Metallogrid Single-Molecule Magnet: Solvent-Induced Nuclearity

May 10, 2016 - Structural assembly and reversible transformation between a metallogrid Dy4 SMM (2) and its fragment Dy2 (1) were established in the di...
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Metallogrid Single-Molecule Magnet: Solvent-Induced Nuclearity Transformation and Magnetic Hysteresis at 16 K Wei Huang,† Fu-Xing Shen,† Shu-Qi Wu,‡ Li Liu,† Dayu Wu,*,† Zhe Zheng,† Jun Xu,† Ming Zhang,† Xing-Cai Huang,† Jun Jiang,† Feifei Pan,† Yao Li,† Kun Zhu,† and Osamu Sato‡ †

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Collaborative Innovation Center of Advanced Catalysis & Green Manufacturing, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ‡ Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Structural assembly and reversible transformation between a metallogrid Dy4 SMM (2) and its fragment Dy2 (1) were established in the different solvent media. The zero-field magnetization relaxation was slowed for dysprosium metallogrid (2) with relaxation barrier of Ueff = 61.3 K when compared to Dy2 (1). Both magnetic dilution and application of a moderate magnetic field suppress ground-state quantum tunneling of magnetization and result in an enhanced Ueff of 119.9 and 96.7 K for 2, respectively. Interestingly, the lanthanide metallogrid complex (2) exhibits magnetic hysteresis loop even up to 16 K at a given field sweep rate of 500 Oe/s.

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INTRODUCTION Rare-earth elements are strategic in the modern industry and play a key role in magnetic materials due to their huge magnetic moments and large anisotropy.1 In fact, many breakthroughs in single-molecule magnetism (SMM) have been made in the f elements, pushing the field to high energy barrier and magnetization blockage at higher temperature regimes.2 The key factor of Ln-SMMs behavior is the significant single ion anisotropy inherent in lanthanide ions, such as DyIII or TbIII, arising from the large unquenched orbital angular momentum and strong spin−orbit coupling.3 This has led to the enhanced thermal energy barriers for the reversal of magnetization, Ueff, when compared to d-block SMMs. Ishikawa et al. initially reported a terbium−phthalocyanine double-decker compound, [Pc2Tb]−, with Ueff of 230 cm−1.4 Recently, Chibotaru, Winpenny, and co-workers reported new competing relaxation pathways in YIII-diluted DyIII cluster with impressive energy barriers for relaxation of magnetization that exceed 800 K.5 More importantly, magnetic hysteresis at relatively high temperature (TB) is crucial to high-performance SMMs that scientists in the field of molecular magnetism have struggled to improve. Hence, fast zero-field quantum tunneling of magnetization (QTM) must be conquered to retain their magnetization at more practical temperatures. Langley et al. reported strong exchange coupling strategy in heterometallic 3d−4f SMM complexes, [CrIII2DyIII2(OMe)2(mdea)2(O2CPh)4(NO3)2], and its trivalent lanthanide substitutes.6 The former displayed the magnetic hysteresis up to 3.5 K and with large coercive fields of 2.7 T at 1.8 K.6a Just recently, Tong © XXXX American Chemical Society

and co-workers reported the symmetry strategy to suppress the QTM and a magnetic hysteresis loop up to 20 K was created as new record.7 Long et al. reported the method of strong exchange coupling between lanthanide centers with the aid of radical bridges to obtain the magnetic hysteresis at 6.5 K.8 Indeed, the N23− radical-bridged TbIII complex exhibits hysteresis up to a high magnetic blocking temperature at 14 K.9 Although the organic radical is effective for creating a strong direct exchange with unpaired 4f electrons of the lanthanide ions, the control of higher nuclearity clusters of more than two employing this highly reactive unit is extremely challenging.10 We therefore chose to explore the possibility of instead employing more controllable organic bridging ligands rather than radicals to effectively suppress quantum tunneling pathways in polynuclear bridging SMM system. In the previous work, we utilized the well-established bridge-forming capabilities of pyrazine to construct the redox- and spin-controllable transition metal metallogrid.11 Herein, we decided to pursue the self-assembly of lanthanide complexes of pyrazine-bridge and described the synthesis, structural transformation, and magnetic switch between tetralanthanide metallogrid and its fragment dilanthanide complexes, the first such species in which zero-field tunneling of magnetization can be controlled by nuclearity transformation. Received: February 29, 2016

