English

• 0.   Introduction

  Single-molecule magnets (SMMs), which are complexes presenting slow relaxation of magnetization below certain temperatures, offer the fascinating possibility of constructing switchable, molecular - based devices that manipulate or store information on tuning their molecular spin^[1-2]. Furthermore, owing to their possible applications in high - density data storage devices, spintronics and quantum computing, SMMs have become one of the most broadly studied fields among coordination compounds^{[1, 3-6]}. Even though thousands of SMMs with distinct topologies and magnetic properties have been described by the researchers worldwide, SMMs are still far from practical implementations^[7-13]. Fortunately, very recently the scientists have greatly succeeded to set the new records for effective energy barrier (U_eff) and blocking temperature (T_B) on lanthanide - based single - ion magnets (SIMs). For example, SIMs of pentagonal bipyramidal geometries were designed systematically by using weak equatorial ligands and strong axial ligands^[14-16] and consequently, the highest U_eff of 2 217 K with a record T_B of 80 K has been achieved^[14].

  Earlier, it was believed that the most suitable strategy to improve the performance of SMMs was to enhance the spin value of the ground state. Soon it was realized that the single-ion magnetic anisotropy is often canceled out within the molecule which gives a week magnetic anisotropy and results in a small U_eff for the whole system^[17]. Alternatively, the mononuclear SIMs, containing only one spin center, allows the scientists to explore and switch the single-ion magnetic anisotropy. Furthermore, it is well-known that the coordination environment of the target compound is usually perturbed through rational changes in synthetic strategies such as pH value, solvent effect, electrostatic potential, or the counter anions around the central metal ion in order to enhance or reduce the electron density^[18-21]. The particular concern here is the tunable counter anions effect that usually brings microenvironment differences in the geometric symmetry which in turn display dynamic SMMs behaviors^[22].

  The coordination chemistry of transition metals based on triaminoguanidine ligands has already been well established^[23-33]. Very recently, the researchers have also started to explore interesting magnetic properties of 3d transition metal complexes carrying rigid triaminoguanidine - backbones^[34-44]. Surprisingly, the magnetochemistry of such ligands with 4f rare earth metals have never been explored before, except only one study that also belongs to our group, where we have expressed a family of Dy(Ⅲ)-SIMs associated with guanidine - based ligands in which the effect of coordinated solvents and lattice counter anions simultaneously^[45]. To the extension of our previous work, we now have succeeded to synthesize two mononuclear Dy - SIMs by using triaminoguanidine - based ligand L (L=tris(2 - hydroxybenzylidene)triaminoguanidine) (Scheme 1) having formulas [DyL₂(H₂O)₂]ClO₄·2H₂O·2CH₃CN· CH₃OH (1) and [DyL₂(H₂O)₂]CF₃SO₃·4H₂O·2CH₃OH (2) with different counter anions. Interestingly, structural and magnetic studies reveal that different counter anions bring small but significant changes in bond lengths and bond angles and affect the axial ligand field which consequently exhibited dynamic singlemolecule magnetic relaxation behaviors with U_eff of 358 K (1), and 309 K (2), respectively, in the absence of a static magnetic field.

  Scheme 1

  

  Scheme 1.  Structural drawing of used ligand along with coordinating pocket

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  1.   Experimental

  1.1   Materials and methods

  All solvents and chemicals were obtained from commercial sources of analytical grade and were used as received. Ligand L was synthesized according to the procedure reported in the literature^{[29, 46-47]}. Elemental analyses for C, H and N were performed on a Perkin - Elmer 2400 analyzer. Magnetic measurements were done on the polycrystalline sample with a Quantum Design MPMS - XL7 SQUID magnetometer from 1.9~ 300 K temperature range equipped with a 7 T magnet. Alternating - current (ac) measurements were obtained at 3.0 Oe with oscillating frequencies from 1~1 000 Hz and direct - current (dc) measurements were measured within 1.9~300 K temperature range with an external magnetic field of 1 000 Oe. The experimental magnetic susceptibility data were corrected for the diamagnetism contribution of all the atoms estimated from Pascal′s tables and sample-holder calibration^[48].

