synthesis, structures, and magnetic properties of a series

INORGANIC CHEMISTRYFRONTIERS

RESEARCH ARTICLE

Cite this: Inorg. Chem. Front., 2014,1, 695

Received 13th August 2014,Accepted 25th September 2014

DOI: 10.1039/c4qi00116h

rsc.li/frontiers-inorganic

Synthesis, structures, and magnetic properties of aseries of new heterometallic hexanuclear Co2Ln4

(Ln = Eu, Gd, Tb and Dy) clusters†

Chong-Bin Tian,a Da-Qiang Yuan,a Yun-Hu Han,a,b Zhi-Hua Li,a Ping Lina andShao-Wu Du*a

A series of new Co–Ln heterometallic clusters formulated as [Co2Ln4(μ3-OH)2(piv)4(hmmp)4(ae)2]·

(NO3)2·2H2O (Ln = Eu (1), Gd (2), Tb (3), Dy (4), H2hmmp = 2-[(2-hydroxyethylimino)methyl]-6-methoxy-

phenol, Hae = 2-aminoethanol, Hpiv = pivalic acid) were synthesized and characterized. X-ray crystallo-

graphy reveals that each of them contains a heterometallic {Ln4Co2} core, which is supported by two μ3-hydroxide, four piv−, four hmmp2− and two ae− ligands. The magnetic investigation indicates that 2 exhi-

bits weak antiferromagnetic interactions between GdIII ions, and a large MCE value of 24.9 J kg−1 K−1,

while 4 shows a fast relaxation of magnetization. The TGA and VT-PXRD measurements suggest that all

the compounds show good thermal stability and can be stable up to about 220 °C. In addition, to address

the influence of the Gd–O–Gd angles on the magnetic properties, compound 2 was compared with a

series of compounds involving different bridges between GdIII ions. The comparison reveals that the tiny

difference in the Gd–O–Gd angles favors different magnetic coupling.

Introduction

The research on nano-sized molecular magnetic materials witha high spin ground state has witnessed in recent years flour-ishing progress due to their aesthetically fascinating structuresand potential applications in many fields such as quantumcomputing, high-density magnetic information storage, mole-cular spintronics and low temperature magnetic refrigeranttechnology.1 One special attractive goal of this research is thesynthesis of molecular magnetic materials having the cryo-genic magnetocaloric effect (MCE) as candidate materials formagnetic refrigerators.1f,2 The MCE is a magneto-thermo-dynamic phenomenon that is characterized by the isothermalentropy change (ΔSM) and by the adiabatic temperaturechange (ΔTad) associated with the magnetic field variation.2,3

Compared with the traditional gas compression–expansiontechnology, adiabatic demagnetization is less harmful to theenvironment, more energy efficient, low-lying noise2,3 andmuch easier to achieve very-low temperatures, even milliKelvin

temperatures.4 These virtues make it particularly attractive toscientists, and hence much effort has been invested inresearch related to materials exhibiting MCE properties.5

Indeed, the MCE is intrinsic in any magnetic material, butonly in a few cases is the entropy change sufficiently large tomake them suitable for practical use. For example, gadoliniumis a suitable candidate for magnetic refrigerant materialsmainly because of its 8S7/2 ground state which provides thelargest entropy per ion, together with a negligible super-exchange interaction. As a result, many Mn–Gd,6 Fe–Gd,7 Co–Gd,8 Ni–Gd,9 Cu–Gd,9b,10 Cr–Gd11 or purely Gd-based5d,12

molecular magnetic cryocooling materials have been reported.However, most of them are coordinated by solvent molecules,which bind to the central metal ions with weaker chemicalbonds compared with the coordinated organic ligands,leading to a lower thermal stability. Very recently, Zheng et al.have reported a series of 3d-Gd clusters without coordinationsolvent molecules by employing phosphonates as ligands,6b,8c,d,9a

which display a fascinating large magnetocaloric effect. Theseclusters are synthesized by a solvothermal method and theirthermal stability has not yet been investigated. For a MCEmaterial, the alignment of randomly oriented magneticmoments by an external magnetic field can result in heating,and this heat may lead to damage of the material. Therefore,good thermal stability is important for this type of material.

On the other hand, Dy clusters, in particular planar “butter-fly” type Dy4 clusters, are of particular interest to the research-

†Electronic supplementary information (ESI) available: Selected bond lengthsand angles, PXRD, TGA, VT-PXRD, IR and other magnetic measurements. CCDC948361–948364. For ESI and crystallographic data in CIF or other electronicformat see DOI: 10.1039/c4qi00116h

aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002,

P. R. China. E-mail: [email protected]; Fax: (+86) 591 83709470bUniversity of Chinese Academy of Sciences, Beijing 100039, P. R. China

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ers in the field of single-molecular magnets (SMMs) owing tothe large magnetic anisotropy and magnetic moment of the Dycomponent. Over the last few years, a large number of Dy4 but-terfly clusters have been reported.13 However, the DyIII ions inthese compounds are mainly eight coordinate, and there areonly two examples in which the coordination numbers of DyIII

ions are greater than eight.13b,c It has been shown that theligand field as well as the coordination geometry stronglyinfluence the local anisotropy of the DyIII ion, and govern theSMM behaviour.14 Therefore, it is interesting to synthesize new“butterfly” type Dy4 clusters with other coordination geome-tries and explore their SMM behaviour.

