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Electrostatic Potential Determined Magnetic Dynamics Observed in Two Mononuclear β‑Diketone Dysprosium(III) Single-Molecule Magnets Pei-Pei Cen,† Sheng Zhang,†,∥ Xiang-Yu Liu,*,‡ Wei-Ming Song,‡ Yi-Quan Zhang,*,§ Gang Xie,† and San-Ping Chen*,† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, China ‡ College of Chemistry and Chemical Engineering, State Key Laboratory Cultivation Base of Natural Gas Conversion, Ningxia University, Yinchuan 750021, China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, China ∥ College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China S Supporting Information *

ABSTRACT: Two β-diketone mononuclear Dy(III) compounds, formulated as Dy(BTFA)3(H2O)2 (1) and Dy(BTFA)3(bpy) (2) (BTFA = 3-benzoyl-1,1,1-trifluoroacetone, bpy = 2,2′-bipyridine), were prepared. Compound 1 can be identified to transform to 2 in the attendance of bpy coligand, when the local geometry symmetry of eight-coordinated Dy(III) ion changes from a dodecahedron (D2d) in 1 to a square antiprism (D4d) in 2. Fine-tuning structure aroused by auxiliary ligand has dramatical impact on magnetic properties of compounds 1 and 2. Magnetic investigations demonstrate that both 1 and 2 display dynamic magnetic relaxation of single-molecule magnets (SMMs) behavior with different effective barriers (ΔE/kB) of 93.09 K for 1 under zero direct-current (DC) field as well as 296.50 K for 1 and 151.01 K for 2 under 1200 Oe DC field, respectively. As noticed, compound 1 possesses higher effective barrier than 2, despite 1 exhibiting a lower geometrical symmetry of the Dy(III) ion. Ab initio studies reveal that the Kramers doublet ground state is predominantly axial with the gz tensors of two compounds matching the Ising-limit factor of 20 anticipated for the pure MJ = ±15/2 state. Electrostatic analysis confirms the uniaxial anisotropy directions, highlighting that the proper electrostatic distribution of the coordination sphere around Ln(III) center is the critical factor to improve the magnetic anisotropy and determine the dynamic behaviors of SMMs.

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pared,5 which is resulted from the unquenched angular moment with a 6H15/2 ground state and an unparalleled uniaxial singleion anisotropy of Dy(III) atom. Thereinto, the exploitation of monometallic Dy(III)-containing molecular magnets or socalled single-ion magnets (SIMs), is especially focused, by reason for these substances possessing explicitly slow magnetic relaxation and ascertaining the synergistic effect between uniaxial magnetic anisotropy and steric configurations in enhancing relaxation barrier.6 Moreover, the relatively simple geometry of mononuclear Dy(III)-containing SIMs is conducive to enhancing the cognition of the magneto-structural correlation.7 The investigations on existing mononuclear Dy(III)-based SMMs verify that the anisotropic magnitude of

INTRODUCTION

Since the single-molecule magnet (SMM) was first discovered in 1993,1 such magnetic matter featuring slow relaxation behavior has been a burgeoning point and explored unremittingly by physical, chemical, and materials scholars.2 Significant easy-axis anisotropy and spin ground state are recognized as two chief components to attain the SMMs with prominent blocking temperature and anisotropic barrier, which acts as key roles in the potential applications of SMMs, including quantum computers, molecular spintronics apparatus, and high-density data memories.3 With plentiful SMMs being designed and prepared, lanthanide ions are encouraged to serve as the highly promising option for SMMs assembly because of their inherent strong spin−orbital coupling effect and significant magnetic anisotropies.4 As reported, mononuclear, multinuclear, and chainlike Dy(III)-containing SMMs have been largely pre© 2017 American Chemical Society

Received: January 12, 2017 Published: March 8, 2017 3644

DOI: 10.1021/acs.inorgchem.7b00057 Inorg. Chem. 2017, 56, 3644−3656

Article

Inorganic Chemistry Dy(III) centers is greatly sensitive to tiny modifications of local geometrical symmetry and ligand field.8 As known, a ligand field that is executed along the axis, such as in low-symmetric SIMs and organometallic, is becoming primary focus on the mononuclear Dy(III)-based SMMs.9 Therefore, selecting suitable ligands is one of the important factors in forming the desirable ligand fields and improving the anisotropy barriers for mononuclear Dy(III)-based SMMs. As an ideal candidate, β-diketone and its derivatives have been widely adopted to trigger the single-ion magnetic anisotropy in mononuclear Dy(III) compounds10 resulting from their intrinsic characteristics of stable bidentate modes chelating to lanthanide ions and offering proper ligand fields.11 As referenced, increasingly Dy(III)-based SMMs or SIMs with eight-coordinated geometry, β-diketone linkers of the class of [Dy(β-diketone)3(L)n] were obtained with the introduction of the capped coligands, for instance, 2,2′-bipyridine, 1,10phenanthroline, and the analogues.12 By and large, the Dy(III) centers with a local symmetry of D4d in the resulting SMMs contribute to relatively larger energy barriers than those lying in a local symmetry of D2d, although a few exceptions have been observed.13 Nevertheless, the integrated and scientific theoretic model of the magnetic relaxation mechanisms in mononuclear Dy(III) SMMs is still ambiguous.14 It remains a significant challenge to design and configure the desired system for clarifying the complicated and crucial issues of Dy(III)-based SMMs, such as the source of slow magnetic relaxation, magnetic dynamics, and the main reasons tuning the anisotropic nature. In recent years, the charge density design around the paramagnetic center to be reached might play an important and constructive role to perturb the uniaxial anisotropy, which is deeply stated and emphasized in recent report.15 In light of this guidance, subtle change of the ligand field would be a promising prospect to modulate the electrostatic distribution around Dy(III) cation, further to regulate and control the magnetic anisotropy and dynamic magnetization processes of the Dy(III)-containing SMMs. In view of the significant development in Dy(III)-based SIMs, a kind of β-diketone, 3-benzoyl-1,1,1-trifluoroacetone (BTFA) ligand, and auxiliary 2,2′-bipyridine (bpy) ligands were employed to further expand the Dy(III)-based SMMs and probe into the correlations between the structure and magnetic property of β-diketone-Dy(III) compounds (Scheme 1).

