English

• 1. INTRODUCTION

  Metal-organic frameworks (MOFs), as one kind of crystal porous materials, have been developed quickly in recent years due to the application of gas adsorption, catalysis, fluorescence sensing, proton conduction and so on^[1-10]. Recently, increasing attention has been paid to the development of MOFs synthetic strategies. In the available proposal, template-directing method as one more effective strategy has been focused on the synthesis of MOFs^[11]. Up to now, different template such as anions (polyoxometalates, acid anions and so on), cations (alkali/alkali earth metal cations, metal complex cations and so on) and neutral molecules (organic species, water clusters and so on) have been adopted for the synthesis of MOFs. Co-templating strategy, as one contemporary template-directing method, has been made few attempts in the synthesis of MOFs^[12-17]. The remarkable example is [Co₄(dpdo)₁₂][H(H₂O)₂₇-(CH₃CN)₁₂][PW₁₂O₄₀]₃, which is co-templated by PW₁₂O₄₀^3- and H⁺(H₂O)₂₇^[17]. Nevertheless, it remains a great challenge for constructing MOFs by the template-direction method, especially the co-templating strategy.

  Another important strategy is the judicious choice of organic ligand and reasonable adoption of metal units^{[18, 19]}. The RCSR (Reticular Chemistry Structure Resource) was built by O'keeffe and Yaghi^[20], and various MOFs can be constructed by using building block of organic ligand and metal unit according to the geometry and topology. As shown in RCSR, the frameworks of MOFs can be simplified to specific networks. So far, typical three-dimensional (3D) nets like such as dia, tbo and bcu have been found in numerous MOFs^[21-23]. As one sort of 3D net, the pillared-layer net which means some linkers and nodes serve as layers and other connectors as pillar has attracted wide attention in recent years^[24-26]. As far as we know, two kinds of pillared-layer MOFs are developed. The first type is that metal node and organic linker are connected to form the layer, and the straight organic ligand can support the adjacent layers, forming the pillared-layer net^[27]. Another case is that inorganic ions can also serve as candidate for the pillar^[28-30]. Compared with the organic connectors as linkers, inorganic ions are not explored adequately for the building of pillared-layer frameworks.

  In our early work, we have prepared a series of metal oxalates by using Co(Ⅲ) complexes as templates^[12-15]. To further understand the influence of template in MOFs, in this work, by using aromatic acids H₃BTC (H₃BTC = 1,3,5-benzenetricarboxylic acid) as the substitute of oxalic acid, HNU-38 (HNU is the abbreviation of Hainan University), [Cd₂(BTC)₂Cl]·Co(NH₃)₆·(H₂O)₃, was prepared by using Co(NH₃)₆³⁺ as the template. HNU-38 presents a three-dimension pillared-layer framework with Cl^- as pillar. Notably, (H₂O)_n chains are found in the channels and play a co-templating role along with the Co(NH₃)₆³⁺ cations in HNU-38. Due to the abundant H₂O molecules and Co(NH₃)₆³⁺ in HNU-38, the proton conduction properties were investigated.

  2. EXPERIMENTAL

  2.1 Materials and methods

  All the solvents and reagents were purchased from commercial sources without further purification. Elemental analysis was performed from Perkin-Elmer 2400 elemental analyzer. Infrared (IR) spectra were obtained with KBr discs in the range of 4000~400 cm^-1 on a Bruker TENSOR-27 IR spectrometer. Thermogravimetric analyses (TGA) were performed on Perkin Elmer TGA7 micro-analyzer under air stream at a heating rate of 10 ℃/min. The proton conduction was performed in the solar1260 +1280. Powder X-ray diffraction was obtained on a Rigaku D/MAX X-ray powder diffractometer (CuKα, 1.5418 Å) at room temperature.

  2.2 X-ray crystallography

  Single-crystal X-ray diffraction data for HNU-38 were carried out on Bruker-AXS at 293 K with Mo-Ka radiation (λ = 0.71073 Å). The program SAINT was used for integration of the diffraction profiles. The structure was solved by direct methods (SHELXS) and refined by a full-matrix least-squares procedure based on F² (SHELXL)^[33]. Non-hydrogen atoms were refined with anisotropic displacement parameters.