The polydentate hydroxyl-rich Schiff bases, derived fromthe reactions of 2-hydroxybenzaldehyde or its derivatives witha range of amino alcohols, have been proven to be versatilechelating and bridging ligands which can react with metalions to afford a number of polynuclear 3d15 or 4f16 clusters. Bycomparison, less work has been done in the preparation of3d–4f clusters with these ligands.6d,17 One of these ligands,namely 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol(H2hmmp), in situ generated from the reaction of 2-hydroxy-3-methoxybenzaldehyde with 2-aminoethanol, has recently beenused to make Mn–Ln clusters.18 However, it has not been usedin the synthesis of Co–Ln clusters. Herein, we report a series ofCo–Ln clusters derived from H2hmmp, formulated as[Co2Ln4(μ3-OH)2(piv)4 (hmmp)4(ae)2]·(NO3)2·2H2O (Ln = Eu 1,Gd 2, Tb 3, Dy 4, Hae = 2-aminoethanol and Hpiv = pivalicacid). The thermal analysis studies show that 1–4 have a highthermal stability up to 220 °C. Magnetic properties of 1–4 arealso investigated. Isothermal magnetization measurementsindicate that the Co2Gd4 cluster exhibits a large MCE. More-over, alternating current susceptibility measurements indicatethat the Co2Dy4 cluster displays fast quantum tunnelingrelaxation.

ExperimentalMaterials and physical measurements

All the materials were purchased from commercial sourcesand used without further purification. Thermogravimetricexperiments were performed using a TGA/NETZSCH STA449Cinstrument heated from 30 to 800 °C (heating rate 10 °Cmin−1, nitrogen stream). Elemental analyses of C, H and Nwere carried out with a Vario EL III elemental analyzer. High-resolution powder XRD patterns were collected using a PANaly-tical X’Pert Pro (Cu-Kα radiation: λ = 1.54056 Å) in the range of5° < 2θ < 60°. IR spectra were recorded on a Perkin-Elmer Spec-trum One using KBr pellets in the range of 4000–400 cm−1.Magnetic susceptibilities were measured on polycrystallinesamples with a Quantum Design PPMS-9 T system. Diamag-netic corrections were made using Pascal’s constants.

Synthesis of compounds 1–4

The same procedure was employed to prepare all the com-pounds and hence only the synthesis of 4 is described here in

detail. A mixture of 2-hydroxy-3-methoxybenzaldehyde(152 mg, 1.00 mmol) and 2-aminoethanol (76 mg, 1.25 mmol)in MeCN (30 mL) was stirred and heated at 80 °C for one hourand the solution turned dark yellow. After cooling, triethyl-amine (405 mg, 4.00 mmol), Dy(NO3)3·6H2O (456 mg,1.00 mmol) and pivalic acid (102 mg, 1.00 mmol) were addedto give a light yellow solution. After stirring for another onehour, Co(NO3)2·6H2O (125 mg, 0.50 mmol) and 3-butyleneglycol (270 mg, 3.00 mmol) were added and the solutionturned dark red. The solution was stirred under ambient con-ditions for another two hours and then filtered. Orange-yellowcrystals of 4 were obtained after one week, washed with coldMeCN (2 × 5 mL) and dried under vacuum (205 mg, yield36%). Elemental analysis calcd (%) for C64H98N8O32Co2Dy4: C34.02, H 4.37, N 4.96; Found: C 33.89, H 4.23, N 4.85. IR (KBr,cm−1): 3626 (v), 3413 (v), 3220 (v), 3115 (v), 2959 (m), 2925 (m),2867 (m), 2695 (vw), 1643 (vw), 1631 (m), 1605 (v), 1563 (s),1538 (m), 1484 (vw), 1471 (m), 1456 (vw), 1423 (s), 1385 (m),1325 (vw), 1297 (m), 1223 (s), 1168 (w), 1062 (m), 970 (w), 920(vw), 894 (vw), 739 (m), 548 (m).

