Effect of Bridging Ligands on Magnetic Behavior in Dinuclear

Dec 28, 2018 - Abstract. Abstract Image. A family of dinuclear dysprosium cores bridged by different ligands within a polyoxometalates (POMs) framewor...
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Effect of Bridging Ligands on Magnetic Behavior in Dinuclear Dysprosium Cores Supported by Polyoxometalates Yu Huo,† Yan-Cong Chen,† Si-Guo Wu,† Jun-Liang Liu,*,† Jian-Hua Jia,† Wen-Bin Chen,† Bao-Lin Wang,‡ Yi-Quan Zhang,‡ and Ming-Liang Tong*,† †

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Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, PR China ‡ Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, PR China S Supporting Information *

ABSTRACT: A family of dinuclear dysprosium cores bridged by different ligands within a polyoxometalates (POMs) framework, (TBA)8.5H1.5[(PW11O39)2Dy2X2(H2O)2]·6H2O (X = OH (1), F (2), OAc (3); TBA = tetra-n-butylammonium), was successfully synthesized and structurally characterized. Magnetic studies indicate that the bridging ligands can significantly affect the magnetic behaviors, with 1 and 3 showing antiferromagnetic coupling and 2 bridged by fluoride ions showing ferromagnetic interaction. 1 and 2 behaved as singlemolecule magnets (SMMs) with the thermally activated energy barrier of 98(5) and 74(6) cm−1 under zero dc filed, respectively, whereas no SMM behavior was observed for 3 bridged by two μ-η1:η2-acetato ligands. Notably, the low-temperature fluorescence spectra of 1−3 provide valuable information on the energy levels, which are consistent with the anisotropic barriers determined by magnetic measurements. These results offer an insight into the magneto-optical correlation. Furthermore, the effective energy barrier of 1 reaches a breakthrough among all POM-based SMMs.

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INTRODUCTION

A survey of the available literatures shows that specific types of bridging moieties, such as hydroxyl, alkoxide and thiolate, have been widely used to achieve Ln-based polynuclear clusters and led toward high energy barrier.6 In contrast, Ln-based complexes bridged by more electronegative F− ions are still rare, especially the pure 4f ones, which may result from the competing formation of highly insoluble LnF3.7 To date, only a 1D zigzag chain bridged by unsupported fluoride anions and a linear lone fluoro-bridges for pure 4f complexes show slow relaxation of magnetization under zero field.8 In order to explore the effects of bridging ligands on magnetic behaviors, it is necessary to keep the ligand field environments structurally comparable except for the bridges and to leave the molecule isolated. The nonmagnetic polyoxometalates (POMs) are among the good candidates because of their rigidity and sizable bulk. Of particular interest is the use of lacunary POMs as “super ligands” for transition metals and Ln ions, affording complexes with novel magnetism and luminescence behavior.9 Lacunary POMs have shown significant advantages for the preparation of SMMs, owing to the flexible and variable coordination lacunary sites enabling facile control of structures accommodating metal cores, and undesired intermolecular magnetic interactions can be prevented by the bulkiness of

Single-molecule magnets (SMMs) are a fascinating class of molecular nanomagnets that exhibit magnetic slow relaxation and can be potentially applicable in ultrahigh-density information storage and molecular spintronics.1 Since the report of the famous [TBA][Tb(Pc)2] (H2Pc = phthalocyanine) showing a SMM behavior,2 lanthanoid (Ln) coordination compounds with significant intrinsic magnetic anisotropy provide important new insight into the field of SMMs. Recently, the monometallic SMMs or single-ion magnets (SIMs) have made remarkable progress with very high effective energy barrier (Ueff) and blocking temperature.3 Fine-tuning the crystal field (CF) or coordination environments around the spin center has been a successful strategy to develop novel SMMs with good performance.4 However, such strategies have a finite ceiling for polynuclear or extended systems, owing to the poor radial extension of the 4f orbitals results in the complicated and weak exchange interactions between lanthanide ions. To our knowledge, magnetic interactions can be strongly influenced by any subtle alterations of the coordination geometry and the ligand field of the lanthanide ion.5 Furthermore, the choice of the bridging group in the case of lanthanide systems is also a key component in controlling the core nature of 4f orbitals and mediating the magnetic coupling between the paramagnetic centers. © XXXX American Chemical Society

