English

• Rare earth metals are called "Industrial Monosodium Glutamate" and "Treasure House" of new materials in today′s society. They can form many different new materials with different properties, and many materials are widely used in all walks of life^[1-5]. As a special kind of inorganic ions, rare earth ions have excellent optical, electrical, and magnetic properties. Wilkinson and Brightan successfully synthesized the first rare earth metal-organic complex (tricycline rare earth compound) in 1954, marking the birth of rare earth metal organic chemistry. After that, many excellent studies on rare earth complexes have been reported, so the research on rare earth coordination compounds is particularly important^[6-9]. Based on the 4f electronic structure of rare earth ions, rare earth complexes have important potential applications in optical, magnetic, and other fields. Among these properties, the magnetic properties of rare earth complexes have been extensively studied^[10-11]. The high magnetic moment and strong magnetic anisotropy of rare earth ions provide the basis for them as the magnetic functional unit of molecular nanomagnets. However, due to the changeable coordination configuration of rare earth ions, the reasonable design and synthesis of rare earth complexes are still very challenging. In this paper, we report our discovery of four new rare earth complexes based on Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ and 5-(4-carboxyphenoxy) nicotinic acid ligand (H₂cpna)^[12-13]. Furthermore, the fluorescence and magnetic properties of the complexes were investigated at room temperature.

  1.   Experimental

  1.1   Material and measurements

  All chemicals of reagent grade were commercially available and used without purification. The H₂cpna ligand was purchased from Jinan Camolai Trading Company (Jinan, China); Dy(NO₃)₃·6H₂O, Ho(NO₃)₃·6H₂O, Er(NO₃)₃·6H₂O, and Tm(NO₃)₃·6H₂O were purchased from Shanghai Aladdin Biological Technology Co., Ltd (Shanghai, China). The experimental water was distilled water.

  Elemental analyses (C, H, and N) were performed using a Perkin-Elmer 2400 element analyzer. IR spectra were recorded in a range of 400-4000 cm^-1 using a VECTOR-22 spectrometer using KBr discs. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker (D8 Advance) diffractometer for a Cu-target tube and a graphite monochromator (voltage: 40 kV, current: 40 mA, incident ray wavelength: 0.154 06 nm, scanning range: 5°-90°). The simulated PXRD patterns were obtained by the single-crystal data and diffraction-crystal module of the Mercury program. A thermal analyzer (TG-DSC, STA449F3, Germany) was applied to investigate the thermal stability of the sample under a nitrogen atmosphere at a heating rate of 10 ℃·min^-1. Fluorescence tests were carried out on a Varian Cary Eclipse fluorescence spectrometer. Magnetic data were collected by a Quantum Design SQUID-VSM magnetometer.

  1.2   Synthesis of complexes [M(Hcpna)(cpna)(H₂O)₃]_n (M=Dy, Ho, Er, Tm)

  A mixture of H₂cpna (0.25 mmol), M(NO₃)₃·6H₂O (0.5 mmol), NaOH (0.45 mmol), and distilled H₂O (10 mL) was stirred magnetically for 5 min at room temperature, then was sealed in a 25 mL Teflon-lined autoclave and heated to 120 ℃ at a rate of 10.8 ℃·h^-1. After maintaining for 72 h, the reaction vessel was cooled to 20 ℃ at a rate of 5 ℃·h^-1. Light yellow crystals were collected.

  [Dy(Hcpna)(cpna)(H₂O)₃]_n (1): 45% yield based on H₂cpna. Elemental Anal. Calcd. for DyC₂₆H₂₁N₂O₁₃(%): C, 42.66; H, 2.89; N, 3.83. Found(%): C, 42.98; H, 2.61; N, 4.59. FTIR (KBr pellets, cm^-1): 1 684, 1 599, 1 551, 1 504, 1 463, 1 408, 1 315, 1 298, 1 259, 1 208, 1 163, 1 024, 963, 909, 878, 853, 794, 763, 707.

