ZHAO Jin?YuZHANG Xue?JinYANG Xiao?MinWEN Jiao?JiaoHUA Yu?Peng

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

(2Ordos Institute of Technology,Ordos,Inner Mongolia 017000,China)

Abstract:By utilizing a polydentate Schiff base ligand(H2L=pyridine?2?carboxylic acid(3,5?di?tert?butyl?2?hydroxy?benzylidene)?hydrazide),reacting with Gd(dbm)3·2H2O(Hdbm=dibenzoylmethane)and Gd(NO)3·6H2O,respectively;two new Gd2complexes formulated as[Gd2(L)2(dbm)2(C2H5OH)2](1)and[Gd2(L)2(HL)2(DMF)]·2CH3CN(2)(DMF=N,N?dimethylformamide)have been obtained by using solvothermal method.The crystal structures and magnetic prop?erties of the two Gd2complexes have been systematically studied.The crystal structures study reveals that each eight?coordinate Gd3+ion in 1 possesses a distorted triangular dodecahedron;the two central Gd（Ⅲ）ions are connected by two μ2?O,resulting in a rhombic?shaped Gd2O2core.However,for 2,each central Gd（Ⅲ） ion is nine?coordinate and their coordination configurations can be described as distorted spherical capped square antiprism,and the two central Gd（Ⅲ） ions are connected by three μ2?O forming a triangular biconical?shaped Gd2O3core.Magnetic investi?gations showed that the two Gd2complexes displayed magnetic refrigeration properties with the magnetic entropy(- ΔSm)of 20.16 J·K-1·kg-1for 1 and 17.14 J·K-1·kg-1for 2 at ΔH=70 kOe and T=2.0 K.CCDC:2111657,1;2111658,2.

Keywords:Gd2complexes;crystal structures;magnetic properties;magnetic refrigeration properties

0 Introduction

In recent years,the studies of lanthanide?based compounds have attracted increasing attention of chem?ists and material scientists not only due to their beauty and fascinating crystal structures[1]but also because of the potential applications in functional materials,including interesting magnetic properties,lumines?cence properties,and catalysis[2?4].Among these poten?tial applications of lanthanide?based compounds,the molecular?based magnetic material is one of the research hotspots for inorganic chemistry and material chemistry[5],and magnetic refrigeration and single?molecule magnets(SMMs)are particularly attractive[6?9].Key to the potential magnetic refrigeration application of a molecular?based magnetic material is its large mag?netocaloric effect(MCE)[10],and an excellent magnetic refrigeration material featuring large MCE should pos?sess negligible magnetic anisotropy and a large magnet?ic density[11].Hence,the isotropic Gd（Ⅲ）ion with a high spin state(S=7/2)is the best candidate for designing and constructing Gd（Ⅲ）?based compounds,which would be a promising magnetic refrigerant material to perform significant MCE[12].Based on this,lots of poly?nuclear 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 Long,Tong,and Zheng′s group have conducted outstanding work on the magnetic refrigeration materials of Gd（Ⅲ）?based clusters[17?19].These studies inspire and promote the synthesis of lanthanide?based compounds with out?standing and excellent magnetic refrigeration materials.

It is well?known that the Schiff base ligand is a type of classical ligand.In the past decade,lots of Ln（Ⅲ）?based compounds with novel topologies and showing outstanding magnetic properties have been constructed by using Schiff base ligands[20?23].Considering the advantage of Schiff base ligands,we design and synthe?size an organic polydentate Schiff base ligand(H2L=pyridine?2?carboxylic acid(3,5?di?tert?butyl?2?hydroxy?benzylidene)?hydrazide,Scheme 1)which possesses abundant coordination sites,strong chelating ability,and various coordination patterns.When H2L reacted with Gd(dbm)3·2H2O (Hdbm=dibenzoylmethane)or Gd(NO)3·6H2O,two new Gd2complexes with the molecular formulas[Gd2(L)2(dbm)2(C2H5OH)2](1)and[Gd2(L)2(HL)2(DMF)]·2CH3CN(2)(DMF=N,N?dimeth?ylformamide)have been synthesized through a solvo?thermal method.The structural and magnetic proper?ties of 1 and 2 were deeply investigated and discussed.The magnetic study revealed that complexes 1 and 2 show MCE with- ΔSmof 20.16 J·K-1·kg-1for 1 and 17.14 J·K-1·kg-1for 2 at ΔH=70 kOe and T=2.0 K.

