Single-molecule magnet behavior in a mononuclear dysprosium(III) complex with 1-methylimidazole

Abstract

Three novel mononuclear lanthanide complexes coordinated with 1-methylimidazole [Ln(hfac)₃·(1-MeIm)₂] (Ln = Dy(1), Tb(2) and Ho(3), hfac = hexafluoroacetylacetonate, 1-MeIm = 1-methylimidazole) have been designed and characterized by single-crystal X-ray diffraction, luminescence and magnetic measurements. Studies indicate that the metal ions of all three mononuclear complexes adopt a slightly distorted D_4d [LnO₆N₂] coordination environment, which is formed by two terminal 1-methylimidazole ligands and three bischelating hfac⁻ anions. Frequency dependent out-of-phase signals were observed for complex 1 in dynamic magnetic susceptibility measurements, which indicates single-molecule magnetic behavior.

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Introduction

Single molecular magnets (SMMs) have been an attractive research field owing to the memory effect exhibited by individual molecules, thus making them candidates for potential applications of high-density data storage technologies, quantum computing and single-spin manipulation.¹ The large spin and strong intrinsic magneto-anisotropy of lanthanide ions (especially Dy^III, Tb^III, Ho^III and Er^III) make them very attractive to construct SMMs of high spin-reversal barriers.² In the past decade, many Ln-based SMMs have been reported, for example, mononuclear,³ dinuclear,⁴ trinuclear,⁵ tetranuclear⁶ pentanuclear⁷ and polynuclear⁸ lanthanide complexes. However, the challenge is to suppress the quantum tunneling of magnetization (QTM) in order to increase the anisotropy barrier, which depends mainly on the crystal field, inter-molecular interactions as well as the hyperfine interactions.^3a,8a,9 Our previous work showed that [Dy(hfac)₃(pz)₂] (pz = pyrazole)¹⁰ was an SMM with a barrier of 28.32 K, and how a small change of the ligand field or intermolecular Dy⋯Dy dipolar coupling affects the magnetic properties of Dy^III SMMs. To further investigate the effect of inter-molecular interactions on SMM behavior, it is interesting to design a similar compound using 1-methylimidazole instead of pyrazole. The presence of the methyl group will affect the intermolecular Ln⋯Ln dipolar coupling of lanthanide complexes, which will give rise to subtle but intriguing changes of the magnetic properties.^11,12

As mentioned above, by utilizing the 1-methylimidazole (1-MeIm) ligand, three novel mononuclear lanthanide complexes [Ln(hfac)₃·(1-MeIm)₂] [Ln = Dy(1), Tb(2) and Ho(3)] have been obtained and characterized. Surprisingly, by using 1-methylimidazole instead of pyrazole to coordinate with Dy(hfac)₃ units in complex 1, it significantly increases the energy barrier of Dy(III) ion from 28.32 to 42.54 K, with the change of ligand field symmetry from D_2d for Dy(III) ion to D_4d. In addition, the expansion of intermolecular Dy–Dy distances from 8.87 Å to 9.10 Å, as we expected, decreases the intermolecular Dy(III) coupling and suppresses QTM to some extent. Complex 1 represents an example of mononuclear lanthanide complexes displaying single-ion magnetic behavior.

Meanwhile, lanthanide compounds can exhibit highly efficient and long-lived luminescence with well-resolved multiline emissions in the UV-Vis.^9a,13 So the luminescence spectra of complexes 1 and 2 are also investigated in the visible spectrum.

Experimental

Materials and physical techniques

All chemicals and reagents used in the syntheses were of analytical grade without further purification. C, H, N analyses were obtained on a Perking-Elmer elemental analyzer model 240. Fourier transform infrared spectroscopy was carried out in the range 400–4000 cm⁻¹ on a Bruker TENOR 27 spectrophotometer using KBr pellets. Luminescence spectra and fluorescence lifetime studies were performed by using solid samples with an Edinburgh FLS-920P spectrophotometer. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was measured with central mental (Dy, Tb, and Ho) by Spectro-Blue. All magnetic data were measured on a SQUID MPMS XL-7 magnetometer using polycrystalline samples. Diamagnetic corrections of experimental magnetic susceptibility data are estimated from Pascal's constants.^9h,14

