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Self-Assembled Zn2+, Co2+ and Ni2+ Complexes Based on Coumarin Schiff Base Ligands: Synthesis, Crystal Structure and Spectral Properties

Geng GUO Juan LI Ya WU Wen-Min DING Shu-Zhen ZHANG Yin-Xia SUN

Citation:  Geng GUO, Juan LI, Ya WU, Wen-Min DING, Shu-Zhen ZHANG, Yin-Xia SUN. Self-Assembled Zn2+, Co2+ and Ni2+ Complexes Based on Coumarin Schiff Base Ligands: Synthesis, Crystal Structure and Spectral Properties[J]. Chinese Journal of Inorganic Chemistry, 2021, 37(6): 1113-1124. doi: 10.11862/CJIC.2021.124 shu

基于香豆素Schiff堿的Zn2+、Co2+和Ni2+配合物的合成、晶體結構及光譜性質

    通訊作者: 孫銀霞, sun_yinxia@163.com
摘要: 合成了3個香豆素Schiff堿單核Zn2+、Co2+和雙核Ni2+的配合物,[Zn(L12](1)(HL1=6-((4-二乙胺基-2-羥基-亞芐基)-氨基)-苯并吡喃-2-酮),[Co(L22](2)(HL2=6-((4-甲氧基-2-羥基-亞芐基)-氨基)-苯并吡喃-2-酮),[Ni2(L32(CH3OH)4)](3)(H2L3=4-羥基-3-((4-甲氧基-2-羥基-亞芐基)-氨基)-苯并吡喃-2-酮)。利用元素分析、紅外光譜、紫外可見光譜、熒光光譜及X射線單晶衍射分析等手段對其進行了表征。X射線單晶衍射分析結果表明:配合物1和配合物2均為單核結構,由1個金屬離子和2個配體單元組成;配合物3具有雙核結構,由2個金屬離子、2個配體單元及4個配位的甲醇分子組成。配合物1、23分別是單斜晶系、三斜晶系和三斜晶系,所屬的空間群分別為C2/c、P1P21/n;中心金屬Zn2+和Co2+離子的空間構型為四配位的四面體,Ni2+離子的空間構型為六配位的扭曲的八面體。此外,通過對配體HL1和HL2的紫外可見光譜性質研究發現,在DMF/H2O(4:1,V/V)溶液中,自由配體HL1和HL2分別可以選擇性識別Hg2+和Zn2+,通過計算得到其檢測限分別為7.45和6.10 μmol·L-1。熒光性質研究發現在DMF/H2O(4:1,V/Vs)溶液中自由配體HL2可以檢測Zn2+,檢測限為2.91 μmol·L-1。

English

  • Coumarin is an excellent fluorescent compound with large Stokes shift, visible light excitation and emission, tunable light wavelength, two-photon effect, stable chemical properties, biocompatibility and high fluorescent rate[1-2]. Different positions of coumarin ring can be replaced by different groups, which can obtain different absorption and fluorescent emission wave-lengths, show different colors, and derive derivatives with strong fluorescence[3-5]. In particular, coumarin modified with amino groups at positions 3 and 6 can react with aldehydes to prepare Schiff base compounds. Schiff base derivatives containing N and O donor site and fluorophore are appealing to tools for designing optical probes for metal ions[6-9]. It is already established that Schiff base alone is weakly or nonfluorescent as the fluorescence is quenched due to the free rotation around C=N bond[10-15]. Recently, numerous papers described the specific detection of various metal ions through the chelation of Schiff base probes[16-18]. Coumarin, as an excellent chromophore, can be invoked as a fluorescent sensor to detect cations, anions and biologically related species[19-22]. Meanwhile, coumarin Schiff base can form a variety of metal complexes because of their abundant coordinating atoms and the ability to participate in coordination is outstanding[23-26]. Herein, three coumarin Schiff bases ligands and their corresponding complexes, [Zn(L1)2] (1) (HL1=6-((4-diethylamino-2-hydroxy-benzylidene)-amino)-benzopyran-2-one), [Co(L2)2] (2) (HL2=6-((4-me-thoxy-2-hydroxy-benzylidene)-amino)-benzopyran-2-one) and [Ni2(L3)2(CH3OH)4] (3) (H2L3=4-hydroxy-3-((4-methoxy-2-hydroxy-benzylidene)-amino)-benzopyran-2-one), were synthesized and characterized. In addition, the photochemical properties of HL1, HL2, H2L3 and their corresponding complexes 1, 2 and 3 were discussed. Hg2+ can be selectively identified in DMF/H2O (4:1, V/V) solution by HL1 based on UV-Vis spectrum, and HL2 can detect Zn2+ in DMSO/H2O (4:1, V/V) solution by UV-Vis spectrum and the fluorescence spectrum.

