English

• 0.   Introduction

  During the last thirty years, 3,4,5-trisubstituted 1,2,4-triazole compounds have been widely adopted as important bridging ligands in coordination chemistry owing to their variable coordinating modes and interesting topological structures^[1]. In particular, some iron(Ⅱ) complexes based on 3,4,5-trisubstituted 1,2,4-triazoles can display intriguing spin-crossover properties^[2], which enable their application across fields ranging from molecular switches and display devices to optical sensors and information storage materials^[3-7].

  Our research group has been devoted to the design and synthesis of various 3,4,5-triaryl-substituted 1,2,4-triazole derivatives for two decades. Up to now, a series of asymmetrically 3,4,5-triaryl-substituted 1,2,4-triazoles containing five-membered aromatic heterocycles such as pyrrole, furan, thiophene, and imidazole; six-membered aromatic rings such as benzene, pyridine, and pyrimidine; and aromatic polycycles such as naphthalene and quinoline have been reported^[8-26]. However, 3-pyridazinyl (Pyd)-substituted 1,2,4-triazole compounds remain exceptionally scarce. To date, there has been only one known example. In 2016, Prof. Brooker reported 4-(4-methylphenyl)-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L) and its spin-crossover binuclear iron(Ⅱ) complex, [Fe₂(L)₂(CH₃CN)₄](BF₄)₄·2CH₃CN^[27-28]. As a continuation of our research on asymmetrically substituted triaryltriazoles, we report herein the first syntheses of three novel 3-Pyd substituted triaryltriazoles: 4-(4-R-phenyl)-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L¹: R=OCH₃; L²: R=H; L³: R=F) (Scheme S1, Supporting information) and their Cu(Ⅱ) complexes, [Cu(L¹)₂(NO₃)₂]·H₂O (1), [Cu(L²)₂ (NO₃)₂] (2) and [Cu(L³)₂(H₂O)₂](NO₃)₂ (3) (Scheme 1). The crystal structures of ligands (L¹)₃, L², L³ and three Cu(Ⅱ) complexes 1-3 have been determined by single-crystal X-ray crystallography, and their intermolecular interactions have been investigated in detail by Hirshfeld surface analysis.

  Scheme 1

  

  Scheme 1.  Synthetic routes of complexes 1-3

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  1.   Experimental

  1.1   Materials and measurements

  Unless otherwise stated, all reagents and solvents were purchased from commercial sources and used without further purification. Melting points were determined with an X-4 digital microscope melting-point apparatus (Beijing) and are uncorrected. Elemental analyses (C, H, N) were performed on a Thermo Finnigan Flash 1112A elemental analyzer. Infrared (IR) spectra were recorded in a range of 4 000-400 cm^-1 on a Nicolet Avatar 380 FTIR spectrophotometer using KBr pellets. Thermogravimetric analysis (TGA) was performed with a simultaneous NETZSCH STA 449C thermal analyzer under flowing nitrogen from 25 to 600 ℃ at a heating rate of 5 ℃·min^-1. ¹H NMR spectra were measured on a Bruker AM 400 MHz spectrometer in DMSO-d₆ solution. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer equipped with Cu Kα radiation (λ=0.154 06 nm) at 40 kV and 40 mA in a range of 5°-50°.

  1.2   Syntheses of ligands L¹-L³

  Ligands L¹-L³ were synthesized according to our reported procedure^[25], and their synthetic route is shown in Scheme S1.

  4-(4-methoxyphenyl)-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L¹): white powder (Yield: 70.5%, m.p. 202.3-203.5 ℃). Anal. Calcd. for C₁₈H₁₄N₆O(%): C, 65.45; H, 4.27; N, 25.44. Found(%): C, 65.64; H, 4.46; N, 25.32. IR (KBr, cm^-1): 3 077(w), 2 957(w), 1 576(s), 1 570(s), 1 241(m), 823(m). ¹H NMR (400 MHz): δ 9.22-9.23(d, 1H, J_H,H=4.0 Hz, Pyd-H), 8.42-8.43(d, 1H, J_H,H=4.0 Hz, Py-H), 8.20-8.22(d, 1H, J_H,H=8.0 Hz, Py-H), 7.92-7.99(m, 2H, Py-H and Pyd-H), 7.83-7.87(m, 1H, Pyd-H), 7.41-7.48(m, 1H, Py-H), 7.21-7.23(d, 2H, J_H,H=8.0 Hz, Ph-H), 7.13-7.15(d, 2H, J_H,H=8.0 Hz, Ph-H), 2.30(s, 3H, OCH₃).

