Influence of Energy Barriers in Triangular Dysprosium Single

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Energy Barriers in Triangular Dysprosium SingleMolecule Magnets through Different Substitutions on a Nitrophenolate-Type Coligand Wei Chin and Po-Heng Lin* Department of Chemistry, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan

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S Supporting Information *

coordination environment because of the variable and high coordination numbers as well as poor directionality of the rareearth metal.5 For this reason, only mononuclear and dinuclear complexes had been shown a direct correlation between the relaxation barriers and electron-withdrawing groups on terminal ligands,6 but examples for multinuclear pure lanthanide complexes were relatively rare.4c,6d,f In particular for moleculebased materials, single-molecule toroics (SMTs) belong to a new class of magnetic materials in which the unusual spin chirality in addition to the slow relaxation of magnetization was observed because eof the toroidal arrangement (or wheelshaped topology) of the local magnetization vector.7 Since the discovery of the toroidal magnetic state on the Dy3 triangle,7b the fascinating magnetic properties of larger dysprosium compounds with Dy3 building blocks, such as Dy3 triangles with different ligands,7e coplanar Dy4 compounds,7f and Dy6 wheel compounds,7g have been reported with toroidal magnetic structure. Inspired by the dedication of previous researches, we investigated the similar trinuclear building blocks of lanthanide SMMs with rational modulation of the anisotropic axes to further understand the magnetostructural correlations. In this contribution, a triangular dysprosium system was synthesized for systematic study by the mixed-ligand strategy, which involved a polydentate Schiff-base ligand and a series of fine-tuning bidentate, monoanionic terminal ligands (Scheme 1). (2-Hydroxy-3-methoxyphenyl)methylene(benzoicotino)hydrazine (H 2 hmb) was selected because of its ideal O,N,O,O-based multichelating site pocket for the encapsulation of DyIII ions,8 and o-vanillin derivatives were suitable ligands for

ABSTRACT: The effect of the directions of the anisotropy axes on the energy barriers of single-molecule magnets (SMMs) was investigated. By introducing nitrophenolate (NP)-type coligands with different substitutions, the energy barrier was significantly changed. The structural and magnetic properties of three novel SMMs based on trinuclear {Dy3O5} phenoxo- and methoxyl-bridged triangular motifs were explored. All complexes share the formula [Dy 3 (Hhmb) 4 (μ 3 OMe)2(OMe)(NP)][Dy3(Hhmb)4(μ3-OMe)2(NP)]·solvent·3Cl, where Hhmb = (2-hydroxy-3-methoxyphenyl)methylene(benzoicotino)hydrazine, secondary ligand NP = 2-nitrophenol (2-NP, complex 1), 2,4-dinitrophenol (2,4-DNP, complex 2), and 2,4,6-trinitrophenol (2,4,6TNP, complex 3), and solvent = 2MeOH·2Et2O (1) and 4MeOH (2 and 3). Magnetic measurements for 1 and 2 revealed observable slow magnetic relaxation behavior with anisotropic energy barriers of 12.18 and 4.96 K, respectively, for SMMs and only the tail of the peaks in the out-of-phase susceptibility, χ″, was observed in complex 3. Comparing a series of NP coligands, we could easily study the correlation between the directions of the anisotropic axes and magnetic properties for this trinuclear SMM system.

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ingle-molecule magnets (SMMs) display slow relaxation of magnetization at the molecular dimension, which has led to worldwide research interest owing to their potential applications in many fields, such as high-density information storage, molecular spintronic devices, and multiferroic materials.1 The observed superparamagnet-like behavior generally results from the presence of a large-spin ground state (ST) and Ising-type magnetoanisotropy (D) in those systems, which is frequently quantified through calculation of their experimentally observed energy barrier to relaxation (Ueff). Recently, lanthanide-based SMMs, such as DyIII, TbIII, and ErIII, have been at the forefront of major advances in the field of SMMs because of their large magnetic moments and strongly inherent magnetic anisotropies caused by strong spin−orbit coupling.2 Because the anisotropy energy can be modulated by a subtle change of the ligand field,3 the correlations among the coordination models, steric effects, electronic effects, and magnetic properties have been studied and offered more information for the design of new lanthanidebased SMM clusters.4 However, any minor difference of the ligand moiety or counterion might dramatically change the © XXXX American Chemical Society

