English

• 0.   Introduction

  In recent years, the study of lanthanide - based complexes has attracted increasing attention of chemists and material scientists not only due to their beautiful and fascinating crystal structures but also because of the potential applications in functional materials, including interesting magnetic properties, luminescence, gas adsorption, and catalysis^[1-4]. Among these potential applications of lanthanide - based complexes, the molecular - based magnetic material is one of the research hotspots for inorganic chemistry and material chemists^[5], and magnetic refrigeration and singlemolecule magnets (SMMs) are particularly attractive^[6-9]. Key to the potential magnetic refrigeration application of molecular-based magnetic materials is its large magnetocaloric effect (MCE) ^[10], and an excellent magnetic refrigeration material featuring large MCE should possess negligible magnetic anisotropy and a large magnetic density^[11]. Hence, the isotropic Gd(Ⅲ) ion with a high spin state (S=7/2) is the best candidate for designing and constructing Ln (Ⅲ) - based complexes, which would be a promising magnetic refrigerant material to perform significant MCE^[12]. Based on this, lots of polynuclear or high-nuclear Gd(Ⅲ)-based clusters with fascinating structures and larger MCE have been reported over the past decade^[13-16]. It is worth mentioning that Zheng and Long's group has conducted outstanding work on the magnetic refrigeration materials of Gd(Ⅲ)-based clusters^[17-18]. These studies promote the design and synthesis of lanthanide - based complexes with outstanding and excellent magnetic properties. On the other hand, lanthanide - based SMMs have caused public attention in recent years^[19], and designing lanthanide-based SMMs with large energy barriers (U_eff) and high blocking temperatures (T_B) is a great challenge^[20]. Given this, lots of Ln(Ⅲ)- based SMMs exhibiting significant magnetic behavior have appeared since the first Ln(Ⅲ)-based SMM (Bu₄N)[Tb(Pc)₂] reported by Ishikawa et al. in 2003^[21].

  To seek and study the magnetic behaviors of lanthanide - based complexes, an attractive polydentate ligand H₂L (Scheme 1) has been selected to construct lanthanide complexes, and three Ln^Ⅲ₂ complexes [Ln₂(L)₂(acac)₂(CH₃OH)₂]·2CH₃OH (Ln(Ⅲ) =Sm (1), Gd (2), and Ho (3), H₂L=pyridine - 2 - carboxylic acid (1 - methyl-3-oxo-butylidene)-hydrazide, Hacac=acetylacetone) have been synthesized. The magnetic refrigeration and slow magnetic relaxation behavior of 1-3 have been studied.

  Scheme 1

  

  Scheme 1.  Structure of ligand H₂L

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  1.   Experimental

  1.1   Material and measurement

  Solvents (methanol, dichloromethane) and starting chemical reagent (Ln(NO₃)₃·6H₂O, Ln=Sm, Gd, and Ho) were purchased commercially and used without further purification. Acetylacetone and 2 - pyridine carboxylic acid hydrazide were purchased from Aladdin Reagent (Shanghai) Co., Ltd. Ln(acac)₃·2H₂O (Ln=Sm, Gd, and Ho) was prepared using a reported method^[22]. The elemental analyses (C, H, and N) of complexes 1-3 were measured on a PerkinElmer 240 CHN elemental analyzer. Powder X-ray diffraction (PXRD) of complexes 1-3 were performed using an Ultima Ⅳ (Rigaku) with Cu Kα radiation in a 2θ range from 5° to 50°. The operating voltage and current were 40 kV and 25 mA, respectively. Thermogravimetric analyses (TGA) of complexes 1-3 were performed on a TG 209 apparatus (Netzsch) under an air atmosphere. Magnetic properties for complexes 1-3 were measured using a Quantum Design MPMS-XL7 and a PPMS-9 ACMS magnetometer. Diamagnetic corrections were estimated with Pascal's constants for all atoms^[23].

  1.2   Preparation of complexes 1-3

  2-Pyridine carboxylic acid hydrazide (0.04 mmol), Ln(acac)₃·2H₂O (0.04 mmol, Ln=Sm (1), Gd (2), and Ho (3)), methanol (8 mL), and dichloromethane (2.0 mL) were enclosed in a 15 mL glass vial, and then the mixture was stirred at room temperature for about 2.0 h. Whereafter, the mixture was heated to 70 ℃ and kept for 48 h, and then the temperature was decreased to room temperature slowly. Yellow block crystals suitable for X-ray diffraction were obtained.