  4-phenyl-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L²): colorless block-shaped crystal (Yield: 73.8%, m.p. 192.5-193.7 ℃). Anal. Calcd. for C₁₇H₁₂N₆(%): C, 67.99; H, 4.03; N, 27.98. Found(%): C, 67.84; H, 4.16; N, 27.76. IR (KBr, cm^-1): 3 060(m), 1 580(m), 1 500(s), 833(m), 775(m). ¹H NMR (400 MHz): δ 9.22-9.23(d, 1H, J_H,H=4.0 Hz, Pyd-H), 8.38-8.39(d, 1H, J_H,H=4.0 Hz, Py-H), 8.23-8.25(d, 1H, J_H,H=8.0 Hz, Py-H), 8.00-8.02(d, 1H, J_H,H=8.0 Hz, Py-H), 7.93-7.97(t, 1H, Pyd-H), 7.41-7.44(m, 1H, Pyd-H), 7.34-7.38(m, 6H, Py-H and Ph-H).

  4-(4-fluorophenyl)-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L³): white powder (yield: 71.8%; m.p. 189.5-190.6 ℃). Anal. Calcd. for C₁₇H₁₁N₆F(%): C, 64.15; H, 3.48; N, 26.40. Found(%): C, 64.31; H, 3.36; N, 26.56. IR (KBr, cm^-1): 3 060(w), 1 590(s), 1 510(s), 1 090(w), 849(m). ¹H NMR (400 MHz): δ 9.22-9.23(d, 1H, J_H,H=4.0 Hz, Pyd-H), 8.40-8.41(d, 1H, J_H,H=4.0 Hz, Py-H), 8.27-8.29(d, 1H, J_H,H=8.0 Hz, Py-H), 8.05-8.07(d, 1H, Py-H), 7.94-7.98(t, 1H, J_H,H=8.0 Hz, Pyd-H), 7.84-7.88(m, 1H, Pyd-H), 7.41-7.48(m, 3H, J_H,H=8.0 Hz, Py-H and Ph-H), 7.19-7.23(t, 2H, J_H,H=8.0 Hz, Ph-H).

  1.3   Syntheses of complexes 1-3

  A solution of Cu(NO₃)₂·3H₂O (24.2 mg, 0.1 mmol) in anhydrous MeOH (3 mL) was added to a solution of L¹ (66.1 mg, 0.2 mmol) in MeOH (4 mL). The mixture was stirred for 6 h at room temperature. Then the green precipitate was separated by filtration, washed with water, and dried under vacuum to obtain complex 1 in a yield of 80.4%. The green block-shaped single crystals suitable for X-ray diffraction were obtained by slow evaporation from a methanol solution of 1. Anal. Calcd. for C₃₆H₃₀CuN₁₄O₉(%): C, 49.91; H, 3.49; N, 22.64. Found(%): C, 49.78; H, 3.32; N, 22.52. FTIR (KBr, cm^-1): 3 420(m, br), 3 060(w), 2 940(w), 1 510(s), 1 450(m), 1 380(vs), 1 260(s), 1 020(m), 837(m).

  The preparation procedure for 2 was the same as that for 1 except that L² (60.1 mg, 0.2 mmol) was used instead of the L¹ ligand. The yield of 2 was 71.6%. A green block-shaped single crystal suitable for X-ray diffraction was obtained by slow evaporation from a methanol solution of 2. Anal. Calcd. for C₃₄H₂₄CuN₁₄O₆(%): C, 51.81; H, 3.07; N, 24.88. Found(%): C, 51.59; H, 3.25; N, 24.76. FTIR (KBr, cm^-1): 3 060(w), 1 500(m), 1 460(m), 1 380(vs), 698(w).