  [Ho(Hcpna)(cpna)(H₂O)₃]_n (2): 76% yield based on H₂cpna. Elemental Anal. Calcd. for HoC₂₆H₂₁N₂O₁₃(%): C, 42.52; H, 2.88; N, 3.81. Found(%): C, 42.38; H, 2.91; N, 4.19. FTIR (KBr pellets, cm^-1): 1 685, 1 602, 1 545, 1 505, 1 460, 1 408, 1 316, 1 300, 1 259, 1 211, 1 179, 1 165, 1 025, 965, 905, 874, 854, 802, 769, 708.

  [Er(Hcpna)(cpna)(H₂O)₃]_n (3): 63% yield based on H₂cpna. Elemental Anal. Calcd. for ErC₂₆H₂₁N₂O₁₃(%): C, 42.38; H, 2.87; N, 3.80. Found(%): C, 42.16; H, 3.06; N, 4.20. FTIR (KBr pellets, cm^-1): 1 682, 1 602, 1 546, 1 506, 1 459, 1 406, 1 316, 1 299, 1 258, 1 213, 1 179, 1 165, 1 025, 965, 905, 872, 857, 799, 766, 706.

  [Tm(Hcpna)(cpna)(H₂O)₃]_n (4): 57% yield based on H₂cpna. Elemental Anal. Calcd. for TmC₂₆H₂₁N₂O₁₃(%): C, 42.29; H, 2.87; N, 3.79. Found(%): C, 42.58; H, 2.49; N, 3.63. FTIR (KBr pellets, cm^-1): 1 687, 1 601, 1 547, 1 507, 1 460, 1 407, 1 316, 1 301, 1 259, 1 215, 1 179, 1 165, 1 027, 965, 904, 875, 854, 801, 769, 706.

  1.3   X-ray data collection and structure determinations

  Single-crystal X-ray diffraction data for complexes 1-4 were collected on a Bruker SMART APEX-Ⅱ CCD diffractometer equipped with a graphite-monochromated Mo Kα radiation (λ=0.071 073 nm) at 293(2) K using an ω-φ scan mode. Absorption corrections were applied by using the SADABS program^[14]. The structure was solved by direct methods and refined by full-matrix least-squares techniques on F² with the SHELX-2018 package^[15]. The H atoms of the ligands were generated theoretically on the specific atoms and refined isotropically with fixed thermal factors. Detailed crystallographic data of complexes 1-4 are summarized in Table 1.

  Table 1

  

  Table 1.  Crystallographic data for complexes 1-4

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  Parameter	1	2	3	4
  Chemical formula	DyC26H21N2O13	HoC26H21N2O13	ErC26H21N2O13	TmC26H21N2O13
  Formula weight	731.95	734.38	736.71	738.38
  Space group	P1	P1	P1	P1
  a / nm	0.593 37(17)	0.591 11(12)	0.587 71(12)	0.587 62(8)
  b / nm	1.252 0(4)	1.247 8(3)	1.244 2(3)	1.245 17(17)
  c / nm	1.801 9(5)	1.795 0(4)	1.788 7(4)	1.789 5(3)
  α / (°)	93.274(5)	93.280(3)	93. 313(4)	93.355(2)
  β / (°)	98.093(5)	98.051(3)	98.104(4)	98.202(2)
  γ / (°)	92.964(5)	92.851(3)	92.894(4)	92.990(2)
  Volume / nm3	1.320 7(7)	1.306 5(5)	1.290 5(5)	1.291 3(3)
  Z	2	2	2	2
  Dc / (g·cm-3)	1.841	1.851	1.880	1.883
  Indices ranges	-7 ≤ h ≤ 6, -14 ≤ k ≤ 14, -16 ≤ l ≤ 21	-6 ≤ h ≤ 7, -11 ≤ k ≤ 14, -21 ≤ l ≤ 20	-6 ≤ h ≤ 6, -14 ≤ k ≤ 14, -21 ≤ l ≤ 19	-6 ≤ h ≤ 6, -14 ≤ k ≤ 12, -21 ≤ l ≤ 20
  F(000)	722	712	714	716
  Reflection collected	6 768	6 723	6 599	6 837
  Unique reflection	4 610	4 562	4 503	4 504
  Rint	0.026 7	0.024 9	0.029 5	0.042 7
  GOF on F2	1.053	1.027	1.048	1.033
  R, wR [I > 2σ(I)]	R1=0.034 6, wR2=0.073 1	R1=0.031 3, wR2=0.079 5	R1=0.035 5, wR2=0.076 6	R1=0.046 1, wR2=0.116 1
  R, wR (all data)	R1=0.041 4, wR2=0.079 2	R1=0.037 0, wR2=0.093 2	R1=0.043 7, wR2=0.081 0	R1=0.050 7, wR2=0.121 5