    4 -hydroxyl coumarin and coumarin from Alfa Aesar were used without further purification. 6-amino-coumarin and 3-amino-4-hydroxy-coumarin were synthesized according to an analogous method reported earlier[27-28]. The other reagent and solvent were analytical grade from Tianjin Chemical Reagent Factory, and were used without further purification.

    C, H and N analysis carried out with a GmbH VariuoEL V3.00 automatic elemental analyzer. FT-IR spectra were recorded on a VERTEX70 FT-IR spectro-photometer with samples prepared as KBr (400~4 000 cm-1) pellets. UV-Vis absorption spectra were recorded on a Hitachi UV -3900 spectrometer. Luminescence spectra in solution were recorded on a Hitachi F-7000 spectrometer. X-ray single crystal structures were determined on a Bruker Smart 1000 CCD area detectors. Melting points were measured by a X-4 microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and were uncorrected.

    The synthetic routes of ligands HL1, HL2 and H2L3 are shown in Scheme 1.

    Scheme 1

    Scheme 1.  Synthetic routes of HL1, HL2 and H2L3
    1.3.1   Synthesis of HL1 and HL2

    6-amino coumarin was synthesized depending on an analogous method reported previously in literature[27]. Firstly, coumarin (1.46 g, 0.010 mol) and sodium nitrate (5.20 g) were placed in a flask, and 5.0 mL of H2O was added to the flask. The mixture was stirred thoroughly until the solid was completely dissolved. Concentrated sulfuric acid (12.0 mL) was slowly added dropwise into the solution under an ice-water bath, and the temperature was kept below 25 ℃. After stirred at room temperature for 4.0 h, the solution was poured into crushed ice, and a light-yellow solid was precipitated. After filtering, 6-nitrocoumarin was obtained by recrystallization with glacial acetic acid. Secondly, 3.5 mL water, 0.86 g iron powder, 0.3 mL glacial acetic acid and an ethanol solution of 6-nitrocoumarin (0.60 g, 3.14 mmol) was added and refluxed at 95 ℃ for 1.5 h. The solution was cooled slightly, then 20.0 mL warm saturated sodium carbonate solution was added. The mixture was filtered while it was hot. Yellow solid 6-aminocoumarin was obtained by vacuum filtration after the filtrate was cooled. Yield: 67.8%, m.p. 172~173 ℃. In the end, 6-amino coumarin (1.00 g, 9.50 mmol) and 4-diethylamino-salicylaldehyde (1.20 g, 6.01 mmol) were placed in a flask, followed by 5.0 mL anhydrous ethanol and 2~3 drops of glacial acetic acid. The mixture was subjected to reflux at 75 ℃ for 6.0 h and cooled to room temperature, and bright yellow solid precipitates were obtained. HL1 was obtained by filtering. Yield: 72.5%, m.p. 155~ 156 ℃. Anal. Calcd. for C18H16N2O3(%): C, 70.12; H, 5.23; N, 9.09. Found(%): C, 70.21; H, 5.19; N, 9.02.

    Ligand HL2 was synthesized by a method analogous to that of HL1 except substituting 4-diethylamino-salicylaldehyde with 4-methoxy-salicylaldehyde. Yield: 77.4%. m. p. 187~188 ℃. Anal. Calcd. for C17H13NO4 (%): C, 69.15; H, 4.44; N, 4.74. Found(%): C, 69.19; H, 4.45; N, 4.72.

    1.3.2   Synthesis of H2L3

    3-amino-4-hydroxy-coumarin (1.23 g, 6.90 mmol) and 4-methoxy-salicylaldehyde (1.00 g, 6.60 mmol) were placed in a flask, then 15.0 mL absolute ethanol and 1~2 drops of acetic acid were added to the flask. The mixture was subjected to reflux at 70 ℃ for 6.0 h. After returning to room temperature, the yellow precipitate was filtered and dried to obtain H2L3. Yield: 78.6%, m. p. 125~126 ℃. Anal. Calcd. for C17H13NO5 (%): C, 65.59; H, 4.21; N, 4.50; Found(%): C, 65.60; H, 4.22, N, 4.54.

