Synthesis, Crystal Structure and Spectral Properties of Binuclear Ni(Ⅱ) and Cubane-like Cu4(μ3-O)4 Cored Tetranuclear Cu(Ⅱ) Complexes Based on Coumarin Schiff Base

Shu-Zhen ZHANG Jian CHANG Hong-Jia ZHANG Ya WU Yin-Xia SUN Yan-Bin WANG

Citation:  ZHANG Shu-Zhen, CHANG Jian, ZHANG Hong-Jia, WU Ya, SUN Yin-Xia, WANG Yan-Bin. Synthesis, Crystal Structure and Spectral Properties of Binuclear Ni(Ⅱ) and Cubane-like Cu4(μ3-O)4 Cored Tetranuclear Cu(Ⅱ) Complexes Based on Coumarin Schiff Base[J]. Chinese Journal of Inorganic Chemistry, 2020, 36(3): 503-514. doi: 10.11862/CJIC.2020.056 shu

基于香豆素Schiff碱双核Ni(Ⅱ)和立方烷型Cu4(μ3-O)4的四核Cu(Ⅱ)配合物的合成、晶体结构及光谱性质

    通讯作者: 孙银霞, sun_yinxia@163.com
  • 基金项目:

    甘肃省重点研发计划项目 18YF1GA054

    兰州交通大学优秀科研平台 201706

    甘肃省重点研发计划项目(No.18YF1GA054),兰州交通大学优秀科研平台(No.201706)和甘肃省高校环境友好复合材料及生物质利用重点实验室开放课题资助项目

摘要: 合成了2个3-氨基-4-羟基香豆素类Schiff碱双核Ni(Ⅱ)和立方烷型Cu4μ3-O)4的四核Cu(Ⅱ)配合物,[Ni(L1)(DMF)(H2O)]21)(H2L1=3-((5-溴-2-羟基-亚苄基)-氨基)-4-羟基-苯并吡喃-2-酮)和[Cu4(L24]·DMF·CH3OH·2H2O(2)(H2L2=4-羟基-3-((2-羟基-3-甲氧基-亚苄基)-氨基)-苯并吡喃-2-酮),并通过元素分析、红外光谱、紫外光谱、荧光光谱及X射线单晶衍射分析等手段进行了表征。X射线单晶衍射分析结果表明:配合物1具有双核结构,由2个金属离子和2个配体单元组成,配合物2具有立方烷型Cu4μ3-O)4的四核结构,由4个金属离子和4个配体单元组成。配合物1是单斜晶系、C2/c空间群;配合物2是四方晶系,I41/a空间群,且中心金属Ni(Ⅱ)和Cu(Ⅱ)离子的空间构型均为六配位的扭曲的八面体。此外,配合物1通过分子间氢键、C-H…πππ作用形成1D超分子链结构,配合物2通过分子内氢键和ππ作用形成3D超分子结构。此外,研究了H2L1,H2L2及其相应的Ni(Ⅱ)和Cu(Ⅱ)配合物的荧光性质。

English

  • 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]. Schiff-base compounds and its derivatives are very important as versatile ligands, properties of interest in materials science. Also, the Schiff base ligands with N- and O- group are strong donors and therefore the oxime-containing ligands were found to efficiently stabilize high oxidation states of metal ions and prepare complexes with different structures and functionalities like Cu(Ⅱ) and Ni(Ⅱ) complexes[34-39]. In recent years, there has been enhanced interest in the synthesis and characterization of such complexes due to their interesting properties and other applications[40-47]. In order to further study the supramolecular of transition metal complexes and Schiff base ligands, we synthesized and analyzed two complexes, [Ni(L1)(DMF)(H2O)]2 (1) (H2L1=3-((5-bromo-2-hydroxy-benzylidene)-amino)-4-hydroxy-benzopyran-2-one) and [Cu4(L2)4]·DMF·CH3OH·2H2O (2) (H2L2=4-hydroxy-3-((2-hydroxy-3-methoxy-benzylidene)-amino)-benzopyran-2-one). Complex 1 is a dinuclear structure and is connected to a 1D supramolecular chain by intermolecular hydrogen bonding, C-H…π and ππ stacking interactions. Complex 2 is a cubane-like Cu4 (μ3-O)4 cored tetranuclear structure and is linked to a 3D network supramolecular structure by intramole-cular hydrogen bonding and ππ stacking interac-tions. The central metal Ni(Ⅱ) and Cu(Ⅱ) ions are all six-coordinated distorted octahedron geometries in complexes 1 and 2, respectively. In addition, the fluorescent properties of the ligands H2L1, H2L2 and their Ni(Ⅱ) complex 1 and Cu(Ⅱ) complex 2 are also studied.

    4-Hydroxyl coumarin from Alfa Aesar was used without further purification. The other reagents and solvents were of analytical grade from Tianjin Chemical Reagent Factory, and were used without further purification.