  [Sm₂(L)₂(acac)₂(CH₃OH)₂]·2CH₃OH (1): Yield based on Sm(acac)₃·2H₂O: 36%. Elemental analysis Calcd. for C₃₆H₅₂N₆O₁₂Sm₂(%): C 40.66, H 4.89, N 7.91; Found (%): C 40.64, H 5.03, N 8.00.

  [Gd₂(L)₂(acac)₂(CH₃OH)₂]·2CH₃OH (2): Yield based on Gd(acac)₃·2H₂O: 32%. Elemental analysis Calcd. for C₃₆H₅₂N₆O₁₂Gd₂(%): C 40.10, H 5.01, N 7.80; Found (%): C 40.21, H 4.97, N 7.75.

  [Ho₂(L)₂(acac)₂(CH₃OH)₂]·2CH₃OH (3): Yield based on Ho(acac)₃·2H₂O: 41%. Elemental analysis Calcd. for C₃₆H₅₂N₆O₁₂Ho₂(%): C 39.57, H 4.76, N 7.69; Found (%): C 39.62, H 4.89, N 7.72.

  1.3   X-ray crystallography

  The X -ray diffraction measurements for complexes 1-3 were performed on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with a graphite monochromatized Mo Kα radiation (λ =0.071 073 nm) by using φ-ω scan mode. Multiscan absorption correction was applied to the intensity data using the SADABS program. The structures were solved by direct methods and refined by full - matrix least - squares on F² using the SHELXTL -2018 program. All non - hydrogen atoms were refined anisotropically. All the other H atoms were positioned geometrically and refined using a riding model. Details of the crystal data and structure refinement parameters for 1-3 are summarized in Table 1, and selected bond lengths and angles of 1-3 are listed in Table S1-S3 (Supporting information).

  CCDC: 2052182, 1; 2052183, 2; 2052184, 3.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for complexes 1-3

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  Parameter	1	2	3
  Empirical formula	C36H52N6O12Sm2	C36H52N6O12Gd2	C36H52N6O12Ho2
  Formula weight	1 062.46	1 075.25	1 091.62
  T / K	150(2)	293(2)	153(2)
  Crystal system	Monoclinic	Monoclinic	Monoclinic
  Space group	P21/c	P21/c	P21/c
  a / nm	0.773 0(5)	0.777 6(6)	0.765 59(3)
  b / nm	2.118 3(3)	2.131 2(6)	2.117 2(3)
  c / nm	2.579 3(7)	2.594 7(1)	2.574 5(7)
  β/(°)	93.961(2)	92.993 0(15)	93.437 5(15)
  V / nm3	4.213 8(5)	4.294 6(2)	4.165 7(3)
  Z	4	4	4
  Cryst size / mm	0.25 × 0.21 × 0.14	0.25 × 0.17 × 0.11	0.27 × 0.21 × 0.14
  Dc / (g·cm-3)	1.574	1.582	1.639
  μ / mm-1	2.816	18.867	3.828
  Limiting indices	-9 ≤ h ≤ 9,	-8 ≤ h ≤ 9,	-9 ≤ h ≤ 9,
  	-26 ≤ k ≤ 26,	-18 ≤ k ≤ 26,	-26 ≤ k ≤ 26,
  	Reflection collected	-32 ≤ l ≤ 31	-27 ≤ l ≤ 32	-32 ≤ l ≤ 32
  	67 691	17 142	52 003
  Unique	8 647	8 364	8 552
  Parameter	485	485	485
  Rint	0.091 5	0.042 2	0.066 8
  GOF on F2	1.006	1.084	1.085
  R1, wR2 [I > 2σ(I)]	0.045 2, 0.100 2	0.068 6, 0.182 5	0.042 7, 0.095 1
  R1, wR2 (all data)	0.073 6, 0.113 7	0.071 3, 0.184 5	0.060 2, 0.101 7

