English

• The programming and construction of polynuclear lanthanide complexes have brought a mass of attention because of their amusing structures and extensive application in the domain of luminescence^[1-4], magnetism^[5-8], gaseous substance separate/storing^[9-10], catalysis^[11-14], and biological activities^[15-16]. In these research areas, the research on the magnetic property of Ln(Ⅲ)-based polynuclear complexes is extremely active due to their potential applications in molecular-based magnetic material, which include single-molecule magnet (SMM) and magnetic refrigerants^[17-19]. Ln(Ⅲ)-based SMM is one of the currently hot research themes because of its utilization potentiality in stored information^[20-21]. The Ln(Ⅲ)-based polynuclear complexes have attracted increasing attention from the inorganic chemist and the material scientist in recent years because they exhibit intriguing SMM behaviors^[22-23]. Along with this research, the literature has reported a range of Ln(Ⅲ)-based polynuclear complexes showing outstanding SMM behaviors, including Dy₃^[24], Dy₄^[25-28], Dy₆^[29-30], Dy₈^[31], Dy₁₀^[32-33], and Dy₁₂ complex^[34]. In the Dy(Ⅲ)-based polynuclear complex SMM, the Tang research group and Tong research group have contributed excellently^[35-39], especially the Dy₃ and Dy₄ complexes SMM. In addition, Liang and Cui groups have also done some interesting works on polynuclear lanthanide complexes^[40-43]. To some extent, these excellent works fan and boost the rapid progress of Ln(Ⅲ)-based polynuclear complexes SMM.

  In view of the above research background for Ln(Ⅲ)-based polynuclear complexes SMM, here, an 8- hydroxyquinoline Schiff base ligand (HL=5-[(4-methylbenzylidene)amino]quinolin-8-ol) has been employed to produce multinuclear Dy(Ⅲ)-based SMM. The Schiff base ligand possesses wonderful structural characteristics: (ⅰ) through O and N ligating atoms, it can lightly bond with Dy(Ⅲ) metal ions; (ⅱ) the Schiff base ligand (HL) which contains phenoxy atoms can play a bridging role, and bridge the Dy(Ⅲ) ions in the center, which can effectively transfer the magnetic interaction. Reacting with the metal salt of Dy(dbm)₃·2H₂O, a novel tetranuclear Dy(Ⅲ)-based complex [Dy₄(dbm)₄(L)₆(μ₃-OH)₂]·CH₃CN (1) (Hdbm=dibenzoylmethane) has been constructed and its structure has been characterized. The magnetic property of complex 1 has been systematically investigated. Magnetic research indicated that 1 displayed extraordinary SMM behavior, the energy barrier U_eff/k_B was 116.7 K, and the pre-exponential τ₀=1.05×10^-8 s.

  1.   Experimental

  Starting materials and physical measurements can be found in Supporting information, as well as the synthesis of 5-amino-8-hydroxylquinoline.

  1.1   Synthesis of HL and complex 1

  1.1.1   Synthesis of HL

  5-Amino-8-hydroxylquinoline (1.6 g, 10 mmol) and 4-methyl benzaldehyde (10 mmol) were added to 50 mL CH₃CH₂OH at 70 ℃, followed by adding five drops of HCOOH as catalyst. Subsequently, the mixed solution was continuously heated at 85 ℃ for 5 h. The product was separated from the mixture and purified by recrystallization with a mixture of CH₃CH₂OH and CH₃COCH₃ (3∶1, V/V). Finally, the purified product was green solid (Yield: 2.1 g, 80.2%) (Scheme 1). Elemental analysis Calcd. for C₁₇H₁₄ON₂(%): C, 77.86; H, 5.34; N, 10.69. Found(%): C, 77.65; H, 5.62; N, 10.40. IR (KBr, cm^-1): 3 679(s), 2 884(w), 1 574(m), 1 500(s), 1 488(m), 1 407(s), 1 372(w), 1 245(s), 1 195(s), 1 141(m), 1 275(w), 1 185(s), 1 046(w), 1 009(w), 974(w), 880(w), 821(m), 785(s), 705(m), 662(m), 578(w), 505(m). The ¹H and ¹³C NMR spectra of HL are shown in Fig.S1. ¹H NMR (400 MHz, CDCl₃): δ 8.99-8.75 (m, 2H), 8.59 (s, 1H), 7.91 (d, J=7.9 Hz, 2H), 7.60-7.15 (m, 6H), 2.47 (s, 3H). ¹³C NMR (101 MHz, CDCl₃): δ 158.93, 150.60, 148.34, 141.85, 140.24, 138.17, 133.97, 133.25, 129.57, 128.81, 124.97, 121.52, 113.68, 109.69, 21.68.