  2.3 Synthesis of HNU-38

  Synthesis of [Cd₂(BTC)₂Cl]·Co(NH₃)₆·(H₂O)₃ (HNU-38): A mixture of H₃BTC (0.5 mmol), Cd(NO₃)₂·4H₂O (0.5 mmol), Co(NH₃)₆Cl₃ (0.1 mmol), H₂O (2.5 mL), DMF (7.5 mL) and HAc (0.5 mL) was sealed in a 20 mL Teflon-lined reaction vessel. After that, it was kept at 100 ℃ for 5 days and cooled at room temperature. The orange block crystals were obtained and collected (yield 20% based on Co(NH₃)₆Cl₃). IR (KBr, cm^-1): 3447 (w), 3307 (w), 3116 (w), 2919 (w), 2850 (w), 1615 (s), 1567 (s), 1436 (s), 1351 (s), 1108 (m), 871 (m), 768 (s), 728 (s), 536 (w).

  3. RESULTS AND DISCUSSION

  3.1 Structure of HNU-38

  HNU-38 crystallizes in the monoclinic system with P2₁/c space group. The structural unit contains two Cd²⁺, two BTC^3-, one Cl^-, three free H₂O and one Co(NH₃)₆³⁺ complex. As shown in Fig. 1a, Cd(1) is coordinated by one Cl^- and six O atoms from carboxylate groups, four of which are in a chelated mode and the other two exhibit monodentate coordination. Cd(2) shows similar coordination mode with Cd(1). Two Cd²⁺ are bonded together by the BTC^3- ligand to form the binuclear Cd₂ unit. The connection modes of BTC^3- ligands are shown in Fig. 1b. Three carboxylate groups of one ligand show monodentate coordination, chelated connection and μ₃: η₁η₂ coordination mode, respectively. The other BTC^3- ligand shows the monodentate and chelated coordination. The Cd₂ units are connected by BTC^3-, forming the 2D layers (Fig. 2a) which are further connected by the Cl^- ions serving as pillars, thus forming the 3D layer-pillar framework. The Co(NH₃)₆³⁺ ions are countered in the channel of MOFs, serving as the template molecule, and the H₂O molecules work as co-template in HNU-38. When the BTC^3- ligand and Cd₂ unit are simplified as nodes, the framework can be regard as the 3,8-c net with the point symbol of {4³}2{4⁶.6¹⁸.8⁴}, as shown in Fig. 2b.

  Figure 1

  

  Figure 1.  (a) Coordination configuration of Cd²⁺ ions in the binuclear unit (grey: C; red: O; pink: Cd; green: Cl, the H atoms were ignored for clarity); (b) Connection mode of BTC^3- ligands

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  Figure 2

  

  Figure 2.  (a) 2D layers composed of Cd₂ units and BTC^3- are further connected by the Cl^- as viewed along a axis (grey: C; red: O; pink: Cd; green: Cl, the H atoms were ignored for clarity); (b) Pillared-layer topology exhibition of HNU-38

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  The 4,4'-bipyridine, pyrazine and other straight ligands are usually used as pillars in the construction of pillared-layer type MOFs^[27], because the terminal sites of metal unit are suitable for this kind of ligands. Moreover, as typical inorganic ions, SiF₆^2- is adopted as pillar ligand by Zaworoko groups^{[28, 29]}, and some pillared-layer MOFs have been successfully synthesized. As far as we know, other inorganic ions which were used as pillar for the building of MOFs were less explored^{[31, 32]}. In the text, Cl^- served as the pillar ligand, and the pillared-layer based MOF is successfully constructed.