Compounds 1–3 were obtained by the same procedures asthat described for 4, using Ln(NO3)3·6H2O (Ln = Eu, Gd andTb) in place of Dy(NO3)3·6H2O. Compound 1: yield: 36%.Elemental analysis calcd (%) for C64H98N8O32Co2Eu4: C 34.67,H 4.46, N 5.05; Found: C 34.72, H 4.49, N 5.11. IR (KBr, cm−1):3620 (v), 3412 (v), 3219 (v), 3113 (v), 2959 (m), 2928 (m), 2866(m), 2691 (vw), 1644 (vw), 1630 (s), 1605 (m), 1558 (s), 1532(m), 1484 (vw), 1456 (vw), 1423 (v), 1384 (v), 1325 (vw), 1297(m), 1220 (s), 1168 (w), 1060 (s), 968 (w), 916 (vw), 893 (vw),739 (m), 545 (v). Compound 2: yield: 43%. Elemental analysiscalcd (%) for C64H98N8O32Co2Gd4: C 34.34, H 4.41, N 5.01;Found: C 34.46, H 4.48, N 5.10. IR (KBr, cm−1): 3622 (v), 3411(v), 3222 (v), 3114 (v), 2959 (m), 2927 (m), 2867 (m), 2691 (vw),1642 (vw), 1631 (s), 1605 (m), 1560 (s), 1535 (m), 1484 (vw),1456 (vw), 1423 (v), 1385 (v), 1325 (vw), 1297 (m), 1221 (s), 1168(w), 1061 (s), 969 (w), 917 (vw), 894 (vw), 739 (m), 546 (v). Com-pound 3: yield: 38%. Elemental analysis calcd (%) forC64H98N8O32Co2 Tb4: C 34.24, H 4.40, N 5.00; Found: C 34.33,H 4.46, N 5.06. IR (KBr, cm−1): 3624 (v), 3412 (v), 3221 (v), 3115(v), 2959 (m), 2926 (m), 2867 (m), 2695 (vw), 1640 (vw), 1631(s), 1605 (m), 1561 (s), 1536 (m), 1484 (vw), 1471 (vw), 1423 (v),1384 (v), 1325 (vw), 1297 (m), 1222 (s), 1168 (w), 1061 (s), 970(w), 918 (vw), 894 (vw), 740 (m), 547 (v).

X-ray crystallography

Suitable single crystals of the compounds were carefullyselected and glued to thin glass fibers with epoxy resin. Inten-sity data were collected at room temperature on a RigakuMercury CCD area-detector diffractometer with a graphitemonochromator utilizing Mo-Kα radiation (λ = 0.71073 Å).CrystalClear software19 was used for data reduction andempirical absorption correction. The structures were solved bydirect methods using SHELXTL and refined by full-matrixleast-squares on F2 using the SHELX-97 program.20 All thenon-hydrogen atoms were refined anisotropically, except forO14, C23, C28, C29 and C30 atoms in 1, O14, O15, C23, C28,

Research Article Inorganic Chemistry Frontiers

696 | Inorg. Chem. Front., 2014, 1, 695–704 This journal is © the Partner Organisations 2014

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C29 and C30 atoms in 2, O14, O15, C28, C29 and C30 atoms in3 and 4. The hydrogen atoms bonded to carbon are generatedgeometrically (C−H 0.97 or 0.93 Å) and U(H) values are set as1.2 times Ueq(C). Since the position of the disordered watermolecules could not be resolved from the Fourier maps,PLATON/SQUEEZE21 was used to compensate the data for theircontribution to the diffraction patterns for 1–4. The finalchemical formulae of 1–4 were calculated from SQUEEZEresults combined with the TGA and elemental analysis data.Crystallographic data and other pertinent information forthese compounds are summarized in Table 1. Selected bondlengths (Å) and angles (°) for compounds 1–4 are listed inTable S1.† CCDC numbers for compounds 1–4 are948361–948364.

Results and discussionSyntheses, thermal stability and spectral analysis

A reaction designed to lead to the in situ formation of a Schiffbase as a ligand has been employed to synthesize compounds1–4. During the reactions, the Schiff base H2hmmp is formedwhich subsequently reacts with LnIII and CoII ions to generatea series of new CoII–LnIII clusters. Considering large magneticanisotropy of the CoII ion, our initial aim was to obtain CoII–LnIII clusters. Unfortunately, the oxidation of CoII to CoIII

occurred during the reactions, resulting in the presence ofCoIII ions in the final products. It should be noted that theaddition of 3-butylene glycol can greatly improve the yields ofthe products. Although a variety of structural types and compo-sitions have been identified so far for Co–Ln clusters,22 to thebest of our knowledge, most of them are prepared in CH3OHor CH3OH–CH2Cl2 solution. As a result, one or more CH3OHmolecules are coordinated to the metal centres. In the prepa-