  Scheme 1

  

  Scheme 1.  Synthetic route of ligand HL

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  1.1.2   Synthesis of complex 1

  Dy(dbm)₃·2H₂O (0.025 mmol) was dissolved in a mixture of 20 mL CH₃CH₂OH and CH₃CN (1∶1, V/V). Then, 5 mL of CH₂Cl₂ solution containing HL (0.025 mmol) was added while stirring, and the mixture was stirred at room temperature for 5 h. Finally, the solution was filtered and stored away from light and condensed slowly by evaporation at room temperature. After approximately 5 d, yellow block single crystals appropriate for single-crystal X-ray diffraction analysis were obtained. Yield: 59% (based on Dy). Elemental analysis Calcd. for C₁₆₄H₁₂₇N₁₃O₁₆Dy₄(%): C, 61.83; H, 4.02; N, 5.72. Found(%): C, 61.68; H, 4.25; N, 5.46. IR (KBr, cm^-1): 3 682(m), 2 861(w), 1 605(m), 1 558(m), 1 515(m), 1 463(m), 1 395(s), 1 311(s), 1 224(w), 1 173(w), 1 091(w), 1 023(w), 924(w), 786(m), 721(w), 606(w), 593(w).

  1.2   Crystallographic analysis

  At 113(2) K, X-ray diffraction measurement for the single crystal of complex 1 was carried out using a CCD X-ray single crystal diffractometer, which possesses a graphite monochromatic Mo Kα radiation, and the wavelength (λ) is 0.071 073 nm. To achieve a more accurate measurement, Lorentz polarization and absorption correction were implemented. The SHELX and Olex2 programs were used to optimize the structure using the F ²-based full-matrix least-squares technique^[44]. The anisotropy parameters were employed for non-hydrogen atoms to achieve precise corrections, while the hydrogen atoms were positioned based on calculations and further refined utilizing the riding model. The solvent molecules were badly confused, the data were processed by squeeze, and the chemical expression of 1 was decided by elemental analysis. Crystallographic data of 1 is detailed in Table 1. In addition, the information on some bond lengths and selected bond angles is shown in Table S1.

  Table 1

  

  Table 1.  Crystallographic data and structure refinements for complex 1

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  Parameter	1		Parameter	1
  Formula	C164H127N13O16Dy4		Dc / (Mg·m-3)	1.747
  Formula weight	3 185.79		μ / mm-1	2.127
  Crystal system	Monoclinic		Rint	0.033 3
  Space group	P21/n		Limiting indices	-19 ≤ h ≤ 19, -21 ≤ k ≤ 21, -38 ≤ l ≤ 38
  a / nm	1.497 6(2)		Reflection collected	89 020
  b / nm	1.648 8(2)		Reflection used	21 459
  c / nm	2.989 4(5)		Number of parameters	904
  β / (°)	103.443(4)		GOF on F 2	1.055
  V / nm3	7.179 5(19)		R1, wR2 [I > 2σ(I)]	0.033 7, 0.108 9
  Z	2		R1, wR2 (all data)	0.040 8, 0.103 9
  Crystal size / mm	0.20×0.18×0.12

  2.   Results and discussion

  2.1   Crystal structures of complex 1

  Complex 1 crystallizes in Monoclinic space group P2₁/n, which is proved through X-ray single crystal diffraction analyses (Table 1), with Z=2. As shown in Fig.1, 1 contains a [Dy₄] core with crystal inversion symmetry (Fig.2), and the four Dy(Ⅲ) ions are strictly coplanar. The structural unit of 1 primarily contains four independent Dy(Ⅲ) ions, six Schiff base ligands (L^-), four dbm^- co-ligands, two μ₃-OH ligands, and a CH₃CN molecule. The nuclear Dy1 ion is coordinated with eight atoms, involving two N atoms, including N1 and N3 from two L^- ions, as well as six O atoms, which are O1, O2, O3, O4, O5, and O8, coming from three L^- ions, one dbm^- ion, and one μ₃-OH ion. The Dy2 ion in the center has the same coordination number as the Dy1 ion, which consists of one nitrogen atom (N5), seven oxygen atoms (O1a, O2a, O3, O6, O7, O8, and O8a) coming from three L^- ions, one dbm^- ion and two μ₃-OH ions. After accurate calculation by the software of Shape 2.0, we get the following results: the Dy1 ion center adopts a dodecahedron configuration, and the Dy2 ion takes a square anti-prism configuration (Table S2).

