English

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  0    Introduction

  1, 2, 4-triazole and its derivatives are important part of heterocyclic compounds and have aroused much interest of people in the past decades due to their specific properties, various structure^[1-9] and broad applications as antifungal, antitumor, fungicidal, herbicide agents and so on^[10-13]. A_t the same time, 1, 2, 4-triazole has many other important applications in different fields such as pharmaceutical synthesis^[14] and materials^[15-16].Moreover, much attention has also been paid to substituted 1, 2, 4-triazole because it displays versatile coordination modes^[17-19] in coordination chem-istry.By introducing different substituents at 3- (or 4-, 5-) positions^{[3, 20-21]}, 1, 2, 4-triazole can be functionalized and has more coordination modes.

  Although many metal complexes with substituted 1, 2, 4-triazoles have been synthesized and character-ized^{[4, 7, 22-24]}, the silver(Ⅰ) complexes with 3-methyl-4-ethyl-5-(2(or 4)-pyridyl)-1, 2, 4-triazoles have not been reported so far.As a continuation of our investigation of the asymmetrical substituted 1, 2, 4-triazole^{[4, 20, 25-26]}, herein we report two Ag(Ⅰ) complexes, [Ag₂( μ-L)₂L₂(NO₃)₂] (1) and {[Ag₂( μ-L′)₂(H₂O)₂](NO₃)₂}_n (2), where L(or L′)=3-methyl-4-ethyl-5-(2(or 4)-pyridyl)-1, 2, 4-triazole, and their crystal structures and spectroscopic properties are studied.

  1    Experimental

  1.1    Materials and measurements

  All chemicals were analytical grade and used without further purification.Melting points were determined using an X4 digital microscopic melting point apparatus and uncorrected.The C, H, N elemental analyses were performed on a Perkin-Elmer 240 analyzer.¹H NMR spectra were measured with a Bruker Avance 300 spectrometer at ambient temperature in CDCl₃ using TMS as an internal reference.IR spectra were recorded from 4 000 to 400 cm^-1 using KBr pellets on a Vector22 Bruker spectrophotometer.UV-Vis spectra were recorded on a Hitachi-4100 UV-Vis absorption spectrophotometer at room temperature in thin acetonitrile solution.Fluorescence spectra data for 2 and L′ were measured on a Fluoromax-4 spec-trofluorometer at room temperature.Thermogravimetric analysis (TGA) measurememts were obtained with a NETZSCH STA49F3 thermal analyzer in a nitrogen atmosphere at a heating rate of 10 K·min^-1.

  1.2    Syntheses of L and L′

  The ligand, 3-methyl-4-ethyl-5-(2-pyridyl)-1, 2, 4-triazole (L) was synthesized by the following reaction: oxalylchloride (5.7 mL, 66 mmol) was added to a solution of N-ethylacetamide (5.22 g, 60 mmol) and 2, 6-lutidine (14.0 mL, 120 mmol) in CH₂Cl₂ (300 mL) at 0 ℃ under nitrogen atmosphere.The mixture was stirred for 40 min, and 2-picolinic acid hydrazide (8.22 g, 60 mmol) was added.The reaction mixture was stirred for 5 h at room temperature, and then the volatiles were removed under reduced pressure.The residue was dissolved in saturated NaHCO₃ solution(300 mL) and refluxed for 3 h at 100 ℃.After cooling to room temperature, the water phase was extracted three times with CHCl₃.The organic phase was dried over MgSO₄, and concentrated.Recrystallization from EtOAc gave a colorless crystals (dried at 80 ℃, 3.6 g, Yield: 31.9%), m.p.90~91 ℃.Anal.Calcd.for C₁₀H₁₂N₄(%): C 63.81, H 6.43, N 29.77.Found(%): C 63.94, H 6.35, N 29.80.¹H NMR(300 MHz, CDCl₃): δ 1.316~1.351(t, 3H), 2.506(s, 3H), 4.478~4.530(m, 2H), 7.260~7.291(t, 1H), 7.739~7.777(t, 1H), 8.218~8.238(d, 1H).8.582~8.591(d, 1H).UV-Vis(λ_max / nm): 297.5.IR (KBr, cm^-1): 3 056, 2 989, 2 933, 1 587, 1 504, 1 438, 1 355, 1 149, 1 064, 993, 798, 730, 707.