  2.   Results and discussion

  2.1   Crystal structures of complexes 1-3

  Single - crystal X - ray diffraction analyses reveal that 1-3 are isostructural and crystallize in the monoclinic space group P2₁/c (Table 1). Hereon, we selected the structure of 2 as a representative for describing. As shown in Fig. 1, the structure of 2 comprises two Gd(Ⅲ) ions, two L^2-, two acac^-, and two coordinated CH₃OH. Each central Gd(Ⅲ) ion is coordinated by six oxygen atoms and two nitrogen atoms formed a [N₂O ₆] coordination environment. As shown in Fig. S1, the Gd1 ion is coordinated by two nitrogen atoms (N3 and N4) and six oxygen atoms (O2, O3, O4, O7, O8, and O9) of two L^2-, one CH₃OH and one acac^-. The eight-coordinated Gd1 ion possesses a square antiprism geometry which is confirmed by using SHAPE 2.0 software (Table 2). The coordination mode of L^2- and acac^- are shown in Fig. 2. The two central Gd(Ⅲ) ions are connected by two μ₂-O atoms forming a parallelogram [Gd₂ O₂] core. In the [Gd₂O₂] core, the Gd1…Gd2 distance is 0.392 8(8) nm, O2—Gd1—O4 bond angle is 62.201 1(9)°, and the Gd1—O2 —Gd2 bond angle is 113.809 2(3)°. Furthermore, the bond distances of Gd—O in complex 2 are in a range of 0.225 0(6)-0.244 5(6) nm, and the Gd1—N3, Gd1—N4, Gd2—N1, and Gd2—N6 bond lengths are 0.255 5(8), 0.243 8(7), 0.242 6(7), and 0.254 1(8) nm, respectively. The O—Gd—O bond angles are in a range of 66.18(19)°-147.9(2)°. These bond lengths and angles of 2 are compared to those of reported Gd₂ complexes^[24-27].

  Figure 1

  

  Figure 1.  Molecular structure of complex 2 shown with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  Table 2

  

  Table 2.  Gd(Ⅲ) geometry analysis by SHAPE 2.0 for complex 2

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  Gd(Ⅲ) ion	D4d SAPR	D2d TDD	C2v JBTPR	C2v BTPR	D2d JSD
  Gd1	2.193	2.379	3.673	2.582	5.883
  Gd2	2.185	2.211	3.656	2.511	5.749
  SAPR-8=square antiprism; TDD -8=triangular dodecahedron; JBTPR - 8=biaugmented trigonal prism J50; BTPR -8=biaugmented trigonal prism; JSD-8=snub diphenoid J84; The number 8 represents eight-coordinated geometrical configuration.

  Figure 2

  

  Figure 2.  (a) Coordination mode of L^2- in 2; (b) Coordination mode of acac^- in 2

  H atoms of C—H bonds are omitted for clarity

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  2.2   PXRD pattern and TGA

  To prove the phase purities of complexes 1-3, the crystalline products of these complexes were measured by PXRD. As shown in Fig. S2, the PXRD patterns of the crystalline samples of 1-3 were in good agreement with their simulated ones, which proves the high phase purity.

  To investigate the thermal stabilities of complexes 1-3, TGA was performed under an air atmosphere. As shown in Fig.S3, the TG curves of 1-3 showed a similar variation trend. Hereon, we selected the TG curve of complex 1 for a detailed description. The weight loss of 6.31% (Calcd. 6.02%) between 40 and 294 ℃ can be attributed to the loss of two free CH₃OH. After that, complex 1 decomposed gradually.

  2.3   Magnetic properties

  The magnetic susceptibility data for the Ln^Ⅲ₂ complexes (1-3) were measured with the polycrystalline samples during a temperature range of 2.0 to 300.0 K and under an external magnetic field of 1.0 kOe. The χ_ΜT vs T plots for complexes 1-3 are shown in Fig. 3. At 300.0 K, the χ_ΜT values of 1-3 were 2.40, 15.73, and 28. 20 cm³·mol^-1·K, respectively. The expected values for two free Ln(Ⅲ) ions are shown as follows: two isolated Sm ions (6H_5/2, g=2/7) gave 0.95 cm³·mol^-1·K for 1; two isolated Gd(Ⅲ) ions (8S_7/2, g=2) gave 15.76 cm³·mol^-1·K for 2; two isolated Ho(Ⅲ) ions (5I₈, g=4/5) gave 28.14 cm³·mol^-1·K for 3. For 1, as the temperature decreased, the χ_ΜT value slowly declined and reached the minor value of 0.26 cm³·mol^-1·K at 2.0 K. For 2, during the temperature range of 20.0-300.0 K, the χ_ΜT value almos remained constant; whereafter, the χ_ΜT value dropped to a minimum of 10.83 cm³·mol^-1·K at 2.0 K. The downtrend suggests that there is an antiferromagnetic interaction between adjacent Gd(Ⅲ) ions in complex 2^[28]. For 3, the χ_ΜT value decreased slowly from 300 to 50 K, then it decreased quickly to reach the minimum of 7.01 cm³·mol^-1·K at 2.0 K. This behavior may be attributed to the thermal depopulation of the Ho(Ⅲ) Stark sublevels or/and the antiferromagnetic interactions between the adjacent Ho(Ⅲ) ions in complex 3^[29].