Received: September 30, 2018

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DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystal Data and Structural Refinement for 1−3 compound

1

2

3

empirical formula Mr (g mol−1) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) μ (mm−1) reflns collected goodness-of-fit on F2 R1, wR2 [I > 2σ(I)]a,b R1, wR2 [all data]

C136H325.5N8.5P2O88Dy2W22 7919.97 triclinic P1̅ 15.0800(5) 15.3402(5) 25.0811(9) 102.356(1) 97.102(1) 107.800(1) 5283.7(3) 1 2.298 12.700 150325 1.026 0.0307, 0.0764 0.0350, 0.0789

C136H323.5N8.5P2O86F2Dy2W22 7923.95 monoclinic C2/c 24.4821(12) 17.9400(12) 48.176(3) 90 93.121(2) 90 21128(2) 4 2.300 12.704 190573 1.132 0.0463, 0.1084 0.0514, 0.1116

C140H329.5N8.5P2O90Dy2W22 8004.05 triclinic P1̅ 15.0186(10) 15.3092(10) 25.3929(18) 102.025(2) 97.727 (2) 107.710(2) 5315.6(6) 1 2.299 12.625 125171 1.156 0.0624, 0.1481 0.0800, 0.1549

R1 = Σ||Fo| − |Fc||/Σ|Fo|. bwR2 = Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]1/2.

a

POM ligands.10 Additionally, POMs are appropriate lightharvesting candidates to sensitize lanthanoid emission, owing to photoexcitation of O → M (M = Mo or W) ligand to metal charge transfer (LMCT) bands of the POM ligands can lead to intramolecular energy transfer from O → M excited states to excited energy levels of the lanthanoid ion.11 The fluorescence spectra can provide an effective means to determine the energy levels of the lanthanoid ions, which is a direct probe, and a further support to those derived from magnetic studies.12 Herein, we report a family of the dinuclear dysprosium cores in POMs (TBA)8.5H1.5[(PW11O39)2Dy2X2(H2O)2]·6H2O (X = OH (1); F (2); OAc (3)), where the Dy(III) ions are bridged by different ligands. Changes of bridging ligands are mainly responsible for the obviously distinct magnetic behaviors observed in 1−3. Furthermore, compounds 1 and 2 exhibit the characteristic Dy(III) ions luminescence associated with the slow relaxation of the magnetization, which give a direct probe to correlate the CF splitting derived from the magnetic data. To our knowledge, compound 1 bridged by two hydroxide groups represents the record Ueff among those of the POMbased SMMs.

