2F_c²)/3) for 3791 observed reflections. (∆/σ)_max = 0.002, (∆ρ)_max = 0.414 and (∆ρ)_min = –0.387 e/ų. The selected important bond distances are listed in Table 1.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure description

  Compound 1 exhibits a 1D double chain. The asymmetric unit of 1 is composed of one Cd(II) ion, one dianionic TFBPDC^2– ligand, one ad ligand, and one coordinated methanol molecule. Cd(1) ion is octahedrally coordinated by an imidazole N and a pyrimidine N from two ad ligands, three carboxylate oxygen atoms of three TFBPDC^2– ligands, and one methanol oxygen atom (Fig. 1). The two N atoms and O(5) as well as O(4B) atoms occupy the equatorial plane, and the O(1) and O(4C) atoms take up the axial positions with the O(1)‒Cd(1)‒O(4C) angle of 168.00(7)° (Fig. 1). The Cd‒N bond distances are 2.290(2) and 2.358(2) Å (Table 1). The Cd‒O bond distances vary from 2.2542(17) to 2.5728(19) Å (Table 1). These bond distances are similar to those in reported Cd(II) compounds^[18]. The ad ligand bridges two Cd(II) atoms through its one imidazole nitrogen (N(1)) and one pyrimidine nitrogen (N(4)) atoms (Fig. 1). Two Cd(II) atoms are bridged by two symmetrically related ad ligands to form a dinuclear [Cd₂(ad)₂] unit, as shown in Fig. 2. Each TFBPDC^2– ligand is coordinated to three Cd(II) atoms with a μ₂-O and an unidentate O atoms from two carboxylate groups (Fig. 2). In the dinuclear [Cd₂(ad)₂] unit, the two Cd(II) atoms are further bridged by a pair of μ₂-O of two TFBPDC^2– ligands with a Cd(1)‒O(4B)‒Cd(1A) angel of 93.55(6)° (Fig. 2). The dinuclear [Cd₂(ad)₂] units are linked by the TFBPDC^2– ligands to give a 1D double chain running along the c axis. Within the chain, π-π interactions between the parallel benzene rings with a center-to-center distance of 3.767 Å were observed (Fig. 2). The adjacent 1D chains are linked by the N(5)–H(5A)⋅⋅⋅O(3)^#1 and N(5)–H(5B)⋅⋅⋅O(2)^#2 hydrogen bonds between the amino group and carboxylate O atoms, and the N(2)–H(2)⋅⋅⋅O(3)^#2 hydrogen bond between the imidazole and carboxylate O atom to generate a 2D layer extended along the ac plane (Fig. 3 and Table 2). The 2D layer is further stabilized by interchain π-π interactions between the parallel imidazole rings with a center-to-center distance of 3.948 Å (Fig. 3). As depicted in Fig. 4, the neighboring 2D layers are linked by the O(5)–H(5)⋅⋅⋅N(3)^#3 hydrogen bond between the coordinated methanol molecule and pyrimidine N atom with the O⋅⋅⋅N separation of 2.777(3) Å to give a 3D framework.

  Figure 1

  

  Figure 1.  Coordination environment for Cd(II) in 1

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  Table 1

  

  Table 1.  Selected Bond Lengths (Å) of 1

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–O(1)	2.2542(17)		Cd(1)–O(4B)	2.5728(19)		Cd(1)–O(4C)	2.3356(15)
  Cd(1)–O(5)	2.353(2)		Cd(1)–N(1)	2.290(2)		Cd(1)–N(4A)	2.358(2)
  Symmetry codes: A: 1–x, –y, 2–z; B: 1–x, 1–y, 1–z; C: x, y–1, z

  Figure 2

  

  Figure 2.  1D double chain of 1 (The dotted lines represent π-π interactions)

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  Figure 3

  

  Figure 3.  View of the 2D hydrogen-bonded layer of 1 (The dotted lines represent hydrogen bonds or π-π interactions)

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  Table 2

  

  Table 2.  Hydrogen Bonds of Compound 1 (Å and °)

  DownLoad: CSV

  D–H⋅⋅⋅A	d(D–H)	d(H⋅⋅⋅A)	d(D⋅⋅⋅A)	< DHA
  O(5)–H(5)⋅⋅⋅N(3)#1	0.85	1.94	2.777(3)	167
  N(2)–H(2)⋅⋅⋅O(3)#2	0.86	1.97	2.764(3)	153
  N(5)–H(5A)⋅⋅⋅O(2)#3	0.86	2.61	3.199(3)	127
  N(5)–H(5B)⋅⋅⋅O(3)#2	0.86	2.05	2.895(3)	165
  Symmetry codes: #1: x, y+1, z; #2: –x+2, –y+1, –z+1; #3: x, y–1, z.

