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Cite this: DOI: 10.1039/c3dt52634h
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Anion effects on the structures and magnetic properties of binuclear lanthanide single-molecule magnets† Fen Yang, Qi Zhou, Guang Zeng, Guanghua Li, Lu Gao, Zhan Shi* and Shouhua Feng Here we report the anion-induced changes of structures and magnetic properties in binuclear lanthanide compounds. Firstly, two Dy3+-based compounds, [Dy2(Mq)4(NO3)6] (1) and [Dy2(Mq)4Cl6](EtOH)2 (2) (Mq = 8-hydroxy-2-methylquinoline), were synthesized and characterized. They contain similar binuclear Dy2O2 cores, while the different peripheral anions lead to quite different coordination environments of the Dy3+ ion. In compound 1, the Dy3+ ion is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment. In compound 2, the Dy3+ ion has a highly distorted six-coordinated octahedral environment. Their Gd3+ analogues, [Gd2(Mq)4(NO3)6] (3) and [Gd2(Mq)4Cl6](EtOH)2 (4), were also studied to investigate the magnetic interaction between metal ions. Variable-temperature dc
Received 23rd September 2013, Accepted 17th October 2013
magnetic susceptibility measurements show that all the compounds are weakly antiferromagnetically coupled. Ac magnetic susceptibility measurements reveal that both compounds 1 and 2 exhibit single-
DOI: 10.1039/c3dt52634h
molecule magnet (SMM) behaviour, while the thermal energy barrier of 2 is significantly higher than
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that of 1 (Δ/kB = 40.0 K for 1 and Δ/kB = 102.4 K for 2).
Introduction Single-molecule magnets (SMMs) characterized by slow magnetic relaxation have recently attracted much research interest since they offer the prospect of storing and processing magnetic information at a molecular level.1 The magnet-like behaviour of SMMs arises from the intrinsic large spin ground state (S) and uniaxial magnetic anisotropy (|D|).2 Lanthanides play a remarkable role in this field due to their large magnetic moments and large anisotropy. Indeed, since the discovery that a single-ion lanthanide compound displays slow relaxation of the magnetization,3 a variety of lanthanide SMMs have been reported, from mono-lanthanide compounds to aggregates based on lanthanide ions.4–10 Among them, binuclear lanthanide compounds are considered as a special group and have been studied extensively.5 They represent the simplest molecular units which allow the study of magnetic interactions between two spin carriers, and they are also crucial in
State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: [email protected]; Fax: +86 431 85168624 † Electronic supplementary information (ESI) available: Crystallographic details in CIF format, selected bond lengths, XRPD patterns, and some magnetic plots. CCDC 923350–923353 for 1, 3, 2, 4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52634h
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understanding the magnetization relaxation characteristics of lanthanide SMMs. It is believed that if a binuclear SMM can be designed and isolated in a controllable manner, one could potentially create systems with higher nuclearity using a bottom up molecular approach and obtain SMMs with significantly higher effective energy barrier.5a Recently, it has been demonstrated that the overall electronic structure of lanthanide ion is very sensitive to its coordination environment. Even subtle ligand changes can drastically influence the overall physical properties of the lanthanide compounds, and the SMM behaviour of lanthanide ions is highly dependent on the geometry of the coordination site.5b,c,11 This brings us a new proposition. As we noticed that there are several binuclear lanthanide compounds with one or more coordinated nitrate anions,12–14 we wonder what will happen if the nitrate anions are replaced by other anions such as chlorine ions. Both Cl− and NO3− ions have a negative charge, but Cl− ion will never coordinate to metal ions with a chelating mode as NO3− ion usually does. It will certainly lead to a quite different coordination environment (coordination numbers and geometries), and in turn, it may make a difference in the magnetic behaviors. To confirm our assumption, we chose a binuclear lanthanide compound, [Dy2(Mq)4(NO3)6] (1) (Mq = 8-hydroxy-2methylquinoline), where there are three NO3− ions coordinated to each Dy3+ centre. The structure of a La3+ analogue has been
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Paper reported earlier.15 When using DyCl3 as the metal source instead of Dy(NO3)3, we obtained a new binuclear Dy3+ compound, [Dy2(Mq)4Cl6](EtOH)2 (2). As we anticipated, Cl− ions took the place of NO3− ions, and the coordination numbers and geometry of the Dy3+ ions in 2 were significantly different from those of 1. The Dy3+ ion in compound 1 is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment, whereas it is six-coordinated and characterized by a distorted octahedral geometry in compound 2. We also synthesized their Gd3+ analogues [Gd2(Mq)4(NO3)6] (3) and [Gd2(Mq)4Cl6] (EtOH)2 (4), respectively, to investigate the magnetic interaction between the lanthanide ions. Magnetic measurements demonstrate that the compounds exhibit weak intra-binuclear antiferromagnetic interaction. Both compounds 1 and 2 display SMM behaviour, while the effective energy gap of 2 is much higher than that of 1. To the best of our knowledge, investigations of anion-perturbed magnetic slow relaxation in lanthanide-based SMMs are extremely rare up to now.16,17
Experimental section Synthesis All reagents and solvents were commercially available and were used without further purification. [Dy2(Mq)4(NO3)6] (1). A mixture of Dy(NO3)3·6H2O (0.1 mmol, 0.045 g) and Mq (0.25 mmol, 0.039 g) in MeOH (10 mL) was stirred for 30 min and then filtered. The yellow filtrate was heated in a Teflon-lined steel bomb at 60 °C for 3 days. Yellow block-shaped crystals formed were collected in 26% yield (based on Dy). IR (KBr, cm−1): 3364(s), 3183(s), 1627(m), 1582(s), 1537(w), 1462(s), 1386(m), 1326(s), 1099(m), 1039(w), 888(m), 827(m), 737(m), 570(m). Elem. anal. calcd (%) for C40H36Dy2N10O22: C, 36.02; H, 2.74; N, 10.50. Found: C, 35.81; H, 2.66; N, 10.27. [Dy2(Mq)4Cl6](EtOH)2 (2). A mixture of DyCl3·6H2O (0.4 mmol, 0.150 g) and Mq (1.0 mmol, 0.159 g) in EtOH (8 mL) and MeCN (2 mL) was stirred for 30 min. The resulting mixture was heated in a Teflon-lined steel bomb at 60 °C for 5 days. Yellow block-shaped crystals formed were collected in 38% yield (based on Dy). IR (KBr, cm−1): 3243(m), 3198(m), 3093(w), 1642(m), 1597(s), 1462(s), 1386(m), 1326(s), 1099(m), 1039(w), 927(m), 873(m), 836(s), 743(m), 576(m). Elem. anal. calcd (%) for C44H48Cl6Dy2N4O6: C, 41.72; H, 3.82; N, 4.42. Found: C, 40.91; H, 3.77; N, 4.36. [Gd2(Mq)4(NO3)6] (3). Yellow block-shaped crystals of 3 were obtained by a procedure similar to that of 1, using Gd(NO3)3·6H2O (0.1 mmol 0.045 g) instead of Dy(NO3)3·6H2O. Yield: 29% (based on Gd). IR (KBr, cm−1): 3348(s), 3183(s), 1642(m), 1597(s), 1552(w), 1477(s), 1387(m), 1310(s), 1114(m), 1039(w), 888(m), 852(m), 752(m), 585(m). Elem. anal. calcd (%) for C40H36Gd2N10O22: C, 36.31; H, 2.74; N, 10.59. Found: C, 36.04; H, 2.62; N, 10.38. [Gd2(Mq)4Cl6](EtOH)2 (4). Yellow block-shaped crystals of 4 were obtained by a procedure similar to that of 2, using