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were corrected for the diamagnetic contribution calculated using the Pascal constants. Synthesis. All chemicals used for synthesis were purchased and without any further purification. Na9[α-PW9O34]·16H2O. The precursor was prepared according to the literature.13 (TBA)8.5H1.5[(PW11O39)2Dy2(OH)2(H2O)2]·6H2O (1). Solid Na9[αPW9O34]·16H2O (0.272 g, 0.1 mmol) and DyCl3·6H2O (0.038 g, 0.1 mmol) were dissolved in H2O (10 mL) and stirred for 5 min at room temperature. Then, tetra-n-butylammonium bromide (TBAB) (0.161 g, 0.5 mmol) was added to this solution (pH 7.8). The resulting slurry was sealed in a 23 mL Teflon-lined autoclave and heated at 140 °C for 48 h, followed by cooling to room temperature at a rate of 10 °C h−1. Colorless crystals were washed with deionized water and dried in air (yield 45% based on W). Elemental analysis calcd for 1: C, 20.62; H, 4.14; N, 1.50. Found: C, 20.68; H, 3.89; N, 1.59. IR data of 1 (KBr, cm−1): 2962 (m), 2872 (m), 1639 (m), 1482 (m), 1377 (m), 1076 (m), 1044(m), 951 (s), 887(s), 814(s). (TBA)8.5H1.5[(PW11O39)2Dy2F2(H2O)2]·6H2O (2). 2 was prepared by a method analogous to that of 1, adding NH4F (0.004 g, 0.1 mmol) to the resulting slurry (yield 32% based on W). Elemental analysis calcd (%) for 2: C, 20.61; H, 4.12; N, 1.50. Found: C, 20.77; H, 3.89; N, 1.60. IR data of 2 (KBr, cm−1): 2961 (m), 2871 (m), 1634 (m), 1484 (m), 1379 (m), 1077 (m), 1046(m), 951 (s), 891(s), 815(s). (TBA)8.5H1.5[(PW11O39)2Dy2(CH3COO)2(H2O)2]·6H2O (3). Solid Na9[α-PW9O34]·16H2O (0.272 g, 0.1 mmol) and DyCl3·6H2O (0.038 g, 0.1 mmol) were dissolved in 10 mL of HOAc/NaOAcbuffer (1 M, pH 4.8) and stirred for 5 min at room temperature. Then, TBAB (0.161 g, 0.5 mmol) was added to this solution. The resulting slurry was sealed in a 23 mL Teflon-lined autoclave and heated at 140 °C for 48 h, followed by cooling to room temperature at a rate of 10 °C h−1. Colorless crystals were washed with deionized water and dried in air (yield 41% based on W). Elemental analysis calcd (%) for 3: C, 21.01; H, 4.15; N, 1.49. Found: C, 21.09; H, 3.99; N, 1.47. IR data of 3 (KBr, cm−1): 2961 (m), 2871 (m), 1634 (m), 1484 (m), 1379 (m), 1077 (m), 1046(m), 951 (s), 891(s), 815(s). X-ray Crystallography. The crystallographic data for 1−3 were given in Table 1. Diffraction data were collected on a Bruker D8 QUEST diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 120 K. Data indexing and integration were carried out using a Bruker given in Table 1. Diffraction data were collected on a Bruker D8 QUEST diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 120 K. Data indexing and integration were carried out using a Bruker APEX3 program. The structures were solved by dual-space algorithm,

EXPERIMENTAL SECTION

Physical Measurements. Elemental analyses were performed by using an Elementar Vario-EL CHN elemental analyzer. The FT-IR spectra were recorded on a Bio-Rad FTS-7 spectrometer. (using KBr pellets) in the range of 4000−400 cm−1. XRPD data were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation in the angular range 2θ = 5−45° at 293 K. Thermogravimetric analyses were measured on a NETZSCH TG209F3 thermogravimetric (TG) analyzer with a heating rate of from 25 to 800 °C in N2 flow (10 °C/min). The luminescence spectra were recorded on an Edinburgh FLS-980 Fluorescence spectrometer equipped with Xenon light, PMT detector, and ALS cryostat down to 10 K. X-ray photoelectron spectroscopy (XPS) data were collected on an ESCALAB 250 X-ray photoelectron spectroscopy, using Mg Kα X-ray as the excitation source. Magnetic susceptibility measurements were performed with a Quantum Design MPMS XL-7 SQUID magneto-meter and a Quantum Design PPMS VSM. Polycrystalline samples were embedded in vaseline to prevent torqueing. Alternating-current (ac) magnetic susceptibility data measurements were performed with a 5 Oe ac oscillation field at frequencies between 1 and 1488 Hz. All data B

DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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extent. As is well-known, Ln-based fluoride complexes are very limited, and 2 represents the rare example of fluoride-bridged Ln-based POMs.17 The presence of F− ions in 2 was further confirmed by XPS (Figure S7). Bond valence sum18 analysis indicates that the oxidation states of all W, P, and Dy centers are +6, +5, and +3, respectively, and the overall charge of 1−3 is −10. On the basis of charge balance considerations, elemental analyses, and TG analyses (Figure S6), 8.5 TBA cations and 1.5 protons are necessary. These protons could not be located by X-ray analysis and are assumed to be delocalized over the entire structure, which is common in POMs.19 Magnetic Measurements. Direct-current (dc) magnetic susceptibility data were carried out on polycrystalline samples of 1−3 in the temperature range 2−300 K under an applied field of 1 kOe. At room temperature, the χMT values of 27.96, 27.62, and 28.42 cm3 K mol−1 for 1−3, respectively, are close to the expected value of two Dy(III) (6H15/2, 2 × 14.17 cm3 K mol−1; Figure 2). Upon cooling, the χMT value of 1 and 3

and all non-hydrogen atoms were refined anisotropically by leastsquares on F2 using the SHELXTL 2014 program suite.14 Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Some tbutyl groups of TBA cations were rotationally disordered and were refined with two sets of partial methyl carbon atoms; suitable restraints were applied to the geometries and to the thermal parameters of the partial atoms. H atoms of TBA cations and organic ligands were fixed in calculated positions; no attempt has been made to locate H atoms of water molecules. The partial TBA cations were found from the Fourier maps. The disordered cations and/or solvent molecules were squeezed15 and determined by the elemental analysis and TG analysis.

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RESULTS AND DISCUSSION Crystal Structure. Single-crystal X-ray diffraction analyses reveal that 1 and 3 crystallize in triclinic space group P1̅, whereas 2 belongs to the monoclinic C2/c space group (Table 1). In spite of the distinction mentioned above, the three compounds are almost isostructural comprising centrosymmetric dimers with an asymmetric unit containing the similar polyanion [(α-PW11O39)Dy(H2O)]4− (Figure 1). The differ-

Figure 1. Combined ball-and-stick/polyhedral representation of the structures of 1, 2 (a), and 3 (b). (c) Capped trigonal prism geometry of the Dy(III) ions in 1 and 2. (d) Biaugmented trigonal prism geometry of the Dy(III) ions in 3. −

Figure 2. Temperature dependence of the molar magnetic susceptibility χMT product at 1000 Oe for compounds 1−3.

gradually decreases to 19.67 and 15.12 cm3 K mol−1 at 2.0 K, indicating the progressive depopulation of the excited Stark sublevels and/or antiferromagnetic exchange interactions. Therefore, the χMT value for 2 gradually decreases with the decrease in temperature to reach a minimum of 22.25 cm3 K mol−1 at 9 K, before increasing to reach values of 23.39 cm3 K mol−1 at 2 K. The increase in χMT at very low temperature suggests the presence of ferromagnetic interactions between the metal centers, as is commonly observed in fluoride-bridged lanthanide compounds.8 The molar magnetizations M(H) for 1−3 show rapid increases at low field and then slowly reach the values of 10.99, 9.94, and 10.05 Nβ, respectively, with the nonsuperimposed M versus H/T curves (Figure S8). This behavior suggests the presence of low-lying magnetic states arising from the crystal-field splitting and/or magnetic couplings. In addition, an observable magnetization hysteresis below 2 K is found in 1 (Figure S9). Alternating-current (ac) susceptibility measurements were performed on 1−3 to probe the slow magnetic relaxation behavior. Both the in-phase (χ′M) and out-of-phase (χ′′M) ac magnetic susceptibility show clear frequency dependence at low temperatures for 1 and 2 under zero dc field (Figure 3), whereas no peaks were shown for 3 under zero and applied fields (Figure S11), indicating no SMM behavior of 3. To our

