English

• Recently, the increasing studies for single - molecule magnets (SMMs) based on lanthanides have elucidated the important role of Ln (Ⅲ)ions in constructing new SMMs with intriguing architectures and fascinating magnetic behaviors^[1-4]. Among these Ln (Ⅲ)ions, Dy (Ⅲ)ion is widely used, because of not only its large spin magnetic moments but also its large intrinsic magnetic anisotropy. So far, some multinuclear Dy - based SMMs exhibiting excellent magnetic properties have been reported, such as Dy₂^[5], Dy₃^[6], Dy₄^[7], Dy₅^[8], Dy₆^[9], and Dy₇₆^[10]. Impressively, a hydroxide - bridged centrosymmetric Dy₂ compound synthesized by Gao group displays slow magnetic relaxation with a large anisotropy energy barrier of 721 K^[11]. Additionally, Powell et al. reported a novel triangular Dy₃ cluster with interesting SMM behaviors despite the almost non-magnetic ground state^[12]. All these excellent works stimulate us to further design and explore other Dy-based SMMs.

  In this context, we prepared a new Dy₂ complex [Dy₂(L)₂(acac)₂(CH₃CH₂OH)₂] (1) by reaction of the multi-dentate Schiff base H₂L (H₂L=N′-(2-hydroxynaphthalen-1-ylmethylene)-2-(hydroxyimino)propanehydrazide, Scheme 1) with the β - diketonate salt Dy(acac)₃·2H₂O (Hacac=acetylacetone). The Schiff base H₂L was selected as ligand due to the excellent structural features: (1) it possesses many N/O donor atoms, which can provide different coordination modes; (2) the carbonyl oxygen atom of H₂L can act as a bridge to connect different Dy (Ⅲ)ions and can also transmit magnetic exchange efficiently. Single - crystal X - ray diffraction analysis reveals that 1 is a centrosymmetric dinuclear Dy (Ⅲ) complex, each Dy (Ⅲ)ion is eight - coordinate and every L^2- adopts μ₂∶η¹η¹η²η¹ binding mode. Magnetic studies indicate that complex 1 exhibited single-molecule magnet behavior with the energy barrier ΔE/k_B=14.52 K and the pre-exponential factor τ₀=7.58×10^-6 s.

  Scheme 1

  

  Scheme 1.  Synthetic route of H₂L

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  1.   Experimental

  1.1   Materials and measurements

  All reagents and solvents were purchased from commercial sources and used as received. The β-diketonate salt (Dy(acac)₃·2H₂O) was prepared accord- ing to a procedure reported previously^[13]. Elemental analyses for C, H, and N were determined by a PerkinElmer 240 CHN analyzer. Magnetic data were obtained using a Quantum Design MPMS - XL7 magnetometer and a PPMS-9 ACMS magnetometer.

  1.2   Synthesis of ligand H₂L and complex 1

  As shown in Scheme 1, the Schiff base ligand H₂L was prepared by a simple condensation reaction of 2-hydroxy-1-naphthaldehyde (0.5 mmol) and 2-(hydroxyimino)propane hydrazide (0.5 mmol) in ethanol. The mixed solution was then stirred at room temperature for 4 h, the obtained yellow precipitates were washed with ethanol.

  Dy(acac)₃·2H₂O (0.05 mmol) and H₂L (0.05 mmol) were stirred together in a EtOH -CH₂Cl₂ mixture (15 mL+3 mL) for 4 h at room temperature and filtered. The resulting mixture was allowed to stand undisturbed and slowly concentrated by evaporation. Block - shaped yellow crystals appeared after five days. Elemental analysis Calcd. for C₄₂H₄₈Dy₂N₆O₁₂(%): C 43.72, H 4.19, N 7.28; Found(%): C 43.60, H 4.25, N 7.21.

  1.3   Single-crystal X-ray crystallography

  Single-crystal X-ray diffraction data for complex 1 were collected on a Rigaku Saturn CCD area detector diffractometer (Mo Kα radiation, λ =0.071 073 nm). The structure of 1 was solved by direct methods and refined by full-matrix least-square against F² using the SHELXL - 2018 crystallographic software package. All non - hydrogen atoms were refined anisotropically. All the H atoms were positioned geometrically and refined using a riding model. A summary of the crystal data and cell parameters for 1 is given in Table 1, and selected bond lengths (nm) and angles (°) for 1 are listed in Table 2.