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DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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EXPERIMENTAL SECTION

Table 1. Data Collection and Structure Refinement Parameters for Complexes 1 and 2

Materials and General Procedures. All the reagents employed were commercially available and used without further purification. Methanol and acetonitrile were dried using standard procedures. 1,1′(Pyrazine-2,5-diyl)diethanone was synthesized according to the reported procedure.12 The IR spectra of polycrystalline solids were performed on a Nicolet Magna-IR 750 spectrophotometer in the 4000−400 cm−1 region (w, weak; b, broad; m, medium; s, strong) by KBr disc. Powder X-ray diffraction (PXRD) was recorded on a RINT2000 vertical goniometer with Cu Kα X-ray source (operated at 40 kV and 100 mA). Elemental analyses (C, H, and N) were conducted with a PerkinElmer 2400 analyzer. Simulations of the powder diffractograms were calculated with CrystalDiffract using crystal structure parameters analyzed by single-crystal X-ray crystallography (Copyright © 2015 CrystalMaker Software Ltd). An accurate yttrium/dysprosium ratio was determined using the inductively coupled plasma (ICP) atomic emission spectra analyzed by a JY/T015−1996 spectrometer. Thermogravimetric (TG) and differential thermal analysis (DTA) curves were recorded on a NETZSCH TG209F3 thermoanalyzer; the samples were filled in alumina crucibles under N2 atmosphere and heated from room temperature to 800 °C at a heating rate of 10 K min−1. Crystal Structure Determination. The diffraction intensity data of complexes 1 and 2 at 120(2) K were collected on a Bruker APEX-2 CCD with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Data collection, data reduction, and cell refinement were performed by using the Bruker Instrument Service v4.2.2 and SAINT V8.34A software.13,14 Structures were solved by direct methods using the SHELXS program, and refinement was performed using SHELXL based on F2 through full-matrix least-squares routine.15 Absorption corrections were applied upon using multiscan program SADABS.16 Hydrogen atoms of organic ligands were generated geometrically by the riding mode, and all the non-hydrogen atoms were refined anisotropically through full-matrix least-squares technique on F2 with the SHELXTL program package.17,18 Note that relatively high R1 and wR2 values for complex 2 may be caused by the heavily disordered solvents, but the diffraction data are sufficient to confirm connectivity. Despite many attempts, the high-quality single-crystal Xray diffraction data were not obtained. A summary of the crystallographic data and refinement parameters is shown in Table 1. Selected bond lengths and bond angles for 1 and 2 are listed in Table S1. Magnetic Measurements. Magnetic susceptibility measurements were obtained using MPMS-XL and VSM Quantum Design SQUID magnetometers, operating between 2.0 and 300 K for direct current applied fields ranging from −5 to 5 T. Microcrystalline samples of 1, 2 and 3 were frozen in eicosane to avoid torquing of the crystallites. Alternating current (ac) susceptibility measurements were performed under an oscillating field of 3 Oe and ac frequencies in the range from 1 to 1500 Hz. The magnetic data were corrected for the sample holder and the intrinsic diamagnetic contributions from Pascal constants.19 Synthetic Procedures. Synthesis of Pyrazine-2, 5-diylbis(ethan1-yl-1-ylidene)di(benzohydrazide) (bzhdep). A mixture ethanolic solution of 1,1′-(pyrazine-2,5-diyl)diethanone (0.8203 g, 5.0 mmol) and benzoylhydrazine (1.3615 g, 10.0 mmol) was refluxed for 3 h under nitrogen atmosphere; after it cooled to room temperature, a pale white solid was obtained by filtration. The crude product was washed with cold ethanol and dried in vacuo. Yield: 90%. IR (KBr pellet cm−1): 635(w), 710(w), 779(w), 924(w), 999(w), 1023(w), 1136(w), 1184(w), 1206(w), 1262(w), 1279(w), 1333(s), 1457(w), 1576(w), 1599(m), 1659(s), 3066(w), 3175(m), 3440(s). Elemental analysis for C22H20O2N6, Calculated: H, 5.03; C, 65.99; N, 20.99. Found: H, 5.11; C, 65.88; N, 21.07%. Preparation of Compound [Dy2(bzhdep-2H) (NO3)4(DMF)4] (1). A solution of Dy(NO3)3·6H2O (45.66 mg, 0.10 mmol) in 3 mL of dimethylformamide (DMF) was added to bzhdep (34.64 mg, 0.10 mmol) in 7 mL of CH2Cl2. The mixture solution was refluxed for 20 min, and then 0.2 mmol triethylamine was added, after which the mixture solution changed from yellow to clear red and was stirred for

