English

• 1. INTRODUCTION

  Compounds of 1,2,4-triazole and their derivatives constitute an important class of organic compounds with diverse agricultural,industrial and biological act ivities^[1-4]. The synthesis of such compounds has received a considerable attention in recent years^[16]. There are numerous researches reporting the method of synthesizing 1,2,4-triazole and their diverse biological activities such as antioxidant,antitumor and antimicrobial^[10-15]. We synthesized a new 1,2,4-triazoles derivative with cyanopyridine and metha-noic acid^[17]. The extensively derived substituted 1,2,4-triazoles have attracted more attention than the unsubstituted one,as they can provide more coor-dination modes,indicating the formation of new coordinated complexes. In this context,we have designed and synthesized the Cu(II) coordination polymer of 3-(pyridin-2-yl)-1,2,4-triazole(Hpt),in thepresence of 3,5-dimethylbenzoic acid (3,5-DMBA) as a secondaryligand. Preliminary results of the luminescence and thermal properties of complex 1 are also reported.

  2. EXPERIMENTAL

  2.1. Materials and instruments

  All commerciallyavailable chemicals were of reagent grade and used as received without further purification. Elemental analysis was performed on a Perkin-Elmer 2400 elemental analyzer. IR spectra were recorded on a Nicolet 6700 FT-IR spectro-photometer in the form of KBr pellets. Thermo-gravimetric analyses were performed on a simulta-neous SPRT-2 pyris1 thermal analyzerwith a heating rate of 10 ℃·min^-1. The luminescence spectra for the powdered samples were measured on an RF-5301PC spectrofluorometer with a xenon arc lamp as the light source. In the measurement of emission and excitation spectra,the pass width is 5 nm,and all the measurements were carried out in the solid state at room temperature.

  2.2. Synthesisof complex 1

  A mixtureof Cu(Ac)₂·H₂O (39.9 mg,0.2mmol),Hpt (14.6 mg,0.1 mmol),3,5-DMBA (40.5 mg,0.1 mmol),NaOH (4.0 mg,0.1 mmol) and 10 mL H₂O was sealed in a 16 mL Teflon-lined stainless-steel vessel and heated at 160 ℃ for 3 days,and then cooled down to room temperature within 12 h,obtaining block blue crystals of 1 (yield: 38% based on Cu). m.p.: 323.7～325.0℃. Anal. Calcd.:(%) for C₁₆H14CuN₄O₂: C,51.53; H,4.32; N,12.65. Found (%): C,52.03; H,4.61; N,12.51. Main IR (KBr, cm^-1): 3448 (s),1655 (s),1617 (m),1561 (s),1499 (w),1387 (s),1291 (m),755 (w),670 (w).

  2.3. Crystal structure determination

  A single crystal with dimensions of 0.15 mm ×0.14 mm × 0.13 mm was put on a Bruker SMART APEX CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ =0.71073 Å) using a φ-ω scan mode at 296(2) K. A total of 2839 reflections were collected in the range of 2.1≤θ≤25.00°,of which 2581 were independent (R_int = 0.0298) and 2839 were observed (I > σ(I)). All data were corrected by Lp factors and empirical absorption. The crystal structure was solved dire ctly by program SHELXS-97,and refined by program SHELXL-97^[18]. The hydrogen and non-hydrogen atoms were corrected by isotropic and anisotropic temperature factors respectively through full-matrix least-squares method. The final R = 0.0298,wR =0.0855 (w = 1/[σ2(Fo ²) + (0.0540P²)+ 0.0755P], where P= (F_o² + 2F 2) /3) ,(∆/σ) = 0.000,S =1.035,(∆ρ) _max = 0.363 and (∆ρ) _min = -0.363 e·Å^-3.

  3. RESULTS AND DISCUSSION

  3.1. Crystal structure of complex 1

  The molecular structure of complex 1 is shown in Fig. 1,and its one-dimensional chain is depicted in Fig. 2. The selected bond lengths and bo nd angles arelisted in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Title Complex

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  Angle (°) Angle (°) Angle (°) O(2) -Cu(1) -N(1) 156.49(8) O(2) -Cu(1) -O(1) 98.18(7) N(1) -Cu(1) -N(2) 80.06(7)

  Bond	Dist.	Bond	Dist.	Bond	Dist.
  Cu(1) -O(1)	2.2200(16)	Cu(1) -N(2A)	2.0519(18)	Cu(1) -N(1A)	1.9916(18)
  Cu(1) -O(2A)	1.9561(15)	Cu(1) -N(3)	1.9776(18)
  O(2) -Cu(1) -N(3)	87.37(7)	N(1) -Cu(1) -O(1)	105.23(7)	N(3) -Cu(1) -N(2)	171.81(7)
  N(1) -Cu(1) -N(3)	93.34(7)	O(2) -Cu(1) -N(2)	96.75(7)	N(2) -Cu(1) -O(1)	93.13(7)
  N(3) -Cu(1) -O(1)	93.29(7)
  Symmetry transformations used to generate the equivalent atoms: A: 2 -x,1 -y,1 -z

  Figure 1

  

  Figure 1.  Molecular structure of complex 1 A: 2 -x,1 -y,1 -z

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  Figure 2

  