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for complex 1

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  Parameter	1		Parameter	1
  Empirical formula	C42H48Dy2N6O12		Dc / (Mg·m-3)	1.719
  Formula weight	1 153.86		Absorption coefficient / mm-1	3.392
  T / K	150.15		Crystal size / mm	0.26×0.21×0.14
  Crystal system	Triclinic		F(000)	1 140
  Space group	P1		θ range for data collection / (°)	2.235-26.449
  a / nm	1.183 41(5)		Index ranges	-14 ≤ h ≤ 14, -15 ≤ k ≤ 15, -23 ≤ l ≤ 23
  b / nm	1.211 11(6)		Reflection collected	48 711
  c / nm	1.851 90(8)		Unique (Rint)	9 171 (0.042 7)
  α / (°)	96.434(2)		Data, restraint, parameter	9 171, 6, 575
  β / (°)	103.827(2)		Goodness-of-fit (GOF) on F2	1.045
  γ / (°)	116.612(2)		Final R indices [I > 2σ(I)]	R1=0.028 7, wR2=0.064 8
  V / nm3	2.229 66(18)		R indices (all data)	R1=0.043 9, wR2=0.072 9
  Z	2		Largest peak and hole / (e·nm-3)	2 083 and -673

  Table 2

  

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

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  Dy1—O2	0.235 9(3)	Dy1—O2a	0.235 3(3)	Dy1—O3	0.220 2(3)
  Dy1—O4	0.232 3(3)	Dy1—O5	0.234 0(3)	Dy1—O6	0.243 5(3)
  Dy1—N1a	0.253 9(4)	Dy1—N3	0.247 5(4)
  O2—Dy1—N1a	127.69(11)	O2a—Dy1—O2	67.14(12)	O2a—Dy1—O6	73.45(10)
  O2—Dy1—O6	76.66(10)	O2—Dy1—N3	63.73(11)	O2a—Dy1—N3	130.84(11)
  O2a—Dy1—N1a	62.10(11)	O3—Dy1—O2a	142.59(10)	O3—Dy1—O2	127.36(11)
  O3—Dy1—O6	77.31(11)	O3—Dy1—O5	77.55(12)	O3—Dy1—N3	73.24(11)
  O3—Dy1—N1a	89.43(12)	O3—Dy1—O4	127.20(11)	O4—Dy1—O2a	88.46(10)
  O4—Dy1—O2	74.66(10)	O4—Dy1—O6	150.33(11)	O4—Dy1—O5	72.35(11)
  O4—Dy1—N3	80.49(11)	O4—Dy1—N1a	114.66(12)	O5—Dy1—O2	146.79(10)
  O5—Dy1—O2a	107.97(11)	O5—Dy1—O6	135.16(10)	O5—Dy1—N3	113.64(12)
  O5—Dy1—N1a	65.45(12)	O6—Dy1—N1a	77.79(12)	O6—Dy1—N3	93.58(11)
  N3—Dy1—N1a	162.07(12)
  Symmetry code: a: -x, -y-1, -z+2.

  CCDC: 2096910.

  2.   Results and discussion

  2.1   Structure description of complex 1

  Single-crystal X-ray diffraction studies reveal that complex 1 crystallizes in the triclinic system with the space group P1 (Z=2). As shown in Fig. 1, the molecular structure of 1 is centrosymmetric and comprised of two Dy (Ⅲ)ions, two ligands (L^2-), two acac^- and two CH₃CH₂OH. Each Dy (Ⅲ)ion is eight - coordinate, connected by six oxygen atoms (O2, O2a, O3, O4, O5, and O6) and two nitrogen atoms (N1a and N3) (Fig. 2a). The eight - coordinate Dy (Ⅲ)ions are located in distorted bicapped trigonal - prismatic geometric configuration (Fig. 2b). Two adjacent Dy(Ⅲ)ions are linked by two μ₂ - O bridges (O2 and O2a) coming from the ketone oxygen atoms of two L^2- ligands. The ligand L^2- adopt μ₂∶ η¹η¹η²η¹ coordination mode (Scheme 2). The Dy1… Dy1a distance is 0.392 59(4) nm, and Dy1—O2—Dy1a angle is 112.86(11)°. The bond lengths of Dy1— O2, Dy1—O2a, Dy1—O3, Dy1—O4, Dy1—O5, and Dy1—O6 are found to be 0.235 9(3), 0.235 3(3), 0.220 2(3), 0.232 3(3), 0.234 0(3), and 0.243 5(3) nm, respectively. Furthermore, the Dy1—N3 and Dy1—N3a bond lengths are 0.247 5(4) and 0.253 9(4) nm. The angles of O—Dy—O are in a range of 67.14(12)°- 150.33(11)°.