Synthesis of [Ln(hfac)₃·(1-MeIm)₂] (Ln = Dy(1), Tb(2), Ho(3))

Dy(hfac)₃·2H₂O (0.05 mmol, 0.041 g) was suspended in 20 ml of n-heptane and heated with stirring. After refluxing for about 2 h and cooling to 60 °C, 1-methylimidazole (0.1 mmol, 0.082 g) in CH₂Cl₂ (5 ml) was added, and the mixture was stirred for 30 minutes. By cooling the solution to room temperature, the filtrate was slowly evaporated at 4 °C. Colorless crystals of the product suitable for X-ray crystallography were obtained after a week. Complex 2 and 3 were synthesized following a similar procedure except that Dy(hfac)₃·2H₂O was substituted by Tb(hfac)₃·2H₂O and Ho(hfac)₃·2H₂O respectively. Yield: 21.8 mg, 45.9%, based on Dy. Anal. calcd (1): C₂₃H₁₅DyF₁₈N₄O₆: C, 29.14; H, 1.60; N, 5.91. Found (1): C, 28.89; H, 1.60; N, 5.95%. ICP-OES anal. Found: Dy, 17.14; calcd. Found: 17.45; IR (KBr, ν/cm⁻¹): 3150(w), 1645(vs), 1535(w), 1485(s), 1248(s), 1189(s), 1138(vs), 1096(s), 935(w), 800(s), 757(w), 664(vs), 580(s). Yield: 21.8 mg, 46.2%, based on Tb. Anal. calcd (2): C₂₃H₁₅TbF₁₈N₄O₆: C, 29.21; H, 1.59; N, 5.95%. Found (2): C, 28.85; H, 1.61; N, 5.94% ICP-OES anal. Found: Tb, 16.83; calcd. Found: Tb, 16.51; IR (KBr, ν/cm⁻¹): 3152(w), 1645(vs), 1535(w), 1468(s), 1256(s), 1189(s), 1138(vs), 1096(s), 935(w), 800(s), 757(w), 656(vs), 582(s) anal. calcd (3): C₂₃H₁₅HoF₁₈N₄O₆: C, 29.07; H, 1.59; N, 5.90. Yield: 21.2 mg, 44.6%, based on Ho. Found (3): C, 29.13; H, 1.63; N, 5.88%. ICP-OES Anal. Found: Ho, 17.36; calcd. Found: Ho, 17.75; IR (KBr, ν/cm⁻¹): 3150(w), 1644(vs), 1535(w), 1476(s), 1255(s), 1189(s), 1137(vs), 1097(s), 944(w), 801(s), 741(w), 655(vs), 580(s).¹⁵

Crystal structure determination and refinement

Suitable crystals were selected and mounted on a Bruker SMART 1000 diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 113 K for 1, 2 and 3. Lorentz-polarization correction was applied to the data. The structures were solved by direct methods using SHELXS-2014 ¹⁶ program and refinements on F² were performed using SHELXL-2014 by full-matrix least-squares procedure with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were added theoretically and refined with riding model position parameters and isotropically refined crystal data and details of structural determination and refinement are summarized in Table 1 and the selected bond lengths and angles have been provided in the ESI, Tables S1–S3.†

Table 1 Crystallographic data and structure refinement for complexes 1, 2, 3

	1	2	3
Chemical formula	C23H15DyF18N4O6	C23H15TbF18N4O6	C23H15HoF18N4O6
Formula weight	947.89	944.32	950.32
Temperature/K	113	113	113
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	C2/c	C2/c	C2/c
a/Å	20.103(4)	20.173(4)	20.079(7)
b/Å	10.649(2)	10.666(2)	10.639(2)
c/Å	18.062(4)	18.074(4)	18.035(9)
α/°	90	90	90
β/°	113.88(3)	113.82(3)	113.89(3)
γ/°	90	90	90
V/Å^−3	3535.7(15)	3557.6(15)	3523(2)
Z	4	4	4
ρ/(g cm^−3)	1.781	1.763	1.792
μ/(mm^−1)	2.248	2.122	2.382
F(000)	1828	1824	1832
Reflections collected	14 331	13 681	14 168
Unique/parameters	3151/322	3136/322	3113/322
R(int)	0.0705	0.0547	0.0705
Max./min. transmission	0.698/0.0.644	0.661/0.702	0.717/0.751
Goodness-of-fit on F^2	1.014	1.049	1.039
R1, wR2 [I > 2σ(I)]	0.0434/0.0987	0.0428/0.1061	0.0423/0.1035
R1, wR2 (all data)	0.0500/0.1032	0.0469/0.1108	0.0468/0.1081