    1.4.1   Synthesis of complex 1

    A colorless methanol solution (3.0 mL) of zinc(Ⅱ) acetate dihydrate (3.0 mg, 0.014 mmol) was added dropwise to a dichloromethane solution (3.0 mL) of HL1 (4.6 mg, 0.014 mmol) at room temperature. The mixing solution turned yellow immediately and was kept at room temperature for about one month. Clear light colorless single crystals of complex 1 suitable for X-ray structural determination were obtained. Anal. Calcd. for C40H38N4O6Zn(%): C, 65.26; H, 5.20; N, 7.61. Found (%): C, 66.58; H, 5.36; N, 7.85.

    1.4.2   Synthesis of complexes 2 and 3

    The complexes 2 and 3 were prepared by the same method as that of the complex 1 except changing the solvent ratios and metal ions. The solvent ratios of complexes 2 and 3 were adjusted from methanol/dichloromethane (3:3, V/V) to methanol/dichloromethane (5: 5, V/V), and the metal salts were adjusted to the corresponding cobalt(Ⅱ) acetate tetrahydrate and nickel(Ⅱ) acetate tetrachloride. Complex 2: Anal. Calcd. for C34H24N2O8Co(%): C, 63.07; H, 3.74; N, 4.33. Found (%): C, 64.80; H, 3.63; N, 4.52. Complex 3: Anal. Calcd. for C38H38N2Ni2O14(%): C, 52.82; H, 4.43; N, 3.24. Found(%): C, 53.83; H, 4.32; N, 3.46.

    The single crystals of dimensions 0.27 mm×0.23 mm×0.21 mm (1), 0.29 mm×0.27 mm×0.25 mm (2) and 0.24 mm×0.22 mm×0.21 mm (3) were used to determine the crystal structures by X-ray diffraction tech-nique on Bruker SMART 1000 CCD area-detector diffractometer. The reflections were collected using graph-ite-monochromatized Mo radiation (λ =0.071 073 nm) at 291, 295 and 293 K, respectively. The SMART and SAINT software packages[29] were used for data collection and reduction, respectively. Absorption corrections based on multi-scans using the SADABS soft-ware[30] were applied. The structures were solved by direct methods and refined by full-matrix least-squares against F2 using the SHELXL program[31]. All non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms were calculated and isotropic fixed in the final refinement. Details of the crystal parameters, data collection, and refinements for complexes 1~3 are summarized in Table 1.

    Table 1

    Table 1.  Crystal data and structure refinement for complexes 1~3
    下載: 導出CSV
    Complex 1 2 3
    Empirical formula C40H36N4O6Zn C34H24CoN2O8 C38H38N2Ni2O14
    Formula weight 734.10 647.48 864.12
    Crystal system Monoclinic Triclinic Monoclinic
    Space group C2/c P1 P21/n
    a/nm 2.571 4(3) 0.931 4(5) 0.967 0(1)
    b/nm 0.982 9(6) 1.231 3(3) 0.791 6(1)
    c/nm 1.380 0(1) 1.302 1(4) 2.398 1(2)
    α/(°) 83.68(6)
    β/(°) 93.90(1) 75.80(6) 99.96(9)
    γ/(°) 82.38(7)
    V/nm3 3.479 9(6) 1.430 2(1) 1.807 9(3)
    Z 4 2 2
    μ/mm-1 0.760 0.659 1.117
    F(000) 1 528.0 666.0 896.0
    θ range/(°) 3.3~26.0 3.3~25.027 3.3~25.027
    Limiting indices -31 ≤ h ≤ 29, -9 ≤ h ≤ 11, -11 ≤ h ≤ 11,
    -12 ≤ k ≤ 7, -15 ≤ k ≤ 15, -9 ≤ k ≤ 5,
    9 ≤ l ≤ 17 -1 6 ≤ l ≤ 16 -29 ≤ l ≤ 27
    Reflection collected, unique 6 930, 1 485 (Rint=0.057 6) 11 299, 4 680 (Rint=0.052 6) 6 239, 3 193 (Rint=0.135)
    Data, restraint, parameter 3 404, 6, 233 11 299, 0, 409 3 193, 6, 262
    GOF on F2 1.039 0.819 0.951
    R1, wR2 [I > 2σ(I)] 0.086 0, 0.193 6 0.068 9, 0.138 7 0.084 2, 0.118 4
    Largest diff. peak and hole/(e·nm-3) 448 and -299 470 and -440 790 and -410

    CCDC: 2034542, 1; 2034547, 2; 2034548, 3.