Scheme 1 Structure of organic polydentate Schiff base H2L

1 Experimental

1.1 Materials and measurements

Gadolinium nitrate hexahydrate(Gd(NO3)3·6H2O)was bought from Energy Chemical Co.,Ltd.DMF,etha?nol,acetonitrile,and other solvents were purchased from Fuchen chemical corporation.Hdbm,picolinohy?drazide,and 3,5?di?tert?butylsalicylaldehyde were pur?chased from Aladdin Reagent(Shanghai)Co.,Ltd.Gd(dbm)3·2H2O and polydentate Schiff base ligand H2L were prepared by using an already reported litera?ture method[24?25].The elemental analyses(C,H,and N)of complexes 1 and 2 were performed on a PerkinElmer 240 CHN elemental analyzer.Magnetic properties for complexes 1 and 2 were measured using a Quantum Design MPMS?XL7 and a PPMS?9 ACMS magnetome?ter.Diamagnetic corrections were estimated with Pascal′s constants for all atoms[26].

1.2 Syntheses of complexes 1 and 2

[Gd2(L)2(dbm)2(C2H5OH)2](1):H2L(0.05 mmol),Gd(dbm)3·2H2O(0.05 mmol),ethanol(6.0 mL),and acetonitrile(5.0 mL)were enclosed in a glass vial(20 mL),and then the mixture was heated to 70℃and keep at this temperature for 72 h,and then the temper?ature was dropped to room temperature slowly.Yellow block crystals suitable for X?ray diffraction were obtained.Yields based on Gd(dbm)3·2H2O:41%.Elemental analysis Calcd.for C76H84Gd2N6O10(%):C 58.61,H 5.40,N 5.40;Found(%):C 58.65,H 5.37,N 5.44.

[Gd2(L)2(HL)2(DMF)]·2CH3CN (2): H2L (0.03 mmol),Gd(NO3)3·6H2O(0.03 mmol),ethanol(3.0 mL),DMF(2.0 mL),and acetonitrile(2.0 mL)were added to a three flask and stirred at room temperature for about 3 h.Then the mixture was sealed in a 15 mL glass bot?tle and heated to 80℃to react for 48 h and then slowly cooled to room temperature subsequently.Yellow block crystals suitable for X?ray diffraction were obtained.Yields based on Gd(NO3)3·6H2O:32%.Elemental anal?ysis Calcd.for C91H115Gd2N15O9(%):C 58.16,H 6.13,N 11.19;Found(%):C 58.11,H 6.17,N 11.25.

1.3 X-ray crystallography

The crystallographic diffraction data for complexes 1 and 2 were collected on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with graphite monochro?matized MoKαradiation(λ=0.071 073 nm)by usingφ?ωscan mode.Multi?scan absorption correction was applied to the intensity data using the SADABS pro?gram.The structures were solved by direct methods and refined by full?matrix least?squares onF2using the SHELXTL(Olex 2)program[27].All non?hydrogen atoms were refined anisotropically.All the other H atoms were positioned geometrically and refined using a riding model.Due to the existence of disordered sol?vent molecules in the crystals of 1 and 2,we remove the disordered solvent molecules by using PLATON/SQUEEZE program.To determine the specific number of free solvent molecules,the thermogravimetric analy?sis(TGA)of the crystal samples for 1 and 2 have been measured.Details of the crystal data and structure refinement parameters for complexes 1 and 2 are sum?marized in Table 1,and selected bond lengths and angles of complexes 1 and 2 are listed in Table S1 and S2(Supporting information).

Table 1 Crystal data and structure refinement parameters for 1 and 2

CCDC:2111657,1;2111658,2.

