A Class of Asymmetric Dinuclear Dysprosium Single-Molecule Magnets Based on Bisacylhydrazone, Their Synthesis Methods and Relaxation Behaviors

Abstract

Based on the excellent magnetism, synthetic difficulty and existing scarcity of asymmetric dinuclear dysprosium single-molecule magnets, a class of asymmetric dinuclear dysprosium single-molecule magnets based on bis-acylhydrazone ligands, their synthesis methods and relaxation behaviors are disclosed. Such magnets are prepared by solvothermal reaction of bis(halogen-substituted salicylaldehyde) -2,6-azoxypyridine dicarboxylic acid hydrazone ligand, dysprosium nitrate and triethylamine in a methanol-acetonitrile mixed solvent. Their common structural feature is that the acylhydrazone pockets on both wings of the ligand chelate one Dy<supgt;III< / supgt; ion respectively, and due to the difference in the ligand coordination sites of counter ions and solvent molecules, the coordination geometries of the two Dy<supgt;III< / supgt> ions in the magnet are different, showing a rare asymmetric dinuclear structure based on symmetry bipolar ligands. The alternating current susceptibility of such asymmetric dinuclear dysprosium magnets exhibits frequency-dependent magnetic relaxation phenomena even in zero field, and turns into a double relaxation process under an external magnetic field, possessing the typical characteristics of single-molecule magnets.

Description

Technical Field

[0001] The present invention relates to the preparation of novel molecular-based magnetic materials, in particular to a class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone, their synthesis methods and relaxation behaviors. Background Art

[0002] Single-molecule magnets are a novel type of molecular-based magnetic materials that can exhibit magnetic bistability and slow magnetic relaxation behaviors at the molecular level. Especially when using dysprosium ions with strong anisotropy as spin carriers, they can exhibit excellent magnetic properties and interesting magnetic relaxation behaviors, and have great research value and application prospects in fields such as high-density information storage and quantum computing. Dysprosium-based single-molecule magnets can be divided into mononuclear dysprosium single-ion magnets, dinuclear dysprosium single-molecule magnets, and polynuclear dysprosium single-molecule magnets according to the number of their spin centers. Among them, dinuclear dysprosium single-molecule magnets not only facilitate the definition and optimization of the single-ion magnetic anisotropy of dysprosium centers through the ligand field, but also can regulate the magnetic exchange interaction between dysprosium ions through the bridging structure of ligands, thereby effectively suppressing the quantum tunneling effect in the system, and having unique structural advantages in the optimization of relaxation behaviors and performance parameters.

[0003] Overall, the vast majority of existing dinuclear dysprosium single-molecule magnets have a symmetric structure, that is, their two dysprosium centers are located in the same ligand field and have the same coordination configuration. Only a very small number of systems have their two dysprosium centers located in different ligand fields and have different coordination geometries. Interestingly, this kind of system called "asymmetric dinuclear dysprosium single-molecule magnet" often exhibits special relaxation behaviors due to its specific asymmetric structure, and has excellent performances in performance parameters such as effective energy barrier and blocking temperature. However, due to the difficulty in synthesis, such asymmetric dinuclear dysprosium single-molecule magnets are still very rare at present. Therefore, it is not only necessary but also very meaningful to explore the synthesis methods and relaxation behaviors of such asymmetric dinuclear dysprosium single-molecule magnets. Summary of the Invention

[0004] In view of the synthesis difficulty and current scarcity of the above-mentioned asymmetric dinuclear dysprosium single-molecule magnets, the present invention provides a class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone, their synthesis methods and relaxation behaviors.

[0005] A class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone, including 2 specific examples. It is characterized in that their chemical compositions are [Dy2(H2fovpho)(NO3)4(MeOH)3(H2O)]·2MeOH·MeCN (Example 1) and [Dy2(H2covpho)(NO3)4(MeOH)3(H2O)]·2MeCN (Example 2) respectively, where H4fovpho and H4covpho represent bis(3-fluorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic acid hydrazone and bis(5-chlorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic acid hydrazone ligands respectively. Example 1 crystallizes in the monoclinic space group P21 / c, and its unit cell parameters are: α = 90°, β = 100.6650(1)°, γ = 90°; Z = 4, ρ calc (g / cm 3 ) = 1.991 g·cm –3 , μ = 3.675 mm –1 , F(000) = 2440.0. And Example 2 crystallizes in the triclinic system, space group, and its unit cell parameters are: α = 82.509(2)°, β = 71.766(2)°, γ = 76.1870(1)°;