Scheme 2. Syntheses of Compounds 1 and 2

behaviors. The uniaxial magnetic anisotropies and magnetostructural correlations, as well as relaxation mechanism, were investigated by ab initio calculations and electrostatic analyses.

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EXPERIMENTAL SECTION

All reagents and starting solvents achieved from commercial channels were of analytical grade. Physical Measurements. Elemental analysis (carbon, hydrogen, and nitrogen) was implemented on a PerkinElmer 2400 CHN analyzer. IR spectra were performed on an EQUINOX55 FT-IR spectro-photometer by using KBr pellets, in the range of 400−4000 cm−1. Powder X-ray diffraction (PXRD) measurements were operated using Cu Kα radiation (λ = 1.5406 Å) on a Rigaku RU200 diffractometer with a step size of 0.02° in 2θ and a scan speed of 5° min−1. Magnetic experiments were accomplished using a Quantum Design MPMS-XL7 SQUID magnetometer on polycrystalline samples for two compounds (restrained in eicosane to prevent torquing at high fields). Diamagnetic corrections were evaluated from Pascal’s Tables. Dy(BTFA)3(H2O)2 (1). A mixture of Et3N (0.014 mL, 0.1 mmol) and BTFA (0.065 g, 0.3 mmol) in methanol (20 mL) was kept stirring for an hour, to which DyCl3·6H2O (0.113 g, 0.3 mmol) was added. The mixed solution above was stirred for 3 h. After filtration, the resultant solution was kept at ambient temperature (Scheme 2). Colorless crystals appeared within two weeks (yield 69%, based on Dy3+). Elemental analysis: (%) calcd for C30H22DyF9O8 (843.98): C, 42.66; H, 2.61. Found: C, 42.62; H, 2.58. Main IR (KBr): 3427 (w), 1614 (s), 1575 (s), 1309 (s), 1291 (s), 1187 (s), 1143 (s), 774 (m), 581 cm−1 (w). Dy(BTFA)3(bpy) (2). Compound 2 was synthesized from the following two ways (Scheme 2): (i) A mixture of Et3N (0.014 mL, 0.1 mmol) and BTFA (0.065 g, 0.3 mmol) in methanol (20 mL) was kept stirring for an hour, to which DyCl3·6H2O (0.113 g, 0.3 mmol) and bpy (0.031 g, 0.2 mmol) were introduced. The mixed system was stirred in a semiclosed beaker for 4 h at normal temperature. Colorless crystals were collected within two weeks (yield 68.2%, based on Dy3+). Elemental analysis: (%) calcd for C40H26DyF9N2O6 (964.13): C, 49.79; N, 2.90; H, 2.70%. Found: C, 49.76; N, 2.87; H, 2.66%. Main IR (KBr): 3087 (w), 2110 (s), 1629 (s), 1527 (s), 1334 (s), 1276 (s), 1242 (m), 1167 (w), 1142 (m), 789 (w), 712 cm−1 (w). (ii) Compound 1 (0.084 g, 0.1 mmol) was added into the C2H5OH (20 mL) of 2,2′-bipyridine (0.031 g, 0.2 mmol). After it was stirred for 5 h, the resultant solution was filtrated at ambient temperature. Slow evaporation of the filtrate also generated the crystals of compound 2. X-ray Crystallography. The X-ray experiments were implemented on a Bruker SMART APEX-CCD-based diffractometer (Mo Kα radiation, λ = 0.710 73 Å) at low temperature. Raw area detector data integration and reduction were performed with SAINT+ program. Absorption correction based on multiscan was processed using the SADABS software.16a All structures were solved by the direct methods and refined by a full-matrix least-squares method on F2 using the SHELXL-97 software.16b Non-hydrogen atoms were refined with anisotropic thermal parameters. Crystallographic data and refinement parameters are listed in Table 1, while selected interatomic distances and angles for compounds 1 and 2 are given in Tables S1 and S2.

Scheme 1. Graphical Representation of BTFA and bpy Ligands

Consequently, two monometallic Dy(III) compounds were prepared and isolated. Compound 1 undergoes a spontaneous self-assembly process to yield 2 that the coordinated water molecules are replaced by bpy ligand (Scheme 2). Single-crystal structure analysis demonstrates that Dy(III) ion in compound 1 is provided with a dodecahedron geometry (D2d), while the central ion in 2 is surrounded by square antiprismatic configuration (D4d). Two compounds perform discriminatively slow relaxation of magnetization and dynamic magnetic 3645


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Kroll−Hess Hamiltonian, in which scalar relativistic contractions were adopted in the basis sets. The spin−orbit coupling was treated individually in the RASSI process. In the CASSCF calculations, the active electrons in seven active spaces contain all f electrons CAS (9 in 7) for two compounds. All the roots were computed in the active space to eliminate possible doubts. The maximal value of spin-free state have been mixed, which might be related with the computer hardware (all from 21 sextets, 128 from 224 quadruplets and 130 from 490 doublets for Dy(III) nodes).