    The molecular structure of complexes 1, 2 and 3 are depicted in Fig. 1. Selected bond lengths and angles for complexes 1~3 are listed in Table S1 (Supporting information). X-ray crystallographic analysis shows that complexes 1 and 2 have similar mononuclear crystal structures. The crystals of complexes 1 and 2 are solved as monoclinic space group C2/c and triclinic space group P1, respectively. Complexes 1 and 2 all consist of one central ion (Zn2+ for 1 and Co2+ for 2) and two units ((L1)- for 1 and (L2)- for 2). The central metal (Zn2+ and Co2+) of complexes 1 and 2 are all four-coordinated by two deprotonated hydroxyl oxygen atoms (O3, O3A in 1 and O3, O7 in 2) and two nitrogen atoms (N1, N1A in 1 and N1, N2 in 2) from the ligand units ((L1)- and (L2)-, respectively), which constitute the [Zn(L1)2] (1) and [Co(L2)2] (2) moieties, respectively (Fig. 1a~1d). Therefore, the coordination geometry around the central Zn2+ and Co2+ of complexes 1 and 2 all can be described as a tetrahedron space configurations.

    Figure 1

    Figure 1.  Molecular structure of complexes 1 (a), 2 (c) and 3 (e) showing 30% probability displacement ellipsoids for Zn2+ (b), Co2+ (d), Ni2+ (f)

    Symmetry codes: A: -x, y, 0.5-z for 1; B: 1-x, -y, 1-z for complex 3

    The Schiff base ligand H2L3 moiety of complex 3 contains phenolate-O atoms that connect two neighboring nickel units by way of two μ-O bridges to form binuclear structures, which crystallizes in monoclinic system P21/n space group, and the asymmetry unit consists one Ni2+ ion, one (L3)2- ligand and two coordinated methanol molecules (Fig. 1e). In complex 3, Ni2+ center is characterized by a twisted octahedron coordination geometry, with the basal donor atoms coming from the phenolate-O (O4), coumarin's hydroxyl-O (O1), and imine-N (N1) atoms of the Schiff base ligand, the symmetry-related phenolate-O (O4B) and the two coordinated methanol-O (O6, O7) (Fig. 1f). The length of Ni—O and Ni—N falls in a range of 0.200 2~0.216 9 nm and 0.200 4 nm (Table S1), which are within the ranges reported for other similar nickel complexes.

    The FT-IR spectra of the ligands HL1, HL2 and H2L3 and their corresponding complexes 1, 2 and 3 exhibited various bands in the 400~4 000 cm-1 region, and the important FT-IR bands are given in Table 2. The broad O—H group stretching bands at 3 419, 3 442 and 3 439 cm-1 for the free ligands HL1, HL2 and H2L3, respectively, disappeared in complexes 1, 2 and 3, which indicates that the oxygen atoms in the phenolic hydroxyl groups are completely deprotonated and coordinated to the metal ion[32-35]. Whereas, the stretching bands at 3 483 cm-1 in complex 3 are attributed to the stretching vibrations of the O—H group of coordinated methanol[36-37]. The Ar—O stretching bands at 1 240, 1 207 and 1 259 cm-1 of complexes 1, 2 and 3 shifted toward lower frequencies by ca. 13, 15 and 24 cm-1, respectively, compared with that of free ligands HL1, HL2 and H2L3 at 1 253, 1 222 and 1 293 cm-1, respectively. The lower frequency of the Ar—O stretching shift indicates that M—O bond is formed between the metal ions and the oxygen atoms of the phenolic groups[38-40]. In addition, the free ligand HL1, HL2 and H2L3 exhibited characteristic band of stretching vibration of C=N group at 1 624, 1 644 and 1 674 cm-1, respectively, and their corresponding complexes 1, 2 and 3 were observed at 1 611, 1 644 and 1 692 cm-1, respectively[41-42]. Compared with ligand HL1 and HL2, the C=N stretching frequency of complexes 1 and 2 shifted to a lower frequency by ca. 13 and 8 cm-1, respectively. While complex 3 shifted to a higher fre-quency by ca. 18 cm-1. It is indicated that the C=N bond frequencies decreased or increased due to the coordination bond between the metal atom and the amino nitrogen lone pair[43-44]. The FT-IR spectrum of complex 1 showed ν(M—N) and ν(M—O) vibration frequencies at 615 and 483 cm-1 (or 505 and 467 cm-1 for complex 2, 582 and 459 cm-1 for complex 3). These assignments are consistent with the frequency values in literature[44-45].

