English

• 0.   Introduction

  The coordination polymers (CPs) or metal‐organic frameworks (MOFs) constructed from metal ions and organic ligands exhibit specific properties and potential applications in anion or cation sensing, absorption or exchange, gas absorption, degradation of organic dyes, storage, transmission or release of drugs, and luminescence^[1-12]. Intermolecular hydrogen bonds and π‐π stacking are common weak interactions and as important as coordination bonds in the field of crystal engineering. The intermolecular hydrogen bonds and π‐π stacking interactions can increase the dimensionality of the complexes and induce some changes in properties^[13-15]. The synthesis method and molar ratio of metal and ligand are important for the synthesis of complexes, and the selection of metal ions and ligands is also crucial. Adopting the d¹⁰ electronic configuration, the Cd(Ⅱ)/Zn(Ⅱ) ions are often used to synthesize CPs because of their potential luminescent properties. Aromatic carboxylate can balance charges and provide multiple coordination sites, which are widely used in the synthesis of complexes. More and more coordination compounds were synthesized with N‐donor ligands assisted by carboxylic acid ligands.

  In previous work, we synthesized some CPs based on a flexible 4‐substituted bis(1, 2, 4‐triazole) ligand 1, 2‐bis(4H‐1, 2, 4‐triazole)ethane (btre) under the assistance of carboxylate ligands^[16-17]. We have synthesized some CPs based on rigid 4‐substituted‐1, 2, 4‐triazole ligand 1, 3‐bis(4H‐1, 2, 4‐triazole)benzene (mbtx) and 1, 4‐bis(4H‐1, 2, 4‐triazole)benzene (btx)^[18-19]. For example, [Zn(btre)_0.5(nbdc)(H₂O)]_n and {[Zn(btre)_0.5(MeOip)(H₂O)₂]·H₂O}_n (nbdc^2-=3‐nitro‐1, 2‐benzenedicarboxylate, MeOip^2-=4‐methoxybenzene‐1, 3‐dicarboxylate) all displays 2D→2D parallel polycatenated consisting of three sets of equivalent 2D (6, 3) networks.

  With this background information, in this paper, we report four new CPs: {[Cd(mbtx)(4OHphCOO)]NO₃}_n (1), {[Zn(mbtx)(1, 4‐bdc)_0.5(H₂O)₂]·(1, 4‐bdc)_0.5·4H₂O}_n (2), {[Cd₂(mbtx)(5NO₂‐bdc)₂(H₂O)₃]·4.5H₂O}_n (3), and {[Zn(H₂O)₆][Zn₂(mbtx)₂(btc)₂(H₂O)₄]·2H₂O}_n (4) based on mbtx and four aromatic carboxylate ligands 4OHphCOOH (p‐hydroxybenzoic acid), 5NO₂‐H₂bdc (5‐nitroisophthalic acid), 1, 4‐H₂bdc (terephthalic acid), and H₃btc (trimesic acid). Ligand mbtx has six different coordination modes, but only shows bis‐monodentate and tetradentate modes in complexes 1‐4 (Scheme 1). The coordination modes of aromatic carboxylate in the four Cd(Ⅱ)/Zn(Ⅱ) complexes are listed in Scheme 2. The syntheses, structures, thermal stabilities, and luminescence properties of 1‐4 were studied.

  Scheme 1

  

  Scheme 1.  Two different coordination modes of mbtx

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  Scheme 2

  

  Scheme 2.  Coordination modes of 5NO₂‐bdc^2-, 4OHphCOO^-, 1, 4‐bdc^2-, and btc^3-