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Results and discussion

Crystal structures

Compared with the previously reported [Dy(hfac)₃·(pz)₂], complex 1 has a similar mononuclear structure and is synthesized by substituting two H₂O molecules by two 1-methylimidazole ligands in a Dy(hfac)₃·2H₂O salt. It crystalizes as a symmetrical mononuclear compound [Dy(hfac)₃·(1-MeIm)₂] in monoclinic C2/c space group (Fig. 1). The Dy^III core is also eight-coordinated in a [DyO₆N₂] environment with Dy–O bond lengths in the range of 2.346(4)–2.358(4) Å, which is shorter than that of Dy–N (2.467(4) Å). The angles of O–Dy–O is ranging from 71.2 to 145.1°, the bond angles O–Dy–N is ranging from 73.6 to 143.8°, and the angle of N(1)¹–Dy(1)–N(1) is 141.7°. The [DyO₆N₂] geometry is studied using the SHAPE software¹⁷ to obtain the minimal deviation parameter S of 0.396 for D_4d symmetry as shown in Table 2, rather than D_2d as in [Dy(hfac)₃·(pz)₂]. The shortest intermolecular Dy⋯Dy separation distance of 9.098(8) Å is larger than that of [Dy(hfac)₃(pz)₂] (8.87 Å). The packing diagram of the compound 1 is given in the ESI, Fig. S1,† which shows no inter- and intramolecular π–π interaction. Hydrogen bonding interactions between the molecules were not found in the complex 1.

Fig. 1 Perspective view of complex [Dy(hfac)₃·(1-MeIm)₂] (H atoms were all omitted for clarity, left) and the coordination polyhedron of the Dy^III ions (right).

Table 2 The minimal deviation parameter S for complex 1, 2, 3 of Ln^III ion geometry analysis by using the SHAPE software

Ln^III	D2d-DD	C2v-TP	D4d-SAP
Dy^III	1.397	2.734	0.396
Tb^III	1.457	1.439	1.971
Ho^III	1.959	0.359	0.380

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The structures of complex 2 and 3 consist of similar mononuclear units as those in complex 1 with only small differences in the bond lengths and angles. Their molecular structures, D_4d-symmetry polyhedrons and packing diagrams are given in the ESI, Fig. S2–S5.†

Luminescence properties

Fig. 2 shows the emission spectrum of the Dy^III and Tb^III complexes recorded at room temperature under irradiation of UV light at 350 and 347 nm, respectively. Well-resolved multiline emissions of Dy^III ion were observed around 480, 573 and 661 nm, which can be attributed to ⁴F_9/2 → ⁶H_J with J = 15/2, 13/2 and 11/2, respectively (Fig. 2a).^13b,d Four characteristic peaks of the Tb^III complex shown in Fig. 2b are attributed to the metal-centered ⁵D₄ excited state to the ⁷F_J ground-state multiplet. Maximum emission peaks at 489, 544, 587 and 621 nm, were observed for the J = 3, 4, 5 and 6 transitions, respectively.^13k

Fig. 2 The luminescence spectra of 1 (a) and 2 (b) in the solid state at room temperature.

The decay lifetime curves reveal single-exponential mode with the lifetimes of τ₁ = 0.4549 μs (100%) for 1, while double-exponential mode was observed for 2, giving lifetime values (17.06 μs (31.66%) and 237.0 μs (68.34%)), as determined by monitoring the ⁵D₄ → ⁷F₅ transition (Fig. S6†). Moreover, for complex 2, the fluorescence lifetime is a little higher than that of [Tb(hfac)₃(pz)₂]¹⁰ (77 μs), showing that the ancillary ligand of 1-methylimidazole has an antenna effect on the luminescence of the Tb^III complex, with the adding of methyl to give more electron density to the aromatic ring.