    C, H and N analyses were carried out with a GmbH Vario EL V3.00 automatic elemental analyzer. FT-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 Hitachi UV-3900 spectrometer. Luminescence spectra in solution were recorded on a Hitachi F-7000 spectrometer. X-ray single crystal structure was determined on a Bruker Smart 1000 CCD area detector. Melting points were measured by an X-4 microscopic melting point apparatus made by Beijing Taike Instrument Limited Company and were uncorrected. 1H NMR spectra were recorded in DMSO-d6 solution at room temperature on a Bruker AV instrument operating at a frequency of 500 MHz and referenced to tetramethylsilane (δ=0.00) as an internal standard. Chemical shift multiplicities are reported as s=singlet, d=doublet, t=triplet and m=multiplet.

    H2L1 and H2L2 were synthesized according to the following synthetic routes shown in Scheme 1.

    Scheme 1

    Scheme 1.  Synthetic routes of H2L1 and H2L2
    1.3.1   Synthesis of 3-amino-4-hydroxycoumarin

    3-Amino-4-hydroxycoumarin was synthesized according to an analogous method reported earlier[48]. Firstly, 0.9 mL concentrated HNO3 and 1.8 mL CH3COOH mixed solution was added dropwise into the 4-hydroxycoumarin (20.5 g, 12.6 mmol) solution containing 5 mL glacial acetic acid over 0.5 h at 85 ℃, and the mixture was subjected to heating at 80 ℃ for 4 h. The mixed solution was allowed to come to room temperature, and placed in an ice-water mixture until a pale yellow solid appeared, which was filtered to give the 3-nitro-4-hydroxycoumarin. Secondly, Na2S2O4 solid (33.0 g, 18.3 mmol) was added in batches to a solution of 3-nitro-4-hydroxycoumarin (10.0 g, 4.8 mmol) in EtOH/H2O (2:1, V/V) mixture solution (60 mL) at 87 ℃, and the reaction continued for 7 h. Then the ethanol was removed via vacuum distillation. The residual Na2S2O4 was removed by reaction with concentrated hydrochloric acid, and neutralized with saturated sodium bicarbonate solution. The product was suction filtered and dried to give a brown 3-amino-4-hydroxycoumarin powder. Yield: 67.8%. m.p. 209~211 ℃. Anal. Calcd. for C9H7NO3(%): C, 61.02; H, 3.98; N, 7.91. Found(%): C, 61.26; H, 3.67; N, 7.72.

    1.3.2   Synthesis of H2L1

    3-Amino-4-hydroxycoumarin (177.0 mg, 1.0 mmol) and 2-hydroxy-5-bromobenzaldehyde (201.21 mg, 1.0 mmol) were placed in a 20 mL flask, and 7 mL of absolute ethanol was added to the flask. The mixture was subjected to reflux at 70 ℃ for 8 h, and allowed to come to room temperature. The light yellow precipitate was filtered and dried to obtain 267.79 mg H2L1. Yield: 61.07%. m.p. > 300 ℃. Anal. Calcd. for C16H10BrNO4(%): C, 53.36; H, 2.80; N, 3.89. Found (%): C, 53.43; H, 2.74; N, 3.79. 1H NMR (500 MHz, DMSO-d6): δ 6.99 (d, J=9.0 Hz, 1H, Ph), 7.24 (td, J=6.5, 1 Hz, 1H, Ph), 7.33 (m, 2H, Ph), 7.62 (m, 2H, Ph), 7.79 (dd, J=3.5 Hz, 1H, Ph), 8.07 (s, 1H, CH=N), 9.86 (s, 1H, OH), 10.99 (s, 1H, OH).

    1.3.3   Synthesis of H2L2

    The ligand H2L2 was synthesized by a method similar to that of H2L1 except substituting 2-hydroxy-5-bromobenzaldehyde with 2-hydroxy-3-methoxybenzal-dehyde. H2L2: 193.21 mg, Yield: 62.23%. m.p. 247~249 ℃. Anal. Calcd. for C17H13NO5(%): C, 65.59; H, 4.21; N, 4.50. Found(%): C, 65.43; H, 4.35; N, 4.72. 1H NMR (500 MHz, DMSO-d6): δ 3.86 (t, J=14.5 Hz, 3H, OCH3), 6.97 (t, J=14.5 Hz, 1H, Ph), 7.08 (s, 1H, Ph), 7.00 (s, 1H, Ph), 7.22 (m, 2H, Ph), 7.4 (s, 1H, Ph), 7.69 (td, J=12.5, 6.5 Hz, 1H, Ph), 7.96 (dd, J=2, 0 Hz, 1H, CH=N), 9.91 (s, 1H, OH), 10.27 (s, 1H, OH).

