含氮氧自由基的Gd，Tb，Dy配合物的合成、結構及磁性

胡鵬　吳燕妮　黃期曉　連思綿　付興慧　何高鵬　陳俠敏(肇慶學院化學化工學院，肇慶526061)

含氮氧自由基的Gd，Tb，Dy配合物的合成、結構及磁性

胡鵬＊吳燕妮黃期曉連思綿付興慧何高鵬陳俠敏(肇慶學院化學化工學院，肇慶526061)

合成了一個新穎的氮氧自由基配體，并用該配體合成了3例未見文獻報道的氮氧自由基-稀土三自旋單核配合物Ln (hfac)3(NIT-Ph-4-OCHCH3CH3)2(Ln=Gd(1),Tb(2),Dy(3);hfac=六氟乙酰丙酮;NIT-Ph-4-OCHCH3CH3=4，4，5，5-四甲基-2-(4′-異丙氧基苯基)-咪唑啉-3-氧化-1-氧基自由基)。單晶結構分析表明配合物1、2、3擁有相似的自由基-稀土-自由基單核結構。對配合物的磁性測試結果表明自由基與稀土之間存在著鐵磁相互作用。自由基與自由基之間存在著反鐵磁相互作用。

氮氧自由基；稀土；晶體結構；磁性

0　Introduction

Single-molecule magnets(SMMs)have attracted many scientists attention in the past two decades[1-4].This type of materials are characterized as slow magnetization relaxation caused by the association of large ground state spin(ST)value with a significant uniaxial(Ising-like) magnetic anisotropy(D),which leads to a significant energy barrier to magnetization reversal(U)[5-7].The SMMs have been found potential applications for the uses of high-density magnetic memories,magnetic refrigeration,quantumcomputingdevicesand spintronics at the molecular level[8-11].Recently 4f metal ions were considered to be good candidates for the construction of SMMs due to their significant magnetic anisotropy arising from the large,unquenched orbital angular momentum.Up to now,a variety of 4f metal ions basedSMMshavebeenreported[12-17].

Apart from the choice of the metal ions,the ligand design also plays an important role.The use of organicradicalligandsinthecreationofnew magnetic molecular compounds have attracted much attention since the discovery of the first radical-4f SMM by Gatteschis group[18].As is well known,the stable radicalligandscangeneratetypicallystronger intramolecular magnetic exchange coupling.The strong exchange coupling between lanthanides and radicals generally leads to superior SMMs.Recently,a binuclear Tbcomplex bridged by a Nradical has been reportedwitharecordblockingtemperatureof 13.9 K[19-20].So far various organic radicals such as nitronyl nitroxide,verdazyl and semiquinone radicals have been reported[21-25].However,researches are focused in particular on the nitronyl nitroxide(NIT)family of radicals,because these type of radicals are relatively stable and easy to obtain derivatives with substituents containing donor atoms.Nitronyl nitroxide radicals can act as bidentate ligands through their identical N-O coordination groups and give rise to complexes with different structures.Unfortunately,NIT radicals are poorly donating ligands,thus utilization of strong electron-withdrawing coligands such as hexafluoroacetylacetonate(hfac)and trifluoroacetylacetonate(tfac) are necessary.However,the steric demand of these coligands restrict the dimensionality of the resulting metal-radical compounds.So,it is easier to get zeroand one-dimensional compounds by this strategy.

To further study the magnetic properties of NIT radical-lanthanide compounds,in this paper we report a novel nitronyl nitroxide radical(Scheme 1)and its corresponding Ln-nitronyl nitroxide compounds Ln (hfac)3(NIT-Ph-4-OCHCH3CH3)2(Ln=Gd(1),Tb(2),Dy (3);hfac=hexafluoroacetylacetonate;NIT-Ph-4-OCHCH3CH3=2-(4-isopropoxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide),their crystal structures and magnetic properties were described in detail.