  The synthesis procedure for 3 was the same as that for 1 except using L³ (63.7 mg, 0.2 mmol) to replace the L¹ ligand. The yield of 3 was 82.6%. A green block-shaped single crystal suitable for X-ray diffraction was obtained by slow evaporation from a methanol solution of 3. Anal. Calcd. for C₃₄H₂₆CuF₂N₁₄O₈(%): C, 47.47; H, 3.05; N, 22.80. Found(%): C, 47.34; H, 3.17; N, 22.62. FTIR (KBr, cm^-1): 3 411(m, br), 3 081(w), 1 580(m), 1 493(s), 1 380(vs), 1 320(m), 1 089(w), 841(m).

  1.4   Crystal structure determination

  Single crystals of (L¹)₃, L², L³ and 1-3 suitable for X-ray diffraction were obtained by slow evaporation of the corresponding solutions in methanol. The single-crystal X-ray diffraction data were collected on a Rigaku XtaLAB Synergy-R DW diffractometer using graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) for L², L³, 1 and 2, but Cu Kα radiation (λ=0.154 184 nm) for (L¹)₃ and 3. The unit cell parameters were determined by the multi-scan program using the ω-2θ method and subjected to empirical absorption correction using the CrysAlisPro program. The structures were solved by direct methods and refined on F² by full-matrix least squares procedures using SHELXTL software^[29]. All non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were calculated and refined as riding modes except water molecules. The F atom in L³ was disordered over two positions with an occupancy of 0.923(9) for F1 and 0.077(9) for F1A. The 4-methoxyphenyl group and the O2 atom of the NO₃^- ion in 1 were also disordered over two sites with an occupancy of 0.618(5) for O1, C12-C18, and 0.382(5) for O1A, C12A-C18A, and 0.65(3) for O2 and 0.35(3) for O2A. The O2 atom of the NO₃^- ion in 2 was disordered over two positions with an occupancy of 0.893(17) for O2 and 0.107(17) for O2A. The 4-fluorophenyl group and three O atoms of the NO₃^- ion in 3 were also disordered over two sites with an occupancy of 0.506(17) for F1, C12-C17 and O1-O3 and 0.494(17) for F1A, C12A-C17A and O1A-O3A. Crystallographic data of (L¹)₃, L², L³ and 1-3 are summarized in Table S1, and selected bond lengths and angles are listed in Table S2 and S3.

  1.5   Hirshfeld surface analysis

  The 3D Hirshfeld surface constructed from the electron distribution calculated by summing spherical atom electron densities can visualize the shape profile of a molecule within a crystal structure. For a given crystal structure with a set of spherical atom electron densities, the 3D Hirshfeld surface is unique. The normalized contact distance (d_norm) derived from the external distance (d_e) and internal distance (d_i) can be used to identify regions of particular importance for intermolecular interactions—where d_e is defined as the distance from a point on the surface to the nearest nucleus outside the surface, and d_i is the distance to the nearest nucleus inside the surface. The 2D fingerprint plot, which combines d_e and d_i, provides a comprehensive overview of the nature and types of all intermolecular contacts in the crystal. In this work, the 3D Hirshfeld surfaces mapped with d_norm and the corresponding 2D fingerprint plots were generated using the Crystal Explorer 3.1 program^[30].

  2.   Results and discussion

  2.1   Synthesis

  Three new 3-Pyd-substituted triaryltriazoles, L¹-L³, have been successfully synthesized by a multi-step organic synthesis (Scheme S1) with yields of 70.5%, 73.8% and 71.8%, respectively. The ligands were soluble in MeOH, MeCN and DMF but slightly soluble in acetone and ethanol. The structures of L¹-L³ have been characterized by FTIR, ¹H NMR, elemental analysis and single-crystal X-ray diffraction.