comp CCDC T (K) formula FW(g/mol) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc.(g/cm3) F(000) R1a (I > 2σ(I)) wR2a (I > 2σ(I)) R1 (all data) wR2 (all data) GOFb

1 1448787 120(2) C34H48Dy2N14O18 1265.86 monoclinic P21/c 11.8667(14) 11.6837(14) 18.7137(17) 90.00 109.413(6) 90.00 2447.1(5) 2 1.718 1252 0.0334 0.1057 0.0425 0.1164 1.081

2 1448788 120(2) C94H116Dy4N28O36 2864.14 orthorhombic Fddd 11.112(7) 36.81(2) 61.22(4) 90.00 90.00 90.00 25 041(26) 8 1.519 11424 0.0973 0.2635 0.1613 0.3083 1.000

R1 = ∑∥F0| − |Fc∥/∑|F0|; wR2 = [∑[w(F02 − Fc2)2]/∑[w(F02)2]]1/2. Goodness-of-fit = [∑[w(F02 − Fc2)2]/(Nobs − Nparams)]1/2, based on the data I > 2σ(I). a b

another 20 min. The suspension was then filtered, and the filtrate was diffused with diethyl ether in a tiny tube. Dark red block-shaped crystals of 1 suitable for X-ray diffraction analysis were obtained after several days. Yield: 39% based on Dy(NO3)3·6H2O. IR (KBr pellet cm−1): 681 (w), 723 (w), 1033 (m), 1042(w), 1295 (m), 1384 (s), 1432 (w), 1469 (w), 1649 (m), 3424 (s). Elemental analysis for Dy2C34H48N14O18, Calculated: H, 3.82; C, 32.26; N, 15.49. Found: H, 3.75; C, 32.19; N, 15.63%. Preparation of Compound [Dy4(bzhdep-2H)4(H2O)4(NO3)4]· 6CH3OH·6H2O (2). A solution of Dy(NO3)3·6H2O (45.66 mg, 0.10 mmol) and bzhdep (34.64 mg, 0.10 mmol) in MeOH (10 mL) was refluxed for 20 min, to which then 0.2 mmol triethylamine was added; the mixture solution changed from yellow to clear red and was stirred for another 20 min. The suspension was filtered, and the filtrate was diffused with diisopropyl ether in a tiny tube. Dark red block-shaped crystals of 2 suitable for X-ray diffraction analysis were obtained after several days. Yield: 46%. IR (KBr pellet cm−1): 679 (w), 717(w), 999(w), 1032(m), 1143(w), 1297(w), 1384(s), 1432(w), 1472(m), 1521(w), 1588(w), 1654(m), 3441(s). Elemental analysis for Dy4C94H116N28O36, Calculated: H, 4.08; C, 39.42; N, 13.69. Found: H, 4.19; C, 39.32; N, 13.76%. Preparation of Compound [Dy0.35Y3.65(bzhdep2H)4(H2O)4(NO3)4]·6CH3OH·6H2O (Dy4@Y4, 3). The magnetically diluted sample, Dy4@Y4 (3), was obtained by combining Dy(NO3)3· 6H2O and Y(NO3)3·6H2O in a 1:10 molar ratio and following the similar procedure described as that of 2. The resulting doping ratio is 0.35:3.65 per tetranuclear unit for Dy/Y, a non-stoichiometric product. The molar ratio of DyIII in 3 is confirmed by the ICP measurement. Elemental analysis for Dy0.35Y3.65C94H116N28O36, Calculated: H, 4.51; C, 43.50; N, 15.11. Found: H, 4.62; C, 43.43; N, 15.24%. Transformation from Complex 1 to 2 (2′). The methanol solution (8 mL) containing complex 1 (30 mg, 0.024 mmol) was refluxed for 3 h. The suspension was then filtered, and the filtrate was diffused with diisopropyl ether in a tiny tube. Dark red block-shaped crystals of product 2 suitable for X-ray diffraction analysis were obtained after several days. Yield: 42%. Elemental analysis for Dy4C94H116N28O36, Calculated: H, 4.08; C, 39.42; N, 13.69. Found: H, 4.71; C, 39.02; N, B

DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Reversible Transformation between a Binuclear Fragment (1) and a Tetranuclear Metallogrid Dysprosium Complex (2) in the Different Solvents

13.78%. The consistency was further confirmed by FT-IR, PXRD data, and unit cell parameters. Transformation from Complex 2 to 1 (1′). Complex 2 (29 mg, 0.01 mmol) and Dy(NO3)3·6H2O (4.566 mg, 0.01 mmol) were combined in DMF (1 mL) and CH2Cl2 (5 mL), which was then refluxed for 3 h. The suspension was then filtered, and the filtrate was diffused with diethyl ether in a tiny tube. Dark red block-shaped crystals of 1 suitable for X-ray diffraction analysis were obtained after several days. Yield: 46%. Elemental analysis for Dy2C34H48N14O18, Calculated: H, 3.82; C, 32.26; N, 15.49. Found: H, 3.11; C, 33.16; N, 15.67%. The consistency was further confirmed by FT-IR, PXRD data, and unit cell parameters.

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RESULTS AND DISCUSSION Reaction of pyrazine-2,5-diyl-bis(ethan-1-yl-1-ylidene)-di(benzohydrazide) (bzhdep, Scheme 1) with 2 equiv of Dy(NO3)3·6H2O in the presence of Et3N in DMF/CH2Cl2 resulted in the binuclear DyIII2 complex ([Dy2(bzhdep2H)(NO3)4(DMF)4] (1), but the reaction in MeOH gave rise to tetranuclear complex [Dy4(bzhdep2H)4(H2O)4(NO3)4]·6CH3OH·6H2O (2). In addition, the binuclear DyIII2 complex 1 and a tetranuclear DyIII4 complex 2 can be reversibly transformed in the different solvent conditions, which can be confirmed by the single-crystal and PXRD analyses (Figure 1). Until now, despite of the numerous DyIII4 SMMs with a [2 × 2] metallogrid structure, the reversible interconversion of the different nuclearities in response to solvent media is rarely reported (Table 2). Consequently, slow magnetic relaxation characteristic of SMM behavior can be switched between them. The tetranuclear metallogrid DyIII4 (2) is a rare example of polynuclear Dy(III)-based SMMs, which opens hysteresis loops even at 16 K. Single-crystal X-ray diffraction analyses revealed that complex 1 crystallized in the monoclinic space group P21/c and that complex 2 crystallized in the orthorhombic space group Fddd.20 As shown in Figure 2, the crystal structure of 1 consists of two DyIII ions with an inversion center, while the asymmetric unit of 2 comprises only one DyIII cations with S4 symmetry in facecentered orthorhombic lattice. For 1, each DyIII ion is coordinated with one fully deprotonated ligand as a N2O pocket, two chelating NO3−, and two DMF molecules to form a nine-coordination N2O7 irregular geometry described as a muffin (Cs, with a CShM value of 2.038, Table S2, Figure 2c).21 In the structure of 2, four coplanar DyIII ions via self-assembly form a tetranuclear [2 × 2] DyIII4 metallogrid (Figure 2b). Each DyIII ion is nine-coordinated by two sets of N2O pockets, one chelating nitrate, and one water molecule to form a capped

Figure 1. Reversible transformation between 1 and 2. (a) PXRD spectrum of complex 1 and 1′. (b) PXRD spectrum of complex 2 and 2′. Note: complex 1′ and 2′ denote the samples obtained from the transformation of 2 and 1 under the conditions, respectively.