  1.1.1   Synthesis of complexes 1 and 2

  Dy(ClO₄)₃·6H₂O (0.10 mmol) and ligand L (0.20 mmol) were dissolved in a 20 mL mixture of methanol and acetonitrile (1∶1, V/V) followed by triethylamine (0.20 mmol). After 3 h of stirring at room temperature, the resultant solution was filtered and put aside for slow evaporation. Well sized yellow color block-like crystals suitable for single crystal measurements were collected after a couple of days. Complex 2 was also obtained by using a similar procedure as for 1, by replacing Dy(ClO₄)₃·6H₂O with Dy(SO₃CF₃)₃·6H ₂O, respectively (Yield: ~70%, based on dysprosium for both the complexes).

  Elemental Anal. Calcd. for C₄₉H₅₆ClDyN₁₄O₁₅ (1, %): C, 46.01; H, 4.41; N, 15.33. Found(%): C, 45.97; H, 4.43; N, 15.31.

  Elemental Anal. Calcd. for C₄₇H₅₈DyF₃N₁₂O₁₇S (2, %): C, 42.94; H, 4.44; N, 12.79. Found(%): C, 43.16; H, 4.03; N, 12.82.

  1.2   Crystallography

  Single crystals of 1 and 2 were mounted on glass fibers under a microscope, and diffraction data were collected at corresponding temperatures (Table S1, Supporting information) using a Bruker AXS D8 Venture single-crystal diffractometer equipped with graphite-monochromatized (Cu Kα λ =0.154 178 nm (1), Mo Kα λ =0.071 073 nm (2)) in liquid N₂. The molecular structures and mean plane analysis were designed from the DIAMOND (version 3.1). The structures were solved by SHELXT (direct methods) and refined by SHELXL (full - matrix least - squares techniques) based on F² in the Olex2 package^[49-50]. Crystal- lographic data, structural refinement parameters and selected bond lengths and angles are given in Table S1~S3.

  2.   Results and discussion

  2.1   Synthesis and structural description

  Complexes 1 and 2 were prepared by reacting ligand L (0.20 mmol) with Dy(ClO₄)₃·6H ₂ O and Dy(SO₃CF₃)₃·6H₂O (0.10 mmol), respectively in the presence of triethylamine (0.20 mmol) in a 20 mL mixture of methanol and acetonitrile (1∶ 1, V/V). Single crystal X-ray diffraction analyses reveal that complexes 1 and 2 crystallize in monoclinic C2/c and triclinic P1 space group, respectively (Table S1).

  Complexes 1 and 2 display similar coordination environments (Fig. 1), only differ in counter anion (ClO₄^- for 1; SO₃CF₃^- for 2) along with some solvent molecules (CH₃OH, 2CH₃CN, 2H₂O for 1 and 2CH₃OH, 4H₂O for 2), respectively. Both compounds are octa - coordinated, six coordination sites (2NNO) from two singly deprotonated ligands and two O donors from two water molecules to complete the coordination sphere along with some solvent molecules in the lattice. The continuous shape measurements^[51-53] indicate that geometry for both the complexes can be best described as triangular dodecahedron D_2d giving the values of 0.739 and 0.915 for 1 and 2, respectively (Fig. S1, Table S2). The Dy1—O1 and Dy1—O2 bond lengths are 0.223 3(3), 0.222 3(3) nm and 0.222 51(3), 0.221 9(3) nm; Dy1—O3w and Dy1—O4w distances are 0.240 9(3), 0.246 7(3) nm and 0.241 1(3), 0.241 8(3) nm for 1 and 2. Whereas the Dy—N bond distances fall in 0.246 2(4) ~0.255 0(4) nm and 0.249 1(3)~0.243(3) nm of 1 and 2 (Table S3), respectively. The axial angles for both the complexes governed by coordinating deprotonated phenolic group of two ligands (O1—Dy1—O2) are 118.29(11)° and 115.26(11)°, respectively. The angles among O3w —Dy1—O4w are 129.77(11)° for 1 and 135.83(10)° for 2 (Table S4). Furthermore, the 3D molecular packing of both the complexes is also different, showing slight differences in intermolecular Dy…Dy interactions along a, b and c axes, caused by different counter anions and some solvent molecules at special positions in the crystal lattice. The closest intermolecular distances of Dy…Dy ions are 0.603 nm for 1 and 0.582 nm for 2, respectively (Fig. S2 and S3). These minor differences of bond lengths and bond angles along with diverse 3D molecular packing of complexes 1 and 2 induced by different counter anions/solvent molecules which in turn affect the anisotropy axes and are mainly responsible for their distinct magnetic properties^[45].