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Dalton Transactions GdCl3·6H2O (0.4 mmol, 0.149 g) instead of DyCl3·6H2O. Yield: 32% (based on Gd). IR (KBr, cm−1): 3249(m), 3195(m), 3073(w), 1637(m), 1576(s), 1469(s), 1401(m), 1309(s), 1103(m), 1034(w), 927(m), 866(m), 828(s), 728(m), 584(m). Elem. anal. calcd (%) for C44H48Cl6Gd2N4O6: C, 42.07; H, 3.85; N, 4.46. Found: C, 41.88; H, 3.74; N, 4.33. Physical measurements Elemental analyses of carbon, nitrogen and hydrogen were carried out on a Perkin-Elmer 240C elemental analyzer. IR spectra as KBr pellets were recorded with a Magna 750 FT-IR spectrophotometer using reflectance technique over the range of 4000–400 cm−1. The phase purity of the bulk or polycrystalline samples was verified by X-ray powder diffraction (XRPD) patterns performed on a Rigaku D/max 2550 X-ray Powder Diffractometer (Fig. S3–S6†). All magnetic data were obtained with a Quantum Design MPMS SQUID VSM magnetometer. Samples were fixed in gelatin capsules and held in a brass sample holder. The variable-temperature magnetic susceptibility was measured with an external magnetic field of 1000 Oe. Alternating current magnetic susceptibility measurements were performed in an oscillating ac field of 3.0 Oe and a zero dc field. Pascal’s constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the molar paramagnetic susceptibilities (χM). X-ray crystallography Suitable single crystals of compounds 1–4 were glued onto a glass fiber. Diffraction intensity data for 1 were collected with a Bruker Smart CCD diffractometer equipped with graphitemonochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. Single-crystal structure determination of 2, 3 and 4 was carried out on a Rigaku RAXIS-RAPID diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The intensity data sets were collected with the ω-scan technique and reduced using CrystalClear software. The structures of the four compounds were solved by direct methods and refined with the full-matrix least squares technique using the program SHELXTL.18 The location of metal atom was easily determined, and Cl, O, N, and C atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The disordered atoms were refined with constrained dimensions. The hydrogen atoms were set in calculated positions. The crystal data, data collection and refinement parameters for compounds 1–4 are listed in Table 1, and selected bond lengths and angles for compounds 1–4 are listed in Table S1.†
Results and discussion Synthesis Compounds 1 and 3 were synthesized at 60 °C in the reaction system with molar composition of 1.0 Dy(NO3)3/Gd(NO3)3 : 2.5 Mq : 246.9 MeOH for 3 days. The La3+ analogue reported
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Table 1
Paper Description of the crystal structures
Crystallographic data for 1–4
Compound
1
2
3
4
Formula
C40H36Dy2N10O22 1333.79 Monoclinic P21/n 10.6892(6) 18.2050(10) 12.5195(9) 90 109.661(3) 90 2294.2(2) 2 1308 1.931 293 0.0384 0.0580
C44H48Cl6Dy2N4O6 1266.56 Monoclinic P21/c 10.910(2) 12.189(2) 18.974(4) 90 105.93(3) 90 2426.3(8) 2 1244 1.734 293 0.0401 0.0715
C40H36Gd2N10O22 1323.29 Monoclinic P21/n 10.716(2) 18.234(4) 12.561(3) 90 109.75(3) 90 2310.1(8) 2 1300 1.902 293 0.0210 0.0523
C44H48Cl6Gd2N4O6 1256.06 Monoclinic P21/c 10.883(2) 12.170(2) 18.594(4) 90 105.43(3) 90 2374.0(8) 2 1236 1.757 293 0.0274 0.0617
Fw (g mol−1) Cryst syst Space grp a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z F(000) Dcalcd (g cm−3) T (K) R1 a wR2 b
R1 = ∑||Fo| − |Fc||/∑|Fo| b wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)]2}1/2; w = 1/[σ2|Fo|2 + (0.0511P)2 + 19.56P], where P = [|Fo|2 + 2|Fc|2]/3.
a
earlier was synthesized by slow evaporation of the solution. Heating at a proper temperature could certainly promote the growth of the crystals. Compounds 2 and 4 were also synthesized at 60 °C, whereas the choice of a suitable solvent is important for the successful synthesis of the compounds. In pure MeOH or EtOH, no crystals were obtained. Adding an appropriate amount of MeCN to EtOH was necessary, but excessive MeCN led to powders rather than block crystals.
Fig. 1 The molecular structure (a) and polyhedral representation of the binuclear Dy3+ core (b) of compound 1.
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Compounds 1 and 3. Compounds 1 and 3 crystallize in the monoclinic space group P21/n, and they are isomorphous with the reported compound [La2(Mq)4(NO3)6].15 The structure of 1 will be described as a representative (Fig. 1). The centrosymmetric binuclear core is composed of two nine-coordinated Dy3+ ions bridged by two oxygen atoms from two Mq ligands, giving rise to a Dy2O2 core with a Dy–Dy distance of 3.914(5) Å and a Dy–O–Dy angle of 112.26(1)°. The coordination sphere of the Dy3+ ion is made up of nine oxygen atoms arising from one terminal Mq ligand (O1), two bridging Mq ligands (O2 and O2A) and three chelating nitrate ions (O3, O4, O6, O7, O9 and O10). The nine-coordinated Dy3+ ions are characterized by a distorted 4,4,4-tricapped trigonal prism environment. The shortest intermolecular Dy–Dy separation distance is 9.598(8) Å from neighbouring Dy2O2 units. For compound 3, the intrabinuclear Gd–Gd distance is 3.969(7) Å and the Gd–O–Gd angle is 112.55(8)°. The shortest intermolecular Gd–Gd separation distance is 9.599(8) Å from neighbouring Gd2O2 units. Compounds 2 and 4. Single-crystal X-ray analysis reveals that compounds 2 and 4 are isomorphous and crystallize in the monoclinic space group P21/c. The structure of 2 will be described as a representative (Fig. 2). It contains a Dy2O2 core very similar to that in compound 1. The biggest difference lies in that the nitrate ions in 1 were replaced by the same amount of chlorine ions. Each Dy3+ ion is coordinated by one terminal Mq ligand (O1), two bridging Mq ligands (O2 and O2A) and three chlorine ions (Cl1, Cl2 and Cl3), leading to a coordination number of six, which exhibits a distorted octahedral
Fig. 2 The molecular structure (a) and polyhedral representation of the binuclear Dy3+ core (b) of compound 2.
