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Cite this: DOI: 10.1039/c2dt31226c

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Connecting single-ion magnets through ligand dimerisation†

Po-Heng Lin, Ilia Korobkov, Tara J. Burchell and Muralee Murugesu*

Received 7th June 2012, Accepted 2nd August 2012DOI: 10.1039/c2dt31226c

A mononuclear as well as dinuclear DyIII complexes of general formula [Dy(hmb)(NO3)2(DMF)2] (1) and[Dy2(hmt)(NO3)4(DMF)4]·DMF (2), where Hhmb: (N′-(2-hydroxy-3-methoxybenzylidene)-benzohydrazide and H2hmt: (N1,N4)-N′1,N′4-bis(2-hydroxy-3-methoxybenzylidene)terephthalohydrazidewere obtained using a synthetic strategy involving a polytopic Schiff base ligand. Single-crystal X-rayanalysis reveals the DyIII ion is in a distorted pentagonal interpenetrating tetrahedral arrangement. Thetwo symmetrical DyIII ions in complex 2 exhibit the same geometry and are well-isolated in the moleculeby an hmt2− ligand. The direct current (dc) and alternating current (ac) magnetic measurements of thecompounds were investigated. Complex 1 did not exhibit any ac signal whereas a frequency dependantsignal was observed for 2 under zero dc field. When an optimum dc field was applied, clear frequencydependant signals were obtained for both complexes indicative of Single-Ion Magnet behaviour withrelaxation barriers of Ueff = 34 and 42 K for 1 and 2, respectively.

Introduction

Ligand design plays a vital role in coordination chemistry allow-ing overall structural features to be engineered at a molecularlevel through ligand modification. Such molecular structure fine-tuning enables chemists to optimize the overall optical and mag-netic properties of metal complexes.1 A ligand’s influence on themagnetic properties is well-known for transition metal com-plexes. However, only recently has their importance been under-lined also in 4f chemistry.2 As such, several research groupshave intensively studied crystal-field effects on lanthanide com-plexes and their magnetism. In some paramagnetic lanthanidecomplexes with the “right” crystal field and coordinationenvironment, slow relaxation of the magnetisation was observed.Such magnet-like behaviour below their blocking temperaturehas even been observed for mononuclear complexes which aretermed Single-Ion Magnets (SIMs).3 Different geometries ofSIMs have been successfully synthesized with polyoxymetala-tes,3f macrocyclic3e and organometallic double-decker struc-tures.3a,i The observed superparamagnet-like behaviour generallyresults from the presence of large spin ground state (ST) andIsing-type magnetoanisotropy (D) in those systems.4 In order tostudy and understand the intriguing slow relaxation of the mag-netisation observed in SIMs it is essential to create a ligandsystem which can be fine-tuned. Schiff base ligands are ideal for

such purposes as they allow simple ligand modifications throughcondensation reactions.

A tridentate oxygen based o-vanillin ligand is an ideal chelatefor coordinating oxophilic lanthanide ions. A decade ago, Costesand co-workers reported a Ln3 complex synthesized using thelatter ligand,5 since then several o-vanillin lanthanide complexeshave been reported.6 Moreover, the aldehyde group on theo-vanillin can serve as a site for simple condensation reactions toallow for ligand modification.7 Therefore, we have taken advan-tage of such ligand motif to create larger polytopic ligands forlanthanide chemistry. The Schiff base reaction of benzhydrazidewith o-vanillin yielded the polytopic ligand N′-(2-hydroxy-3-methoxybenzylidene)benzohydrazide (Hhmb) (Scheme 1).The latter chelate contains ideal coordination pocketsfor the encapsulation of DyIII ions.7b,d Similarly, when

Scheme 1 Polytopic ligands (N′-(2-hydroxy-3-methoxybenzylidene)-benzohydrazide (Hhmb) (Top) and N′1,N′4-bis(2-hydroxy-3-methoxy-benzylidene)terephthalohydrazide (H2hmt) (Bottom). Symmetric ligand(H2hmt) promoting the formation of the centrosymmetric complex 2.

†Electronic supplementary information (ESI) available: Additionalmagnetic data are given (Fig. S1). CCDC 885486 and 885487. Forcrystallographic data in CIF or other electronic format see DOI:10.1039/c2dt31226c

Department of Chemistry, University of Ottawa, 10 Marie-Curie,Ottawa, ON K1N 6N5, Canada. E-mail: m.murugesu@uottawa.ca;Tel: +1 613 562 5800-2733

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terephthalohydrazide is employed in the presence of o-vanillin,a condensation reaction yields the dimerised form of Hhmb,N′1,N′4-bis(2-hydroxy-3-methoxybenzylidene) terephthalohydra-zide (H2hmt). Hence, it can be envisioned that a centrosymmetriccomplex can be isolated using the H2hmt ligand.

