English

• 1. INTRODUCTION

  Metal-organic frameworks^{[1, 2]} (MOFs) are considered as a greatly developed class of inorganic-organic hybrid materials, which have attracted a mass of efforts in the past decades. It is not only owning to MOFs' various compositions and networks, but also to their potential applications, including small gas separation^{[3, 4]}, drug delivery system^[5-7], sensor^[8-13], emitting light devices^[14-16], heterogeneous catalysis^[17-20], biomacro-molecule immobilization^[21-24], etc. A great amount of attention has been devoted to the design and synthesis of novel MOFs in recent years. From the early extensive literatures, various synthesis conditions will directly affect the crystal growth and structure, such as metal source, ligand, pH value, solvent system, temperature, pressure, template, and counter ion^[25-30]. In view of crystal engineering, interpenetrated MOFs are very interesting and worthy of study because of their high stabilities and intriguing structures. The flexible organic linkers are excellent candidates for the preparation of interpenetrated MOFs. Generally, mixing different linkers with long length and flexibility are very important factors to generate interpenetrated MOFs^[31-34]. The flexible segment of the -O–X–O-chain can be introduced in organic linkers to serve as long and flexible building blocks to construct interpenetrated MOFs^[35]. On the other hand, another neutral N-donor ligand is another very important element for the construction of MOFs. Various interpenetrated MOFs always have been designed and prepared by mixing these organic linkers^[36]. However, it is still a great challenge to rationally construct interpenetrated MOFs and investigate their functional properties.

  In this study, we chose two flexible organic linkers, 1, 3-bis(4-carboxyphenoxy) propane (H₂bcp) and 1, 4-bis(4-pyri-dylmethyl) piperazine (bpp), as bridging linkers, which are both assembled with Ni(II) ion to construct a novel three dimensional (3D) MOF, [Ni(bcp)(bpp)]_n (namely complex 1). Complex 1 can be prepared by heating Ni(NO₃)₂·6H₂O, H₂bcp, bpp, and NaOH in water at 160 ℃ for four days. As we expected, the as-synthesized complex 1 has a rare [3+3+3] nine-fold interpenetrated diamond network. The generated sample was characterized by many technologies, including elemental analysis, powder X-ray diffraction (PXRD), and thermal gravimetric analysis (TGA). Meanwhile, the as-synthesized 1 has a potential application for chemical transformation of CO₂ and epoxide to cyclic carbonate with excellent stability and recyclability.

  2. EXPERIMENTAL

  2.1 General materials and methods

  All chemical solvents and reagents in this work were obtained from commercial sources and used without any further purification. The PXRD profiles were achieved on a Riguku D/MAX2550 diffractometer with Cu-Kα (λ = 1.5418 Å) at 40 kV and 200 mA in a wide 2θ range from 5° to 50° at room temperature. Elemental analyses of C, H and N were performed on a Perkin-Elmer 240 analyzer. The TGA curve in the temperature range of 30~800 ℃ was measured on a TGA Q500 thermal analyzer with a heating rate of 10 ℃·min^-1 in air. All catalytic yields can be calculated by the GC method on a TRACE 1300 instrument.

  2.2 Synthesis

  The mixture of Ni(NO₃)₂·6H₂O (0.5 mmol, 0.145 g), H₂bcp (0.5 mmol, 0.160 g), bpp (0.5 mmol, 0.134 g), NaOH (1 mmol, 0.04 g) and 12 mL of water was heated to 160 ℃ for four days, and then cooled to room temperature. The green crystals were obtained in pure phase, washed with water and ethanol, and dried at room temperature (Yield: 42% based on Ni). Anal. Calcd. (%) for C₃₃H₃₂NiN₄O₆: C, 62.00; H, 5.05; N, 8.76. Found (%): C, 62.05; H, 5.06; N, 8.77.

  2.3 Single-crystal X-ray crystallography

  The single crystal result of the as-synthesized 1 was achieved on a Bruker SMART CCD diffractometer with MoKα radiation (λ = 0.71073 Å) at room temperature. The crystal structure can be directly located by SHELXS-97 program^[37] and refined for several cycles by full-matrix least-squares on F² of the SHELXL-97^[38]. All non-hydrogen atoms were made sure positions and further refined by anisotropic parameters. The hydrogen atoms were positioned geometrically according to the theoretical model and allowed to ride on their parent atoms. The selected bond lengths and bond angles of complex 1 are summarized in Table 1, and the corresponding crystal data are provided in Table 2.