Table 1. Crystallographic Data and Refinement Parameters for 1 and 2 formula Mr cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z calculated density De, mg·m−3 absorption coeff μ, mm−1 Rint Θ range, deg total/unique parameters refined R1 [I > 2σ(I)] wR2 (all data) temp (K)

1

2

C30H22DyF9O8 843.98 orthorhombic P2(1) 10.7364(13) 13.0536(15) 22.849(3) 90 90 90 3202.3(7) 4 1.751 2.434 0.0696 2.46 to 25.00 16358/5627 433 0.0394 0.0725 100(2)

C40H26DyF9N2O6 964.13 monoclinic P2(1)/n 11.102(7) 22.737(13) 15.916(9) 90 103.020(12) 90 3914(4) 4 1.636 2.000 0.1347 2.53 to 25.00 19471/6869 523 0.0578 0.1034 296(2)

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RESULTS AND DISCUSSION Description of the Structures. X-ray determination suggests that both compounds are mononuclear. Compound 1 crystallizes in the orthorhombic P21 space group, in which the eight-coordinated Dy(III) core is surrounded by six O atoms from three BTFA ligands and two water oxygen atoms (Figure 1a). The average length between Dy(III) ion and BTFA oxygen atoms is 2.344 Å, which is less than the distances between Dy(III) center and two water oxygen atoms (2.385 and 2.421 Å). The neutral [Dy(BTFA)3(H2O)2] units are integrated by hydrogen bonds, leading to the closest intermolecular Dy···Dy separations being 6.047 Å (Figure S1a). Compound 2 crystallizes in the monoclinic space group P21/n, being compared with 1, two H2O molecules are substituted by bpy ligand. Accordingly, the Dy(III) center is configured with two bpy nitrogen atoms and six BTFA oxygen atoms (Figure 1c). The average Dy−O bond length (2.310 Å) in compound 2 is shorter than that in compound 1, whereas the lengths between Dy(III) ion and two N atoms (2.520 and 2.542 Å) are distinctly greater than the bond distances composed of Dy(III) node and coordinated water O atoms in 1. The smallest intermolecular Dy···Dy separations in 2 is 8.382 Å. Evidently, the steric hindrance of H2O in 1 is far less than the capping bpy ligand in 2, leading to the shorter intermolecular distance in 1 compared

Computational Details. Theoretical studies using CASSCF calculations on the Dy(III) nodes (see Figure 1 for the structural model of compounds 1 and 2) in two compounds based on the singlecrystal diffraction measured geometries were performed by MOLCAS 8.0 software package.17 For the calculations, the basis sets are atomic natural orbitals from the MOLCAS ANO-RCC library: ANO-RCCVTZP for Dy(III) cation; VTZ for close N atom and O atom; VDZ for distant atoms. The calculations used the second-order Douglas−

Figure 1. Coordination environment of the Dy centers in compounds 1 (a) and 2 (c) and polyhedrons around the Dy(III) ions for 1 (b) and 2 (d). H atoms are omitted for clarity. 3646


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Figure 2. χMT vs T plots for 1 (a) and 2 (b). Solid lines represent the simulation from ab initio calculation. (inset) Plots of M vs H/T at different temperatures for 1 (left) and 2 (right).

Figure 3. Temperature dependence of χ′ and χ″ susceptibilities for 1 without static field.

lower than the expected value of 14.17 cm3 K mol−1 for a single Dy(III) ion (6H15/2, g = 4/3). Such a situation might be resulted from the point that the states of crystal field splitting within the ground Russell−Saunders multiplet are not evenly filled even at normal temperature, since χmT value keeps increasing upon heating near normal temperature.19 When cooled, the χmT curves for compounds 1 and 2 reduce tardily in the range from 300 to 100 K. Subsequently, the χmT products decrease sharply below 100 K to the minimums of 11.86 cm3 K mol−1 for 1 and 11.75 cm3 K mol−1 for 2 at 1.8 K, respectively. These behaviors could be ascribed to crystal field splits, particularly the progressive quenching of excited Dy(III) Stark sublevels and/or weak intermolecular dipole−dipole effects.20 The M versus H curves were determined from zero DC field to 5 T at 2, 3, 5 K for 1 and at 1.8, 3, 5 K for 2, respectively (Figure S3). The two compounds exhibit analogous tendency and obtain the values of 4.82 Nβ for 1 and 6.07 Nβ for 2 at the base temperature. The maximum values at 5 T largely deviate from the expected saturation point of 10 Nβ, consisting with the magnetic anisotropism and crystal field effects at the dysprosium center that dispel the 16-fold degeneration of the 6 H15/2 ground state.21 Moreover, the divergence of the M versus H/T plots for two compounds at different temperatures implies the occurrence of dominant single-ion anisotropism and/or low-lying excitation states (Figure 2, inset).22 To explore the magnetization dynamics of the anisotropic magnetic moments, alternating-current (AC) magnetic suscept-