[0006] Z = 2, ρ calc (g / cm 3 ) = 1.918 g·cm –3 , μ = 3.625 mm –1 , F(0

[0007] 00) = 1224.0. Detailed crystallographic data are shown in Table 1 below:

[0008] Table 1. Crystallographic parameters of the magnet

[0009]

[0010] A synthesis method of a class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone, and its specific steps are as follows:

[0011] (1) Weigh pyridine-2,6-dicarboxylic acid hydrazide (0.844 g, 4 mmol), and disperse it fully in 70 mL of methanol solution. Under the reflux condition at 80 °C, slowly add the solution of 3-fluorosalicylaldehyde or 5-chlorosalicylaldehyde (8 mmol) dissolved in 10 mL of methanol to the methanol solution of pyridine-2,6-dicarboxylic acid hydrazide. After refluxing the reaction solution for 12 h, let it stand and cool to room temperature. Filter the pale yellow solid precipitated in the solution by suction, wash it with cold methanol, and dry it at room temperature to obtain the bis(3-fluorosalicylaldehyde)pyridine-2,6-dicarboxylic acid hydrazone ligand (H4fovpho) or bis(5-chlorosalicylaldehyde)pyridine-2,6-dicarboxylic acid hydrazone ligand (H4covpho).

[0012] (2) Weigh the ligand H4fovpho (0.0220 g, 0.05 mmol) and Dy(NO3)3·6H2O (0.06849 g, 0.15 mmol), and place them in a 20 mL vial. Then add 5 mL of methanol, 1.5 mL of acetonitrile and sodium hydroxide, stir at room temperature for 6 h and then seal. Place it in an oven at 75 °C for reaction for 48 h, and then cool it to room temperature at a cooling rate of 5 °C / hour. Orange rectangular crystals of Example 1 will precipitate at the bottom of the vial.

[0013] (3) Weigh the ligand H4covpho (0.0227 g, 0.05 mmol) and Dy(NO3)3·6H2O (0.2283 g, 0.5 mmol) and place them in a 10 mL vacuum solvent tube. Then add 1 mL of methanol, 0.5 mL of acetonitrile and 3 drops of triethylamine, and shake well to mix evenly. After vacuum sealing the solvent tube, place it in an oven at 80 °C for reaction for 72 h. Then cool it to room temperature at a cooling rate of 10 °C / hour to obtain the red square crystals of Example 2.

[0014] A class of asymmetric dinuclear dysprosium single-molecule magnets based on bis-hydrazone, and the spatial structure characteristics of its molecules are as follows:

[0015] Its molecule consists of 2 Dy III ions, 1 partially deprotonated ligand (H2fovpho 2- or H2covpho 2- ), 4 NO3 - ions, 3 coordinated methanol molecules and 1 coordinated water molecule. The 2 Dy III ions are respectively located in the coordination pockets on both wings of the ligand hydrazone. There are 2 NO3 III ions, 1 coordinated methanol molecule and 1 coordinated water molecule on one Dy - ion. And the other Dy III ion is connected with 2 NO3 -ions and two coordinated methanol molecules. Therefore, these molecules all exhibit the rare asymmetric dinuclear structure with two Dy III centers in different coordination fields.

[0016] The advantages of the present invention are as follows:

[0017] Based on the excellent magnetic properties of the asymmetric dinuclear dysprosium single-molecule magnet, especially in view of its scarcity caused by difficult synthesis, through the exploration of synthesis conditions, a class of asymmetric dinuclear dysprosium single-molecule magnets is provided, as well as a method for effectively synthesizing such molecules using bis-acylhydrazone ligands. Therefore, the present invention is innovative both in the type of molecules provided and the synthesis method. Brief Description of the Drawings

[0018] Figure 1 It is the molecular structure diagram of the bis(3-fluorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic acid hydrazone ligand used in the present invention.

[0019] Figure 2 It is the molecular structure diagram of Example 1 described in the present invention.

[0020] Figure 3 It is the variable-temperature magnetic susceptibility diagram of Example 1 described in the present invention.

[0021] Figure 4 It is the magnetization intensity curve diagram of Example 1 described in the present invention.

[0022] Figure 5 It is the AC magnetic susceptibility diagram of Example 1 described in the present invention under zero field.