Scheme 1. Syntheses of Complexes 1−3

Received: June 19, 2018

A

DOI: 10.1021/acs.inorgchem.8b01667 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry trapping of Dy3 triangles.7c,9 A series of nitrophenolate (NP)type secondary ligands possessing nitro substituents with ortho and/or para positions attached to the phenolate rings were expected to find-tune one of the three coordination geometries of the DyIII ions. The syntheses, structures, and magnetic properties of three trinuclear DyIII complexes were reported, and consequently magnetostructural correlations were investigated. The structure of the trinuclear complex, [Dy3(Hhmb)4(μ3OMe) 2 (OMe)(2-NP)][Dy 3 (Hhmb) 4 (μ 3 -OMe) 2 (2-NP)]· 2MeOH·2Et2O·3Cl (1·2MeOH·2Et2O·3Cl), crystallized in the triclinic P1̅ space group (Figure 1). Four ligands coordinated

the Supporting Information. The molecules of 2 and 3 were also linked through hydrogen bonds between the Cl− ions and the N atoms of the ligands (Figures S9 and S10 for complexes 2 and 3, respectively). The temperature dependence of the direct-current (dc) magnetic susceptibility (3.0−300 K) was measured under an applied dc field of 1000 Oe. The χMT versus T plot (Figure 2)

Figure 1. Molecular structure of complex 1. Color code: Dy, yellow; O, red; N, blue; C, gray. The H and Cl atoms were omitted for clarity.

Figure 2. Temperature dependence of the χMT product of complexes 1−3 at 1000 Oe. Inset: Field dependence of magnetization data at 2−8 K for complex 1.

with DyIII in three different coordination modes (Figure S1). The O,N,O pocket of the last ligand coordinated to Dy1 to achieve an eight-coordinate environment. Three DyIII ions were also bridged by the two μ3-methoxy anions (O16 and O17) from the deprotonated methanol solvents to form a triangular core structure. The coordination number of both the Dy2 and Dy3 ions is 9. One terminal methoxyl group (50% chemical occupancy in each molecule; Figure S2) filled the coordination site of Dy2, and the rest of the coordination sites of Dy3 were occupied by two O atoms (O13 and O14) from 2-NP. The coordination geometries of Dy1, Dy2, and Dy3 were determined by SHAPE 2.1 (Table S1 and Figure S3). Charge-balance considerations for the molecule indicate that all of the ligands are in the keto form (Table S3), which is similar to the mononuclear and dinuclear complexes in the literature.5c,d Moreover, three Cl− counterions in the lattice were required for charge balance. The molecules of 1 were also linked through hydrogen bonds between the Cl− ions and N (N1 and N5) atoms of the ligands (Figure S4). 2,4-Dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (2,4,6-TNP) were also selected in the syntheses in order to replace the coordinated 2-NP ligand and fine-tune the core geometry of Dy3. The structures of both complexes 2 and 3 were contained the same coordination modes as complex 1 (Figure S5). Comparisons of the Dy−O bond distances and Dy−O−Dy angles in complexes 1−3 (Figure S4 and Table S3) reflected the similar {Dy3O5} cores for the three complexes. The polyhedra of DyIII ions in complexes 2 and 3 were compared with that of complex 1, resulting in almost the same geometry features of Dy1 and Dy2 in the three complexes, but the coordination geometries of the Dy3 centers in complexes 2 and 3 are different from that of complex 1. More details of the crystallographic data and bond lengths and angles of complexes 1−3 are reported in

suggests the presence of magnetic anisotropy arising from the DyIII ions. The room temperature values of the χMT products for complexes 1−3 were 42.40, 42.98, and 45.36 cm3 K mol−1, respectively. The theoretical spin value for a single DyIII ion is 14.17 cm3 K mol−1 (6H15/2, S = 5/2, L = 5, g = 4/3) and thus is 42.51 cm3 K mol−1 for three DyIII ions, which are both near the experimental values observed. The χMT values of complexes 1− 3 dropped to values of 30.11, 36.80, and 29.01 cm3 K mol−1 at 3.0 K, respectively, which is likely indicative of weak antiferromagnetic interactions between the three DyIII ions and/or inherent magnetic anisotropy from the successive perturbation of electron−electron repulsion, spin−orbital coupling, and the presence of a crystal field. The isotherm magnetization (M) measurements below 8 K of complexes 1−3 reveal a rapid increase in magnetization at low magnetic fields (Figures 2, inset, and S11−S14). In order to assess the relaxation dynamics of the complex, alternatingcurrent (ac) susceptibility measurements were performed with an ac field of 3.50 Oe oscillating at frequencies up to 10000 Hz. We have measured the frequency dependence of the in-phase (χ′) and out-of-phase (χ′′) magnetic susceptibilities under zero applied dc fields, where clear peaks are seen to shift to lower frequencies as the temperature is decreased, indicative of slow magnetic relaxation and SMM behavior of complexes 1 and 2 in Figure 3. The relaxation times, τ, are extracted from these data, allowing calculation of a thermally induced anisotropic energy barrier, Ueff, between the magnetic ground states [τ = τ0 exp(Ueff/kT)]. The effective energy barriers obtained from the fitting procedure (Figures S15 and S16) are Ueff = 12.18 K (τ0 = 3.89 × 10−6 s) for 1 and 4.96 K (τ0 = 3.54 × 10−6 s) for 2. A frequency-dependent tail of the peak is observed in the χ′′ versus frequency plots (Figure S17) below 6 K for complex 3, indicating potential SMM behavior at very low temperature and/or high frequency; however, it is difficult to quantify the B

