English

• 0.   Introduction

  In recent decades, complex has attracted lots of attention, due to its multi-functional crystalline materials and interesting structure, which are based on the coordination bonding interaction between metal ions/ clusters and bridging organic linkers. They stand out among polymer materials owing to their brilliant properties, for instance, large surface area^[1], adjustable structure^[2-3], and high porosity^[4-5]. Considerable efforts have been made in synthesizing and researching new complexes, not only thanks to their intriguing structures and distinctive topologies, but also due to their promising applications as functional materials in many fields as luminescence^[6-8], catalysis^[9-11], chemical sensors^[12-14], magnetism^[15-18], gas storage and separation^[19-21], and biology^[22].

  The fluorescent properties of d¹⁰ metal complexes have attracted the interest of many researchers. Especially, the d orbital of Cd(Ⅱ) ion was filled with electrons, which can effectively reduce the energy loss caused by d - d transitions, when connected to a π conjugated organic framework. It may exhibit good fluorescence properties^[23-24].

  In this work, we used 2-(4-carboxy-phenyl)-imidazole-4, 5-dicarboxylic acid (H₃L) to connect with Cd(Ⅱ) and added auxiliary ligands to adjust the structure to construct complexes, The sp² hybridization of N on the ring can increase the π electronic density. The introduction of a functional carboxyphenyl group at 2 position of the imidazole ring (H₃L) can produce more coordination modes^[25-26]. Both the O on the carboxyl group and the N on the imidazole can coordinate with Cd(Ⅱ), and N and O can form chelating coordination effect. So, H₃L shows more advantages than other N- or Odonor ligands. Here, under the same conditions, we synthesized two new Cd(Ⅱ) complexes (Scheme 1) through introduce auxiliary ligand, and studied their properties.

  Scheme 1

  

  Scheme 1.  Synthesis routes of complexes 1 and 2

  1,10-phen=1,10-phenanthroline, dib=1,4-bis(1-imidazolyl)benzene

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  1.   Experimental

  1.1   Materials and general methods

  H₃L was prepared according to the reported procedure^[27-28]. All chemical reagents and solvents were purchased from a commercial source, which was used without undergoing the purification process. The FT - IR spectra were recorded in a range of 4 000-400 cm^-1 on a Bruker VERTEX 70 spectrometer using KBr pellets. Elemental analyses (C, H, and N) were carried out on a VxRio EL elemental analyzer. Powder X-ray diffraction (PXRD) patterns were collected in a 2θ range of 5°-45° on a Philips PW 1710-based diffractometer with Cu Kα radiation (λ=0.154 184 nm) at room temperature, operated at 40 kV and 100 mA. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TG-7 analyzer heated from room temperature to 800 ℃ under nitrogen at a heating rate of 10 ℃ ·min^-1. Fluorescent analyses of the complexes were performed on an F-7100 Fluorescence spectrometer. UV - Vis DRS (UV - Vis Diffuse Reflectance Spectroscopy) spectra were recorded on a U-3900H spectrophotometer.

  1.2   Synthesis

  1.2.1   Synthesis of [Cd₂(HL)₂(1, 10-phen)₂(H₂O)₂] (1)

  A mixture of Cd(NO₃)₂·4H₂O (30.8 mg, 0.10 mmol), H₃L (13.8 mg, 0.05 mmol), 1, 10-phen (9.9 mg, 0.05 mmol), and H₂O (8 mL) was placed in a 25 mL Teflon - lined autoclave and heated to 160 ℃ for 3 d. When the mixture was cooled to room temperature, light yellow block - shaped crystals of 1 were obtained with 65% yield (based on H₃L). Analysis Calcd. for C₄₈H₃₂Cd₂N₈O₁₄(%): C 49.29, H 2.75, N, 9.58; Found (%): C 49.33, H 2.76, N 9.51. IR (KBr, cm^-1): 3 428(s), 2 989(m), 2 826(w), 2 355(w), 2 066(w), 1 620(s), 1 486 (m), 1 395(s), 1 365(s), 1 176(m), 1 005(m), 790(m), 728(w), 621(m).

  1.2.2   Synthesis of {[Cd(HL)(dib)_0.5(H₂O)₂]·2H₂O}_n (2)

  The synthesis of 2 used the same condition as 1 except that 1, 10-phen was replaced by dib. Clear colorless block crystals of 2 were obtained in 63% yield (based on H₃L). Analysis Calcd. for C₁₈H₁₈CdN₄O₁₀(%): C 38.41, H 3.22, N 9.96; Found(%): C 38.43, H 3.23, N 9.99. IR (KBr, cm^-1): 3 420(s), 2 985(m), 2 831(w), 2 716(w), 2 362(w), 2 062(w), 1 620(s), 1 490(m), 1 422 (m), 1 398(m), 1 365(s), 1 175(m), 1 049(w), 1 005(m), 882(w), 793(m).