−

ences lie in that the bridged ligand of 1−3 is OH , F , and OAc−, respectively. Each Dy(III) ion in 1 and 2 has the similar capped trigonal-prismatic coordination geometry (Table S2) composed of four terminal oxido ligands from the lacunary faces of the {α-PW11O39} unit, an aqua ligand and two bridging OH− (O1, O1A)/ F− (F1, F1A), with Dy−O1/O1A and Dy− F1/F1A bond lengths of 2.451(4)/2.483(4) Å and 2.325(6)/ 2.326(6) Å, respectively. A similar dinuclear dysprosium motif was also observed in [Dy2(μ-OH)2(SiW10O36)2]12− without aqua ligand.10e The Dy(III) ions in 3 were bridged by bidentate acetate ligand expending their coordination environment to biaugmented trigonal-prismatic (Table S2) with Dy− O1/O1A lengths of 2.423(9)/2.408(9) Å, which is similar to the previously reported [(CH3)4N]10[(PW11O39)2Dy2(OAc)2(H2O)2].16 The Dy··· Dyintra distances in 1−3 are 4.0671(6) Å, 3.8549(10) Å and 3.9530(13) Å, respectively, with the associated angle Dy1− O1−Dy1A of 111.02(17)° for 1, Dy1−F1−Dy1A of 112.0(3)° for 2, and Dy1−O1−Dy1A of 109.2(6)° for 3. The shortest Dy···Dy distance between the neighboring molecules is respectively 15.0800(8) Å for 1, 15.1762(11) Å for 2, and 15.0186(15) Å for 3, which are well-separated and undesired intermolecular magnetic interactions can be prevented to some C

DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Plots of ln(τ) versus T−1 for 1 (a) and 2 (b) under zero dc field. The solid lines are the best fits to the Arrhenius law, and the dashed lines are the best fits to the multiple relaxation equation. Inset: Cole−Cole plots for the ac susceptibilities under zero dc field. The solid lines are the best fit to modified generalized Debye model. Figure 3. Temperature and frequency dependence of the out-of-phase (χ′′M) products under zero dc field for 1 (a) and 2 (b) with ac frequencies of 1−1488 Hz. The lines are guide to the eyes.

carefully examine the relaxation mechanisms, the data can be analyzed using the equation τ−1 = CTn + τ0−1 exp(−Ueff/kBT), involving the Raman and Orbach process for zero dc field (Figure 4). The multiple process fitting gave the C = 3.2(5) s−1 K−1.60, n = 1.60(9), Ueff = 98(5) cm−1 and τ0 = 8.74 × 10−9 s for 1, C = 117(12) s−1 K−1.02, n = 1.02(7), Ueff = 74(6) cm−1, and τ0 = 1.55 × 10−8 s for 2, respectively. These parameters of Ueff and τ0 are close to those obtained from Arrhenius law at high temperatures. To the best of our knowledge, the effective energy barrier of 1 reached a breakthrough among the POMbased SMMs (Table S4). As described above, the different Dy−ligand interactions and change of the coordination sphere around the Dy(III) ion can largely affect the magnetic relaxation behavior. In order to estimate the orientation of the magnetic anisotropy, Magellan software that takes into account the electrostatic optimization of the aspherical electron density distribution method was employed to probe the anisotropy axis direction on the Dy(III) ions.21 The main magnetic axes are calculated as shown in Figure 5, with the angles between the Dy1−O1w bond and the anisotropy axis being 83.54 (for 1), 87.94 (for 2), and 89.71 (for 3), respectively. As 1−3 are all centrosymmetric, the magnetic axes of the two Dy(III) must be perfectly antiparallel/parallel. Considering magnetic dipolar interactions, eq 2 provides the simplified form of the potential energy (Edip) in the case of parallel interacting moments for a dimer:6d,22

knowledge, 2 is the first example of fluoride-bridged POMbased SMMs. It is noteworthy that two maxima can be observed around 10 and 15 K for 1 and 5 and 11 K for 2 (Figure 3a,b), respectively, indicating the existence of two regimes of relaxation, which may result from the highly disordered TBA cations in molecules.20 In the temperature range of 2−11 K for 1 and 4−8 K for 2, the frequencydependent ac magnetic susceptibilities can be successfully fitted using the sum of two generalized Debye functions (eq 1). Unfortunately, the peaks of the fast relaxation of 1 and 2 were significantly broadened and too fast, leading to meaningless relaxation times; thus, only the slow relaxation of 1 and 2 was fully explored, with α parameters in the range 0.24−0.31 for 1 and 0.22−0.34 for 2. χ1 − χ0 χ2 − χ1 χac (ω) = + + χ0 (1 − α2) 1 + (iϖτ2) 1 + (iϖτ1)(1 − α1) ω = 2πν (1)