  2.   Results and discussion

  2.1   Crystal-structural description

  The structures of complexes 1, 2, 3, and 4 are the same, so structural analysis of the four complexes is performed by representing the central atoms of M. Single-crystal X-ray diffraction analysis reveals that the complex crystallizes in the triclinic crystal system with the P1 space group. The asymmetric unit comprises one M³⁺ ion, one fully deprotonated ligand cpna^2-, one partially deprotonated ligand Hcpna^-, and three coordinated water molecules. Thus, a nine-coordinated structure is formed. As shown in Fig. 1a, M³⁺ is arranged in a distorted single-cap square inverse prism, O3, O4, O5, and O9A form the bottom of the tetragonal body, O3, O6, O7, and O10A form the top of the tetragonal body and O2 is the top of the single cap. The completely deprotonated cpna^2- ligand adopts μ₂∶η¹, η¹, η¹, η¹ coordination mode, and the partially deprotonated ligand Hcpna^- ligand adopts μ₁∶η¹, η¹ coordination mode (Scheme 1). M³⁺ ion is coordinated by two O atoms (O3 and O4) from cpna^-, four O atoms (O1, O2, O9A, and O10A) from cpna^2- and three O atoms (O5, O6, and O7) from water molecules. Each M³⁺ ion linked two adjacent M³⁺ ions through fully deprotonated cpna^2- and formed infinite zigzag chains along the b-axis. (Fig. 1b)

  Figure 1

  

  Figure 1.  (a) Coordination environment of M³⁺ (M=Dy, Ho, Er, Tm) in complexes 1-4; (b) 1D chain structure of complexes 1-4

  Hydrogen atoms are omitted for clarity; Symmetry code: A: x, 1+y, z.

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  Scheme 1

  

  Scheme 1.  Coordination modes of cpna^2- (left) and Hcpna^- (right) anions

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  In the complexes, the O—M—O angles are in the ranges of 51.06(11)°-160.16(14)° (1), 51.45(12)°-159.80(14)° (2), 51.63(12)°-159.76(14)° (3), and 51.58(15)°-159.18(19)° (4). The M—O bond lengths change from 0.233 1(4) to 0.289 7(5) nm for 1, from 0.230 8(4) to 0.287 9(6) nm for 2, from 0.229 4(4) to 0.278 5(6) nm for 3, and from 0.230 1(4) to 0.285 8(7) nm for 4. The organic ligand exhibits two kinds of coordination modes: one carboxyl group on the benzene ring of the Hcpna^- ligand is not coordinated and another COO^- group on the pyridine ring adopts a bidentate mode. The Hcpna^- ligand shows a dihedral angle (between pyridyl and benzene rings) of 53.20° (1), 52.69° (2), 52.92° (3), and 52.50° (4), and the C—O_ether—C angle between two aromatic rings is 120.50° (1), 120.40° (2), 120.22° (3), and 121.34° (4). For the cpna^2- ligand, two COO^- groups adopt bidentate mode. The cpna^2- ligand shows a dihedral angle (between pyridyl and benzene rings) of 66.89° (1), 66.84° (2), 67.26° (3), and 67.63° (4), and the C—O_ether—C angle between two aromatic rings is 119.64° (1), 120.14° (2), 119.58° (3), and 120.03° (4). The distance between adjacent M³⁺ atoms is 1.252 nm (1), 1.248 nm (2), 1.244 nm (3), and 1.245 nm (4).