  1.3   X-ray data collection and structure refinement

  The diffraction data were collected at 296(2) K for 1, with a Bruker APEX-Ⅱ CCD area detector diffractometer using φ and ω rotation scans and Mo Kα radiation (λ=0.071 073 nm). The crystallographic data of 2 were collected with Cu Kα radiation (λ=0.154 184 nm) on SuperNova, Dual, Cu at zero, Eos at 296(2) K. Then absorption corrections were carried out. The structures were solved by the direct method and refined by the full - matrix least - squares on F² using the SHELX and Olex2 program^[29-31]. The data of 1 were processed by squeeze to remove disordered solvent water molecules. Non-hydrogen atoms were refined with anisotropic, and the hydrogen atoms were included in the final refinement by using geometrical restrains and refined isotropically using the riding model. Crystal data and structure refinements for 1 and 2 and selected bond distances and angles are given in Table 1 and 2, respectively.

  Table 1

  

  Table 1.  Crystallographic data and structure refinement parameters for 1 and 2

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  Parameter	Complex
  	1	2
  Chemical formula	C48H32Cd2N8O14	C18H19CdN4O10
  Formula weight	1 169.61	563.77
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	0.864 41(5)	0.881 5(4)
  b / nm	1.063 69(5)	1.134 57(4)
  c / nm	1.449 67(8)	1.139 38(5)
  α/(°)	105.947(2)	104.252(3)
  β/(°)	91.012(2)	100.969(4)
  γ/(°)	100.591(2)	95.618(3)
  V / nm3	1.256 51(12)	1.072 20(8)
  Z	1	2
  Dc / (g·cm-3)	1.546	1.746
  μ / mm-1	0.919	8.741
  θ range / (°)	4.062-54.104	8.212-133.172
  Crystal size / mm	0.17×0.14×0.11	0.21×0.17×0.15
  Rint	0.023 0	0.027 9
  F(000)	584.0	566.0
  GOF	1.058	1.048
  R1a, wR2b [I > 2σ(I)]	0.029 3, 0.070 1	0.037 0, 0.098 8
  R1, wR2 (all data)	0.033 7, 0.072 5	0.038 5, 0.101 0
  aR1=∑||Fo|-|Fc||/∑|Fo|; bwR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  Table 2

  

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

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  1
  Cd1—O5	0.231 60(18)	Cd1—N4	0.235 7(2)	Cd1—O6	0.256 81(18)
  Cd1—O7	0.231 66(19)	Cd1—N3	0.235 9(2)
  Cd1—N2#1	0.233 64(19)	Cd1—O2#1	0.247 43(17)
  O5—Cd1—O7	85.58(7)	O7—Cd1—N3	85.35(9)	N3—Cd1—O2#1	81.49(7)
  O5—Cd1—N2#1	115.68(7)	N2#1—Cd1—N3	150.92(8)	O5—Cd1—O6	53.37(6)
  O7—Cd1—N2#1	85.13(7)	N4—Cd1—N3	70.69(10)	O7—Cd1—O6	128.90(7)
  O5—Cd1—N4	107.76(7)	O5—Cd1—O2#1	164.70(6)	N2#1—Cd1—O6	87.26(6)
  O7—Cd1—N4	152.34(8)	O7—Cd1—O2#1	80.63(7)	N4—Cd1—O6	76.70(7)
  N2#1—Cd1—N4	109.07(8)	N2#1—Cd1—O2#1	69.87(6)	N3—Cd1—O6	119.73(8)
  O5—Cd1—N3	90.86(8)	N4—Cd1—O2#1	82.29(7)	O2#1—Cd1—O6	141.79(6)
  2
  Cd1—O0AA	0.237 1(3)	Cd1—O6#1	0.229 8(3)	Cd1—N3	0.224 7(3)
  Cd1—O1	0.246 3(3)	Cd1—O7	0.246 2(4)
  Cd1—O5#1	0.261 6(4)	Cd1—N1	0.233 3(3)
  O0AA—Cd1—O1	79.17(11)	O6#1—Cd1—O7	80.07(13)	N1—Cd1—O7	88.32(12)
  O0AA—Cd1—O5#1	75.50(12)	O6#1—Cd1—N1	95.75(11)	N3—Cd1—O0AA	94.83(12)
  O0AA—Cd1—O7	162.35(13)	O7—Cd1—O1	83.51(13)	N3—Cd1—O1	82.30(11)
  O1—Cd1—O5#1	149.07(11)	O7—Cd1—O5#1	122.02(13)	N3—Cd1—O5#1	82.44(14)
  O6#1—Cd1—O0AA	115.80(11)	N1—Cd1—O0AA	82.62(10)	N3—Cd1—O6#1	109.97(12)
  O6#1—Cd1—O1	158.63(12)	N1—Cd1—O1	70.07(10)	N3—Cd1—O7	86.16(14)
  O6#1—Cd1—O5#1	52.17(12)	N1—Cd1—O5#1	122.96(13)	N3—Cd1—N1	152.27(11)
  Symmetry codes: #1: 1-x, 1-y, 1-z for 1; #1: 2-x, 2-y, 2-z; #2: 1-x, 1-y, -z for 2.

