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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*

Downloaded by University of Ottawa on 11 September 2012 Published on 03 August 2012 on http://pubs.rsc.org | doi:10.1039/C2DT31226C

Received 7th June 2012, Accepted 2nd August 2012 DOI: 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)terephthalohydrazide were obtained using a synthetic strategy involving a polytopic Schiff base ligand. Single-crystal X-ray analysis reveals the DyIII ion is in a distorted pentagonal interpenetrating tetrahedral arrangement. The two symmetrical DyIII ions in complex 2 exhibit the same geometry and are well-isolated in the molecule by an hmt2− ligand. The direct current (dc) and alternating current (ac) magnetic measurements of the compounds were investigated. Complex 1 did not exhibit any ac signal whereas a frequency dependant signal was observed for 2 under zero dc field. When an optimum dc field was applied, clear frequency dependant signals were obtained for both complexes indicative of Single-Ion Magnet behaviour with relaxation barriers of Ueff = 34 and 42 K for 1 and 2, respectively.

Introduction Ligand design plays a vital role in coordination chemistry allowing overall structural features to be engineered at a molecular level through ligand modification. Such molecular structure finetuning enables chemists to optimize the overall optical and magnetic properties of metal complexes.1 A ligand’s influence on the magnetic properties is well-known for transition metal complexes. However, only recently has their importance been underlined also in 4f chemistry.2 As such, several research groups have intensively studied crystal-field effects on lanthanide complexes and their magnetism. In some paramagnetic lanthanide complexes with the “right” crystal field and coordination environment, slow relaxation of the magnetisation was observed. Such magnet-like behaviour below their blocking temperature has even been observed for mononuclear complexes which are termed Single-Ion Magnets (SIMs).3 Different geometries of SIMs have been successfully synthesized with polyoxymetalates,3f macrocyclic3e and organometallic double-decker structures.3a,i The observed superparamagnet-like behaviour generally results from the presence of large spin ground state (ST) and Ising-type magnetoanisotropy (D) in those systems.4 In order to study and understand the intriguing slow relaxation of the magnetisation observed in SIMs it is essential to create a ligand system which can be fine-tuned. Schiff base ligands are ideal for

Department of Chemistry, University of Ottawa, 10 Marie-Curie, Ottawa, ON K1N 6N5, Canada. E-mail: [email protected]; Tel: +1 613 562 5800-2733 † Electronic supplementary information (ESI) available: Additional magnetic data are given (Fig. S1). CCDC 885486 and 885487. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt31226c

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such purposes as they allow simple ligand modifications through condensation reactions. A tridentate oxygen based o-vanillin ligand is an ideal chelate for coordinating oxophilic lanthanide ions. A decade ago, Costes and co-workers reported a Ln3 complex synthesized using the latter ligand,5 since then several o-vanillin lanthanide complexes have been reported.6 Moreover, the aldehyde group on the o-vanillin can serve as a site for simple condensation reactions to allow for ligand modification.7 Therefore, we have taken advantage of such ligand motif to create larger polytopic ligands for lanthanide chemistry. The Schiff base reaction of benzhydrazide with o-vanillin yielded the polytopic ligand N′-(2-hydroxy3-methoxybenzylidene)benzohydrazide (Hhmb) (Scheme 1). The latter chelate contains ideal coordination pockets for 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-methoxybenzylidene)terephthalohydrazide (H2hmt) (Bottom). Symmetric ligand (H2hmt) promoting the formation of the centrosymmetric complex 2.

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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) terephthalohydrazide (H2hmt). Hence, it can be envisioned that a centrosymmetric complex can be isolated using the H2hmt ligand. Herein, we present a unique synthetic approach to isolate a dysprosium based SIM and its molecular dimerised version. These molecules exhibit slow relaxation of the magnetisation at low temperatures with a relaxation barrier that increases significantly under an applied external static field.

temperature. Light yellow powder was collected through suction filtration 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).

Experimental General methods

All chemicals were purchased from Thermofisher Scientific and STREM chemicals and used without further purification. Infrared analyses were obtained using a Nicolet Nexus 550 FT-IR spectrometer in the 4000–650 cm−1 range. The spectra were recorded in the solid state by preparing KBr pellets. NMR spectra were acquired on a Bruker AVANCE spectrometer, operating at 400 MHz for 1H.

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

Crystals were mounted in inert oil and transferred to the cold gas stream of the diffractometer. Unit cell measurements and intensity data were collected at 200 K on a Bruker-AXS SMART 1 k CCD and SMART APEX2 CCD diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å). The data reduction included a correction for Lorentz and polarization effects, with an applied multi-scan absorption correction (SADABS).8 The crystal structure was solved and refined using the SHELXTL9 program suite. Direct methods yielded all nonhydrogen atoms, which were refined with anisotropic thermal parameters. All hydrogen atom positions were calculated geometrically and were riding on their respective atoms.