Scheme 1Molecule structure of NIT-Ph-4-OCHCH3CH3

1　Experimental

1.1Materials and physical measurements

All the starting chemicals were obtained from Aldrich and used without further purification.The radical ligand NIT-Ph-4-OCHCH3CH3was prepared according to literature method[26].Elemental analyses (C,H,N)were determined by Perkin-Elmer 240 elemental analyzer.The infrared spectra was recorded from KBr pellets in the range of 4 000～400 cm-1with a Bruker Tensor 27 IR spectrometer.The magnetic measurementswerecarriedoutwithMPMSXL-7 SQUID magnetometer.Diamagnetic corrections were made with Pascals constants for all the constituent atoms.

1.2Synthesis of Complex 1

A solution of Gd(hfac)3·2H2O(0.05 mmol)in 25 mL dry heptane was heated to reflux for 2 h.Then the solution was cooled to about 60℃,a solution of NITPh-4-OCHCH3CH3(0.1 mmol)in 2 mL of CH2Cl2was added.The resulting solution was stirred for about 3 min and then cooled to room temperature.The filtrate was allowed to stand at room temperature for slow evaporation.After three days,some blue crystals were collected.Yield:31.4 mg(45.5%based on Gd). Elemental analysis calculated for C47H49F18N4O12Gd(%): C:41.47;H:3.63;N:4.12.Found(%):C:41.88;H: 3.69;N:4.22.

1.3ynthesis of Complex 2

Complex2wassynthesizedwiththesame procedure for complex 1 using Tb(hfac)3·2H2O instead of Gd(hfac)3·2H2O.Yield:32.7 mg(47.9%).Elemental analysis calculated for C47H49F18N4O12Tb(%):C:41.42; H:3.62;N:4.11.Found(%):C:40.69;H:3.57;N: 4.22.

1.4Synthesis of Complex 3

Complex3wassynthesizedwiththesame procedure for complex 1 using Dy(hfac)3·2H2O instead of Gd(hfac)3·2H2O.Yield:29.3 mg(42.9%).Elementalanalysis calculated for C47H49F18N4O12Dy(%):C:41.31; H:3.61;N:4.10.Found(%):C:40.91;H:3.77;N:4.17.

1.5Crystal Structure Determination and Refinement

X-ray single-crystal diffraction data for complexes 1,2 and 3 were collected using a Rigaku Saturn CCD diffractometer at 113(2)K with Mo Kα radiation(λ= 0.071 073 nm).The structure was solved by direct methods by utilizing the program SHELXS-97[27]and refined by full-matrix least-squares methods on F2with the use of the SHELXL-97 program package[28]. Anisotropic thermal parameters were assigned to all non-hydrogen atoms.The hydrogen atoms were set in calculated positions and refined as riding atoms with a common fixed isotropic thermal parameter.Disordered carbon atoms were observed in the hfac ligands for both compounds and disorders were also observed for some fluorine atoms.Pertinent crystallographic data andstructurerefinementparametersforthese complexes were listed in Table 1.Selected bond lengths and bond angles of complexes 1,2 and 3 are listed in Table 2.

CCDC:985443,1;1043098,2;985444,3.

Table 1Crystal data and structure refinement for 1,2 and 3

aR1=∑(‖Fo|-|Fc‖)/∑|Fo|;bwR2=[∑w

Table 2Selected bond distances(nm)and Angles(°)for 1,2 and 3

Continued Table 2

2Results and discussion

2.1Crystal Structure of Complex 1

Single-crystal X-ray diffraction analyses reveal that all these three compounds show similar radical-Ln-radical structures,which are composed of one Ln(hfac)3unit and two NIT-Ph-4-OCHCH3CH3radicals. Compounds 1 and 3 are isostructural and crystallize in thespace group,while compound 2 crystallizes in the P21/c space group.

Fig.1Molecular structure(left)and crystal packing diagram of complex 1(right)

In complex 1,the central Gdions are eightcoordinate with eight oxygen atoms.Two radical ligands bond to one Gdion via the oxygen atoms of N-O coordination groups.The bond length of Gd(1)-O(6)is 0.234 8 nm while the bond length of Gd(1)-O(3) is 0.237 8 nm.The N(2)-O(3)and N(3)-O(6)bond lengths of nitronyl nitroxide radicals are 0.130 4 nm and 0.130 7 nm,respectively.The uncoordinated N(1) -O(2)and N(4)-O(5)bond lengths are 0.127 1 nm and 0.127 2 nm,respectively,which are comparable to thoseofreportedtri-spinradical-Ln-radical complexes[29-35].The other six oxygen atoms are from three hfacs with the Gd-O bond lengths in the range of 0.236 0～0.242 9 nm.The nearest Gd…Gd distance between the adjacent molecules is 1.024 9 nm(Fig.1).