with that in 2. Notably, there are only hydrogen bonds in 1, while π−π interactions between bpy rings and hydrogen bonds coexist in 2, yielding a three-dimensional supramolecular structure (Figure S1b). The different types of weak interactions between the neutral molecules would produce different dipole− dipole interactions, finally influencing the magnetic behaviors. The geometrical configuration of Dy(III) center in two compounds was investigated with the SHAPE 2.1 software18 based on the structural parameters; the typical geometric polyhedrons are depicted in Figure 1. In principle, the data extracted from the package tend to zero, responding to the optimal geometry, whereas a greater value presents a major deviation from the optimal polyhedron. For 1, the result of the triangular dodecahedron architecture (TDD-8, D2d, 0.149) is smaller than that for the square antiprism (SAPR-8, D4d, 2.869) and snub diphenoid J84 (JSD-8, D2d, 2.942); therefore, the coordinated geometry for compound 1 is the TDD-8 structure (Table S3). Like 1, the geometric configuration around Dy(III) ion in compound 2 is classified as eightcoordinated square antiprism structure (SAPR-8, D4d, 0.580; Table S3). Magnetic Studies. The magnetic experiments of 1 and 2 were performed on polycrystalline samples. PXRD results of 1 and 2 support the pure state of the bulk materials (Figure S2 in the Supporting Information). Under a direct current (DC) field of 1 kOe, the χmT products at 300 K are 13.87 cm3 K mol−1 for 1 and 13.92 cm3 K mol−1 for 2 (Figure 2), which are mildly 3647


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Figure 4. Frequency dependence of χ′ and χ″ susceptibilities for 1 without static field.

Figure 5. Cole−Cole plots at HDC = 0 for 1 (a) and 2 (b). The solid lines represent the best fit to the measured results.

Figure 6. Frequency dependence of χ′ and χ″ susceptibilities for 2 without static field.

excavated in Ln(III)-based SMMs or SIMs.23 Moreover, out-ofphase peaks in 1 appear in all the applied frequencies except for 1 Hz, illustrating that a higher energy barrier would be expected. Furthermore, the frequency-dependent AC data for compound 1 were characterized in the absence of a DC field at various temperatures; the peaks of the χ″ plots gradually transfers with the frequency sequence from low to high, explicating that the χ″ of compound 1 always manifests frequency dependence in selected temperature range (Figure 4). The relaxation time τ value was obtained depending on the frequency-dependent AC susceptibility and obtained by the χ″

ibility experiments for two title compounds were performed under 0 DC field. For 1, the χ″ versus T plots are measured from 25 to 1.8 K at different frequencies; both the real (χ′) and imaginary (χ″) parts present significant frequency and temperature dependence. As shown in Figure 3, the χ′ and χ″ plots for 1 reveal significantly temperature-dependent maximums at comparatively high-temperature region. The character definitely suggests the slow relaxation of magnetization. When cooled, χ′ and χ″ raise again at lower temperatures; such a situation could be resulted from the emergence of quantum tunneling of magnetization (QTM) without extra DC field, which is often 3648


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Figure 7. Frequency dependence of χ′ and χ″ susceptibilities for 1 at applied DC fields of 1200 Oe.

Figure 8. Frequency dependence of χ′ and χ″ susceptibilities for 2 at applied DC fields of 1200 Oe.

maximum values following the Arrhenius fit [τ = τ0 exp(ΔE/ kBT)] (ΔE = energy gap; τ0 = pre-exponential factor; τ = relaxation time; Figure S4). The effective barrier (ΔE/kB) and the τ0 factor are extracted to be 93.09 K and 1.10 × 10−6 s for 1. The resulting barrier value and the relaxation time are approximate with those for known Dy(III) devices.24 It is noteworthy that ln(τ) value of 1 becomes slightly dependent on 1/T with the cooling. The characteristic above represents an alternation from an Orbach process, which is dominant at high temperature, to a Raman process.25 On the basis of the frequency-dependent AC data, the Cole−Cole plots could be analyzed to the generalized Debye model (Figure 5a and Table S4).26 The fitted αCole parameters (0.41 at 3 K and 0.03 at 18.5 K for 1) demonstrate that multifold relaxation processes or a wide distribution of relaxation times might be considered; such a feature has been expounded that the potential QTM procedure is more sensitive to the geometrical symmetry or disorder than the Orbach relaxation mechanism. Also, AC magnetic susceptibility studies under 0 Oe DC field for compound 2 were implemented at 1.8−15 K and the frequencies of 10, 100, and 1000 Hz. Unfortunately, no maxima in out-of-phase emerges in the available temperature window (Figure S5). χ′ and χ″ values increase along with reducing the temperature in 2, signifying the commencer of pure QTM effects, as generally observed in reported Ln(III) SIMs.23,24 However, for the frequency dependence of AC susceptibility (Figure. 6), the real (χ′) and imaginary (χ″) parts of 2 behave as frequency dependencies at high-frequency zones with the proceeding of warming, which illuminates the occurrence of slow relaxation process in 2 as well. There is weak dependence

at low temperature, predicting a definite cross operation from the temperature dependence related to thermally activated Orbach mechanism to the temperature independence associated with a quantum tunneling process upon decreasing the temperature. The magnetic effective barrier and the preexponential factor of 2 could not be extracted from the Arrhenius relation due to the absence of the maximal values in the χ″ part at high temperature range. The data plotted as Cole−Cole curves exhibit asymmetrical semicircles based on the frequency-dependent AC susceptibility data (Figure 5b), revealing the slow relaxation of magnetization triggered by the uniaxial magnetic anisotropy of single Dy(III) ion. It could be simulated from the generalized Debye model, with αCole factors below 0.05 in the range of 2.0−11 K (Table S5), expressing an extremely narrow distribution of relaxation time.27 Both compounds present typical of slow relaxation of the magnetization depending on SIM behavior in the absence of DC field. Such a situation is inconsistent with that for the obtained analogs [Dy(hafc) 3 (H 2 O) 2 ] 2 8 a and [Dy(hafc)3(bpy)],28b as well as [Dy(TFI)3(H2O)2]29 and [Dy(TFI)3(bpy)],29 where the magnetism switches from frequency independence to frequency dependence while coordinated H2O fragments are substituted by bpy ligands. To restrain the quantum tunneling, the AC magnetic data measurements for two compounds were further performed under an extra 1200 Oe DC field. The thermal dependence of AC susceptibility plots of 1 and 2 were employed at the same frequencies of 1, 10, 33, 100, 333, and 1000 Hz (Figures S6 and S7). Both χ′ and χ″ susceptibilities in two compounds show significantly thermal-dependent maximums at relatively low3649


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Inorganic Chemistry

Figure 9. Plots of ln τ vs T−1 for compounds 1 (a) and 2 (b) under 1200 Oe static field, respectively. The red lines represent the Arrhenius fit. (inset) Plots of ln τ vs T−1 are fitted for multiple relaxation processes at an applied DC field of 1200 Oe (green lines).