2 Results and discussion

2.1 Crystal structures of complexes 1 and 2

Single?crystal X?ray diffraction analyses reveal that both complexes 1 and 2 crystallize in the mono?clinic space groupP21/c(Table 1).As shown in Fig.1,the structure of 1 contains two Gd（Ⅲ）ions,two L2-ions,two dbm-ions,and two coordinated C2H5OH mole?cules.Each central Gd（Ⅲ） ion in complex 1 is coordinat?ed by six oxygen atoms(O1,O1a,O2,O3,O4,and O5)and two nitrogen atoms(N1 and N3)forming an O6N2coordination environment(Fig.S1).As shown in Fig.S2,the eight?coordinate Gd1 ion shows a distorted triangu?lar dodecahedron(D2h)coordination geometry.It is also confirmed by using SHAPE 2.0 software(Table 2)[28].The coordination modes of L2-and dbm-are shown in Fig.2.L2-adopts a quad?dentate chelation model to connect the central Gd（Ⅲ） ion,and dbm-adopts a biden?tate chelation model to connect the central Gd（Ⅲ）ion.The Gd1 and Gd1a ions are connected by twoμ2?O(O1 and O1a)atoms forming a parallelogram Gd2O2core.The Gd1…Gd1a distance is 0.405 2(9)nm,which is slightly larger than those of some reported Gd2com?plexes[29?32].In addition,the Gd1—O1—Gd1a angle in the Gd2O2core is 114.90(7)°.In 1,the Gd—O distances fall in a range of 0.222 3(2)?0.243 1(7)nm,and the Gd1—N1,Gd1—N3 bond lengths are 0.257 9(2)and 0.2476(2)nm,respectively.TheO—Gd—Obondangles fall in a range of 65.09(7)°?146.46(6)°.

Fig.1 Molecular structure of complex 1 shown with 50% probability displacement ellipsoids

Fig.2 Coordination mode of L2-(a)and dbm-(b)in 1

Different from 1,complex 2 is mainly composed of two Gd（Ⅲ）ions,two L2-ions,two HL-ions,and one coordinated DMF molecule(Fig.3).Both Gd1 and Gd2 ions in complex 2 are nine?coordinate,and each Gd（Ⅲ）ion is coordinated by six oxygen atoms and three nitro?gen atoms forming an N3O6coordination environment(Fig.S3).Accordingly,as shown in Fig.S4,both of the nine?coordinate Gd（Ⅲ） centers lie in a distorted spheri?cal capped square antiprism(C4v)which also can be cal?culated by using SHAPE 2.0 software(Table 3).There are two coordination modes for L2-and HL-in 2(Fig.4):quad?dentate or tri?dentate chelation model to connect the central Gd（Ⅲ） ion,respectively.The two Gd（Ⅲ） ions are connected by threeμ2?O(O3,O5,and O9)atoms forming a triangular biconical?shaped Gd2O3core.The distance of the two central Gd（Ⅲ）ions is 0.392 3(3)nm,which is smaller than that of complex 1.The Gd1—O3—Gd2,Gd1—O9—Gd2,and Gd1—O9—Gd2 angles in the Gd2O3core are 105.68(3)°,98.64(6)°,and 106.95(4)°,respectively,which are also smaller than those of complex 1.The Gd—O bond lengths are in a range of 0.225 3(3)?0.260 7(4)nm,while the average Gd—N distance is 0.261 2(1)nm.The O—Gd—O bond angles fall in a range of 61.96(10)°?149.73(12)°.

Table 2 GdⅢion geometry analysis by SHAPE 2.0 for 1*

Fig.3 Molecular structure of the complex 2 shown with 50% probability displacement ellipsoids

Table 3 GdⅢion geometry analysis by SHAPE 2.0 for 2*

Fig.4 Coordination mode of L2-(a)and HL-(b)in 2

2.2 TGA of complexes 1 and 2

To study the thermal stabilities of complexes 1 and 2,TGA was performed and the curves are shown in Fig.S5 and S6.For 1,the weight loss of 5.78%(Calcd.5.91%)between 26 and 285℃can be attributed to the loss of two coordinated EtOH molecules.After that complex 1 started to decompose.For 2,the weight loss of 4.11% from 26 to 245℃is attributed to the loss of two free CH3CN molecules(Calcd.4.36%).Thereafter,a weight loss of 3.92%(Calcd.3.88%)occurred,which is attributed to the loss of a coordinated DMF mole?cule.Subsequently,complex 2 gradually decomposed in a temperature range of 280?800 ℃.