ration of 1–4, excess 2-aminoethanol was used to prevent thecoordination of solvent molecules and we successfullyobtained four Co–Ln clusters without any coordinated solventmolecules at the metal centres. Powder X-ray diffraction(PXRD) patterns measured in the solid state have been used tocheck the phase purity of the bulk samples. The measuredPXRD patterns of 1–4 are in good agreement with the simu-lated ones generated from the results of single-crystal diffrac-tion data, indicating the phase purity of the as-synthesizedsamples (Fig. S1†). TGA experiments were performed to deter-mine the thermal stability of 1–4 under a nitrogen atmospherein the temperature range of 30–800 °C. TG analyses indicatethat all these compounds have high thermal stability andexhibit similar thermal behaviour (Fig. 1a and S2†). Thereforeonly the thermal stability of 4 was discussed in detail. TheTGA curve of 4 shows a weight loss of 1.54% from 40 to230 °C, which can be attributed to the loss of two lattice watermolecules (calcd = 1.59%). And then, it begins to decomposedue to the collapse of organic ligands. The high thermal stabi-lity of 1–4 was further confirmed from the temperatureresolved XRD patterns, which were performed after calcinationof the sample at elevated temperatures in the range of100–240 °C (Fig. 1b and S2†). The powder XRD patterns of 4 fit

Fig. 1 (a) TGA curve of 4. (b) Variable-temperature powder XRD of 4.

Table 1 Crystallographic data for 1–4

1 (Eu) 2 (Gd) 3 (Tb) 4 (Dy)

Formula C64H94N8O30Co2Eu4 C64H94N8O30Co2Gd4 C64H94N8O30Co2Tb4 C64H94N8O30Co2Dy4Formula mass 2183.53 2202.33 2209.01 2223.33Crystal system Triclinic Triclinic Triclinic TriclinicSpace group P1̄ P1̄ P1̄ P1̄a/Å 11.579(5) 11.628(4) 11.568(5) 11.576 (2)b/Å 13.984(6) 13.997(4) 13.976(6) 14.014 (3)c/Å 15.517(7) 15.441(5) 15.460(7) 15.380 (3)α/° 76.066(17) 74.478(14) 75.651(15) 74.549 (9)β/° 68.290(16) 68.117(12) 68.137(14) 67.993 (9)γ/° 70.713(18) 69.735(12) 70.478(15) 69.888 (9)V/Å3 2183.0(17) 2159.8(12) 2164.5(17) 2144.8 (7)Z 1 1 1 1μ/mm−1 3.277 3.479 3.675 3.895Dcalcd/g cm−3 1.659 1.693 1.695 1.727F(000) 1080 1084 1108 1092Reflns measured 17 096 23 180 19 203 22 834Independent reflns 7435 9808 7584 9662Observed reflns 6380 7955 6335 7402R1

a [I > 2σ(I)] 0.0408 0.0490 0.0475 0.0476wR2

b [I > 2σ(I)] 0.1062 0.1304 0.1135 0.1209GOF on F2 1.096 1.077 1.099 1.012

a R1 = ∑||Fo| − |Fc||/∑|Fo|.bwR2 = [∑w(Fo

2 − Fc2)2/∑w(Fo

2)2]0.5.

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well with the simulated data below 220 °C, which indicate thatit remains crystalline below this temperature (Fig. 1b). At240 °C, the long-range order of the structure was lost and theamorphous phase was formed. The thermal stability of 4 iscomparable to metal–organic frameworks (MOFs) that gener-ally decompose in the temperature range of 200–350 °C.23 Asshown in Fig. S3,† the IR spectra of these compounds arenearly identical. The absorption bands resulting from the skel-etal vibrations of the aromatic ring in 1–4 were found in therange of 1400–1600 cm−1. The broad peaks around 3410 cm−1

indicate the presence of free water molecules. The sharp peakaround 3625 cm−1 suggests the presence of hydroxyl groups inthese compounds. The stretching bands for –NH2 groupsappear at about 3220 and 3113 cm−1. Additionally, the IRspectra of 1–4 display absorption bands at about 2960, 2925,2867 and 1223 cm−1, which can be assigned to the character-istic signals of the But groups. The above vibration bands areconsistent with the single-crystal structure analyses.

Crystal structure of compounds 1–4

Single crystal X-ray diffraction analysis reveals that 1–4 are iso-structural to each other. Therefore, only the description of theDy analogue will be given here. The asymmetric unit of 4 con-tains one CoIII ion, two DyIII ions, one OH−, one ae−, two piv−,two hmmp2− and one NO3