Magnetic properties

Static magnetic properties of 1. Magnetic susceptibility measurement of 1 was carried out under an external direct current (dc) magnetic field of 0.1 T at 2–300 K. At 300 K, the χ_MT value of 14.60 cm³ K mol⁻¹ were obtained, which is slightly higher than the expected value of 14.17 cm³ K mol⁻¹ for one uncoupled Dy^III cation (Dy^III:S = 5/2, L = 5, ⁶H_15/2, g = 4/3). The χ_MT value keeps roughly constant from 300 K to 50 K but decreases more and more rapidly as temperature drops from 50 K and reaches a minimum value of 13.13 cm³ K mol⁻¹ at 2 K (Fig. 3), which probably results from the progressive depopulation of the Dy^III stark sublevels and/or intermolecular interactions. Reduced magnetization data for complex 1 was determined at different temperatures (inset of Fig. 3). The magnetization values show poor superposition even at the highest available fields. Moreover, the maximum value of 8.86 Nβ is considerably lower than the expected value, which indicates the presence of a significant magnetic anisotropy and/or low-lying excited states.^3i,9h,18

Fig. 3 Temperature dependency of χ_MT in the range of 2–300 K in 0.1 T for 1. Inset: M vs. H/T plot measured in different fields below 7 T.

Dynamic magnetic properties of 1. The frequency- and temperature-dependent alternating current (ac) susceptibilities for 1 were obtained under zero-dc field (Fig. 4 and S7†). With the increasing temperature the peaks of the χ′′_M data move to higher frequency, which reveals the typical features associated with the SMM behavior. To further ensure the complex 1 is a SMM but not a spin-glass (0.01 < ϕ < 0.08), the indicative parameter of spin disorder ϕ of 0.21 was obtained based on the data of the temperature dependencies of χ′_M, using Mydosh's formula in eqn (1).¹⁹

ϕ = (ΔT_p/T_p)/Δ(log ω) (1)

Fig. 4 Frequency dependence of the in-phase (left) and out-of-phase (right) ac susceptibility of 1 below 11 K, under zero-dc field.

The magnetization relaxation time in the form of ln τ derived from the frequency-dependence measurements is plotted as a function of 1/T between 2 and 11 K (Fig. 5). Above 7.7 K, the relaxation follows a thermally activated process permitting the estimation of an energy barrier of 42.54 K with a pre-exponential factor (τ₀) of 1.13 × 10⁻⁶ s based on the Arrhenius law, which is consistent with the expected τ₀ of 10⁻⁶ to 10⁻¹¹ for a SMM.^3g–i At lower temperatures a gradual crossover to a temperature-independent regime is observed and below about 3.5 K, a dominant quantum regime with a τ value of 0.0004 s explains the absence of M vs. H hysteresis effect at 2 K (Fig. S8†), which is likely due to the hyperfine coupling and dipolar spin–spin interactions of the metal cations. As a result, fast quantum tunneling of magnetization was observed.

Fig. 5 Magnetization relaxation time, ln τ vs. T⁻¹ plot for 1 under zero-dc field. The solid line is with the fitting to the Arrhenius law.

Cole–cole plots (Fig. 6) in the temperature range of 2–10 K with the form of χ′′_M vs. χ′_M, which can be well fitted by the generalized Debye model,^20a,b exhibit good semicircle shapes and small distribution parameters α have been obtained (Table S4†). All of these magnetic parameters reveal a single relaxation process in complex 1.^20c,d All the magnetic analyses as discussed above support the SMM nature of the mononuclear Dy complex 1. Compared with [Dy(hfac)₃(pz)₂]¹⁰, the use of 1-methylimidazole instead of pyrazole to coordinate with Dy(hfac)₃ units leads to the improvement of the ligand field (from D_2d for lanthanide ion to D_4d), and further enlarges the energy barrier of Dy ion from 28.32 to 42.54 K. The higher symmetric coordination environments (as an example (D_4d)^3a,g,21) reduce the admixing of states on opposite sides of the barrier and successfully suppress QTM, which proved the conclusion already obtained by Tong et al. that SIMs with the five symmetries (C_∞v, D_∞h, S₈ (I₄), D_5h, and D_4d) usually showing higher energy barriers.^3a,c,e,h,22 And the intermolecular Dy–Dy distance changes from 8.87 Å to 9.10 Å, also decreases the intermolecular Dy ions dipolar coupling then minimize the QTM to some extent.