    A solution of Ni(Ⅱ) acetate monohydrate (2.48 mg, 0.01 mmol) in methanol (1 mL) was added dropwise to a solution of H2L1 (3.6 mg, 0.01 mmol) in acetone/DMF (7 mL, 6:1, V/V). The color of the mixture turned to yellow immediately, and then 2 drops of triethyla-mine were added in it. The mixture was stirred for 1 h at room temperature, filtered, and the filtrate was allowed to stand at room temperature for about two weeks. The solvent was partially evaporated, and pale yellow needle-like single crystals of complex 1 suitable for X-ray crystallographic analysis were obtained. Anal. Calcd. for C38H34Br2N4Ni2O12(%): C, 49.43; H, 3.37; N, 5.52. Found(%): C, 49.22; H, 3.75; N, 5.62.

    The synthesis of Cu(Ⅱ) complex 2 was same as above to obtain yellow needle-like single crystals suitable for X-ray crystallographic analysis. Anal. Calcd. for C72H59Cu4N5O24(%): C, 52.97; H, 3.64; N, 4.29. Found(%): C, 53.20; H, 3.40; N, 4.51.

    The single crystals with approximate dimensions of 0.22 mm×0.26 mm×0.28 mm (1) and 0.22 mm×0.25 mm×0.27 mm (2) were placed on a Bruker Smart 1000 CCD area detector. The reflections were collected using graphite-monochromatized Mo radiation (λ=0.071 073 nm) at 296(1) K and 296(2) K, respectively. The Lp corrections were applied to the SAINT program[49]and semi-empirical correction were applied to the SADABS program[50]. The crystal structures were solved by the direct methods (SHELXS-2014)[51]. 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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    Complex 1 2
    Empirical formula C38H34Br2N4Ni2O12 C72H59Cu4N5O24
    Formula weight 1 015.87 1 491.27
    Crystal system Monoclinic Tetragonal
    Space group C2/c I41/a
    a/nm 1.799 3(4) 1.663 3(4)
    b/nm 0.921 0(2) 1.663 3(4)
    c/nm 2.496 9(6) 2.342 9(4)
    β/(°) 110.249(2)
    V/nm3 3.881 8(2) 6.482(3)
    Z 4 4
    μ/mm-1 3.098 1.374
    F(000) 2 048 3 024
    θ range/(°) 1.70~25.0 2.5~25.0
    Limiting indices -21 ≤ h ≤ 19, -10 ≤ k ≤10, -29 ≤ l ≤ 29 -18 ≤ h ≤ 19, -19 ≤ k ≤ 18, -27 ≤ l ≤ 27
    Reflection collected, unique 14 365, 3 413 (Rint=0.174) 20 340, 2 862 (Rint=0.072)
    Completeness to θ/% 100 99.9
    Data, restraint, parameter 3 413, 0, 263 2 862, 0, 218
    GOF on F2 1.07 1.06
    R1, wR2 [I>2σ(I)] 0.044 4, 0.108 31 0.039 6, 0.100 2
    Largest diff. peak and hole/(e·nm-3) 820 and -810 860 and -890

    CCDC: 1922258, 1; 1922309, 2.

    The molecular structure of complexes 1 and 2 are shown in Fig. 1 and 2, respectively, and selected bond lengths and angles are listed in Table 2. X-ray crystallographic analysis shows that complex 1 crystallizes in the monoclinic system, and the space group is C2/c. Complex 1 can be described as a binuclear Ni(Ⅱ) complex, consisting of two Ni(Ⅱ) ions, two (L1)2- units and two coordinated solvent molecules H2O and DMF. In complex 1, two deprotonated hydroxyl oxygen (O3, O4) atoms and one oxime nitrogen (N1) atoms come from the (L1)2- unit, as well as two oxygen (O5, O6) atoms of the coordinated solvent molecules H2O and DMF, respectively, which constitute the [Ni(L1)(H2O)(DMF)] moiety. And then the O4 and O4a atoms bridge the two [Ni(L1)(H2O)(DMF)] moieties to form the binuclear structure [Ni(L1)(H2O)(DMF)]2 (1). Thus, the central Ni(Ⅱ) ions are hexa-coordinated and their coordination sphere is best described as a slightly distorted octahedron.