2.2Crystal structure of complex 2

The crystal structure of complex 2 shows that the central Tbions are eight-coordinated with eight oxygen atoms.Two radical ligands bond to one Tbion via the oxygen atoms of N-O coordination groups. The bond length of Tb(1)-O(4)is 0.234 1 nm while the bond length of Tb(1)-O(2)is 0.234 5 nm.The N(3) -O(4)and N(2)-O(2)bond lengths of nitronyl nitroxide radicals are 0.131 6 nm and 0.131 8 nm respectively. The uncoordinatedN(1)-O(1)and N(4)-O(5)bond lengths are 0.128 3 nm and 0.127 3 nm,respectively, which are comparable to those of reported tri-spin radical-Ln-radical complexes[29-35].The other six oxygen atoms are from three hfacs with the Tb-O bond lengths in the range of 0.234 6～0.241 7 nm.The nearestTb…Tbdistancebetweentheadjacent molecules is 1.080 8 nm,which is a little bit longer than that for complex 1(Fig.2).

Fig.2Molecular structure(left)and crystal packing diagram of complex 2(right)

2.3Crystal Structure of Complex 3

Compound 3 is isostructural to compound 1 and the bond lengths of Dy-O are in the range of 0.232 1～0.239 3 nm,which are a little shorter than the bond lenths of Gd-O.

2.4Magnetic Properties of Complex 1

Variable-temperature magnetic susceptibilities of complexes 1,2,and 3 were measured from 300 to 2.0 K in an applied field of 1 kOe.The χMT vs T plot for 1 are shown in Fig.3.At 300 K,the χMT value is 8.77 cm3·K·mol-1,close to the theoretical value of 8.63 cm3·K·mol-1(Uncoupled one Gdion,f7electron configuration,χMT=7.88 cm3·K·mol-1)plus two organic radicals(S=1/2,χMT=0.375 cm3·K·mol-1)).Upon cooling,the χMT value of complex 1 increases steadily to a maximum of 10.04 cm3·K·mol-1at 15 K, afterward decreases to 9.28 cm3·K·mol-1at 2.0 K.

As shown in Scheme 2,there are two kinds of magnetic interactions in this radical-Gd-radical complex at the same time.The first one is Gd-radical interaction and the second one is radicalradical interaction.

The magnetic interactions between Gdand the radicals can be described by isotropic exchange interaction.Thus the experimental data for complex 1 can be analyzed with an expression derived from a spin Hamiltonian.Considering the g value range of the radical and Gdion,we assume that the radical and Gdion have the same g value.Thus the variable-temperature magnetic susceptibility data for complex 1 can be analyzed by a theoretical expression (Eq.2)deduced from a spin Hamiltonian(Eq.1)[30-35].

The best fitting leads to g=2.00,JRad-Gd=2.57 cm-1, JRad-Rad=-9.98 cm-1for complex 1.The positive value of JRad-Gdindicates that there is a weak ferromagnetic interaction between the Gdand the radicals in the molecule.ThenegativeJRad-Radindicatesthe antiferromagneticinteractionbetweenthetwo intramolecularradicals.TheobtainedJvalueis comparable with the previously reported GdIII-radicals compounds[30-35].

Fig.3　Temperature dependence of χMT for complex 1

Scheme 2　Model of intramolecular interactions

2.5Magnetic properties of complex 2

While for complex 2(Fig.4),at 300 K,the χMT value is 13.08 cm3·K·mol-1,close to the theoretical value 12.57 cm3·K·mol-1in uncoupled system of one Tbion(f9electron configuration,χMT=11.82 cm3· K·mol-1)plus two organic radical(S=1/2,χMT=0.375 cm3·K·mol-1).Upon cooling,the χMT value of complex 2 increases steadily to a maximum of 28.62 cm3·K· mol-1at 3.0 K,afterward the value decreases to 28.54 cm3·K·mol-1at 2.0 K.The increase of χMT suggests the presence of ferromagnetic interaction between the Tband the organic radical.The decrease of χMT at lowtemperatureindicatestheantiferromagnetic interaction between the two intramolecular radicals. The magnetic properties of complex 2 are similar to those of previously reported[29-35].