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  1.   Experimental

  1.1   Materials and methods

  The ligand mbtx was synthesized according to the literature method^[20]. All other reagents (such as Cd(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, 5NO₂‑H₂bdc, 4OHphCOOH, 1, 4‐H₂bdc, and H₃btc) were of analytical grade and used without further purification. Elemental analyses for C, H, and N were performed on a Perkin‐Elmer 240C analyser. FTIR spectra were recorded from KBr pellets in a 4 000‐400 cm^-1 range on a Thermo NICOEF iS10 spectrometer. Powder X‐ray diffraction (PXRD) was performed on an X Pert‐Pro MPD diffractometer with the Cu Kα radiation (λ= 0.154 06 nm, U=40 kV, I=40 mA) over a 2θ range of 5°‐50° at room temperature. The luminescence measurements were carried out in the solid state at room temperature, and the spectra were collected with a Perkin‐Elmer LS50B spectrofluorimeter. Thermogravimetric analysis (TGA) was carried out using a Thermal Analyst 2100 TA Instrument and SDT 2960 Simultaneous TGA‐DTA instrument in flowing nitrogen with a heating rate of 10 ℃·min^-1. UV‐Vis Diffuse reflectance spectra of the solid samples were collected using a U‐3900 spectrophotometer.

  1.2   Synthesis of complex 1

  The pH value of a solution of 4OHphCOOH (0.2 mmol, 28 mg) in 10 mL of H₂O was adjusted to 6 with 1.0 mol·L^-1 NaOH solution, then Cd(NO₃)₂·4H₂O (0.2 mmol, 62 mg) was added with stirring. The mbtx (0.2 mmol, 42 mg) was dissolved in 5 mL of DMF and added to the mixture, and the solution was continuously stirred for 10 min. The mixed solution was filtered and stood for two weeks to give colorless single crystals of complex 1. Anal. Calcd. for C₁₇H₁₃CdN₇O₆(%): C, 38.99; H, 2.50; N, 18.72. Found(%): C, 38.91; H, 2.48; N, 18.76. IR data (cm^-1): 3 443w, 3 115w, 1 670m, 1 596 m, 1 542s, 1 390s, 1 238m, 1 093m.

  1.3   Synthesis of complex 2

  The synthesis of complex 2 was similar to that of complex 1, except that the ligand 4OHphCOOH was replaced with 1, 4‑H₂bdc (0.2 mmol, 33 mg), and Cd(NO₃)₂·4H₂O was replaced with Zn(NO₃)₂·6H₂O (0.2 mmol, 60 mg). Colorless single crystals of 2 were collected. Anal. Calcd. for C₁₈H₂₄ZnN₆O₁₀(%): C, 39.32; H, 4.40; N, 15.29. Found(%): C, 39.41; H, 4.35; N, 15.33. IR data (cm^-1): 3 411s, 1 648w, 1 572s, 1 501m, 1 405 s, 1 384s, 1 363s, 1 309m, 1 145w, 1 108w, 1 016w, 856 m, 747w, 584w.

  1.4   Synthesis of complex 3

  The synthetic method of complex 3 was similar to that of complex 1, except that 4OHphCOOH was replaced by 5NO₂‑H₂bdc (0.2 mmol, 42 mg). Anal. Calcd. for C₂₆H₂₉Cd₂N₈O_19.5(%): C, 31.53; H, 2.95; N, 11.31. Found(%): C, 31.45; H, 2.93; N, 11.33. IR data (cm^-1): 3 406s, 3 120w, 1 615s, 1 564s, 1 536s, 1 450 m, 1 372s, 1 221w, 1 087w, 1 050w, 741s.

  1.5   Synthesis of complex 4

  The synthetic method of complex 4 was similar to that of complex 1, except that 4OHphCOOH was replaced by H₃btc (0.2 mmol, 42 mg), and Cd(NO₃)₂·4H₂O was replaced with Zn(NO₃)₂·6H₂O (0.2 mmol, 60 mg). Anal. Calcd. for C₃₈H₄₆Zn₃N₁₂O₂₄(%): C, 36.48; H, 3.71; N, 13.44. Found(%): C, 36.41; H, 3.66; N, 13.45. IR data (cm^-1): 3 282s, 1 614s, 1 560s, 1 446s, 1 375s, 1 212w, 1 112w, 759w, 735m.

  1.6   X‐ray crystallography

  Single‐crystal X‐ray diffraction analyses of the four complexes were carried out. The diffraction data of complexes 1‐4 were collected on the CCD area detector and Rigaku Saturn/Mercury diffractometer with graphite monochromated Mo Kα (λ=0.071 070 nm for 1 and 2 and 0.071 075 nm for 3 and 4) radiation. Intensities were collected by the ω scan technique for 1‐4. The structures were solved by direct methods with SHELXS‐97 and refined with full‐matrix least‐squares on F ² with SHELXL‑97. All non‑hydrogen atoms were refined anisotropically, and hydrogen atoms on the ligands were placed at the calculated positions and were refined as a riding model. Active hydrogen atoms were located by difference Fourier syntheses. The parameters of crystal data collection and refinement of 1‐4 are given in Table 1. Selected bond lengths and bond angles of 1‐4 are listed in Table S1 (Supporting information).