  3-Methyl-4-ethyl-5-(4-pyridyl)-1, 2, 4-triazole (L′) was synthesized according the same process as L.Colorless (dried at 80 ℃, 4.2 g, Yield: 37.2%), m.p.128~129 ℃.Anal.Calcd.for C₁₀H₁₂N₄(%): C 63.81, H 6.43, N 29.77. Found(%): C 63.87, H 6.31, N 29.64.¹H NMR(300 MHz, CDCl₃): δ 1.333~1.369(t, 3H) 2.547(s, 3H), 4.013~4.056(m, 2H), 7.543~7.563(d, 2H), 8.762~8.769(d, 2H).UV-Vis(λ_max / nm): 289.0.IR (KBr, cm^-1): 3 039, 2 983, 2 935, 1 600, 1 525, 1 483, 1430, 1 359, 1 414, 835, 721, 679.

  1.3    Synthesis of complex 1

  A solution of AgNO₃ (0.170 g, 1 mmol) was added to a warm solution of L (0.376 g, 2 mmol) in 30 mL acetonitrile.A colorless mixture was formed and filtered.The filtrate was left to stand at room temperature for evaporation.Several days later a colorless crystals was collected (0.447 g, Yield: 81.8%).The crystals are stable in air and a single crystal suitable for X-ray was picked.Anal.Calcd.for C₄₀H₄₈Ag₂N₁₈O₆(%): C 43.97, H 4.43, N 23.07.Found(%): C 44.12, H 4.57, N 22.68.UV-Vis (λ_max / nm): 294.5.IR (KBr, cm^-1): 3 056, 2 979, 2 939, 1 631, 1 539, 1 519, 1 489, 1 452, 1 384, 1 329, 1 282, 1 092, 995, 966, 800, 731, 621.

  1.4    Synthesis of complex 2

  A solution of AgNO₃ (0.170 g, 1 mmol) was added to a warm solution of L′ (0.376 g, 2 mmol) in 30 mL acetonitrile.The mixture turned turbidity slightly.The mixture gradually become clear after 2.5 mL of water was added dropwise.A colorless mixture was formed and filtered.The filtrate was left to stand at room temperature for evaporation.Several days later a colorless crystals was collected (0.337 g, Yield: 89.6%), and a single crystal suitable for X-ray was picked.Anal.Calcd. for C₂₀H₂₈Ag₂N₁₀O₈(%): C 31.93, H 3.75, N 18.62. Found(%): C 31.47, H 3.92, N 18.24.UV-Vis (λ_max /nm): 290.5.IR (KBr, cm^-1): 3 430, 2 993, 2960, 1 641, 1 612, 1 537, 1 498, 1 384, 1 066, 1003, 964, 836, 740, 700, 632.

  1.5    Crystal structure determination

  Table1. Crystal data and structure refinement for L′,1 and 2

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  Table1. Crystal data and structure refinement for L′,1 and 2

  CCDC: 1046706, L′； 1046707, 1； 1046708, 2.

  Well-shaped single crystals of L′, 1 and 2 were selected for X-ray diffraction studies.The data were collected at 296(2) K on a Bruker Smart APEX Ⅱ CCD diffractometer with a detector distance of 5 cm and frame exposure time of 10s using graphite-monochromated Mo Kα (λ=0.071 073 nm) radiation.The structures were solved by direct methods and refined on F ² by full-matrix least squares procedures using SHELXTL software^[27]. All non-hydrogen atoms were anisotropically refined, and all hydrogens on carbon atoms were generated geometrically and allowed to ride on their parent atoms, but not refined.The hydrogen atoms in water were found from the Fourier difference map, but not refined anisotropically.Crystal data and structure refinement for L′, 1 and 2 are listed in Table 1.Selected bond lengths and angles for L′, 1 and 2 are listed in Table 2.The molecular graphics were made by using the DIAMOND 3.1 program.

  Table2. Selected bond lengths (nm) and angles (°) for L′,1 and 2

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  Table2. Selected bond lengths (nm) and angles (°) for L′,1 and 2

  2    Results and discussion

  2.1    Description of crystal structure

  

  Figure 3. 2D structure of L′Symmetry codes:ⁱ1-x,2-y,1-z

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  Figure 8. Segment of the polymeric chain of 2

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  Figure 10. 3D structure of 2

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  Figure 2. H-bonding and π…π interactions in L′

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  Table4. Hydrogen bonding geometry and π…π interactions for 1

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  Table4. Hydrogen bonding geometry and π…π interactions for 1
  Table5. Hydrogen bonding geometry and π…π interaction for 2

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  Table5. Hydrogen bonding geometry and π…π interaction for 2
  Table3. Hydrogen bonding geometry and π…π interactions for L′

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  Table3. Hydrogen bonding geometry and π…π interactions for L′
  