  Figure 1

  

  Figure 1.  Molecular structures of 1 (a) and 2 (b)

  Color codes: Dy, light green; C, gray; O, red; Cl, dark green; N, blue; F, bright green; S, yellow; Lattice solvent molecules are omitted for clarity

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  2.2   Magnetic properties

  The dc magnetic susceptibility (χ_MT) experiments were performed on polycrystalline samples for both the complexes in a temperature range of 1.9~300 K in an applied field of 1 kOe (Fig. 2). The measured χ_ΜT values at room temperature were 14.18 and 13.99 cm³·K· mol^-1 for complexes 1 and 2, respectively, which were both well consistent with the expected value of 14.17 cm³·K·mol^-1 for a free Dy(Ⅲ) ion (S=5/2, L =5, g=4/3 and ⁶H_15/2). Upon cooling, the χ_MT values decreased gradually until 20 K, and then decreased rapidly, reaching 9.59 and 9.47 cm³·K·mol^-1 at 1.9 K, respectively. The decrease in the χ_MT value in both the complexes is due to crystal field splitting, mainly the thermal depopulation of the Dy(Ⅲ) Stark sublevels and/or weak intra/intermolecular interactions^[54-57]. Comparatively, the χ_MT curves for both the complexes were slightly different at low temperatures, very similar to recently reported analogous Dy(Ⅲ) - SIMs^[45] which can be attributed to different intermolecular Dy…Dy distances, causing distinct antiferromagnetic dipole- dipole interactions among the molecules.

  Figure 2

  

  Figure 2.  Temperature dependence of χ_MT values of 1 and 2 under 1 kOe dc field

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  The field - dependent magnetization (M vs H) of 1 and 2 were examined at 1.9~5.0 K in the field range of 0~70 kOe (Fig. S4). The maxima of magnetization for both the complexes were 5.87μ_B and 5.58μ_B, respectively. These experimental values were well lower than the expected saturation value of 10μ_B for one uncorrelated Dy(Ⅲ) ion, but agreed with the mean value of 5μ_B under considerable crystal field, which imply a well - isolated ground Kramers doublet state. Further, non-superimposed M vs HT^-1 (Inset of Fig. S4) and lack of saturation of M vs H plots at different temperatures indicating significant magnetic anisotropy^[54-58]. Another important tool of the magnetic bi - stability is magnetic hysteresis which was experimentally measured for both the complexes. Interestingly, both the complexes displayed well defined butterfly - shaped hysteresis loops at 1.9 K (Fig.S5).

  The in - phase (χ′) and the out - of - phase (χ″) ac magnetic susceptibility measurements as a function of the frequency of 1 and 2 were performed under a zero dc field with a small ac field of 3 Oe. The frequency dependence χ′ and χ″ magnetic susceptibilities showed typical SMM behaviors for 1 and 2 (Fig. 3 and S6). Both the complexes revealed a series of well - defined χ″ maxima in a temperature range of 9~24 K for 1 and 5~22 K for 2, respectively. As seen from Fig. S7, another increase of the ac response was observed in the low-temperature region (below 9 K for 1 and 5 K for 2), probably indicating the onset of pure quantum tunneling^[59].

  Figure 3

  

  Figure 3.  Frequency dependence of in-phase (χ′) and out-of-phase (χ″) plots of 1 (left) and 2 (right)

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  The Cole - Cole fitting graphs (Fig. 4) of 1 and 2, based on frequency - dependent data with the generalized Debye model^[54-58] show nonsymmetrical semicircles. The obtained parameters are in the range of 0.024~0.3 (9~24 K) for 1 and 0.055~0.18 (5~22 K) for 2 respectively, giving a narrow distribution of relaxation times (τ) (Table S5 and S6). The Arrhenius plots (ln τ vs T^-1, Fig. 5) were fitted considering the Orbach and Raman process to obtain the corresponding effective energy barrier of relaxation U_eff=358.18 K (τ₀= 2.27×10^-11 s) for 1 and U_eff=309.19 K (τ₀ =2.64×10 ^-9 s) for 2^[55]. The insets of Fig. 5 shows the detail for the corresponding extracted parameters for both the complexes.