  Table 1

  

  Table 1.  Crystallographic data and structure refinements of complexes 1‐4

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  Parameter	1	2	3	4
  Formula	C17H13CdN7O6	C18H24ZnN6O10	C26H29Cd2N8O19.5	C38H46Zn3N12O24
  Formula weight	523.74	549.80	990.37	1 250.98
  T / K	293(2)	293(2)	223(2)	293(2)
  Crystal system	Monoclinic	Triclinic	Triclinic	Triclinic
  Space group	P21/n	P1	P1	P1
  a / nm	1.478 40(19)	0.975 2(3	1.062 1(2)	1.200 9(3)
  b / nm	0.774 55(10)	1.106 9(4)	1.289 3(2)	1.316 8(3)
  c / nm	1.713 0(2)	1.179 8(4)	1.327 4(3)	1.5717(4)
  α / (°)		106.171(5)	85.295(8)	93.916(4)
  β / (°)	94.734(3)	101.963(7)	83.664(6)	100.587(5)
  γ / (°)		104.732(4)	85.406(7)	108.232(4)
  V / nm3	1.954 8(4)	1.128 7(7)	1.795 8(6)	2.299 2(10)
  Z	4	2	2	2
  F(000)	1 040	568	986	1 280
  Dc / (g·cm-3)	1.780	1.618	1.832	1.807
  μ / mm-1	1.170	1.156	1.277	1.658
  Reflection collected	17 582	11 108	15 852	22 388
  Unique reflection	3 568(Rint=0.038 1)	4 116(Rint=0.030 4)	7 636(Rint=0.021 6)	10 360(Rint=0.036 0)
  Goodness of fit on F 2	1.022	1.030	1.036	1.024
  Final R indices [I > 2σ(I)]	R1=0.039 7, wR2=0.101 9	R1=0.038 0, wR2=0.087 9	R1=0.032 9, wR2=0.102 3	R1=0.053 3, wR2=0.112 4
  R indices (all data)	R1=0.047 8, wR2=0.108 1	R1=0.047 3, wR2=0.093 8	R1=0.044 1, wR2=0.106 6	R1=0.101 5, wR2=0.135 1
  (Δρ)max, Δρ)min / (e·nm-3)	1 228, -515	361, -484	1 857, -464	365, -523

  2.   Results and discussion

  2.1   Crystal structure description for complex 1

  Single‐crystal diffraction shows that complex 1 crystallizes in the monoclinic system with P2₁/n space group. The asymmetric unit of 1 consists of one Cd(Ⅱ) ion, one 4OHphCOO^- ion, one mbtx molecule, and one NO₃^- ion. The Cd(Ⅱ) ion is seven‐coordinated by three carboxylate oxygen atoms (O1, O1A, O2A) from two 4OHphCOO^- ligands, four nitrogen atom (N1, N2A, N4C, N5B) from four mbtx ligands to give a [ZnO₃N₄] distorted pentagonal bipyramid (Fig. 1a). The dihedral angle between two triazole rings of mbtx is 62.889°. The dihedral angles between the two triazole rings and the benzene ring are 43.437° and 78.681°, respectively. Each mbtx ligand shows a tetradentate coordinated mode, and bridges four Cd(Ⅱ) ions to form a 2D network [Cd(mbtx)]_n (Fig. 1b). [Cd(mbtx)(4OHphCOO)]_n does not increase in dimension because 4OHphCOO^- connects adjacent two Zn(Ⅱ) ions in 1 (Fig. 1c).