  CCDC: 2044127, 1; 2044128, 2.

  2.   Results and discussion

  2.1   Description of crystal structure

  2.1.1   Structural description for 1

  Single X - ray crystallography shows that 1 based on H₃L, 1, 10-phen, and Cd(Ⅱ) crystallizes in the triclinic system and P1 space group. The asymmetric unit of 1 contains two crystallographically independent Cd(Ⅱ) cation, two (HL)^2- ligands, two 1, 10 - phen ligand, and two coordinated water molecules. As depicted in Fig. 1a, the Cd1 center is coordinated by three nitrogen atoms of one 1, 10 - phen ligand (N3 and N4) and one (HL)^2- ligand (N2#1), four oxygen atoms of two different (HL)^2- ligands (O2#1, O5, and O6), and one coordinated H₂O (O7), adopting a distorted pentagonal bipyramid geometry. Two main ligands and two metal ions form a ring structure and then coordinate with the two chelating auxiliary ligands, and the remaining N and O of the ligand are not further coordinated with Cd which leads to a 0D structure (Fig. 1b). Intermolecular hydrogen bonding interactions (O—H…O, Table 3) between a coordinated water molecule and the carboxylate of (HL)^2- ligands and a C—H… π interaction (Table 4) between the benzene ring and the imidazole ring of (HL)^2- ligands are showed in Fig. 1c. The interaction of O—H…O and C—H…π (Fig. 1c, 1d), the π…π stacking interactions(Table 5) between benzene rings with the aid of 1, 10-phen (Fig. 1e) form supramolecular framework structure of complex 1 (Fig. 1f).

  Figure 1

  

  Figure 1.  (a) Coordination environment around Cd(Ⅱ) ions in 1 drawn with 30% probability displacement ellipsoids; (b) Unit cell of 1; (c) Hydrogen bonding in crystal of 1; (d) Supramolecular structure of 1 formed by hydrogen bonding; (e) π…π stacking in crystal of 1; (f) Supramolecular framework structure of 1 formed by hydrogen bonding and π…π stacking

  Cd: purple, N: blue, O: red, C: gray; Hydrogen atoms are omitted for clarity; Symmetry code: #1: 1-x, 1-y, 1-z

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  Table 3

  

  Table 3.  Hydrogen bond parameters of 1 and 2

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  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  O3—H3…O1	0.082	0.162	0.243 7(3)	172
  O7—H7A…O3#2	0.088	0.184	0.271 1(3)	167
  O7—H7B…O4#1	0.085	0.185	0.269 6(3)	173
  C22—H2…O7	0.093	0.259	0.319 5(5)	123
  2
  O0AA—H0AA…O3#2	0.087	0.202	0.284 6(4)	159
  N2—H2…O9#4	0.086	0.200	0.286 1(5)	173
  O2—H2…O3	0.090(9)	0.155(9)	0.245 1(5)	173(15)
  O0AA—H0AB…O1#1	0.087	0.201	0.286 5(5)	167
  O0AA—H0AB…O2#1	0.087	0.251	0.317 5(5)	134
  O7—H7A…O9#3	0.087	0.235	0.314 0(6)	151
  O7—H7B…O8#3	0.087	0.197	0.278 8(8)	156
  O9—H9B…O6#5	0.085	0.191	0.275 7(5)	173
  C13—H13…O1	0.093	0.254	0.309 1(5)	118
  C13—H13…O0AA#1	0.093	0.258	0.350 3(6)	171
  Symmetry codes: #1: 1+x, -1+y, z; #2: -x, 1-y, 1-z for 1; #1: 1-x, 1-y, 1-z; #2: 1+x, y, z; #3: x, -1+y, z; #4: 1-x, 2-y, 2-z; #5: 2-x, 2-y, 2-z for 2.

