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  Generally, both Cu(Ⅱ) and Zn(Ⅱ) ions are closely related to biochemistry, clinical diagnostics as well as environmental pollution^{[1, 11]}.Furthermore, a large amount of Zn(Ⅱ) complexes with Schiff bases have been reported for their fluorescence properties^[7].Therefore, in this paper, Cu(Ⅱ) and Zn(Ⅱ) complexes with a semicarbazone ligand derived from 2-acetyl-3-ethylpyrazine and 4-phenylsemicarbazide have been synthesized and structural determined by single-crystal X-ray diffraction.In addition, the fluorescence properties of three compounds in methanol solution were investigated.

  It is well known that Schiff bases are an important class of ligands in coordination chemistry and have been found extensive application in different fields^[1-2].As one of the most promising systems, a sizable number of transition metal complexes with acylhydrazones and thiosemicarbazones derived from acetyl-pyridine/pyrazine have been extensively investi-gated as potential anticancer agents^[4-11]. However, semicarbazones, as their structurally analogous, have been paid much less attention ^[12].

  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 FTIR spectrophotometer.¹H NMR spectra of HL 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 and in the measurements of emission and excitation spectra the pass width is 5 nm.

  1.2    Preparations of the ligand HL, complexes 1 and 2

  The complexes 1 and 2 were generated by reaction of the ligand HL (5 mmol) with equimolar of Cu(NO₃)₂ and ZnCl₂ in methanol solution (10 mL), respectively.Crystals suitable for X-ray diffraction analysis were obtained by evaporating the corresponding reaction solutions at room temperature.

  

  Figure Scheme 1. Synthesis route of HL

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  2: Colorless needles.Anal.Calcd.for C₁₅H₁₇Cl₂N₅OZn(%): C: 42.93; H: 4.08; N: 16.69.Found(%): C: 42.76; H: 4.22; N: 16.78.FTIR(cm^-1): (C=O) 1 694, (C=N) 1578, (C=N)_pyrazine 1 550.

  1: Green blocks.Anal.Calcd.for C15H19N7O₈Cu (%): C: 36.85; H: 3.92; N: 20.05.Found(%): C: 36.75; H: 4.02; N: 19.94.FTIR(cm^-1): ν(C=O) 1 687, ν(C=N) 1 582, ν(C=N)_pyrazine 1 550, ν₁(NO₃) 1 494, ν₄(NO₃)1 384 and 1 301.

  As shown in Scheme 1, the ligand HL was produced by condension of 2-acetyl-3-ethylpyrazine (1.50 g, 0.01 mol) and 4-phenylsemicarbazide (1.51 g, 0.01 mol) in ethanol solution (30 mL) with continuous stirring at room temperature for 5 h.The white solid was filtered and washed three times by cold ethanol.Yield: 2.15 g (76%).m.p.121~124 ℃.Elemental analysis Calcd.for C15H17N5O(%): C: 63.59; H: 6.05; N: 24.72.Found(%): C: 63.46; H: 6.18; N: 24.57.FTIR (cm^-1): ν(C=O) 1 705, ν(C=N) 1 609, ν(C=N)_pyrazine 1 595.¹H NMR (400 MHz, DMSO-d₆): 10.00 (1H, s, NH), 8.71 (1H, s, NH), 8.46~8.51 (2H, m, pyrazine-H), 7.47~7.56 (2H, m, phenyl-H), 7.25~7.27 (2H, m, phenyl-H), 6.95~6.98 (1H, m, phenyl-H), 3.04~3.10 (2H, d, CH₂), 2.28 (3H, s, CH₃), 1.21~1.25 (3H, s, CH₃).

  1.3.1    X-ray crystallography

  Table1. Crystal data and structure refinement for complexes 1 and 2

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  Table1. Crystal data and structure refinement for complexes 1 and 2

  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 ichscan mode.Semi-empirical absorption correction was applied to the intensity data using the SADABS program^[13].The structures were solved by direct methods and refined by full matrix least-square on F 2 using the SHELXTL-97 program^[14].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.

  CCDC: 1497370, 1; 1497371, 2.