− anion (Fig. S4†). The Dy1 and Dy2atoms are both nine-coordinated in a distorted tri-capped tri-gonal prismatic geometry but with different coordinationenvironments (Fig. 2a): Dy1 is coordinated by one nitrogenatom and eight oxygen atoms with the Dy1–N bond length of2.49 Å and the average Dy1–O bond length of 2.45 Å, while Dy2is surrounded by nine oxygen atoms with the average Dy2–Obond length of 2.43 Å. The Co1 atom has a slightly distortedoctahedral arrangement with the Co1–LN,O bond lengthsranging from 1.874(6) to 1.942(5) Å. The DyIII ions make up aDy4O2 core, in which the four DyIII ions exhibit a planar butter-fly structure. At the centre of the Dy4 metallic core are two µ3-hydroxide ligands, which bridge the central hinge DyIII (Dy1and Dy1A) to the outer wing-tip DyIII ions (Dy2 and Dy2A). Thetwo (µ3-OH)− ions are located above and below the Dy4 planeby 0.859 Å (Fig. S5†). Such a Dy4O2 core is bridged to two CoIII

ions by two µ3-O atoms (one from an ae− ligand and the otherfrom an hmmp2− ligand), generating a Dy4Co2O12 oxo clustercore (Fig. 2b). Around the periphery of the oxo cluster are twoae−, four piv− and four hmmp2− ligands (Fig. 2c). Both of theae− ligands adopt a µ3–η1η3 bonding mode. The four piv−

ligands are all coordinated to the DyIII ions in the syn–syn andµ–η2 bonding modes, with the former bridging the hinge ionsto the wing-tip ions and the latter chelating to the wing-tipions. The hmmp2− ligands exhibit two bridging modes,µ4–η1η2η1η3 and µ3–η1η2η1η2, to connect DyIII and CoIII ions(Scheme 1). It should be noted that the coordination geome-tries of DyIII ions in 4 are all tri-capped trigonal prismatic(nine-coordinate) which are different from those of otherreported Dy4 butterfly clusters,13,16a where the DyIII ions aremainly in a distorted square-antiprismatic geometry (eight-coordinate).

Magnetic properties

The variable-temperature dc magnetic susceptibilities for 1–4were measured in the temperature range of 300–2 K under anapplied field of 1000 Oe (Fig. 3a). For 1, the χmT value is5.27 cm3 mol−1 K at 300 K, which is much larger than thevalue of 0 cm3 mol−1 K expected for four EuIII ions ( J = 0). Dueto the presence of the thermally populated excited states of the

Fig. 2 (a) Coordination polyhedron for DyIII ions in 4. (b) TheDy4Co2O12 oxo cluster core in 4. (c) Coordination environments for DyIII

and CoIII ions in 4.

Scheme 1 Coordination modes of the hmmp2− ligand.



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EuIII ion, the magnetic properties of 1 remain difficult toexplain even at room temperature.24 However, its χmT value isclose to zero (0.05 cm3 mol−1 K) at 2 K, indicating a non-magnetic ground state (7F0) for the EuIII ion. This thermallypopulated diamagnetic ground state of 1 leads to a value ofmagnetization close to zero even at 8 T (Fig. 3b). Consideringthe nonmagnetic ground state (7F0) and thermally populatedexcited states (7FJ, J = 0, 1, 2,…, 6) of the EuIII ion, the magneticdata of 1 can be analyzed using eqn (1) which is deduced fromfour magnetically non-interacting EuIII ions.25

xm ¼ðNβ 2=3KTxÞ½24þ ð27x=2þ 3=2Þe�x þ ð135x=2� 5=2Þe�3x

þ ð189x� 7=2Þe�6x þ ð405x� 9=2Þe�10x

þ ð1485x=2� 11=2Þe�15x

þ ð2457x=2� 13=2Þe�21x�=½1þ 3e�x

þ 5e�3x þ 7e�6x þ 9e�10x þ 11e�15x þ 13e�21x�ð1Þ

where x = λ/KT and λ is the spin–orbit coupling parameter. Theleast-squares fitting in the whole temperature range leads toλ = 365 cm−1 and R = 8.3 × 10−4 (Fig. 3a). The value of λ is com-parable to other reported EuIII complexes,26 but is smaller

than 370 cm−1 deduced from the average value of three com-ponents arising from the 7F1 state according to the spectro-scopic data.25 This divergence could be attributed to thedifferent crystal field effect.

For 2, the Gd4 displays an χmT value of 30.80 cm3 mol−1 Kat room temperature, which is in good agreement with that forfour isolated GdIII ions (31.52 cm3 mol−1 K). This valueremains constant as the temperature is decreased untilca. 30 K where it sharply decreases to a value of 14.87 cm3

mol−1 K at 2 K. This behaviour is indicative of the occurrenceof weak antiferromagnetic interactions between the GdIII ions.In order to quantify the magnitude of the magnetic exchangewithin the Gd4, the exchange mode with three coupling con-stants is taken into account (Fig. S6†). The fitting to the mag-netic susceptibility curve was performed using the MAGPACKprogram27 based on the spin-Hamiltonian shown in eqn (2).

