Dy(III) zig-zag chains assembled in a 3D framework with single-molecule magnet behaviour

Chao Bai , Chuan-Ti Li , Huai-Ming Hu *, Bin Liu , Jin-Dian Li and Ganglin Xue
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710127, China. E-mail: chemhu1@nwu.edu.cn; Fax: +86-29-81535026; Tel: +86-29-81535026

Received 7th September 2018 , Accepted 24th September 2018

First published on 26th September 2018


A new lanthanide-based framework, [Dy(STP)(1,2-bdc)]n (1), was constructed. It represents dysprosium(III) 1D zigzag chains in a 3D framework and displays single-molecule magnet (SMM) behaviour with an energy barrier of 55.76 K under zero dc field.


Single-molecule magnets (SMMs) have attracted considerable attention in recent years across many fields, including chemistry, physics, and materials science.1 Among the SMM family, lanthanide ions are widely used as excellent magnetic building blocks (MBBs) owing to their intrinsic large spin state and large magnetic anisotropy derived from spin–orbit coupling and the crystal field effect, which are not easily satisfied in transition-metal compounds.2 Hence, the introduction of lanthanide ions into SMMs shed new light on this field, especially Tb3+, Dy3+, Ho3+ and Er3+. Hitherto, plenty of lanthanide SMMs, particularly the Dy3+-based SMMs, with mononuclear,3 dinuclear,4 trinuclear,5 polynuclear,6 and 1D polymeric7 structures, have been well documented in the literature.

As far as we know, there are some kinds of 3D lanthanide-based frameworks that have one-dimensional lanthanide coordination chains and exhibit SMM behaviours.8 For example, Z. Chen and coworkers8a reported two 3D lanthanide–organic frameworks: a Gd3+-framework and a Dy3+-framework. In these 3D structures, an {Ln3} motif as a repeat unit was connected to a one-dimensional belt-like chain by carboxylate groups. The Dy3+-framework shows slow magnetic relaxation behaviour and ferromagnetic coupling, meanwhile, which is rather rare in 3D lanthanide-based frameworks. Q.-Y. Liu and Y.-L. Li et al.8b constructed a 3D dysprosium-based framework. The 3D framework is constructed by 1D rod-shaped Dy-carboxylate secondary building units. The magnetic analysis displays a typical behaviour of SMMs. Moreover, K. Liu and H. Li et al.8c synthesized two 3D Dy3+-based frameworks where 1D carboxylate-bridged dysprosium chains can be observed. Both Dy3+-based frameworks exhibit magnetization relaxation that is mainly from the symmetry-related single-ion behaviour.

Herein we report the preparation and magnetic studies of a new 3D framework: [Dy(STP)(1,2-bdc)]n (1) (NaSTP = sodium 2-(2,2′:6′,2′′-terpyridin-4′-yl)benzenesulfonate, H2(1,2-bdc) = benzene-1,2-dicarboxylic acid). The 3D framework contains dysprosium(III) 1D zig-zag chains and presents SMM behaviour under zero dc field.

The rigid NaSTP ligand was synthesized by the reaction of sodium 2-formylbenzesulfonate and 2-acetylpyridine. Compound 1 was obtained through hydrothermal synthesis of NaSTP, Dy(NO3)3·6H2O, H2(1,2-bdc) and H2O at 180 °C for 3 days. The details of the crystal parameters, structural refinement and selected bond lengths and angles are given in Tables S1 and S2 (ESI). Good agreement between the simulated and experimental powder X-ray diffraction (PXRD) patterns verifies the purity of the as-synthesized sample (Fig. S1, ESI). Thermogravimetric analysis (TGA) measurements demonstrated that 1 has excellent thermal stability as no strictly clean weight loss step occurred below 460 °C (Fig. S2, ESI). X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic P2(1)/n space group and exhibits a 3D network structure. The asymmetric unit of 1 contains one crystallographically independent Dy3+ ion, one STP ligand, and one (1,2-bdc)2− anion. Dy1 shows a distorted triangular dodecahedron coordination geometry (Table S3, ESI) of three nitrogen atoms (N1, N2 and N3) and one oxygen atom (O1A) from one STP ligand, and four oxygen atoms (O4, O5B, O6 and O7C) from three different (1,2-bdc)2− anions, as shown in Fig. 1a and 1b. The Dy–O bond lengths vary from 2.258(8) to 2.381(8) Å and the Dy–N bond lengths range from 2.501(1) to 2.546(1) Å. The angles of O–Dy–O and O–Dy–N range from 70.1(3) to 147.7(3)°and 74.5(3) to 146.1(3)°, respectively. In 1, the STP ligand adopts a μ2–η31 bridging coordination mode and the auxiliary (1,2-bdc)2− ligand employs a μ3–η211 coordination fashion to construct a 2D grid-shaped layer (Fig. 1c), and then the resulting layers are further pillared by (1,2-bdc)2− anions to give rise to an infinite 3D framework structure (Fig. 1d).


image file: c8dt03617a-f1.tif
Fig. 1 (a) Coordination environment of Dy3+ in 1. All hydrogen atoms have been omitted for clarity (symmetry codes: A = 1.5 − x, 0.5 + y, 0.5 − z; B = 1 − x, 2 − y, 1 − z; C = 2 − x, 2 − y, 1 − z). (b) Distorted triangular dodecahedron coordination sphere of the Dy3+ ion. (c) View of the 2D layer structure of 1. (d) View of the 3D framework of 1. (e) Schematic view of the 3,5T1 topological network: the Dy3+ cations and (1,2-bdc)2− anions are marked as blue and pink, respectively.