Herein, we present a unique synthetic approach to isolate adysprosium based SIM and its molecular dimerised version.These molecules exhibit slow relaxation of the magnetisation atlow temperatures with a relaxation barrier that increases signifi-cantly under an applied external static field.

Experimental

General methods

All chemicals were purchased from Thermofisher Scientific andSTREM chemicals and used without further purification. Infra-red analyses were obtained using a Nicolet Nexus 550 FT-IRspectrometer in the 4000–650 cm−1 range. The spectra wererecorded in the solid state by preparing KBr pellets. NMRspectra were acquired on a Bruker AVANCE spectrometer, oper-ating at 400 MHz for 1H.

X-Ray crystallography

Crystals were mounted in inert oil and transferred to the cold gasstream of the diffractometer. Unit cell measurements and inten-sity data were collected at 200 K on a Bruker-AXS SMART 1 kCCD and SMART APEX2 CCD diffractometer using graphitemonochromated MoKα radiation (λ = 0.71073 Å). The datareduction included a correction for Lorentz and polarizationeffects, with an applied multi-scan absorption correction(SADABS).8 The crystal structure was solved and refined usingthe SHELXTL9 program suite. Direct methods yielded all non-hydrogen atoms, which were refined with anisotropic thermalparameters. All hydrogen atom positions were calculated geome-trically and were riding on their respective atoms.

Magnetic measurements

The magnetic susceptibility measurements were obtained using aQuantum Design SQUID magnetometer MPMS-XL7 operatingbetween 1.8 and 300 K for dc-applied fields ranging from −7 to7 T. Direct current (dc) analyses were performed on polycrystal-line samples of 7.8 and 9.6 mg for complexes 1 and 2, respect-ively, restrained in a polyethylene membrane. Ac susceptibilitymeasurements were carried out under an oscillating ac field of3 Oe and ac frequencies ranging from 1 to 1500 Hz. The magne-tisation data were collected at 100 K to check for ferromagneticimpurities that were absent in all samples. A diamagnetic correc-tion was applied for the sample holder and the sample.

Synthesis of N′-(2-hydroxy-3-methoxybenzylidene)benzohydrazide (Hhmb)

To a solution of benzhydrazide (20.0 mmol, 2.72 g) in methanol(10 ml), a solution of o-vanillin (20.0 mmol, 3.32 g) in methanol(10 ml) was added. The mixture was stirred for 24 h at room

temperature. Light yellow powder was collected through suctionfiltration and washed with a small amount of methanol. Yield =85%. IR (KBr, cm−1): 3421(br), 3079(m), 1655(s), 1605(m),1572(m), 1534(w), 1465(s), 1448(w), 1411(w), 1379(s),1346(s), 1249(s), 1165(m), 1096(m), 1074(s), 1028(w), 972(m),957(w), 953(m), 890(w), 873(w), 834(w), 804(s), 787(s),775(m), 736(s), 717(s), 702(w). 1H-NMR (DMSO-d6,400 MHz): δ(ppm) 12.11 (s, 1H), 11.10 (s, 1H), 8.68 (s, 1H),7.96 (d, J = 7.3 Hz, 2H), 7.62 (t, J = 7.3 Hz, 1H), 7.55 (t, J =7.1 Hz, 2H), 7.16 (dd, J = 7.8, 0.7 Hz, 1H), 7.05 (dd, J = 7.1,1.0 Hz, 1H), 6.88 (t, J = 7.8 Hz, 1H), 3.83 (s, 3H).

Synthesis of N′1,N′4-bis(2-hydroxy-3-methoxybenzylidene)terephthalohydrazide (H2hmt)

In order to synthesize the terephthalohydrazide precursor for thepreparation of H2hmt, a solution of dimethyl terephthalate10

(10 mmol, 1.94 g) was added to a solution of N2H2·H2O(4.0 mmol, 224 μl) in MeOH (15 ml). The solution was refluxedfor 1 day. After being cooled to room temperature, white powderwas collected through suction filtration and washed with a smallamount of methanol. Yield = 68%. This product with a similarsynthetic procedure has been reported previously in theliterature.11