H ¼ �2J1ðS1S2 þ S3S4Þ � 2J2ðS2S3 þ S1S4Þ � 2J3S1S3 ð2Þ

The best fit gave the parameters g = 1.98, J1 = −0.07 cm−1,J2 = −0.05 cm−1 and J3 = −0.029 cm−1 (Fig. 3a). The agreementfactor R, defined as Σ[(χmT )obsed − (χmT )calcd]

2/Σ(χmT )2obsed, isequal to 2.53 × 10−5. The very small negative J values clearlysuggest the weak antiferromagnetic couplings among the GdIII

ions. The field dependence of magnetization of 2 shows a sat-uration value of 27.46Nβ at 2 K and H = 8 T (Fig. 3b), which isclose to the expected theoretical value of 28.00Nβ.

The bridges between GdIII ions in 2 can be divided intothree types: two μ2-O bridges (type I), three μ2-O bridges (typeII) and mixed bridges of one syn–syn carboxylate and two μ2-Obridges (type III) (Fig. S6†). Their values of magnetic couplingare compared with those of other magnetostructurally charac-terized GdIII complexes (Table 2). It seems that there exists acorrelation between the Gd–O–Gd angle (or Gd⋯Gd distance)and the magnetic interaction of GdIII ions. For example, alarge Gd–O–Gd angle with type I12c,28 or III12c,29 favours a ferro-magnetic interaction. Accordingly, the small Gd–O–Gd anglesof types I and III in 2 may lead to an antiferromagnetic inter-action between the GdIII ions. It is also found that the type IIbridge usually results in antiferromagnetic interactionsbetween the GdIII ions,13d,30 and the smaller the Gd–O–Gdangle, the larger the antiferromagnetic interaction. This obser-vation is in good agreement with the fact that the type IIbridge in 2 which has a medium Gd–O–Gd angle shows thepresence of a moderate antiferromagnetic interaction.

The χmT products for 3 and 4 display room temperaturevalues of 46.61 and 55.88 cm3 mol−1 K, respectively, which areclose to the expected theoretical values using the free ionapproximation (47.28 cm3 mol−1 K for 3 and 56.68 cm3 mol−1

K for 4) for four non-interacting lanthanide ions: TbIII ( J = 6and g = 3/2) and DyIII ( J = 15/2 and g = 4/3). For 3, the χmTvalue slowly increases from 46.60 cm3 mol−1 K at 300 K to54.62 cm3 mol−1 K at about 40 K. Upon further cooling, itdecreases to reach 45.16 cm3 mol−1 K at 2 K. The χmT versus Tcurve shows a broad protuberance at about 40 K, indicatingthe presence of ferromagnetic interactions between TbIII ions.

Fig. 3 (a) Temperature dependence of the χmT for 1–4 with an appliedfield of 1000 Oe. The solid line represents the best fit for 1 and 2. (b)Field dependence of magnetization of 1–4 at 2 K.



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This kind of interaction is strong enough to compensate thedecrease of χmT that resulted from the depopulated Starkstates. The ferromagnetic interaction resulting in the increaseof χmT upon cooling may be ascribed to the dipole–dipoleinteraction between TbIII ions.31 For 4, the χmT productremains roughly constant with decreasing temperature downto about 50 K, and then drops to a minimum value of23.53 cm3 mol−1 K at 2 K. The decrease of χmT in 4 is mostlikely due to a combination effect of the thermal depopulationof Stark sublevels and antiferromagnetic interactions.32 Due tothe presence of the unquenched orbital moment of the TbIII orDyIII center, it is difficult to fit the magnetic data of 3 and 4.

The field-dependent magnetizations for 3 and 4 are nearlythe same (Fig. 3b), both showing a regular increase in magneti-zation below about 1 T. The magnetization of 3 exhibits aplateau at high magnetic field, whereas that of 4 shows alinear increase at high magnetic field without complete satur-ation even at 8 T. The magnetization values of 26.36Nβ for 3and 29.19Nβ for 4 at 8 T are lower than their theoreticallyderived values (36Nβ for 3 and 40Nβ for 4). Furthermore, theM versus H/T curves at low temperatures for 3 and 4 are notsuperposed (Fig. 4 and S7c†), indicating the presence of mag-netic anisotropy. The divergence of M versus H/T curves of 4 ismore apparent than that of 3, suggesting that 4 displays amore significant magnetic anisotropy.33

In view of the weak interactions as well as the isotropicnature of GdIII ions in 2, the MCE of 2 is investigated accord-ing to the isothermal magnetization curves measured in anapplied field of up to 8 T and the temperature range of 2–7 K

(Fig. 5a). The magnetic entropy changes −ΔSM can bedescribed by the Maxwell equation as follows:3

�ΔSmðTÞΔH ¼ð½@MðT ;HÞ=@T �HdH ð3Þ

Using eqn (3), magnetic entropy changes ΔSm were calcu-lated from the experimental magnetization data (Fig. 5b). Itcan be seen that the maximum value of −ΔSm is 24.9 J kg−1

K−1 for ΔH = 8 T at 2 K, which is lower than the expected valueof 30.90 J kg−1 K−1 according to the equation nR ln(2S + 1) forfour isolated GdIII ions. This deviation may be caused by theweak intra-cluster anti-ferromagnetic interactions in 2.7,34

Table 2 Selected magnetostructural data of GdIII complexes

Complexa Gd⋯Gd/Å Gd–O–Gd/° Jb/cm−1 Bridge Ref.