  Figure 1

  

  Figure 1.  Molecular structure of complex 1

  Hydrogen atoms have been omitted for clarity; Symmetry code: a: -x, -y-1, -z+2

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  Figure 2

  

  Figure 2.  Core structural representation (a) and coordination polyhedra of Dy ions (b) in 1

  Symmetry code: a: -x, -y-1, -z+2

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  Scheme 2

  

  Scheme 2.  Binding mode of L^2- ligand in 1

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  2.2   Magnetic properties

  The direct - current (dc) magnetic susceptibility of complex 1 was investigated in a temperature range of 300.0 - 2.0 K under an applied magnetic field of 1 000 Oe. The χ_MT vs T plot is shown in Fig. 3. The room - temperature χ_MT value for 1 was 28.55 cm³·K·mol^-1, which was close to 28.34 cm³·K·mol^-1 of two uncoupled Dy (Ⅲ)ions (⁶H_15/2, g=4/3). Upon lowering the temperature, the χ_MT value of 1 decreased slowly from 300 to 50 K and then declined abruptly to a minimum value of 9.29 cm³·K·mol^-1 at 2 K. This decline tendency can be ascribed to the thermal depopulation of the Dy (Ⅲ) Stark sublevels and/or the weak antiferromagnetic interactions between the adjacent Dy(Ⅲ)ions in 1.

  Figure 3

  

  Figure 3.  Plot of χ_MT vs T at 1 000 Oe for complex 1

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  To deeply study the dynamic magnetic behaviors of complex 1, the alternating - current (ac) susceptibilities were measured at various frequencies under a zerodc field. Interestingly, both in - phase (χ′) and out - of - phase (χ″) ac magnetic susceptibilities data display frequency dependence under the zero - dc field and obvious peaks in the χ″ signals can be detected (Fig. 4), which is typical features of SMMs behavior. To further investigate the dynamic magnetic behaviors of 1, the frequency - dependent susceptibility of 1 was also measured. As shown in Fig. 5, the χ′ and χ″ signals both show obvious temperature dependences, further confirming the presence of slow relaxation of the magnetization in 1. Notably, two peaks in the χ″ signals can be observed in the low - temperature zone, suggesting two relaxation processes may exist in 1. The Cole - Cole plots for 1 derived from the plots of χ″ vs χ′ are shown in Fig. 6, which could be fitted based on the generalized Debye model, giving the parameter α =0.05 - 0.54. But some Cole - Cole plots exhibited two approximate semicircles and cannot be well fitted. The relaxation time τ data extracted from the higher temperature χ″ peaks can be well fitted with the Arrhenius law τ = τ₀exp[ΔE/(k_BT)]^[14], yielding the effective energy barrier ΔE/k_B=14.52 K and pre - exponential factor τ₀=7.58×10^-6 s (Fig. 7). The obtained τ₀ was in the normal range of the reported Dy-based SMMs^[15]. By contrast with some previously reported Dy₂ complexes constructed based on Schiff base ligands, the obtained value of ΔE/k_B for 1 is larger than those of some Dy₂ compounds (Table 3). The diverse magnetic properties in them can be attributed to the different coordinated Schiff bases and β - diketone co-ligands, which inspire us to further explore the relationship between the structures and magnetic properties of lanthanide SMMs.