square antiprism (the CShM value of 1.103, C4v, Table S2, Figure 2d).13 Two MeOH and two H2O solvent guests were found to reside in the core of grid. The neighboring DyIII ions are separated by the pyrazine block with the intramolecular Dy···Dy distances of 7.8815(7) Å in 1 and 7.8276(5) Å along the edges of the grid 2, which are longer than the distances in the reported Dy4 SMMs10a−e,g (Table 2) and Dy2 SMM,22 and suggest the magnetic exchange interaction is weak or negligible within the metallogrid. Direct-current magnetic susceptibilities of both 1 and 2 were performed on polycrystalline samples in the temperature range of 2−300 K under 2500 Oe magnetic field (Figure 3). The χMT values at 300 K for 1 and 2 are 28.99 and 54.98 cm3 mol−1 K, which are in agreement with the expected values of 28.34 cm3 C

DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Reported Gridlike Dy4 Single-Molecule Magnets in the Literature molecular formula −

coordination number

ranges of Dy···Dy intramolecular

shortest Dy···Dy intermolecular

Dy···Dy···Dy angle ranges

1

[Dy4(μ4−OH) (Hhpch )8)]

9

3.536−3.573

11.833

88.86−90.91

2

[Dy4(L1−2H)2(L1-H)2(OH)4]2−

8

3.76−3.79

9.586

89.51−90.33

3−

[Dy4(L1−2H)2)(L1-H)2(N3)4(O)]

9

3.66−3.69

9.203

88.28−92.13

3

[(L2−H)3(L2)Dy4(N3)4(O)]3−

9

3.65−3.71

9.594

89.70−90.30

4

[(L3−2H)2(L3-H)2(L3)2Dy4(OH)4] [{(Me3Si)2N}3Dy(μ-Cl)Li(thf)3]

8 6

3.75−3.77 3.915−4.001

13.077 11.577

89.89−90.02 89.62−90.35

5 6 7

[Dy4(HL3−)4(MeOH)4 [Dy4(H2L)4(H2O)8·2DMF]4− [Dy4(OH)2(bpt)4(NO3)4(OAc)2]

8 8 8

3.823−3.877 8.998−9.064 4.183−5.062

9.663 10.655 7.623

85.07−87.35 83.13−96.33 89.43−90.57

8

[Dy4(bzhdep-2H)4(H2O)4(NO3)4]·6CH3OH·6H2O

9

7.828

8.067

89.99

Ueff, cm−1 (K) 30.3K (0 Oe) 92K (1 kOe)

refsa 10a 10b

91K (0 Oe) 270 K (1.6 kOe) 110 K (1.8 kOe) 66.3 K (0 Oe) 70.6 K(2.0 kOe) 16K (0.9 kOe) 24.9K (2 kOe) 134 K, 206 K (0 Oe) 136 K,189 K (1.5 kOe) 61.3 K (0 Oe)

10c

10d

10e 10f 10g

this work

96.7 K(1 kOe) a

Note: 10(a) H2hpch: (2-hydroxybenzylidene)isonicotinohydrazide, 10(b) L1: (1-(pyridin-2-yl)ethylidene)-2-(1-(pyridin-2-yl)ethylidene)hydrazine-1-carbohydrazide, 10(c) L1: (1-(pyridin-2-yl)ethylidene)-2-(1-(pyridin-2-yl)ethylidene)hydrazine-1-carbohydrazide, L3:1-(pyrazin-2yl)ethylidene)-2-(1-(pyrazin-2-yl)ethylidene)hydrazine-1-carbohydrazide, 10(e) H4L: (butane-2,3-diylidene)bis(2-hydroxy-3-methoxybenzohydrazide), 10(f) H4L ((4,6-dihydroxy-5-methyl-1,3-phenylene)bis(methanylylidene)) di(isonicotinohydrazide), 10(g) Hbpt: 3,5-bis(pyridin-2-yl)-1,2,4trizole.