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geometry. In the centrosymmetric Dy2O2 unit, the Dy–Dy distance and Dy–O–Dy angle are of 3.836(8) Å and 110.80(1)°, respectively. The shortest intermolecular Dy–Dy separation distance is 9.50(1) Å from neighbouring Dy2O2 units. For compound 4, the intra-binuclear Gd–Gd distance is 3.883(2) Å and
Dalton Transactions the Gd–O–Gd angle is 111.21(8)°. The shortest intermolecular Gd–Gd separation distance is 9.29(8) Å from neighbouring Gd2O2 units. So far, a certain number of lanthanide compounds consisting of similar Ln2O2 parallelograms have been reported, for example, [Dy2(hfac)6(PyNONIT)4],19 [Dy2(hfac)4(NITPhO)2],20 [Dy2(hmi)2(NO3)2(MeOH)2],13 and [Gd2(Hsabhea)2(NO3)2].21 In most of them, the Ln3+ ion is eight- or seven-coordinated. Interestingly, for compounds 2 and 4, Ln3+ ions with the coordination number of six are observed. There are only a few precedents with similar structures, for example [Eu2(Odip)4(THF)2] and [Dy2(OH)2(SiW10O36)2],22 and the magnetic properties of six-coordinated lanthanide compounds are little known. Our successful replacement of NO3− by Cl− may provide an effective approach towards obtaining lanthanide compounds with lower coordination number. Magnetic properties
Fig. 3 Temperature dependence of the χMT product for compounds 1–4 at 1000 Oe. The red lines are simulations of the experimental data for compounds 3 and 4.
Solid-state, variable-temperature magnetic susceptibility measurements were carried out for the compounds in an applied dc field of 1000 Oe in the temperature range of 2–300 K. The plots of χMT vs. T are shown in Fig. 3, where χM represents the molar magnetic susceptibility. For compounds 1 and 2, the χMT values are 28.03 and 27.37 cm3 K mol−1 at room temperature, respectively, which
Fig. 4 Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility and frequency dependence of in-phase (c) and out-of-phase (d) ac susceptibilities for compound 1 under a zero dc field.
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Dalton Transactions are in good agreement with the expected value of 28.34 cm3 K mol−1 for two magnetically non-interacting Dy3+ ions (6H15/2 ground term, S = 5/2, L = 5, J = 15/2, g = 4/3). Upon cooling, the χMT values decrease gradually until about 25 K, and then more rapidly to reach values of 11.47 and 11.14 cm3 K mol−1 at 2 K, respectively. This behavior is generally indicative of intramolecular antiferromagnetic coupling of the metal centers. However, it may also arise from the thermal depopulation of the Stark sub-levels and/or from the presence of large magnetic anisotropy in the system.23 For 3 and 4, the χMT values are 15.02 and 15.39 cm3 K mol−1 at 300 K, respectively, which agree well with the expected value of 15.76 cm3 K mol−1 for two uncoupled Gd3+ ions (8F7/2 ground term, S = 7/2, g = 2). On lowering the temperature, the χMT values stay almost constant until about 50 K, and then below 50 K χMT decrease rapidly to values of 8.01 and 8.52 cm3 K mol−1 at 2 K, respectively. The absence of spin–orbit at the first order in Gd3+ compounds allows the direct correlation of the curve shape with the nature of the exchange interaction. Therefore, the decrease of the χMT when lowering the temperature for 3 and 4 reveals the presence of antiferromagnetic interaction between the Gd3+ ions. Moreover, by applying the van Vleck equation to Kambe’s vector coupling scheme by using the spin-only Hamiltonian H = −2JaS1S224 ( Ja is the magnitude of the exchange interaction
Paper between spins S1 and S2), the interaction can be quantified. The best-fit parameters obtained are Ja = −0.15 cm−1 and g = 2.00 for 3, Ja = −0.12 cm−1 and g = 2.00 for 4. These data indicate that the exchange interaction is rather weak, which is consistent with other pure lanthanide systems,25 and can be rationalized in terms of the core-like nature of their valence 4f orbits. Field-dependence measurements of the magnetization up to 5 T were performed at 2 K for compounds 1–4, as shown in Fig. S7.† For compounds 1 and 2, the values of the magnetization at 5 T are 10.30 and 10.12μB, respectively. Both of them are far lower than the expected saturation value of 20μB for two isolated DyIII ions, indicating a strong contribution from the ligand field.26 The lack of saturation of the magnetization confirms the presence of a significant magnetic anisotropy and/ or low lying excited states. Additionally, the absence of the M vs. H hysteresis loop above 2 K in compound 1 is caused by the presence of a relatively fast zero-field relaxation. It is worth noting that there is a “double butterfly” shaped hysteresis loop in 2 (Fig. S8†). The rather small coercive field is due either to antiferromagnetic coupling between the Dy3+ ions or to the quantum tunneling of the magnetization (QTM) at a zero field. As the field is increased, the hysteresis loop opens. Its further narrowing (at about 2000 Oe) may be ascribed to the level
Fig. 5 Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility and frequency dependence of in-phase (c) and out-of-phase (d) ac susceptibilities for compound 2 under a zero dc field.
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crossing between the first excited state and the ground state of the dimer.27 For 3 and 4, the magnetizations reveal a saturation of 14.04 and 14.11μB, respectively, in good agreement with the expected value 14.00μB for two weakly antiferromagnetically coupled Gd3+ ions. In order to investigate the presence of slow relaxation of the magnetization which may originate from SMM behavior, alternating-current (ac) susceptibility measurements were performed on compounds 1 and 2 with a zero dc field and a 3.0 Oe ac field at frequencies between 10 and 800 Hz. For compound 1, both the in-phase (χ′) and out-of-phase (χ″) components of ac susceptibility exhibit frequency and temperature dependent signal, and maxima in the χ″ curves are observed below 11 K (Fig. 4). This indicates the presence of slow magnetic relaxation at low temperature, and thus probable SMM behavior. Additionally, in the χ″ vs. T plot there is a clear tail of a peak at temperatures below 4 K, indicative of QTM through tunneling between degenerate MJ states.13,28 The magnetization relaxation time (τ) is derived from the frequency-dependence measurements and is plotted as a function of 1/T in Fig. 7. The best fit of the experimental data to the Arrhenius equation, 1/Tp = −kB/Δ[ln(2πf ) + logτ0], gives an energy gap of Δ/kB = 40.01 K and a pre-exponential factor of τ0 = 5.44 × 10−6 s.
