English

• 1. INTRODUCTION

  Metal-organic frameworks (MOFs) as functional materials have attracted more and more research interest owing to their potential applications in the fields of luminescence sensing, gas storage and separation, heterogeneous catalysis and so on^[1-5]. The typical strategy for the construction of MOFs is to exploit the coordination covalent bonds to bridge metal ions via various polydentate ligands. In addition, there also exist some weak non-covalent interactions, such as hydrogen bonds, halogen-halogen interactions, metal-metal interactions, π-π interactions, van der Waals forces, etc., which can influence the structural topologies and properties of MOFs^[6-8]. Therefore, it is very important to understand and utilize these weak non-covalent interactions for the development of supramolecular chemistry and the control of MOFs structures^[9-12].

  Melamine, first obtained by Liebig, has three ring N atoms and three amidos that can act as hydrogen-bond acceptors and donors to form abundant hydrogen bonding network. This unique structural characteristic makes it been widely used for the construction of supramolecular coordination polymers in the past few decades^[13-15]. According to recent literatures, melamine can simultaneously undergo in situ reactions of hydrolysis, deaminization with the participation of Cu(Ⅱ) ions under hydrothermal conditions^{[16, 17]}. Using this in situ reaction, in this work, we successfully obtained a new luminescent Cu(Ⅰ) coordination polymer, 1. It is a 3D supramolecular framework connected via the cooperation of coordination covalent bonds, hydrogen bonds and π-π interactions, and can be simplified into a 5-connected BN topological network with the point symbol of {4⁶, 6⁴}.

  2. EXPERIMENTAL

  2.1 Materials and equipments

  All the starting reagents and solvents employed in this work were commercially purchased and used without further purification. Elemental analyses of C, H and N were performed on a Perkin-Elmer 240 elemental analyzer. The IR spectrum was recorded on a Nicolet Magna 750FT-IR spectrometer in the range of 400~4000 cm^–1. Power X-ray diffraction was recorded with a Bruker AXS D8 advanced automated diffractometer with CuKα radiation (λ = 1.5406 Å) at room temperature. Thermogravimetric analysis was carried out on a NetzschSTA499C integration thermal analyzer under a nitrogen atmosphere from 30 to 800 ℃ at a heating rate of 10 ℃/min. Optical diffuse reflectance spectra were measured at room temperature with a computer-controlled PE Lambda 900 UV/Vis spectrophotometer. The fluorescence measurement was performed on an Edinbergh Analytical instrument FLS920.

  2.2 Synthesis of [Cu₄(MA)₂(HCA)₂]_n

  A mixture of CuCl₂·2H₂O (0.2 mmol, 0.034 g), MA (1.0 mmol, 0.12 g), triethylamine (5 drops) and H₂O (12 ml) was placed in a 23 ml Teflon-lined stainless-steel reactor under autogenous pressure at 180 ℃ for 72 h and then cooled to room temperature at a rate of 2 ℃/min. Yellow block crystals were in 23% yield based on CuCl₂·2H₂O. Anal. Calcd. (%) for 1 C₁₂H₁₄Cu₄N₁₈O₆: C, 18.93; H, 1.84; N, 33.13. Found (%): C, 18.89; H, 1.87; N, 33.10. IR (KBr pellet, cm^–1) for 1: 3422(s), 3125(vs), 1664(m), 1635(m), 1582(m), 1487(vs), 1435(m), 1091(m), 856(vw), 772(w), 714(vw).

  2.3 Structure determination

  Structural determination was performed on a Riguka Saturn724 diffractometer equipped with a graphite-monochromatized MoKα radiation with a graphite-monochromatic radiation (λ = 0.71073 Å) at 293(2) K. A total of 9505 reflections were collected with 4200 unique ones (R_int = 0.0395) in the range of 2.57 < θ < 27.50º by using an ω-2θ scan mode. The structure was solved by direct methods and refined on F² by full-matrix least-squares methods using the SHELXL-97 program package^[18]. The final R = 0.0479, wR = 0.1243 (w = 1/[σ²(F_o²) + (0.0725P)² + 0.0000P], where P = (F_o² + 2F_c²)/3), S = 1.072 and (Δ/σ)_max = 0.001 for 3172 observed reflections (I > 2σ(I)). The maximum and minimum peaks on the final difference Fourier map are 0.583 and –0.764 e/ų, respectively. All non-hydrogen atoms were refined anistropically, and all hydrogen atoms attached to carbon were placed at their ideal positions. Selected bond lengths and bond angles of 1 are given in Table 1 and the detailed hydrogen bond lengths and bond angles are listed in Table 2.

