English

• 0.   Introduction

  Schiff base compounds and their transition metal complexes are playing an important part in the development of coordination chemistry^[1-5] because of their potential application in catalysis^[6], bioscience^[7-11], magnetic materials^[12-17], luminescent^[18-24], electrochem-ical systems^[25-26] and constructing supramolecular stru-ctures building^[27-33]. There have been reports of the use of Schiff base ligands synthesized from salicylaldehyde and various amines to form complexes with transition metals such as complexes of nickel(Ⅱ)^[35] and copper(Ⅱ)^[36-38]. In recent years, there has been enhanced interest in the synthesis and characterization of such complexes due to their interesting properties and other applications^[39-46]. Herein, in order to further study the supramolecular of the transition metal complexes with the Schiff base ligand, we synthesized and analyzed two complexes, [Ni(L¹)(DMF)]₂ (1) (H₂L¹=4-hydroxy-3-((3-hydroxy-pyridin-2-ylimino)-methyl)-chromen-2-one) and [Zn(L²)(H₂O)]₂·2DMF (2) (H₂L²=2-((3, 5-dibromo-2-hydroxy-benzylidene)-amino)-pyridin-3-ol), which was named complexes 1 and 2. Complexes 1 and 2 are all binuclear structures and links some other molecules into an infinite 3D network supramolecular structure via intermolecular hydrogen bond, C-H…π or π…π stacking interactions. The ligands H₂L¹ and H₂L² exhibit blue emission and complexes 1 and 2 all show green emission with λ_em=543 and 538 nm.

  1.   Experimental

  1.1   Materials

  4-hydroxyl coumarin, POCl₃, 2-amino-3-hydroxy-pyridine (≥98%) from Alfa Aesar was used without further purification. The other reagents and solvents were of analytical grade from Tianjin Chemical Reagent Factory.

  1.2   Methods

  C, H and N analyses were carried out with a GmbH VariuoEL V3.00 automatic elemental analyzer. IR spectra were recorded on a Vertex70 FT-IR spectrophotometer, with samples prepared as KBr (400~4 000 cm^-1) pellets. UV-Vis absorption spectra were recorded on a Shimadzu UV-3900 spectrometer. Luminescence spectra in solution were recorded on a Hitachi F-7000 spectrometer. Melting points were measured by the use of a microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and were uncorrected.

  1.3   Syntheses of ligands H₂L¹ and H₂L²

  HL¹ and HL² were synthesized according to the following synthetic routes shown in Scheme 1 and 2.

  Scheme 1

  

  Scheme 1.  Synthetic route of H₂L¹

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  Scheme 2

  

  Scheme 2.  Synthetic route of H₂L²

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  H₂L¹: 3-Formyl-4-hydroxyl-comnarin was synthes-ized according to the analogous method^[47]. POCl₃ (10 mL, 2.00 mmol) was dropped in the anhydrous DMF solution (10 mL) of 4-hydroxyl coumarin (3.8 g, 20.00 mmol) with the reaction temperature sustained lower than 5 ℃. The reaction mixture was kept at room temperature for 2 h, then it was heated in the steam bath for 1 h at 70 ℃. After the mixed solution come to room temperature, it was mixed with ice water and neutralized with sodium carbonate, and the resulting yellow solid was collected and recrystallized using ethyl alcohol. Yield: 78.6%; m.p. 115~116 ℃. Anal. Calcd. for C₁₀H₆O₄(%): C, 63.16; H, 3.18. Found(%): C, 63.68; H, 3.21.

  A solution of 3-formyl-4-hydroxyl comnarin (190.03 mg, 1.00 mmol) in ethanol (2 mL) was added to a solution of 2-amino-3-hydroxy pyridine (110.05 mg, 1.00 mmol)) in ethanol (2 mL) and the mixture was subjected to heating at 70 ℃ for 12 h. After the mixed solution come to room temperature, the resulting white solid was collected. After cooling to room temperature the light yellow precipitate was filtered and washed successively with ethanol/petroleum ether (1: 4, V/V). The product was dried under reduced pressure to obtain 222.00 mg H₂L¹. Yield: 74.5%. m.p. 214~215 ℃. Anal. Calcd. for C₁₅H₁₀N₂O₄(%): C, 63.83; H, 3.57; N, 9.93. Found(%): C, 63.52; H, 4.22; N, 9.80.

