English

• 1. INTRODUCTION

  In the past two decades, metal-organic frameworks (MOFs) have drawn lots of attention from chemists and material scientists due to their fascinating structural aesthetics, rich optical, electrical and magnetic phenomena as well as their promising applications in gas storage and separation, catalysis, sensors, semiconductor devices, multiferroic materials, etc^[1-14]. They can always bring us various kinds of surprises. For example, in 2017, Yaghi group reported a porous metal-organic framework [Zr₆O₄(OH)₄(fumarate)₆]}, MOF-801, which is capable of harvesting 2.8 liters of water per kilogram of MOF-801 daily at relative humidity levels as low as 20%, and requires only the input of energy from natural sunlight^[7]. More recently, MOFs materials have been combined with another star material perovskite. In 2019, Fei group reported the intrinsic white-light-emitting MOFs [Pb₂X₃⁺][^-O₂C(C₆H₄)CO₂^-]₂[(CH₃)₂NH₂⁺]₃ (X = Cl, Br, or I) with excellent stability, tunability and moisture resistance, in which the structurally deformable haloplumbate units often observed in organolead halide perovskites have been successfully incorporated into MOFs^[15].

  Metal-organic frameworks are made up of metal ions or metal clusters and organic ligands, in which the types of metal ions are very limited when compared with the organic ligands. The structures and properties of the complexes are affected or even directly determined by organic ligands. Therefore, the design and syntheses of organic ligands play a vital role in the development of MOFs. The fluorene and carbazole molecules and their derivatives are excellent electron donors and have been used extensively in organic photoelectric functional materials^{[16, 17]}. Our group has been committed to the syntheses and performance studies of coordination compounds with the fluoreneand carbazole-based ligands for ten years^{[2, 18-30]}. We have synthesized eight fluoreneand carbazole-based ligands successively, which are 9, 9-dimethylfluorene-2, 7-dicarboxylic acid, 9, 9-dimethylfluorene-2, 7-diphosphonic acid, 4, 4΄-(9, 9-dimethyl-9H-fluorene-2, 7-diyl)dibenzoic acid, 9-methyl-9-hydroxy-fluorene-2, 7-dicarboxylic acid, 9-fluorenone-2, 7-dicarboxylic acid, 9-(pyridin-4-yl)-9H-carbazole-3, 6-dicarboxylic acid, 9-(pyridin-4-yl)-9H-carbazole-3, 6-dicarboxylic acid and 9H-carbazole-2, 7-dicarboxylic acid, respectively. In addition, lanthanide ions possess excellent optical, electrical and magnetic properties such as rare earth catalysts, single-ion and single-molecule magnets, photo-emitting materials, and so on^[31-35].

  In this work, we continued to synthesize a new carbazole-based ligand 9-(2, 6-dicarboxy-pyridin-4-yl)-9H-carbazole-3, 6-dicarboxylic acid (H₄L). Based on this ligand, a metal-organic framework [Tb(HL)(H₂O)] (TbL) was obtained by solvothermal synthesis method. Here, we report the syntheses of H₄L ligand and MOF TbL, structure and property of TbL.

  2. EXPERIMENTAL

  2.1 Materials and general methods

  Unless otherwise indicated, chemicals and solvents were purchased from commercial suppliers or purified by standard techniques, and all reactions were carried out in standard operation. Diethyl 4-iodopyridine-2, 6-dicarboxylate and 3, 6-diiodo-9H-carbazole (1) were prepared according to previous literatures^{[36, 37]}. Reactions were monitored by TLC on silica gel plates (GF254). ¹H NMR and ¹³C NMR spectra were recorded on Bruker Avance instruments (400 MHz and 100 MHz, respectively) and internally referenced to tetramethylsilane signal or residual protio solvent signals (s = singlet, d = doublet, dd = double doublet, t = triplet, q = quartet, m = multiplet). Flash column chromatography was carried out using silica gel (200~300 mesh) eluting with ethyl acetate/petroleum ether or dichloromethane/methanol. FT-IR spectra were measured in the range of 400~4000 cm^-1 with a PerkinElmerSpectrum on KBr pellets. Powder X-ray diffraction patterns (PXRD) data of all samples were collected in the 2θ range of 5~50° with a scan step width of 0.02° on a Bruker D8 Advance A25 diffractometer (Cu-Kα, λ = 1.5418 Å). Photoluminescence spectra were carried out using a RF-5301PC spectrofluorophotometer at the room temperature.