To a solution of terephthalohydrazide (10 mmol, 1.94 g) inmethanol (10 ml), a solution of o-vanillin (20 mmol, 3.32 g) inmethanol (10 ml) was added. The mixture was stirred for 24 h atroom temperature. The product, a light yellow powder, waswashed with cold methanol and filtered in vacuo for 2 h. Yield =75%. IR (KBr, cm−1): 3425(br), 3037(w), 1718(m), 1650(s),1604(m), 1572(m), 1463(m), 1433(w), 1403(w), 1380(w),1359(w), 1326(w), 1277(s), 1246(s), 1197(w), 1148(w),1106(m), 1076(m), 1017(m), 962(m), 896(w), 870(w), 828(w),783(w), 737(m) and 716(m). 1H-NMR (DMSO-d6, 400 MHz):δ(ppm) 12.22 (s, 2H), 10.87 (s, 2H), 8.69 (d, J = 5.3 Hz, 2H),8.21 (m, 4H), 7.19 (dd, J = 7.8, 1.2 Hz, 2H), 7.06 (dd, J = 8.0,1.2 Hz, 2H), 6.82 (t, J = 8.0 Hz, 2H), 3.83 (s, 6H).

Synthesis of [Dy(hmb)(NO3)2(DMF)2] (1)

A solution of Dy(NO3)3·6H2O (0.25 mmol, 0.11 g) in DMF(5 ml) was added slowly to a solution of Hhmb (0.25 mmol,0.07 g) and pyridine (0.50 mmol, 40 μl) in CHCl3 (25 ml). Themixture was stirred for 5 min at room temperature and thenfiltered. After three days, X-ray-quality light brown needle crys-tals were isolated. All crystals have the same morphology, colourand high crystallinity. The sample was maintained in contactwith the mother liquor to prevent deterioration of the crystals,which were identified crystallographically. Yield 24%. IR (KBr,cm−1): 3259(br), 2847(w), 1641(s), 1605(s), 1569(s), 1545(w),1448(s), 1383(s), 1324(m), 1282(s), 1217(s), 1172(w), 1108(m),1030(s), 975(w), 899(w), 859(m), 795(w), 745(m) and 680(w).

Synthesis of [Dy2(hmt)(NO3)4(DMF)4]·DMF (2)

A solution of Dy(NO3)3·6H2O (0.25 mmol, 0.11 g) in DMF(5 ml) was added slowly to a solution of H2hmt (0.125 mmol,0.06 g) and pyridine (0.50 mmol, 40 μl) in THF (25 ml).

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The mixture was stirred for 5 min at room temperature and thenfiltered. After three days, X-ray-quality red plate crystals wereisolated. All crystals have the same morphology, colour and highcrystallinity. The sample was maintained in contact with themother liquor to prevent deterioration of the crystals, which wereidentified crystallographically. Yield 30%. IR (KBr, cm−1):3436(br), 1663(s), 1604(s), 1545(w), 1459(m), 1442(m),1414(w), 1383(s), 1295(m), 1250(m), 1219(m), 1170(w),1115(m), 1086(m), 1058(w), 1030(w), 979(w), 887(w), 859(w),815(w), 742(m) and 718(w).

Results and discussion

The mononuclear complex, [Dy(hmb)(NO3)2(DMF)2], (1), wasobtained through the reaction of a polydentate Schiff baseligand, Hhmb (1 equiv.), and Dy(NO3)3·6H2O (1 equiv.) in thepresence of pyridine (4 equiv.) as base in a mixture of DMF–CHCl3. The employed ligand : metal : base ratio proved to beideal for obtaining X-ray quality needle shaped light brown crys-tals of 1.

The obtained compound crystallizes in the triclinic P1̄ spacegroup. A partially labelled X-ray structure of 1 is shown inFig. 1 (top) and the X-ray information including cell parametersis given in Table 1. Selected bond distances and angles are givenin Table 2.

The DyIII ion of the mononuclear complex adopts a nine-coor-dinate distorted pentagonal interpenetrating tetrahedral geometry,where one ligand, two nitrate and two coordinated DMF solventmolecules occupy the coordination sites. Four oxygen atoms(O4, O6, O7, O9) from two nitrate groups form a distorted tetra-hedron which interpenetrates the distorted pentagonal planeformed by N1, O2, O3 from the ligand and O10, O11 from the

DMF molecules (Fig. 2). The polydentate Schiff base ligandcoordinates to the Dy centre via two O atoms (O2 and O3) andone N atom (N1). The magnetic properties of nine-coordinateDy SIMs have been studied recently and the geometry of the Dyions was shown to play an important role in the direction of theanisotropic axis.3b,h In comparison with the other reported com-plexes, the aforementioned coordination geometry of the metalcentre proves to be unique.