  Figure 2.  One-dimensional chain of complex 1

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  Complex 1 is a one-dimensional (1-D) chain coordination polymer. As shown in Fig. 1,each Cu(II) atom is coordinated by three nitrogen atoms from two different Hpt ligands and two oxygen atoms from two 3,5-DMBAmolecules with the Cu-N bond distances ranging from 1.9776(18) to 2.0519(18) Å (Table 1) . Two additional positions are occupied by two oxygen atoms from two 3,5-DMBA-ligands with the O-Cu-O bond angle of 98.18(7) ° and Cu-O bond distances falling in the 1.9561(15) ～ 2.2200(16) Å range. Therefore,the five-coordinated Cu(II) center has a slightly distorted square pyramidal coordination geometry with a N3O2 donor set. Each 3,5-DMBA-ligand in turn uses its carboxylate group to connect two metal centers with μ₂-η₁:η₁-bridge coordination modes. Two Cu(II) and two 3,5-DMBA-ligands form aM₂L₂ macrocyclic ring with the Cu···Cu distance of 4.7646(5) Å,longer than that in the other same Cu complexes (Cu₃(Hpt)₃ (Cu-Cu = 3.457 Å)^[19],{[Cu(Hpt)₂(H₂O)1.5]n (Cu-Cu = 4.018 Å)^[20],{[Cu₃(Hpt)₃]n (Cu-Cu = 3.472 Å)^[21] and (Cu₃(μ₃-O)(L2) 3·(H₂O)3] BF4·3H₂O (Cu-Cu = 3.439Å)^[22]. Such M₂L₂ macrocyclic rings are further connected by Cu-N coordination bonds to give an infinite 1-D chain structure (Fig. 2) .

  3.2. Spectroscopic characterization

  In the IR spectrumof 1,the characteristic car-boxylate bands of νas(COO-) and νs(COO-) appear at 1655 and 1387 cm^-1,respectively. A splitting of 268 cm^-1 (separation between νas and νs) indicates that the carboxylate group coordinates with Cu(II) in a monodentate mode^[23],which is consistent with that observed in the crystalstructure of 1 (Fig. 1) .

  Coordination polymers have been reported to have the ability to adjust the emission wavelength of organic materialsthrough the incorporation of metal centers. It is of great significance to investigate the luminescence properties of coordination polymers in view of their potential applications as light-emitting diodes (LEDs). The emission spectra of Cu(II) com-plexes 1 were studied in the solid state at room temperature and depicted in Fig. 3. There are emis-sion bands at 430 nm (λ_ex = 318 nm) for 1. Such fluorescence emissions may be assigned to intra-ligand π-π* transitions,since the free Hpt ligand exhibits a similar broad emission at 450 nm upon excitation at 340 nm. The emission band of complex 1 is respectively 19 nm blue shifted,compared with theHpt ligand,which are attributed to the coor-dination interactions between the metal atom and the ligand,and such emission bands may be tentatively assigned to ligand-to-metal charge transfer (LMCT)^[23]. This suggeststhat the complexesmay be good candidates of blue-fluorescent materials,since they are highly thermally stable and insoluble incommon solvents.

  Figure 3

  

  Figure 3.  Spectraof complex 1A. Emission spectrum (λmax = 430 nm) B. Excitation spectrum (λmax = 318 nm);

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  3.3. Thermal stability

  Thermal gravimetric analyses (TGA) were per-formed to verify the thermal stability of the com-plexes,and the results are shown in Fig. 4. For complex 1,no weight loss is found below 320.0℃,and above this temperature,there are two weight-loss stages from room temperature to 600℃. The first stagetakes place from 320 to 356℃ with the weight loss of 40.50%,corresponding to the release of Hpt molecules (calcd.: 40.56%). A strong endothermic peak near 323.7℃ can be attributed to the endothermic meltingof the complex,which agreeswith the melting point of the compound. The second stage occurs between 356 and 542℃ with the weight loss of two coordinated 3,5-DMBA molecules. The final product in air is copper oxide with the final residue residual rate to be about 22.29% (calcd.: 22.22%).

  Figure 4

  

  Figure 4.  TG and DTG curves of complex 1

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• 

  Figure 1  Molecular structure of complex 1 A: 2 -x,1 -y,1 -z

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  Figure 2  One-dimensional chain of complex 1

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  Figure 3  Spectraof complex 1A. Emission spectrum (λmax = 430 nm) B. Excitation spectrum (λmax = 318 nm);

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  Figure 4  TG and DTG curves of complex 1

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) of the Title Complex

  Angle (°) Angle (°) Angle (°) O(2) -Cu(1) -N(1) 156.49(8) O(2) -Cu(1) -O(1) 98.18(7) N(1) -Cu(1) -N(2) 80.06(7)

  Bond	Dist.	Bond	Dist.	Bond	Dist.
  Cu(1) -O(1)	2.2200(16)	Cu(1) -N(2A)	2.0519(18)	Cu(1) -N(1A)	1.9916(18)
  Cu(1) -O(2A)	1.9561(15)	Cu(1) -N(3)	1.9776(18)
  O(2) -Cu(1) -N(3)	87.37(7)	N(1) -Cu(1) -O(1)	105.23(7)	N(3) -Cu(1) -N(2)	171.81(7)
  N(1) -Cu(1) -N(3)	93.34(7)	O(2) -Cu(1) -N(2)	96.75(7)	N(2) -Cu(1) -O(1)	93.13(7)
  N(3) -Cu(1) -O(1)	93.29(7)
  Symmetry transformations used to generate the equivalent atoms: A: 2 -x,1 -y,1 -z

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•