  2.2   PXRD analyses

  PXRD analyses of complexes 1-4 were performed before the fluorescence and magnetic properties test. As shown in Fig. 2, the experimental data are in good agreement with the simulated pattern of single-crystal X-ray diffraction data, which proves that the phase purity of the test product is sufficient for the next step of the experiment.

  Figure 2

  

  Figure 2.  Experimental and simulated PXRD patterns of complexes 1 (a), 2 (b), 3 (c), and 4 (d)

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  2.3   TG analyses

  To determine the thermal stability of the complexes, TGA experiments were carried out. As shown in Fig. 3, the results of the TGA indicate that complexes 1-4 have almost the same thermodynamic stability. The complexes had two steps of decomposition in a range of 22-900 ℃. The first major weight loss occurred at temperatures between 22 and 200 ℃ corresponding to the removal of the water molecules. There was nearly no weight loss at the temperatures between 200 and 320℃. The second weight loss occurred at temperatures between 320 and 900 ℃, corresponding to the decomposition of the complex skeleton. TGA indicates complexes 1-4 have good thermal stability.

  Figure 3

  

  Figure 3.  TGA curves of complexes 1-4

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  2.4   Fluorescence spectrum

  The solid-state fluorescence spectra of H₂cpna and complexes 1-4 were recorded at room temperature on a Varian Cary Eclipse fluorescence spectrometer under an excitation of 280 nm. As shown in Fig. 4, the H₂cpna ligand had a wide emission band between 350 and 415 nm, and the maximum emission peak was about 390 nm. The emission peak of the ligand is attributed to the charge transfer within the ligand, that is, the electron transition of n-π* or π-π*. The fluorescence test results show that complexes 1-4 had similar fluorescence emission bands with the H₂cpna ligand. Therefore, it is considered that the fluorescence emission peak of the complexes comes from the ligand. The difference was that the fluorescence emission intensity of 1-4 were weaker than that of the ligand. After the formation of the complex, the lone pair electrons of the coordination atoms on the ligand form a coordination bond with the metal ions, which changes the electron distribution on the ligand. Coupled with the heavy atom effect of metal ions, the fluorescence intensity of the complex is reduced.

  Figure 4

  

  Figure 4.  Fluorescence spectra of H₂cpna and complexes 1-4

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  2.5   Magnetic properties

  The magnetism of complexes 1-4 were studied on a Quantum Design SQUID-VSM magnetometer by solid-state magnetic susceptibility measurements in the 2-300 K range at 1 kOe dc field. Plots of the χ_mT versus T and magnetic analysis of complexes 1-4 are shown in Fig. 5.

  Figure 5

  

  Figure 5.  Plots of χ_mT vs T for complexes 1-4

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  For complex 1, χ_mT was almost a constant from 14.04 cm³·mol^-1·K at 300 K, corresponding to the expected values (14.18 cm³·mol^-1·K) for one uncoupled Dy(Ⅲ)) ion^[16-17]. As the temperature decreased, χ_mT gradually decreased; reaching 7.56 cm³·mol^-1·K at 2 K. For complex 2, χ_mT was almost a constant from 14.15 cm³·mol^-1·K at 300 K, which corresponds to the expected values (14.45 cm³·mol^-1·K) for one uncoupled Ho(Ⅲ) ion^[18-19]. When the temperature decreased, χ_mT gradually decreased and reached 2.54 cm³·mol^-1·K at 2 K. For complex 3, χ_mT value was 11.08 cm³·mol^-1·K at room temperature, which is consistent with the value (11.86 cm³·mol^-1·K) of one independent Er(Ⅲ) ion^[20-21]. χ_mT gradually decreased as the temperature decreased, and finally reached 5.78 cm³·mol^-1·K at 2 K. For complex 4, χ_mT was almost a constant from 6.83 cm³·mol^-1·K at 300 K, which is consistent with the value (7.15 cm³·mol^-1·K) of one independent Tm(Ⅲ) ion^[22-23]. χ_mT decreased with decreasing temperature; reaching 0.57 cm³·mol^-1·K at 2 K. The above data analysis indicates that the magnetism of all the complexes exhibits weak antiferromagnetic behavior. From the structural diagrams of the four complexes, it can be seen that the ligands in the complexes are relatively large, resulting in a longer distance between rare earth ions, thus there is almost no magnetic interaction between adjacent rare earth ions. At the same time, rare earth ions are difficult to magnetically interact with neighboring ions due to f-electrons. So χ_mT of the four complexes decreasing with decreasing temperature is not related to the coupling effect. This trend of change is completely caused by the de-heating population of the Stark energy levels of rare earth ions^[24].