  Figure 4

  

  Figure 4.  Temperature dependence of in-phase (a) and out-of-phase (b) ac magnetic susceptibilities for 1 in zero-dc field

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  Figure 5

  

  Figure 5.  Frequency dependence of in-phase (a) and out-of-phase (b) ac magnetic susceptibility for 1 in zero-dc field

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  Figure 6

  

  Figure 6.  Cole-Cole plots for complex 1 at T=2.0-10.0 K

  Solid lines indicate the best fits to the experiments with the generalized Debye model

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  Figure 7

  

  Figure 7.  Plot of ln τ vs T^-1 for 1

  Solid red line is the best fit of the Arrhenius law

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  Table 3

  

  Table 3.  Magnetic relaxation barriers and pre-exponential factors for selected Dy₂ complex under a zero-dc field

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  Complex	ΔE/kB / K	τ0 / s	Ref.
  [Dy2(L1)2(hfac)6]·C7H16	0.72	6.28×10-5	[16]
  [Dy2(dbm)4(H2L2)(CH3O)(CH3OH)]	0.87	1.3×10-5	[17]
  [Dy2(hfac)4(L3)2]	6.77	9.12×10-6	[18]
  [Dy2(L4)2(hfac)6]·C7H16	6.81	2.56×10-5	[16]
  [Dy2(hfac)4(L5)2]	9.91	1.99×10-6	[19]
  [Dy2(L)2(acac)2(CH3CH2OH)2]	14.52	7.58×10-6	This work
  [Dy2(dbm)5(H2L2)]	18.68	1.1×10-6	[17]
  [Dy2(tfac)4(L3)2]	19.83	7.62×10-8	[18]
  [Dy2(hfac)4(L6)2]	20.57	2.71×10-6	[19]
  [Dy2(dbm)4(L7)2]	22.04	2.204×10-4	[20]
  HL1=5-(2-thenylidene)-8-hydroxylquinoline; H3L2=(2E, N′E)-2-(hydroxyimino)-N′-(pyridin-2-plmethylene)propanehydrazide; HL3=2-(((4-fluorophenyl)imino) methyl)-8-hydroxyquinoline; HL4=5-(4-methoxylbenzylidene)-8-hydroxyquinoline; HL5=2-((4-methylaniline-imino)methyl)-8-hydroxyquinoline; HL6=2-(((3, 4-dimethylaniline)-imino)methyl)-8-hydroxyquinoline; HL7=2-(((4-fluorophenyl)imino)methyl)-8-hydroxyquinoline.

  3.   Conclusions

  In summary, one new Dy₂ complex 1 was successfully obtained by the reaction of Dy³⁺ ions and a Schiff base ligand. The structures and magnetic properties of 1 were investigated in detail. Structural analysis shows that 1 contains a centrosymmetric dinuclear dysprosium center. Magnetic investigations reveal that slow magnetic relaxation behaviors can be observed in 1 with the effective energy barrier ΔE/k_B=14.52 K and the pre-exponential factor τ₀=7.58×10^-6 s.

  
  
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• 

  Scheme 1  Synthetic route of H₂L

  下载: 全尺寸图片 幻灯片
  

  Figure 1  Molecular structure of complex 1

  Hydrogen atoms have been omitted for clarity; Symmetry code: a: -x, -y-1, -z+2

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  Figure 2  Core structural representation (a) and coordination polyhedra of Dy ions (b) in 1

  Symmetry code: a: -x, -y-1, -z+2

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  Scheme 2  Binding mode of L^2- ligand in 1

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  Figure 3  Plot of χ_MT vs T at 1 000 Oe for complex 1

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  Figure 4  Temperature dependence of in-phase (a) and out-of-phase (b) ac magnetic susceptibilities for 1 in zero-dc field

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  Figure 5  Frequency dependence of in-phase (a) and out-of-phase (b) ac magnetic susceptibility for 1 in zero-dc field

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  Figure 6  Cole-Cole plots for complex 1 at T=2.0-10.0 K

  Solid lines indicate the best fits to the experiments with the generalized Debye model

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  Figure 7  Plot of ln τ vs T^-1 for 1

  Solid red line is the best fit of the Arrhenius law

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  Table 1.  Crystal data and structure refinement parameters for complex 1