  Figure 4

  

  Figure 4.  View of the 3D framework of 1, showing the interlayered hydrogen bonds with dotted lines

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  3.2 Powder X-ray pattern, IR spectrum and thermogravimetric analysis

  The purity of the bulk samples of 1 was checked by powder X-ray pattern (PXRD). The experimental PXRD pattern for compound 1 is close to that of the simulated pattern generated from the single-crystal diffraction data (Fig. S1), which indicates the bulk of crystals of 1 are pure product. The thermal behavior of 1 was studied. Compound 1 is stable from room temperature to 130 ℃ (Fig. S2). When above 130 ℃, the loss of weight was observed. The first weight loss between 130 to 235 ℃ corresponded to the removal of methanol molecules (observed 5.93%, calculated 5.41%). The combustion of the organic ligands occurred at 280 ℃.

  The O–H stretching vibrations of methanol appeared at 3534 cm^–1 in the IR spectrum of 1. The stretching vibration of the hydroxyl group of methanol was found at 1697 cm^–1. The strong peaks at 1626 and 1382 cm^–1 can be assigned to the asymmetric and symmetric stretching vibrations of carboxylate groups for the TFBPDC^2– ligand. The peak at 3443 cm^–1 can be attributed to the amine group of adenine ligand. The strong absorption at 1030 cm^–1 is the C–F stretching.

  3.3 Photoluminescence properties

  The d¹⁰ transition metal ions without d-d transition have intrinsic electronic properties. Thus, their coordination compounds possess various emitting levels. The solid state excitation and emission spectra of 1 were studied under room temperature. As depicted in Fig. 5, the excitation spectrum of 1 shows an excited band between 320 and 400 nm with a maximum peak at 350 nm, which is mainly assigned to the π-π* transition of organic ligand. Upon excitation at 350 nm, the solid sample of 1 displayed an emission band from 400 to 550 nm with a centered peak at 425 nm. It has been reported that the adenine ligand showed the emission band between 395 and 550 with the maximum peak at 400 nm^[19]. Thereby, the photoluminescent emission of compound 1 can be attributed to the adenine ligand centered π-π* transition. Compared with the emission peak of adenine ligand, the emission peak for compound 1 is bathochromically shifted, resulting from the coordination of adenine to the metal ions.

  Figure 5

  

  Figure 5.  Excitation and emission spectra for 1 (λ_ex = 350 nm) in the solid state

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  4. CONCLUSION

  In conclusion, a coordination polymer of [Cd₂(TFBPDC)₂(ad)₂(CH₃OH)₂]_n (1) constructed from 3, 3΄, 5, 5΄-tetrafluorobiphenyl-4, 4΄-dicarboxylate and adenine mixed ligand has been reported. Compound 1 has been characterized by FT-IR spectrum, single-crystal X-ray diffraction, PXRD, and TG analysis. Compound 1 is a 1D ribbon structure, which is further packed into a 3D framework through hydrogen bonds. Compound 1 shows ligand centered emission.

  
  
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• 

  Figure 1  Coordination environment for Cd(II) in 1

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  Figure 2  1D double chain of 1 (The dotted lines represent π-π interactions)

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  Figure 3  View of the 2D hydrogen-bonded layer of 1 (The dotted lines represent hydrogen bonds or π-π interactions)

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  Figure 4  View of the 3D framework of 1, showing the interlayered hydrogen bonds with dotted lines

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  Figure 5  Excitation and emission spectra for 1 (λ_ex = 350 nm) in the solid state

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  Table 1.  Selected Bond Lengths (Å) of 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–O(1)	2.2542(17)		Cd(1)–O(4B)	2.5728(19)		Cd(1)–O(4C)	2.3356(15)
  Cd(1)–O(5)	2.353(2)		Cd(1)–N(1)	2.290(2)		Cd(1)–N(4A)	2.358(2)
  Symmetry codes: A: 1–x, –y, 2–z; B: 1–x, 1–y, 1–z; C: x, y–1, z

  下载: 导出CSV

  Table 2.  Hydrogen Bonds of Compound 1 (Å and °)

  D–H⋅⋅⋅A	d(D–H)	d(H⋅⋅⋅A)	d(D⋅⋅⋅A)	< DHA
  O(5)–H(5)⋅⋅⋅N(3)#1	0.85	1.94	2.777(3)	167
  N(2)–H(2)⋅⋅⋅O(3)#2	0.86	1.97	2.764(3)	153
  N(5)–H(5A)⋅⋅⋅O(2)#3	0.86	2.61	3.199(3)	127
  N(5)–H(5B)⋅⋅⋅O(3)#2	0.86	2.05	2.895(3)	165
  Symmetry codes: #1: x, y+1, z; #2: –x+2, –y+1, –z+1; #3: x, y–1, z.

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•