  Figure 1

  

  Figure 1.  (a) Coordination environment of Cd(Ⅱ) ions in complex 1; (b) 2D network [Cd(mbtx)]_n in 1; (c) 2D network [Cd(mbtx)(4OHphCOO)]_n in 1; (d) 3D network constructed by π‐π stacking interactions (bright green dotted line) in 1

  Symmetry codes: A: -x+3/2, y+1/2, -z+1/2; B: x-1/2, -y-1/2, z+1/2; C: -x+2, -y, -z

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  There are strong π‑π stacking interactions between two benzene rings (Cg7: C11‐C16) of the 4OHphCOO^- ligands from the adjacent 2D network. The center‐to‐center distance and dihedral angle are 0.377 3 nm and 0°, respectively. The 2D networks [Cd(mbtx)(4OHphCOO)]_n are further connected to a 3D network by the strong π‑π stacking interactions (Fig. 1d).

  2.2   Crystal structure description for complex 2

  Complex 2 crystallizes in the triclinic system with P1 space group. The asymmetric unit consists of one Zn(Ⅱ) ion, one mbtx ligand, half a coordinated 1, 4‐bdc^2- ligand, half a free 1, 4‐bdc^2- ligand, two coordinated water molecules, and four lattice water molecules. The Zn(Ⅱ) ion is six‐coordinated by two carboxylate oxygen atoms (O1, O2) from 1, 4‐bdc^2- ligand, two coordination water (O5 and O6) and two nitrogen atoms (N1, N4A) from two mbtx ligands to give a [ZnO₄N₂] distorted octahedron geometry (Fig. 2a). Each mbtx adopts bis‐monodentate coordinated mode and bridges two Zn(Ⅱ) ions with a distance of 1.179 81(4) nm to form 1D chain [Zn(mbtx)]_n. The two carboxyl groups (O1, O2, O1C, and O2C) of coordinated 1, 4‐bdc^2- ligand show bidentate chelated coordinated mode and connect two Zn(Ⅱ) ions with a distance of 1.087 00(3) nm. Ligand 1, 4‐bdc^2- bridges adjacent [Zn(mbtx)]_n to form a 1D chain [Zn(mbtx)(1, 4‐bdc)_0.5]_n (Fig. 2b). The 1D chains are further extended into a 2D network by hydrogen bonds and strong π‐π stacking interactions. Hydrogen bonds are observed between coordinated water molecules and carboxylate oxygen atoms from adjacent 1D chains (O5…O2E 0.275 5(3) nm, Symmetry code: E: -x, -y, -z+2). Strong π‐π stacking interactions exist between the triazole ring of mbtx and the benzene ring of the coordinated 1, 4‐bdc^2- ligand from adjacent 1D chains. The center‐to‐center distance and dihedral angle of Cg3 (N4‐N5‐C10‐N6‐C9) and Cg5 (C11‐C12‐C13‐C11C‐C12C‐C13C) are 0.365 6 nm and 3.844°, respectively (Fig. 2c).

  Figure 2

  

  Figure 2.  (a) Coordination environment of Zn(Ⅱ) ions in complex 2; (b) 1D chain constructed by coordinated 1, 4‐bdc^2- and mbtx in 2; (c) 2D network constructed by hydrogen bonds and π‐π stacking interactions in 2; (d) 1D chain constructed by free 1, 4‐bdc^2- and lattice water; (e) 3D network of 2

  Symmetry codes: A: x, y, z+1; C: -x+1, -y, -z+2; F: x+1, y+1, z; G: -x, -y, -z+1; I: 1-x, 1-y, 1-z; In panels c‐e, bright green and orange dotted lines represent hydrogen bonds and π‐π stacking interactions, respectively.

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  There are half a free 1, 4‐bdc^2- ligand and four lattice water molecules in the asymmetric unit of 2. So, there are abundant intermolecular hydrogen bonds. The free 1, 4‐bdc^2- ligand and four lattice water molecules are connected to form a 1D chain by intermolecular hydrogen bonds (Fig. 2d). The parameters of hydrogen bonds are listed in Table S2. 1D chain constructed by hydrogen bond passes through the 2D network to form a 3D structure (Fig. 2e). There are strong π‐π stacking interactions between the benzene ring of free 1, 4‐bdc^2- ligand and the other triazole ring of mbtx (Fig.S1), simultaneously. The center‐to‐center distance and dihedral angle of Cg2 (N1‐N2‐C8‐N3‐C7) and Cg6 (C15‐C16‐C17‐C15D‐C16D‐C17D) are 0.369 4 nm and 15.486°, respectively.