  Figure 3

  

  Figure 3.  Temperature dependence of χ_MT at 1.0 kOe for 1-3

  Red solid line stands for the best fitting of complex 2 by using Eq.1

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  The Curie - Weiss law was used for fitting the magnetic susceptibility of 2 (Fig.S4). Two parameters C (15.83 cm³·mol^-1·K) and θ (-0.73 K) were obtained (R =0.999 78). The small and negative θ value of 2 further suggests that there is an antiferromagnetic interaction between adjacent Gd(Ⅲ) ions in 2^[30]. For further exploring the magnetic interaction between the adjacent Gd(Ⅲ) ions in complex 2, we fitted the χ_MT vs T curve of 2 by using the Hamiltonian: ${\hat H_{{\rm{G}}{{\rm{d}}_2}}} = - J({\hat S_{{\rm{G}}{{\rm{d}}_1}}}{\hat S_{{\rm{G}}{{\rm{d}}_2}}}) - g{\mu _{\rm{B}}}\hat H({\hat S_{{\rm{G}}{{\rm{d}}_1}}} + {\hat S_{{\rm{G}}{{\rm{d}}_2}}})\;({\bf{1}})$^[31], and the two significant parameters, J=-0.06 cm^-1 and g=2.05, were obtained. The negative J value further proves that there is an antiferromagnetic coupling between the neighboring Gd(Ⅲ) ions in 2.

  The magnetization data for complex 2 were studied at 2.0 - 10.0 K in a 0-70 kOe field. As depicted in Fig. 4, the magnetization M for complex 2 rapidly increased blow 20 kOe and then steadily increased to 14.02Nβ at 70 kOe, which is very close to the saturation value of 14Nβ for two free Gd(Ⅲ) (S=7/2, g=2) ions.

  Figure 4

  

  Figure 4.  Plots of M vs H at 2.0-10.0 K for 2

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  According to the previous literature^[32-34], due to the presence of the isotropic Gd(Ⅲ) ion with a high -spin ground state, hereon, the magnetocaloric effect of complex 2 was investigated. The maximum magnetic entropy change (- ΔS_max) could be calculated by using the Maxwell equation: $\Delta {S_{\max }}(T){\rm{ }} = \int {{{[\partial M(T, H)/\partial T]}_H}{\rm{d}}H} $^[35]. As shown in Fig. 5, at ΔH=70 kOe and T=2.0 K, the observed -ΔS_max was 31.9 J·K^-1·kg^-1, which was smaller than the theoretical value of 34.2 J·K^-1·kg^-1 based on the equation: ΔS_max=2Rln(2S+1) (S_Gd=7/2 and R= 8.314 J·mol^-1·K^-1). The difference between experimental and theoretical magnetic entropy change (- ΔS_max) may be due to the weak antiferromagnetic interactions between Gd(Ⅲ) ions in complex 2^[36]. It is worth mentioning that the -ΔS_max of complex 2 was larger than those of mostly reported Gd₂ complexes^[37-39]. With the larger - ΔS_max value, complex 2 may be a good candidate for potential application in magnetocaloric materials.

  Figure 5

  

  Figure 5.  Plots of -ΔS_max vs T for 2

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  Figure 6

  

  Figure 6.  Temperature dependence of in-phase (χ') and out-of-phase (χ″) components of ac magnetic susceptibility for 3 in 0 Oe field with an oscillation of 3.0 Oe

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  In order to study the dynamic magnetic behavior of complex 3, the alternating current (ac) magnetic susceptibility measurements were performed during a temperature range of 2.0-15.0 K at various frequencies. Clear frequency dependence of out - of - phase (χ″) signals was observed which suggests that slow magnetic relaxation occurred in 3^[40]. However, no χ″ peaks were observed until T=2.0 K, and χ″ values gradually increased in the lower temperature region, which can be ascribed to the quantum tunneling effect (QTE) ^[41]. This phenomenon commonly occurred in most Ln(Ⅲ) - based complexes^[42].

  To check the quantum tunneling of magnetization (QTM) effect above 2.0 K in complex 3, under H_dc=2 500 Oe, the variable - temperature ac susceptibilities were determined. As shown in Fig. S5, remarkable and peak shapes were observed, which show that the QTE in complex 3 was pronounced, and the QTM effect was basically suppressed when it was under an external 2 500 Oe dc field. The ln τ vs T^-1 plot is shown in Fig. 7. The relaxation time τ obeys the Arrhenius law: τ = τ₀exp[ΔE/(k_BT)]. Two key parameters, energy barrier ΔE/k_B=17.99 K and pre - exponential factor τ₀=9.55× 10^-7 s, were obtained. The τ₀ of complex 3 was consistent with the reported values of 10^-6 -10^-12 s for Ln(Ⅲ) - based SMMs^[43-44].