  Table 1

  

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

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–N(7)	1.888(7)		Cu(1)–N(3)	1.903(8)		Cu(2)–N(9)	1.897(7)
  Cu(2)–N(13)	1.899(8)		Cu(3)–N(19)	1.896(7)		Cu(3)–N(14)	1.911(7)
  Cu(4)–N(2)	1.899(7)		Cu(4)–N(21)a	1.904(7)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(7)–Cu(1)–N(3)	164.4(3)		N(9)–Cu(2)–N(13)	165.9(3)		N(19)–Cu(3)–N(14)	163.2(3)
  N(2)–Cu(4)–N(21)a	165.1(3)
  Symmetry code: a: x, y+1, z

  Table 2

  

  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (º) of Compound 1

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  D–H⋅⋅⋅A	D–H/Å	H⋅⋅⋅A/Å	D⋅⋅⋅A/Å	D–H⋅⋅⋅A/º
  N(1)–H(1)⋅⋅⋅N(8)ⅰ	0.86	2.04	2.894(1)	175.0
  N(10)–H(10A)⋅⋅⋅O(6)ⅱ	0.86	2.14	2.998(1)	178.9
  N(11)– H(11A) ⋅⋅⋅O(6	0.86	2.07	2.908(1)	165.6
  N(12)– H(12A) ⋅⋅⋅O(4)ⅱ	0.86	2.00	2.864(1)	177.9
  N(12)–H(12B)⋅⋅⋅O(17)	0.86	2.12	2.956(1)	163.0
  N(15)–H(15)⋅⋅⋅N(20)ⅲ	0.86	2.09	2.950(1)	177.6
  N(22)–H(22B)⋅⋅⋅O(18)	0.86	2.08	2.924(1)	165.5
  N(23)–H(23A)⋅⋅⋅O(16)ⅳ	0.86	2.03	2.886(1)	177.7
  N(23)–H(23B)⋅⋅⋅O(5)ⅴ	0.86	2.08	2.911(1)	161.8
  N(24)–H(24B)⋅⋅⋅O(18)ⅳ	0.86	2.14	3.002(1)	179.6
  Symmetry codes: (ⅰ) 1 + x, 1 – y, –1/2 + z; (ⅱ) –1 + x, 1 – y, 1/2 + z; (ⅲ) 1 + x, –y, –1/2 + z; (ⅳ) –1 + x, –y, 1/2 + z; (ⅴ) x, –1 + y, z

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure

  X-ray structural analysis revealed that the asymmetric unit of 1 contains four independent Cu(Ⅰ) ions, two MA ligands and two HCA^2– ligands. The HCA^2– ligand was obtained by the in situ hydrolysis and deaminization of MA ligand, and the Cu(Ⅰ) ion was obtained by the in situ reduction of Cu(Ⅱ) ion. As shown in Fig. 1, each Cu(Ⅰ) ion displays a linear coordination mode, coordinated by two N atoms from one MA ligand and one HCA^2– ligand. The N–Cu–N bond angles are 164.9º, 165.6º, 164.3 and 164.2º, respectively. As displayed in Fig. 2a, the Cu(Ⅰ) ions are linked by alternating melamine and cyanuric acid ligands into a 1D zigzag chain structure. Additionally, the adjacent 1D zigzag chains are glued together by abundant N–H···O and N–H···N hydrogen bonds, generating a 2D layered hydrogen bonding network (Fig. 2b). It is noteworthy that there exist strong π-π interations (center···center distance: 3.539(3) Å) between the aromatic rings of MA and HCA^2– from two adjacent layers. Therefore, the adjacent layers are connected with each other via π-π interactions, affording a 3D supramolecular framework (Fig. 2c). In the 3D supramolecular framework, if the MA and HCA^2– ligands were reduced as 5-connected nodes, and the Cu(Ⅰ) ions were regarded as linear linkers, the whole 3D supramolecular framework can be regarded as a 5-connected BN topological network with the point symbol of {4⁶, 6⁴} (Fig. 2d).

  Figure 1

  

  Figure 1.  An ORTEP drawing of the molecular structure of compound 1 (50% thermal ellipsoids). All hydrogen atoms were omitted for clarity. Symmetry code: (a) x, y + 1, z

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  Figure 2

  

  Figure 2.  (a) 1D zigzag chain structure. (b) 2D hydrogen bonded layer of 1. Intermolecular and intramolecular hydrogen bonds are shown in dotted lines. (c) 3D supramolecular framework connected by hydrogen bonds and π-π interactions. (d) Schematic representation of 5-connected BN topological network for 1

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  3.2 Powder X-ray diffraction (PXRD) pattern, IR spectroscopy and thermal analysis (TGA)

  The purity of the compound was confirmed by the powder X-ray diffraction experiment, which has been carried out at room temperature. As shown in Fig. S1a, the main diffraction peaks match well with the simulated one obtained from the single-crystal diffraction data, indicating the good phase purity of compound 1.

  The peaks around 3422 and 3125 cm^–1 should be ascribed to the stretching vibrations of N–H. The peaks at about 1664~1091 cm^–1 can be attributed to stretching vibrations of the 1,3,5-triazine aromatic rings.

  In addition, thermal stability of the compound was also investigated in the temperature range of 30~800 ℃ under N₂ atmosphere. As shown in Fig. S1b, no obvious weight loss was observed in the temperature range of 30~272 ℃. Then, the framework of 1 begins to collapse above 272 ℃ owing to the decomposition of the organic ligands. The final residue of 36.9% may be Cu₂O (Calcd.: 37.8%).

