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  Schiff bases as an important class of ligands play a crucial role in coordination chemistry and have been widely applied in different fields mainly due to their wide-spectrum biological applications ^[1-7]. In recent years, there are many reports about the acylhydrazones, thiosemicarbazones derived from actyl-pyridine/pyrazine, and their transition metal complexes because of high biological and pharmaceutical activities ^{[3, 8-10]}. By contrast, as their structurally analogous, carbazates (R-O-CO-NH-NH₂) and their metal complexes have been paid much less attention ^[6].

  Generally, Cd (Ⅱ) ion is closely related to biochemistry, clinical diagnostics as well as environ-mental pollution ^[11]. Furthermore, a large amount of Cd (Ⅱ) complexes have been reported for their fluore-scence properties ^[12-13]. Therefore, in this paper, Ni (Ⅱ) and Cd (Ⅱ) complexes with a Schiff base ligand derived from 2-acetyl pyrazine and methyl hydrazinocarbox-ylate have been synthesized and structural determined by single-crystal X-ray diffraction. In addition, the thermal stability and luminescent properties of the complexes are also investigated.

  1   Experimental

  1.1   Materials and measurements

  Solvents and starting materials for synthesis were purchased commercially and used as received. Elemental analysis was carried out on an Elemental Vario EL analyzer. The IR spectra (ν=4 000~400 cm^-1) were determined by the KBr pressed disc method on a Bruker V70 FT-IR spectrophotometer. ¹H NMR spectra of L was acquired with Bruker AV400 NMR instrument in DMSO-d₆ solution with TMS as internal standard. The UV spectra were recorded on a Purkinje General TU-1800 spectrophotometer. Fluorescence spectra were determined on a Varian CARY Eclipse spectrophotometer.

  1.2   Preparations of the ligand, complexes 1 and 2

  As shown in Scheme 1, the ligand HL was produced by condension of 2-acetyl pyrazine (1.22 g, 0.01 mol) and methyl hydrazinocarboxylate (0.90 g, 0.01 mol) in anhydrous methanol solution (30 mL) with continuous stirring at room temperature for 3 h. The white solid was filtered and washed three times by cold methanol. Yield: 1.13 g (58%). m.p. 183~185 ℃. Elemental analysis Calcd. for C₈H₁₀N₄O₂(%): C: 49.48; H: 5.19; N: 28.85; Found (%): C: 49.26; H: 5.34; N: 29.00. FT-IR (cm^-1): ν(C=O) 1 748, ν(C=N) 1 613, ν(C=N)_pyrazine 1 558. ¹H NMR (400 MHz): δ 10.52(¹H, s, NH), 9.09(¹H)/8.55~8.58(2H) for pyrazine-H, 3.70 (3H, s, CH₃), 2.24(3H, s, CH₃).

  Figure Scheme 1.  Synthesis route of HL

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  The complexes 1 and 2 were generated by reaction of the ligand HL (5 mmol) with equimolar of Ni (NO₃)₂ ·6H₂O or Cd (NO₃)₂·6H₂O in methanol solution (10 mL), respectively. Crystals suitable for X-ray diffraction analysis were obtained by evaporating the correspon-ding reaction solutions at room temperature.

  1: Brown blocks. Anal. Calcd. for C₁₆H₁₈N₈O₄Ni (%): C: 43.18; H: 4.08; N: 25.18. Found (%): C: 43.05; H: 4.00; N: 25.33. FT-IR (cm^-1): ν(N=C-O) 1 606, ν(C=N) 1 594, ν(C=N)_pyrazine 1 535.

  2: Colorless blocks. Anal. Calcd. for C₉H₁₄N₆O₉Cd (%): C: 23.36; H: 3.05; N: 18.16. Found (%): C: 23.22; H: 3.17; N: 18.27. FT-IR (cm^-1): ν(O-H) 3 532, ν(C=O) 1 720, ν(C=N) 1 579, ν(C=N)_pyrazine 1 523. ν₁(NO3) 1 494, ν₄(NO3) 1 311.