  2.2 Preparation of 2

  To the mixture of 3, 6-diiodo-9H-carbazole (1, 4.2 g, 10.0 mmol), diethyl 4-iodopyridine-2, 6-dicarboxylate (3.5 g, 10.0 mmol), copper(I) iodide (190 mg, 1.0 mmol), L-proline (230 mg, 2.0 mmol) and K₂CO₃ (276 mg, 2.0 mmol) in Schlenk flask was added 20 mL dimethyl sulfoxide (DMSO). The flask was degassed in vacuum and backfilled with argon and heated to 90 ℃ for 24 h. After cooling to room temperature, the reaction mixture was poured into water and extracted with dichloromethane. The combined organic phases were dried over Na₂SO₄ and the solvent was removed afterwards. Purification of the residue by flash chromatography afforded the desired product 2 as a light yellow solid (4.73 g, 74% yield). 8.72 (s, 2H), 8.40 (s, 2H), 7.76 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.8 Hz, 2H), 4.41 (dd, J = 14.0, 6.8 Hz, 4H), 1.35 (t, J = 6.8 Hz, 6H). ¹³C-NMR (100 MHz, DMSO): δ (ppm) 163.5, 150.2, 145.9, 138.1, 135.2, 129.7, 124.9, 124.1, 112.1, 85.1, 61.8, 14.0.

  2.3 Preparation of 3

  A mixture of the diethyl 4-(3, 6-diiodo-9H-carbazol-9-yl)pyridine-2, 6-dicarboxylate (2, 0.64 g, 1 mmol) and dried copper(I) cyanide (0.45 g, 5 mmol) in dry N-methyl pyrrolidone (NMP) (5 mL) was heated in a sealed tube at 140 ℃ overnight. After cooling to room temperature, a mixture of water (18 mL), HCl (6 mL) and FeCl₃ (1.45 g, 9 mmol) was poured into the reaction mixture and the solution was stirred for 1 h. The resulting brown precipitate was filtered and washed with water. The solid was re-dissolved in dichloromethane (DCM) and washed with water. The combined organic phases were dried over Na₂SO₄ and the solvent was removed at reduced pressure to give the crude product which was further purified by flash chromatography to give the desired product 3 as a white solid (0.13 g, 30% yield). 8.96 (s, 2H), 8.52 (s, 2H), 7.97 (d, J = 10.8 Hz, 2H), 7.77 (d, J = 11.2 Hz, 2H), 4.42 (dd, J = 14.0, 6.8 Hz, 4H), 1.34 (t, J = 7.2 Hz, 6H). ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 163.7, 150.5, 145.1, 142.1, 131.0, 126.5, 125.4, 122.9, 119.5, 111.8, 104.0, 61.8, 14.1.

  2.4 Preparation of H₄L

  A mixture of diethyl 4-(3, 6-dicyano-9H-carbazol-9-yl)pyridine-2, 6-dicarboxylate (3, 88 mg, 0.2 mmol) and KOH (0.22 g, 4 mmol) in water (1 mL) and ethanol (5 mL) was heated at reflux for 3 d. The reaction mixture was cooled at room temperature, diluted with water (20 mL) and acidified with HCl 2M to pH = 1. The resulted precipitate was filtered, washed with water and crystallized from ethanol to give pure H₄L as a off-white solid (58.8 mg, 70% yield). ¹H-NMR (400 MHz, CDCl₃): δ (ppm) 8.93 (s, 2H), 8.43 (s, 2H), 8.07 (dd, J = 8.8, 1.6 Hz, 2H), 7.65 (d, J = 8.8 Hz, 2H) 3.82~3.17 (m, 4H). ¹³C-NMR (100 MHz, CDCl₃): δ (ppm) 167.3, 165.0, 150.9, 145.8, 142.3, 128.5, 124.5, 124.3, 123.3, 123.0, 110.0.

  Scheme 1

  

  Scheme 1.  Synthetic route for the preparation of H₄L

  DownLoad: Full-Size Img PowerPoint

  2.5 Preparation of TbL

  H₄L (0.04 mmol, 0.0172 g) and Tb(NO₃)₃·6H₂O (0.04 mmol, 0.0180 g) were dissolved in 2 mL N, N-dimethylformamide (DMF), and then 2 mL deionized water was added. The mixture was sealed in a 15 mL Teflon-lined stainless-steel vessel, and heated at 180 ℃ for 72 h. After cooling to room temperature, the colorless crystals were obtained, washed by DMF, and then put in a glass desiccator. IR data (KBr, cm^-1): 3428 (b, m), 1602 (s), 1538 (w), 1395 (s), 1136 (w), 1023 (w), 784 (w), 741 (w), 645 (w).