  3.3 Diffuse reflectance UV-vis spectroscopy and luminescent property

  The reflectance diffusion spectrum of compound 1 is shown in Fig. 3a, and it displays intense absorption in the UV range (310~395 nm) that may be assigned to d-s charge transfer of Cu(Ⅰ) ions, and almost no absorption in the visible region (400~800 nm). Solid-state luminescent property of compound 1 was also studied at room temperature. As shown in Fig. 3b, upon excitation of 330 nm, compound 1 exhibits an intense emission band centered at 478 nm, which is similar to that of the inorganic solid containing isolated Cu(Ⅰ) ions^[19]. In order to ascertain the luminescence origin for 1, the luminescent properties of the free MA ligand and H₃CA ligand were also investigated at the same conditions. No obvious emission bands were observed in the visible light range. Therefore, the luminescence of compound 1 may be attributed to the d-s transition of Cu(Ⅰ) ions^[17]. The quantum yield (QY) of compound 1 is 28.75%, and the decay time (τ) is 18 ns.

  Figure 3

  

  Figure 3.  (a) Reflectance diffusion spectrum of compound 1. (b) Solid-state luminescent spectrum of compound 1 at room temperature

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  4. CONCLUSION

  In summary, a new luminescent Cu(Ⅰ) coordination polymer [Cu₄(MA)₂(HCA)₂]_n (1) was successfully synthesized via the simultaneous in situ hydrolysis, deaminization, reduction and self-assemble reaction under hydrothermal conditions. In compound 1, the Cu(Ⅰ) ions are alternately bridged by MA and HCA^2- ligands, affording a 1D zigzag chain structure. Adjacent 1D chains are further connected together via intermolecular hydrogen bonds, generating a 2D hydrogenbonded layer. Finally, these 2D hydrogen bonded layers are glued together via π-π interations, leading to a 3D supramolecular framework with 5-connected BN topology.

  
  
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• 

  Figure 1  An ORTEP drawing of the molecular structure of compound 1 (50% thermal ellipsoids). All hydrogen atoms were omitted for clarity. Symmetry code: (a) x, y + 1, z

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  Figure 2  (a) 1D zigzag chain structure. (b) 2D hydrogen bonded layer of 1. Intermolecular and intramolecular hydrogen bonds are shown in dotted lines. (c) 3D supramolecular framework connected by hydrogen bonds and π-π interactions. (d) Schematic representation of 5-connected BN topological network for 1

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  Figure 3  (a) Reflectance diffusion spectrum of compound 1. (b) Solid-state luminescent spectrum of compound 1 at room temperature

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–N(7)	1.888(7)		Cu(1)–N(3)	1.903(8)		Cu(2)–N(9)	1.897(7)
  Cu(2)–N(13)	1.899(8)		Cu(3)–N(19)	1.896(7)		Cu(3)–N(14)	1.911(7)
  Cu(4)–N(2)	1.899(7)		Cu(4)–N(21)a	1.904(7)
  Angle	(°)		Angle	(°)		Angle	(°)
  N(7)–Cu(1)–N(3)	164.4(3)		N(9)–Cu(2)–N(13)	165.9(3)		N(19)–Cu(3)–N(14)	163.2(3)
  N(2)–Cu(4)–N(21)a	165.1(3)
  Symmetry code: a: x, y+1, z

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  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (º) of Compound 1

  D–H⋅⋅⋅A	D–H/Å	H⋅⋅⋅A/Å	D⋅⋅⋅A/Å	D–H⋅⋅⋅A/º
  N(1)–H(1)⋅⋅⋅N(8)ⅰ	0.86	2.04	2.894(1)	175.0
  N(10)–H(10A)⋅⋅⋅O(6)ⅱ	0.86	2.14	2.998(1)	178.9
  N(11)– H(11A) ⋅⋅⋅O(6	0.86	2.07	2.908(1)	165.6
  N(12)– H(12A) ⋅⋅⋅O(4)ⅱ	0.86	2.00	2.864(1)	177.9
  N(12)–H(12B)⋅⋅⋅O(17)	0.86	2.12	2.956(1)	163.0
  N(15)–H(15)⋅⋅⋅N(20)ⅲ	0.86	2.09	2.950(1)	177.6
  N(22)–H(22B)⋅⋅⋅O(18)	0.86	2.08	2.924(1)	165.5
  N(23)–H(23A)⋅⋅⋅O(16)ⅳ	0.86	2.03	2.886(1)	177.7
  N(23)–H(23B)⋅⋅⋅O(5)ⅴ	0.86	2.08	2.911(1)	161.8
  N(24)–H(24B)⋅⋅⋅O(18)ⅳ	0.86	2.14	3.002(1)	179.6
  Symmetry codes: (ⅰ) 1 + x, 1 – y, –1/2 + z; (ⅱ) –1 + x, 1 – y, 1/2 + z; (ⅲ) 1 + x, –y, –1/2 + z; (ⅳ) –1 + x, –y, 1/2 + z; (ⅴ) x, –1 + y, z

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