    Figure 1

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

    Symmetry code: a: 3/2-x, 3/2-y, 1-z

    Figure 2

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

    Symmetry codes: a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z; c:-x, 3/2+y, z

    Table 2

    Table 2.  Selected bond lengths(nm) and bond angles(°) for complexes 1 and 2
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    1
    Ni1-O3 0.205 0(2) Ni1-O4 0.201 6(3) Ni1-O5 0.208 4(2)
    Ni1-O6 0.210 0(2) Ni1-N1 0.201 2(3) Ni1-O4a 0.206 4(2)
    O3-Ni1-O4 172.93(8) O3-Ni1-O5 87.32(7) O3-Ni1-O6 93.89(8)
    O1-Ni1-N1 82.01(8) O1-Ni1-O4a 105.58(8) O4-Ni1-O5 90.81(7)
    O4-Ni1-O6 88.92(7) O4-Ni1-N1 91.42(9) O4-Ni1-O4a 80.95(8)
    O5-Ni1-O6 171.90(9) O5-Ni1-N1 96.12(8) O5-Ni1-O4a 83.42(8)
    O6-Ni1-N1 91.99(9) O6-Ni1-O4a 88.54(8) N1-Ni1-O4a 172.34(8)
    2
    Cu1-O1 0.234 7(2) Cu1-O2 0.198 5(2) Cu1-O2a 0.196 1(2)
    Cu1-O3a 0.194 5(2) Cu1-N1a 0.195 1(2) Cu1b-O2 0.196 1(2)
    Cu1b-O3 0.194 5(2) Cu1b-N1 0.195 1(2) Cu1-O2b 0.268 2(2)
    O1-Cu1-O2 75.51(7) O1-Cu1-O2a 100.98(7) O2-Cu1-O2a 88.76(8)
    O1-Cu1-O3a 84.41(7) O2-Cu1-O3a 91.75(7) O2A-Cu1-O3a 174.54(7)
    O3A-Cu1-N1a 85.16(8) O1-Cu1-N1a 114.44(7) O2-Cu1-N1a 169.13(8)
    O2A-Cu1-N1a 93.35(8) Cu1-O2-Cu1b 111.26(8)
    Symmetry codes: a: 3/2-x, 3/2-y, 1-z for 1; a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z for 2.

    Complex 2 crystallizes in the tetragonal system, and the space group was I41/a. Complex 2 can be described as a cubane-like Cu4(μ3-O)4 cored tetranu-clear Cu(Ⅱ) complex, and consist of four Cu(Ⅱ) ions and four (L2)2- units, in which the ligand (L2)2- is both chelating and bridging after double deprotonation of the phenolic hydroxyls. A [Cu(L2)] moiety was constituted by two deprotonated hydroxyl oxygen (O2, O3) atoms, one oxime nitrogen (N1) atoms from one of the ligand units (L2)2-, and the oxygen (O1) atom of the methoxy group coming from this (L2)2- units coordinated to the adjacent [Cu(L2)] moiety (Fig. 3a). By self-assembly, four such monomeric [Cu(L2)] entities eventually are linked through alkoxo (O2 and O2a) bridges to produce the tetranuclear cubane Cu4(μ3-O)4 core. This Cu4(μ3-O)4 core consists of four alkoxo-bridged Cu(Ⅱ) centers approximately arranged in a cuboid geometry of an alternating array of Cu and O atoms that occupy the corners of the cube (Fig. 3b). This structure could alternatively be seen as two interpenetrated Cu4 and O4 tetrahedrons. The four pincer ligands (L2)2- are all in the binding mode μ3-η1:η1:η3, and all four bridging alkoxo oxygens are located at the four corners of the cube, each bridging three Cu(Ⅱ) ions. Four Cu(Ⅱ) ions are crystallographically equivalent and have the same coordination spheres, which is six coordinated by one nitrogen atom (N1a), five oxygen atoms (O1, O2, O2a, O2b, O3a) from the (L2)2- units, and all are in distorted [CuN1O5] octahedral geometries. Observed Cu-N bond distances lie in the normal range of 0.195 1(2) nm. The equatorial Cu-O bonds at each Cu center are shorter (ranging from 0.194 5(2) to 0.198 5(2) nm) than the axial Cu-O bond (in a range of 0.234 7(2)~0.268 2(2) nm)[52]. The metal-metal intramolecular bond distances range from 0.325 69(6) to 0.359 12(9) nm.

    Figure 3

    Figure 3.  (a) Structure of [Cu(L2)] moiety of complex 2 showing 30% probability displacement ellipsoids; (b) Distorted Cu4O4 core of the cubane-like isomer (30% probability ellipsoids) with no crystallographic constraint

    Symmetry codes: a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z; c:-x, 3/2+y, z

    The intra- and intermolecular interactions data of complexes 1 and 2 are shown in Table 3 and 4. The structure of complex 1 was stabilized by three intra-molecular hydrogen bonds of O5-H5C…O6, C15-H15…O3, C10-H10…O2 (Fig. 4). An intermolecular hydrogen bonds O5-H5B…O2 (Fig. 5) and an inter-molecular non-classical C5-H5A…πcentroid(C11-C16) interac-tions (Fig. 6) link complex 1 molecules to form an infinite 1D supramolecular chain. Synchronously, this linkage is further stabilized via the intermolecular πcentroid(C1-C6)πcentroid(C11-C16) stacking interactions between the benzene ring of adjacent complex 1 molecules with the distance of 0.428 1(2) nm (Fig. 6)[53-57]. Consequently, the intermolecular classical and non-classical hydrogen-bonding and ππ stacking interactions plays a very important role in the construction of supramolecular networks structure[58-64].