Cole–Cole diagrams of 1 are shown in Fig. 6a. They exhibit a quasi-semicircle shape that can be fitted to the generalized Debye model with α < 0.1 (between 5.0 and 9.0 K), indicating the narrow distribution of relaxation times at these temperatures.29 For compound 2, the SMM behavior is also apparent. As shown in Fig. 5, the temperature and frequency dependent ac susceptibility signal was observed below 30 K. Compared with that of 1, the maxima in χ″ shift to higher temperature and a clear single-relaxation peak without the presence of a tail. The relaxation follows a thermally activated mechanism with an energy gap of 102.4 K and a pre-exponential factor of τ0 = 3.54 × 10−5 s (Fig. 7). The energy gap of 2 is significantly higher than that of 1, among the highest in lanthanide compounds consisting of similar Ln2O2 parallelograms.5a Cole– Cole plots of 2 can be fitted to the generalized Debye model with α parameters below 0.1 (Fig. 6b), indicating the presence of a single relaxation process.
Fig. 6 Cole–Cole plots measured in a zero dc field for compound 1(a) and 2(b). The solid lines are the best fits to the experimental data.
Fig. 7 Relaxation time, ln(τ), versus T−1 plot for 1 and 2 under a zero dc field. The solid line is fitted with Arrhenius law.
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Conclusions In summary, we reported the structures and magnetic properties of a set of binuclear lanthanide compounds, which are derivatives of a related, previously synthesized compound [La2(Mq)4(NO3)6]. Compounds 1 and 2 contain similar Dy2O2 cores, while the different peripheral anions lead to quite different coordination environments of the Dy3+ ions. In compound 1, where there are three nitrate ions coordinated to each Dy3+ ion with a chelating mode, the Dy3+ ion is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment. In compound 2, the nitrate ions are replaced by chlorine ions, and the Dy3+ ion has a highly distorted six-coordinated octahedral environment. Their Gd3+ analogues were also studied in order to understand the nature of magnetic interactions between metal ions. Magnetic measurements show that all the compounds exhibit weak intra-molecular antiferromagnetic interactions, and the two
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Dalton Transactions Dy3+-based compounds show frequency-dependent ac-susceptibility indicative of SMM behavior. As the slow magnetic relaxation for SMMs is affected by local molecular symmetry and is sensitive to subtle distortions of the coordination geometry of 4f ions, their dynamic magnetic properties show obvious difference. The thermal energy barrier to magnetization relaxation for 1 is of 40.0 K. The energy barrier for 2 is of 102.4 K, significantly higher than that for 1. We can deduce that the design and modulation of the coordination environment of lanthanide ions are crucial in directly tuning the energy barriers of corresponding SMMs. In addition, our compounds could serve as a good example of how the dynamic magnetic properties of lanthanide-based SMMs can be improved by chemical modifications.
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Acknowledgements This work was supported by the Foundation of the National Natural Science Foundation of China (no. 21371969), Specialized Research Fund for the Doctoral Program of Higher Education (no. 20110061110015) and National High Technology Research and Develop Program (863 program) of China (no. 2013AA031702).
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M. L. Tong, S. Gao, L. Ungur and L. F. Chibotaru, Chem.– Eur. J., 2011, 17, 2458. (a) Y. Bi, X. T. Wang, W. Liao, X. Wang, R. Deng, H. Zhang and S. Gao, Inorg. Chem., 2009, 48, 11743; (b) P. H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wernsdorfer and M. Murugesu, Angew. Chem., Int. Ed., 2008, 48, 948; (c) Y. N. Guo, G. F. Xu, P. Gamez, L. Zhao, S. Y. Lin, R. Deng, J. Tang and H. J. Zhang, J. Am. Chem. Soc., 2010, 132, 8538. (a) M. T. Gamer, Y. Lan, P. W. Roesky, A. K. Powell and R. Clérac, Inorg. Chem., 2008, 47, 6581; (b) R. J. Blagg, C. A. Muryn, E. J. McInnes, F. Tuna and R. E. P. Winpenny, Angew. Chem., Int. Ed., 2011, 50, 6530. (a) I. J. Hewitt, J. Tang, N. T. Madhu, C. E. Anson, Y. Lan, J. Luzon, M. Etienne, R. Sessoli and A. K. Powell, Angew. Chem., Int. Ed., 2010, 49, 6352; (b) S. Y. Lin, W. Wernsdorfer, L. Unger, A. K. Powell, Y. N. Guo, J. Tang, L. Zhao, L. F. Chiotaru and H. J. Zhang, Angew. Chem., Int. Ed., 2012, 51, 12767; (c) S. Sanz, R. D. McIntosh, C. M. Beacers, S. J. Teat, M. Evangelisti, E. K. Brechin and S. J. Dalgerno, Chem. Commun., 2012, 48, 1449. (a) J. W. Sharples, Y. Z. Zheng, F. Tuna, E. J. L. McInnes and D. Collison, Chem. Commun., 2011, 47, 7650; (b) H. Tian, L. Zhao, Y. N. Guo, Y. Guo, J. Tang and Z. Liu, Chem. Commun., 2012, 48, 708; (c) P. P. Yang, X. F. Gao, H. B. Song, S. Zhang, X. L. Mei, L. C. Li and D. Z. Liao, Inorg. Chem., 2011, 50, 720. (a) T. Rajeshkumar and G. Rajaraman, Chem. Commun., 2012, 48, 7856; (b) G. Cucinotta, M. Perfetti, J. Luzon, M. Etienne, P. E. Car, A. Caneschi, G. Calvez, K. Bernot and R. Sessoli, Angew. Chem., Int. Ed., 2012, 51, 1606; (c) S. Sakaue, A. Fuyuhiro, T. Fukuda and N. Ishikawa, Chem. Commun., 2012, 48, 5337. J. Long, F. Habib, P. H. Lin, I. Korobkov, G. Enright, L. Ungur, W. Wernsdorfer, L. F. Chibotaru and M. Murugesu, J. Am. Chem. Soc., 2011, 133, 5319. P. H. Lin, T. J. Burchell, R. Clérac and M. Murugesu, Angew. Chem., Int. Ed., 2008, 47, 8848. (a) L. Zou, L. Zhao, P. Chen, Y. N. Guo, Y. Guo, Y. H. Li and J. K. Tang, Dalton Trans., 2012, 41, 2966; (b) C. Platas, F. Avecilla, A. de Blas, T. Rodrigez-Blas, C. F. G. Geraldes, E. Toth, A. E. Merbach and J. C. Bunzli, J. Chem. Soc., Dalton Trans., 2000, 611. Y. Fazaeli, M. M. Amini and S. W. Ng, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2009, 65, m711. Y. Z. Zheng, Y. Lan, C. E. Anson and A. K. Powell, Inorg. Chem., 2008, 47, 10813. P. F. Shi, G. Xiong, B. Zhao, Z. Y. Zhang and P. Cheng, Chem. Commun., 2013, 49, 2338. G. M. Sheldrick, SADABS, the Siemens Area Detector Absorption Correction, University of Göttingen, Göttingen, Germany, 2005. T. Han, W. Shi, X. P. Zhang, L. L. Li and P. Cheng, Inorg. Chem., 2012, 51, 13009. R. Liu, C. Zhang, L. Li, D. Liao and J. P. Sutter, Dalton Trans., 2012, 41, 12139.