  H₂L²: The ligand H₂L² was synthesized by a method similar to that of H₂L¹ except substituting 3-aldehyde-4-hydroxy coumarin with 3, 5-dibrominesa-licylaldehyde. Yield: 351.33 mg, 76.8 %. m.p. 204~205 ℃. Anal. Calcd. for C₁₂H₈Br₂N₂O₂(%): C, 38.74; H, 2.17; N, 7.53. Found(%): C, 38.81; H, 2.16; N, 7.49.

  1.2.2   Syntheses of complexes 1 and 2

  Complex 1: A solution of Ni(Ⅱ) acetate monohy-drate (0.7 mg, 0.003 mmol) in ethanol (2 mL) was added dropwise to a solution of H₂L¹ (0.95 mg, 0.005 mmol) in acetone (4 mL). The color of the mixture turned to brown immediately, and then 2 drops of DMF were added in it, stirred for 1 h at room temperature. The mixture was filtered, and the filtrate was allowed to stand at room temperature for about a week. The solvent was partially evaporated, and single crystals suitable for X-ray crystallographic analysis were obtained. Yield: 25.6%. Anal. Calcd. for C₁₈H₁₅N₃NiO₅(%): C, 52.47; H, 3.67; N, 10.20. Found(%): C 53.47, H, 3.67; N, 10.87.

  Complex 2: The complex 2 was synthesized with the similar method for complex 1. Yield: 28.6%. Anal. Calcd. for C₁₅H₁₅Br₂N₃O₄Zn(%): C, 34.22; H, 2.87; N, 7.98. Found(%): C, 34.78; H, 2.87; N, 8.38.

  1.3   Crystal structure determinations of complexes 1 and 2

  The single crystals of the complexeswith approxi-mate dimensions of 0.40 mm× 0.11 mm× 0.08 mm (1) and 0.30 mm× 0.27 mm× 0.15 mm (2) were placed on a Bruker Smart 1000 CCD area detector. The reflec-tions were collected using graphite-monochromatized Mo Kα radiation (λ=0.071 073 nm). The Lp correc-tions were applied to the SAINT program^[48] and semi-empirical correction were applied to the SADABS program^[49]. The crystal structures were solved by the direct methods (SHELXS-2014)^[50]. The hydrogen atoms of water molecules in the complex 2 were located from difference Fourier maps, and the other hydrogen atoms were generated geometrically. Details of the crystal parameters, data collection and refinements for complexes 1 and 2 are summarized in Table 1.

  Table 1

  

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

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  Empirical formula	C18H15N3NiO5	C12H8Br2N2O3Zn·C3H7NO
  Formula weight	412.04	526.49
  Temperature/K	297.16(10)	293(2)
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a/nm	0.991 21(4)	1.159 66(6)
  b/nm	1.411 92(6)	1.394 83(4)
  c/nm	1.240 16(5)	1.135 06(6)
  β/(°)	102.395(5)	105.547(5)
  Volume/nm3	1.695(13)	1.768(14)
  Z	4	4
  Dc/(Mg·m-3)	1.615	1.977
  μ/mm-1	1.18	5.93
  F(000)	848	1 032
  θ range/(°)	3.566 0~28.422 0	3.679 0~23.951 0
  Limiting indices	-16 ≤ h ≤ 17, -12 ≤ k ≤ 13, -15 ≤ l ≤ 14	-14≤ h ≤ 7, -17 ≤ k ≤ 16, -13≤ l ≤ 14
  Reflection collected, unique	6 323, 2 737 (Rint=0.026)	7 039, 2 306 (Rint=0.047)
  Completeness to θ=26.32°/%	99.67	99.74
  Data, restraint, parameter	3 335, 0, 246	3 478, 0, 229
  GOF on F2	1.047	0.912
  R1, wR2 [I > 2σ(I)]	0.036 1, 0.081 5	0.053 3, 0.133 0
  Largest diff. peak and hole/(e·nm-3)	520 and -322	836 and -766

  CCDC: 1855900, 1; 1855901, 2.