  As shown in Scheme 1, the ligands L¹-L³ react with Cu(NO₃)₂·3H₂O in a 2∶1 molar ratio in methanol, affording three mononuclear Cu(Ⅱ) complexes: trans-[Cu(L¹)₂(NO₃)₂]·H₂O (1), trans-[Cu(L²)₂(NO₃)₂] (2) and trans-[Cu(L³)₂(H₂O)₂](NO₃)₂ (3) with a yield of 80.4%, 71.6% and 82.6%, respectively. Three complexes are air-stable, and their compositions have been confirmed by elemental analyses. 1 contains one Cu(Ⅱ) cation, two L¹ ligands, two NO₃^- ions and one water molecule, 2 comprises one Cu(Ⅱ) ion, two L² ligands and two NO₃^- ions, while 3 consists of one Cu(Ⅱ) ion, two L³ ligands, two coordinated water molecules and two NO₃^- counterions. The complexes were slightly soluble in MeOH, EtOH and DMF but poorly soluble in H₂O, Me₂CO, MeCN, THF and CH₂Cl₂.

  2.2   Crystal structures of ligands (L¹)₃, L², L³

  The X-ray structure analyses indicate that the supramolecular trimer (L¹)₃ crystallizes in the triclinic P1 space group, while L² and L³ crystallize in the monoclinic space group P2₁/c and P2₁/n, respectively (Table S1). These ligands all consist of three aromatic rings and one central 1,2,4-triazole ring (Fig.1). Furthermore, the substituted phenyl (Ph), pyridyl (Py), and Pyd rings are located in a propeller arrangement around the central 1,2,4-triazole ring (Trz). The relevant dihedral angles are listed in Table 1. Their bond lengths and angles are within the normal ranges observed for the related triaryltriazoles (Table S2)^{[15, 25]}.

  Figure 1

  

  Figure 1.  ORTEP views of (L¹)₃, L², L³ with 20% thermal ellipsoids probability

  The disordered atoms are omitted for clarity.

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  Table 1

  

  Table 1.  Dihedral angles (°) for (L¹)₃, L², L³ and 1-3

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  Compounds	Py/Trz	Ph/Trz	Pyd/Trz
  (L1)3 (O1)	11.2(2)	78.5(4)	11.7(1)
  (L1)3 (O2)	26.0(2)	83.2(4)	23.1(3)
  (L1)3 (O3)	35.7(1)	87.1(2)	24.2(4)
  L2	29.1(3)	77.2(1)	41.4(4)
  L3	21.7(3)	87.7(2)	23.6(1)
  1	5.5(2)	79.9(3)	3.6(2)
  2	6.6(1)	83.0(2)	2.9(2)
  3	5.8(3)	69.9(1)	29.4(2)

  Notably, in the asymmetric unit of L¹, there is a hydrogen-bonded trimer (L¹)₃ (Fig.1), which has not been observed in any of the known triaryltriazoles until now^{[1, 5, 10, 12, 14-15, 20-25]}. These interactions include six kinds of C—H…N hydrogen bonds, one type of C—H…O hydrogen bond, four kinds of edge-to-face C—H…π interactions and two types of π…π stacking interactions [π(N5-pyd)…π(N11-pyd), Cg1…Cg3 0.381 2(3) nm/1.43°; π(N7-py)…π(N17-pyd), Cg2…Cg4 0.386 2(2) nm/14.24°] (Table S4). Among them, the L¹ with O1 and the L¹ with O3 are first linked by C9—H9…N15 and C45—H45…N3 hydrogen bonds to form a dimer, and then this dimer further connects the L¹ containing O2 through C31—H31…π(Trz) interaction and π(Pyd)…π(Pyd) and π(Py)…π(Pyd) stacking interactions to form a trimer (Fig.S1). Ultimately, two trimers interconnect with each other by other hydrogen bonds to produce a 3D supramolecular network (Fig.2). Additionally, the hydrogen-bond interactions link the molecules of L² into a 1D chain, while a 2D layered structure is observed in L³ (Fig.S2). Obviously, the different substituted groups on the 4-R-phenyl ring (R=OCH₃, H, F) can influence the crystal packing of the corresponding ligands L¹-L³.

  Figure 2

  

  Figure 2.  Three-dimensional network of (L¹)₃

  Symmetry code: ^ⅴ 1-x, 1-y, 1-z.