Figure 10. Cole−Cole plots under 1200 Oe for 1 (a) and 2 (b). The solid lines represent the best fit to the measured results.

αCole parameters are below 0.18 at the temperature region of 6− 18 K, responding that the thermally sensitized dynamic behavior possesses a simplex relaxation procedure and that the QTM is effectively suppressed in the presence of 1200 Oe static field. For 2, Cole−Cole curves increase prominently in low-temperature region and exhibit a correspondingly wide semicircle in high-temperature region, αCole parameters below 0.30 point out that a mainly simplex relaxation procedure with a nonwide distribution of τ factor may be involved in the current relaxation mode under the extra DC field. Furthermore, a combination of multiple relaxation mechanisms, that is, Raman and Orbach mechanisms, is taken into account so as to clarify the ln(τ) vs 1/T behaviors of two compounds by using eq 1.31 The best fits are obtained giving B = 1.95 × 10−4, n = 5.70, τ0 = 6.27 × 10−12 s, ΔE/kB = 296.50 K for 1, as well as B = 3.01 × 10−3, n = 5.72, τ0 = 3.14 × 10−10 s, ΔE/kB = 151.01 K for 2 (Figure 9 inset). In both compounds, the two processes contribute cooperatively to the overall relaxation rates, and the Orbach mechanism prevails in the high-temperature region, whereas the Raman process dominates in the low-temperature range.

temperature region, which explicitly declares the slow magnetic relaxation, and the probable relaxation behavior through the QTM process is substantially depressed under a static field of 1200 Oe. Meanwhile, the frequency-dependent AC magnetic data experiments were characterized ranging from 3 to 20 K for 1 and from 2 to 11 K for 2 (Figures 7 and 8). Both χ′ and χ″ components of two compounds occur as frequency dependencies. Obviously, the maximums of the χ″ curves in both compounds slowly move from lower frequency to higher frequency with increasing temperature. Also, the magnetization relaxation times (τ) in the form of ln(τ) are depicted to be a function of 1/T in Figure 9. The effective barriers (ΔE/kB) yield by fitting the behavior from Arrhenius law on account of high-temperature curves. The resulting ΔE/kB values and the τ0 factors are 169.33 K and 8.79 × 10−9 s for 1 and 90.48 K and 4.45 × 10−8 s for 2, which are comparable to the typical τ0 factors from 1 × 10−6 to 1 × 10−11 for SMMs.30 Note that the energy barrier height for 1 is predominantly larger than that for 2, and the energy barrier of 1 under a static DC field is in excess of the front value extracted from the plots at zero DC field, further illumining that the potential quantum tunneling process is efficiently subdued. To probe the dynamic magnetic patterns, Cole−Cole curves of compounds 1 and 2 were provided in the presence of 1200 Oe static field. Consequently, two individual semicircular shapes are evidently noticed in the motifs for 1 (Figure 10a) and 2 (Figure 10b), which could be well-fitted by the generalized Debye model (Tables S6 and S7).26 For 1, the

τ −1 = BT n + τ0−1 exp( −ΔE /T )

(1)

The dynamic behaviors of the magnetization reveal distinct differences in present two compounds, demonstrating the influence of the ligand field on variant relaxation mechanisms. 3650


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Figure 11. M vs H data of compounds 1 (a) and 2 (b) at 1.8 K highlighting the hysteresis.

for 2 at a zero DC field. Herein, diverse dipole−dipole interactions and exchange bias also have an influence on the magnetization dynamics of 1 and 2. Theoretical Analysis. Ab initio methods have been demonstrated to be calculable in confirming the direction of the single-ion anisotropic axes of lanthanide SMMs.34 The calculations had been performed on the electronic structure of isolated Dy(III) fragments using MOLCAS 8.0 package17 without taking into account the weakly magnetic interaction between Dy(III) ions. The calculated eight lowest Kramer’s doublets (KDs) and the g factors of 1 and 2 using CASSCF/ RASSI were summarized in Table 2. The calculated effective gz tensors are of 19.431 (gx,y = 0.005, 0.007) and 19.411 (gx,y = 0.014, 0.022) for 1 and 2, respectively, which are approximately