  Table 4

  

  Table 4.  Structural parameters of C—H…π interaction in crystal of 1*

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  X—H…π	d(H…Cg) / nm	γ/(°)	∠X—H…Cg / (°)	d(X…Cg) / nm	σ/(°)
  C10—H10…Cg(1)#1	0.299	13.10	117	0.351 1(3)	35
  *γ: angle between Cg-H vector and ring; σ: angle of X—H bond with π-plane (i.e., Perpendicular: 90°, Parallel: 0°); Symmetry codes: #1: -x, 1-y, 1-z; Rings: Cg(1): C2, C3, N2, C4, N1, C5.

  Table 5

  

  Table 5.  Structural parameters of π…π interactions in crystal of 1*

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  Cg…Cg	α/(°)	DC / nm	β/(°)	DZ / nm	S / nm
  Cg(3)…Cg(3)#1	0.00(19)	0.364 5(1)	21.8	0.338 51(16)	0.135 3
  Cg(5)…Cg(5)#2	0.00(2)	0.368 3(2)	19.4	0.347 3(2)	0.122 6
  Cg(2)…Cg(3)#2	1.90(2)	0.437 8(2)	35.9	0.346 07(19)	0.256 7
  Cg(4)…Cg(4)#3	0.00(11)	0.459 42(15)	47.3	0.311 32(10)	0.337 8
  *α: dihedral angle between mean planes of the rings; DC: distance between ring centroids; β: angle between DC vector and normal to plane (i); DZ: perpendicular distance of the centroids of ring (i) on plane of ring (j); S: distance between ring centroid (i) and perpendicular projection of ring centroid (j) on ring (i); Symmetry codes: #1:2-x, 1-y, 2-z; #2:1-x, 1-y, 2-z; #3:1-x, 1-y, 1-z; Rings: Cg(2): N3, C22, C21, C20, C19 C23; Cg(3): N4, C13, C14, C15, C16, C24; Cg(4): C6, C7, C8, C9, C10, C11; Cg(5): C16, C17, C18, C19, C23, C24.

  2.1.2   Structural description for 2

  The single - crystal X - ray diffraction analysis reveals that 2 crystallizes in the triclinic crystal system with the space group of P1. The fundamental building unit of 2 consists of one crystallographically independent Cd(Ⅱ) ion, one (HL)^2- ligands, half of dib ligand, and two coordinated water molecules. Cd1 adopts a distorted pentagonal bipyramid geometry, coordinating to three carboxylate oxygen atoms (O1, O5#2, and O6#2) from two different (HL)^2- ligands, two water oxygen atoms (O7 and O0AA), and two nitrogen atoms from one dib ligand (N3) and one (HL)^2- ligand (N1), respectively (Fig. 2a). Under the regulation of the auxiliary ligands, 2 formed a different structure from 1, but 2 has the same ring as 1, also composed of two main ligands and two metal ions. The difference between 1 and 2 is that the two coordination sites of 1, 10-phen in 1 are replaced by dib and water molecular in 2, and the dib acts as a bridging molecule to connect two rings to form a 1D chain structure (Fig. 2b). Intermolecular hydrogenbonding interactions (O—H…O and C—H…O, Talbe 3) between the coordinated water molecule and the carboxylate of (HL)^2- ligand and (N—H…O, Talbe 3) between lattice water and (HL)^2- are shown in Fig. 2c, and the intermolecular weak interaction forming supramolecular structure are shown in Fig. 2d.

  Figure 2

  

  Figure 2.  (a) Coordination environment around Cd(Ⅱ) ions in 2 drawn with 30% probability displacement ellipsoids; (b) 1D chain structure of 2; (c) Hydrogen bonding in crystal of 2; (d) Supramolecular framework structure of 2 formed by hydrogen bonding

  Cd: purple, N: blue, O: red, C: gray; Hydrogen atoms are omitted for clarity; Symmetry codes: #1: 1-x, 1-y, -z; #2: 2-x, 2-y, 2-z

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  2.2   PXRD analyses

  The PXRD was carried out to test the phase purity of 1 and 2 (Fig. 3), and the experimental results are well consistent with the simulated data, which indicates the high purity of two complexes. The difference in peak intensity may be due to the different orientations of the sample.