  3.   Conclusions

  In summary, four 1D chain structure isomorphic rare earth complexes have been synthesized by the solvothermal method based on the polyacid ligand (H₂cpna) and rare earth metal ions Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺. The fluorescence and magnetism of complexes 1-4 were studied at room temperature. The fluorescence test results show that the fluorescence intensities of complexes 1-4 were all lower than that of the ligand, and magnetic results indicate that the magnetism of all four complexes exhibits weak antiferromagnetic behavior.

  
  
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• 

  Figure 1  (a) Coordination environment of M³⁺ (M=Dy, Ho, Er, Tm) in complexes 1-4; (b) 1D chain structure of complexes 1-4

  Hydrogen atoms are omitted for clarity; Symmetry code: A: x, 1+y, z.

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  Scheme 1  Coordination modes of cpna^2- (left) and Hcpna^- (right) anions

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  Figure 2  Experimental and simulated PXRD patterns of complexes 1 (a), 2 (b), 3 (c), and 4 (d)

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  Figure 3  TGA curves of complexes 1-4

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  Figure 4  Fluorescence spectra of H₂cpna and complexes 1-4

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  Figure 5  Plots of χ_mT vs T for complexes 1-4

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  Table 1.  Crystallographic data for complexes 1-4

  Parameter	1	2	3	4
  Chemical formula	DyC26H21N2O13	HoC26H21N2O13	ErC26H21N2O13	TmC26H21N2O13
  Formula weight	731.95	734.38	736.71	738.38
  Space group	P1	P1	P1	P1
  a / nm	0.593 37(17)	0.591 11(12)	0.587 71(12)	0.587 62(8)
  b / nm	1.252 0(4)	1.247 8(3)	1.244 2(3)	1.245 17(17)
  c / nm	1.801 9(5)	1.795 0(4)	1.788 7(4)	1.789 5(3)
  α / (°)	93.274(5)	93.280(3)	93. 313(4)	93.355(2)
  β / (°)	98.093(5)	98.051(3)	98.104(4)	98.202(2)
  γ / (°)	92.964(5)	92.851(3)	92.894(4)	92.990(2)
  Volume / nm3	1.320 7(7)	1.306 5(5)	1.290 5(5)	1.291 3(3)
  Z	2	2	2	2
  Dc / (g·cm-3)	1.841	1.851	1.880	1.883
  Indices ranges	-7 ≤ h ≤ 6, -14 ≤ k ≤ 14, -16 ≤ l ≤ 21	-6 ≤ h ≤ 7, -11 ≤ k ≤ 14, -21 ≤ l ≤ 20	-6 ≤ h ≤ 6, -14 ≤ k ≤ 14, -21 ≤ l ≤ 19	-6 ≤ h ≤ 6, -14 ≤ k ≤ 12, -21 ≤ l ≤ 20
  F(000)	722	712	714	716
  Reflection collected	6 768	6 723	6 599	6 837
  Unique reflection	4 610	4 562	4 503	4 504
  Rint	0.026 7	0.024 9	0.029 5	0.042 7
  GOF on F2	1.053	1.027	1.048	1.033
  R, wR [I > 2σ(I)]	R1=0.034 6, wR2=0.073 1	R1=0.031 3, wR2=0.079 5	R1=0.035 5, wR2=0.076 6	R1=0.046 1, wR2=0.116 1
  R, wR (all data)	R1=0.041 4, wR2=0.079 2	R1=0.037 0, wR2=0.093 2	R1=0.043 7, wR2=0.081 0	R1=0.050 7, wR2=0.121 5

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