[0023] Figure 6 It is the Cole-Cole diagram of Example 1 described in the present invention under zero field.

[0024] Figure 7 It is the AC magnetic susceptibility diagram of Example 1 described in the present invention under an external field of 2.0 kOe.

[0025] Figure 8 It is the Cole-Cole diagram of Example 1 described in the present invention under an external field of 2.0 kOe.

[0026] Figure 9 It is the molecular structure diagram of the bis(5-chlorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic acid hydrazone ligand used in the present invention.

[0027] Figure 10 It is the molecular structure diagram of Example 2 described in the present invention.

[0028] Figure 11 It is the variable-temperature magnetic susceptibility diagram of Example 2 described in the present invention.

[0029] Figure 12Magnetization intensity curve graph of Example 2 of the present invention.

[0030] Figure 13 AC susceptibility graph of Example 2 of the present invention under zero field.

[0031] Figure 14 Cole-Cole graph of Example 2 of the present invention under zero field.

[0032] Figure 15 AC susceptibility graph of Example 2 of the present invention under an external magnetic field of 0.4 kOe.

[0033] Figure 16 Cole-Cole graph of Example 2 of the present invention under an external magnetic field of 0.4 kOe.

[0034] Figure 17 Molecular structure comparison graph of Example 1 and Example 2 of the present invention. Detailed implementation method

[0035] Example 1:

[0036] 1. Preparation method of an asymmetric dinuclear dysprosium single-molecule magnet based on bis(3-fluorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic hydrazone, and its specific steps are as follows:

[0037] (1) Weigh 2,6-azoxypyridine dicarboxylic hydrazide (0.844 g, 4 mmol) into a flask containing 70 mL of methanol solution. Under reflux conditions at 80 °C, slowly drip a solution of 3-fluorosalicylaldehyde (1.121 g, 8 mmol) dissolved in 10 mL of methanol into the above flask. As the reaction proceeds, a large amount of light yellow solid begins to form in the solution. After continuing the reaction for 12 h, let it stand and cool to room temperature. A large amount of light yellow solid precipitates in the solution. Filter by suction, wash with cold methanol, and then dry at room temperature by suction to obtain a light yellow solid of bis(3-fluorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic hydrazone (H4fovpho). The molecular structure of this H4fovpho is shown in the appendix Figure 1 .

[0038] (2) Weigh Dy(NO3)3·6H2O (0.06849 g, 0.15 mmol) and H4fovpho (0.0220 g, 0.05 mmol), place them in a 20 mL vial, and then sequentially add 5 mL of methanol, 1.5 mL of acetonitrile, and 0.004 g of sodium hydroxide. Stir at room temperature for 6 h, then seal it and transfer it to an oven at 75 °C for reaction for 48 h. After the reaction is completed, cool it down to room temperature at a rate of 5 °C / hour in a gradient manner. Orange rectangular crystals of an asymmetric dinuclear dysprosium single-molecule magnet based on the bis(3-fluorosalicylaldehyde) azoxypyridine-2,6-dicarboxylic hydrazone ligand are obtained at the bottom of the vial. Calculated based on the ligand, its yield is approximately 54%.

[0039] 2. Crystal Structure Characterization of an Asymmetric Binuclear Dysprosium Single-Molecule Magnet Based on Bis(3-fluorosalicylaldehyde)azineoxypyridine-2,6-dicarboxylic Hydrazone:

[0040] Using a Bruker Smart Apex CCD single-crystal diffractometer, graphite-monochromated Cu-Kα radiation was used to collect single-crystal X-ray diffraction data of this magnet in the ω-scan mode. All data were corrected for empirical absorption. The structure of the complex was solved by the heavy-atom method, refined by full-matrix least-squares method after hybrid hydrogenation. The calculations were carried out on a PC using the SHELXS and SHELXL programs incorporated in the Olex2 software.

[0041] It can be seen from Figure 2 that this asymmetric binuclear dysprosium single-molecule magnet based on bis(3-fluorosalicylaldehyde)azineoxypyridine-2,6-dicarboxylic hydrazone consists of 2 Dy III ions, 1 H2fovpho 2- ligand, 4 NO3 - ions, 3 methanol molecules, and 1 water molecule. The 2 Dy III ions are located in the chelating pockets on both wings of the hydrazone of the H2fovpho 2- ligand respectively, and are connected by the azineoxypyridine moiety of the ligand, with a distance of Although both show a nine-coordinate single-capped square antiprismatic geometry, due to the differences in their coordinating atoms, an asymmetric binuclear structure is presented.