  Co(NH₃)₆³⁺ molecules locate at the space between the layers and Cl^- pillars, serving as the template agent, as shown in the blue octahedra (Fig. 3). Meanwhile, the H₂O molecules in HNU-38 are also filled in the channel. Three H₂O molecules of O(1W), O(2W) and O(3W) are the composition units, with the distances of adjacent H₂O molecules falling in the range of 3.12~3.24 Å. The adjacent H₂O molecules gather together through hydrogen bond as well as intermolecular interaction with the framework and Co(NH₃)₆³⁺ ions. The 1D zigzag chain is displayed through the periodic arrangement of H₂O molecules (Fig. 3). The Co(NH₃)₆³⁺ range two sides of H₂O chain, and two kinds of molecules serve as the template molecules. In most cases of the reported results, single template such as amine and inorganic ion is always used in the synthesis of MOFs, and co-template is not common in the development of template-based MOFs^[12-17].

  Figure 3

  

  Figure 3.  Distribution of H₂O chains (H₂O molecules with imaginary yellow bond) and Co(NH₃)₆³⁺ molecules with octahedral configuration in HNU-38 (H atoms were ignored for clarity)

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  3.2 PXRD and TG

  The PXRD measurement was performed to evaluate the phase state of synthesized sample. As shown in Fig. 4, the PXRD pattern of as-synthesized sample fits well with the simulated data derived from the crystallographic data, indicating the pure phase of the obtained crystal sample.

  Figure 4

  

  Figure 4.  PXRD pattern of HNU-38

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  The TG was performed to investigate the weight loss behavior of the complex. As shown in Fig. 5, the first step weight loss of TG plot from 20 to 170 ℃ can be attributed to the three free H₂O molecules (experiment: 6.17%; calculated: 6.07%). From 170 to 322 ℃, the weight loss can correspond to the six NH₃ molecules of Co(NH₃)₆³⁺ (experiment: 11.29%, calculated: 10.19%). Then, the framework begins to decompose and the complex decomposes completely at 616 ℃.

  Figure 5

  

  Figure 5.  TG plot of HNU-38

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  3.3 Proton conduction

  It is well known that [(CH₃)₂NH₂]⁺, H₂O and so on can serve as the carriers to transport the delocalized protons. Therefore, the presence of abundant Co(NH₃)₆³⁺ and H₂O molecules in the structure of HNU-38 promotes us to investigate the proton conduction. Subsequent AC impedance measurements were performed at free of any additional humidity in the range of 25~80 ℃. Typical impedance plots have been observed, which contain a partial semicircle at high frequencies and a tail at lower frequencies. The conductivity increased along with temperature, up to 4.35 × 10^-4 S/cm at 25 ℃. At 80 ℃, the conductivity reached a maximum of 1.45 × 10^-3 S/cm (Fig. 6).

  Figure 6

  

  Figure 6.  Impedance plots of 2 at 25~80 ℃ without additional humidity

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  4. CONCLUSION

  A layer-pillar MOF was built by the assembly of H₃BTC, Cl^- and metal units, and the Cl^- ions serve as the pillar. In the structure, the Co(NH₃)₆³⁺ and H₂O molecules were served as co-template, both of which fill in the channel of framework, supporting the architecture. Due to the Co(NH₃)₆³⁺ and H₂O in the framework, the proton conduction was investigated, and conductivity is 1.45 × 10^-3 S/cm at 80 ℃. The co-template strategy involving Co(NH₃)₆³⁺ and H₂O opens a new way for the proton conduction of MOFs-based materials.

  Table 1

  