  2.   Results and discussion

  2.1   Crystal structures of complexes 1 and 2

  The single crystal structures and the coordination pattern diagram of complexes 1 and 2 are shown in Fig. 1 and 2. The selected bond lengths and angles of complexes 1 and 2 are listed in Table 2. X-ray crystallographic analysis reveals that both complexes 1 and 2 crystallize in the monoclinic system with space group P2₁/c. Complexes 1 and 2 can be described as binuclear M(Ⅱ) complexes (M=Ni or Zn), consist of two M(Ⅱ) ions, two (L^2-) units and two coordinated solvent molecules (DMF for 1 and H₂O for 2), in which the difference is that complex 2 contains two free DMF molecules.

  Figure 1

  

  Figure 1.  (a) Molecular structure of complex 1 showing 30% probability displacement ellipsoids; (b) Coordination geometry diagram for Ni(Ⅱ) ions of complex 1

  Symmetry codes: #1:-x, 0.5+y, 0.5-z

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  Figure 2

  

  Figure 2.  Molecular structure of complex 2 showing 30% probability displacement ellipsoids; (b) Coordination geometry diagram for Zn(Ⅱ) ions of complex 2

  Symmetry codes: #1:-x, 0.5+y, 0.5-z

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  Table 2

  

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

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  Complex 1
  Ni1-O3	0.194 0(2)	Ni1-O5	0.197 8(2)	Ni1-O4#1	0.235 8(2)
  Ni1-O4	0.194 4(2)	Ni1-N1	0.192 9(2)
  O3-Ni1-O5	94.25(8)	O3-Ni1-O4	177.77(7)	O3-Ni1-O4#1	92.27(7)
  O5-Ni1-O4#1	89.16(7)	O4-Ni1-O5	87.94(7)	O4-Ni1-O4#1	87.37(7)
  N1-Ni1-O3	93.03(8)	N1-Ni1-O5	161.42(8)	N1-Ni1-O4#1	107.62(7)
  N1-Ni1-O4	84.99(8)	Ni1-O4-Ni1#1	92.63(7)	C7-O3-Ni1	126.33(2)
  Complex 2
  Zn1-O1	0.196 5(5)	Zn1-O2#1	0.200 0(4)	Zn1-O2	0.209 8(4)
  Zn1-O4	0.201 7(5)	Zn1-N1	0.206 1(5)	O2-Zn1#1	0.200 0(4)
  O1-Zn1-O2#1	100.8(2)	O1-Zn1-O2	161.30(2)	O1-Zn1-O4	101.39(2)
  O1-Zn1-N1	90.62(2)	O2#1-Zn1-O2	78.33(2)	O2#1-Zn1-O4	103.22(2)
  O2#1-Zn1-N1	140.7(2)	O4-Zn1-O2	96.95(2)	O4-Zn1-N1	111.22(2)
  Symmetry codes: #1: -x, 0.5+y, 0.5-z for complexes 1 and 2.

  As presented in Fig. 1 and Fig. 2, the M(Ⅱ) ion (Ni for 1 and Zn for 2) was coordinated by one oxime nitrogen (N1) atoms and two deprotonated hydroxyl oxygen (O3, O4 in 1 and O1, O2 in 2) atoms of (L)^2- units (L=L¹ or L², as well as one oxygen (O5 in 1 and O4 in 2) atom of the coordinated solvent molecule (DMF for 1 and H₂O for 2), which constitute the [M(L)(solvent)] moiety. And then the O4 and O4A in 1 (and O2 and O2A in 2) atoms bridge the two [M(L)(solvent)] moieties to form the binuclear structure [Ni(L¹)(DMF)]₂ (1) and [Zn(L²(H₂O)]₂ (2). Thus, the central M(Ⅱ) ions are penta-coordinated and their coordination sphere is best described as a distorted tetragonal pyramid. In order to get the geometry adopted by M(Ⅱ) ions, the τ value was estimated to be τ=0.222 8 for 1 and τ=0.133 3 for 2^[51-52]. The difference between complexes 1 and 2 is that the two [M(L)] moieties in complex 2 are almost planar with the distance of 0.007 7 nm but those in 1 are paralleled with the distance of 0.251 0 nm. As well as in complex 1 the coordinated DMF molecule is almost planar with [Ni(L¹)] moiety but the the coordinated H₂O molecule in complex 2 is almost perpendicular to the [Zn(L²] moiety.