  Figure 3

  

  Figure 3.  XRD patterns of 1 and 2

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  2.3   Thermal analyses

  To test the thermal stability of 1 and 2, their thermal behaviors were investigated under a nitrogen atmosphere by TGA. As shown in Fig. 4, the TGA curve of 1 showed a slow weight loss near 100 ℃ which is consistent with the loss of lattice water (removed by SQUEEZE) molecules and one coordinated water molecules, and upon further heating, the structure was stable up to 275 ℃. The continuous heating up led to a decomposition. The TGA curve of 2 was similar to 1 and exhibited thermal stability up to 275 ℃, and then the structure started collapsing. The remaining weight was 23.66% (Calcd. 22.81%) at 800 ℃.

  Figure 4

  

  Figure 4.  TGA curves of 1 and 2

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  2.4   Luminescent properties

  The solid - state photoluminescent properties of 1 and 2 were investigated at room temperature with the microcrystalline samples, and the luminescence spectra of free H₃L, 1, 10-phen, and dib were also measured at the same condition. As shown in Fig. 5, the maximum emission of 1, 2 and ligand H₃L, 1, 10-phen and dib are at 377 nm (λ_ex=289 nm), 383 nm (λ_ex=280 nm), 394 nm (λ_ex=318 nm), 435 nm (λ_ex=373 nm), 326 nm (λ_ex=293 nm), respectively. The emission of 1 showed a blue shift relative to free 1, 10 - phen ligand (58 nm) and 2 showed a red shift relative to dib ligand (47 nm), both of them show blue shifts relative to H₃L (17 nm for 1 and 11 nm for 2). Due to that the Cd(Ⅱ) ions with d¹⁰ configurations are difficult to oxidize or reduce, the emission of 1 and 2 couldn't be attributed to metal-toligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) ^[15]. Their photoluminescence could be assigned to intra-ligand fluorescence emission (π* → π) or ligand localized emission (π* →n), which are consistent with the reported Cd(Ⅱ) complexes^[32-33]. The shift of the emission peaks may be ascribed to the coordination action of the ligands to Cd(Ⅱ) ions, which increases the energy between the ground state and excited state^[34-35]. Although 1 and 2 have the same main ligand and center metal ions, 1 showed fluorescence quenching, while 2 showed fluorescence enhancement. The protons on the uncoordinated carboxyl group of the ligands in 1 are not removed, and the carboxyl group is used as a fluorescence quenching group, which may cause the fluorescence of 1 to be quenched^[8]. The enhanced fluorescence of 2 may be due to the coordination between the ligands and the metal ion, which increases the rigidity of the ligands and reduces the non -radiative energy loss^[36].

  Figure 5

  

  Figure 5.  Solid-state excitation and emission spectra of the ligands and 1 and 2

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  2.5   Hirshfeld surface analysis of 1 and 2

  To analyze intermolecular interaction and surface electron distribution, we used crystalExplorer software to calculate the Hirshfeld surface explanation of the immediate environment of a molecile in the crystal. The 3D Hirshfeld surface and 2D fingerprint plots of 1 and 2 are shown in Fig. 6, and the 3D Hirshfeld surface was mapped with d_norm, shape index, and curvedness with standard high resolution. As shown in Fig. 6, the red regions on d_norm of 1 and 2 suggest that high electron densities may be due to the strong interaction, generally representing the formation of hydrogen bond or coordination bond; the blue regions have low electron densities and no obvious interaction; the white regions have moderate electron densities, which correspond to slightly weaker interaction, and commonly regarded as π…π stacked regions^[37-38]. The shape index is an obvious indication of a subtle change of Hirshfeld's surface. The curvedness is the measurement of"how much shape"; the flat regions represent a low value of curvedness, while the sharp regions represent a high value of curvedness, indicating an interaction between adjacent molecules^[39].

  Figure 6

  

  Figure 6.  Hirshfeld surfaces mapped with d_norm, shape index, curvedness, and 2D fingerprint plots of 1 and 2

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  The 2D fingerprint plot can easily recognize the intermolecular interaction on the molecular surface. The C…H, O…H, N…H, and H…H interactions of 1 and 2 are shown in Fig. 6. For 1, the H…H, C…H/H… C, O…H/H…O and N…H/H…N interactions accounted for 32.2%, 22.2%, 26.3%, and 3.0% of Hirshfeld surface, respectively. For 2, the H…H, C…H/H…C, O…H/H…O and N…H/H…N interactions accounted for 26.4%, 17.2%, 34.8%, and 1.4% of Hirshfeld surface respectively. It is the interaction in 1 and 2 that makes them more stable.

  2.6   Density functional theory calculation

  To prove the theoretical stability of 1 and 2, we applied Guassian09 software to optimize the structure and calculate the energies of 1, 2, and H₃L, based on the method of density functional theory (DFT) with the level of B3LYP. Non-metal atoms were described by 631G and metal atoms were treated by LANL2DZ basis sets, which didn't consider the solvent effect^[40-41]. Since the structures of 1 and 2 have been determined in the single crystal X-ray diffraction, it is possible to directly import data (CIF) and directly perform energy calculations without structural optimization.