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

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–Cl(1)	2.6444(13)		Cd(1)–O(1)	2.212(3)		Cd(1)–O(6a)	2.283(3)
  Cd(1)–O(11d)	2.490(3)		Cd(1)–O(12d)	2.400(3)		Cd(1)–O(3h)	2.454(3)
  Cd(2)–Cl(1)	2.5767(13)		Cd(2)–O(7)	2.233(3)		Cd(2)–O(10b)	2.214(3)
  Cd(2)–O(4f)	2.365(3)		Cd(2)–O(12g)	2.389(3)		Cd(2)–O(3f)	2.570(3)
  Co(1)–N(3)	1.947(4)		Co(1)–N(4)	1.960(4)		Co(1)–N(5)	1.967(3)
  Co(1)–N(6)	1.967(3)		Co(1)–N(1)	1.968(3)		Co(1)–N(2)	1.968(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  Cl(1)–Cd(1)–O(1)	92.35(9)		Cl(1)–Cd(1)–O(6a)	88.50(9)		Cl(1)–Cd(1)–O(11d)	79.72(8)
  Cl(1)–Cd(1)–O(12d)	102.85(8)		Cl(1)–Cd(1)–O(3h)	171.49(8)		O(1)–Cd(1)–O(6a)	115.75(11)
  O(1)–Cd(1)–O(11d)	117.37(11)		O(1)–Cd(1)–O(12d)	159.16(11)		O(1)–Cd(1)–O(3h)	89.56(11)
  O(6a)–Cd(1)–O(11d)	125.85(10)		O(6a)–Cd(1)–O(12d)	79.27(10)		O(3h)–Cd(1)–O(6a)	83.20(11)
  O(11d)–Cd(1)–O(12d)	53.19(10)		O(3h)–Cd(1)–O(11d)	106.71(10)		O(3h)–Cd(1)–O(12d)	77.52(10)
  Cl(1)–Cd(2)–O(7)	99.53(9)		Cl(1)–Cd(2)–O(10b)	90.97(9)		Cl(1)–Cd(2)–O(3f)	97.35(8)
  Cl(1)–Cd(2)–O(4f)	89.64(8)		Cl(1)–Cd(2)–O(12g)	170.34(8)		O(7)–Cd(2)–O(10b)	113.69(11)
  O(3f)–Cd(2)–O(7)	126.43(10)		O(4f)–Cd(2)–O(7)	76.91(10)		O(7)–Cd(2)–O(12g)	90.00(11)
  O(3f)–Cd(2)–O(10b)	116.44(10)		O(4f)–Cd(2)–O(10b)	169.10(11)		O(10b)–Cd(2)–O(12g)	86.60(11)
  O(3f)–Cd(2)–O(4f)	52.72(10)		O(3f)–Cd(2)–O(12g)	75.49(10)		O(4f)–Cd(2)–O(12g)	91.00(10)
  N(3)–Co(1)–N(6)	89.27(15)		N(4)–Co(1)–N(5)	90.02(16)		N(4)–Co(1)–N(6)	91.66(15)
  N(5)–Co(1)–N(6)	91.40(18)		N(1)–Co(1)–N(2)	90.30(18)		N(1)–Co(1)–N(3)	91.85(16)
  N(1)–Co(1)–N(4)	89.75(16)		N(1)–Co(1)–N(5)	179.75(19)		N(1)–Co(1)–N(6)	88.49(18)
  N(2)–Co(1)–N(3)	89.35(15)		N(2)–Co(1)–N(4)	89.76(15)		N(2)–Co(1)–N(5)	89.81(18)
  N(2)–Co(1)–N(6)	178.14(16)		N(3)–Co(1)–N(4)	178.17(14)		N(3)–Co(1)–N(5)	88.38(16)
  Symmetry transformation: a: –1+x, y, z; b: 1+x, y, z; c: 1–x, –1/2+y, 3/2–z; d: 1–x, 1/2+y, 3/2–z; e: 2–x, –1/2+y, 3/2-z; f: 2–x, 1/2+y, 3/2–z; g: 1–x, –y, 1–z; h: 2–x, –y, 2–z

  
  
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• 

  Figure 1  (a) Coordination configuration of Cd²⁺ ions in the binuclear unit (grey: C; red: O; pink: Cd; green: Cl, the H atoms were ignored for clarity); (b) Connection mode of BTC^3- ligands

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  Figure 2  (a) 2D layers composed of Cd₂ units and BTC^3- are further connected by the Cl^- as viewed along a axis (grey: C; red: O; pink: Cd; green: Cl, the H atoms were ignored for clarity); (b) Pillared-layer topology exhibition of HNU-38

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  Figure 3  Distribution of H₂O chains (H₂O molecules with imaginary yellow bond) and Co(NH₃)₆³⁺ molecules with octahedral configuration in HNU-38 (H atoms were ignored for clarity)

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  Figure 4  PXRD pattern of HNU-38

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  Figure 5  TG plot of HNU-38

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  Figure 6  Impedance plots of 2 at 25~80 ℃ without additional humidity