  Figure 7

  

  Figure 7.  ln τ vs T-1 plot for complex 3

  Red solid lines represent the best fit of the experimental data to the Arrhenius law

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  3.   Conclusions

  In summary, we have synthesized three new Ln^Ⅲ₂ complexes [Ln₂(L)₂(acac)₂(CH₃OH)₂]·2CH₃OH (Ln=Sm (1), Gd (2), Ho (3)). Complexes 1-3 are all isostructural and contain a parallelogram [Ln₂O₂] core. Magnetic measurements imply that Gd₂ complex 2 displayed significant magnetic refrigeration property with a larger - ΔS_max of 31.9 J·K^-1·kg^-1 (ΔH=70 kOe and T=2.0 K); while Ho₂ complex 3 shows slow magnetic relaxation behavior.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Scheme 1  Structure of ligand H₂L

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  Figure 1  Molecular structure of complex 2 shown with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  Figure 2  (a) Coordination mode of L^2- in 2; (b) Coordination mode of acac^- in 2

  H atoms of C—H bonds are omitted for clarity

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  Figure 3  Temperature dependence of χ_MT at 1.0 kOe for 1-3

  Red solid line stands for the best fitting of complex 2 by using Eq.1

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  Figure 4  Plots of M vs H at 2.0-10.0 K for 2

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  Figure 5  Plots of -ΔS_max vs T for 2

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  Figure 6  Temperature dependence of in-phase (χ') and out-of-phase (χ″) components of ac magnetic susceptibility for 3 in 0 Oe field with an oscillation of 3.0 Oe

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  Figure 7  ln τ vs T-1 plot for complex 3

  Red solid lines represent the best fit of the experimental data to the Arrhenius law

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  Table 1.  Crystal data and structure refinement parameters for complexes 1-3

  Parameter	1	2	3
  Empirical formula	C36H52N6O12Sm2	C36H52N6O12Gd2	C36H52N6O12Ho2
  Formula weight	1 062.46	1 075.25	1 091.62
  T / K	150(2)	293(2)	153(2)
  Crystal system	Monoclinic	Monoclinic	Monoclinic
  Space group	P21/c	P21/c	P21/c
  a / nm	0.773 0(5)	0.777 6(6)	0.765 59(3)
  b / nm	2.118 3(3)	2.131 2(6)	2.117 2(3)
  c / nm	2.579 3(7)	2.594 7(1)	2.574 5(7)
  β/(°)	93.961(2)	92.993 0(15)	93.437 5(15)
  V / nm3	4.213 8(5)	4.294 6(2)	4.165 7(3)
  Z	4	4	4
  Cryst size / mm	0.25 × 0.21 × 0.14	0.25 × 0.17 × 0.11	0.27 × 0.21 × 0.14
  Dc / (g·cm-3)	1.574	1.582	1.639
  μ / mm-1	2.816	18.867	3.828
  Limiting indices	-9 ≤ h ≤ 9,	-8 ≤ h ≤ 9,	-9 ≤ h ≤ 9,
  	-26 ≤ k ≤ 26,	-18 ≤ k ≤ 26,	-26 ≤ k ≤ 26,
  	Reflection collected	-32 ≤ l ≤ 31	-27 ≤ l ≤ 32	-32 ≤ l ≤ 32
  	67 691	17 142	52 003
  Unique	8 647	8 364	8 552
  Parameter	485	485	485
  Rint	0.091 5	0.042 2	0.066 8
  GOF on F2	1.006	1.084	1.085
  R1, wR2 [I > 2σ(I)]	0.045 2, 0.100 2	0.068 6, 0.182 5	0.042 7, 0.095 1
  R1, wR2 (all data)	0.073 6, 0.113 7	0.071 3, 0.184 5	0.060 2, 0.101 7

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  Table 2.  Gd(Ⅲ) geometry analysis by SHAPE 2.0 for complex 2

  Gd(Ⅲ) ion	D4d SAPR	D2d TDD	C2v JBTPR	C2v BTPR	D2d JSD
  Gd1	2.193	2.379	3.673	2.582	5.883
  Gd2	2.185	2.211	3.656	2.511	5.749
  SAPR-8=square antiprism; TDD -8=triangular dodecahedron; JBTPR - 8=biaugmented trigonal prism J50; BTPR -8=biaugmented trigonal prism; JSD-8=snub diphenoid J84; The number 8 represents eight-coordinated geometrical configuration.

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