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  2.3   Crystal structures of complexes 1-3

  The ORTEP views of 1-3 are shown in Fig.3 together with the atomic labeling system. Because 1 and 2 crystallize in the same monoclinic P2₁/c space group (Table S1) and have an almost similar structure except for the lattice water, herein, only the structure of 1 is discussed in detail. The asymmetric unit of 1 includes one Cu(Ⅱ) ion (the occupancy factor is 0.5), one L¹ ligand, one coordinated NO₃^- ion, and a half H₂O molecule. The Cu(Ⅱ) ion in 1 displays a distorted trans-[CuN₄O₂] octahedron with four N atoms from two L¹ ligands in the equatorial plane and two NO₃^- ions in the axial positions, as confirmed by the SHAPE software with the continuous symmetry measures (CSM) of 1.22^[31]. Each L¹ ligand coordinates to the Cu(Ⅱ) ion via the N1 atom of the pyridyl and N2 atom of the triazole, very similar to the coordinated modes of the triaryltriazoles in the reported mononuclear Cu(Ⅱ) complexes^{[11, 13, 22-24, 26]}. The 3-pyridazinyl group of the L¹ ligand is uncoordinated. The Cu—N bond lengths are within the normal ranges observed for the triaryltriazole-based Cu(Ⅱ) complexes (Table S3)^[22-24]. The triazole forms dihedral angles of 5.5(2)°, 79.9(3)° and 3.6(2)° with the Py, Ph and Pyd rings, respectively (Table 1), revealing that the pyridyl, triazole and 3-pyridazinyl rings of the L¹ ligand in 1 are almost coplanar. Moreover, the dihedral angle between the coordinated pyridyl and the triazole ring in 1 becomes quite smaller than that in the free L¹ ligand (Table 1), unveiling that the L¹ ligand in 1 adopts an optimal conformation in favor of coordination^[10]. In 1, there are two kinds of strong hydrogen bond interactions involving lattice H₂O [O1W— H1WA…N6: 0.292 5(4) nm/179° and O1W—H1WB…O1^iv: 0.312 7(3) nm/155°] and four kinds of weak C— H…O hydrogen bond interactions involving the NO₃^- ions [C2—H2…O2^ⅰ=0.328 9(6) nm/132°, C9—H9…O2^ⅱ=0.327 1(1) nm/153°, C10—H10…O3^ⅲ: 0.333 3(4) nm/136° and C11—H11…O4^ⅲ: 0.314 6(3) nm/155°] (Table S4, Fig.S6). Additionally, there are a kind of C—H…π interaction [C4—H4…π(Ph): 0.368 9(4) nm/150°] and a kind of lone-pair p-electron…π interaction [O4^ⅴ…π(Trz): 0.318 8(3) nm] (Table S5). These interactions further link the mononuclear unit and lattice water to create a 3D supramolecular framework (Fig.4).

  Figure 3

  

  Figure 3.  ORTEP views of 1-3 with 30% thermal ellipsoids probability

  All H and disordered atoms are omitted for clarity; Symmetry codes: ^ⅰ 1-x, 1-y, 1-z for 1 and 2; ^ⅰ 1-x, 1-y, -z for 3.

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  Figure 4

  

  Figure 4.  Three-dimensional supramolecular frameworks of 1-3

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  Different from 1 and 2, complex 3 crystallizes in the triclinic P1 space group (Table S1) and its asymmetric unit consists of a Cu(Ⅱ) cation (the occupancy factor is 0.5), one L³ ligand, one coordinated H₂O molecule, and one NO₃^- ion. The Cu(Ⅱ) ion also exhibits a distorted trans-[CuN₄O₂] octahedron (CSM=1.18) but contains four N atoms from two L³ ligands in the equatorial plane and two H₂O molecules in the axial positions. Each L³ ligand coordinates to the Cu(Ⅱ) ion via the N1 atom of the pyridyl and N2 atom of the triazole, while the 3-pyridazinyl group is also uncoordinated, which is very similar to the coordinated modes of L¹ in 1. The NO₃^- ion only acts as the counterion and does not involve coordination. The Cu—N and Cu—O bond distances are all within the normal ranges found for the related Cu(Ⅱ) complexes with the triaryltriazoles (Table S3)^{[22, 24, 26]}. The triazole ring makes dihedral angles of 5.8(3)°, 69.9(1)° and 29.4(2)° with the Py, Ph and Pyd rings, respectively (Table 1), disclosing that the pyridyl and triazole rings of the L³ ligand in 3 are almost coplanar to make a better coordination^[10]. In 3, there are two kinds of strong hydrogen bond interactions involving the coordinated H₂O [O1W—H1WA…O1: 0.275 3(5) nm/132°; O1W—H1WB…O2^ⅳ: 0.287 3(2) nm/158°] and three kinds of weak C—H…O hydrogen bond interactions involving the NO₃^- ions [C10—H10…O1^ⅱ: 0.336 4(2) nm/144°; C13—H13…O3: 0.339 9(3) nm/164°; C17—H17…O3^ⅲ: 0.330 6(1) nm/173°] (Table S4, Fig.S8). In addition, there are a kind of C—H…π interaction [C4—H4…π(Ph): 0.386 7(3) nm/149°] and a kind of lone-pair p-electron…π interaction [O2^ⅴ…π(Trz): 0.293 3(2) nm] (Table S5). These interactions further connect the mononuclear units and NO₃^- ions to generate a 3D framework (Fig.4).