Fig.4Temperature dependence of χMT for complex 2

Alternating current(ac)susceptibility measurementsforcomplex2werecarriedoutinlow temperature regime under a zero dc field to investigate the dynamics of the magnetization.As shown in Fig.5, there are no obvious frequency dependent out-ofphase signals.We do not think that complex 2 express SMM behavior at low temperature.This may due tothe small energy barrier which could not prevent the inversion of spin.

Fig.5Temperature dependence of the in-phase and out-of-phase components of ac susceptibility for 2 in zero dc field with an oscillation of 3.5 Oe

2.6Magnetic properties of complex 3

Complex 3 shows similar magnetic properties with complex 1(Fig.6).At 300 K,the χMT value is 15.01 cm3·K·mol-1,close to the theoretical value of 14.92 cm3·K·mol-1.Upon cooling,the χMT value of complex3increasessteadilytoamaximumof 19.79 cm3·K·mol-1at 15.4 K,afterward decreases to 16.97 cm3·K·mol-1at 2.0 K.The plot also suggests the presence of ferromagnetic interaction between the Dyand the coordinated N-O groups of the organic radicals and the antiferromagnetic interaction between the two intramolecular radicals.

Fig.6　Temperature dependence of χMT for complexes 3

Alternating current(ac)susceptibility measurements for complex 3 were also carried out in low temperature regime under a zero dc field.The result (Fig.7)shows that there are no obvious frequency dependentout-of-phasesignals.Likecomplex2, complex 3 doesnt express SMMs behavior at low temperature.

Fig.7　Temperature dependence of the in-phase and out-of-phase components of ac susceptibility for 3 in zero dc field with an oscillation of 3.5 Oe

3　Conclusions

A novel nitron yl nitroxide radical and its three correspondingmononucleartri-spincompounds Ln(hfac)3(NIT-Ph-4-OCHCH3CH3)2(Ln=Gd(1),Tb(2), Dy(3).)have been synthesized and characterized.The magnetic studies reveal that ferromagnetic interactions (between the intramolecular Ln and radical)and antiferromagnetic interactions(between the intramolecularradicals)arecoexistinthesecomplexes. Complexes 2 and 3 do not have SMMs behavior at low temperature,this may due to the small energy barrier which could not prevent the inversion of spin.

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Gd,Tband DyComplexes Based on a Nitronyl Nitroxide Radical:Syntheses, Structures and Magnetic Properties

HU Peng＊WU Yan-NiHUANG Qi-XiaoLIAN Si-MianFU Xing-HuiHE Gao-PengCHEN Xia-Min
(College of Chemistry and Chemical Engineering,Zhaoqing University,Zhaoqing,Guangdong 526061,China)

One novel nitronyl nitroxide radical and its three mononuclear tri-spin compounds Ln(hfac)3(NIT-Ph-4-OCHCH3CH3)2(Ln=Gd(1),Tb(2),Dy(3);hfac=hexafluoroacetylacetonate;NIT-Ph-4-OCHCH3CH3=2-(4-isopropoxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide)havebeensuccessfullypreparedand characterized.Single crystal X-ray crystallographic analyses reveal that complexes 1,2,and 3 shows similar radical-Ln-radical structures,which ar composed of one Ln(hfac)3units and two NIT-Ph-4-OCHCH3CH3radicals. Magneticstudiesrevealthatferromagneticinteractions(betweenintramolecularLnandradical)and antiferromagnetic interactions(between the intramolecular radicals)are coexist in these complexes.CCDC:985443, 1;1043098,2;985444,3.

nitronyl nitroxide radical;lanthanides;crystal structure;magnetic properties

O614.33+9;O614.341;O614.342

A

1001-4861(2016)02-0297-08

10.11862/CJIC.2016.032

2015-06-20。收修改稿日期：2015-12-01。

廣東省高校創新強校項目(No.504-20000158)、廣東省教育廳科技創新項目(No.2013KJCX0193)和廣東省大創項目(No.603-60300346)資助。

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