Figure 2. Molecular structure of dinuclear Dy(III) compound 1 (a) and tetranuclear Dy(III) one 2 (b), and the coordination polyhedron around DyIII centers with muffin (Cs) for 1 (c) and a capped square antiprism (C4v) for 2 (d).

mol−1 K for 1 and 56.68 cm3 mol−1 K for 2. When cooled, the χMT values for 1 and 2 gradually decrease to 20.66 and 45.61 cm3 mol−1 K at 2.0 K. This behavior is mainly due to the thermal depopulation of the DyIII Stark sublevels as well as the possibility of anti-ferromagnetic coupling. As shown in Figure 3, fitting the 1/χM versus T plots with Curie−Weiss law in the

range of 300−2 K affords the Curie constant (C = 29.37 cm3 mol−1 K for 1 and 55.07 cm3 mol−1 K for 2) and small Weiss temperature (θ = −3.72 K for 1 and −0.96 K for 2), which is indicative of the weak or negligible anti-ferromagnetic interactions between DyIII ions. The magnetization of both 1 and 2 at 2 K increases to 11.06 and 19.14 μB at 50 kOe (Figure D

DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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tunneling relaxation was effectively suppressed in compound 2. The Cole−Cole plots (Figure 6a) of 2 at temperatures from 2.0 to 21.0 K exhibit a semicircular shape and can be fitted to the generalized Debye model with α parameters in the range of 0.08−0.19 (Table S4, Supporting Information). According to the temperature dependence of relaxation time (black dots in Figure 7b), the relaxation processes could be considered for dominant Orbach process in high temperature region and QTM at the low temperatures. Fitting the data to the Arrhenius law in the high-temperature region gives the energy barrier Ueff = 61.3 K and the relaxation time τ0 = 6.9 × 10−6 s. Both application of a dc magnetic field and magnetic dilution of sample were performed to suppress ground-state quantum tunneling in 2. With application of a 1000 Oe dc field, the lowtemperature increments nearly disappeared, indicating the QTM effect was efficiently suppressed (Figure 4d). The field effect can be further supported by the magnetic hysteresis observations described in the next paragraph. These waistrestricted curves are indicative of faster relaxation near zero field compared with slower relaxation in applied fields, which is commonly attributed to the faster QTM owing to the lack of an external field. As shown in Figure 5c,d, the magnetic dilution and external field move peaks in χM″ versus frequency to lowerfrequency region, indicating the slower relaxation was aroused. Under 1000 Oe dc field, the relaxation times in Cole−Cole plots (Figure 6b) of 2 were extracted by the generalized Debye model, fitting the data to the Arrhenius law in the hightemperature region (above 14 K) yields Ueff = 96.7 K with τ0 = 2.0 × 10−6 s (red dots in Figure 7b). The dilution in cognate Y(III) matrix, Dy4@Y4,24 gave rise to Ueff = 119.9 K with τ0 = 1.5 × 10−6 s, respectively (blue dots in Figure 7b). Despite our efforts, both magnetic dilution and application of dc field could not fully suppress the QTM effects and instead arouse competing Orbach and Raman processes, because the low temperature relaxation times start to gradually diverge from the Arrhenius law.25 In fact, they tend to obey the power law as τ ≈ T−n within the intermediate temperature region of 6−14 K with the best fits of n = 5.7 under a 1000 Oe dc field and n = 3.8 for magnetically diluted sample under a zero dc field, respectively (dashed lines in Figure 7b). It is recognized that n = 9 is expected for the two-phonon Raman process for Kramers ions such as DyIII; however, the smaller n values in the range of 1−6 can be considered as acceptable for an optical acoustic Raman process.26 Variable-field magnetization measurements were performed with polycrystalline samples to reveal magnetic hysteresis at the potential high temperature, which is practically crucial to information storage and magnetic memory.27,28 When the dc magnetization is measured at 2 K ± 0.002 within ±50 kOe, no obvious hysteresis loop is detected for 1 due to the very fast relaxation times indicated in the ac experiments (Figure S13, Supporting Information). However, a remanence of 16 μB and a coercive field of 2540 Oe were observed at 2.0 K ± 0.002 at sweep rates of 0.05 T/s for compound 2. As shown in Figure 8a, the sweep-rate dependence of hysteresis loops at 2 K confirms the presence of QTM effect, and magnetic hysteresis loops of 2 were still visible at temperatures up to 16 K ± 0.002 (Figure 8b). It is well-known that QTM is typically observed in lanthanide-containing single-ion magnets, resulting usually in very small coercive fields. However, in complex 2, the increasing coercivities with decreasing temperature suggest undermined quantum tunneling under low-lying states as they progressively depopulate. In fact, there are only limited

Figure 3. Temperature dependence of χMT product and 1/χM vs T plots with Curie−Weiss fitting as the solid line at 2500 Oe for 1 (a) and 2 (b).