  2.4   PXRD analysis and IR spectra of 1-3

  As shown in Fig.S9-S11, the PXRD patterns of as-synthesized samples of 1-3 are in good agreement with the simulated ones from the single crystal data, displaying the high phase purity of the bulk samples.

  In the IR spectra of 1-3 (Fig.S12-S14), a medium-broad peak at 3 420 cm^-1 (1) and 3 411 cm^-1 (3) can be attributed to the O—H stretching vibrations of water molecules^[26]. A strong band at 1 510 cm^-1 (1), 1 500 cm^-1 (2) and 1 580 cm^-1 (3) can be assigned to the coordinated pyridyl ring vibration^{[10, 24]}. A very strong band at 1 380 cm^-1 is assigned to characteristic N=O stretching vibrations of the NO₃^- ion in 1-3^{[22, 24, 26]}. Two bands at 1 260 and 1 020 cm^-1, respectively, are due to asymmetric and symmetric stretching vibrations of the O—CH₃ group in 1^[8]. In addition, the stretching vibration of C—F at 1 089 cm^-1 can be found in 3^[24]. These features are in agreement with the X-ray analysis results.

  2.5   Thermal stability of 1-3

  As shown in Fig.S15, the first weight loss of 2.2% between 25 and 135 ℃ for 1 was observed due to the loss of one lattice water molecule (Calcd. 2.1%). Then 1 started to decompose, and the remaining weight of 9.5% after heating to 485 ℃ is owing to the final residue of CuO (Calcd. 9.2%). Due to the lack of any guest molecule, 2 showed high thermal stability. Practically no weight loss was observed up to 290 ℃ (Fig.S16). Then an abrupt decomposition was observed. The remaining weight of 10.5% after heating to 490 ℃ is owing to the final residue of CuO (Calcd. 10.1%). In 3, the first weight loss of 4.4% between 25 and 175 ℃ was found because of the loss of two coordinated water molecules (Calcd. 4.2%). Then 3 began to collapse, and the remaining weight of 9.3% after heating to 500 ℃ is due to the final residue of CuO, in agreement with the calculated value of 9.2% (Fig.S17). Notably, the differences in substituted groups on the 4-R-phenyl ring (R=OCH₃, H, F) of ligands L¹-L³ can also influence the thermal stabilities of the corresponding complexes, with decomposition temperatures of 235, 290 and 258 ℃ for 1-3, respectively.

  2.6   Hirshfeld surface analyses

  Hirshfeld surface analysis serves as a powerful visualization tool for probing weak intermolecular interactions, such as hydrogen bonds, C—H…π contacts and π…π stacking interactions^[25-26]. To elucidate the intermolecular interactions in the three ligands (L¹)₃, L², L³ and their complexes 1-3, 3D Hirshfeld surface analyses were performed. The corresponding 2D fingerprint plots provide a clear quantitative analysis of each type of interaction present in the six compounds^[31]. Fig.5 shows the 3D Hirshfeld surfaces of (L¹)₃, L², L³ and 1-3 mapped with d_norm, respectively. The surface is shown with a red-white-blue color: the red spots represent close contacts such as O—H…O, O—H…N and C—H…N interactions, while the white and blue regions stand for weak contacts, like C—H…π and p…π interactions. For (L¹)₃, there are six strong red dots close to the 4-methoxyphenyl group and pyridyl ring. For L², there are two strong red dots close to the pyridazinyl group and triazole ring. For L³, five strong red dots are found near the pyridazinyl group and pyridyl ring. For 1, there are five strong red dots close to the water molecule, one methoxyphenyl group, and one NO₃^- ion. For 2, there are four strong red dots close to the phenyl group and the NO₃^- ion. For 3, seven strong red dots are found near the water molecule, the 4-fluorophenyl group, and the NO₃^- ion.