In order to synthesize the terephthalohydrazide precursor for the preparation 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 refluxed for 1 day. After being cooled to room temperature, white powder was collected through suction filtration and washed with a small amount of methanol. Yield = 68%. This product with a similar synthetic procedure has been reported previously in the literature.11 To a solution of terephthalohydrazide (10 mmol, 1.94 g) in methanol (10 ml), a solution of o-vanillin (20 mmol, 3.32 g) in methanol (10 ml) was added. The mixture was stirred for 24 h at room temperature. The product, a light yellow powder, was washed 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).

Magnetic measurements

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

The magnetic susceptibility measurements were obtained using a Quantum Design SQUID magnetometer MPMS-XL7 operating between 1.8 and 300 K for dc-applied fields ranging from −7 to 7 T. Direct current (dc) analyses were performed on polycrystalline samples of 7.8 and 9.6 mg for complexes 1 and 2, respectively, restrained in a polyethylene membrane. Ac susceptibility measurements were carried out under an oscillating ac field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. The magnetisation data were collected at 100 K to check for ferromagnetic impurities that were absent in all samples. A diamagnetic correction was applied for the sample holder and the sample.

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). The mixture was stirred for 5 min at room temperature and then filtered. After three days, X-ray-quality light brown needle crystals were isolated. All crystals have the same morphology, colour and high crystallinity. The sample was maintained in contact with 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).

X-Ray crystallography

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

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

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

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 then filtered. After three days, X-ray-quality red plate crystals were isolated. All crystals have the same morphology, colour and high crystallinity. The sample was maintained in contact with the mother liquor to prevent deterioration of the crystals, which were identified 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), was obtained through the reaction of a polydentate Schiff base ligand, Hhmb (1 equiv.), and Dy(NO3)3·6H2O (1 equiv.) in the presence of pyridine (4 equiv.) as base in a mixture of DMF– CHCl3. The employed ligand : metal : base ratio proved to be ideal for obtaining X-ray quality needle shaped light brown crystals of 1. The obtained compound crystallizes in the triclinic P1ˉ space group. A partially labelled X-ray structure of 1 is shown in Fig. 1 (top) and the X-ray information including cell parameters is given in Table 1. Selected bond distances and angles are given in Table 2. The DyIII ion of the mononuclear complex adopts a nine-coordinate distorted pentagonal interpenetrating tetrahedral geometry, where one ligand, two nitrate and two coordinated DMF solvent molecules occupy the coordination sites. Four oxygen atoms (O4, O6, O7, O9) from two nitrate groups form a distorted tetrahedron which interpenetrates the distorted pentagonal plane formed by N1, O2, O3 from the ligand and O10, O11 from the

Table 1

Crystallographic data for 1 and 2

Formula Fw Crystal system Space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dc/g cm−3 R1, wR2 (I > 2σ(I)) R1, wR2 (all data)

Table 2

1

2

C21H27DyN6O11 701.99 Triclinic P1ˉ 7.6044(1) 10.2506(2) 18.9048(3) 104.1310(10) 90.7570(10) 107.6510(10) 1355.67(4) 2 1.720 0.0262, 0.0628 0.0315, 0.0649

C42H62Dy2N14O24 1237.79 Triclinic P1ˉ 11.145(3) 11.145(3) 17.813(3) 81.334(2) 75.852(2) 84.688(2) 2357.2(7) 2 1.744 0.0332, 0.0909 0.0396, 0.0940

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

Dy1–O2 Dy1–O3 Dy1–O4 Dy1–O6 Dy1–O7 Dy1–O9 Dy1–O10 Dy1–O11 Dy1–N1 N3–Dy1–N4

1

2

2.203(3) 2.409(3) 2.468(4) 2.515(4) 2.498(4) 2.461(4) 2.361(4) 2.346(4) 2.555(4) 169.62(16)

2.231(22) 2.384(2) 2.447(2) 2.535(3) 2.479(2) 2.447(3) 2.327(2) 2.345(2) 2.533(2) 174.89(9)

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

Fig. 1 Top: Partially labelled molecular crystal structure of complex 1 with 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).


DMF molecules (Fig. 2). The polydentate Schiff base ligand coordinates to the Dy centre via two O atoms (O2 and O3) and one N atom (N1). The magnetic properties of nine-coordinate Dy SIMs have been studied recently and the geometry of the Dy ions was shown to play an important role in the direction of the anisotropic axis.3b,h In comparison with the other reported complexes, the aforementioned coordination geometry of the metal centre proves to be unique. Dalton Trans.