    Table 3

    Table 3.  Putative 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)/(°)
    1
    O5-H5C…O6a 0.082 0.213 0.291 1(3) 158
    C10-H10…O2 0.093 0.220 0.285 6(4) 127
    C15-H15…O3a 0.093 0.223 0.310 6(4) 156
    O5-H5B…O2a 0.086 0.183 0.267 9(3) 172
    C5-H5A…Cg1a 0.298 0.294 0.336 9(4) 107
    2
    C8-H8…O5 0.93 2.17 2.856(3) 130
    Symmetry code: 1-x, 2-y, 1-z; Cg1 is the centroids of C11~C16 in benzene ring.

    Table 4

    Table 4.  ππ stacking interactions for complexes 1 and 2
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    Ring (I) Ring (J) d(Cg…Cg)/nm α/(°) d(Cg(I)-perp)/nm d(Cg(J)-perp)/nm Slippage/nm
    1
    Cg2 Cg1a 0.428 1(2) 26.58(2) 0.283 5(1) 0.394 4(1) 0.961 0
    2
    Cg3 Cg4a 0.329 8(2) 13.77(1) 0.275 51(8) 0.307 81(9) 0.118 3
    Cg3 Cg5b 0.329 8(2) 13.77(1) 0.275 51(8) 0.307 81(9) 0.118 3
    Cg4 Cg6b 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg4 Cg7c 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg5 Cg6a 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg5 Cg7b 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg7 Cg6a 0.312 5(1) 15.47(8) 0.303 72(8) 0.303 71(8) 0.073 7
    Cg6 Cg7b 0.312 5(1) 15.47(8) 0.303 72(8) 0.303 71(8) 0.073 7
    Symmetry codes: a: 1-x, 2-y, 1-z for 1; a: 3/4-y, 3/4+x, 3/4-z; b: -x, 3/2-y, z; c: -3/4+y, 3/4-x, 3/4-z for 2; α=dihedral angle between planes I and J; d(Cg…Cg)=distance between ring centroids; d(Cg(I)-perp)=perpendicular distance of Cg(I) on ring J; d(Cg(J)-perp)=perpendicular distance of Cg(J) on ring I; Slippage=distance between Cg(I) and perpendicular projection of Cg(J) on ring I; Cg1, Cg2 are the centroids of C11~C16 and C1~C6 in benzene ring, respectively; Cg3, Cg4, Cg5, Cg6, Cg7 are the centroids of ring Cu1-O1-C2-C7-O2, Cu1a-O3c-C10c-C9c-N1c, O3-C10-C9-N1-Cu1b, Cu1-O2a-C7a-C6a-C8a-N1a and O2b-C7b-C6b-C8b-N1b-Cu1c, respectively.

    Figure 4

    Figure 4.  Intramolecular hydrogen bonding of complex 1

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

    Figure 5

    Figure 5.  Intermolecular hydrogen bonding of complex 1

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

    Figure 6

    Figure 6.  View of 1D supramolecular chain linked by C-H…π and ππ stacking interaction of complex 1

    In complex 2, the structure were only stabilized by an intramolecular non-classical hydrogen bonds of C8-H8…O5 (Fig. 7 and Table 3), but there are eight complicated intermolecular ππ stacking interac-tions (Cg3…Cg4a, Cg3…Cg5b, Cg4…Cg6b, Cg4…Cg7c, Cg5…Cg6a, Cg5…Cg7b, Cg7…Cg6a, Cg6…Cg7b), with the distance of 0.329 7(7), 0.329 7(7), 0.357 3(11), 0.357 3(11), 0.357 3(0), 0.357 3(0), 0.312 5(4), 0.312 54(4) nm, respectively, which linked the neighboring molecules into a 3D network supramolecular structure (Table 4).