  2.2   Intermolecular interactions of complexes 1 and 2

  The intra- and intermolecular interactions data of complexes 1 and 2 are shown in Table 3 and 4. The crystal structure of complex 1 is stabilized by a pair of intramolecular non-classic hydrogen bonds C16-H16…O3 (Fig. 3, Table 3). Meanwhile, two pairs of intermolecular C18-H18B…O2 and C12-H12…O4 hydrogen bonds link neighboring molecules into 2D supramolecular network structure parallel to the bc planes (Fig. 4). Synchronously, complex 1 molecules are further linked by three pairs of intermolecular π…π stacking interactions(Cg1…Cg4, Cg2…Cg4 and Cg2…Cg3) between the benzene ring of adjacent complex 1 molecules to form the other 1D infinite chain along a axis (Fig. 5, Table 4). Thus, complex 1 molecules self-assemble via the intermolecular non-classic hydrogen-bonding and π…π stacking interac-tions to form the 3D supramolecular networks structure (Fig. 6). Consequently, the intermolecular non-classical hydrogen-bonding and π…π stacking interactions plays a very important role in the construction of supramolecular networks structure^[53-59].

  Table 3

  

  Table 3.  Hydrogen-bonding interactions for complexes 1 and 2

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  D-H…A	d(D-H)/nm	d(H…A)/nm	d(D…A)/nm	∠D-H…A/(°)
  Complex 1
  C16-H16…O3	0.093	0.241	0.297 4(3)	119
  C12-H12…O4#1	0.093	0.246	0.337 6(3)	169
  C18-H18B…O2#2	0.096	0.252	0.346 2(4)	166
  Complex 2
  C5-H5…O3#1	0.093	0.247	0.335 6(9)	159
  O4-H4A…O3#2	0.086	0.183	0.265 0(9)	158
  O4-H4B…N2#3	0.086	0.202	0.275 9(7)	143
  C15-H15C…Cg5#4	0.096	0.271	0.361 5(11)	161
  C14-H14C…Cg6#5	0.096	0.300	0.381 3(10)	144
  Cg5 and Cg6 are the centroids of benzene ring C1#4~C6#4 and the chelate ring C8#5-C12#5-O2#5-Zn1#5-N1#5 of complex 2, respectively; Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: x, y, 1+z for 1; #1: x, 1/2-y, -1/2+z; #2: -x, -y, 2-z; #3: -x, -1/2+y, 3/2-z; #4: 1-x, -y, 2-z; #5: -x, -y, 2-z for 2.

  Table 4

  

  Table 4.  π…π stacking interactions for complex 1

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  Ring (i)	Ring (j)	d(Cg…Cg)/nm	d(Cg(i)-perp)/nm	d(Cg(j)-perp)/nm
  Cg1	Cg4#3	0.360 38(15)	0.332 92(9)	0.327 87(11)
  Cg2	Cg4#3	0.349 87(14)	0.330 91(8)	0.331 93(11)
  Cg2	Cg3#3	0.367 44(13)	0.328 74(9)	0.330 08(10)
  Cg1, Cg2, Cg3#3 and Cg4#3 are the centroids of ring C11-N1-Ni1-O4-C15, C7-O3-Ni1-N1-C10, C1#3-O1#3-C9#3 and C1#3~C6#3, respectively; Symmetry codes: #3: 1-x, -y, 1-z.

  Figure 3

  

  Figure 3.  Intramolecular hydrogen bonding of complex 1

  Hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity

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  Figure 4

  

  Figure 4.  Part of 2D supramolecular structure of complex 1 linked by intermolecular hydrogen bonds parallel to the bc planes

  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: x, y, 1+z

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  Figure 5

  

  Figure 5.  Part of 1D supramolecular structure linked by π… π stacking interactions along the a axis of complex 1

  Symmetry codes: #3: 1-x, -y, 1-z

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  Figure 6

  