S7, Supporting Information), and the values are clearly lower than the expected saturation value, 20 and 40 μB, suggesting the presence of magnetic anisotropy and/or low-lying excited states. Alternating-current susceptibility measurements were performed to investigate the dynamic magnetic behavior. Under a zero dc field, no peaks in either the frequency- or temperaturedependent out-of-phase ac susceptibility (χM″) were detected for 1 within the available frequencies of superconducting quantum interference device (SQUID) apparatus (Figures 4a and S8, Supporting Information), which is due to very fast quantum tunnelling of magnetization (QTM). However, upon applying a 1000 Oe dc field for 1 to suppress the QTM effect, both in-phase (χM′) and out-of-phase (χM″) show obvious temperature and frequency dependence. (Figures 4b and 5a) The Cole−Cole plots of 1 can be extracted from the frequency dependent χM′ and χM″ in the temperature range of 3.0−8.0 K (Figure S9, Supporting Information) and can be fitted to a generalized Debye model with α parameters in the range of 0.006−0.099 (Table S3, Supporting Information), indicative of a narrow distribution of relaxation times.23 From the resulting temperature dependence of the relaxation time, the relaxation process follows a thermally activated relaxation process (Orbach). Fitting the data to the Arrhenius law (τ = τ0 exp(Ueff/kBT)) in the whole temperature range affords the energy barrier Ueff = 29.5 K and the relaxation time τ0 = 2.6 × 10−6 s (Figure 7a). The dynamic magnetic behavior varied with the lanthanide nuclearities from 1 (binuclear DyIII2) to 2 (tetranuclear DyIII4). Under zero dc field, both in-phase (χM′) and out-of-phase (χM″) susceptibilities of 2 show obvious temperature and frequency dependence in the temperature range from 2 to 21 K (Figure 4c, Figure 5b), indicating the zero-field quantum E

DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Temperature-dependent in-phase (top) and out-of-phase (bottom) ac susceptibility data for 1 measured under zero dc field (a) and 1000 Oe dc field (b) and for 2 measured under zero dc field (c) and 1000 Oe dc field (d) in the range of 1−1488 Hz (Hac = 3 Oe). The solid lines are guided for eyes.

molecules shown to have a higher maximum temperature for magnetic hysteresis when measured under similar field sweep rates, such as Dy4(OH)2(bmh)2(msh)4Cl2 at 7 K,29 TbiPc2 at 7 K,30 [(Cp*2Dy)2(μ-bpym·)]+ at 6.5 K,8 and (Cp*)Er(COT) at 5 K.31 Interestingly, Long’s group developed the high hysteresis temperature of 14 K in a dinuclear lanthanide compound {[(Me3Si)2N]2(THF)TbIII}2(μ-η2:η2-N2) (THF = tetrahydrofuran).8 Tong, Ungur, and co-workers reported the new record of 20 K in the pentagonal bipyramidal compound [Dy(Cy3PO)2(H2O)5]Br3·2(Cy3PO)·2H2O·2EtOH, (Cy3PO = tricyclohexyl phosphine oxide).7 However, the magnetic hysteresis at blocking temperature up to 16 K ± 0.002 for the metallogrid without the aid of radical is also noteworthy, especially among the reported DyIII4 SMMs.10 On measuring the diluted sample Dy4@Y4, (3), under the same conditions, the hysteresis loops were recorded as well to show little differences (Figure S14, Supporting Information), indicating the single-ion anisotropy rather than the long-range ordering or dipolar interactions responsible for the hysteresis loops.32−34

Hence, tetranuclear Dy(III) grid complex displayed rare instances of open hysteresis loops for lanthanide-containing SMMs at relatively high temperature; however, this is not observed for dinuclear Dy(III) compound despite the identical bridging mode. Clearly, there is a significant change in the dynamic relaxation behavior between them. The reasons for the difference of magnetic hysteresis between 1 and 2 are important questions to be answered. Very weak spin−spin interactions and the different coordination sphere maybe contributed to the difference. However, to renew our knowledge of metallogrid magnetic systems and to quantitatively explain the experimental magnetic behavior, a full theoretical analysis utilizing ab initio and density functional theory calculations should be undertaken in the future. Our aim is to determine the single-ion anisotropies, the nature and magnitude of magnetic coupling and to understand the mechanism of slow magnetic relaxation in these systems. This, however, is not a trivial task, and results will be reported in due course. It was semiquantitatively reasoned that the differences in the magnetic relaxation F

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Figure 5. Frequency dependence of out-of-phase (χM″) ac susceptibilities for 1 (a), 2 (b), 2 (c) under 1000 Oe dc field, and diluted samples, 3 (d) under zero dc field. The solid lines are guides.