Actually, the slight changes of the geometrical configuration of the Ln(III) cations dramatically regulates the relaxation process of the magnetization.32 As reported in previous references,24 the different local symmetry and the bond distances would contribute to differing ligand fields, further resulting in diverse dynamic magnetic behaviors. For instance, the energy barrier of compound D4d with square antiprismatic Dy(III) ion is higher than that of compound D2d with dodecahedral Dy(III) ion. Nevertheless, the results in present cases are in contrast with the explanation above, which verifies that the relaxation magnetism simply depends on the coordination symmetry of Dy(III) centers. Accordingly, the magnetism of dysprosiumbased SMMs might be simultaneously and coefficiently dominated by complicated factors including local symmetry, electrostatics, spin−orbit coupling, etc.14 As it is another significant character of the magnetic bistability of a magnet, magnetic hysteresis loops were examined for 1 and 2. The results indicate that the hysteresis becomes more obscure in the order of 1 to 2. Compound 1 behaves as a butterfly-shaped loop only at 1.8 K, whereas 2 displays tiny hysteresis openings at the same temperature (Figure 11), supporting the more explicit hysteresis in 1 than 2 depends on the disparity of quantum tunneling rates. The chief reason for the different hysteresis loops for two compounds is that the magnetization reversal time of 2 is shorter than that of 1 in the magnetic quantum tunneling regime, implying a faster relaxation behavior in 2. Correspondingly, the more legible motif of slow relaxation observed in 1 under 0 Oe alludes to the slower QTM effect that existed in 1, in accord with the energy barrier tendency in the sequence of 1 > 2. May be tiny but definitely important distinction for the respective architectures impacts the essence of high-order transversal anisotropy. In other words, the magnitude of energy barrier could not only depend on the local symmetry of Dy(III) ion but also, and most importantly, rely on the attendance of axial crystalline field around the Dy(III) center, which might be achieved by regulating the electron density of the donor atoms. Additionally, even extremely weak dipole−dipole interactions are effective for decreasing QTM and slowing the magnetic relaxation of SMMs, which has been manifested by some cases.33 In view of the nearest Dy···Dy distances in present compounds (6.047 Å for 1, 8.382 Å for 2), it is predicted that the dipole−dipole interaction for 1 is 2.7 times larger than that for 2, which is enough to affect the difference of exchange bias between 1 and 2, facilitating a slower QTM rate for 1 than that

Table 2. Calculated Energy Levels (cm−1), g (gx, gy, gz) Tensors, and mJ Values of the Minimum Kramer’s Doublets of the Dy Motifs in Compounds 1 and 2 1 KDs

3651

−1

E (cm )

1

0.0

2

172.8

3

238.5

4

265.2

5

302.3

6

383.7

7

456.9

8

464.4

2 −1

g

mJ

0.005 0.007 19.431 0.269 0.493 15.234 8.778 6.560 3.812 10.481 5.868 0.197 2.662 2.997 13.985 0.619 0.640 17.078 0.018 0.750 16.826 0.131 0.770 16.946

±15/2

E (cm ) 0.0

±13/2

110.0

±5/2

144.8

±3/2

176.9

±1/2

212.8

±7/2

270.9

±9/2

351.2

±11/2

450.9

g

mJ

0.014 0.022 19.411 1.001 2.391 15.436 4.746 5.986 8.237 0.087 3.958 9.316 1.327 2.058 12.400 0.697 0.988 16.303 0.119 0.263 18.847 0.020 0.039 19.542

±15/2

±13/2

±9/2

±7/2

±5/2

±3/2

±1/2

±11/2


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Figure 12. Anisotropic axes are depicted for compounds 1 (a) and 2 (b).

ion and a couple of oxygen atoms from coordination H2O moieties, while the magnetic axis in 2 is perpendicular to the facet constituted from metal ion and a pair of N atoms from alternated bpy ligand. To further acquire the deep understanding for the variously anisotropic nature of two compounds, the charge distributions of Dy(III) ions and coordination atoms are considered concurrently (Table S8). Calculation results have illuminated that the strong uniaxial anisotropy is observed, while only the low-lying ground state, | ±15/2⟩ Kramer’s doublet, is dominantly occupied.36 The Kramer’s doublet (|±15/2⟩) tends to the ligand field that the negative charges populate below and above the equatorial plane to weaken the repulsive potential between the f-electron cloud and ligand, which impels the |±15/2⟩ Kramer’s doublet to be more stable, further realizing significant anisotropic behavior. Actually, the desirable ligand field could be approximated by organizing the ligand with lower positive charge and smaller Dy-ligand bond lengths below and above the equatorial plane to enhance the axial anisotropism of magnetization. For compound 2, the bond lengths between dysprosium node and a pair of bpy nitrogen atoms (Dy−N1 = 2.542 Å, Dy−N2 = 2.520 Å) are almost equivalent, and two N donors contribute to the lower negative charges (N1 = −0.351, N2 = −0.349), suggesting that the |±15/2⟩ Kramer’s doublet would not be steadied in the ligand field. Sequentially, 2 possesses relatively weaker uniaxial anisotropy. On the contrary, in the device of 1, the lengths between Dy(III) center and two O atoms of coordination H2O groups (Dy−O7 = 2.384 Å, Dy−O8 = 2.421 Å) are significantly shorter than that of Dy−N bonds in 2, and the charge distributions on the two O atoms (O7 = −0.731, O8 = −0.687) are evidently more negative than that of N atoms in 2, clarifying that the oxygen-containing linkers exclude the felectron cloud more intensively than the nitrogen-containing organics and yielding the dramatically diverse configuration of the static potential. Consequently, compound 1 owns more prominent magnetic anisotropy. The anisotropic distinctions of 1 and 2 prove that proper integration of large and small Dyligand distance in the ligand field would promote the stabilization of the |±15/2⟩ Kramer’s doublet, which inversely gives rise to significant anisotropic performance. To deeply inquire into the principle of relaxation process, the effective relaxation pathways from the maximum magnetized state in the ground-state doublets to the time-reversed states were investigated with reverse magnetizations for compounds 1 and 2 (Figure S8).37 The Kramer’s doublets were arranged according to the counts of the magnetic moments (bold black lines in Figure S8), which are the functions of the magnetic moments along the axes of the magnetization. The digits with