  Figure 6.  Part of a self-assembling 3D supramolecular structure of complex 1

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  Complex 2 contains crystallizing DMF molecules and the oxygen atoms (O3) of the crystallizing DMF molecules have hydrogen-bond interactions with the coordinated water molecules and benzene rings of the L^2- unit of one neighboring complex molecule, respe-ctively. Meanwhile, the coordinated water molecules are bonded to the neighboring complex molecule. Thus, the complex molecules and the crystallizing DMF molecules are linked by intermolecular hydrogen bonds to form a 2D-layer supramolecular structure parallel to the bc planes (Fig. 7, Table 3). Furthermore, this linkage is further stabilized by two pairs of intermolecular hydrogen bonds interactions (C15-H15C…Cg5 and C14-H14C…Cg6), which interlink the neighboring molecules into the other 1D infinite chain along the a axis (Fig. 8, Table 3). Thus, the crystal packing of complex 2 shows that a 3D supramolecular networks are formed through intermolecular O-H…O, O-H…N, C-H…O and C-H…π hydrogen bonding interactions^[60-66] (Fig. 9).

  Figure 7

  

  Figure 7.  Part of 2D supramolecular structure linked by intermolecular hydrogen bonds parallel to the bc planes of complex 2

  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2:-x, -y, 2-z; #3: -x, -1/2+y, 3/2-z

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  Figure 8

  

  Figure 8.  Part of 1D supramolecular structure linked by C- H…π stacking interactions along the a axis of complex 2

  Symmetry codes: #4: 1-x, -y, 2-z; #5:-x, -y, 2-z

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  Figure 9

  

  Figure 9.  Part of self-assembling 3D supramolecular structure of complex 2

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  2.3   IR spectra analyses

  The FT-IR spectra of H₂L¹, H₂L² and their corresponding complexes 1 and 2 exhibit various bands in the 400~4 000 cm^-1 region. The most impor-tant FT-IR bands for H₂L¹, H₂L² and its Ni(Ⅱ)and Zn(Ⅱ) complexes are listed in Table 5.

  Table 5

  

  Table 5.  Main bands in IR spectra of H₂L¹, H₂L² and their Ni(Ⅱ) and Zn(Ⅱ) complexes

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  cm-1
  Compound	ν(O-H)	ν(C=N)	ν(Ar-O)	ν(M-N)	ν(M-O)
  H2L1	3 447	1 619	1 200	—	—
  1	3 442	1 605	1 193	478	438
  H2L2	3 460	1 607	1 203	—	—
  2	3 437	1 600	1 196	576	506

  The characteristic C=N stretching band of the free ligand H₂L¹ and H₂L² appeared at 1 619 and 1 607 cm^-1, respectively, while those of complexes 1 and 2 were observed in the 1 605 and 1 600 cm^-1, respe-ctively^[67-75]. The C=N stretching frequencies were all shifted to lower frequencies by ca. 14 and 7 cm^-1 upon complexation respectively, indicating a decrease in the C=N bond order due to the coordinated bond of the metal atom with the imino nitrogen lone pair^[76-78]. The Ar-O stretching band of the ligands H₂L¹ and H₂L² occured at 1 200 and 1 203 cm^-1, respectively, and those at 1 193 and 1 196 cm^-1 for complexes 1 and 2. The lower frequency of the Ar-O absorption shift indicates that M-O bond is formed between the metal ions and the oxygen atoms of the phenolic groups^[59]. In addition, the broad O-H group stretching band at 3 447 and 3 460 cm^-1 in the free ligands H₂L¹ and H₂L² disappeared in complexes 1 and 2, indicating the oxygen atoms in the phenolic hydroxyl groups have been completely deprotonated and coordinated to the metal ions. Whereas, the stretching band at 3 437 cm^-1 in the complex 2 is attributed to the stretching vibrations of the O-H group of the coordinated water. The FT-IR spectrum of complex 1 showed ν(M-N) and ν(M-O) vibration absorption frequencies at 478 and 438 cm^-1 (or 576 and 506 cm^-1 for complex 2), respe-ctively. These assignments are consistent with the literature frequency values.