  Table 1

  

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

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  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ni(1)–N(1)	2.0737(19)		Ni(1)–N(4)	2.0687(18)		Ni(1)–O(1)	2.2306(18)
  Ni(1)–O(2)	2.0455(18)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Ni(1)–O(1)	61.64(6)		O(2)–Ni(1)–N(1)	96.15(7)		O(2)–Ni(1)–N(4)	97.74(8)
  N(1)–Ni(1)–O(1)	157.79(7)		N(4)–Ni(1)–O(1)	88.62(8)		N(4)–Ni(1)–N(1)	95.01(8)

  Table 2

  

  Table 2.  Crystal Data and Structure Refinement for Complex 1

  DownLoad: CSV

  Compound	Complex 1	V (Å3)	3120(2)
  Empirical formula	C33H32O6N4Ni	Z	4
  Formula weight	622.37	ρ(calc g·cm-3)	1.361
  Crystal system	Orthorhombic	μ(mm-1)	0.672
  a (Å)	11.592(5)	Nref	7215
  b (Å)	12.538(5)	Rint	0.0198
  c (Å)	21.466(5)	Goodness-of-fit on F2	1.042
  α (º)	90	R, wR (I > 2σ(I))	0.0268, 0.0752
  β (º)	90	R, wR (all data)	0.0294, 0.0772
  γ (º)	90

  2.4 Catalytic experiment

  As-synthesized complex 1 was obtained and further dried in air. This CO₂ cycloaddition reaction was measured in a reaction system with propylene oxide (25 mmol), catalyst (complex 1, 100 mg), and tetra-n-tertbutylammonium bromide (TBABr, 0.3 g) under 2 MPa CO₂ at 80 ℃ for 3 hours. All the yields of this reaction were calculated by the GC method. The recyclability can be performed by recollecting samples and washing with fresh CH₂Cl₂.

  3. RESULTS AND DISCUSSION

  3.1 Structural description for complex 1

  As shown in Fig. 1a, Ni(II) atom is coordinated by four carboxyl and two nitrogen atoms. The bcp^2- linker bridges Ni(II) atoms by a bis-chelating coordination mode to form an infinite helical chain along the b axis. The helical chains have a pitch of 34.776 Å, and adjacent helical chains are further pillared by bpp ligands to generate a 3D open framework. From the topological point, if Ni(II) atom, bpp ligand and bcp^2- linker are all considered as four-connected nodes, this framework can be simplified into a four-connected diamond net with short Schläfli symbols. An adamantane unit is shown in Fig. 1b. The Ni–Ni–Ni angles are in the range of 81.51~129.80^o, which represents a significant distortion from the ideal tetrahedral angle of 109.5^o. The long spacer between the nodes results in a very large cavity within the adamantane unit, which exhibits maximum dimensions of 34.776 × 37.614 × 21.466 (the intracage diagonal Ni–Ni distances along the crystallographic axes). It has been well established that diamond networks tend to interpenetrate to fill the voids with a single net, but n-fold interpenetrated examples (n > 6) are still limited. In the absence of appropriate guest molecules in 1, the large voids in a single net are filled via the mutual interpenetration of eight equivalent nets. Each adamantane cage of a net is thread by other eight nets.

  Figure 1

  

  Figure 1.  (a) Coordination environment of Ni(II) atom in complex 1; (b) 3D structure; (c) A set of 3-fold interpenetrating nets; (d) [3+3+3] interpenetration between three sets of three interpenetrating nets (All hydrogen atoms are removed for clarity)

  DownLoad: Full-Size Img PowerPoint

  In a normal n-fold interpenetrated diamond net, the interpenetrating nets are related by the translation of 1/n of the distance across the adamantane unit along a shared two-fold axis. The nine-fold interpenetration in 1 is different, and can be best described as the interpenetration of three sets of normal 3-fold interpenetrated nets. In each set, the neighbouring nets are related by unit cell translations along the two-fold axis in a direction (1+x, y, z) (Fig. 1c), and the neighbouring nets from different sets are related by the crystallographic glide planes normal to a direction (1+x, 1+y, z). This 'abnormal' interpenetration mode is called as [3+3+3] interpenetration (Fig. 1d).

  3.2 TGA and PXRD analyses

  As shown in Fig. 2a, the TGA data of complex 1 in solid state were collected in air condition at a heating rate of 10 ℃·min^-1 in the temperature range from room temperature to 800 ℃. It is found that no weight loss happened under 400 ℃, which exhibits excellent stability for 1 at high temperature. With further increasing the heating temperature, the 3D interpenetrated structure began to collapse gradually. Finally, the residue after heating to 630 ℃ is NiO, which is about 12.9% and close to the theoretical value of 12%. As displayed in Fig. 2b, the measured PXRD data of as-synthesized complex 1 match very well with those of the simulated result from the single crystal data. The result evidently ascertains the phase purity of the as-synthesized bulk crystals.