  The ligand ion L^-, acting as a bidentate ligand, is chelated to the central Dy1 ion via a phenolic hydroxyl O atom (O1/O2/O3) and a pyridyl N atom (N1/N3), and the O atom (O3) bridges to the other Dy2 ion. In the coplanar Dy₄ nucleus, the angle of Dy1—O3—Dy2 is 108.447(102)°, and the angle of Dy1—O8—Dy2 is 111.362(100)°. The distances of Dy…Dy are 0.355 3(5), 0.385 89(4), and 0.635 09(18) nm, respectively. The Dy—O bond length scope is 0.228 91(18)-0.240 49(29) nm, and the distance scope of Dy—N bond is 0.252 78(27)-0.255 98(35) nm. These bond length and bond angle values are in accord with those of lanthanide complexes already publicly reported in the literature^[45-47].

  Figure 1

  

  Figure 1.  (a) Molecular structure of complex 1; (b) [Dy₄] core structure

  Ellipsoid probability: 30%; Symmetry code: a: -x+2, -y+2, -z.

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  Figure 2

  

  Figure 2.  Coplanar graph of the Dy^Ⅲ ion in complex 1

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  The structure reported in this paper is isomorphic to those of the Eu, Tb, and Ho complexes reported in reference^[16]. The metal centers are all octa-coordinated, and the types and numbers of coordination atoms are the same as those reported in the reference. The difference is that the coordination geometry of the two central Dy ions reported in this paper are square anti-prism and dodecahedron configurations, respectively, while the two central metal ions in the literature are square anti-prism coordination geometry.

  2.2   Powder X-ray diffraction analysis

  The phase purity of the crystal powder of complex 1 was determined using powder X-ray diffraction. The experimental peaks were highly consistent with the simulation result of single crystal data, manifesting that the solid sample is of high purity (Fig.S2). The disparities in strength may be because of the preferred orientation of samples which crystalize in solid.

  2.3   Thermogravimetric analysis

  The thermogravimetric analysis of complex 1 was performed from room temperature to 800 ℃. As shown in Fig.S3, from room temperature to about 190 ℃, the weight loss of the test sample was about 1.59%, corresponding to the loss of one free CH₃CN molecule in the crystal (Calcd. 1.29%). From 190 to about 350 ℃, there was almost no weight loss. Above about 350 ℃, the sample lost weight rapidly, indicating that the organic ligands are decomposed and the structure of 1 is destroyed.

  2.4   Magnetic properties

  The magnetic susceptibility of microcrystalline samples of complex 1 was measured at a magnetic field strength of 1 000 Oe and in a temperature scope from 300 to 2 K. The molar susceptibility (χ_MT) vs temperature (T) curve is displayed in Fig.3. The χ_MT of 1 was 56.54 cm³·mol^-1·K at room temperature, which is on the border of the corresponding theoretical calculating value of 56.68 cm³·mol^-1·K, obtaining based on four uncoupled Dy^Ⅲ ions in the normal state (⁶H_15/2, g=4/3). In the temperature scope of 300-50 K, the χ_MT value of 1 showed a gradually decreasing trend. When the temperature dropped to 2 K, this value rapidly dropped to a minimum of 37.43 cm³·mol^-1·K. Such behavior may be attributed to the thermal depopulation of the Dy^Ⅲ ions Stark sublevels and/or the weak antiferromagnetic coupling between the neighboring Dy^Ⅲ ions inside 1^[48]. Furthermore, the magnetic susceptibility of 1 could be fitted to the Curie-Weiss law (Fig.S4)^[49], and the significant parameters θ=-3.25 K and C=56.59 cm³·mol^-1·K for 1 have been obtained.

  Figure 3

  

  Figure 3.  Magnetic susceptibility (plot of χ_MT vs T) for complex 1

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  The magnetization data of complex 1 have been measured between 2.0 and 7.0 K at a field of 0.0-8.0 T. As shown in Fig.S5, under T=2.0 K, the M vs H curve showed a gradual increase with the increasing field. However, it still has not reached complete saturation up to 8.0 T (Theoretical saturated value: 40Nβ for four independent Dy³⁺ ions). It can be mainly attributed to magnetic anisotropy for Dy³⁺ ions in 1^[30]. Moreover, the M vs HT^-1 curves at T=2.0-7.0 K showed non-superimposed magnetization curves for 1, which also demonstrates the presence of anisotropy and/or low-lying excited states^[30].

  To inquire into the magnetization dynamics, we have successfully measured the temperature-dependent susceptibility of alternating current (ac) for complex 1 at a temperature scope of 2.0-18.0 K, as well as under the frequency range of 111-2 311 Hz, and in a 3.0 Oe ac and 0 dc magnetic field. Interestingly, complex 1 displayed a clear frequency dependence and remarkable peaks were detected (Fig.4), indicative of the possibility of SMM behavior^[50-51].