  1.3   X-ray crystallography

  The X-ray diffraction measurement for complexes 1 and 2 were performed on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with a graphite monochromatized Mo Kα radiation (λ=0.071 073 nm) by using φ-ω scan mode. Semi-empirical absorption correction was applied to the intensity data using the SADABS program ^[14]. The structures were solved by direct methods and refined by full matrix least-square on F² using the SHELXTL-97 program ^[15]. All non-hydrogen atoms were refined anisotropically. All the H atoms were positioned geometrically and refined using a riding model. Details of the crystal parameters, data collection and refinements for complexes 1 and 2 are summarized in Table 1.

  Table 1.  Crystal data and structure refinement for complexes 1 and 2

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  Table 1.  Crystal data and structure refinement for complexes 1 and 2

  CCDC: 1424257, 1; 1424258, 2.

  2   Results and discussion

  2.1   Crystal structures description

  Selected bond distances and angles, hydrogen bonds information for both complexes are listed in Table 2 and 3, respectively. As shown in Fig. 1a, the central Ni (Ⅱ) ion in complex 1 is surrounded by two independent anionic ligands with N₂O donor set, thus possesses a distorted octahedral coordination geometry. The enolization of C=O bond of the ligand can be confirmed by the bond lengths of C-O being 0.125 3(4) and 0.124 1(4) nm, which are in excellent agreement with previously known semicarbazone complexes in the literature ^[6-7]. The distances of Ni-N/O bonds were in the range of 0.197 1(3)~0.209 5(2) nm, comparable with those in some reported complexes with similar donor set ^[3]. As expected, there exist none classic hydrogen bonds in the crystal of 1. However, certain C-H…N and C-H…O interactions are helpful to construct three dimensional net work.

  Table 2.  Selected bond lengths (nm) and angles (°) in complexes 1 and 2

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  Table 2.  Selected bond lengths (nm) and angles (°) in complexes 1 and 2
  Table 3.  Hydrogen bonds information in complexes 1 and 2

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  Table 3.  Hydrogen bonds information in complexes 1 and 2
  Figure 1.  Diamond drawing of 1 (a) and 2 (b) with 30% thermal ellipsoids; (c) Coordination enviorment of Cd (Ⅱ) ion in complex 2 (coordination atoms shown with 30% thermal ellipsoids); (d) Extend 2D supromolecular structure along c axis in complex 2

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  By contrast, the molar ratio of the ligand HL and metal is 1:1, and the ligand is neutral tridentate in complex 2 (Fig. 1b) with C-O bond length being 0.121 1(5) nm. The Cd (Ⅱ) ion is also coordinated with two bidentate nitrate anions and one methanol molecule, giving bicapped-triangular prism geometry (Fig. 1c). In the crystal, intermolecular O9-H9…N₂^iv and N4-H₄…O₄^v (Symmetry codes: ^iv x-1/2, -y+1, z+1/2; ^v x, y-1, z) hydrogen bonds link the complexes into extended 2D supromolecular structure (Fig. 1d).

  2.2   IR spectra

  The ν(C=O) of the free ligand is 1 748 cm^-1, it shifts to lower frequency value in complex 2, confir-ming the coordination of the carbonyl group ^[3]. How-ever, such absorption is disappeared in complex 1, meanwhile, new (N=C-O) stretching vibration absorp-tion is observed at 1 606 cm^-1, revealing that the C=O in O=C-N moiety has enolizated and the oxygen atom coordinates to the Ni (Ⅱ) ion ^[6-7]. The ν(C=N) bands of the imine group and pyrazine ring in the ligand HL shift to lower frequency values in the complexes, indicating that the N atoms of both units take part in the coordination ^[8]. Meanwhile, the bands at 1 494 and 1 311 cm^-1 in complex 2 could be assigned to the two split bands ν₁ and ν₄ of the coordinated nitrate group, respectively, showing that the nitrate group is bidentate ^[16]. It is in accordance with the crystal struc-ture study.