  Figure 5

  

  Figure 5.  Three-dimensional Hirshfeld surfaces of (L¹)₃, L², L³ and 1-3 mapped with d_norm

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  The 2D fingerprint plots of (L¹)₃, L², L³ and 1-3 are shown in Fig.S18-S23, respectively. As summarized in Table S6, the maximum contribution is from H…H contacts for all the compounds: 45.8% in (L¹)₃ and 35.9% in 1, 39.7% in L² and 31.7% in 2, 27.3% in L³ and 27.0% in 3. The next contribution is from C…H/H…C contacts (23.5%) for (L¹)₃ but O…H/H…O contacts (24.8%) for 1; N…H/H…N contacts (29.0%) for L² but O…H/H…O contacts (22.6%) for 2; C…H/H…C contacts (24.8%) and N…H/H…N contacts (24.7%) for L³ but N…H/H…N contacts (17.5%) and O…H/H…O contacts (16.7%) for 3. These results reveal that there are some important changes in the types of weak intermolecular interactions between the free ligands (L¹)₃, L², L³ and their corresponding Cu(Ⅱ) complexes 1-3.

  3.   Conclusions

  Three novel 3-pyridazinyl-substituted triaryltriazoles, 4-(4-R-phenyl)-3-(2-pyridyl)-5-(3-pyridazinyl)-1,2,4-triazole (L¹: R=OCH₃; L²: R=H; L³: R=F), and their Cu(Ⅱ) complexes, trans-[Cu(L¹)₂(NO₃)₂]·H₂O (1), trans-[Cu(L²)₂(NO₃)₂] (2) and trans-[Cu(L³)₂(H₂O)₂](NO₃)₂ (3), have been synthesized and characterized by single crystal X-ray crystallography. It is first observed that L¹ shows a hydrogen-bonded trimer (L¹)₃ in all known triaryltriazoles. Three complexes have a similar [CuN₄O₂] octahedron with two NO₃^- ions in the trans-positions in both 1 and 2, but two H₂O molecules in the trans-positions in 3.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Scheme 1  Synthetic routes of complexes 1-3

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  Figure 1  ORTEP views of (L¹)₃, L², L³ with 20% thermal ellipsoids probability

  The disordered atoms are omitted for clarity.

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  Figure 2  Three-dimensional network of (L¹)₃

  Symmetry code: ^ⅴ 1-x, 1-y, 1-z.

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  Figure 3  ORTEP views of 1-3 with 30% thermal ellipsoids probability

  All H and disordered atoms are omitted for clarity; Symmetry codes: ^ⅰ 1-x, 1-y, 1-z for 1 and 2; ^ⅰ 1-x, 1-y, -z for 3.

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  Figure 4  Three-dimensional supramolecular frameworks of 1-3

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  Figure 5  Three-dimensional Hirshfeld surfaces of (L¹)₃, L², L³ and 1-3 mapped with d_norm

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  Table 1.  Dihedral angles (°) for (L¹)₃, L², L³ and 1-3

  Compounds	Py/Trz	Ph/Trz	Pyd/Trz
  (L1)3 (O1)	11.2(2)	78.5(4)	11.7(1)
  (L1)3 (O2)	26.0(2)	83.2(4)	23.1(3)
  (L1)3 (O3)	35.7(1)	87.1(2)	24.2(4)
  L2	29.1(3)	77.2(1)	41.4(4)
  L3	21.7(3)	87.7(2)	23.6(1)
  1	5.5(2)	79.9(3)	3.6(2)
  2	6.6(1)	83.0(2)	2.9(2)
  3	5.8(3)	69.9(1)	29.4(2)

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