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Paper 21 W. Plass and G. Z. Fries, Z. Anorg. Allg. Chem., 1997, 623, 1205. 22 (a) S. Hamidi, G. B. Deancon, P. C. Junk and P. Neumann, Dalton Trans., 2012, 41, 3541; (b) K. Suzuki, R. Sato and N. Mizuno, Chem. Sci., 2013, 4, 596. 23 (a) P. P. Yang, X. F. Gao, H. B. Song, S. Zhang, X. L. Mei, L. C. Li and D. Z. Liao, Inorg. Chem., 2011, 50, 720; (b) Y. J. Gao, G. F. Xu, L. Zhao, J. K. Tang and Z. L. Liu, Inorg. Chem., 2009, 48, 11495; (c) Y. N. Guo, G. F. Xu, W. Wernsdorfer, L. Ungur, Y. Guo, J. Tang, H. J. Zhang, L. F. Chibotaru and A. K. Powell, J. Am. Chem. Soc., 2011, 133, 11948. 24 (a) F. Pointillart, Y. L. Gal, S. Golhen, O. Cador and L. Ouahab, Chem.–Eur. J., 2011, 17, 10397; (b) L. E. Roy and T. Hughbanks, J. Am. Chem. Soc., 2006, 128, 568. 25 (a) Z. S. Meng, F. S. Guo, J. L. Liu, J. D. Leng and M. L. Tong, Dalton Trans., 2012, 41, 2320; (b) S. V. Eliseeva, M. Ryazanov, F. Gumy, S. I. Troyanov, L. S. Lepnev, J.-C. L. Bunzli and N. P. Kuzmina, Eur. J. Inorg. Chem., 2006, 4809.
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Dalton Transactions 26 S. Osa, T. Kido, N. Matsumoto, N. Re, A. Pochaba and J. Mrozinski, J. Am. Chem. Soc., 2004, 126, 420. 27 (a) X. Yi, K. Bernot, F. Pointillart, G. Poneti, G. Calvez, C. Daiguebonne, O. Guillou and R. Sessoli, Chem.–Eur. J., 2012, 18, 11379; (b) F. Pointillart, B. L. Guennic, O. Maury, S. Golhen, O. Cador and L. Ouahab, Inorg. Chem., 2013, 52, 1398; (c) P. H. Lin, T. J. Burchell, L. Ungur, L. F. Chibotaru, W. Wernsdorfer and M. Murugesu, Angew. Chem., Int. Ed., 2009, 48, 9489. 28 (a) R. A. Layfield, J. J. W. McDouall, S. A. Sulway, F. Tuna, D. Collison and R. E. P. Winpenny, Chem.–Eur. J., 2010, 16, 4442; (b) F. Tuna, C. A. Smith, M. Bodensteiner, L. Ungur, L. F. Chibotaru, E. J. L. McInnes, R. E. P. Winpenny, D. Collison and R. A. Layfield, Angew. Chem., Int. Ed., 2012, 51, 6976. 29 (a) K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341; (b) C. J. Bottcher, Theory of Electric Polarisation, Elsevier, Amsterdam, 1952; (c) S. M. Aubin, Z. Sun, L. Pardi, J. Krzystek, K. Folting, L. C. Brunel, A. L. Rheingold, G. Christou and D. N. Hendrickson, Inorg. Chem., 1999, 38, 5329.
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PAPER
Cite this: DOI: 10.1039/c3dt52634h
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Anion effects on the structures and magnetic properties of binuclear lanthanide single-molecule magnets† Fen Yang, Qi Zhou, Guang Zeng, Guanghua Li, Lu Gao, Zhan Shi* and Shouhua Feng Here we report the anion-induced changes of structures and magnetic properties in binuclear lanthanide compounds. Firstly, two Dy3+-based compounds, [Dy2(Mq)4(NO3)6] (1) and [Dy2(Mq)4Cl6](EtOH)2 (2) (Mq = 8-hydroxy-2-methylquinoline), were synthesized and characterized. They contain similar binuclear Dy2O2 cores, while the different peripheral anions lead to quite different coordination environments of the Dy3+ ion. In compound 1, the Dy3+ ion is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment. In compound 2, the Dy3+ ion has a highly distorted six-coordinated octahedral environment. Their Gd3+ analogues, [Gd2(Mq)4(NO3)6] (3) and [Gd2(Mq)4Cl6](EtOH)2 (4), were also studied to investigate the magnetic interaction between metal ions. Variable-temperature dc
Received 23rd September 2013, Accepted 17th October 2013
magnetic susceptibility measurements show that all the compounds are weakly antiferromagnetically coupled. Ac magnetic susceptibility measurements reveal that both compounds 1 and 2 exhibit single-
DOI: 10.1039/c3dt52634h
molecule magnet (SMM) behaviour, while the thermal energy barrier of 2 is significantly higher than
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that of 1 (Δ/kB = 40.0 K for 1 and Δ/kB = 102.4 K for 2).