  2.6 X-ray crystallography

  X-ray diffraction data of the single crystal of TbL were collected on a Bruker Smart Apex CCD area detector diffractometer (Germany) using graphite-monochromated Mo-Kαradiation (λ = 0.71073 Å). Absorption corrections were applied using the SADABS supplied by Bruker^[38]. Structures were solved by direct methods and refined by full-matrix least-squares method protocols on F² using the program SHELXL-2014/7 program package^[39]. The positions of metal atoms and their first coordination spheres were located from direct-method E maps; other non-hydrogen atoms were found using alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, were refined anisotropically. The hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of U_iso. Because the isolated solvents within the channels were not crystallographically well-defined, the data were treated with the SQUEEZE routine within PLATON^[40]. The selected bond lengths and bond angles are listed in Table 1.

  Table 1

  

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

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Tb(1)–O(1)#1	2.349(5)		Tb(1)–O(2)#2	2.367(5)		Tb(1)–O(3)#3	2.418(6)
  Tb(1)–O(4)#3	2.402(5)		Tb(1)–O(6)	2.357(6)		Tb(1)–O(8)	2.370(6)
  Tb(1)–O(9)	2.408(5)		Tb(1)–N(2)	2.485(7)		Tb(1)–Tb(1)#4	3.861(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)#1–Tb(1)–O(6)	80.7(2)		O(8)–Tb(1)–O(6)	129.5(2)		O(1)#1–Tb(1)–O(2)#2	116.7(2)
  O(8)–Tb(1)–O(2)#2	76.4(2)		O(6)–Tb(1)–O(2)#2	151.8(2)		O(1)#1–Tb(1)–O(9)	74.7(2)
  O(8)–Tb(1)–O(9)	132.9(2)		O(6)–Tb(1)–O(9)	88.9(2)		O(2)#2–Tb(1)–O(9)	76.0(2)
  O(1)#1–Tb(1)–O(4)#3	146.1(3)		O(8)–Tb(1)–O(4)#3	129.1(2)		O(6)–Tb(1)–O(4)#3	76.4(2)
  O(2)#2–Tb(1)–O(4)#3	77.6(2)		O(9)–Tb(1)–O(4)#3	80.1(2)		O(1)#1–Tb(1)–O(3)#3	150.7(2)
  O(8)–Tb(1)–O(3)#3	80.5(2)		O(6)–Tb(1)–O(3)#3	89.3(2)		O(2)#2–Tb(1)–O(3)#3	84.2(2)
  O(9)–Tb(1)–O(3)#3	133.0(1)		O(4)#3–Tb(1)–O(3)#3	53.8(2)		O(1)#1–Tb(1)–O(8)	84.8(3)
  O(8)–Tb(1)–N(2)	64.8(2)		O(6)–Tb(1)–N(2)	64.8(2)		O(2)#2–Tb(1)–N(2)	140.6(2)
  O(9)–Tb(1)–N(2)	137.0(2)		O(4)#3–Tb(1)–N(2)	121.5(2)		O(3)#3–Tb(1)–N(2)	82.5(2)
  O(1)#1–Tb(1)–N(2)	68.3(2)
  Symmetry transformation: #1 1 – y, 1 – x, 4/3 – z; #2 –1 + x, –1 + y, z; #3 –x + y, –1 + y, 5/3 – z; #4 –y, –x, 4/3 – z

  3. RESULTS AND DISCUSSION

  The compound TbL is obtained under the solvothermal condition.

