Solvent-Induced Syntheses,Crystal Structures and Magnetic Properties of Two Mononuclear Er（Ⅲ）Complexes

XI Jing MA Xiu-Fang XU Yan-Hong ZHOU Hui-Liang ZHOU Yu-Ting LIU Xiang-Yu

(State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering,National Demonstration Center for Experimental Chemistry Education,College of Chemistry and Chemical Engineering,Ningxia University,Yinchuan 750021,China)

Abstract:By adjusting the ratio of the solvent components,two mononuclear erbium complexes,[Er(bpad)3]·CH3OH·H2O(1)and[Er(bpad)2(H2O)2]NO3·3H2O(2)(Hbpad=N3-benzoylpyridine-2-carboxamidrazone),have been successfully synthesized.In the mixure of methanol and water solvent(2∶1,V/V),yellow crystals of 1 were prepared by solution reaction of Er(NO3)3·6H2O,Hbpad and triethylamine.Interestingly,complex 2 was obtained by the similar way of 1,while the volume ratio of CH3OH to H2O is altered to be 1∶2.The resulting complexes were characterized by IR spectrum,elemental analysis.Two complexes are air stable,and can maintain the crystalline integrities at ambient conditions.Crystal structure analysis indicates that the Er（Ⅲ）ions in 1 and 2 present nonacoordinated and octa-coordinated environments in different coordination geometries,respectively.Monocapped square antiprism(C4v)is formed in 1,while biaugmented trigonal prism(C2v)is observed in 2.The direct-current(dc)and alternating-current (ac)magnetic susceptibilities of 1 and 2 have been analyzed as well.CCDC:1478582,1;1478579,2.

Keywords:erbium（Ⅲ）complex;solvent effect;crystal structure;magnetic property

After the discovery of the first single-ion magnet[TbPc2]-(Pc2-=pthalocyanine dianion)[1], lanthanide based complexes have gained momentum in the area of molecular magnetism as they exhibit slow relaxation of magnetization at low temperatures[2-6].Thanks to their unquenched orbital angular momentum and inherent magnetic anisotropy, the number of lanthanide-based single-molecule magnets (SMMs)reported to-date is increasing exponentially[7-14].Such molecules hold tremendous application potential in high-density data storage and/or quantum computation due to their magnetic moment bistability.By using geometric design principles to minimize electronic repulsions between the electron densities of the lanthanide ions and the ligands,researchers have prepared SMMs with extremely high barriers and magnetic hysteresis as compared to previous examples[15].Especially,a breakthrough has been made by Layfield and co-workersin Dy-SIMs,exhibiting magnetic hysteresis at temperatures up to 80 K and a high effective energy barrier of 1 541 cm-1[16].While most examples of the rare-earth SMMs are based on the oblate terbium and dysprosium ions(and particularly the latter[17-19]),emerging work has revealed that prolate ions can also engender SMM behavior,with the erbium（Ⅲ）ion being the main choice for such systems[20-22].The first reported erbium（Ⅲ）mononuclear SMM is the organometallic complex[(Cp)Er(COT)] (Cp=pentamethylcyclopentadienide;COT=cyclooctatetraenide),reported by Gao and co-workers,which exhibits two relaxation processes with energy barriers ΔEeff/kBof 197 and 323 K and a butterfly hysteresis loop as high as 5 K[23].The lack of SMM behavior in the majority of the Er（Ⅲ）complexes and the presence of large TBvalues in some other Er（Ⅲ）complexes have attracted our attention.

Existing researches pointoutthatdifferent environments ofthe magnetic centers generating various crystal fields usually bring about distinct dynamic magnetic relaxation processes;even subtle changes of the coordination pattern can drastically affect the overall magnetic properties of SMMs.It is well-known that the structural formation of a target complex is often perturbed by rational changes of the synthetic conditions such as solvent effect,pH,counterions[24-31].Of particular concern is the tunable solvent effect that usually motivates variable coordination behaviors of metal cations,even in a same reaction system,which can modulate the coordination microenvironment and the geometric symmetry of the metallic core[32].Herein,two monometallic erbium complexes,[Er(bpad)3]·CH3OH·H2O(1)and[Er(bpad)2(H2O)2]NO3·3H2O(2),have been isolated by changing the solvent ratio,with which the Er（Ⅲ）ion is surrounded,leading to a nona-coordinated monocapped square antiprism (C4v)and octa-coordinated biaugmented trigonalprism (C2v)geometries,respectively.In addition,the magnetic properties of complexes 1 and 2 have been investigated.