  Parameter	1		Parameter	1
  Empirical formula	C42H48Dy2N6O12		Dc / (Mg·m-3)	1.719
  Formula weight	1 153.86		Absorption coefficient / mm-1	3.392
  T / K	150.15		Crystal size / mm	0.26×0.21×0.14
  Crystal system	Triclinic		F(000)	1 140
  Space group	P1		θ range for data collection / (°)	2.235-26.449
  a / nm	1.183 41(5)		Index ranges	-14 ≤ h ≤ 14, -15 ≤ k ≤ 15, -23 ≤ l ≤ 23
  b / nm	1.211 11(6)		Reflection collected	48 711
  c / nm	1.851 90(8)		Unique (Rint)	9 171 (0.042 7)
  α / (°)	96.434(2)		Data, restraint, parameter	9 171, 6, 575
  β / (°)	103.827(2)		Goodness-of-fit (GOF) on F2	1.045
  γ / (°)	116.612(2)		Final R indices [I > 2σ(I)]	R1=0.028 7, wR2=0.064 8
  V / nm3	2.229 66(18)		R indices (all data)	R1=0.043 9, wR2=0.072 9
  Z	2		Largest peak and hole / (e·nm-3)	2 083 and -673

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

  Dy1—O2	0.235 9(3)	Dy1—O2a	0.235 3(3)	Dy1—O3	0.220 2(3)
  Dy1—O4	0.232 3(3)	Dy1—O5	0.234 0(3)	Dy1—O6	0.243 5(3)
  Dy1—N1a	0.253 9(4)	Dy1—N3	0.247 5(4)
  O2—Dy1—N1a	127.69(11)	O2a—Dy1—O2	67.14(12)	O2a—Dy1—O6	73.45(10)
  O2—Dy1—O6	76.66(10)	O2—Dy1—N3	63.73(11)	O2a—Dy1—N3	130.84(11)
  O2a—Dy1—N1a	62.10(11)	O3—Dy1—O2a	142.59(10)	O3—Dy1—O2	127.36(11)
  O3—Dy1—O6	77.31(11)	O3—Dy1—O5	77.55(12)	O3—Dy1—N3	73.24(11)
  O3—Dy1—N1a	89.43(12)	O3—Dy1—O4	127.20(11)	O4—Dy1—O2a	88.46(10)
  O4—Dy1—O2	74.66(10)	O4—Dy1—O6	150.33(11)	O4—Dy1—O5	72.35(11)
  O4—Dy1—N3	80.49(11)	O4—Dy1—N1a	114.66(12)	O5—Dy1—O2	146.79(10)
  O5—Dy1—O2a	107.97(11)	O5—Dy1—O6	135.16(10)	O5—Dy1—N3	113.64(12)
  O5—Dy1—N1a	65.45(12)	O6—Dy1—N1a	77.79(12)	O6—Dy1—N3	93.58(11)
  N3—Dy1—N1a	162.07(12)
  Symmetry code: a: -x, -y-1, -z+2.

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  Table 3.  Magnetic relaxation barriers and pre-exponential factors for selected Dy₂ complex under a zero-dc field

  Complex	ΔE/kB / K	τ0 / s	Ref.
  [Dy2(L1)2(hfac)6]·C7H16	0.72	6.28×10-5	[16]
  [Dy2(dbm)4(H2L2)(CH3O)(CH3OH)]	0.87	1.3×10-5	[17]
  [Dy2(hfac)4(L3)2]	6.77	9.12×10-6	[18]
  [Dy2(L4)2(hfac)6]·C7H16	6.81	2.56×10-5	[16]
  [Dy2(hfac)4(L5)2]	9.91	1.99×10-6	[19]
  [Dy2(L)2(acac)2(CH3CH2OH)2]	14.52	7.58×10-6	This work
  [Dy2(dbm)5(H2L2)]	18.68	1.1×10-6	[17]
  [Dy2(tfac)4(L3)2]	19.83	7.62×10-8	[18]
  [Dy2(hfac)4(L6)2]	20.57	2.71×10-6	[19]
  [Dy2(dbm)4(L7)2]	22.04	2.204×10-4	[20]
  HL1=5-(2-thenylidene)-8-hydroxylquinoline; H3L2=(2E, N′E)-2-(hydroxyimino)-N′-(pyridin-2-plmethylene)propanehydrazide; HL3=2-(((4-fluorophenyl)imino) methyl)-8-hydroxyquinoline; HL4=5-(4-methoxylbenzylidene)-8-hydroxyquinoline; HL5=2-((4-methylaniline-imino)methyl)-8-hydroxyquinoline; HL6=2-(((3, 4-dimethylaniline)-imino)methyl)-8-hydroxyquinoline; HL7=2-(((4-fluorophenyl)imino)methyl)-8-hydroxyquinoline.

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