  The frontier molecular orbit energy levels, HOMO and LUMO, are two important indicators of molecular stability, where HOMO represents the ability to donate an electron, and LUMO represents the ability to accept an electron. The energy gaps (ΔE) between LUMO and HOMO represent the chemical stability of the molecule. The HOMO and LUMO levels of 1, 2, and H₃L are shown in Fig. 7. The LUMO, HOMO, and ΔE of 1, 2, and H₃L were -4.51, -5.41, and 0.90 eV; -4.57, -5.93, and 1.36 eV; -2.60, -6.84, and -4.24 eV, respectively. The energies of frontier orbit HOMO-LUMO are all negative, demonstrating that 1, 2, and H₃L have chemical stability^[42-43]. All the above indicates that 1, 2, and H₃L are stable on the ground state, and the calculation method of DFT with B3LYP basis set is reasonable.

  Figure 7

  

  Figure 7.  Energy level diagrams of frontier orbitals of 1, 2, and H₃L

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  2.7   Optical band gaps

  To test the candidate properties of 1 and 2 as semiconductors, the UV-Vis DRS spectra to determine the band gap (E_g) were tested (photoresponse wavelength region), based on Kubelka - Munk (K - M) function, using absorbance as the ordinate and energy as the abscissa to draw a graph and fit the linear section to obtain a linear equation. Its intercept on the x-axis is the band gap energy^[44-45]. As shown in Fig. 8, E_g of 1 and 2 had similar values: 3.55 and 3.58 eV, respectively. The wide band gaps of 1 and 2 indicate that they are potential wide gap semiconductors materials^[46].

  Figure 8

  

  Figure 8.  Plots of K-M function vs energy for 1 and 2

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  2.8   FT-IR analysis

  The FT-IR spectra of H₃L, 1, and 2 are shown in Fig. 9. 1 and 2 had similar infrared absorption peaks to H₃L. The absorption peaks at 3 420-3 430 cm^-1 could be ascribed to the stretching vibration of N—H bond from H₃L, and the absorption peaks between 1 400 and 1 630 cm^-1 are attributed to the breathing vibrations of the aromatic ring and imidazole ring^[47]. There was a strong absorption peak at 1 726 cm^-1 for H₃L but 1 and 2 had no absorption peak in the same region. This absorption peak could be attributed to the stretching vibration of C=O bond, and the absence of this absorption peak for 1 and 2 may be due to the coordination of the oxygen of carbonyl group with the metal ion.

  Figure 9

  

  Figure 9.  FT-IR spectra of 1, 2 and H₃L

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  3.   Conclusions

  In summary, two new Cd(Ⅱ) complexes were successfully synthesized by hydrothermal method. The structure analysis result shows that auxiliary ligand has an important effect on the crystal structure: bridge ligands can easier to form 1D structure, but chelating ligands tend to generate 0D structure. Through PXRD, TGA, luminescent, Hirshfeld surface analyses, and DFT calculation, we verified the phase purity of the complexes, tested their thermal stability and photoluminescence, calculated the intermolecular interaction, surface electron distribution, and the chemical stability of them. Besides, 1 and 2 showed wide optical band gaps, which indicates that they have potential application on wide band gap semiconductors materials.

  
  Acknowledgements: This work was supported by the Research Projects of Colleges and Universities in Gansu Province (Grant No.2019A-032), Natural Science Foundation of China (Grant No. 21761030), Tianjin University - Lanzhou Jiaotong University Independent Innovation Fund Cooperation Project (Grant No. 2020060), and College Student Innovation Training Program (Grant No.2020033).
• 1. [1]

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• 

  Scheme 1  Synthesis routes of complexes 1 and 2

  1,10-phen=1,10-phenanthroline, dib=1,4-bis(1-imidazolyl)benzene

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  Figure 1  (a) Coordination environment around Cd(Ⅱ) ions in 1 drawn with 30% probability displacement ellipsoids; (b) Unit cell of 1; (c) Hydrogen bonding in crystal of 1; (d) Supramolecular structure of 1 formed by hydrogen bonding; (e) π…π stacking in crystal of 1; (f) Supramolecular framework structure of 1 formed by hydrogen bonding and π…π stacking

  Cd: purple, N: blue, O: red, C: gray; Hydrogen atoms are omitted for clarity; Symmetry code: #1: 1-x, 1-y, 1-z