At high temperatures, the relaxation times obey the Arrhenius law corresponding to the Orbach process. For 1, this affords a thermally activated Ueff = 101(5) cm−1 with the pre-exponential factor τ0 = 6.39 × 10−9 s, and for 2, this affords Ueff = 75(4) cm−1 with τ0 = 1.05 × 10−8 s (Figure 4). To D

DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a−c) Orientation of the anisotropy axis of complexes 1−3 according to the MAGELLAN software package. The antiparallel/parallel alignments of the magnetic axes are calculated based on magnetic dipole field.

Figure 6. (a, c, and e) Fluorescence spectra for 1−3 at 10 K. (b, d, and f) Fluorescence spectra of 1−3 showing the 4F9/2 → 6H15/2 peaks. The dotted line indicates the last peak of the excitation spectrum and overlaps perfectly with the first peak of the emission spectrum for clarity.

Edip = −

μ0 μ2 [3 cos2 θ − 1] 4π r 3

the emission spectra for 1−3, which are assigned as the ground-to-ground transition of 4F9/2 ↔6H15/2 (Figure 6b,d,f). The well-resolved emission spectrum of 1 (Figure 7a) can be deconvoluted into a 10-component multi-Gaussian function. Since the maximum Stark splitting of the 6H15/2 levels is eight (peak 1−8), the two additional transitions (peaks a and b) should be arisen from “hot” bands involving the first Stark component of the 7F9/2 level. Then, the energy diagram of the Stark-sublevels can be successively determined from the fine structure of the 4F9/2 → 6H15/2 emission (Figure S14b). The energy difference between the first two lowest doublets is 92(3) cm−1, which matches well with the energy barrier of 98(5) cm−1 for 1 (Figure 7b). As shown in Figure 7c, the fine spectrum of the 4F9/2 → 6H15/2 transition of 2 is fitted by Gaussian function convolution of 8 components assumed to be representative of the crystal-field splitting of the ground 6H15/2 state. The energy peak positions of mJ levels were determined (Table S7), and the energy gap from the ground doublet to the first excited doublet is 98(8) cm−1, which is also estimated from the dynamics of the magnetization 74(6) cm−1 (Figure 7d). It is similar to 1 that the high-resolution emission of the 4 F9/2 → 6H15/2 transition for 3 can also be fitted by a 10component multi-Gaussian function (Figure 7e), whose energy peak positions of mJ levels is listed in Table S8. The energy gap between the ground and first excited state of the 6H15/2 crystal field splitting is 85(3) cm−1 (Figure 7f). However, the energy

(2)

where θ is the angle between the orientation of the magnetic moments and the vector connecting two interacting center. The sign of the interaction depends on θ, leading to antiferromagnetic coupling for θ > 54.75 and ferromagnetic coupling for θ < 54.75. The θ of 1−3 is 71.5, 51.9, and 89.0°, respectively, which confirms that the Dy−Dy interaction in 1− 3 are both antiferromagnetic while 2 is ferromagnetically coupling. As a result, the anisotropy axes on two DyIII for 2 are parallel, and those of 1 and 3 are antiparallel (Figure 5). Luminescence Study. Photoluminescence spectrum has recently been exploited as an experimental probe for elucidating the magnetic anisotropy of the Ln-based SMMs.12 1−3 show good fluorescence spectra both at room temperature and at low temperature upon irradiation of UV light at 387 nm (for 1), 365 nm (for 2), and 366 nm (for 3), respectively, which provides valuable information on the energy levels. The emission spectra of 1−3 show the characteristic peaks for Dy(III) around 20 000, 17 450, and 15 100 cm−1, corresponding to 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, and 4F9/2 → 6H11/2 transitions, respectively (Figure 6a,c,e). The accurate and fine luminescence spectra of 1−3 were achieved at 10 K. In terms of the 4F9/2 → 6H15/2 transition, the last peaks of the excitation spectra overlap with the peaks of E

DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.8b02788. Crystallographic details, structure, characterization details, magnetic and luminescence characterization for 1− 3 (PDF) Accession Codes

CCDC 1818031, 1818035, and 1818038 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]. uk, 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 Authors

*E-mail: [email protected] (J.-L.L.). *E-mail: [email protected] (M.-L.T.). ORCID

Yan-Cong Chen: 0000-0001-5047-3445 Jun-Liang Liu: 0000-0002-5811-6300 Wen-Bin Chen: 0000-0002-8396-7035 Yi-Quan Zhang: 0000-0003-1818-0612 Ming-Liang Tong: 0000-0003-4725-0798

Figure 7. (a, c, e) Magnification of the 4F9/2 → 6H15/2 transition measured for 1 (λex = 387 nm), 2 (λex = 365 nm) and 3 (λex = 366 nm) at 10 K. Blue points are experimental data; the lines are as follows: green lines, Gaussian components; red line, sum of the components. (b, d) Energy levels determined by the fluorescence spectra of 1 and 2 compared with the magnetic energy barrier. (f) Energy levels determined by the fluorescence spectra of 3.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key Research and Development Program of China (2018YFA0306001), the NSFC (Grant nos. 21701198, 21822508 and 21620102002), and the Pearl River Talent Plan of Guangdong (2017BT01C161).

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barrier for 3 cannot be determined by magnetic measurements, owing to the very fast magnetic relaxation. This observation of 1 and 2 suggests that the first excited Kramers doublet is very likely to be involved in the Orbach-type relaxation for 1 and 2.

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REFERENCES

(1) (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141− 143. (b) Vincent, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Balestro, F. Electronic read-out of a single nuclear spin using a molecular spin transistor. Nature 2012, 488, 357−360. (c) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Magnetic memory of a single-molecule quantum magnet wired to a gold surface. Nat. Mater. 2009, 8, 194−197. (d) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013, 113, 5110−5148. (e) Liu, J.-L.; Chen, Y.C.; Tong, M.-L. Symmetry strategies for high performance lanthanidebased single-molecule magnets. Chem. Soc. Rev. 2018, 47, 2431−2453. (f) Feng, M.; Tong, M.-L. Single Ion Magnets from 3d to 5f: developments and strategies. Chem. - Eur. J. 2018, 24, 7574−7594. (2) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S. Y.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (3) (a) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (b) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular magnetic hysteresis at 60 K in dysprosocenium. Nature 2017, 548, 439−442. (c) Guo, F.S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield,

CONCLUSIONS

In summary, a family of three POM-based dinuclear dysprosium cores with different bridging ligands has been successfully synthesized and structurally characterized. The magnetic measurement and analysis demonstrate that changes of the bridged ligands impact not only the couplings between the Dy(III) ions and the arrangement of the main magnetic axes but also the SMM behavior. As a result, 1 and 2 show SMM behavior with the thermally activated energy barrier of 98 and 74 cm−1 under zero dc field, respectively, while 3 indicates no SMM behavior. Moreover, 1 exhibits the highest effective energy barrier of reversal of magnetization among POM-based SMMs, and 2 represents the first fluoride-bridged POM-based SMMs. Additionally, the anisotropic barriers of 1 and 2 extracted from magnetic measurements are estimated to the energy gaps from the ground doublet to the first excited doublet of the 6H15/2 term. F

DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02788 Inorg. Chem. XXXX, XXX, XXX−XXX