of the Ising-limit value of 20, illustrating that the two examples present significantly axial anisotropy for Dy(III) fragments. It is gratifying that the calculated χMT motifs for both 1 and 2 are in well accordance with the experimental values in high-temperature range (Figure 2), while the experimental curves slightly overtop the calculated ones in low-temperature region, responding to the presence of significant uniaxial anisotropy and the direction of the easy axis in low-temperature region. In general, the effective barrier of the thermal-assisted Orbach relaxation mechanism is corresponding to the energy gap between the ground state and first excitation state. Thus, the CASSCF/RASSI procedure was applied to determine the fine energy spectrum of compounds 1 and 2 (Table 2). The calculated energy gaps of 239 K (173 cm−1) and 152 K (110 cm−1) between the ground and first excitation states in two compounds are visibly deviation from the experimental results fitted by the Arrhenius law (169 K for 1, 90 K for 2) under a static field of 1200 Oe. Noteworthily, although the relaxation barriers from calculations are larger than the experimental data, the calculated values are in the identical order of the experiment results, and the difference of 87 K between the two calculated values for 1 and 2 is well consistent with the ones of 79 K from the magnetic measurements. The origination of this deviation between experiments and calculations is probably connected with the existence of extra relaxation behaviors (for instance, quantum tunneling effect in ground state due to dipole−dipole interaction or vibrational exchange) that are considerable for the obtained experiment effective energy barriers, Ueff values.35 In addition, the barrier values in 1 and 2 should be carefully processed because of the case that the maximal peaks in hightemperature range are restricted in the extraction of barriers from the Arrhenius analysis. Furthermore, the quantum tunneling effect is commonly quantized by the transversal anisotropic magnitude. The obtained parameters of gx 0.014, gy 0.022 for compound 2 is clearly greater than gx 0.005, gy 0.007 for 1 (Table 2). Ulteriorly, for the first excitation Kramer’s doublets, the transverse magnitudes still represent relatively low values for 1 (gx,y = 0.269, 0.493), while the opposite case appears on 2 (gx,y = 1.001, 2.391), the gx,y tensors of higher excitation states raise obviously. Comparing with 1, the larger transversal anisotropy components in 2 would propel more conspicuous QTM process, which is responsible for the experimental results. As depicted in Figure 12, the orientations of magnetic easy axis for two compounds are illustrated, in which the main magnetic axis on the central ions point to almost accordant directions in two compounds. The easy axes of magnetization in 1 is oriented vertically to the triangular facet composed of Dy 3652


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the information unveiled is conducive to in-depth understanding of magneto-structural correlation and relaxation pathways, the ultimate goal of which is to advance the deliberate tailoring of SMMs. Further studies following this guideline are actually ongoing.

the arrows are the average absolute values of matrix elements of transition magnetic moments (μB) correlating to the relevant states. The transverse magnetic moments of the Kramer’s doublets in the ground state arise to be modest in both compounds (green arrows in Figure 13, ca. 1 × 10−2 μB), revealing that the diagonal quantum tunneling is operational for the Kramer’s doublets in the ground state. The nonlevel arrows express the spin phonon transition pathways. The nondiagonal items of the transverse magnetic moments (0.017 μB for 1 and 0.098 μB for 2) respond to the Orbach mechanism between the ground and first excitation states of inverse magnetizing (blue arrows in Figure 13), which are still modest and expand ulteriorly relaxation pathways. The tunneling gaps of the diagonal and nondiagonal items between ground state and first excitation state of 2 are greater than those of 1; therefore, 2 signifies the more rapid QTM in two compounds, which is corresponding to the AC magnetic data and hysteresis experiments. On the basis of a recent proposal by Chibotaru,37a the blocking barriers were determined by the closest pathways with the maximal transition magnetic moments, the most likely paths for relaxation processes of magnetization in compounds 1 and 2 are anticipated to relate to the red arrows in Figure S8. Thereby we can explicate that the behaviors of two Dy(III) compounds are close to the typical characteristics of the Ln(III) molecular nanomagnet. However, it is unpractical to provide an evaluation of the energetic magnitude for the reverse barrier in 2 in the absence of an applied field, because the relaxation pathways included a faster quantum effect in the ground-state Kramer’s doublet. Thus, the effective barrier for the magnetic relaxation process of compound 2 would be acquired from the Arrhenius equation under an external static field owing to the quantum tunneling effect being enormously repressed, whereas the effective relaxation energy barriers of 1 could be unambiguously extracted at zero and applied fields, respectively.

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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.7b00057. Molecular stacking charts, PXRD patterns, M versus H curves, plot of relaxation time versus T-1, temperature dependance of in-phase and out-of-phase susceptibilities under varied DC field, magnetism blocking barriers, selected bond lengths and angles, Dy(III) ion geometry analysis, relaxation fitting parameters, natural bond order charges per atom (PDF) Crystal structures (CIF) Crystal structures (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (X.-Y.L.) *E-mail: [email protected]. (Y.-Q.Z.) *E-mail: [email protected]. (S.-P.C.) ORCID

Xiang-Yu Liu: 0000-0001-8864-3411 Notes

The authors declare no competing financial interest. CCDC numbers are 1504089 (1) and 1504090 (2).

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the NSFC (21673180, 21463020, and 21473135), the NSF of Ningxia Province (NZ16035), the NSF of Jiangsu Province (BK20151542), the Foundation for Fostering Outstanding Young Teachers of Ningxia Higher Education Institutions (NGY2016063), and the Foundation of State Key Laboratory of Coal Clean Utilization and Ecological Chemical Engineering (2016-14).