  2.3   Crystal structure description for complex 3

  Single‐crystal diffraction shows that complex 3 crystallizes in the triclinic system with P1 space group. The asymmetric unit of 3 consists of Cd1 and Cd2 ions, two 5NO₂‐bdc^2- ions, one mbtx molecule, three coordination water molecules, and four and a half lattice water molecules. The Cd1 is six‐coordinated by three carboxylate oxygen atoms (O1, O9A, O10A) from two 5NO₂‐bdc^2- ligands, O13 and O14 from two water molecules, and N1 from the mbtx ligand to give a [CdO₅N] distorted octahedron geometry. The Cd2 is seven‐coordinated by five carboxylate oxygen atoms (O2, O3C, O4C, O7, O8) from three 5NO₂‐bdc^2- ligands, O15 from the water molecule, N4B from the mbtx ligand to give a [CdO₆N] distorted pentagonal bipyramid geometry (Fig. 3a). The Cd—O/N bond lengths are in the normal range of 0.219 9(2)‐0.259 2(3) nm, which is similar to those observed in the other Cd—O/N complexes^[21]. In 3, 5NO₂‐bdc^2- has two kinds of coordination modes. Two carboxylate groups (O7, O8, O9, and O10) of one 5NO₂‐bdc^2- ligand adopt a chelated mode and connect two Cd(Ⅱ)ions with a distance of 0.933 96 nm. Two carboxylate groups (O1, O2, O3, and O4) of the other 5NO₂‐bdc^2- ligand adopt a bis‐monodentate and chelated coordinated mode, and bridge three Cd(Ⅱ) ions. The carboxylate group of O1 and O2 connects two Cd(Ⅱ) ions with a distance of 0.501 88 nm. Two kinds of 5NO₂‐bdc^2- ligand alternately connect Cd1 and Cd2 to form a 1D double stranded chain [Cd₂(5NO₂‐bdc)₂]_n (Fig. 3b). Ligand mbtx exhibits bis‐monodentate coordinated mode and connects the adjacent 1D chain [Cd₂(5NO₂‐bdc)₂]_n to form a 2D network (Fig. 3c). The topological analysis of 3 has been performed by considering both the metal ion and the ligand as topological nodes. Each Cd1 ion coordinates with one mbtx ligand and two 5NO₂‐bdc^2- ligands and is a 3‐connected node. Each Cd2 ion coordinates with three 5NO₂‐bdc^2- ligands and one mbtx ligand and is 4‐connected. Two kinds of 5NO₂‐bdc^2- ligands exhibit the 2‐and 3‐connected, respectively. Each mbtx ligand is a 2‐connected node. The 2D network of 3 can be simplified as a (3, 3, 4)‐connected network with point symbol (4·6·8)(6²·8)(4·6·8⁴) (Fig. 3d).

  Figure 3

  

  Figure 3.  (a) Coordination environment of Cd(Ⅱ) ions in complex 3; (b) 1D chain constructed by Cd(Ⅱ) and 5NO₂‐bdc^2-; (c) 2D network of 3; (d) Schematic depiction of 2D (3, 3, 4)‐connected network in 3

  Symmetry codes: A: -x, -y+1, -z; B: -x, -y+2, -z+1; C: -x+1, -y+2, -z; In panel d, the dark and bright green lines represent 2‐connected 5NO₂‐bdc^2- and mbtx, respectively, and the red dots represent 3‐connected 5NO₂‐bdc^2-.

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  2.4   Crystal structure description for complex 4