[Gd2(OAc)6(H2O)4]·4H2O 4.206 115.5 0.03 I 28a[Gd2(Hsal)6(H2O)2] 4.250 116.12 0.025 I 28b[Gd2(mal)3(H2O)6]n 4.276 116.71 0.024 I 28c[Gd(Hcit)(H2O)2]n·nH2O 4.321 118.49 0.02 I 28d[Gd2(OAc)2(Ph2acac)4(MeOH)2] 4.128 113.65 0.019 I 12c[Gd2(OAc)6(H2O)4]·2H2O 4.159 115.47 0.016 I 28e[Gd4(OAc)4(acac)8(H2O)4] 4.271/4.334 114.45/117.73 0.012 I 12c[Gd2(tpac)6(H2O)4] 4.126 112.5 −0.006 I 28f[Gd2(pac)6(H2O)4] 4.112 113.16 −0.016 I 28f2 3.975 112.17 −0.029 I This work[Gd2(L)(NO3)2]·(NO3)·1.5H2O 3.500 92.48/94.93/95.12 −0.097 II 30a[Gd2(Hppa)2(H2ppa)Cl(py)(H2O)] 3.813 98.86/102.93/103.36 −0.021 II 30b[Gd4(μ3-OH)2L2(acac)6]·4CH3CN 3.689 93.52/100.67/102.44 −0.02 II 13d[Gd2(hfac)5(O2CPhCl)(tpno)3]·2H2O 3.92 102.14/103.38/105.37 −0.005 II 30c2 3.77 96.28/99.60/104.21 −0.05 II This work[Gd(H2sal)(Hsal)(sal)(H2O)]n 4.187 111.85/114.29 0.019 III 29a[Gd(OAc)3(MeOH)]n 4.055 110/112.62 0.017 III 12c[Gd(mta)(H2O)]n·nH2O 4.065 110/112.3 0.013 III 29b[Gd2(succinate)3(H2O)2]n·0.5nH2O 4.059 109.96/112.24 0.01 III 29c[Gd(OAc)3(H2O)]n 4.027/4.034 106.5/108.4 −0.006 III 12c2 3.819 104.40/105.87 −0.07 III This work

a Abbreviations used in this table: OAc = acetate, H2sal = salicylic acid, H2mal = 1,3-propanedioic acid, H4cit = citric acid, Ph2acacH =dibenzoylmethane, acacH = acetylacetone, tpac = 3-thiopheneacetate, pac = pentanoate, H3L = N[(CH2)2NvCH-R-CHvN–(CH2)2]3N (R = 1,3-(2-OH-5-Me-C6H2)), H3ppa = 6-(3-oxo-3-(2-hydroxyphenyl) propionyl)-2-pyridinecarboxylic acid, py = pyridine, hfac− = 1,1,1,5,5,5-hexafluoroacetylacetonate anion, tpno = tetrathiafulvaleneamido-2-pyridine-N-oxide, H3mta = methanetriacetic acid. b The spin Hamiltonian isdefined as H = −2JSASB.

Fig. 4 Plots of M vs. H/T for 4 at different temperatures below 5 K.



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Among the 3d-Gd based magnetic cryocooling materials, amagnetic entropy change −ΔSm larger than 24 J kg−1 K−1 isrelatively rare (Table 3). The maximum value of −ΔSm in 2 islarger than most of the reported Co–Gd clusters. Nevertheless,it is surpassed only by four known Co–Gd clusters,{Co10Gd42},

8a {Co4Gd10},8b {Co6Gd8}

8c and {Co16Gd24},8e with

the −ΔSm values of 41.3, 32.6, 28.6 and 28.0 J kg−1 K−1 forΔH = 7 T, respectively. This is not surprising as 2 contains dia-magnetic CoIII ions that reduce the magnetic density, resultingin a low magnetic entropy change.