    Table 2

    Table 2.  Important IR bands of free ligands HL1, HL2, H2L3 and their corresponding complexes 1, 2, 3?cm-1
    下載: 導出CSV
    Compound n(O—H) n(C=N) n(Ar—O) n(M—N) n(M—O)
    HL1 3 419 1 624 1 253
    Complex 1 1 611 1 240 615 483
    HL2 3 442 1 644 1 222
    Complex 2 1 636 1 207 485 467
    H2L3 3 439 1 674 1 283
    Complex 3 3 483 1 692 1 259 482 459

    The absorption spectra of ligands HL1, HL2, H2L3 and their corresponding complexes 1, 2, 3 in 50 μmol· L-1 DMSO solution are shown in Fig. 2. As shown in Fig. 2, the typical absorption peaks of HL1 (HL2 or H2L3) at ca. 276 nm (ca. 285 nm or ca. 282 and 314 nm) was clearly observed. The peaks can be attributed respectively to the π-π* transitions of the benzene ring[46-47]. Whereas the peak at 382 nm (342 or 411 nm) may be attributed to π-π * transition of the C=N group in the ligand[48-49]. Upon coordination of Zn2+ ion with HL1 (Co2+ ion with HL2 or Ni2+ ion with H2L3), the absorption peaks had evidently changed to 277 nm in complex 1 (287 nm in complex 2 or 277 and 312 nm in complex 3). Compared with free ligands HL1 at 276 nm (285 nm in HL2 or 282 and 314 nm in H 2 L3), the absorption peak was red-shifted by about 1 nm in complex 1 (2 nm in complex 2), and 282 and 314 nm were slightly shifted hypochromic to 277 and 312 nm in complex 3. Additionally, another absorption peak at 382 nm of ligand HL1 (342 nm of HL2 or 411 nm of H2L3) disappeared in the UV-Vis absorption spectrum of complex 1 (complex 2 or complex 3), which indicates that the imine nitrogen atoms of the ligands HL1 (HL2 or H2L3) have coordinated to the metal Zn2+ (Co2+ or Ni2+) ions. The new peak at 467 nm of complex 3 is assigned to transition of ligand to metal charge-transfer[50].

    Figure 2

    Figure 2.  UV-Vis spectra of (a) HL1 and its Zn2+ complex 1, (b) HL2 and its Co2+ complex 2, (c) H2L3 and its Ni2+ complex 3

    The UV-Vis specific responses of HL1 or HL2 to partial common cations (Ag+, Ba2+, Ca2+, Cd2+, Cr3+, Cu2+, Fe3+, Mg2+, Mn2+, Hg2+ and Zn2+) were carried out in DMF/H2O (4:1, V/V) or DMSO/H2O (4:1, V/V) solutions (c=50 μmol·L-1). The UV-Vis spectra of HL1 solution had a significant blue - shift from 383 to 348 nm when Hg2+ ion was added, and the other measured ions could not lead to apparent changes of the spectrum, except for Hg2+ ion and the self-absorption of Cu2+ and Fe3+ ions (Fig. 3a). In order to study the variation of absorption peak intensity with time in UV-Vis spectra of HL1, the UV-Vis absorption intensity was measured at 1 min interval after the addition of Hg2+. It can be seen from Fig. 3b that the UV-Vis peak position of HL1 solu-tion was significantly red-shifted from 383 to 424 nm after adding Hg2+. With the passage of time, the peak at 424 nm gradually disappeared and the peak intensity at 348 nm gradually increased, and remained stable after 18 min. As for HL2, the UV-Vis spectra of HL2 solution had a significant red-shift from 342 to 382 nm when the Zn2+ ion was added, and the other tested ions could not lead to obvious changes of spectra except for Zn2+ ion and the self-absorption of Cu2+ and Fe3+ ions (Fig. 4a). In the natural light, the color changes from yellow (or colorless) to colorless (or yellow) after adding Hg2+ (or Zn2+) to the DMF/H2O (4:1, V/V) (or DMSO/ H2O (4:1, V/V)) solution of HL1 (or HL2), and the color changes of other ions except Fe3+ are not obvious, as shown in Fig. 3d and Fig. 4c. The results showed that the HL1 and HL2 represented excellent selectivity for Hg2+ and Zn2+, respectively.