  Complex 4 crystallizes in the triclinic system with P1 space group. The asymmetric unit consists of three Zn(Ⅱ) ions, two mbtx ligands, two btc^3- ligands, ten coordinated water molecules, and two lattice water molecules. The Zn1 and Zn2 are all five‐coordinated by one carboxylate oxygen atom (O1 or O7) from the btc^3- ligand, two coordination water molecules (O13 and O14, or O15 and O16), and two nitrogen atoms (N1 and N7, or N4 and N10A) from two mbtx ligands to give a [ZnO₃N₂] distorted trigonal bipyramid geometry. Zn3 is six‐coordinated by six water molecules (Fig. 4a). Two carboxyl groups of each btc^3- are free, and the third carboxyl group shows a monodentate coordinated mode. Ligand mbtx adopts a bis‐monodentate coordinated mode and bridges two Zn(Ⅱ) ions with the distance of 1.171 19(3) and 1.179 45(3) nm to form a 1D chain (Fig. 4b). There are strong π‐π stacking interactions between benzene rings of mbtx and benzene rings of btc^3- (Fig.S2). The center‐to‐center distance and dihedral angle of Cg5 (C1‐C6) and Cg7 (C21‐C26), Cg6 (C11‐C16) and Cg7, Cg5 and Cg8 (C30‐C35), Cg6 and Cg8 are 0.364 5 nm and 3.409°, 0.363 1 nm and 2.279°, 0.367 3 nm and 1.889°, 0.365 5 nm and 0.839°, respectively. The adjacent 1D chains are connected to a 2D network by π‐π stacking interactions (Fig. 4c). The intermolecular hydrogen bonds between water molecules coordinated to Zn3 and carboxylate oxygen atoms connect the adjacent 2D network into a 3D network (Fig. 4d). The parameters of hydrogen bonds are listed in Table S3.

  Figure 4

  

  Figure 4.  (a) Coordination environment of Zn(Ⅱ) ions in complex 4; (b) 1D chain in 4; (c) 2D network constructed by π‐π stacking interaction in 4; (d) 3D network of 4

  Symmetry code: A: x+2, y+1, z; In panels c and d, the bright green and bright orange dotted lines express hydrogen bonds and π‐π stacking interactions, respectively.

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  2.5   PXRD and TGA

  Both measured and simulated PXRD patterns of complexes 1‐4 are shown in Fig.S3, which are in agreement with each other, indicating that the crystalline samples of 1‐4 have high phase purity. The thermal stability of 1‐4 was studied by the TG technique from room temperature to 800 ℃ with a heating rate of 10 ℃·min^-1 under a nitrogen atmosphere, and the corresponding plots are shown in Fig. 5. Before 300 ℃, 1 decomposed very slowly, but the framework showed thermostability. After 300 ℃, the decomposition happened quickly until 344 ℃ with a weight loss of 37.8%, which may be due to the release of coordinated NO₃^- and p‐hydroxybenzoate (38.0% calculated from the formula). Then the decomposition happened continuously until 800 ℃. The main residue should be CdO (Calcd. 24.5%, Obsd. 28.7%). For 2, lattice water molecules and free 1, 4‐bdc^2- were lost from 27‐273 ℃ (Calcd. 28.03%, Obsd. 26.2%). The framework of 2 began to decompose after 273 ℃. At about 553 ℃, the weight loss curve has leveled off, indicating that 2 was completely decomposed, and a white product was obtained. The total weight loss was 84.3%, which is comparable with the value of 85.2% expected for 2 degrading to ZnO. The first step of weight loss for 3 appeared in a range of 26.5‐284 ℃ with a weight loss of 12.3%, which corresponds to the release of lattice water and coordinated water molecules (Calcd. 13.6%). The framework of 3 was thermally stable up to 284 ℃. Then the decomposition happened continuously until 800 ℃. The main residue should be CdO (Calcd. 25.9%, Obsd. 25.8%). For 4, lattice water molecules and coordinated water molecules were lost from 31‐150 ℃ (Calcd. 17.3%, Obsd. 16.1%). The framework of 4 began to decompose slowly from 150 to 285 ℃. After 285 ℃, the decomposition happened quickly until 800 ℃. The main residue was black powder, which indicates that 4 has not completely decomposed.