    Figure 7

    Figure 7.  Intramolecular hydrogen bonding of complex 2

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

    The FT-IR spectra of H2L1, H2L2, and their corresponding complexes 1 and 2 exhibited various bands in the 400~4 000 cm-1 region, and the most important FT-IR bands for H2L1, complex 1 and H2L2, complex 2 are given in Table 5. The free ligand H2L1 and H2L2 exhibited characteristic stretching bands of C=N group at 1 670 and 1 676 cm-1, respectively[65-73], while that of their corresponding complexes 1 and 2 were observed at 1 610 and 1 682 cm-1, respectively. Compared with the ligand, the C=N stretching frequency of complex 1 shifted to a lower frequency by ca. 60 cm-1, while that of complex 2 shifted to a higher frequency by ca. 6 cm-1. It is indicated that the C=N bond sequence is decreased or increased due to the coordination bond between the metal atom and the imino nitrogen lone pair[74-76]. In addition, the broad O-H group stretching bands at 3 403 and 3 438 cm-1 for the free ligands H2L1 and H2L2, disappeared for complexes 1 and 2, indicating the oxygen atoms in the phenolic hydroxyl groups were completely deprotonated and coordinated to the metal ions. Whereas, the stretching bands at 3 409 and 3 476 cm-1 in comp-lexes 1 and 2 are attributed to the stretching vibrations of the O-H group of coordinated water or methanol. The Ar-O stretching bands at 1 230 and 1 229 cm-1 of complexes 1 and 2 shifted toward lower frequencies by ca. 35 and 12 cm-1, respectively, compared with that of the free ligands H2L1 and H2L2 at 1 265 and 1 241 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[77]. The FT-IR spectrum of complex 1 showed ν(M-N) and ν(M-O) vibration frequencies at 514 and 467 cm-1 (or 538 and 467 cm-1 for complex 2), respectively. These assignments are consistent with the frequency values in literature[78].

    Table 5

    Table 5.  Main bands in IR spectra of H2L1, H2L2 and complexes 1 and 2 cm-1
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    Compound ν(O-H) ν(C=N) ν(Ar-O) ν(M-N) ν(M-O)
    H2L1 3 403 1 670 1 265
    [Ni(L1)(DMF)(H2O)]2 (1) 3 409 1 610 1 230 514 467
    H2L2 3 438 1 676 1 241
    [Cu4(L2)4]·DMF·CH3OH·2H2O (2) 3 476 1 682 1 229 538 467

    The absorption spectra of ligands H2L1, H2L2 and their corresponding Ni(Ⅱ) and Cu(Ⅱ) complexes 1 and 2 were determined in diluted DMSO solution, respectively. As shown in Fig. 8, compared with complex 1, an important feature of the absorption spectrum of H2L1 is shown that three absorption peaks were observed at 445, 469, 498 nm attributed to the intra-ligand π-π* transition of the C=N bonds and the conjugated aromatic chromophore, in which the absorption peak at 498 nm was absent in the spectrum of complex 1. The absorption peaks at 445, 469 nm were blue-shifted by 2 nm and red-shifted 2 nm, respectively, indicating that Ni(Ⅱ) ion coordinates with the O and N atoms of the deprotonated ligand units. And the absorption peak at 349 nm assigned to the π-π* transitions of the phenyl rings in H2L1 was shifted to 351 nm in complex 1, indicating the coordination of Ni(Ⅱ) atom with (L1)2-.

    Figure 8

    Figure 8.  UV-Vis absorption spectra of H2L1 and complex 1 in diluted DMSO solution at room temperature

    The electronic absorption spectrum of free ligand H2L2 exhibited three absorption peaks at approxi-mately 389, 412 and 437 nm (Fig. 9). The former absorption peaks at 389 nm can be assigned to the π-π* transition of benzene rings and the latter at 412 and 437 nm can be attributed to the intra-ligand π-π* transition of C=N group[79]. Upon coordination of the ligand, the absorption peaks at 412 and 437 nm were red-shifted to 416 and 438 nm, respectively, indicating that the amino nitrogen is involved in coordination with Cu(Ⅱ) ion[80]. The intraligand π-π* transitions of the benzene ring were bathochromically shifted to 343 and 360 nm in complex 2, indicating the coordination of Cu(Ⅱ) ion with deprotonated (L2)2- unit.

    Figure 9

    Figure 9.  UV-Vis absorption spectra of H2L2 and complex 2 in diluted DMSO solution at room temperature

    The fluorescence emission spectra of the ligands H2L1, H2L2 and their corresponding Ni(Ⅱ) and Cu(Ⅱ) complexes 1 and 2 were determined at room temp-erature in a diluted DMSO solution. As shown in Fig. 10, the free ligand H2L1 showed stronger fluorescence emission at 413, 439 and 518 nm with the excitation at 370 nm, respectively, which could be assigned to the intraligand π*-π transition. Compared with H2L1, the emission peaks of complex 1 was slightly blue-shifted 1~2 nm, and the fluorescence intensity at 412, 437 nm tended to increase and that at 518 nm tended to decrease significantly, indicating that the Ni(Ⅱ) ion coordinates with the N and O atoms and electron transition occurs.

    Figure 10

    Figure 10.  Emission spectra of H2L1 and complex 1 in diluted DMSO at room temperature

    Meanwhile, the ligand H2L2 exhibited a relatively strong emission peak at ca. 507 nm and a weaker emission peak at 414 nm upon excitation at 370 nm (Fig. 11), which could be assigned to the intraligand π*-π transition. Compared with the H2L2, the emission intensity at 507 nm reduced obviously and that at 414 nm tended to increase for complex 2, indicating that the Cu(Ⅱ) ion coordinates with the N and O atoms and electron transition occurs. The fluorescence intensity of the free ligands H2L2 is probably enhanced via the occurrence of a photoinduced electron transfer process owing to the presence of a lone pair of nitrogen atoms. The process is prevented by the complexation of the free ligand H2L2 with the Cu(Ⅱ)ions. Therefore, the coordination of Cu(Ⅱ) ions can effectively reduce the fluorescence intensities[81].