  Figure 2

  

  Figure 2.  (a) TGA curve and (b) the PXRD pattern of as-synthesized sample

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  3.3 Catalytic performance

  Thanks to high stability and Ni^II canter, the as-synthesized sample can be thought as a favourable heterogeneous catalyst for the chemical fixation of CO₂ and epoxides to generate cyclic carbonates without organic solvents, which is similar with lots of reported MOFs^[39~46]. Before catalysing this CO₂ cycloaddition reaction, as-synthesized 1 was filtered and dried at 50 ℃. This reaction was carried out at 85 ℃ and 2 MPa CO₂ for 3 hours by adding propylene oxide (25 mmol), catalyst (complex 1, 100 mg), and tetra-n-tertbutylammonium bromide (TBABr, 0.3 g). The catalytic yield can be accurately calculated by the GC approach. As seen in Table 3, it summarizes the catalytic yields of CO₂ and various epoxides with different groups. As shown in entry 1, the catalytic yield of propylene oxide and CO₂ reaches up to 98.6%. However, the corresponding catalytic results of only 1 or TBABr were only 5.3% and 11.2% under the same reaction conditions (entries 2 and 3). The above results clearly illustrate that complex 1 owns preferable catalytic performance with the co-catalysis TBABr. Furthermore, complex 1 was used to explore the reaction of CO₂ with various epoxides with other groups in the presence of TBABr. The corresponding yields sharply reduce to 93.8%, 67.9% and 27.5% for 2-(chloro-methyl)oxirane, styrene oxide, and cyclohexene oxide, respecttively (entries 4~6). This low catalytic performance is probably due to the steric hindrance to decrease the interaction of catalytic centres and epoxides, which is similar with many reported MOFs for this CO₂ cycloaddition reaction^[47~50].

  Table 3

  

  Table 3.  CO₂ Cycloaddition Reaction with Various Epoxides Catalyzed by Complex 1

  DownLoad: CSV

  Entry	Substrate	Product	Yield (%)
  1			98.6
  2			5.3
  3			11.2
  4			93.8
  5			67.9
  6			27.5

  On the other hand, the recyclability of complex 1 is also investigated and evaluated in detail by using chemical transformation of CO₂ and propylene oxide. Complex 1 can be directly collected and washed for several times with fresh CH₂Cl₂ after each reaction. Then the reused sample further catalyzed the CO₂ reaction again. As shown in Fig. 3, the catalytic yields at different recycles are the same with the original result within four times. These results sufficiently show that the as-synthesized 1 is an excellent heterogeneous catalysis for generating cyclic carbonates from CO₂ and epoxides through the free-solvent synthesis.

  Figure 3

  

  Figure 3.  Catalytic yields of reused sample at different recycle times

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  4. CONCLUSION

  In summary, we successfully design and synthesize a novel 3D Ni(II) MOF with a rare [3+3+3] nine-fold interpenetrated diamond topology based on two flexible linkers. The resultant complex 1 has outstanding stability and can be used as a heterogeneous catalyst for the chemical fixation of CO₂ and epoxide. We hope that this work can provide a useful method to design and obtain interpenetrated MOFs based on these mixed flexible ligands.

  
  
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      Parmar, B.; Patel, P.; Pillai, R. S.; Tak, R. K.; Kureshy, R. I.; Khan, N. H.; Suresh, E. Cycloaddition of CO₂ with an epoxide-bearing oxindole scaffold by a metal-organic framework-based heterogeneous catalyst under ambient conditions. Inorg. Chem. 2019, 58, 10084–10096. doi: 10.1021/acs.inorgchem.9b01234

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  Figure 1  (a) Coordination environment of Ni(II) atom in complex 1; (b) 3D structure; (c) A set of 3-fold interpenetrating nets; (d) [3+3+3] interpenetration between three sets of three interpenetrating nets (All hydrogen atoms are removed for clarity)

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  Figure 2  (a) TGA curve and (b) the PXRD pattern of as-synthesized sample

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  Figure 3  Catalytic yields of reused sample at different recycle times

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Ni(1)–N(1)	2.0737(19)		Ni(1)–N(4)	2.0687(18)		Ni(1)–O(1)	2.2306(18)
  Ni(1)–O(2)	2.0455(18)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Ni(1)–O(1)	61.64(6)		O(2)–Ni(1)–N(1)	96.15(7)		O(2)–Ni(1)–N(4)	97.74(8)
  N(1)–Ni(1)–O(1)	157.79(7)		N(4)–Ni(1)–O(1)	88.62(8)		N(4)–Ni(1)–N(1)	95.01(8)

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  Table 2.  Crystal Data and Structure Refinement for Complex 1

  Compound	Complex 1	V (Å3)	3120(2)
  Empirical formula	C33H32O6N4Ni	Z	4
  Formula weight	622.37	ρ(calc g·cm-3)	1.361
  Crystal system	Orthorhombic	μ(mm-1)	0.672
  a (Å)	11.592(5)	Nref	7215
  b (Å)	12.538(5)	Rint	0.0198
  c (Å)	21.466(5)	Goodness-of-fit on F2	1.042
  α (º)	90	R, wR (I > 2σ(I))	0.0268, 0.0752
  β (º)	90	R, wR (all data)	0.0294, 0.0772
  γ (º)	90

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  Table 3.  CO₂ Cycloaddition Reaction with Various Epoxides Catalyzed by Complex 1

  Entry	Substrate	Product	Yield (%)
  1			98.6
  2			5.3
  3			11.2
  4			93.8
  5			67.9
  6			27.5

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