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  Figure 2  (a) Coordination environment around Cd(Ⅱ) ions in 2 drawn with 30% probability displacement ellipsoids; (b) 1D chain structure of 2; (c) Hydrogen bonding in crystal of 2; (d) Supramolecular framework structure of 2 formed by hydrogen bonding

  Cd: purple, N: blue, O: red, C: gray; Hydrogen atoms are omitted for clarity; Symmetry codes: #1: 1-x, 1-y, -z; #2: 2-x, 2-y, 2-z

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  Figure 3  XRD patterns of 1 and 2

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  Figure 4  TGA curves of 1 and 2

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  Figure 5  Solid-state excitation and emission spectra of the ligands and 1 and 2

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  Figure 6  Hirshfeld surfaces mapped with d_norm, shape index, curvedness, and 2D fingerprint plots of 1 and 2

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  Figure 7  Energy level diagrams of frontier orbitals of 1, 2, and H₃L

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  Figure 8  Plots of K-M function vs energy for 1 and 2

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  Figure 9  FT-IR spectra of 1, 2 and H₃L

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  Table 1.  Crystallographic data and structure refinement parameters for 1 and 2

  Parameter	Complex
  	1	2
  Chemical formula	C48H32Cd2N8O14	C18H19CdN4O10
  Formula weight	1 169.61	563.77
  Crystal system	Triclinic	Triclinic
  Space group	P1	P1
  a / nm	0.864 41(5)	0.881 5(4)
  b / nm	1.063 69(5)	1.134 57(4)
  c / nm	1.449 67(8)	1.139 38(5)
  α/(°)	105.947(2)	104.252(3)
  β/(°)	91.012(2)	100.969(4)
  γ/(°)	100.591(2)	95.618(3)
  V / nm3	1.256 51(12)	1.072 20(8)
  Z	1	2
  Dc / (g·cm-3)	1.546	1.746
  μ / mm-1	0.919	8.741
  θ range / (°)	4.062-54.104	8.212-133.172
  Crystal size / mm	0.17×0.14×0.11	0.21×0.17×0.15
  Rint	0.023 0	0.027 9
  F(000)	584.0	566.0
  GOF	1.058	1.048
  R1a, wR2b [I > 2σ(I)]	0.029 3, 0.070 1	0.037 0, 0.098 8
  R1, wR2 (all data)	0.033 7, 0.072 5	0.038 5, 0.101 0
  aR1=∑||Fo|-|Fc||/∑|Fo|; bwR2=[∑w(Fo2-Fc2)2/∑w(Fo2)2]1/2.

  下载: 导出CSV

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

  1
  Cd1—O5	0.231 60(18)	Cd1—N4	0.235 7(2)	Cd1—O6	0.256 81(18)
  Cd1—O7	0.231 66(19)	Cd1—N3	0.235 9(2)
  Cd1—N2#1	0.233 64(19)	Cd1—O2#1	0.247 43(17)
  O5—Cd1—O7	85.58(7)	O7—Cd1—N3	85.35(9)	N3—Cd1—O2#1	81.49(7)
  O5—Cd1—N2#1	115.68(7)	N2#1—Cd1—N3	150.92(8)	O5—Cd1—O6	53.37(6)
  O7—Cd1—N2#1	85.13(7)	N4—Cd1—N3	70.69(10)	O7—Cd1—O6	128.90(7)
  O5—Cd1—N4	107.76(7)	O5—Cd1—O2#1	164.70(6)	N2#1—Cd1—O6	87.26(6)
  O7—Cd1—N4	152.34(8)	O7—Cd1—O2#1	80.63(7)	N4—Cd1—O6	76.70(7)
  N2#1—Cd1—N4	109.07(8)	N2#1—Cd1—O2#1	69.87(6)	N3—Cd1—O6	119.73(8)
  O5—Cd1—N3	90.86(8)	N4—Cd1—O2#1	82.29(7)	O2#1—Cd1—O6	141.79(6)
  2
  Cd1—O0AA	0.237 1(3)	Cd1—O6#1	0.229 8(3)	Cd1—N3	0.224 7(3)
  Cd1—O1	0.246 3(3)	Cd1—O7	0.246 2(4)
  Cd1—O5#1	0.261 6(4)	Cd1—N1	0.233 3(3)
  O0AA—Cd1—O1	79.17(11)	O6#1—Cd1—O7	80.07(13)	N1—Cd1—O7	88.32(12)
  O0AA—Cd1—O5#1	75.50(12)	O6#1—Cd1—N1	95.75(11)	N3—Cd1—O0AA	94.83(12)
  O0AA—Cd1—O7	162.35(13)	O7—Cd1—O1	83.51(13)	N3—Cd1—O1	82.30(11)
  O1—Cd1—O5#1	149.07(11)	O7—Cd1—O5#1	122.02(13)	N3—Cd1—O5#1	82.44(14)
  O6#1—Cd1—O0AA	115.80(11)	N1—Cd1—O0AA	82.62(10)	N3—Cd1—O6#1	109.97(12)
  O6#1—Cd1—O1	158.63(12)	N1—Cd1—O1	70.07(10)	N3—Cd1—O7	86.16(14)
  O6#1—Cd1—O5#1	52.17(12)	N1—Cd1—O5#1	122.96(13)	N3—Cd1—N1	152.27(11)
  Symmetry codes: #1: 1-x, 1-y, 1-z for 1; #1: 2-x, 2-y, 2-z; #2: 1-x, 1-y, -z for 2.