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cd(1)–Cl(1)	2.6444(13)		Cd(1)–O(1)	2.212(3)		Cd(1)–O(6a)	2.283(3)
  Cd(1)–O(11d)	2.490(3)		Cd(1)–O(12d)	2.400(3)		Cd(1)–O(3h)	2.454(3)
  Cd(2)–Cl(1)	2.5767(13)		Cd(2)–O(7)	2.233(3)		Cd(2)–O(10b)	2.214(3)
  Cd(2)–O(4f)	2.365(3)		Cd(2)–O(12g)	2.389(3)		Cd(2)–O(3f)	2.570(3)
  Co(1)–N(3)	1.947(4)		Co(1)–N(4)	1.960(4)		Co(1)–N(5)	1.967(3)
  Co(1)–N(6)	1.967(3)		Co(1)–N(1)	1.968(3)		Co(1)–N(2)	1.968(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  Cl(1)–Cd(1)–O(1)	92.35(9)		Cl(1)–Cd(1)–O(6a)	88.50(9)		Cl(1)–Cd(1)–O(11d)	79.72(8)
  Cl(1)–Cd(1)–O(12d)	102.85(8)		Cl(1)–Cd(1)–O(3h)	171.49(8)		O(1)–Cd(1)–O(6a)	115.75(11)
  O(1)–Cd(1)–O(11d)	117.37(11)		O(1)–Cd(1)–O(12d)	159.16(11)		O(1)–Cd(1)–O(3h)	89.56(11)
  O(6a)–Cd(1)–O(11d)	125.85(10)		O(6a)–Cd(1)–O(12d)	79.27(10)		O(3h)–Cd(1)–O(6a)	83.20(11)
  O(11d)–Cd(1)–O(12d)	53.19(10)		O(3h)–Cd(1)–O(11d)	106.71(10)		O(3h)–Cd(1)–O(12d)	77.52(10)
  Cl(1)–Cd(2)–O(7)	99.53(9)		Cl(1)–Cd(2)–O(10b)	90.97(9)		Cl(1)–Cd(2)–O(3f)	97.35(8)
  Cl(1)–Cd(2)–O(4f)	89.64(8)		Cl(1)–Cd(2)–O(12g)	170.34(8)		O(7)–Cd(2)–O(10b)	113.69(11)
  O(3f)–Cd(2)–O(7)	126.43(10)		O(4f)–Cd(2)–O(7)	76.91(10)		O(7)–Cd(2)–O(12g)	90.00(11)
  O(3f)–Cd(2)–O(10b)	116.44(10)		O(4f)–Cd(2)–O(10b)	169.10(11)		O(10b)–Cd(2)–O(12g)	86.60(11)
  O(3f)–Cd(2)–O(4f)	52.72(10)		O(3f)–Cd(2)–O(12g)	75.49(10)		O(4f)–Cd(2)–O(12g)	91.00(10)
  N(3)–Co(1)–N(6)	89.27(15)		N(4)–Co(1)–N(5)	90.02(16)		N(4)–Co(1)–N(6)	91.66(15)
  N(5)–Co(1)–N(6)	91.40(18)		N(1)–Co(1)–N(2)	90.30(18)		N(1)–Co(1)–N(3)	91.85(16)
  N(1)–Co(1)–N(4)	89.75(16)		N(1)–Co(1)–N(5)	179.75(19)		N(1)–Co(1)–N(6)	88.49(18)
  N(2)–Co(1)–N(3)	89.35(15)		N(2)–Co(1)–N(4)	89.76(15)		N(2)–Co(1)–N(5)	89.81(18)
  N(2)–Co(1)–N(6)	178.14(16)		N(3)–Co(1)–N(4)	178.17(14)		N(3)–Co(1)–N(5)	88.38(16)
  Symmetry transformation: a: –1+x, y, z; b: 1+x, y, z; c: 1–x, –1/2+y, 3/2–z; d: 1–x, 1/2+y, 3/2–z; e: 2–x, –1/2+y, 3/2-z; f: 2–x, 1/2+y, 3/2–z; g: 1–x, –y, 1–z; h: 2–x, –y, 2–z

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