Fig. 1 Top: Partially labelled molecular crystal structure of complex 1with hydrogen atoms and carbon labels omitted for clarity. Bottom:molecular crystal structure of complex 2. Colour code: yellow (Dy),red (O), blue (N), grey (C).

Table 1 Crystallographic data for 1 and 2

1 2

Formula C21H27DyN6O11 C42H62Dy2N14O24Fw 701.99 1237.79Crystal system Triclinic TriclinicSpace group P1̄ P1̄a/Å 7.6044(1) 11.145(3)b/Å 10.2506(2) 11.145(3)c/Å 18.9048(3) 17.813(3)α/° 104.1310(10) 81.334(2)β/° 90.7570(10) 75.852(2)γ/° 107.6510(10) 84.688(2)V/Å3 1355.67(4) 2357.2(7)Z 2 2Dc/g cm−3 1.720 1.744R1, wR2 (I > 2σ(I)) 0.0262, 0.0628 0.0332, 0.0909R1, wR2 (all data) 0.0315, 0.0649 0.0396, 0.0940

Table 2 Selected bond distances (Å) and angles (°) for 1 and 2

1 2

Dy1–O2 2.203(3) 2.231(22)Dy1–O3 2.409(3) 2.384(2)Dy1–O4 2.468(4) 2.447(2)Dy1–O6 2.515(4) 2.535(3)Dy1–O7 2.498(4) 2.479(2)Dy1–O9 2.461(4) 2.447(3)Dy1–O10 2.361(4) 2.327(2)Dy1–O11 2.346(4) 2.345(2)Dy1–N1 2.555(4) 2.533(2)N3–Dy1–N4 169.62(16) 174.89(9)

Fig. 2 Coordination sphere of the DyIII ion in a unique distorted penta-gonal interpenetrating tetrahedral arrangement.

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Similarly, the dinuclear [Dy2(hmt)(NO3)4(DMF)4]·DMFcomplex, (2), was obtained through the reaction of H2hmt(1 equiv.), Dy(NO3)3·6H2O (2 equiv.) and pyridine (4 equiv.) inTHF and DMF. The same compound can be obtained when ratioof H2hmt : Dy(NO3)3·6H2O from 1 : 2 to 1 : 1 is varied, however,the yield is much lower. Compared to the previous reaction, THFwas used as a co-solvent in order to isolate X-ray quality singlescrystals of 2. Single crystal X-Ray diffraction reveals thatcomplex 2 crystallizes in the triclinic P1̄ space group. The struc-ture of complex 2 is shown in Fig. 1, bottom and selected bonddistances and angles are given in Table 2. The two symmetricalDyIII ions are well-isolated in the molecule by the phenyl groupspacer of the rigid hmt2− ligand with an intramolecular distanceof 12.00 Å. This complex could be described as two mono-nuclear units bridged by the phenyl ring of the ligand. EachDyIII centre in 2 was shown to exhibit a similar coordinationenvironment to complex 1 with the same atom labels. As seen inTable 2, the difference in bond distances between complexes 1and 2 does not exceed 0.04 Å.

The packing arrangement along the b axis of complex 1 is pre-sented in Fig. 3. All mononuclear complexes are well-isolatedwith the closest intermolecular Dy⋯Dy distance being 7.60 Å.The phenyl rings of the hmb−1 ligands of different layers are par-ticipating in π–π stacking with a distance of 4.29 Å. The packingarrangement along the a axis of complex 2 is presented in Fig. 4.Close inspection reveals that all dinuclear complexes are well-isolated with the closest intermolecular Dy⋯Dy distance being8.15 Å which is shorter than the intramolecular Dy1⋯Dy1a dis-tance of 12.00 Å.

The magnetic susceptibility for both complexes was measuredin an applied magnetic field of 1000 Oe in the range of 1.8 K to300 K using polycrystalline samples (Fig. 5). At room temp-erature, the χT values of 13.55 and 28.62 cm3 K mol−1 for com-plexes 1 and 2, respectively, are reasonably close to the expectedvalues of 14.17 and 28.34 cm3 K mol−1 for one and twouncoupled DyIII ions (S = 5/2, L = 5, 6H15/2, g = 4/3), respect-ively. The χT product remains relatively constant above 60 Kand decreases at lower temperatures reaching 12.19 and26.63 cm3 K mol−1 for 1 and 2 at 2 and 1.8 K, respectively.