  2    Results and discussion

  2.1    Crystal structures description

  Table2. Selected bond lengths (nm) and angles (°) in complexes 1 and 2

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  Table2. Selected bond lengths (nm) and angles (°) in complexes 1 and 2

  However, most bond angles are highly deviated from those of the ideal geometry.The basal plane of the square-pyramid is made up of N1, N3, O1 and O₂.The bond lengths from Cu(Ⅱ) center to these atoms are in the range of 0.192 2(3)~0.201 3(3) nm.The fifth coordination site is occupied by O₅ atom located axially at 0.221 2(3) nm. In addition, the second O atom (O₃) of the coordinated nitrate group occupied a sixth-coordinate position at a distance greater than 0.26 nm, namely 0.261 6 nm, to give the (4+1+1^*) type coordination^[11].In the solid state, the complexes were linked into a one-dimensional chain along a axis (Fig.1b) by intermolecular N-HO and O-HO hydrogen bonds (Table 3).

  Similarly, the semicarbazone HL acts as a neutral tridentate ligand in complex 2 (Fig.1c).Coordinated by two additional chloride anions, the Zn(Ⅱ) ion adopts a distorted square pyramid coordination geometry (τ=0.169)^[15].In the crystal, two pairs of intermolecular N-HCl hydrogen bonds link two complex molecules into a centrosymmetric dimer (Fig.1d).

  

  Figure 1. Diamond drawing of 1 (a) and 2 (c) with 30% thermal ellipsoids and H atoms being omitted for clarity; Extend chain-like structure along a axis in complex 1(b) and the centrosymmetric dimer in complex 2 (d)

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  Table3. Hydrogen bonds information in complexes 1 and 2

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  Table3. Hydrogen bonds information in complexes 1 and 2

  A diamond drawing of complexes 1 and 2 is shown in Fig.1.Selected bond distances and angles are listed in Table 2.As shown in Fig.1a, complex 1 contains one discrete cationic Cu (Ⅱ) complex and one free nitrate as counter anion in the unit cell.The center Cu(Ⅱ) ion is penta-coordinated by one neutral semicarbazone with N₂O donor set, one coordinated water molecule and one monodentate nitrate, thus giving a distorted square pyramid coordination geometry (τ=0.318)^[15].

  2.2    IR spectra

  The FTIR spectral region for both complexes is more or less similar due to the similar coordination modes of the ligands.The ν(C=O), ν(C=N) and ν(C=N)_pyrizine bands are at 1 705, 1 609 and 1 595 cm^-1, respectively.They shift to lower frequency values in the spectra of complexes, indicating that the carbonyl O, imine N and pyrizine N atoms take part in the coordination^[12].In addition, the intense absorption bands in the spectra of complex 1 associated with the asymmetric stretching appear at 1 384 and 1 301 cm^-1 (ν₄) as well as 1 494 cm^-1 (ν₁), clearly establishing the existence of monodentate and bidentate NO₃^- ligands, respectively^{[12, 15]}.It is in accordance with the crystal structure study.

  2.3    UV spectra

  

  Figure 2. UV spectra of the ligand HL,complexes 1 and 2 in the methanol solution at room temperature

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  The UV spectra of the ligand HL, complexes 1 and 2 in methanol solution (c=1×10^-5 mol·L^-1) were measured at room temperature (Fig.2).The spectra of HL features one main band located around 275 nm (ε=6 929 L·mol^-1·cm^-1) and a shoulder at 282 nm (ε=6 567 L·mol^-1·cm^-1), which could be assigned to characteristic π-π^* transition of benzene and pyrazine units, respectively^[12].Both bands have no shift while with hyperchromic effect in the spectra of 2 (275 nm, ε=8 863 L·mol^-1·cm^-1 and 282 nm, ε=8 331 L·mol^-1·cm^-1).The new absorbance band at 411 nm (ε=1 101 L·mol^-1·cm^-1) of 2 may be from the n→π^* transition of the intra-ligand^[12]. By contrast, complex 1 exhibits two bands at 310 nm (ε=8 863 L·mol^-1·cm^-1) and 432 nm (ε=6 173 L·mol^-1·cm^-1), which should be assigned to π→π^* transition of imine bond and the ligand-to-metal charge transfer (LMCT), respectively^{[12, 16]}.