  Figure 5. H-bonding and π…π interactions in 1

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  Complex 2 is a polymeric Ag(Ⅰ) complex (Fig.8) crystallizing in the triclinic system P1.Each Ag(Ⅰ) ion is coordinated by three nitrogen atoms and one oxygen atom from the water molecule, and two nitrogen atoms are from two different triazole rings and another nitrogen atom from pyridyl ring.These nitrogen atoms and oxygen atom make the center Ag(Ⅰ) ion form a tetrahedron geometry [AgN₃O] (Fig.7).Every repeat unit comprises one [Ag₂( μ-L′)2(H₂O)₂]²⁺ and two NO₃^- ions.For every L′ in complex 2, two nitrogen atoms (N1, N₂) from the same triazole ring coordinate with two different Ag(Ⅰ) ions, while the N4 atom from the pyridyl ring coordinates with the third Ag(Ⅰ) ion.The distance of two adjacent Ag(Ⅰ) ions (Ag1-Ag1ⁱ) in the repeat units is 0.383 9 nm.The bond lengths of Ag1-O4W is 0.269 7(3) nm, and Ag1-N1i, Ag1-N₂ and Ag1-N4ii are 0.225 7(2), 0.230 9(3) and 0.229 7(2) nm, respectively.The dihedral angle of the triazole ring (N1, N2, C3, N3 and C2) and pyridyl ring (C4, C5, C6, C7, C8 and N4) is 46.80°.

  In complex 2 there are five kinds of hydrogen bond interactions.A face-to-face π…π interaction was found between the pyridyl rings with a centroid-centroid distance of 0.388 23 nm (Table 5 and Fig.9).There is also one edge-to-face C-H…π interaction of C1-H1B…Cg1 involving the triazole ring.All these interactions link the complex 2 into a three-dimensional structure (Fig.10).

  In complex 1, there exist two intramolecular hydrogen bonds (C11-H11A…O₂ and C19-H19B…N8) and several intermolecular hydrogen bonds (Table 4).There are also three face-to-face π…π interactions and one edge-to-face C-H…π interaction of C17-H17…Cg1 (Table 4 and Fig.5).All these interactions link complex 1 into a three-dimensional structure (Fig.6).

  

  Figure 1. Structure of L′ with atomic labeling

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  Figure 4. Structure of 1 with atomic labeling

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  Figure 6. 3D structure of 1

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  Compound 1 is a binuclear Ag(Ⅰ) complex bridged by two ligands of L, which comprises one [Ag₂( μ-L)₂L₂]²⁺ ion and two NO₃^- ligands.Each Ag(Ⅰ) ion is coordinated by three nitrogen atoms and one oxygen atom, where the nitrogen atoms are from three different triazole rings.All these nitrogen atoms and oxygen atom make the center Ag(Ⅰ) ion form a tetrahedron geometry [AgN₃O] (Fig.4).The bond lengths of Ag1-O1, Ag1-N1i, Ag1-N₂ and Ag1-N5 are 0.244 3(3), 0.223 8(3), 0.234 5(3) and 0.223 2(2) nm, respectively.The dihedral angle between the triazole ring (N1, N2, C3, N3 and C2) and pyridyl ring (C4, C5, C6, C7, C8 and N4) is 41.47°, while that between the triazole ring (N5, N6, C13, N7 and C12) and pyridyl ring (C14, C15, C16, C17, C18 and N8) is 19.81°.

  X-ray diffraction structure analyses reveals that L′ crystallizes in the monoclinic system with space group P2_1/c, and it comprises one 1, 2, 4-triazole molecule and three lattice water molecules (Fig.1).Both of the triazole ring and pyridyl ring of L′ are planar.The dihedral angle between the triazole ring and pyridyl ring is 37.18°.Three hydrogen bond interactions are found among the water molecules: O1W-H1WA…OO3W, OO3W-H3WB…O1W and O3W-H3WA…O2W.There also exist three hydrogen bonds among water molecules and nitrogen atoms: O1W-H1WB…N1, OO2W-HO2WB…N2 and OO2W-HO2WA…N4 (Table 3 and Fig.2).In addition, two weak face-to-face π…π interactions exist between the triazole ring and pyridyl ring (Fig.2).All these interactions link the L′ molecules into a two-dimensional structure (Fig.3).

  

  Figure 7. Structure of 2 with atomic labeling

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  Figure 9. H-bonding and π…π interactions in 2

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  2.2    Spectral characterization

  The photoluminescent properties of the free ligand L′ and complex 2 have been studied in the solid state at room temperature.The emission spectra for L′ and complex 2 showed a broad emission maximum at 491 nm and 465 nm with excitation at 413 nm and 400 nm, respectively.Their photolumine-scent properties are attributed to the π-π^* electronic transition.As shown in Fig.11, it can be observed that the fluorescence intensity of complex 2 is weaker than the free ligand L′, and the emission peak of complex 2 has blue-shift.As the nitrogen atoms of L′ in complex 2 coordinated to Ag(Ⅰ) ions, it makes the conjugation extent of L′ in 2 weaker than that of the free L′, and the weak interactions of mental atoms could exist due to the short distance of silver atoms, which leads to a higher energy under the excited status.

