WANG QinLIU Yuan-YuanNIU LiYAN JingHOU Yin-Ling

(1Department of Chemistry,Taiyuan Normal University,Jingzhong,Shanxi 030619,China)

(2School of Sciences,Kaili University,Kaili,Guizhou 556011,China)

Abstract:A new Dy2complex[Dy2(L)2(acac)2(CH3CH2OH)2](1)has been synthesized based on a multidentate Schiff base H2L(H2L=N′-(2-hydroxynaphthalen-1-ylmethylene)-2-(hydroxyimino)propanehydrazide)and β-diketone coligand Hacac(Hacac=acetylacetone).Complex 1 was characterized by single-crystal X-ray diffraction,elemental analysis and further investigated by magnetic measurements.Structural analysis reveals that complex 1 is a centrosymmetric dinuclear dysprosium complex and the asymmetric unit contains a Dy（Ⅲ）ion,one L2-ligand,one acac-coligand,and an ethanol molecule.Each of the Dy（Ⅲ）centers is eight-coordinate and adopts a distorted bicapped trigonal-prismatic coordination geometry.Magnetic studies suggest the existence of weak antiferromagnetic interactions between the Dy（Ⅲ）ions in 1.In addition,the slow magnetic relaxation behaviors are observed in 1 with the magnetic energy barrier ΔE/kB=14.52 K and the pre-exponential factor τ0=7.58×10-6s.CCDC：2096910.

Keywords:dinulcear Dy（Ⅲ）complex;eight-coordinate;bicapped trigonal-prismatic;antiferromagnetic interaction;slow magnetic relaxation behavior

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 Dy2compound 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 Dy3cluster 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 Dy2complex[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 L2-adopts μ2∶η1η1η2η1binding mode.Magnetic studies indicate that complex 1 exhibited single-molecule magnet behavior with the energy barrier ΔE/kB=14.52 K and the pre-exponential factor τ0=7.58×10-6s.

Scheme 1 Synthetic route of H2L

1 Experimental

1.1 Materials and measurements

All reagents and solvents were purchased from commercial sources and used as received.The βdiketonate salt(Dy(acac)3·2H2O)was prepared according 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-CH2Cl2mixture(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 F2using 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 Crystal data and structure refinement parameters for complex 1

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

CCDC：2096910.

2 Results and discussion

2.1 Structure description of complex 1

Fig.1 Molecular structure of complex 1

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

Scheme 2 Binding mode of L2-ligand in 1

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 χMT vs T plot is shown in Fig.3.The roomtemperature χMT value for 1 was 28.55 cm3·K·mol-1,which was close to 28.34 cm3·K·mol-1of two uncoupled Dy（Ⅲ）ions(6H15/2,g=4/3).Upon lowering the temperature,the χMT value of 1 decreased slowly from 300 to 50 K and then declined abruptly to a minimum value of 9.29 cm3·K·mol-1at 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.

Fig.3 Plot of χMT vs T at 1 000 Oe for complex 1

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-ofphase(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/(kBT)][14],yielding the effective energy barrier ΔE/kB=14.52 K and pre-exponential factor τ0=7.58×10-6s(Fig.7).The obtained τ0was in the normal range of the reported Dy-based SMMs[15].By contrast with some previously reported Dy2complexes constructed based on Schiff base ligands,the obtained value of ΔE/kBfor 1 is larger than those of some Dy2compounds(Table 3).The diverse magnetic properties in them can be attrib-uted to the different coordinated Schiff bases and βdiketone co-ligands,which inspire us to further explore the relationship between the structures and magnetic properties of lanthanide SMMs.

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

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

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

Fig.7 Plot of ln τ vs T-1for 1

Table 3 Magnetic relaxation barriers and pre?exponential factors for selected Dy2complex under a zero?dc field

3 Conclusions

In summary,one new Dy2complex 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/kB=14.52 K and the pre-exponential factor τ0=7.58×10-6s.