  2.3   Thermal decomposition process of complexes 1 and 2

  For detecting the thermal stabilities of complexes 1 and 2, thermal gravimetric (TG) analyses were carried out from the room temperature to 800 ℃ with the linear heating rate of 10 ℃·min^-1 under argon atmosphere. Complex 1 (Fig. 2a) is thermally stable up to about 300 ℃, indicating there exist no solvent molecules in the complex. A sharp weight loss for complex 1 could be observed from 300 to 330 ℃, then gradually smooth until to about 700 ℃, corre-sponding to the decomposition of two organic ligands. However, the first stage occurs with weight loss of 3.62% below 150 ℃ for complex 2, contributing to the loss of one coordinated methanol molecule (Calcd. 6.89%). The second process of weight loss appears between 150 to 780 ℃, considered as the decomposi-tion of the ligand L and two nitrate anions. The remainders of the complexes 1 and 2 might be the metal oxides because the residue weights (11.53% and 15.08%) are agreement with the calculated values of 10.12%, and 15.72 %, respectively.

  Figure 2.  TG curves for complexes 1 (a) and 2 (b)

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  2.4   UV spectra

  The UV spectra of HL, complexes 1 and 2 in CH₃OH solution (concentration: 1×10^-5 mol·L^-1) were measured at room temperature (Fig. 3). The spectra of HL features only one main band located around 290 nm (ε=10 674 L·mol^-1·cm^-1), which could be assigned to characteristic π-π^* transition of pyrazine unit ^[3]. Similar bands are observed at 289 nm (ε=12 422 L·mol^-1·cm^-1) in the complex 2. However, there are three bonds in spectra of 1 at 252 (ε=7914 L·mol^-1·cm^-1), 296 (ε=16 464 L·mol^-1·cm^-1) and 385 nm (ε=12 538 L·mol^-1·cm^-1). The former two could be contributed to the characteristic π-π^* transition of pyrazine and imine unit, respectively ^[16], while the final one is probably due to the ligand-to-metal charge transfer (LMCT) ^[3]. This indicates that an extended conjugation is formed in anionic ligand after complexation in complex 1.

  Figure 3.  UV spectra of the ligand HL (a), 1 (b) and 2 (c) in CH₃OH solution at room temperature

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  Figure 4.  Fluorescence emission spectra of the ligand L (a), 1 (b) and 2 (c) in CH₃OH solution at room temperature

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  2.4   Fluorescence spectra

  The fluorescence spectra of the ligand HL, complexes 1 and 2 have been studied in CH₃OH solution (concentration: 1×10^-5 mol·L^-1) at room temperature. The results show that the emission spectra of all three compounds exhibit one emission peak at 380 nm when excited at 340 nm, while complex 2 shows another peak at 550 nm. The behavior of Cd (Ⅱ) ion coordinated to the ligand is regarded as that of emissive species resulted in a CHEF (chelation enhancement of the fluorescence emission) effect ^[17].

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• 

  Scheme 1  Synthesis route of HL

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  Figure 1  Diamond drawing of 1 (a) and 2 (b) with 30% thermal ellipsoids; (c) Coordination enviorment of Cd (Ⅱ) ion in complex 2 (coordination atoms shown with 30% thermal ellipsoids); (d) Extend 2D supromolecular structure along c axis in complex 2

  In (a)~(c): H atoms are omitted for clarity; In (d): H atoms of C-H bonds are omitted for clarity, Symmetry codes: ^iv x-1/2, -y+1, z+1/2; ^v x, y-1, z

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  Figure 2  TG curves for complexes 1 (a) and 2 (b)

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  Figure 3  UV spectra of the ligand HL (a), 1 (b) and 2 (c) in CH₃OH solution at room temperature

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  Figure 4  Fluorescence emission spectra of the ligand L (a), 1 (b) and 2 (c) in CH₃OH solution at room temperature

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  Table 1.  Crystal data and structure refinement for complexes 1 and 2

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  Table 2.  Selected bond lengths (nm) and angles (°) in complexes 1 and 2

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  Table 3.  Hydrogen bonds information in complexes 1 and 2

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