[0042] 3. Static Magnetic Behavior of an Asymmetric Binuclear Dysprosium Single-Molecule Magnet Based on Bis(3-fluorosalicylaldehyde)azineoxypyridine-2,6-dicarboxylic Hydrazone:

[0043] Figure 3 is the temperature-dependent magnetic susceptibility of this magnet measured at an external field of 1.0 kOe in the temperature range of 1.8 - 300 K. Its χ M T value at room temperature is 29.78 cm 3 K mol -1 , which is close to the corresponding theoretical value (28.34 cm III Kmol 3 ) of 2 independent Dy -1 ions. As the temperature decreases, the χ M T value begins to decrease and finally reaches a minimum value of 19.38 cm 3 K mol -1 at 2 K.

[0044] Figure 4It is the magnetization intensity diagram of the magnet at different temperatures. In the low-field region, the magnetization intensity (M) of the magnet increases significantly with the increase of the external field (H). When the external field is further increased, the growth rate of the magnetization intensity becomes relatively gentle. Finally, its magnetization intensity (M) reaches the maximum value of 9.07 Nμ at 1.8 K and 7 T. B In addition, the magnetization intensity curves of the magnet at each temperature do not overlap.

[0045] 4. Dynamic magnetic behavior of an asymmetric dinuclear dysprosium single-molecule magnet based on bis(3-fluorosalicylaldehyde)azoxypyridine-2,6-diformylhydrazone:

[0046] Figure 5 It is the AC susceptibility diagram of the magnet measured at zero field and a temperature range of 1.8 - 8 K. The real part (χ′) and the imaginary part (χ″) of its AC susceptibility both show obvious dependence on frequency within the measured temperature range, but no peaks are observed. Figure 6 It is the Cole-Cole diagram obtained by fitting the AC susceptibility data based on the Debye model. On this basis, using the Arrhenius formula to fit the ln(τ)-T -1 data of the relaxation process, the effective energy barrier (U eff ) of the magnet at zero field can be obtained as 3.59 cm -1 , and the relaxation time (τ0) is 4.24×10 -6 s.

[0047] Figure 7 It is the AC susceptibility diagram of the magnet measured at an external magnetic field of 2.0 kOe and a temperature range of 1.8 - 12 K. Compared with the AC susceptibility measured at zero field above, the frequency dependence of the real part (χ′) and the imaginary part (χ″) of the magnet at an external field of 2.0 kOe is more obvious. At the same time, signals appear in both the high and low frequency regions of its imaginary part, showing the characteristics of dual magnetic relaxation behavior with both fast and slow relaxation processes. The Cole-Cole diagram ( Figure 8 ) obtained by fitting the AC susceptibility data according to the Debye model further verifies the field-induced dual relaxation behavior of the magnet. Using the Arrhenius formula to fit the ln(τ)-T -1 data of the fast and slow relaxation processes respectively, the effective energy barrier (U eff ) of the slow relaxation process can be obtained as 44.58 cm -1 , and the relaxation time (τ0) is 1.47×10 -7 s; for the fast relaxation process, its effective energy barrier (U eff ) is 32.26 cm -1 , and the relaxation time (τ0) is 2.59×10 -5 s.

[0048] Example 2:

[0049] 1. Preparation method of an asymmetric dinuclear dysprosium single-molecule magnet based on bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone, and the specific steps are as follows:

[0050] (1) Weigh azoxypyridine-2,6-dicarbohydrazide (0.844 g, 4 mmol) and dissolve it in a round-bottom flask containing 70 mL of methanol. Under the condition of refluxing in a water bath at 80 °C, slowly add a methanol solution (10 mL) of 5-chlorosalicylaldehyde (1.329 g, 8 mmol) to the above methanol solution of azoxypyridine-2,6-dicarbohydrazide. As the reaction proceeds, a large amount of yellow solid begins to form in the colorless solution. After continuing to reflux for 12 h, let it stand and cool to room temperature. Filter the yellow solid by suction, wash it with cold methanol, and then continue to dry it by suction to obtain a solid of light yellow bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone (H4covpho). The molecular structure of this H4covpho ligand is shown in the appendix Figure 9 .