Fig. 6 Cole–cole plots measured at 2–11 K in zero-dc field.

Static magnetic properties for 2 and 3. As depicted in Fig. 7 and 8, the temperature dependence of magnetic susceptibilities for 2 and 3 are also studied over the range from 300 to 2 K. At room temperature, the χ_MT values are 11.85 and 13.85 cm³ K mol⁻¹ for 2 and 3, respectively. The value of complex 2 is in good agreement with the expected value of 11.81 for one uncoupled Tb^III ion with ⁷F₆ ground state, but 3 is slightly lower than 14.06 cm³ K mol⁻¹ for one uncoupled Ho^III ion with ⁵I₈ ground state. Upon cooling, the χ_MT value of 2 goes down slowly and reaches a minimum of 11.57 cm³ K mol⁻¹ at 77 K, then gradually rises to a maximum of 11.75 at 53.4 K and further declines more and more rapidly to 10.13 cm³ K mol⁻¹ at 2 K. However, the χ_MT value for 3 gradually deceases over the temperature range 120–300 K and further goes down dramatically to reach a minimum value of 5.89 cm³ K mol⁻¹ at 2 K. To obtain a rough quantitative estimation of the magnetic interaction between paramagnetic species,²³ the magnetic data of 2 and 3 can be analyzed by the approximate model,¹¹ which is given in the ESI, eqn (2)–(4)† using an anisotropic exchange Hamiltonian H = ΔJ_z². The intermolecular interactions were considered by the term zj′.

Fig. 7 Temperature dependence of χ_M and χ_MT product in the range of 2–300 K in 0.1 T for 2.

Fig. 8 Temperature dependence of χ_M and χ_MT product in the range of 2–300 K in 0.1 T for 3.

The best fitting results yielded, zj′ = 0.106 cm⁻¹ (Δ = 0.526 cm⁻¹) with the g-factor of 1.516 for Tb complex, and zj′ = −0.230 cm⁻¹ (Δ = −0.154 cm⁻¹) with the g-factor of 1.267 for Ho complex. The small value of zj′ is indicative of the very weak magnetic interaction between Ln^III ions in 2 and 3.

Variable temperature ac susceptibility data for complexes 2 and 3 are also collected, which are given in the ESI, Fig. S9 and S10.† No frequency-dependent signals of imaginary component χ′′_M have been observed for two complexes even at 2 K. That might be due to the Tb and Ho ions spin states are more sensitive to crystal-field, which usually give Tb and Ho ions very small J grand state and/or QTM, to induce their complexes have too small energy barrier to prevent the inversion of the spin.

Conclusion

Using 1-methylimidazole ligand in association with Dy^III, we successfully synthesized a symmetrical mononuclear complex of [Dy(hfac)₃·(1-MeIm)₂], which exhibits slow relaxation of magnetization considered as one of the examples of a SMM as well as the Dy^III characteristic emission at room temperature. The change of ligand field symmetry from D_2d for lanthanide ion to D_4d resulted in a larger energy barrier of Dy ion from 28.32 to 42.54 K in complex 1 where 1-methylimidazole was coordinated, as compared to [Dy(hfac)₃(pz)₂]. The higher symmetry coordination environments reduce the admixing of states on opposite sides of the barrier, which controls QTM by tuning the local symmetry of the metal centers. The expansion of intermolecular Dy–Dy distance from 8.87 Å to 9.10 Å as we expected probably decreases the intermolecular Dy ions dipolar coupling and suppresses QTM effectively as well. It is obvious that the variation in ligands leads to the change of ligand field and magnetic interaction, which affect the magnetic properties of lanthanide complexes. This proves to be a powerful approach to understand how ligand field symmetry and magnetic interactions affect the SMM behaviors of lanthanide-based systems.

Acknowledgements

This work was supported by the National Natural Science Foundation of China 21471084, 21471083, 21371104. National Undergraduates Innovating Experimentation Project 201410055097.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic data for complexes 1–3 in CIF format. Selected bond lengths and angles, figures of crystal structures, luminescence measurements, and magnetic measurements. CCDC 800739, 798521 and 1515832. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra25038f

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