  3.1 Structural description of complex TbL

  Single-crystal X-ray analysis on TbL reveals a chiral neutral three-dimensional (3D) framework. In the asymmetric unit of TbL, there are one Tb³⁺ ion, one HL^3- ligand and one coordinated H₂O molecule (Fig. 1a). The Tb³⁺ ion is eight-coordinated by O(6), O(8) and N(2) from one tri-dentate chelating 2, 6-pyridinedicarboxylate group of a HL^3- ligand, O(3)^#3 and O(4)^#3 from a di-dentate chelated carboxylate group of another HL^3- ligand, O(1)^#1 and O(2)^#2 from the third and fourth HL^3- ligands, and O(9) from the μ₂-H₂O molecule. The eight coordinated atoms form the distorted bicapped trigonal prismatic coordination geometry around the Tb³⁺ ion. The Tb–O bond distances range from 2.349(5) to 2.418(6) Å and the Tb–N bond distance is 2.485(7) Å in TbL, which are comparable to those reported for other Tb³⁺ complexes^{[41, 42]}. The HL^3- ligand links four Tb³⁺ ions by 2, 6-pyridinedicarboxylate moiety tri-dentate chelating to a Tb³⁺ ion, a carbazole carboxylate group chelating to the second Tb³⁺ ion, another carbazole carboxylate group coordinating to the third and fourth Tb³⁺ ions with each oxygen atom coordinating to a Tb³⁺ ion. In the HL^3- ligand, the dihedral angle between the carbazole and pyridine rings is 61.803(2)º. Two carboxylate groups and a water molecule link two Tb³⁺ ions in a tri-bridge mode to form a binuclear Tb₂ unit with the Tb···Tb distance of 3.861(1) Å. Each two neighboring binuclear Tb₂ units are bibridged by two carbazole-3, 6-dicarboxylate moieties from two HL^3- ligands to lead to a serials of homochiral parallel arranged helical chains extending along the c axis (Fig. 1b). In each helical chain, there exist π···π stacking interactions between two carbazole-3, 6-dicarboxylate moieties as a bibridge with the centroid-to-centroid distance in the range of 3.698(7)~3.779(5) Å (Table 2 and Fig. 1b). These homochiral helical chains are further connected by 2, 6-pyridinedicarboxylate moieties to produce the chiral neutral 3D framework of TbL (Fig. 1c and 1d). In the framework, there are three classes of 1D channels. One possesses the hexagonal window and exists within the helical chain; one possesses the triagonal window and exists among the helical chains (Fig. 1c); and the last one possesses the fish-mouth-like window and extends along the a, b axes and [110] direction, respectively (Fig. 1d). The solvent accessible space for the desolvated TbL is 2783.1 ų per unit cell or 53.3% of the total volume, calculated using the PLATON routine^[43].

  Figure 1

  

  Figure 1.  (a) View of the asymmetric unit of TbL with thermal ellipsoids drawn at the 30% probability level. (b)Helical chains. Views of the 3D framework along the c (c) and a (d) axes. All hydrogen atoms are omitted for clarity. Symmetry transformation: ^#1 1 – y, 1 – x, 4/3 – z; ^#2 –1 + x, –1 + y, z; ^#3 –x + y, –1 + y, 5/3 – z

  DownLoad: Full-Size Img PowerPoint

  Table 2

  

  Table 2.  Defined Rings and Relative Parameters of the π···π Interactions for Complex TbL

  DownLoad: CSV

  	Dist. centroids (Å)	Dihedral angle (°)	CgI_Perp. dist. (Å)	CgJ_Perp. dist. (Å)	Slip dist. (Å)
  Cg(1)→Cg(1)#5	3.779(5)	0.00	0.775(7)	3.586(5)	1.190
  Cg(1)→Cg(2)#5	3.698(7)	0.00	0.711(7)	3.432(5)	1.377
  Cg(2)→Cg(3)#5	3.739(5)	0.00	0.267(7)	3.507(5)	1.296
  Cg(1): N(1)– > C(6)– > C(7)– > C(8)– > C(14); Cg(2): C(8)– > C(9)– > C(10)– > C(12)– > C(13)– > C(14); Cg(3): C(2)– > C(3)– > C(4)– > C(5)– > C(6)– > C(7). Symmetry transformation: #5 1 – x + y, y, 5/3 – z

  3.2 PXRD and IR

  In order to check the phase purity of TbL, the PXRD of the compound was recorded at room temperature. As shown in Fig. 2, the peak positions of the simulated pattern closely match those of the experimental one, indicating phase purity of the as-synthesized samples. In IR spectrum of TbL

  Figure 2

  

  Figure 2.  PXRD patterns of TbL

  DownLoad: Full-Size Img PowerPoint

  (Fig. S2), the strong absorption peaks locating at around 1602~1395 cm^-1 are characteristic of the expected absorption for asymmetric and symmetric vibrations of the carboxylate groups. The broad band observed at around 3428 cm^-1 should result from the stretching vibrations of H₂O molecules in TbL.

  3.3 Photoluminescence property

  As shown in Fig. 3, the solid-state photoluminescence properties of TbL and free H₄L ligand were studied at room temperature. The free H₄L ligand emits blue light with the broad emission peak and the emission maximum at 455 nm upon excitation at the maximum excitation wavelength of 378 nm. The emission spectrum of TbL in the solid state excited at 374 nm exhibits the luminescence peaks at 497, 552, 591 and 628 nm, which could be attributed to the characteristic transitions of the Tb³⁺ ion: ⁵D₄ → ⁷F_J (J = 6, 5, 4 and 3), respectively. In the emission spectrum of TbL, the strong blue light emission in free H₄L completely disappear, suggesting that the ligands effectively transfer energy to the Tb(Ⅲ) ions.