Topologically,9 (1,2-bdc)2− anions and Dy3+ cations can be regarded as 3- and 5-connected nodes, respectively, STP as linkers, and 1 possesses a 2-nodal (3,5)-connected novel 3,5T1 net with the point symbol of (42·65·83)(42·6), which is calculated by TOPOS (Fig. 1e). Astonishingly, Dy3+ ions are linked to each other through pairs of carboxylate groups to form a one-dimensional dysprosium zigzag chain (Fig. 2). The adjacent intrachain Dy–Dy distances are 5.349 and 5.675 Å, respectively. The closest Dy⋯Dy distance between different dysprosium zigzag chains is 10.603 Å.


image file: c8dt03617a-f2.tif
Fig. 2 The 1D dysprosium zigzag chains in the 3D structure of 1.

The direct-current (dc) magnetic susceptibility of 1 was measured for microcrystalline samples in the temperature range of 300–2.0 K under an applied field of 1 kOe (Fig. 3). At room temperature, the observed χMT value of 15.33 cm3 K mol−1 is slightly greater than the expected value (14.17 cm3 K mol−1) for one Dy3+ ion with the ground state 6H15/2 and g = 4/3. Upon cooling, the χMT product of 1 gradually descends reaching a minimum of 11.94 cm3 K mol−1 at about 16.0 K, this behaviour is often observed in 4f paramagnetic ions and is attributed to the depopulation of Stark sublevels.10 Then it increases to a value of 14.3 cm3 K mol−1 drastically when the temperature is further decreased to 2.0 K, indicating the presence of ferromagnetic coupling in 1. Moreover, the ln(χT) versus 1/T plot (Fig. S3, ESI) features a linear region in the range of 10–16 K with Δξ equal to 2.43 K, which indicates that there is a weak ferromagnetic coupling between Dy3+ ions in the 1D zigzag chain.11 The field dependence of magnetization for 1 has been determined at 2.0 K in the range of 0–50 kOe (Fig. 3 inset). Up to 50 kOe the magnetization is 6.4, which is lower than the theoretical saturation value due to the presence of magnetic anisotropy, crystal-field effect and low-lying excited states.12


image file: c8dt03617a-f3.tif
Fig. 3 Plot of χMT vs. T at 1 kOe of 1 (inset: field-dependent magnetization at 2.0 K).

Dynamic alternating-current (ac) magnetization measurements have been performed in order to further investigate the nature of the magnetic states of 1.

Ac susceptibility measurements in the temperature range of 2.0–12.0 K were carried out at several frequencies between 1 and 1000 Hz with zero dc field and an oscillating 3.0 Oe ac field. As shown in Fig. 4a and 4b, both the in-phase (χ′) and out-of-phase (χ′′) components of the ac susceptibility show significant temperature-dependent peaks. This characteristic clearly indicates that slow relaxation of magnetization exists in 1. Additionally, the frequency dependencies of the ac susceptibility were measured under zero dc field in the 2.0–8.5 K interval (Fig. 4c and 4d). The in-phase (χ′) and out-of phase (χ′′) signals of 1 show strong frequency dependencies. Obviously, with increasing temperature, the peaks of the out-of-phase (χ′′) ac susceptibility in 1 gradually shift from a low frequency to a high frequency. The relaxation of magnetization for 1 is described as a SMM and primarily derived from the individual dysprosium ion. In 1, these data were extracted from the peak frequencies of out-of phase (χ′′) and can be fitted well by the Arrhenius law. The best-fit relaxation time and the corresponding energy barrier are τ0 = 1.40 × 10−7 s and Δτ/kB = 55.76 K, respectively (Fig. 5). The data plotted as Cole–Cole plots (Fig. 5 inset) can be fitted to the generalized Debye model which gives rise to α parameters of 0.05–0.18, indicating the narrow distribution of the relaxation times, which is typical for the reported SMMs.13


image file: c8dt03617a-f4.tif
Fig. 4 The in-phase (χ′) (a) and out-of-phase (χ′′) (b) signals of temperature-dependent ac susceptibility plots for 1 in the 2.0–12.0 K interval at different frequencies with Hdc = 0 and Hac = 3.0 Oe; The frequency dependence of in-phase (χ′) (c) and out-of-phase (χ′′) (d) for 1 under zero dc field in the range of 1–1488 Hz.

image file: c8dt03617a-f5.tif
Fig. 5 Peak frequencies of out-of phase (χ′′) fitted by the Arrhenius law. (Inset: Cole–Cole plots measured for 1 under zero-dc field. The solid lines represent the best-fit calculated values with the Debye model.)

In summary, a new Dy(III) zigzag chain-based 3D framework was successfully achieved. Magnetic studies revealed that SMM behaviour occurred in 1 under zero dc field with an energy barrier of 55.76 K. To the best of our knowledge, 1 is very rare that represents SMM behaviour with dysprosium(III) 1D zig-zag chains assembled in a 3D framework.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21473133 and 21173164).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental section, crystal data, additional figures and tables. CCDC 1860308. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt03617a

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