[0051] (2) Weigh Dy(NO3)3·6H2O (0.228 g, 0.5 mmol) and H4covpho (0.0227 g, 0.05 mmol) according to a molar equivalent of 10:1, place them in a 10 mL vacuum solvent tube, then add 1 mL of methanol, 0.5 mL of acetonitrile, and 3 drops of triethylamine, and shake to mix them evenly. Then use an oil pump to evacuate the solvent tube and seal it. After that, transfer it to an oven at 80 °C and react for 72 h, and cool it down to room temperature at a rate gradient of 10 °C / hour. Red square crystals of an asymmetric dinuclear dysprosium single-molecule magnet based on bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone will precipitate at the bottom of the tube.

[0052] 2. Crystal structure characterization of an asymmetric dinuclear dysprosium single-molecule magnet based on bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone:

[0053] The single-crystal structure diffraction data of this magnet was collected using a Bruker Smart Apex CCD single-crystal diffractometer with graphite-monochromated Cu-Kα radiation in -ω scan mode. All data were corrected for empirical absorption. The structure of the complex was solved by the heavy-atom method, hydrogen atoms were added by mixing, and the structure was refined by full-matrix least-squares method. The calculation work was completed on a PC using the SHELXS and SHELXL programs embedded in the Olex2 software.

[0054] As Figure 10 can be seen, the chemical composition of this magnet includes 2 Dy III ions, 1 partially deprotonated H2covpho 2-Ligand, four terminally coordinated NO3 - ions, three methanol molecules and one water molecule. H2covpho 2- The ligand coordinates in a μ-η 1 :η 1 :η 1 :η 1 :η 1 :η 1 coordination mode, using the acylhydrazone wings coordination groups to chelate two Dy III ions respectively. The two Dy III ions are connected by the pyridine-N-oxide parent of H2covpho 2- ligand, with a distance of Among them, the Dy1 ion is completed to its nine-coordinate three-capped trigonal prism geometry by two NO3 - ions, one methanol molecule and one water molecule. On the Dy2 ion, there are two terminally coordinated NO3 - ions and two methanol molecules, presenting a nine-coordinate Muffin geometry. Therefore, this magnet presents an asymmetric binuclear structure.

[0055] 3. Static magnetic behavior of the asymmetric binuclear dysprosium single-molecule magnet based on bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone:

[0056] Figure 11 is the temperature-dependent magnetic susceptibility measured for this magnet under an external field of 1.0 kOe in the temperature range of 2 - 300 K. Its χ M T value at room temperature is 30.3 cm 3 K mol -1 , which is close to the corresponding theoretical value (28.34 cm III Kmol 3 of two independent Dy -1 ions. As the temperature decreases, the χ M T value begins to decrease and finally reaches the lowest value of 23.43 cm 3 K mol -1 at 2 K.

[0057] Figure 12 is the magnetization intensity diagram of this magnet at different temperatures. The magnetization intensity of this magnet increases rapidly with the increase of the external field at low fields, and the growth rate slows down relatively after 1 T, and finally reaches the maximum value of 11.68 Nμ B at 1.8 K and 7 T. Moreover, the magnetization intensity curves at each temperature do not coincide.

[0058] 4. Dynamic magnetic behavior of the asymmetric binuclear dysprosium single-molecule magnet based on bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarbohydrazone:

[0059] Figure 13 The AC susceptibility graph of this magnet was measured under zero field. Its real part (χ′) and imaginary part (χ″) both show a dependence on frequency within the temperature range of 1.8 - 8 K during the test, but no peak appears. Figure 14 Shown is the Cole - Cole graph obtained by fitting the AC susceptibility data based on the Debye model. According to the data obtained from the fitting, the Arrhenius formula is used to fit the ln(τ)-T -1 data, and the effective energy barrier (U eff ) and relaxation time (τ0) of this magnet under zero field are 0.32 cm -1 and 1.28×10 -4 s, respectively.

[0060] As Figure 15 shown, under an external field of 0.4 kOe, the real part (χ′) and imaginary part (χ″) of this magnet show a more obvious frequency - dependent behavior. Moreover, the imaginary part of the AC susceptibility has a peak in the low - frequency region, and a rising signal appears at the same time in the high - frequency region. This indicates that this magnet has field - induced double relaxation behavior. The obvious double - semicircular Cole - Cole graph ( Figure 16 ) further verifies this double relaxation behavior. On this basis, the Arrhenius formula is used to fit the ln(τ)-T -1 data of the slow and fast relaxation processes respectively, and the following parameters can be obtained: the effective energy barrier (U eff ) and relaxation time (τ0) of the slow relaxation process are 40.09 cm -1 and 4.83×10 -6 s respectively; while the effective energy barrier (U eff ) and relaxation time (τ0) of the fast relaxation process are 0.22 cm -1 and 2.81×10 -5 s respectively.