    Figure 3

    Figure 3.  (a) UV-Vis spectra of HL1 upon addition of different cations (50 eq.) in DMF/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) UV-Vis absorption intensity of HL1 varying with time; (c) UV-Vis spectra of HL1 in DMF/H2O (4:1, V/V) solution (c=50 μmol·L-1) by adding 0~1.15 eq. of Hg2+; (d) Discoloration photos of HL1 in DMF/H2O (4:1, V/V) solution after adding metal cations under natural light

    Figure 4

    Figure 4.  UV-Vis spectra of HL2 upon addition of different cations (50 eq.) in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) UV-Vis spectra of HL2 in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) by adding 0~0.6 eq. of Zn2+, (c) Discoloration photos of HL2 in DMSO/H2O (4:1, V/V) solution after adding metal cations under natural light

    The identification characteristics of HL1 (or HL2) as Hg2+ (or Zn2+) probe were confirmed by UV-Vis titration in DMF/H2O (4:1, V/V) (or DMSO/H2O (4:1, V/V)) solution (Fig. 3c and 4b). The absorption band of HL1 (or HL2) decreased at 383 nm (or 342 nm), and a new absorption band appeared gradually at 348 nm (or 382 nm) when the concentration of Hg2+ (or Zn2+) increased from 0 to 1.15 (or 0.6) eq., which signs the deproton-ation of hydroxyl moiety and coordination with the Hg2+ (or Zn2+) ions. The change in absorbance of HL1 (or HL2) at 348 nm (382 nm) had a linear relationship with Hg2+ (or Zn2+) ion. In the low concentration range, the detection limit of 7.45 μmol·L-1 (or 6.10 μmol·L-1) for Hg2+ (or Zn2+) was calculated (Fig. S7 and Fig. S8) [51-54]. The modified Benesi - Hildebrand formula was used to calculate LOD (limit of detection) and LOQ (limit of quantitation) [55] : LOD=3σ/S, LOQ=10σ/S (N=20). As shown in Table S4, we compared probe HL1 and HL2 of our work with the previously reported Hg2+ and Zn2+ probes. The probes in the table all have lower detection limits and are applied in relatively less toxic solvents. The detection limit of HL1 (or HL2) is similar with previously reported detection limit, and the obtained LOD value is below the value (76 μmol·L-1 for Hg2+ and 46 μmol·L-1 for Zn2+) approved by World Health Organization (WHO)[56-57].

    The emission spectra of free ligands HL1, HL2, H2L3 and its corresponding complexes 1, 2, 3 in 50 μmol·L-1 DMSO solution at room temperature are shown in Fig. 5. As shown in Fig. 5a and 5b, the free ligand HL1 (or HL2) exhibited a relatively strong emis-sion peak at ca. 546 nm (or 498 nm) and a weaker emission peak at 458 nm (or 384 nm) upon excitation at 380 nm (or 340 nm), which could be assigned to the intra-ligand π*-π transition[58-59]. Compared with the free ligand HL1 (or HL2), the emission intensity of complex 1 (or 2) decreased significantly at 546 nm (or 498 nm), but increased at 458 nm (or 384 nm), indicating that the introduction of M2+ ions (Zn2+ or Co2+) affected the fluorescence properties. These transitions may be related to the coordination of ligand HL1 or HL2 and Zn2+ or Co2+ ions, which allows the ligand to develop towards a more stable complex[60]. Both H2L3 and its complex 3 showed strong fluorescence emission at 482 and 396 nm with excitation at 300 nm, respectively (Fig. 5c). Compared with H2L3, the fluorescence intensity of complex 3 reduced slightly and blue-shifted ca. 86 nm, which might be related to the coordination of the N and O atom to the metal ions, and allows H2L3 to develop towards a more stable complex.