    Figure 11

    Figure 11.  Emission spectra of H2L2 and complex 2 in diluted DMSO at room temperature

    Based on two Schiff base ligands, dinuclear complex 1 and cubane-like Cu4(μ3-O)4 core tetranuclear complex 2 were synthesized and their structural chara-cterization and fluorescence properties were carried out. The crystal structure analysis of complexes 1 and 2 shows that the atomic configurations of Ni(Ⅱ) and Cu(Ⅱ) are all six-coordinated distorted octahedrons. Complex 1 self-assembles into a 1D supramolecular chain through intermolecular hydrogen bonding, C-H…π and ππ stacking interactions, and complex 2 self-assembles into a 3D supramolecular network structure through intramolecular hydrogen bonding and ππ stacking interactions. Furthermore, the optical properties of complexes 1 and 2 indicate that the fluorescence variation of H2L1 and H2L2 is due to the coordination of the metal ions Ni(Ⅱ) and Cu(Ⅱ).


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  • Scheme 1  Synthetic routes of H2L1 and H2L2

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

    Symmetry code: a: 3/2-x, 3/2-y, 1-z

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

    Symmetry codes: a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z; c:-x, 3/2+y, z

    Figure 3  (a) Structure of [Cu(L2)] moiety of complex 2 showing 30% probability displacement ellipsoids; (b) Distorted Cu4O4 core of the cubane-like isomer (30% probability ellipsoids) with no crystallographic constraint

    Symmetry codes: a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z; c:-x, 3/2+y, z

    Figure 4  Intramolecular hydrogen bonding of complex 1

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

    Figure 5  Intermolecular hydrogen bonding of complex 1

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

    Figure 6  View of 1D supramolecular chain linked by C-H…π and ππ stacking interaction of complex 1

    Figure 7  Intramolecular hydrogen bonding of complex 2

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

    Figure 8  UV-Vis absorption spectra of H2L1 and complex 1 in diluted DMSO solution at room temperature

    Figure 9  UV-Vis absorption spectra of H2L2 and complex 2 in diluted DMSO solution at room temperature

    Figure 10  Emission spectra of H2L1 and complex 1 in diluted DMSO at room temperature

    Figure 11  Emission spectra of H2L2 and complex 2 in diluted DMSO at room temperature

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

    Complex 1 2
    Empirical formula C38H34Br2N4Ni2O12 C72H59Cu4N5O24
    Formula weight 1 015.87 1 491.27
    Crystal system Monoclinic Tetragonal
    Space group C2/c I41/a
    a/nm 1.799 3(4) 1.663 3(4)
    b/nm 0.921 0(2) 1.663 3(4)
    c/nm 2.496 9(6) 2.342 9(4)
    β/(°) 110.249(2)
    V/nm3 3.881 8(2) 6.482(3)
    Z 4 4
    μ/mm-1 3.098 1.374
    F(000) 2 048 3 024
    θ range/(°) 1.70~25.0 2.5~25.0
    Limiting indices -21 ≤ h ≤ 19, -10 ≤ k ≤10, -29 ≤ l ≤ 29 -18 ≤ h ≤ 19, -19 ≤ k ≤ 18, -27 ≤ l ≤ 27
    Reflection collected, unique 14 365, 3 413 (Rint=0.174) 20 340, 2 862 (Rint=0.072)
    Completeness to θ/% 100 99.9
    Data, restraint, parameter 3 413, 0, 263 2 862, 0, 218
    GOF on F2 1.07 1.06
    R1, wR2 [I>2σ(I)] 0.044 4, 0.108 31 0.039 6, 0.100 2
    Largest diff. peak and hole/(e·nm-3) 820 and -810 860 and -890
    下载: 导出CSV