Claims

1. A class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone, characterized in that, The chemical formulas of the two included examples are: [Dy2(H2fovpho)(NO3)4(MeOH)3(H2O)]·2MeOH·MeCN (Example 1) and [Dy2(H2covpho)(NO3)4(MeOH)3(H2O)]·2MeCN (Example 2); where H4fovpho and H4covpho represent bis(3-fluorosalicylaldehyde)azoxypyridine-2,6-dicarboxylic acid hydrazone and bis(5-chlorosalicylaldehyde)azoxypyridine-2,6-dicarboxylic acid hydrazone ligands respectively; the molecular structures of H4fovpho and H4covpho are as follows:

2. The class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone according to claim 1, characterized in that, Example 1 crystallizes in the space group P21 / c of the monoclinic system, and its unit cell parameters are as follows: α = 90°, β = 100.6650(1)°, γ = 90°; Z = 4, ρ calc (g / cm 3 ) = 1.991 g·cm –3 , μ = 3.675 mm –1 , F(000) = 2440.0; Example 2 crystallizes in the space group of the triclinic system, and its unit cell parameters are as follows: α = 82.509(2)°, β = 71.766(2)°, γ = 76.1870(1)°; Z = 2, ρ calc (g / cm 3 ) = 1.918 g·cm –3 , μ = 3.625 mm –1 , F(000) = 1224.

0.

3. A class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone according to claim 1, characterized in that, The molecular structural features of the two examples are as follows: The ligand chelates two Dy ions with its acylhydrazone wing groups respectively; III Two NO3 ions, one methanol molecule and one water molecule are contained on the Dy ion on one side, while two NO3 ions and two methanol molecules are contained on the Dy ion on the other side; - Two NO3 ions, one methanol molecule and one water molecule are contained on the Dy ion on one side, while two NO3 ions and two methanol molecules are contained on the Dy ion on the other side; - Thus, the molecule presents an asymmetric binuclear structure with different coordination geometries of the two Dy ions.

4. A synthesis method of Example 1 in a class of bisacylhydrazone-based asymmetric dinuclear dysprosium single-molecule magnets as described in any one of claims 1-3, characterized in that The specific process is as follows: Weigh 0.15 mmol of Dy(NO3)3·6H2O and 0.05 mmol of H4fovpho ligand, place them in a 20 mL reactor, and then sequentially add 5 mL of methanol, 1.5 mL of acetonitrile and 0.004 g of sodium hydroxide; stir at room temperature for 6 h and then seal it, transfer it to an oven at 75 °C and react for 48 h; after the reaction is completed, cool it down to room temperature at a rate of 5 °C / hour.

5. The synthesis method according to claim 4: The reactor is a microbottle, the molar ratio of H4fovpho to Dy(NO3)3·6H2O is 3 / 1; the volume ratio of methanol to acetonitrile is 10:3, and the amount of sodium hydroxide used as the deprotonating base is 0.004 g.

6. A synthesis method of Example 2 in a class of bisacylhydrazone-based asymmetric dinuclear dysprosium single-molecule magnets as described in any one of claims 1-3, characterized in that The specific process is as follows: Weigh 0.5 mmol of Dy(NO3)3·6H2O (and 0.05 mmol of H4covpho ligand, place them in a 10 mL reactor, add 1 mL of methanol, 0.5 mL of acetonitrile and 3 drops of triethylamine, shake to mix them evenly; then use an oil pump to evacuate the solvent tube and seal it; then transfer it to an oven at 80 °C and react for 72 h, and cool it down to room temperature at a rate of 10 °C / hour.

7. The synthesis method according to claim 6: The reactor is a vacuum solvent tube, the molar ratio of H4covpho to Dy(NO3)3·6H2O is 1:10; the volume ratio of methanol to acetonitrile is 2:1, and the amount of triethylamine used as the deprotonating base is 3 drops.

8. The class of asymmetric dinuclear dysprosium single-molecule magnets based on bisacylhydrazone according to claim 1, characterized in that: Its ac susceptibility shows frequency-dependent single-molecule magnet behavior at zero field; And under an applied dc field, double relaxation behaviors are presented.

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