  下载: 导出CSV

  Table 3.  Hydrogen bond parameters of 1 and 2

  D—H…A	d(D—H) / nm	d(H…A) / nm	d(D…A) / nm	∠DHA / (°)
  1
  O3—H3…O1	0.082	0.162	0.243 7(3)	172
  O7—H7A…O3#2	0.088	0.184	0.271 1(3)	167
  O7—H7B…O4#1	0.085	0.185	0.269 6(3)	173
  C22—H2…O7	0.093	0.259	0.319 5(5)	123
  2
  O0AA—H0AA…O3#2	0.087	0.202	0.284 6(4)	159
  N2—H2…O9#4	0.086	0.200	0.286 1(5)	173
  O2—H2…O3	0.090(9)	0.155(9)	0.245 1(5)	173(15)
  O0AA—H0AB…O1#1	0.087	0.201	0.286 5(5)	167
  O0AA—H0AB…O2#1	0.087	0.251	0.317 5(5)	134
  O7—H7A…O9#3	0.087	0.235	0.314 0(6)	151
  O7—H7B…O8#3	0.087	0.197	0.278 8(8)	156
  O9—H9B…O6#5	0.085	0.191	0.275 7(5)	173
  C13—H13…O1	0.093	0.254	0.309 1(5)	118
  C13—H13…O0AA#1	0.093	0.258	0.350 3(6)	171
  Symmetry codes: #1: 1+x, -1+y, z; #2: -x, 1-y, 1-z for 1; #1: 1-x, 1-y, 1-z; #2: 1+x, y, z; #3: x, -1+y, z; #4: 1-x, 2-y, 2-z; #5: 2-x, 2-y, 2-z for 2.

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  Table 4.  Structural parameters of C—H…π interaction in crystal of 1*

  X—H…π	d(H…Cg) / nm	γ/(°)	∠X—H…Cg / (°)	d(X…Cg) / nm	σ/(°)
  C10—H10…Cg(1)#1	0.299	13.10	117	0.351 1(3)	35
  *γ: angle between Cg-H vector and ring; σ: angle of X—H bond with π-plane (i.e., Perpendicular: 90°, Parallel: 0°); Symmetry codes: #1: -x, 1-y, 1-z; Rings: Cg(1): C2, C3, N2, C4, N1, C5.

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  Table 5.  Structural parameters of π…π interactions in crystal of 1*

  Cg…Cg	α/(°)	DC / nm	β/(°)	DZ / nm	S / nm
  Cg(3)…Cg(3)#1	0.00(19)	0.364 5(1)	21.8	0.338 51(16)	0.135 3
  Cg(5)…Cg(5)#2	0.00(2)	0.368 3(2)	19.4	0.347 3(2)	0.122 6
  Cg(2)…Cg(3)#2	1.90(2)	0.437 8(2)	35.9	0.346 07(19)	0.256 7
  Cg(4)…Cg(4)#3	0.00(11)	0.459 42(15)	47.3	0.311 32(10)	0.337 8
  *α: dihedral angle between mean planes of the rings; DC: distance between ring centroids; β: angle between DC vector and normal to plane (i); DZ: perpendicular distance of the centroids of ring (i) on plane of ring (j); S: distance between ring centroid (i) and perpendicular projection of ring centroid (j) on ring (i); Symmetry codes: #1:2-x, 1-y, 2-z; #2:1-x, 1-y, 2-z; #3:1-x, 1-y, 1-z; Rings: Cg(2): N3, C22, C21, C20, C19 C23; Cg(3): N4, C13, C14, C15, C16, C24; Cg(4): C6, C7, C8, C9, C10, C11; Cg(5): C16, C17, C18, C19, C23, C24.

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