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

    1
    Ni1-O3 0.205 0(2) Ni1-O4 0.201 6(3) Ni1-O5 0.208 4(2)
    Ni1-O6 0.210 0(2) Ni1-N1 0.201 2(3) Ni1-O4a 0.206 4(2)
    O3-Ni1-O4 172.93(8) O3-Ni1-O5 87.32(7) O3-Ni1-O6 93.89(8)
    O1-Ni1-N1 82.01(8) O1-Ni1-O4a 105.58(8) O4-Ni1-O5 90.81(7)
    O4-Ni1-O6 88.92(7) O4-Ni1-N1 91.42(9) O4-Ni1-O4a 80.95(8)
    O5-Ni1-O6 171.90(9) O5-Ni1-N1 96.12(8) O5-Ni1-O4a 83.42(8)
    O6-Ni1-N1 91.99(9) O6-Ni1-O4a 88.54(8) N1-Ni1-O4a 172.34(8)
    2
    Cu1-O1 0.234 7(2) Cu1-O2 0.198 5(2) Cu1-O2a 0.196 1(2)
    Cu1-O3a 0.194 5(2) Cu1-N1a 0.195 1(2) Cu1b-O2 0.196 1(2)
    Cu1b-O3 0.194 5(2) Cu1b-N1 0.195 1(2) Cu1-O2b 0.268 2(2)
    O1-Cu1-O2 75.51(7) O1-Cu1-O2a 100.98(7) O2-Cu1-O2a 88.76(8)
    O1-Cu1-O3a 84.41(7) O2-Cu1-O3a 91.75(7) O2A-Cu1-O3a 174.54(7)
    O3A-Cu1-N1a 85.16(8) O1-Cu1-N1a 114.44(7) O2-Cu1-N1a 169.13(8)
    O2A-Cu1-N1a 93.35(8) Cu1-O2-Cu1b 111.26(8)
    Symmetry codes: a: 3/2-x, 3/2-y, 1-z for 1; a: 3/4+x, 3/4-y, 3/4-z; b: 3/4-x, -3/4+y, 3/4-z for 2.
    下载: 导出CSV

    Table 3.  Putative 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)/(°)
    1
    O5-H5C…O6a 0.082 0.213 0.291 1(3) 158
    C10-H10…O2 0.093 0.220 0.285 6(4) 127
    C15-H15…O3a 0.093 0.223 0.310 6(4) 156
    O5-H5B…O2a 0.086 0.183 0.267 9(3) 172
    C5-H5A…Cg1a 0.298 0.294 0.336 9(4) 107
    2
    C8-H8…O5 0.93 2.17 2.856(3) 130
    Symmetry code: 1-x, 2-y, 1-z; Cg1 is the centroids of C11~C16 in benzene ring.
    下载: 导出CSV

    Table 4.  ππ stacking interactions for complexes 1 and 2

    Ring (I) Ring (J) d(Cg…Cg)/nm α/(°) d(Cg(I)-perp)/nm d(Cg(J)-perp)/nm Slippage/nm
    1
    Cg2 Cg1a 0.428 1(2) 26.58(2) 0.283 5(1) 0.394 4(1) 0.961 0
    2
    Cg3 Cg4a 0.329 8(2) 13.77(1) 0.275 51(8) 0.307 81(9) 0.118 3
    Cg3 Cg5b 0.329 8(2) 13.77(1) 0.275 51(8) 0.307 81(9) 0.118 3
    Cg4 Cg6b 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg4 Cg7c 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg5 Cg6a 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg5 Cg7b 0.357 3(2) 13.08(9) 0.300 81(9) 0.309 50(8) 0.178 5
    Cg7 Cg6a 0.312 5(1) 15.47(8) 0.303 72(8) 0.303 71(8) 0.073 7
    Cg6 Cg7b 0.312 5(1) 15.47(8) 0.303 72(8) 0.303 71(8) 0.073 7
    Symmetry codes: a: 1-x, 2-y, 1-z for 1; a: 3/4-y, 3/4+x, 3/4-z; b: -x, 3/2-y, z; c: -3/4+y, 3/4-x, 3/4-z for 2; α=dihedral angle between planes I and J; d(Cg…Cg)=distance between ring centroids; d(Cg(I)-perp)=perpendicular distance of Cg(I) on ring J; d(Cg(J)-perp)=perpendicular distance of Cg(J) on ring I; Slippage=distance between Cg(I) and perpendicular projection of Cg(J) on ring I; Cg1, Cg2 are the centroids of C11~C16 and C1~C6 in benzene ring, respectively; Cg3, Cg4, Cg5, Cg6, Cg7 are the centroids of ring Cu1-O1-C2-C7-O2, Cu1a-O3c-C10c-C9c-N1c, O3-C10-C9-N1-Cu1b, Cu1-O2a-C7a-C6a-C8a-N1a and O2b-C7b-C6b-C8b-N1b-Cu1c, respectively.
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    Table 5.  Main bands in IR spectra of H2L1, H2L2 and complexes 1 and 2 cm-1

    Compound ν(O-H) ν(C=N) ν(Ar-O) ν(M-N) ν(M-O)
    H2L1 3 403 1 670 1 265
    [Ni(L1)(DMF)(H2O)]2 (1) 3 409 1 610 1 230 514 467
    H2L2 3 438 1 676 1 241
    [Cu4(L2)4]·DMF·CH3OH·2H2O (2) 3 476 1 682 1 229 538 467
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  • 发布日期:  2020-03-10
  • 收稿日期:  2019-06-18
  • 修回日期:  2019-11-14
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