  2.4   UV-Vis spectra analyses

  The absorption spectra of ligands H₂L¹, H₂L² and theirs corresponding Ni(Ⅱ) and Zn(Ⅱ) complexes 1 and 2 were determined in diluted DMF solution as shown in Fig. 10 and 11. The electronic absorption spectrum of free ligand H₂L¹ exhibited two absorption peaks at approximately 271 and 390 nm (Fig. 10). The former absorption peaks at 271 nm can be assigned to the π-π^* transition of the benzene rings and the latter at 390 nm can be attributed to the intraligand π-π^* transition of the C=N group^[79]. Upon coordination of the ligand, the relatively intense absorption at 390 nm disappeared from UV-Vis spectra of complex 1, indicating that the amino nitrogen is involved in coordination with Ni(Ⅱ) ion^[80]. The intraligand π-π^* transition of the benzene ring is bathochromically shifted to 310 nm in complex 1, indicating the coordination of Ni(Ⅱ) ion with deprotonated L^- unit. The new peak at 437 nm of complex 1 is assigned to L→M charge-transfer transition.

  Figure 10

  

  Figure 10.  UV-Vis absorption spectra of H₂L¹ and complex 1 in diluted DMF solution at room temperature

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  The absorption of complex 2 was obviously different from that of H₂L² owing to complexation (Fig. 11). For the free ligand there were two intense peaks centered at around 275 and 399 nm, assigned to π-π^* transitions of the benzene rings of the benzaldehyde and C=N groups, respectively. Compared with the absorption peak of the free ligand, the absorption at 275 nm was slightly shifted hypsochromically to 272 nm of complex 2, indicating the coordination of Zn(Ⅱ)ion with deprotonated L unit. Meanwhile, the absorption peak at 399 nm disappeared from the UV-Vis spectrum of complex 2, which indicates that the amino nitrogen atom is involved in coordination to the metal atom. In addition, a new absorption peak was observed at 460 nm in complex 2, which is assigned to the L→M charge-transfer transition. This is chara-cteristic of a transition metal complex with Schiff base ligand^[80].

  Figure 11

  

  Figure 11.  UV-Vis absorption spectra of H₂L² and complex 2 in diluted DMF solution at room temperature

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  2.5   Emission spectra of complexes 1 and 2

  The emission spectra of ligands H₂L¹ and H₂L², complexes 1 and 2 were determined in diluted DMF solution at room temperature as shown in Fig. 12 and 13. The ligands H₂L¹ and H₂L² exhibited the relatively weak emission at 457 and 473 nm upon excitation at 321 and 351 nm, respectively. The blue emission should be assigned to intraligand π-π^* transition^[81-82]. Compared with the free ligands H₂L¹ and H₂L², an intense green emission at 543 and 538 nm for complexes 1 and 2 were observed upon excitation at 321 and 351 nm, respectively, which indicates that the addition of metal ions Ni(Ⅱ) and Zn(Ⅱ) induces the change of the fluorescence characteristics of the ligand, which may be due to the destruction of the intramolecular hydrogen bonding of the ligand and resulting in the enhancement of the planarity of the conjugated system^[83-85]. The Stokes shift between the maximum wavelength of the fluorescence emission and the fluorescence excitation for H₂L¹, complex 1, H₂L² and complex 2 is 136, 222, 122 and 182 nm, respe-ctively, which indicates that the introduction of coumarin group is beneficial to the luminescence of ligand and its metal complex. And the red shifts in emission wavelength of complexes 1 and 2 compared with H₂L¹ and H₂L² might be related to the coor-dination of the metal ions to the ligands and increases of the rigidity of ligands, which can diminish the loss of energy via vibrational motions and increase the emission efficiency.

  Figure 12

  

  Figure 12.  Emission spectra of H₁L¹ and complex 1 in diluted DMF at room temperature

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  Figure 13

  

  Figure 13.  Emission spectra of H₁L² and complex 2 in diluted DMF at room temperature

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  3.   Conclusions

  The syntheses, structural characterizations, and fluorescence properties of the Schiff base ligands H₂L¹, H₂L² and their corresponding binuclear Ni(Ⅱ) and Zn(Ⅱ) complexes 1 and 2 were discussed. Crystal structure analyses of complexes 1 and 2 showed that the Ni(Ⅱ) and Zn(Ⅱ) atoms all have penta-coordinate environ-ments and adopt distorted square pyramidal geome-tries. Complexes 1 and 2 are self-assembled into the 3D supramolecular networks through intermolecular hydrogen bonding, C-H…π and π…π stacking interactions. Furthermore, the optical properties of complexes 1 and 2 indicated that the coordination of metal ions Ni(Ⅱ) and Zn(Ⅱ) leads to the fluorescence enhancement of H₂L¹ and H₂L². Moreover, the Stokes shifts of H₂L¹ and complex 1 is larger than those of H₂L² and complex 2, which indicates that the intro-duction of coumarin group is beneficial to the luminescence of ligand and its metal complex.