This behaviour is generally indicative of weak antiferromagneticcoupling between the metal centres. However, due to the largephysical separation between DyIII ions, this decrease is mostlikely due to the thermal depopulation of the Stark sub-levelsand/or the presence of large anisotropy in the system. Thereduced magnetisation plots, M vs. H/T, (Fig. S1† and Fig. 6 forcomplexes 1 and 2, respectively) at different temperatures showmagnetisation curves that are not super imposable on a singlemaster curve. These two figures are indicative of the presence ofsignificant magnetoanisotropy and/or low-lying excitation statespresent in the molecules.

In order to investigate the possibility of SIM behaviour, acmagnetic susceptibility measurements were carried out underzero dc field (Fig. 7 for complex 2). A frequency dependent tailof a peak is observed in the out-of-phase susceptibility, χ′′,below 15 K for complex 2 indicating potential SMM behaviourat very low temperature; however, it is difficult to quantify the

Fig. 4 Packing arrangement along the crystallographic a axis forcomplex 2 with hydrogen atoms omitted for clarity. Colour code: yellow(Dy), red (O), blue (N), grey (C).

Fig. 5 Temperature dependence of the χT product at 1000 Oe for com-plexes 1 and 2 (with χ = M/H normalized per mol).

Fig. 3 Packing arrangement along the crystallographic b axis forcomplex 1 with hydrogen atoms omitted for clarity. Colour code: yellow(Dy), red (O), blue (N), grey (C).

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energy barrier without a full peak with maxima. No signal in theχ′′ vs. T plot was observed for complex 1. Such behaviour gener-ally indicates that the slow relaxation of the magnetisation ishighly influenced by the quantum tunnelling of the magnetisa-tion (QTM) through the spin reversal barrier, which is verycommon in mononuclear SMMs. Moreover, in order to shortcutthe QTM, ac measurements need to be carried out under anoptimum dc field. Therefore, we initially carried out ac measure-ments under various dc fields to determine the optimum field forwhich the QTM will be reduced or suppressed (Fig. 8). Theoptimum dc field was found to be 1800 Oe for both complexes.

Ac measurements under the applied optimum field of 1800Oe reveal a frequency dependent signal with a clear out-of-phase(χ′′) peak (Fig. 9, bottom for complex 1 and Fig. 10, bottom forcomplex 2). Such behaviour is indicative of super paramagnet-like slow magnetisation relaxation of a SMM. The thermallyactivated relaxation follows an Arrhenius-like behaviour (τ =τ0exp(Ueff/kT)) where the anisotropic energy barriers are calcu-lated to be Ueff = 34 (1) K (τ0 = 3.2 (3) × 10−6 s) (Fig. 9, inset)for complex 1 and 42 (2) K (τ0 = 1.6 (2) × 10−6 s) (Fig. 10,

Fig. 6 M vs. H/T plot measured between 1.8 K and 8 K for 2.

Fig. 7 Frequency dependence of the out-of-phase ac susceptibility for2 between 10 and 1500 Hz at Hdc = 0 Oe.

Fig. 8 Field dependence of the characteristic frequency (maximum ofχ′′) as a function of the applied dc field for complexes 1 (black) and 2(red) at 8 K. Line is guide for the eyes and the optimum field is observedat 1800 Oe for both complexes.

Fig. 9 Temperature dependence of the in-phase (top) and out-of-phase(bottom) ac susceptibility for 1 between 1 and 1500 Hz at Hdc = 1800Oe. Inset: Relaxation time of the magnetisation ln(τ) vs. T−1 (Arrheniusplot using temperature-dependent ac data). The solid line corresponds tothe fit.

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inset) for complex 2. Slight difference in the energy barrier canbe attributed to minor changes around the coordination environ-ment of the metal ions (Table 2).

Conclusion

In summary, we have designed and successfully synthesized amononuclear DyIII field-induced SIM and extended it to a well-isolated centrosymmetric dinuclear structure through liganddimerisation. The slow relaxation of the magnetisation underoptimum field in complex 1 confirms the SIM nature with an ani-sotropic energy barrier of 34 K. Similar magnetic properties arealso obtained for complex 2 with an anisotropic energy barrier of42 K. This synthetic approach allows us to envision a new meth-odology to promote formation of larger molecules while retain-ing their physical properties via controlled ligand modification.

Acknowledgements

We thank the University of Ottawa, the Canada Foundation forInnovation (CFI), FFCR, NSERC (Discovery and RTI grants)for financial support.

Notes and references

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Fig. 10 Temperature dependence of the in-phase (top) and out-of-phase (bottom) ac susceptibility for 2 between 1 and 1500 Hz at Hdc =1800 Oe. Inset: Relaxation time of the magnetisation ln(τ) vs. T−1

(Arrhenius plot using temperature-dependent ac data). The solid line cor-responds to the fit.

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