Figure 6. Cole−Cole plots of 2 measured under zero dc field (a) and under 1000 Oe dc field (b), respectively. The solid lines are the best fitting according to the generalized Debye model. Figure 7. Plots of τ vs 1/T for 1 (a) under 1000 Oe dc field and for 2 (b) under the conditions including zero dc field (black dots), 1000 Oe dc field (red dots), and dilution (blue dots). The solid lines represent the fitting by the Arrhenius law for all the data of 1 and for the high temperature region data of 2. The dashed lines are the best fits to the power laws in 6−14 K region.

observed between 1 and 2 were a main consequence of contributions from the ligand field effect. In the DyIII case, the ligand environment was shown to stabilize 6H15/2 ground state due to the pseudo-axial potential where the size of the Ueff barrier could be influenced by the strength of the competing equatorial field. The displacement of nitrate and DMF in 1 by nitrogen donors in 2 would lead to the stronger ligand field in 1, which is evidenced by the shorter Ln−O in 1 than Ln−N G

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00500. The IR spectra characterization, TG and DTA data analysis, additional magnetic and crystallographic data. (PDF) X-ray crystallographic information for 1−2. (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the Priority Academic Program Development of Jiangsu Higher Education Institutions. This experimental work is financially supported by the National Natural Science Foundation of China (Grant Nos. 21471023 and 21371010) and sponsored by Jiangsu Provincial QingLan Project. The magnetic hysteresis was undertaken on a conventional SQUID VSM apparatus in SKLCC of Nanjing University, China.

Figure 8. (a) Sweep rate dependence of hysteresis loops of reduced magnetization vs μ0H at 2 K ± 0.002. (b) Hysteresis loops at different temperatures for polycrystalline sample 2 collected at a sweep rate of 0.05 T/s. The red line represents the hysteresis loop measured on the diluted sample Dy4@Y4 (3) at 2 K ± 0.002. (inset) Expanded views of magnetic hysteresis to 16 K ± 0.002 for 2.

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REFERENCES

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bond in 2 in the crystallographic analysis (Table S1, Supporting Information). As a result, the increased electron density of equatorial field would result in a less axial anisotropic ground state for 1; therefore, a higher barrier of reorientation of the magnetization was obtained in 2. However, DyIII ion in 1 has different electron density distributions for the high angular momentum states of prolate−spheroidal character, which are not likely to be stabilized in this pseudoaxial environment. The absence of out-of-phase ac χM″ peaks for 1 in zero field is assumed to originate from fast QTM relaxation in the ground exchange states due to the lack of single-ion bistability. Hence, the degree of oblate character of the high angular momentum states for DyIII in 2 is greater than that in 1; this is presumably at the root of the poorer hysteresis for compound 1.

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CONCLUSIONS In summary, we described the synthesis and reversible transformation of two polymetallic dysprosium SMMs, that is, a metallogrid compound and its binuclear fragment in the different solvent media. The magnetic studies revealed that magnetization relaxation was effectively slowed, and energy barrier was enhanced by switching the dinuclear dysprosium complex to the [2 × 2] grid-like dysprosium complex. Remarkably, the dysprosium metallogrid compound displayed high-temperature magnetic hysteresis loops at temperature higher than 16 K. The system provides an alternative approach for the design of high-performance SMMs and opens the possibility of blocking the zero-field tunneling of magnetization without the magnetic coupler in bridging polynuclear systems. Future efforts will be focused on the development of the radical-bridged metallogrid SMM to enhance the blocking temperature in this system. H

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DOI: 10.1021/acs.inorgchem.6b00500 Inorg. Chem. XXXX, XXX, XXX−XXX