1 Experimental

1.1 Materials and methods

The Hbpad ligand was synthesized following a previous reference[33].Other chemicals were reagent grade and commercially available.Fourier transform infrared (FT-IR)spectra were recorded in a range of 400～4 000 cm-1using KBr pellets on an EQUINOX55 FT/IR spectrophotometer.Elemental analysis(carbon,hydrogen and nitrogen)was implemented on a Perkin Elmer 2400 CHN analyzer.Magnetic measurements were accomplished using a Quantum Design MPMS-XL7 SQUID magnetometer on polycrystalline samples(restrained in eicosane to prevent torquing at high fields).The diamagnetic corrections for the complexes were estimated using Pascals constants.

1.2 Synthesis of[Er(bpad)3]·CH3OH·H2O(1)

A mixed CH3OH/H2O solution(15 mL,2∶1,V/V)of triethylamine (0.007 mL,0.05 mmol)and Hbpad(0.146 g,0.6 mmol)was stirred for 1 h,and then Er(NO3)3·6H2O (0.091 g,0.2 mmol)was added.The mixture above was allowed to be stirring for 5 h and shelved after filtration.Yellow crystals of 1 were isolated after 4 days (Yield:72%,based on Er3+).Anal.Calcd.for C40H39ErN12O5(%):C,51.38;H,4.20;N,17.98.Found(%):C,51.42;H,4.25;N,18.02.IR(KBr,cm-1):3 442(w),1 633(m),1 560(s),1 521(s),1 472(s),1 187(s),1 436(m),1 362(s),1 165(w),1 029(w),805(w),713(m).

1.3 Synthesis of[Er(bpad)2(H2O)2]NO3·3H2O(2)

Complex 2 was prepared by the similar way of 1,while the ratio of CH3OH to H2O is altered to be 1∶2(V/V).Pale yellow crystals of 2 were obtained after 5 days (Yield:68%,based on Er3+).Anal.Calcd.for C26H32ErN9O10(%):C,39.14;H,4.04;N,15.80.Found(%):C,39.18;H,4.07;N,15.75.IR (KBr,cm-1):3 436(w),1 635(s),1 585(s),1 565(s),1 521(s),1 479(s),1 436(m),1 393(s),1 168(m),1 063(m),912(w),743(m),710(s).

1.4 Crystallographic data collection and refinement

Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker SMART APEX CCD diffractometer equipped with graphite-monochromated Mo Kα radiation (λ=0.071 073 nm).Data processing and absorption corrections were accomplished using SAINT and SADABS[34].The structures were solved by direct methods and refined against F2by full-matrix least-squares with SHELXTL program[35].All non-hydrogen atoms were refined with anisotropic thermal parameters.All hydrogen atoms were positioned geometrically and refined using a riding model.Crystallographic data and refinement parameters are listed in Table 1,while selected interatomic distances and angles for complexes 1 and 2 are given in Table S1(Supporting information).

CCDC:1478582,1;1478579,2.

Table 1 Crystallographic data and refinement parameters for 1 and 2

2 Results and discussion

2.1 Crystal structure

Fig.1 Crystal structures of 1(a)and 2(b)and local coordination geometries of the Er（Ⅲ）ions for 1(c)and 2(d)

X-ray determination suggests that both complexes are mononuclear structures.Complexes 1 and 2 belong to the monoclinic C2/c and P21/n space groups,respectively.The Er（Ⅲ）site in the asymmetric unit of 1 is nona-coordinated by three O atoms and six N atoms from three tridentate bpad-ligands(Fig.1).The average Er-O and Er-N bond lengths are 0.236 1 and 0.254 6 nm,respectively(Table S1).The shortest Er…Er distance between neutral[Er(bpad)3]molecules is 0.955 9 nm.Being compared with 1,a tridentate bpadligand are substituted by two coordinated H2O molecules,forming the configuration of 2.Accordingly,the metal center in 2 is bonded by two tridentate bpad-anions and two coordinated H2O units,thus completing the ErN4O4coordination group(Fig.1).The average Er-O and Er-N bond lengths are 0.236 5 and 0.247 3 nm,respectively.The closest Er…Er separation between two isolated molecules in 2 is 0.697 1 nm.Evidently,the steric hindrance of H2O in 2 is far less than the bpad-ligand in 1,giving rise to the shorter intermolecular distance in 2 compared to that in 1.Hydrogen bonds and π-π interactions coexist in 1 and 2, yielding a three-dimensional supramolecular structure(Fig.S1).