2.3 Magnetic properties of complexes 1 and 2

Direct current(dc)magnetic susceptibility mea?surements for the two Gd2complexes 1 and 2 were per?formed on polycrystalline samples during a tempera?ture range of 300.0?2.0 K under an applied field of 1 kOe.TheχΜTvsTplots for complexes 1 and 2 are shown in Fig.5.The room?temperatureχΜTproducts of 1 and 2 were 15.80 and 15.78 cm3·K·mol-1,respective?ly,which are in good agreement with the expected value(15.76 cm3·K·mol-1)for two uncoupled Gd （Ⅲ）ions(8S7/2,g=2).As the temperature decreased,theχΜTvalues of 1 and 2 slowly declined during the tempera?ture range of 300.0?25.0 K.Thereafter,theχΜTvalues of 1 and 2 quickly dropped to a minimum of 11.58 and 5.68 cm3·K·mol-1at 2.0 K.The downward trend of the χΜT vs T curves implies that there is an antiferromag?netic(AF)interaction between adjacent Gd（Ⅲ）ions in the two Gd2complexes 1 and 2[33].

Fig.5 Temperature dependence of χMT products at 1.0 kOe for 1(a)and 2(b)

The Curie?Weiss law was used for fitting the mag?netic susceptibility of complexes 1 and 2(Fig.S7 and S8).The two parameters,C=15.84 cm3·K·mol-1and θ=-1.84 K(R2=0.999 9)for 1 and C=15.91 cm3·K·mol-1and θ=-5.43 K(R2=0.999 51)for 2 were obtained.The negative θ values of 1 and 2 further suggest that there is an antiferromagnetic interaction between adjacent Gd（Ⅲ）ions in 1 and 2[34].

The magnetization data for the two Gd2complexes 1 and 2 were collected at 2.0?10.0 K in the 0?70 kOe field.As depicted in Fig.S9,the M values for complex?es 1 and 2 rapidly increased below 20 kOe and then steadily increased to 14.13Nβ for 1 and 14.07Nβ for 2 at 70 kOe,which are very close to the saturation value of 14Nβ for two isolated Gd（Ⅲ） (S=7/2,g=2)ions.

According to the previously reported litera?ture[35?37],because of the larger isotropic and high?spin ground state of Gd（Ⅲ）ion,the MCE of both 1 and 2 was studied.The maximum magnetic entropy change(-ΔSm)of 1 and 2 were calculated by using the Maxwell equation: ΔSm(T)=∫[?M(T,H)/?T]HdH[38].The- ΔSmvs T curves of 1 and 2 are shown in Fig.6.The observed-ΔSmvalues of 1 and 2 were 20.16 and 17.14 J·K-1·kg-1at ΔH=70 kOe and T=2.0 K,which were smaller than the theoretical values of 22.22 J·K-1·kg-1for 1 and 18.83 J·K-1·kg-1for 2(based on the equation-ΔSm=2Rln(2S+1)/Mr,SGd=7/2,and R=8.314 J·mol-1·K-1).The difference between experimental and theoretical-ΔSmvalues may be due to the antiferromagnetic interac?tion between Gd（Ⅲ） ions in 1 and 2[39].To better com?pare and display the-ΔSmvalues of Gd2complexes,the-ΔSmvalues of recently reported dinuclear Gd（Ⅲ）?based complexes are listed in Table 4[40?49].The-ΔSmof 2 was smaller than some reported Gd2complexes,how?ever,it is worth mentioning that the-ΔSmof complex 1 was larger than those of some dinuclear Gd（Ⅲ）?based complexes.The reason for the larger-ΔSmof complex 1 may be due to the weak antiferromagnetic interaction and the smallerMr/NGdratio of 1.

Fig.6 Plots of-ΔSmvs T for 1(a)and 2(b)

Table 4 Comparison of-ΔSmvalues for complexes 1,2 and some reported Gd2complexes

3 Conclusions

In summary,we have synthesized two new Gd2complexes [Gd2(L)2(dbm)2(C2H5OH)2] (1) and[Gd2(L)2(HL)2(DMF)]·2CH3CN(2).Both complexes 1 and 2 are binuclear structures with different coordina?tion environments of central Gd（Ⅲ） ions.Magnetic mea?surements imply that the two Gd2complexes display magnetic refrigeration properties.Our present work provides a new approach to design and construct Gd（Ⅲ）?based magnetic refrigeration materials.Magnetic refrig?eration studies of other poly?nuclear or high?nuclear Gd（Ⅲ）?based clusters are underway in our group.

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