    Figure 5

    Figure 5.  Emission spectra of (a) HL1 and complex 1 (λex=380 nm), (b) HL2 and complex 2 (λex=340 nm), (c) H2L3 and complex 3 (λex=300 nm) in DMSO solutions at room temperature

    c=50 μmol·L-1

    Under the excitation wavelength of 380 nm, the fluorescence response of HL2 to various metal ions (Ba2+, Ni2+, Pb2+, Cu2+, Cd2+, Co2+, Cr3+, Mn2+, Hg2+, Ca2+, Sn2+ and Zn2+) was studied. The DMSO/H2O (4:1, V/V) solution of HL2 (50 μmol·L-1) was added to 50 eq. of the solution with above metal ions (10 mmol·L-1). As shown in Fig. 6a, the experimental results showed that when Zn2+ ion was added, the fluorescence intensity of HL2 solution changed significantly, and a new strong fluorescence emission peak appeared at 464 nm, which may be caused by the π*-π transition in the molecules. After the addition of other metal cations, the fluorescence intensity of HL2 solution did not change, which indicates that HL2 has a single and efficient recognition of Zn2+ ion.

    Figure 6

    Figure 6.  Fluorescence emission spectra of HL2 upon addition of different cations (50 eq.) in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) Fluorescence titration spectra of HL2 with increasing concentration of Zn2+ (0~0.5 eq.) in DMSO/H2O (4:1, V/V) solution

    λex=380 nm

    Fluorescence titration experiments were used to detect the binding affinity of HL2 to Zn2+ ion. The Zn2+ ion solution was gradually added to the DMSO/H2O (4:1, V/V) solution of HL2. With the increase of the amount of Zn2+ ion, the fluorescence emission intensity of HL2 gradually increased at 464 nm until the amount of Zn2+ ion reached 0.5 eq. After adding Zn2+, the fluorescence intensity did not change any more, as shown in Fig. 6b. It is worth noting that the change in fluorescence intensity of HL2 at 464 nm had a linear relationship with Zn2+ ion in a range of 0 ~0.5 eq. The experimental results show that the coordination ratio of HL2 to Zn2+ is 2:1.

    In addition, the binding constant of Zn2+ ion to HL2 was determined by Hill's equation to be Ksv=8.91× 104 L·mol-1 (Fig.S9a). The calculated LOD (2.91 μmol· L-1) and LOQ (0.808 μmol·L-1) of HL2 for Zn2+ ions were calculated from the titration spectra of HL2 containing Zn2+, as shown in Fig. S9b. The detection limit was also lower than the WHO approved LOD value.

    Two mononuclear complexes 1, 2 and one binuclear complex 3 based on coumarin Schiff base ligands (HL1, HL2 and H2L3) were synthesized and structural characterized. The crystal structure analyses of complexes 1 and 2 illustrated that the Zn2+ and Co2+ are all four-coordinated distorted tetrahedrons, and the Ni2+ in complex 3 are six-coordinated distorted octahedrons. Meanwhile, the optical properties of complexes 1, 2 and 3 indicate that the fluorescence and UV-Vis varia-tions of HL1, HL2 and H2L3 are due to the coordination of Zn2+, Co2+ and Ni2+, respectively. UV-Vis studies demonstrate that the free ligands HL1 (in DMF/H2O (4:1, V/V) solution) and HL2 (in DMSO/H2O (4:1, V/V) solution) could selectively recognize Hg2+ and Zn2+, respectively. The detection limits were calculated to be 7.45 and 6.10 μmol·L-1, respectively. Fluorescence studies indicate that HL2 exhibits selective recognition ability of Zn2+ against other common cationic including Ag+, Ba2+, Ca2+, Cd2+, Cr3+, Cu2+, Fe3+, Mg2+, Mn2+ and Hg2+ in DMSO/H2O (4:1, V/V). The sensing capability of HL2 for Zn2+ against other common cationic with a low limit of 2.91 μmol·L-1 renders it a candidate probe for Zn2+ detection.