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

Similarly, the dinuclear [Dy2(hmt)(NO3)4(DMF)4]·DMF complex, (2), was obtained through the reaction of H2hmt (1 equiv.), Dy(NO3)3·6H2O (2 equiv.) and pyridine (4 equiv.) in THF and DMF. The same compound can be obtained when ratio of H2hmt : Dy(NO3)3·6H2O from 1 : 2 to 1 : 1 is varied, however, the yield is much lower. Compared to the previous reaction, THF was used as a co-solvent in order to isolate X-ray quality singles crystals of 2. Single crystal X-Ray diffraction reveals that complex 2 crystallizes in the triclinic P1ˉ space group. The structure of complex 2 is shown in Fig. 1, bottom and selected bond distances and angles are given in Table 2. The two symmetrical DyIII ions are well-isolated in the molecule by the phenyl group spacer of the rigid hmt2− ligand with an intramolecular distance of 12.00 Å. This complex could be described as two mononuclear units bridged by the phenyl ring of the ligand. Each DyIII centre in 2 was shown to exhibit a similar coordination environment to complex 1 with the same atom labels. As seen in Table 2, the difference in bond distances between complexes 1 and 2 does not exceed 0.04 Å. The packing arrangement along the b axis of complex 1 is presented in Fig. 3. All mononuclear complexes are well-isolated with the closest intermolecular Dy⋯Dy distance being 7.60 Å. The phenyl rings of the hmb−1 ligands of different layers are participating in π–π stacking with a distance of 4.29 Å. The packing arrangement along the a axis of complex 2 is presented in Fig. 4. Close inspection reveals that all dinuclear complexes are wellisolated with the closest intermolecular Dy⋯Dy distance being 8.15 Å which is shorter than the intramolecular Dy1⋯Dy1a distance of 12.00 Å. The magnetic susceptibility for both complexes was measured in an applied magnetic field of 1000 Oe in the range of 1.8 K to 300 K using polycrystalline samples (Fig. 5). At room temperature, the χT values of 13.55 and 28.62 cm3 K mol−1 for complexes 1 and 2, respectively, are reasonably close to the expected values of 14.17 and 28.34 cm3 K mol−1 for one and two uncoupled DyIII ions (S = 5/2, L = 5, 6H15/2, g = 4/3), respectively. The χT product remains relatively constant above 60 K and decreases at lower temperatures reaching 12.19 and 26.63 cm3 K mol−1 for 1 and 2 at 2 and 1.8 K, respectively. Dalton Trans.

Fig. 4 Packing arrangement along the crystallographic a axis for complex 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 complexes 1 and 2 (with χ = M/H normalized per mol).

This behaviour is generally indicative of weak antiferromagnetic coupling between the metal centres. However, due to the large physical separation between DyIII ions, this decrease is most likely due to the thermal depopulation of the Stark sub-levels and/or the presence of large anisotropy in the system. The reduced magnetisation plots, M vs. H/T, (Fig. S1† and Fig. 6 for complexes 1 and 2, respectively) at different temperatures show magnetisation curves that are not super imposable on a single master curve. These two figures are indicative of the presence of significant magnetoanisotropy and/or low-lying excitation states present in the molecules. In order to investigate the possibility of SIM behaviour, ac magnetic susceptibility measurements were carried out under zero dc field (Fig. 7 for complex 2). A frequency dependent tail of a peak is observed in the out-of-phase susceptibility, χ′′, below 15 K for complex 2 indicating potential SMM behaviour at very low temperature; however, it is difficult to quantify the This journal is © The Royal Society of Chemistry 2012


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Fig. 6

M vs. H/T plot measured between 1.8 K and 8 K for 2. Fig. 8 Field dependence of the characteristic frequency (maximum of χ′′) 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 observed at 1800 Oe for both complexes.

Fig. 7 Frequency dependence of the out-of-phase ac susceptibility for 2 between 10 and 1500 Hz at Hdc = 0 Oe.

energy barrier without a full peak with maxima. No signal in the χ′′ vs. T plot was observed for complex 1. Such behaviour generally indicates that the slow relaxation of the magnetisation is highly influenced by the quantum tunnelling of the magnetisation (QTM) through the spin reversal barrier, which is very common in mononuclear SMMs. Moreover, in order to shortcut the QTM, ac measurements need to be carried out under an optimum dc field. Therefore, we initially carried out ac measurements under various dc fields to determine the optimum field for which the QTM will be reduced or suppressed (Fig. 8). The optimum dc field was found to be 1800 Oe for both complexes. Ac measurements under the applied optimum field of 1800 Oe reveal a frequency dependent signal with a clear out-of-phase (χ′′) peak (Fig. 9, bottom for complex 1 and Fig. 10, bottom for complex 2). Such behaviour is indicative of super paramagnetlike slow magnetisation relaxation of a SMM. The thermally activated relaxation follows an Arrhenius-like behaviour (τ = τ0exp(Ueff/kT)) where the anisotropic energy barriers are calculated 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, This journal is © The Royal Society of Chemistry 2012

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 = 1800 Oe. Inset: Relaxation time of the magnetisation ln(τ) vs. T−1 (Arrhenius plot using temperature-dependent ac data). The solid line corresponds to the fit.

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Notes and references

Fig. 10 Temperature dependence of the in-phase (top) and out-ofphase (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 corresponds to the fit.

inset) for complex 2. Slight difference in the energy barrier can be attributed to minor changes around the coordination environment of the metal ions (Table 2).

Conclusion In summary, we have designed and successfully synthesized a mononuclear DyIII field-induced SIM and extended it to a wellisolated centrosymmetric dinuclear structure through ligand dimerisation. The slow relaxation of the magnetisation under optimum field in complex 1 confirms the SIM nature with an anisotropic energy barrier of 34 K. Similar magnetic properties are also obtained for complex 2 with an anisotropic energy barrier of 42 K. This synthetic approach allows us to envision a new methodology to promote formation of larger molecules while retaining their physical properties via controlled ligand modification.

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

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