Introduction Single-molecule magnets (SMMs) characterized by slow magnetic relaxation have recently attracted much research interest since they offer the prospect of storing and processing magnetic information at a molecular level.1 The magnet-like behaviour of SMMs arises from the intrinsic large spin ground state (S) and uniaxial magnetic anisotropy (|D|).2 Lanthanides play a remarkable role in this field due to their large magnetic moments and large anisotropy. Indeed, since the discovery that a single-ion lanthanide compound displays slow relaxation of the magnetization,3 a variety of lanthanide SMMs have been reported, from mono-lanthanide compounds to aggregates based on lanthanide ions.4–10 Among them, binuclear lanthanide compounds are considered as a special group and have been studied extensively.5 They represent the simplest molecular units which allow the study of magnetic interactions between two spin carriers, and they are also crucial in
State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, P. R. China. E-mail: [email protected]; Fax: +86 431 85168624 † Electronic supplementary information (ESI) available: Crystallographic details in CIF format, selected bond lengths, XRPD patterns, and some magnetic plots. CCDC 923350–923353 for 1, 3, 2, 4. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52634h
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understanding the magnetization relaxation characteristics of lanthanide SMMs. It is believed that if a binuclear SMM can be designed and isolated in a controllable manner, one could potentially create systems with higher nuclearity using a bottom up molecular approach and obtain SMMs with significantly higher effective energy barrier.5a Recently, it has been demonstrated that the overall electronic structure of lanthanide ion is very sensitive to its coordination environment. Even subtle ligand changes can drastically influence the overall physical properties of the lanthanide compounds, and the SMM behaviour of lanthanide ions is highly dependent on the geometry of the coordination site.5b,c,11 This brings us a new proposition. As we noticed that there are several binuclear lanthanide compounds with one or more coordinated nitrate anions,12–14 we wonder what will happen if the nitrate anions are replaced by other anions such as chlorine ions. Both Cl− and NO3− ions have a negative charge, but Cl− ion will never coordinate to metal ions with a chelating mode as NO3− ion usually does. It will certainly lead to a quite different coordination environment (coordination numbers and geometries), and in turn, it may make a difference in the magnetic behaviors. To confirm our assumption, we chose a binuclear lanthanide compound, [Dy2(Mq)4(NO3)6] (1) (Mq = 8-hydroxy-2methylquinoline), where there are three NO3− ions coordinated to each Dy3+ centre. The structure of a La3+ analogue has been
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Paper reported earlier.15 When using DyCl3 as the metal source instead of Dy(NO3)3, we obtained a new binuclear Dy3+ compound, [Dy2(Mq)4Cl6](EtOH)2 (2). As we anticipated, Cl− ions took the place of NO3− ions, and the coordination numbers and geometry of the Dy3+ ions in 2 were significantly different from those of 1. The Dy3+ ion in compound 1 is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment, whereas it is six-coordinated and characterized by a distorted octahedral geometry in compound 2. We also synthesized their Gd3+ analogues [Gd2(Mq)4(NO3)6] (3) and [Gd2(Mq)4Cl6] (EtOH)2 (4), respectively, to investigate the magnetic interaction between the lanthanide ions. Magnetic measurements demonstrate that the compounds exhibit weak intra-binuclear antiferromagnetic interaction. Both compounds 1 and 2 display SMM behaviour, while the effective energy gap of 2 is much higher than that of 1. To the best of our knowledge, investigations of anion-perturbed magnetic slow relaxation in lanthanide-based SMMs are extremely rare up to now.16,17
Experimental section Synthesis All reagents and solvents were commercially available and were used without further purification. [Dy2(Mq)4(NO3)6] (1). A mixture of Dy(NO3)3·6H2O (0.1 mmol, 0.045 g) and Mq (0.25 mmol, 0.039 g) in MeOH (10 mL) was stirred for 30 min and then filtered. The yellow filtrate was heated in a Teflon-lined steel bomb at 60 °C for 3 days. Yellow block-shaped crystals formed were collected in 26% yield (based on Dy). IR (KBr, cm−1): 3364(s), 3183(s), 1627(m), 1582(s), 1537(w), 1462(s), 1386(m), 1326(s), 1099(m), 1039(w), 888(m), 827(m), 737(m), 570(m). Elem. anal. calcd (%) for C40H36Dy2N10O22: C, 36.02; H, 2.74; N, 10.50. Found: C, 35.81; H, 2.66; N, 10.27. [Dy2(Mq)4Cl6](EtOH)2 (2). A mixture of DyCl3·6H2O (0.4 mmol, 0.150 g) and Mq (1.0 mmol, 0.159 g) in EtOH (8 mL) and MeCN (2 mL) was stirred for 30 min. The resulting mixture was heated in a Teflon-lined steel bomb at 60 °C for 5 days. Yellow block-shaped crystals formed were collected in 38% yield (based on Dy). IR (KBr, cm−1): 3243(m), 3198(m), 3093(w), 1642(m), 1597(s), 1462(s), 1386(m), 1326(s), 1099(m), 1039(w), 927(m), 873(m), 836(s), 743(m), 576(m). Elem. anal. calcd (%) for C44H48Cl6Dy2N4O6: C, 41.72; H, 3.82; N, 4.42. Found: C, 40.91; H, 3.77; N, 4.36. [Gd2(Mq)4(NO3)6] (3). Yellow block-shaped crystals of 3 were obtained by a procedure similar to that of 1, using Gd(NO3)3·6H2O (0.1 mmol 0.045 g) instead of Dy(NO3)3·6H2O. Yield: 29% (based on Gd). IR (KBr, cm−1): 3348(s), 3183(s), 1642(m), 1597(s), 1552(w), 1477(s), 1387(m), 1310(s), 1114(m), 1039(w), 888(m), 852(m), 752(m), 585(m). Elem. anal. calcd (%) for C40H36Gd2N10O22: C, 36.31; H, 2.74; N, 10.59. Found: C, 36.04; H, 2.62; N, 10.38. [Gd2(Mq)4Cl6](EtOH)2 (4). Yellow block-shaped crystals of 4 were obtained by a procedure similar to that of 2, using
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Dalton Transactions GdCl3·6H2O (0.4 mmol, 0.149 g) instead of DyCl3·6H2O. Yield: 32% (based on Gd). IR (KBr, cm−1): 3249(m), 3195(m), 3073(w), 1637(m), 1576(s), 1469(s), 1401(m), 1309(s), 1103(m), 1034(w), 927(m), 866(m), 828(s), 728(m), 584(m). Elem. anal. calcd (%) for C44H48Cl6Gd2N4O6: C, 42.07; H, 3.85; N, 4.46. Found: C, 41.88; H, 3.74; N, 4.33. Physical measurements Elemental analyses of carbon, nitrogen and hydrogen were carried out on a Perkin-Elmer 240C elemental analyzer. IR spectra as KBr pellets were recorded with a Magna 750 FT-IR spectrophotometer using reflectance technique over the range of 4000–400 cm−1. The phase purity of the bulk or polycrystalline samples was verified by X-ray powder diffraction (XRPD) patterns performed on a Rigaku D/max 2550 X-ray Powder Diffractometer (Fig. S3–S6†). All magnetic data were obtained with a Quantum Design MPMS SQUID VSM magnetometer. Samples were fixed in gelatin capsules and held in a brass sample holder. The variable-temperature magnetic susceptibility was measured with an external magnetic field of 1000 Oe. Alternating current magnetic susceptibility measurements were performed in an oscillating ac field of 3.0 Oe and a zero dc field. Pascal’s constants were used to estimate the diamagnetic corrections, which were subtracted from the experimental susceptibilities to give the molar paramagnetic susceptibilities (χM). X-ray crystallography Suitable single crystals of compounds 1–4 were glued onto a glass fiber. Diffraction intensity data for 1 were collected with a Bruker Smart CCD diffractometer equipped with graphitemonochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. Single-crystal structure determination of 2, 3 and 4 was carried out on a Rigaku RAXIS-RAPID diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. The intensity data sets were collected with the ω-scan technique and reduced using CrystalClear software. The structures of the four compounds were solved by direct methods and refined with the full-matrix least squares technique using the program SHELXTL.18 The location of metal atom was easily determined, and Cl, O, N, and C atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The disordered atoms were refined with constrained dimensions. The hydrogen atoms were set in calculated positions. The crystal data, data collection and refinement parameters for compounds 1–4 are listed in Table 1, and selected bond lengths and angles for compounds 1–4 are listed in Table S1.†
Results and discussion Synthesis Compounds 1 and 3 were synthesized at 60 °C in the reaction system with molar composition of 1.0 Dy(NO3)3/Gd(NO3)3 : 2.5 Mq : 246.9 MeOH for 3 days. The La3+ analogue reported
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Table 1
Paper Description of the crystal structures
Crystallographic data for 1–4
Compound
1
2
3
4
Formula
C40H36Dy2N10O22 1333.79 Monoclinic P21/n 10.6892(6) 18.2050(10) 12.5195(9) 90 109.661(3) 90 2294.2(2) 2 1308 1.931 293 0.0384 0.0580
C44H48Cl6Dy2N4O6 1266.56 Monoclinic P21/c 10.910(2) 12.189(2) 18.974(4) 90 105.93(3) 90 2426.3(8) 2 1244 1.734 293 0.0401 0.0715
C40H36Gd2N10O22 1323.29 Monoclinic P21/n 10.716(2) 18.234(4) 12.561(3) 90 109.75(3) 90 2310.1(8) 2 1300 1.902 293 0.0210 0.0523
C44H48Cl6Gd2N4O6 1256.06 Monoclinic P21/c 10.883(2) 12.170(2) 18.594(4) 90 105.43(3) 90 2374.0(8) 2 1236 1.757 293 0.0274 0.0617
Fw (g mol−1) Cryst syst Space grp a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z F(000) Dcalcd (g cm−3) T (K) R1 a wR2 b
R1 = ∑||Fo| − |Fc||/∑|Fo| b wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)]2}1/2; w = 1/[σ2|Fo|2 + (0.0511P)2 + 19.56P], where P = [|Fo|2 + 2|Fc|2]/3.