  Figure 3

  

  Figure 3.  Excitation and emission spectra of TbL and H₄L

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In conclusion, based on a new synthetic ligand 9-(2, 6-dicarboxy-pyridin-4-yl)-9H-carbazole-3, 6-dicarboxylic acid (H₄L), a metal-organic framework [Tb(HL)(H₂O)] (TbL) has been obtained. TbL possesses the chiral neutral 3D framework constructed by the interconnected homochiral parallel arranged helical chains. Three kinds of 1D channels are observed in the framework. TbL exhibits intense characteristic green emission of Tb³⁺ ions in the solid state at room temperature.

  
  
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  Scheme 1  Synthetic route for the preparation of H₄L

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  Figure 1  (a) View of the asymmetric unit of TbL with thermal ellipsoids drawn at the 30% probability level. (b)Helical chains. Views of the 3D framework along the c (c) and a (d) axes. All hydrogen atoms are omitted for clarity. Symmetry transformation: ^#1 1 – y, 1 – x, 4/3 – z; ^#2 –1 + x, –1 + y, z; ^#3 –x + y, –1 + y, 5/3 – z

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  Figure 2  PXRD patterns of TbL

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  Figure 3  Excitation and emission spectra of TbL and H₄L

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

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Tb(1)–O(1)#1	2.349(5)		Tb(1)–O(2)#2	2.367(5)		Tb(1)–O(3)#3	2.418(6)
  Tb(1)–O(4)#3	2.402(5)		Tb(1)–O(6)	2.357(6)		Tb(1)–O(8)	2.370(6)
  Tb(1)–O(9)	2.408(5)		Tb(1)–N(2)	2.485(7)		Tb(1)–Tb(1)#4	3.861(1)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)#1–Tb(1)–O(6)	80.7(2)		O(8)–Tb(1)–O(6)	129.5(2)		O(1)#1–Tb(1)–O(2)#2	116.7(2)
  O(8)–Tb(1)–O(2)#2	76.4(2)		O(6)–Tb(1)–O(2)#2	151.8(2)		O(1)#1–Tb(1)–O(9)	74.7(2)
  O(8)–Tb(1)–O(9)	132.9(2)		O(6)–Tb(1)–O(9)	88.9(2)		O(2)#2–Tb(1)–O(9)	76.0(2)
  O(1)#1–Tb(1)–O(4)#3	146.1(3)		O(8)–Tb(1)–O(4)#3	129.1(2)		O(6)–Tb(1)–O(4)#3	76.4(2)
  O(2)#2–Tb(1)–O(4)#3	77.6(2)		O(9)–Tb(1)–O(4)#3	80.1(2)		O(1)#1–Tb(1)–O(3)#3	150.7(2)
  O(8)–Tb(1)–O(3)#3	80.5(2)		O(6)–Tb(1)–O(3)#3	89.3(2)		O(2)#2–Tb(1)–O(3)#3	84.2(2)
  O(9)–Tb(1)–O(3)#3	133.0(1)		O(4)#3–Tb(1)–O(3)#3	53.8(2)		O(1)#1–Tb(1)–O(8)	84.8(3)
  O(8)–Tb(1)–N(2)	64.8(2)		O(6)–Tb(1)–N(2)	64.8(2)		O(2)#2–Tb(1)–N(2)	140.6(2)
  O(9)–Tb(1)–N(2)	137.0(2)		O(4)#3–Tb(1)–N(2)	121.5(2)		O(3)#3–Tb(1)–N(2)	82.5(2)
  O(1)#1–Tb(1)–N(2)	68.3(2)
  Symmetry transformation: #1 1 – y, 1 – x, 4/3 – z; #2 –1 + x, –1 + y, z; #3 –x + y, –1 + y, 5/3 – z; #4 –y, –x, 4/3 – z

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  Table 2.  Defined Rings and Relative Parameters of the π···π Interactions for Complex TbL

  	Dist. centroids (Å)	Dihedral angle (°)	CgI_Perp. dist. (Å)	CgJ_Perp. dist. (Å)	Slip dist. (Å)
  Cg(1)→Cg(1)#5	3.779(5)	0.00	0.775(7)	3.586(5)	1.190
  Cg(1)→Cg(2)#5	3.698(7)	0.00	0.711(7)	3.432(5)	1.377
  Cg(2)→Cg(3)#5	3.739(5)	0.00	0.267(7)	3.507(5)	1.296
  Cg(1): N(1)– > C(6)– > C(7)– > C(8)– > C(14); Cg(2): C(8)– > C(9)– > C(10)– > C(12)– > C(13)– > C(14); Cg(3): C(2)– > C(3)– > C(4)– > C(5)– > C(6)– > C(7). Symmetry transformation: #5 1 – x + y, y, 5/3 – z

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