Due to the presence of significant magnetic anisotropy in 4,both the temperature and frequency dependences of ac mag-netic susceptibilities were performed under a zero dc field(Fig. 6 and S8†). As illustrated in Fig. 6 and S8,† a frequencydependence of the ac susceptibility was observed with an out-of-phase signal that is weak in intensity and does not exhibit amaximum within the experimental temperature (above 2 K)and frequency (50 to 104 Hz) windows, which indicates thepresence of fast relaxation via quantum tunneling (QTM) asobserved in other low symmetry lanthanide based-SMMs.22j

Hence, in order to bypass any possible quantum tunnelingeffects, ac susceptibility measurements were further performedunder applied dc fields of 5000 and 8000 Oe (Fig. S9†). It isfound that 4 shows peaks in the temperature dependence ofx″m under applied dc fields, which is due to the suppression ofthe quantum tunneling of magnetization. The relaxation

follows a thermally activated mechanism, giving energy bar-riers of ΔE/KB = 25.2 and 32.4 K, relaxation times τ0 = 1.3 ×10−6 and 4.2 × 10−7 s at 5000 and 8000 Oe, respectively, basedon the Arrhenius law τ(Tp) = τ0exp(Δ/Tp) with a linear corre-lation of 1/Tp vs. ln(2πf ) (Fig. S10†). These energy barriers arecomparable to those reported for other compounds exhibitingthe field induced SMM behaviour.13d,35 It should be noted thatthe fast quantum tunneling relaxation observed in the case of

Fig. 5 (a) The field dependence of magnetization of 2 at 2–7 K. (b)Calculated −ΔSM using the magnetization data of 2 at various fields(0.5–8 T).

Table 3 −ΔSM of 2 and related 3d–Gd compounds

Compounds −ΔSM/J kg−1 K−1 Ref.

Co10Gd42 41.3 (7 T) 8aNi10Gd42 38.2 (7 T) 8aNi12Gd36 36.3 (7 T) 9dMn4Gd6 33.7 (7 T) 6bCo4Gd10 32.6 (7 T) 8bCu5Gd4 31.0 (9 T) 10aCo6Gd8 28.6 (7 T) 8cMn9Gd9 28.0 (7 T) 6bFe5Gd8 26.7 (7 T) 7Ni6Gd6 26.5 (7 T) 9aCo16Gd24 26.0 (7 T) 8e2 24.9 (8 T) This workCo4Gd6 23.6 (7 T) 9bCu6Gd6 23.5 (7 T) 10cCo8Gd4 22.3 (7 T) 8bNi8Gd4 22.0 (7 T) 9bNi12Gd5 21.8 (7 T) 9eCo8Gd8 21.4 (7 T) 8cCo8Gd4 21.1 (7 T) 9bCu36Gd24 21.0 (7 T) 10bZn8Gd4 20.8 (7 T) 9bCo8Gd8 20.4 (7 T) 8dCo4Gd2 20.0 (7 T) 8cCo4Gd6 19.9 (7 T) 8dCo6Gd4 19.7 (7 T) 8bMn4Gd4 19.0 (7 T) 6aNi6Gd2 17.6 (7 T) 9cMn12Gd6 17.0 (7 T) 6dCu8Gd4 14.6 (7 T) 9bCu8Gd2 12.8 (7 T) 10dCo8Gd2 11.8 (7 T) 8cCr2Gd2 11.4 (9 T) 11bCr7Gd 5.1 (7 T) 11a

Fig. 6 Frequency dependence in the zero dc field of the (x’’m) ac sus-ceptibility component at different temperatures for 4.



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4 may be attributed to the low symmetric coordination geome-try of the local DyIII sites as well as the weak intramolecularinteractions between the adjacent DyIII ions.36,37

Conclusions

In summary, using the in situ synthetic route, we have syn-thesized a series of new Co–Ln heterometallic clusters, inwhich the {Co2Ln4} core is bridged by two μ3-hydroxide, fourpiv−, four hmmp2− and two ae− ligands. Due to the absence ofany coordinated solvent molecules at the metal centres, allthese compounds have a high thermal stability and can bestable up to 220 °C. The magnetic measurements show thatthe Co2Gd4 cluster exhibits very weak antiferromagnetic inter-actions among the GdIII ions, as well as a large magnetocaloriceffect at low temperatures, showing a potential application inmagnetic cooling technology in the very-low temperaturerange. Replacement of GdIII ions with anisotropic DyIII ionsgives the Dy4Co2 cluster, which displays significantly fast QTMin a zero dc field that can be slowed down by application of anexternal dc field. Moreover, the magnetostructural studyreveals that the tiny change in Gd–O–Gd angles will result indifferent magnetic interactions. This work will be helpful forthe rational design and synthesis of highly thermally stable3d–4f based molecular magnetic cryogenic materials.

Acknowledgements

We thank the National Basic Research Program of China (973program, 2012CB821702), the National Natural Science Foun-dation of China (21233009 and 21173221) and the State KeyLaboratory of Structural Chemistry, Fujian Institute ofResearch on the Structure of Matter, Chinese Academy ofSciences for financial support.

Notes and references

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synthesis, structures, and magnetic properties of a series

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