To ascertain the precise geometries around the metallic centers and the degree of the distortion from the ideal model for both complexes,the geometric spheres of Er（Ⅲ）cations are calculated by using the SHAPE 2.1 program[36]based on the structural parameters,and the typical geometric polyhedrons are depicted in Fig.1.Aslisted in Table S2,the calculated values illuminate that the Er（Ⅲ）ion in 1 is best described as a monocapped square-antiprism(C4v)with moderate distortions from the ideal geometry.By contrast,the Er（Ⅲ）nodes in 2 represent a biaugmented trigonal prism(C2v).

2.2 Magnetic studies

Fig.2 Plots of χMT vs T for 1(a)and 2(b)

In order to understand the consequence of this ligand architecture on the local anisotropy of Er（Ⅲ）ion, magnetic susceptibility measurements were performed with a SQUID magnetometer on freshly prepared ground polycrystalline samples of 1 and 2 sealed under N2to prevent sample degradation.Direct current magnetic susceptibility measurements were conducted in a temperature range of 1.8～300 K under an applied dc field of 0.1 T (Fig.2).The room temperature χMT values of 11.58 and 11.37 cm3·K·mol-1for 1 and 2,respectively,are in good agreement with the theoretical value of 11.48 cm3·K·mol-1for a mononuclear Er（Ⅲ）(4I15/2,S=3/2,L=6,gJ=6/5)complex.Upon decrease of the temperature,the χMT product remained nearly constant then decreased gradually between 100 and 50 K.Below 50 K,there was a sharp decrease reaching the minimums of 5.25 and 5.58 cm3·K·mol-1for 1 and 2 at 1.8 K,respectively.Such behavior is resulted from significant anisotropy as seen in some highly anisotropic complexes or weak antiferromagnetic dipolar coupling between the monotnuclear groups[37-38].Isothermal magnetization curves at 1.8 K exhibited saturation for 1 and 2 with values of 4.77Nβ and 4.44Nβ (Fig.S2),respectively,which prominently deviate from the saturated value of 10Nβ.The non-saturation as well as the non-superimposition of iso-temperature lines in the M vs H/T plot also confirm the presence of significant magnetic anisotropy in 1 and 2(Fig.2,Inset).Therefore,alternating-current magnetic susceptibility measurements for complexes 1 and 2 were carried out under 0 dc field in a range of 1.8～20 K and a frequency of 1 000 Hz(Fig.S3).Unfortunately,no out-of-phase(χM″)signal was observed until the temperature dropped to 1.8 K,suggesting a fast quantum tunneling of magnetization(QTM)through the spin-reversal barrier and the inexistence of SMM behavior in the absence of extra field.To gain insight into the relationship between configuration and magnetic relaxation behavior,structural and magnetic parameters of some Er（Ⅲ）complexes are summarized in Table S3[23,39-47].It is observed that the ligand field design plays a crucial role for the observance of SMM behaviorin Er（Ⅲ）complexes.The sandwich-type ligand geometry leads to a higher symmetry environment around the Er（Ⅲ）sites,thus exhibiting a superior magnetic property.The case with low-coordinated Er（Ⅲ）center may also act as a potential candidate for high-performant SMMs.In contrast,the seven or eight coordinated Er ions adverse to the zero-field slow magnetic relaxation.

3 Conclusions

This work reports the synthesis of two mononuclear Er（Ⅲ）complexes by means of solvent effect and the characterizations of them.For 1,the coordination geometry of Er（Ⅲ）core in N6O3motif could be best identified as a monocapped squareantiprism (C4v),while the configuration of Er（Ⅲ）ion in 2 is surrounded by a biaugmented trigonal prism(C2v)with N4O4group.Magnetic studies unveil that the performance of slow magnetic relaxation is absent in these systems,owing to the possible QTM which can usually be efficiently suppressed by a DC field.The related follow-up tests are under way.The impetus for the case presented herein is to exploit the potential of synthesizing Er-based single-molecule magnets,further studies following this guideline are in progress.

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