  To further deeply study the dynamic of the magnetic behavior of complex 1, frequency-dependent ac magnetic susceptibility (Fig.S6) was gauged at 2.0-16.0 K in 0 Oe dc field conditions. The out-of-phase peaks (χ″) of frequency-dependent further validate that the marked SMM behavior appears on complex 1^[52]. The Cole-Cole plot of 1 is shown in Fig.S7, which we attempt to fit using the generalized Debye model. Under the temperature scope of 2.0-16.0 K, we succeeded in obtaining the significant parameter α, whose value ranges from 0.15 to 0.63. The distribution number α values of 1 is relatively large, indicating that the relaxation time is widely distributed in 1^[53].

  Figure 4

  

  Figure 4.  χ′-T (a) and χ″-T (b) plots for complex 1 in a 0 Oe dc field

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  The relaxation time τ of complex 1 could be predicted by analyzing the χ″ peaks data, and the Arrhenius law τ=τ₀exp[ΔE/(k_BT)]^[54] fitted this result well. According to the fitting results, we got the energy barrier U_eff/k_B=116.7 K and the value of the pre- exponential coefficient τ₀ was 1.05×10^-8 s (Fig.5). What needs illustration is that the plot of ln τ vs 1/T under H_dc=0 Oe field showed an evident curvature, which indicates that perhaps multi-relaxation pathway occurred in 1. Based on this, we fitted the ln τ vs 1/T plot with the equation: ln τ=-ln{AT+B+CTⁿ+τ₀^-1exp[U_eff/(k_BT)]}^[55]. Hereon, AT+B, CTⁿ, and τ₀^-1exp[U_eff/(k_BT)] represent the Direct, Raman, and Orbach relaxation processes, respectively. These critical parameters have been obtained for 1 (U_eff/k_B=121 K, τ₀=7.01×10^-8 s, n=5.1, A=0.311 5 s^-1·K^-5.1, and C=0.057 36). The small parameters of A and C suggest that the relaxation process can be dominated by the Orbach mechanism at high temperatures and the Raman mechanism at low temperatures. The procured τ₀ identifies with the previously reported 10^-6-10^-12 s of Dy₄-based SMM^[56-59].

  Figure 5

  

  Figure 5.  ln τ vs T^-1 plot for complex 1

  The red solid line represents the best fits of the Arrhenius law; the pink solid line represents the fits under multiple magnetic relaxation processes.

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  3.   Conclusions

  In conclusion, one new tetranuclear Dy(Ⅲ)-based polynuclear complex based on 5-[(4-methylbenzylidene)amino]quinolin-8-ol has been successfully constructed. The crystal structure and magnetic properties of complex 1 have been minutely characterized. Single crystal structure analysis shows that 1 possesses a rhombic Dy₄ nucleus. Intriguingly, 1 displayed a remarkable SMM behavior, which was corroborative through the χ″ peaks. To our knowledge, it is a larger spin-reversal energy barrier (U_eff/k_B=116.7 K) for 1 in comparison with that of the reported Dy₄ SMM. Overall, this work provides an instance of a self-assembled polynuclear Dy(Ⅲ)-based complex with significant SMM behavior, and the magnetic behaviors research of other multinuclear Dy(Ⅲ)-based complexes is underway in our team.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Scheme 1  Synthetic route of ligand HL

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  Figure 1  (a) Molecular structure of complex 1; (b) [Dy₄] core structure

  Ellipsoid probability: 30%; Symmetry code: a: -x+2, -y+2, -z.

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  Figure 2  Coplanar graph of the Dy^Ⅲ ion in complex 1

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  Figure 3  Magnetic susceptibility (plot of χ_MT vs T) for complex 1

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  Figure 4  χ′-T (a) and χ″-T (b) plots for complex 1 in a 0 Oe dc field

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  Figure 5  ln τ vs T^-1 plot for complex 1

  The red solid line represents the best fits of the Arrhenius law; the pink solid line represents the fits under multiple magnetic relaxation processes.

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  Table 1.  Crystallographic data and structure refinements for complex 1

  Parameter	1		Parameter	1
  Formula	C164H127N13O16Dy4		Dc / (Mg·m-3)	1.747
  Formula weight	3 185.79		μ / mm-1	2.127
  Crystal system	Monoclinic		Rint	0.033 3
  Space group	P21/n		Limiting indices	-19 ≤ h ≤ 19, -21 ≤ k ≤ 21, -38 ≤ l ≤ 38
  a / nm	1.497 6(2)		Reflection collected	89 020
  b / nm	1.648 8(2)		Reflection used	21 459
  c / nm	2.989 4(5)		Number of parameters	904
  β / (°)	103.443(4)		GOF on F 2	1.055
  V / nm3	7.179 5(19)		R1, wR2 [I > 2σ(I)]	0.033 7, 0.108 9
  Z	2		R1, wR2 (all data)	0.040 8, 0.103 9
  Crystal size / mm	0.20×0.18×0.12

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