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  Scheme 1  Synthetic route of H₂L¹

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  Scheme 2  Synthetic route of H₂L²

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  Figure 1  (a) Molecular structure of complex 1 showing 30% probability displacement ellipsoids; (b) Coordination geometry diagram for Ni(Ⅱ) ions of complex 1

  Symmetry codes: #1:-x, 0.5+y, 0.5-z

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  Figure 2  Molecular structure of complex 2 showing 30% probability displacement ellipsoids; (b) Coordination geometry diagram for Zn(Ⅱ) ions of complex 2

  Symmetry codes: #1:-x, 0.5+y, 0.5-z

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  Figure 3  Intramolecular hydrogen bonding of complex 1

  Hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity

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  Figure 4  Part of 2D supramolecular structure of complex 1 linked by intermolecular hydrogen bonds parallel to the bc planes

  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: x, y, 1+z

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  Figure 5  Part of 1D supramolecular structure linked by π… π stacking interactions along the a axis of complex 1

  Symmetry codes: #3: 1-x, -y, 1-z

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  Figure 6  Part of a self-assembling 3D supramolecular structure of complex 1

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  Figure 7  Part of 2D supramolecular structure linked by intermolecular hydrogen bonds parallel to the bc planes of complex 2

  Symmetry codes: #1: x, 1/2-y, -1/2+z; #2:-x, -y, 2-z; #3: -x, -1/2+y, 3/2-z

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  Figure 8  Part of 1D supramolecular structure linked by C- H…π stacking interactions along the a axis of complex 2

  Symmetry codes: #4: 1-x, -y, 2-z; #5:-x, -y, 2-z

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  Figure 9  Part of self-assembling 3D supramolecular structure of complex 2

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  Figure 10  UV-Vis absorption spectra of H₂L¹ and complex 1 in diluted DMF solution at room temperature

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  Figure 11  UV-Vis absorption spectra of H₂L² and complex 2 in diluted DMF solution at room temperature

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  Figure 12  Emission spectra of H₁L¹ and complex 1 in diluted DMF at room temperature

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  Figure 13  Emission spectra of H₁L² and complex 2 in diluted DMF at room temperature

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

  Empirical formula	C18H15N3NiO5	C12H8Br2N2O3Zn·C3H7NO
  Formula weight	412.04	526.49
  Temperature/K	297.16(10)	293(2)
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a/nm	0.991 21(4)	1.159 66(6)
  b/nm	1.411 92(6)	1.394 83(4)
  c/nm	1.240 16(5)	1.135 06(6)
  β/(°)	102.395(5)	105.547(5)
  Volume/nm3	1.695(13)	1.768(14)
  Z	4	4
  Dc/(Mg·m-3)	1.615	1.977
  μ/mm-1	1.18	5.93
  F(000)	848	1 032
  θ range/(°)	3.566 0~28.422 0	3.679 0~23.951 0
  Limiting indices	-16 ≤ h ≤ 17, -12 ≤ k ≤ 13, -15 ≤ l ≤ 14	-14≤ h ≤ 7, -17 ≤ k ≤ 16, -13≤ l ≤ 14
  Reflection collected, unique	6 323, 2 737 (Rint=0.026)	7 039, 2 306 (Rint=0.047)
  Completeness to θ=26.32°/%	99.67	99.74
  Data, restraint, parameter	3 335, 0, 246	3 478, 0, 229
  GOF on F2	1.047	0.912
  R1, wR2 [I > 2σ(I)]	0.036 1, 0.081 5	0.053 3, 0.133 0
  Largest diff. peak and hole/(e·nm-3)	520 and -322	836 and -766