a
earlier was synthesized by slow evaporation of the solution. Heating at a proper temperature could certainly promote the growth of the crystals. Compounds 2 and 4 were also synthesized at 60 °C, whereas the choice of a suitable solvent is important for the successful synthesis of the compounds. In pure MeOH or EtOH, no crystals were obtained. Adding an appropriate amount of MeCN to EtOH was necessary, but excessive MeCN led to powders rather than block crystals.
Fig. 1 The molecular structure (a) and polyhedral representation of the binuclear Dy3+ core (b) of compound 1.
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Compounds 1 and 3. Compounds 1 and 3 crystallize in the monoclinic space group P21/n, and they are isomorphous with the reported compound [La2(Mq)4(NO3)6].15 The structure of 1 will be described as a representative (Fig. 1). The centrosymmetric binuclear core is composed of two nine-coordinated Dy3+ ions bridged by two oxygen atoms from two Mq ligands, giving rise to a Dy2O2 core with a Dy–Dy distance of 3.914(5) Å and a Dy–O–Dy angle of 112.26(1)°. The coordination sphere of the Dy3+ ion is made up of nine oxygen atoms arising from one terminal Mq ligand (O1), two bridging Mq ligands (O2 and O2A) and three chelating nitrate ions (O3, O4, O6, O7, O9 and O10). The nine-coordinated Dy3+ ions are characterized by a distorted 4,4,4-tricapped trigonal prism environment. The shortest intermolecular Dy–Dy separation distance is 9.598(8) Å from neighbouring Dy2O2 units. For compound 3, the intrabinuclear Gd–Gd distance is 3.969(7) Å and the Gd–O–Gd angle is 112.55(8)°. The shortest intermolecular Gd–Gd separation distance is 9.599(8) Å from neighbouring Gd2O2 units. Compounds 2 and 4. Single-crystal X-ray analysis reveals that compounds 2 and 4 are isomorphous and crystallize in the monoclinic space group P21/c. The structure of 2 will be described as a representative (Fig. 2). It contains a Dy2O2 core very similar to that in compound 1. The biggest difference lies in that the nitrate ions in 1 were replaced by the same amount of chlorine ions. Each Dy3+ ion is coordinated by one terminal Mq ligand (O1), two bridging Mq ligands (O2 and O2A) and three chlorine ions (Cl1, Cl2 and Cl3), leading to a coordination number of six, which exhibits a distorted octahedral
Fig. 2 The molecular structure (a) and polyhedral representation of the binuclear Dy3+ core (b) of compound 2.
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geometry. In the centrosymmetric Dy2O2 unit, the Dy–Dy distance and Dy–O–Dy angle are of 3.836(8) Å and 110.80(1)°, respectively. The shortest intermolecular Dy–Dy separation distance is 9.50(1) Å from neighbouring Dy2O2 units. For compound 4, the intra-binuclear Gd–Gd distance is 3.883(2) Å and
Dalton Transactions the Gd–O–Gd angle is 111.21(8)°. The shortest intermolecular Gd–Gd separation distance is 9.29(8) Å from neighbouring Gd2O2 units. So far, a certain number of lanthanide compounds consisting of similar Ln2O2 parallelograms have been reported, for example, [Dy2(hfac)6(PyNONIT)4],19 [Dy2(hfac)4(NITPhO)2],20 [Dy2(hmi)2(NO3)2(MeOH)2],13 and [Gd2(Hsabhea)2(NO3)2].21 In most of them, the Ln3+ ion is eight- or seven-coordinated. Interestingly, for compounds 2 and 4, Ln3+ ions with the coordination number of six are observed. There are only a few precedents with similar structures, for example [Eu2(Odip)4(THF)2] and [Dy2(OH)2(SiW10O36)2],22 and the magnetic properties of six-coordinated lanthanide compounds are little known. Our successful replacement of NO3− by Cl− may provide an effective approach towards obtaining lanthanide compounds with lower coordination number. Magnetic properties
Fig. 3 Temperature dependence of the χMT product for compounds 1–4 at 1000 Oe. The red lines are simulations of the experimental data for compounds 3 and 4.
Solid-state, variable-temperature magnetic susceptibility measurements were carried out for the compounds in an applied dc field of 1000 Oe in the temperature range of 2–300 K. The plots of χMT vs. T are shown in Fig. 3, where χM represents the molar magnetic susceptibility. For compounds 1 and 2, the χMT values are 28.03 and 27.37 cm3 K mol−1 at room temperature, respectively, which
Fig. 4 Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility and frequency dependence of in-phase (c) and out-of-phase (d) ac susceptibilities for compound 1 under a zero dc field.