  The IR spectrum of the free L (or L′) shows medium-intensity bands at 1 587, 1 504 and 1 438 cm^-1 (or 1 600, 1 525 and 1 430 cm^-1 for L′), which are attributed to pyridine and triazole rings vibrations.As the nitrogen atoms coordinate to metal atoms, the bands for L (or L′) shift about 14 cm^-1 (or 12 cm^-1)^[24].We can find a band at 1 601, 1 519 and 1 452 cm^-1 (or 1612, 1537 and 1 445 cm^-1) in the spectrum of 1 (or 2), which can be assigned to the coordinated triazole rings (or pyridyl rings and triazole rings), the bands at 1 384 cm^-1 is attributed to the NO₃^- in complex 1 and 2.These features are in consistent with the results of X-ray analyses.

  

  Figure 11. Solid-state emission spectra of free ligand L′ and complex 2 at room temperature

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  2.3    Thermal analysis

  Thermal gravimetric analyses (TGA) of complexes 1 and 2 were performed in the temperature range of 40~800 ℃ in nitrogen atmosphere with a heating rate of 10 K·min^-1 to investigate their thermal stabilities (Fig.12).In the TGA curve of complex 1, it is stable when the temperature is below 207 ℃.As the temperature was raised, complex 1 decomposed intensely between 207 and 349 ℃, corresponding to the loss of the ligand L and NO₃^-.The total weight loss is about 76.82% at 490 ℃, which indicates the residual is Ag₂O (Calcd.78.79%).In the curve of the complex 2, the first weight loss between 64 and 102 ℃ is 4.15% due to the removal of crystal water molecules(Calcd.4.79%).As the temperature was raised, complex 2 decomposed intensely from 214 to 271 ℃, corresponding to the loss of the ligand L and NO₃^-.The total weight loss is about 66.23% at 580 ℃, which indicates the residual is Ag₂O (Calcd.69.19%).The results of thermal decompositions of 1 and 2 are consistent with their crystal structures.

  

  Figure 12. TGA curves of complexes 1 and 2

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  3    Conclusions

  Two Ag(Ⅰ) complexes, [Ag₂( μ-L)₂L₂(NO₃)₂] (1) and {[Ag₂( μ-L′)2(H₂O)2](NO₃)₂}n (2), where L (or L′)=3-methyl-4-ehtyl-5-(2(or 4)-pyridyl)-1, 2, 4-triazole were synthesized and characterized.Both complex 1 and 2 have distorted tetrahedral cores [AgN₃O] with one NO₃^- in 1 but one coordinated H₂O molecule in 2.Complex 1 is a binuclear silver(Ⅰ) complex bridged through the nitrogen atoms of triazole rings.Complex 2 is a polymeric silver(Ⅰ) complex bridged by the nitrogen atoms of pyridine and triazole rings.

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• 

  Figure 1  Structure of L′ with atomic labeling

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  Figure 2  H-bonding and π…π interactions in L′

  Symmetry codes:ⁱ1-x,1-y,1-z; ⁱⁱ 1-x,-1/2+y,1/2-z; ⁱⁱⁱ 1+x,y,1+z

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  Figure 3  2D structure of L′Symmetry codes:ⁱ1-x,2-y,1-z

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  Figure 4  Structure of 1 with atomic labeling

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  Figure 5  H-bonding and π…π interactions in 1

  Symmetry codes:ⁱ2-x,2-y,1-z; ⁱⁱ 1-x,1-y,1-z; ⁱⁱⁱ 1+x,y,z; iv -x,1-y,1-z; v: x,y,1+z

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  Figure 6  3D structure of 1

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  Figure 8  Segment of the polymeric chain of 2

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  Figure 7  Structure of 2 with atomic labeling

  Symmetry codes:ⁱ1-x,2-y,2-z; ⁱⁱ 1-x,2-y,1-z

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  Figure 9  H-bonding and π…π interactions in 2

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  Figure 10  3D structure of 2

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  Figure 11  Solid-state emission spectra of free ligand L′ and complex 2 at room temperature

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  Figure 12  TGA curves of complexes 1 and 2

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  Table 1.  Crystal data and structure refinement for L′,1 and 2

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  Table 2.  Selected bond lengths (nm) and angles (°) for L′,1 and 2

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  Table 3.  Hydrogen bonding geometry and π…π interactions for L′

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  Table 4.  Hydrogen bonding geometry and π…π interactions for 1

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  Table 5.  Hydrogen bonding geometry and π…π interaction for 2

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•