DOI: 10.1021/acs.inorgchem.8b01667 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

anisotropic axes on the Dy1 and Dy2 ions for all three complexes are very similar but not on Dy3 (Figure S25). Indeed, the coordinated NP ligands could find-tune the local Dy3 geometry because of the different positions of the nitro groups, leading to the different directions of the anisotropic axes. In conclusion, a new family of trinuclear dysprosium complexes, comprising three congeners, have been assembled by adopting a mixed-ligand strategy based on a Schiff-base ligand (H2hmb) and three NP-type ligands with different substitutions [2-NP (1), 2,4-DNP (2), and 2,4,6-TNP (3)]. Complexes 1 and 2 exhibited slow relaxation of magnetization under zero applied field with energy barriers of 12.18 and 4.96 K, respectively, and a temperature-dependent tail of the peak was observed in complex 3. Indeed, complexes 1−3 did not satisfy two requirements of the typical SMTs: planar arrangement of the local anisotropic axes and the cyclic symmetry of the multinuclear lanthanide complex, leading to the low energy barrier. The result still provided clear evidence that the obvious differences of the magnetic properties were correlated to the anisotropic axes of the Dy3 atoms by changing the series of NP ligands. This synthetic strategy of rationally modulating the dynamic magnetic relaxation of the lanthanide SMMs could offer fundamental knowledge for future investigations of SMTs.

Figure 3. Frequency dependence of the out-of-phase (χ′′) susceptibility under zero dc field for complexes 1 (left) and 2 (right).

energy barrier without a full peak. Such behavior generally indicates that slow relaxation of magnetization is highly influenced by the quantum tunneling of magnetization (QTM) through the spin reversal barrier. In particular, for triangular systems, the magnetic moment could arise from the toroidal arrangement of the local magnetization vectors along the DyIII ions, which can interact with the dc current through magnetoelectric coupling. In order to probe the temperature and frequency dependence of the data, the χ′ and χ′′ magnetic susceptibilities were measured with an applied dc magnetic field up to 5000 Oe (Figures S19−S21). The addition of a low static field was found to have only a small effect, and/or the peaks were shifted to high frequencies as the static fields were increased. Thus, we may conclude that zero-field QTM is not efficient within these complexes. Furthermore, the Cole−Cole plots are shown in Figures S22 and S23, and good fittings were not able to be achieved by using a generalized Debye equation. This phenomenon would be explained by phonon-mediated hybridization in that plenty of low-lying weakly coupled states lead to a multitude of available direct relaxation mechanisms. In a dinuclear system,6a the difference of the energy barriers can be correlated to the Dy−Dy coupling. The electron density on the coordinating O atoms of the terminal ligands could be effected by the electron-withdrawing groups, which induce a stronger chemical bonding between the DyIII ions and bridging O atoms. This leads to shorter Dy−O bond lengths and a larger energy barrier. However, this explanation does not apply to our triangular system. The electronic effects induced by the nitro groups did not change the Dy−O bond distances between Dy3− O13 and Dy3−O14 as well as all of the Dy−O bonds of the {Dy3O5} core structures (Table S3 and Figure S4). On the other hand, the different energy barrier may be based on the direction of the anisotropic axes. Owing to our interest in multinuclear DyIII complexes, we employed electrostatic modeling in order to investigate the orientations of the magnetic anisotropy axes for complexes 1−3 using Magellan magnetic software.10 The three anisotropic axes in complexes 1−3 are all pointed in different directions because of the three crystallographically nonequivalent DyIII ions, leading to the low energy barriers in comparison to other triangular systems with toroidal arrangement (Figure S24). In the derivation of the magnetostructural correlation, it is essential to look at the details of each DyIII ion. A comparison of the directions of the anisotropy axes is illustrated in Table S4, indicating that the directions of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01667. Experimental details, including X-ray crystallographic studies, magnetic measurement, and syntheses, additional structures, table summary of SHAPE analysis, crystallographic data, selected bond angles and lengths, field dependence magnetization data, M versus H/T plots, Arrhenius plots, frequency dependence of the out-ofphase susceptibility, Cole−Cole plots, and plots of the anisotropic axes (PDF) Accession Codes

CCDC 1847244, 1847246, and 1847248 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: + 886-4-22840411, ext. 724. ORCID

Po-Heng Lin: 0000-0002-0893-0806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from National Chung Hsing University and the Ministry of Science and Technology, Taiwan (Grants MOST 105-2113-M-005-011 and 107-2113-M-005-003). We also thank Prof. Hui-Lien Tsai from the National Cheng Kung University for assistance with the ac magnetic measurements. C

DOI: 10.1021/acs.inorgchem.8b01667 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b01667 Inorg. Chem. XXXX, XXX, XXX−XXX