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Dalton Transactions are in good agreement with the expected value of 28.34 cm3 K mol−1 for two magnetically non-interacting Dy3+ ions (6H15/2 ground term, S = 5/2, L = 5, J = 15/2, g = 4/3). Upon cooling, the χMT values decrease gradually until about 25 K, and then more rapidly to reach values of 11.47 and 11.14 cm3 K mol−1 at 2 K, respectively. This behavior is generally indicative of intramolecular antiferromagnetic coupling of the metal centers. However, it may also arise from the thermal depopulation of the Stark sub-levels and/or from the presence of large magnetic anisotropy in the system.23 For 3 and 4, the χMT values are 15.02 and 15.39 cm3 K mol−1 at 300 K, respectively, which agree well with the expected value of 15.76 cm3 K mol−1 for two uncoupled Gd3+ ions (8F7/2 ground term, S = 7/2, g = 2). On lowering the temperature, the χMT values stay almost constant until about 50 K, and then below 50 K χMT decrease rapidly to values of 8.01 and 8.52 cm3 K mol−1 at 2 K, respectively. The absence of spin–orbit at the first order in Gd3+ compounds allows the direct correlation of the curve shape with the nature of the exchange interaction. Therefore, the decrease of the χMT when lowering the temperature for 3 and 4 reveals the presence of antiferromagnetic interaction between the Gd3+ ions. Moreover, by applying the van Vleck equation to Kambe’s vector coupling scheme by using the spin-only Hamiltonian H = −2JaS1S224 ( Ja is the magnitude of the exchange interaction
Paper between spins S1 and S2), the interaction can be quantified. The best-fit parameters obtained are Ja = −0.15 cm−1 and g = 2.00 for 3, Ja = −0.12 cm−1 and g = 2.00 for 4. These data indicate that the exchange interaction is rather weak, which is consistent with other pure lanthanide systems,25 and can be rationalized in terms of the core-like nature of their valence 4f orbits. Field-dependence measurements of the magnetization up to 5 T were performed at 2 K for compounds 1–4, as shown in Fig. S7.† For compounds 1 and 2, the values of the magnetization at 5 T are 10.30 and 10.12μB, respectively. Both of them are far lower than the expected saturation value of 20μB for two isolated DyIII ions, indicating a strong contribution from the ligand field.26 The lack of saturation of the magnetization confirms the presence of a significant magnetic anisotropy and/ or low lying excited states. Additionally, the absence of the M vs. H hysteresis loop above 2 K in compound 1 is caused by the presence of a relatively fast zero-field relaxation. It is worth noting that there is a “double butterfly” shaped hysteresis loop in 2 (Fig. S8†). The rather small coercive field is due either to antiferromagnetic coupling between the Dy3+ ions or to the quantum tunneling of the magnetization (QTM) at a zero field. As the field is increased, the hysteresis loop opens. Its further narrowing (at about 2000 Oe) may be ascribed to the level
Fig. 5 Temperature dependence of the in-phase (a) and out-of-phase (b) ac susceptibility and frequency dependence of in-phase (c) and out-of-phase (d) ac susceptibilities for compound 2 under a zero dc field.
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crossing between the first excited state and the ground state of the dimer.27 For 3 and 4, the magnetizations reveal a saturation of 14.04 and 14.11μB, respectively, in good agreement with the expected value 14.00μB for two weakly antiferromagnetically coupled Gd3+ ions. In order to investigate the presence of slow relaxation of the magnetization which may originate from SMM behavior, alternating-current (ac) susceptibility measurements were performed on compounds 1 and 2 with a zero dc field and a 3.0 Oe ac field at frequencies between 10 and 800 Hz. For compound 1, both the in-phase (χ′) and out-of-phase (χ″) components of ac susceptibility exhibit frequency and temperature dependent signal, and maxima in the χ″ curves are observed below 11 K (Fig. 4). This indicates the presence of slow magnetic relaxation at low temperature, and thus probable SMM behavior. Additionally, in the χ″ vs. T plot there is a clear tail of a peak at temperatures below 4 K, indicative of QTM through tunneling between degenerate MJ states.13,28 The magnetization relaxation time (τ) is derived from the frequency-dependence measurements and is plotted as a function of 1/T in Fig. 7. The best fit of the experimental data to the Arrhenius equation, 1/Tp = −kB/Δ[ln(2πf ) + logτ0], gives an energy gap of Δ/kB = 40.01 K and a pre-exponential factor of τ0 = 5.44 × 10−6 s.
Cole–Cole diagrams of 1 are shown in Fig. 6a. They exhibit a quasi-semicircle shape that can be fitted to the generalized Debye model with α < 0.1 (between 5.0 and 9.0 K), indicating the narrow distribution of relaxation times at these temperatures.29 For compound 2, the SMM behavior is also apparent. As shown in Fig. 5, the temperature and frequency dependent ac susceptibility signal was observed below 30 K. Compared with that of 1, the maxima in χ″ shift to higher temperature and a clear single-relaxation peak without the presence of a tail. The relaxation follows a thermally activated mechanism with an energy gap of 102.4 K and a pre-exponential factor of τ0 = 3.54 × 10−5 s (Fig. 7). The energy gap of 2 is significantly higher than that of 1, among the highest in lanthanide compounds consisting of similar Ln2O2 parallelograms.5a Cole– Cole plots of 2 can be fitted to the generalized Debye model with α parameters below 0.1 (Fig. 6b), indicating the presence of a single relaxation process.
Fig. 6 Cole–Cole plots measured in a zero dc field for compound 1(a) and 2(b). The solid lines are the best fits to the experimental data.
Fig. 7 Relaxation time, ln(τ), versus T−1 plot for 1 and 2 under a zero dc field. The solid line is fitted with Arrhenius law.
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Conclusions In summary, we reported the structures and magnetic properties of a set of binuclear lanthanide compounds, which are derivatives of a related, previously synthesized compound [La2(Mq)4(NO3)6]. Compounds 1 and 2 contain similar Dy2O2 cores, while the different peripheral anions lead to quite different coordination environments of the Dy3+ ions. In compound 1, where there are three nitrate ions coordinated to each Dy3+ ion with a chelating mode, the Dy3+ ion is nine-coordinated and characterized by a distorted 4,4,4-tricapped trigonal prism environment. In compound 2, the nitrate ions are replaced by chlorine ions, and the Dy3+ ion has a highly distorted six-coordinated octahedral environment. Their Gd3+ analogues were also studied in order to understand the nature of magnetic interactions between metal ions. Magnetic measurements show that all the compounds exhibit weak intra-molecular antiferromagnetic interactions, and the two
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Dalton Transactions Dy3+-based compounds show frequency-dependent ac-susceptibility indicative of SMM behavior. As the slow magnetic relaxation for SMMs is affected by local molecular symmetry and is sensitive to subtle distortions of the coordination geometry of 4f ions, their dynamic magnetic properties show obvious difference. The thermal energy barrier to magnetization relaxation for 1 is of 40.0 K. The energy barrier for 2 is of 102.4 K, significantly higher than that for 1. We can deduce that the design and modulation of the coordination environment of lanthanide ions are crucial in directly tuning the energy barriers of corresponding SMMs. In addition, our compounds could serve as a good example of how the dynamic magnetic properties of lanthanide-based SMMs can be improved by chemical modifications.
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Acknowledgements This work was supported by the Foundation of the National Natural Science Foundation of China (no. 21371969), Specialized Research Fund for the Doctoral Program of Higher Education (no. 20110061110015) and National High Technology Research and Develop Program (863 program) of China (no. 2013AA031702).
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