    Supporting information is available at http://www.wjhxxb.cn
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  • Scheme 1  Synthetic routes of HL1, HL2 and H2L3

    Figure 1  Molecular structure of complexes 1 (a), 2 (c) and 3 (e) showing 30% probability displacement ellipsoids for Zn2+ (b), Co2+ (d), Ni2+ (f)

    Symmetry codes: A: -x, y, 0.5-z for 1; B: 1-x, -y, 1-z for complex 3

    Figure 2  UV-Vis spectra of (a) HL1 and its Zn2+ complex 1, (b) HL2 and its Co2+ complex 2, (c) H2L3 and its Ni2+ complex 3

    Figure 3  (a) UV-Vis spectra of HL1 upon addition of different cations (50 eq.) in DMF/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) UV-Vis absorption intensity of HL1 varying with time; (c) UV-Vis spectra of HL1 in DMF/H2O (4:1, V/V) solution (c=50 μmol·L-1) by adding 0~1.15 eq. of Hg2+; (d) Discoloration photos of HL1 in DMF/H2O (4:1, V/V) solution after adding metal cations under natural light

    Figure 4  UV-Vis spectra of HL2 upon addition of different cations (50 eq.) in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) UV-Vis spectra of HL2 in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) by adding 0~0.6 eq. of Zn2+, (c) Discoloration photos of HL2 in DMSO/H2O (4:1, V/V) solution after adding metal cations under natural light

    Figure 5  Emission spectra of (a) HL1 and complex 1 (λex=380 nm), (b) HL2 and complex 2 (λex=340 nm), (c) H2L3 and complex 3 (λex=300 nm) in DMSO solutions at room temperature

    c=50 μmol·L-1

    Figure 6  Fluorescence emission spectra of HL2 upon addition of different cations (50 eq.) in DMSO/H2O (4:1, V/V) solution (c=50 μmol·L-1) at room temperature; (b) Fluorescence titration spectra of HL2 with increasing concentration of Zn2+ (0~0.5 eq.) in DMSO/H2O (4:1, V/V) solution

    λex=380 nm

    Table 1.  Crystal data and structure refinement for complexes 1~3

    Complex 1 2 3
    Empirical formula C40H36N4O6Zn C34H24CoN2O8 C38H38N2Ni2O14
    Formula weight 734.10 647.48 864.12
    Crystal system Monoclinic Triclinic Monoclinic
    Space group C2/c P1 P21/n
    a/nm 2.571 4(3) 0.931 4(5) 0.967 0(1)
    b/nm 0.982 9(6) 1.231 3(3) 0.791 6(1)
    c/nm 1.380 0(1) 1.302 1(4) 2.398 1(2)
    α/(°) 83.68(6)
    β/(°) 93.90(1) 75.80(6) 99.96(9)
    γ/(°) 82.38(7)
    V/nm3 3.479 9(6) 1.430 2(1) 1.807 9(3)
    Z 4 2 2
    μ/mm-1 0.760 0.659 1.117
    F(000) 1 528.0 666.0 896.0
    θ range/(°) 3.3~26.0 3.3~25.027 3.3~25.027
    Limiting indices -31 ≤ h ≤ 29, -9 ≤ h ≤ 11, -11 ≤ h ≤ 11,
    -12 ≤ k ≤ 7, -15 ≤ k ≤ 15, -9 ≤ k ≤ 5,
    9 ≤ l ≤ 17 -1 6 ≤ l ≤ 16 -29 ≤ l ≤ 27
    Reflection collected, unique 6 930, 1 485 (Rint=0.057 6) 11 299, 4 680 (Rint=0.052 6) 6 239, 3 193 (Rint=0.135)
    Data, restraint, parameter 3 404, 6, 233 11 299, 0, 409 3 193, 6, 262
    GOF on F2 1.039 0.819 0.951
    R1, wR2 [I > 2σ(I)] 0.086 0, 0.193 6 0.068 9, 0.138 7 0.084 2, 0.118 4
    Largest diff. peak and hole/(e·nm-3) 448 and -299 470 and -440 790 and -410
    下載: 導出CSV

    Table 2.  Important IR bands of free ligands HL1, HL2, H2L3 and their corresponding complexes 1, 2, 3?cm-1

    Compound n(O—H) n(C=N) n(Ar—O) n(M—N) n(M—O)
    HL1 3 419 1 624 1 253
    Complex 1 1 611 1 240 615 483
    HL2 3 442 1 644 1 222
    Complex 2 1 636 1 207 485 467
    H2L3 3 439 1 674 1 283
    Complex 3 3 483 1 692 1 259 482 459
    下載: 導出CSV
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  • 發布日期:  2021-06-10
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