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

  Complex 1
  Ni1-O3	0.194 0(2)	Ni1-O5	0.197 8(2)	Ni1-O4#1	0.235 8(2)
  Ni1-O4	0.194 4(2)	Ni1-N1	0.192 9(2)
  O3-Ni1-O5	94.25(8)	O3-Ni1-O4	177.77(7)	O3-Ni1-O4#1	92.27(7)
  O5-Ni1-O4#1	89.16(7)	O4-Ni1-O5	87.94(7)	O4-Ni1-O4#1	87.37(7)
  N1-Ni1-O3	93.03(8)	N1-Ni1-O5	161.42(8)	N1-Ni1-O4#1	107.62(7)
  N1-Ni1-O4	84.99(8)	Ni1-O4-Ni1#1	92.63(7)	C7-O3-Ni1	126.33(2)
  Complex 2
  Zn1-O1	0.196 5(5)	Zn1-O2#1	0.200 0(4)	Zn1-O2	0.209 8(4)
  Zn1-O4	0.201 7(5)	Zn1-N1	0.206 1(5)	O2-Zn1#1	0.200 0(4)
  O1-Zn1-O2#1	100.8(2)	O1-Zn1-O2	161.30(2)	O1-Zn1-O4	101.39(2)
  O1-Zn1-N1	90.62(2)	O2#1-Zn1-O2	78.33(2)	O2#1-Zn1-O4	103.22(2)
  O2#1-Zn1-N1	140.7(2)	O4-Zn1-O2	96.95(2)	O4-Zn1-N1	111.22(2)
  Symmetry codes: #1: -x, 0.5+y, 0.5-z for complexes 1 and 2.

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  Table 3.  Hydrogen-bonding interactions for complexes 1 and 2

  D-H…A	d(D-H)/nm	d(H…A)/nm	d(D…A)/nm	∠D-H…A/(°)
  Complex 1
  C16-H16…O3	0.093	0.241	0.297 4(3)	119
  C12-H12…O4#1	0.093	0.246	0.337 6(3)	169
  C18-H18B…O2#2	0.096	0.252	0.346 2(4)	166
  Complex 2
  C5-H5…O3#1	0.093	0.247	0.335 6(9)	159
  O4-H4A…O3#2	0.086	0.183	0.265 0(9)	158
  O4-H4B…N2#3	0.086	0.202	0.275 9(7)	143
  C15-H15C…Cg5#4	0.096	0.271	0.361 5(11)	161
  C14-H14C…Cg6#5	0.096	0.300	0.381 3(10)	144
  Cg5 and Cg6 are the centroids of benzene ring C1#4~C6#4 and the chelate ring C8#5-C12#5-O2#5-Zn1#5-N1#5 of complex 2, respectively; Symmetry codes: #1: x, 1/2-y, -1/2+z; #2: x, y, 1+z for 1; #1: x, 1/2-y, -1/2+z; #2: -x, -y, 2-z; #3: -x, -1/2+y, 3/2-z; #4: 1-x, -y, 2-z; #5: -x, -y, 2-z for 2.

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  Table 4.  π…π stacking interactions for complex 1

  Ring (i)	Ring (j)	d(Cg…Cg)/nm	d(Cg(i)-perp)/nm	d(Cg(j)-perp)/nm
  Cg1	Cg4#3	0.360 38(15)	0.332 92(9)	0.327 87(11)
  Cg2	Cg4#3	0.349 87(14)	0.330 91(8)	0.331 93(11)
  Cg2	Cg3#3	0.367 44(13)	0.328 74(9)	0.330 08(10)
  Cg1, Cg2, Cg3#3 and Cg4#3 are the centroids of ring C11-N1-Ni1-O4-C15, C7-O3-Ni1-N1-C10, C1#3-O1#3-C9#3 and C1#3~C6#3, respectively; Symmetry codes: #3: 1-x, -y, 1-z.

  下载: 导出CSV

  Table 5.  Main bands in IR spectra of H₂L¹, H₂L² and their Ni(Ⅱ) and Zn(Ⅱ) complexes

  cm-1
  Compound	ν(O-H)	ν(C=N)	ν(Ar-O)	ν(M-N)	ν(M-O)
  H2L1	3 447	1 619	1 200	—	—
  1	3 442	1 605	1 193	478	438
  H2L2	3 460	1 607	1 203	—	—
  2	3 437	1 600	1 196	576	506

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