  Figure 5

  

  Figure 5.  TG curves of complexes 1‐4

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  2.6   Solid‐state luminescence and UV‐Vis absorption spectra

  The solid‐state luminescent properties of complexes 1‐4 and the ligands were investigated at room temperature. No emission of 3 was observed at room temperature. The UV‐Vis diffuse reflectance absorption spectra of 1, 2, 4, and free ligands were investigated in the solid state. As shown in Fig.S4, the UV‐Vis absorption spectra of free ligands mbtx, 4OHphCOOH, and H₃btc consisted of two intense absorption bands, 215 and 254 nm for mbtx, 213 and 278 nm for 4OHphCOOH, and 212 and 289 nm for H₃btc. UV‐Vis absorption spectra of 1, 4‐H₂bdc consisted of two intense and one weak absorption bands. Both absorption bands are dominated by a π‐π* transition^[22-23]. The absorption bands of 1, 2, and 4 were similar. The absorption bands were centred at 267, 272, and 262 nm for 1, 2, and 4, respectively (Fig. 6). Compared with the free ligands, the UV absorption of 1, 2, and 4 exhibited increase in intensity and absorption bands were between O‐donor ligand and N‐donor ligand, which may be attributed to the intraligand and ligand‐to‐ligand charge^[24]. As shown in Fig. 7, 1 and 2 exhibited the emission band maximum at 405 and 354 nm upon excitation at 313 and 325 nm. 4 showed two emission peaks with a strong emission at 418 nm and weak emission peaks at 520 nm upon excitation at 330 nm. The free ligand mbtx^[25], 1, 4‐H₂bdc^[26], and H₃btc^[22] exhibited the emissions at 390, 388, and 325 nm, respectively. The free 4OHphCOOH exhibited the emission at 418 nm upon excitation at 310 nm (Fig.S5). For the free mbtx, 4OHphCOOH, 1, 4‐H₂bdc, and H₃btc, the chromophores are the aromatic rings, and the observed emission is assigned to the π‐π* transition^[27-28]. Fluorescent emission of 1 exhibited a slight red shift compared to mbtx and a blue shift compared to 4OHphCOOH. The emission at 405 nm for 1 can be attributed to the emission of the mbtx and 4OHphCOO combination^[29]. Emissions of 2 and 4 exhibited blue shift and red shift compared to the two kinds of ligand, respectively. Due to the d¹⁰ configuration, Zn(Ⅱ) ion is difficult to oxidize or reduce, and the emissions are neither metal‐to‐ligand charge transfer (MLCT) nor ligand‐to‐metal charge transfer (LMCT). The emissions can be attributed to the ligand‐to‐ligand charge transitions (LLCT) of the mbtx and carboxylate ligands^{[24, 30]}.

  Figure 6

  

  Figure 6.  Solid state UV‐Vis absorption spectra of complexes 1, 2, and 4

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  Figure 7

  

  Figure 7.  Solid state photoluminescent spectra of complexes 1, 2, and 4 at room temperature

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  3.   Conclusions

  In short, four Cd(Ⅱ)/Zn(Ⅱ) CPs were synthesized using the rigid bi(triazole) ligand 1, 3‐bis(4H‐1, 2, 4‐triazol)benzene and four carboxylate ligands at room temperature. Complex 3 presents a (3, 4, 4)‐connected 2D network with a point symbol of (4·6·8)(6²·8)(4·6·8⁴). There are π‐π stacking interactions in complexes 1, 2, and 4. At the same time, there are abundant intermolecular hydrogen bonds in 2 and 4. In 2, a 3D network is formed by a 1D chain (constructed by hydrogen bonds) and a 2D network (constructed by π‐π stacking interactions). In 4, 1D chains are connected into a 3D network due to intermolecular hydrogen bonding and π‐π interactions. All of these indicate the importance of intermolecular hydrogen bonds and π‐π interactions in constructing 3D structures of complexes. The successful synthesis of these four complexes indicates that the structures of CPs/MOFs can be tuned with different multicarboxylates and rigid bis(triazole) ligands.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Two different coordination modes of mbtx

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  Scheme 2  Coordination modes of 5NO₂‐bdc^2-, 4OHphCOO^-, 1, 4‐bdc^2-, and btc^3-

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  Figure 1  (a) Coordination environment of Cd(Ⅱ) ions in complex 1; (b) 2D network [Cd(mbtx)]_n in 1; (c) 2D network [Cd(mbtx)(4OHphCOO)]_n in 1; (d) 3D network constructed by π‐π stacking interactions (bright green dotted line) in 1

  Symmetry codes: A: -x+3/2, y+1/2, -z+1/2; B: x-1/2, -y-1/2, z+1/2; C: -x+2, -y, -z