  2.4    Fluorescence spectra

  

  Figure 3. Fluorescence emission spectra of the ligand HL,complexes 1 and 2 in the methanol solution at room temperature

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  The fluorescence spectra of the ligand HL, complexes 1 and 2 have been studied in methanol solution (c=1×10^-5 mol·L^-1) at room temperature.As shown in Fig.3, the emission spectra of the ligand HL and complex 2 are quite similar, each of them exhibits two indistinct peaks at 311 and 328 nm when excited at 285 nm.The emissions may be assigned to π*→π or π^*→n transition of the intra-ligand^{[12, 16]}.However, complex 1 shows a remarkable peak at about 333 nm under the same tested condition.Compared with the emission of free ligand HL, a red-shift of 5 nm was observed for complex 1, primarily due to the metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT)^[16].

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     Ye X P, Zhu T F, Wu W N, et al. Inorg. Chem.Commun., 2014,47:60-62

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     陈延民,解庆范,刘金花,等.无机化学学报, 2015,31:74-80 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20150111&journal_id=wjhxxbcnCHEN Yan-Min , XIE Qing-Fan, LIU Jin-Hua, et al. Chinese J. Inorg. Chem. , 2015,31:74-80 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20150111&journal_id=wjhxxbcn

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     Singh P, Singh D P, Singh V P. Polyhedron, 2014,81:56-65

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     Milenkovi M, Bacchi A, Cantoni G, et al. Inorg. Chim. Acta, 2013,395:33-43

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     Chang H Q, Jia L, Xu Z Q, et al. Inorg. Chem.Commun., 2015,57:8-10

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     Li M X, Zhang L Z, Yang M, et al. Bioorg. Med. Chem. Lett., 2012,22:2418-2433

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     Li M X, Zhang L Z, Zhang D, et al. Eur. J. Med. Chem., 2011,46:4383-4390

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      Qi J, Liang S, Gou Y, et al. Eur. J. Med. Chem., 2015,96: 360-368

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      毛盼东,韩学峰,吴伟娜,等.无机化学学报, 2016,32:161-166 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20160123&journal_id=wjhxxbcnMAO Pan-Dong, HAN Xue-Feng, WU Wei-Na, et al. Chinese J. Inorg. Chem. , 2016,32:161-166 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20160123&journal_id=wjhxxbcn

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      Sheldrick G M. SADABS, University of Göttingen, Germany, 1996.

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      Sheldrick G M. SHELX-97, Program for the Solution and the Refinement of Crystal Structures, University of Göttingen, Germany, 1997.

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      吴伟娜,王元,唐宁.无机化学学报, 2012,28: 425-428 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20120236&journal_id=wjhxxbcnWU Wei-Na, WANG Yuan , TANG Ning . Chinese J. Inorg. Chem., 2012,28: 425-428 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20120236&journal_id=wjhxxbcn

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      王秀丽,隋芳芳,林宏艳,等.无机化学学报, 2014,30:2626-2634 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20141125&journal_id=wjhxxbcnWANG Xiu-Li, SUI Fang-Fang, LIN Hong-Yan, et al. Chinese J. Inorg. Chem. , 2014,30:2626-2634 http://www.wjhxxb.cn/wjhxxbcn/ch/reader/view_abstract.aspx?flag=1&file_no=20141125&journal_id=wjhxxbcn

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  Scheme 1  Synthesis route of HL

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  Figure 1  Diamond drawing of 1 (a) and 2 (c) with 30% thermal ellipsoids and H atoms being omitted for clarity; Extend chain-like structure along a axis in complex 1(b) and the centrosymmetric dimer in complex 2 (d)

  Symmetry codes:ⁱ-0.5+x,1.5-y,-z; ⁱⁱ -1+x,y,z; ⁱⁱⁱ -x,-y,1-z

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  Figure 2  UV spectra of the ligand HL,complexes 1 and 2 in the methanol solution at room temperature

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  Figure 3  Fluorescence emission spectra of the ligand HL,complexes 1 and 2 in the methanol 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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