  Figure 4

  

  Figure 4.  Cole-Cole plots of 1 (a) and 2 (b) with blue solid lines as Debye fits within indicated temperatures

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  Figure 5

  

  Figure 5.  Fitting of frequency-dependent of relaxation time of 1 (a) and 2 (b)

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  2.3   Structural correlation

  Complexes 1 and 2 are actually analogous to each other but with different ac magnetic properties. Detailed insight into the structures of 1 and 2, the axial O1—Dy1—O2 angle of 118.29° for complex 1 is larger than 2 of 115.26°. Meanwhile, the axial Dy1—O1 and Dy1 —O2 average bond lengths in 1 (0.228 nm) are also slightly shorter than that detected in 2 (0.235 nm, Table 1). The presence of different counter anions (ClO₄^- and CF ₃SO₄^-) and solvent molecules (CH₃OH, 2CH₃CN, 2H₂O and 2CH ₃OH, 4H₂O) of 1 and 2 in the crystal lattice results in slight structural changes and different Dy…Dy intermolecular interactions along a, b and c axes (Fig.S2 and S3). The different intermolecular Dy…Dy interactions along a, b and c axes may bring different dipole-dipole interactions that are influencing on the relaxation rate of incoherent quantum tunneling of the magnetization to obtain different effective relaxation barriers. Collectively, the large angle and short bond lengths at the axial position incorporate with different intermolecular interactions in both the complexes may induce the stronger axial magnetic anisotropy which is mainly responsible for higher energy barrier of 358 K in 1 than that observed in complex 2 of 309 K.

  Table 1

  

  Table 1.  Important bond lengths (nm) and angles (°) of axially coordinated O of L for 1 and 2

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  Complex	1			2
  Length/nm	Dy1—O1	0.223 3(3)		Dy1—O1	0.225 1(3)
  	Dy1—O2	0.222 3(3)		Dy1—O2	0.221 9(3)
  	average	0.228(3)		average	0.235(3)
  Angle/(°)	O2—Dy1—O1	118.29(11)		O2—Dy1—O1	115.26(11)
  τ0/s	2.27×10-11			2.64×10-9
  Ueff/K	358.18			309.19

  3.   Conclusions

  In summary, two new triaminoguniadine-based mononuclear Dy(Ⅲ) complexes with formulas [DyL₂(H₂O)₂]ClO ₄·solvent (1) and [DyL₂(H₂O)₂]CF₃SO₃· solvent (2) have been successfully prepared by altering the reaction conditions. Both the complexes display almost analogous structures featuring eight coordinated N₄O₄ environment each that can be best described as triangular dodecahedron D_2d symmetry. The experimental ac magnetic studies revealed the single-ion magnetic behavior under zero dc applied field with effective energy barriers (U_eff) of 358 K (1) and 309 K (2), respectively. The diverse ac magnetic behaviors may result from different types of lattice counter anions (ClO₄^- for 1 and CF₃SO₄^- for 2), which slightly but significantly affect the axial ligand field parameters and coordination geometries. These results offer an interesting and possible opportunity for tuning the magnetic properties of SIMs by affecting the axial ligand field through introducing different counter anions into the crystal lattice.

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

  
  Acknowledgements: We thank the National Natural Science Foundation of China (Grants No.21871247, 21801237), the Natural Science Foundation of Jilin Province of China (Grant No. 20200201244JC) and Key Research Program of Frontier Sciences, CAS (Grant No. ZDBS-LY-SLH023) for financial support. ALI Basharat is grateful for the support through CAS-TWAS President′s Fellowship. Conflicts of Interest: The authors declare no conflict of interest.
  
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• 

  Scheme 1  Structural drawing of used ligand along with coordinating pocket

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  Figure 1  Molecular structures of 1 (a) and 2 (b)

  Color codes: Dy, light green; C, gray; O, red; Cl, dark green; N, blue; F, bright green; S, yellow; Lattice solvent molecules are omitted for clarity

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  Figure 2  Temperature dependence of χ_MT values of 1 and 2 under 1 kOe dc field

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  Figure 3  Frequency dependence of in-phase (χ′) and out-of-phase (χ″) plots of 1 (left) and 2 (right)

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  Figure 4  Cole-Cole plots of 1 (a) and 2 (b) with blue solid lines as Debye fits within indicated temperatures

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  Figure 5  Fitting of frequency-dependent of relaxation time of 1 (a) and 2 (b)

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  Table 1.  Important bond lengths (nm) and angles (°) of axially coordinated O of L for 1 and 2

  Complex	1			2
  Length/nm	Dy1—O1	0.223 3(3)		Dy1—O1	0.225 1(3)
  	Dy1—O2	0.222 3(3)		Dy1—O2	0.221 9(3)
  	average	0.228(3)		average	0.235(3)
  Angle/(°)	O2—Dy1—O1	118.29(11)		O2—Dy1—O1	115.26(11)
  τ0/s	2.27×10-11			2.64×10-9
  Ueff/K	358.18			309.19

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