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  Figure 2  (a) Coordination environment of Zn(Ⅱ) ions in complex 2; (b) 1D chain constructed by coordinated 1, 4‐bdc^2- and mbtx in 2; (c) 2D network constructed by hydrogen bonds and π‐π stacking interactions in 2; (d) 1D chain constructed by free 1, 4‐bdc^2- and lattice water; (e) 3D network of 2

  Symmetry codes: A: x, y, z+1; C: -x+1, -y, -z+2; F: x+1, y+1, z; G: -x, -y, -z+1; I: 1-x, 1-y, 1-z; In panels c‐e, bright green and orange dotted lines represent hydrogen bonds and π‐π stacking interactions, respectively.

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  Figure 3  (a) Coordination environment of Cd(Ⅱ) ions in complex 3; (b) 1D chain constructed by Cd(Ⅱ) and 5NO₂‐bdc^2-; (c) 2D network of 3; (d) Schematic depiction of 2D (3, 3, 4)‐connected network in 3

  Symmetry codes: A: -x, -y+1, -z; B: -x, -y+2, -z+1; C: -x+1, -y+2, -z; In panel d, the dark and bright green lines represent 2‐connected 5NO₂‐bdc^2- and mbtx, respectively, and the red dots represent 3‐connected 5NO₂‐bdc^2-.

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  Figure 4  (a) Coordination environment of Zn(Ⅱ) ions in complex 4; (b) 1D chain in 4; (c) 2D network constructed by π‐π stacking interaction in 4; (d) 3D network of 4

  Symmetry code: A: x+2, y+1, z; In panels c and d, the bright green and bright orange dotted lines express hydrogen bonds and π‐π stacking interactions, respectively.

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  Figure 5  TG curves of complexes 1‐4

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  Figure 6  Solid state UV‐Vis absorption spectra of complexes 1, 2, and 4

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  Figure 7  Solid state photoluminescent spectra of complexes 1, 2, and 4 at room temperature

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  Table 1.  Crystallographic data and structure refinements of complexes 1‐4

  Parameter	1	2	3	4
  Formula	C17H13CdN7O6	C18H24ZnN6O10	C26H29Cd2N8O19.5	C38H46Zn3N12O24
  Formula weight	523.74	549.80	990.37	1 250.98
  T / K	293(2)	293(2)	223(2)	293(2)
  Crystal system	Monoclinic	Triclinic	Triclinic	Triclinic
  Space group	P21/n	P1	P1	P1
  a / nm	1.478 40(19)	0.975 2(3	1.062 1(2)	1.200 9(3)
  b / nm	0.774 55(10)	1.106 9(4)	1.289 3(2)	1.316 8(3)
  c / nm	1.713 0(2)	1.179 8(4)	1.327 4(3)	1.5717(4)
  α / (°)		106.171(5)	85.295(8)	93.916(4)
  β / (°)	94.734(3)	101.963(7)	83.664(6)	100.587(5)
  γ / (°)		104.732(4)	85.406(7)	108.232(4)
  V / nm3	1.954 8(4)	1.128 7(7)	1.795 8(6)	2.299 2(10)
  Z	4	2	2	2
  F(000)	1 040	568	986	1 280
  Dc / (g·cm-3)	1.780	1.618	1.832	1.807
  μ / mm-1	1.170	1.156	1.277	1.658
  Reflection collected	17 582	11 108	15 852	22 388
  Unique reflection	3 568(Rint=0.038 1)	4 116(Rint=0.030 4)	7 636(Rint=0.021 6)	10 360(Rint=0.036 0)
  Goodness of fit on F 2	1.022	1.030	1.036	1.024
  Final R indices [I > 2σ(I)]	R1=0.039 7, wR2=0.101 9	R1=0.038 0, wR2=0.087 9	R1=0.032 9, wR2=0.102 3	R1=0.053 3, wR2=0.112 4
  R indices (all data)	R1=0.047 8, wR2=0.108 1	R1=0.047 3, wR2=0.093 8	R1=0.044 1, wR2=0.106 6	R1=0.101 5, wR2=0.135 1
  (Δρ)max, Δρ)min / (e·nm-3)	1 228, -515	361, -484	1 857, -464	365, -523

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