CONCLUSIONS Two mononuclear Dy(III) compounds, Dy(BTFA)3(H2O)2 (1) and Dy(BTFA)3(bpy) (2), have been prepared with a βdiketone ligand (BTFA) and auxiliary ligand (bpy). X-ray crystallographic studies reveal that the ancillary ligand (bpy) acts as a key factor in manipulating the geometrical configurations of Dy(III) centers. The coordination environment with O8 motif in 1 could be best indentified as a triangular dodecahedron (D2d), while the configuration in 2 is surrounded by a square antiprismatic (D4d) N2O6 group. Magnetic studies demonstrate that the two compounds exhibit discriminatively slow relaxation with QTM in the absence of an extra magnetic field. The quantum tunneling process is effectively suppressed after applying an optimal static field, generating the anisotropy barrier ΔE/kB = 296.50 K for 1 and the barrier ΔE/kB = 151.01 K for 2. As explored by ab initio calculations and electrostatic analysis, significantly uniaxial magnetic anisotropies exist in two title compounds, and the easy axis orientation is insensitive to different local geometrical symmetries. The comparison of the electronic structures clarifies that different charge distributions around the Dy(III) ions in two compounds compensate for the discrepancy of the geometrical symmetries, which is responsible for the great disparities of magnetic anisotropy, as well as energy barrier and slow relaxation behavior among the two compounds. The present work provides further evidence of the effects of electrostatic perturbations on single-ion anisotropy and magnetization dynamics in β-diketone-Dy(III) SMMs, and

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REFERENCES

(1) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141−143. (2) (a) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press, 2006. (b) Van Slageren, J. New directions in electron paramagnetic resonance spectroscopy on molecular nanomagnets. Top. Curr. Chem. 2011, 321, 199−234. (c) Liu, K.; Shi, W.; Cheng, P. Toward heterometallic single-molecule magnets: synthetic strategy, structures and properties of 3d−4f discrete complexes. Coord. Chem. Rev. 2015, 289, 74−122. (3) (a) Gatteschi, D.; Sessoli, R. Quantum tunneling of magnetization and related phenomena in molecular materials. Angew. Chem., Int. Ed. 2003, 42, 268−297. (b) Bogani, L.; Wernsdorfer, W. Molecular spintronics using single-molecule magnets. Nat. Mater. 2008, 7, 179− 186. (c) 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. (d) Liu, X. Y.; Sun, L.; Zhou, H. L.; Cen, P. P.; Jin, X. Y.; Xie, G.; Chen, S. P.; Hu, Q. L. Single-Ion-Magnet Behavior in a Two-Dimensional Coordination Polymer Constructed from CoII Nodes and a Pyridylhydrazone Derivative. Inorg. Chem. 2015, 54, 8884−8886. (e) Liu, X. Y.; Liu, H. 3653


Article

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ligands. Inorg. Chem. 2013, 52, 8933−8940. (c) Sun, W. B.; Han, B. L.; Lin, P. H.; Li, H. F.; Chen, P.; Tian, Y. M.; Murugesu, M.; Yan, P. F. Series of dinuclear and tetranuclear lanthanide clusters encapsulated by salen-type and β-diketionate ligands: single-molecule magnet and fluorescence properties. Dalton Trans. 2013, 42, 13397−13403. (d) Li, D. P.; Zhang, X. P.; Wang, T. W.; Ma, B. B.; Li, C. H.; Li, Y. Z.; You, X. Z. Distinct magnetic dynamic behavior for two polymorphs of the same Dy(III) complex. Chem. Commun. 2011, 47, 6867−6869. (11) (a) Bernot, K.; Luzon, J.; Bogani, L.; Etienne, M.; Sangregorio, E. C.; Shanmugam, M.; Caneschi, A.; Sessoli, R.; Gatteschi, D. Magnetic anisotropy of dysprosium(III) in a low-symmetry environment: a theoretical and experimental investigation. J. Am. Chem. Soc. 2009, 131, 5573−5579. (b) Ma, Y.; Xu, G. F.; Yang, X.; Li, L. C.; Tang, J. K.; Yan, S. P.; Cheng, P.; Liao, D. Z. Pyrazine-bridged Dy2 single-molecule magnet with a large anisotropic barrier. Chem. Commun. 2009, 46, 8264−8266. (12) (a) Chen, G. J.; Guo, Y. N.; Tian, J. L.; Tang, J. K.; Gu, W.; Liu, X.; Yan, S. P.; Cheng, P.; Liao, D. Z. Enhancing anisotropy barriers of dysprosium(III) single-ion magnets. Chem. - Eur. J. 2012, 18, 2484− 2487. (b) Norel, L.; Bernot, K.; Feng, M.; Roisnel, T.; Caneschi, A.; Sessoli, R.; Rigaut, S. A carbon-rich ruthenium decorated dysprosium single molecule magnet. Chem. Commun. 2012, 48, 3948−3950. (c) da Cunha, T. T.; Jung, J.; Boulon, M. E.; Campo, G.; Pointillart, F.; Pereira, C. L. M.; Le Guennic, B. L.; Cador, O.; Bernot, K.; Pineider, F.; Golhen, S.; Ouahab, L. Magnetic Poles Determinations and Robustness of Memory Effect upon Solubilization in a DyIII-Based Single Ion Magnet. J. Am. Chem. Soc. 2013, 135, 16332−16335. (d) Pointillart, F.; Bernot, K.; Golhen, S.; Le Guennic, B. L.; Guizouarn, T.; Ouahab, L.; Cador, O. Magnetic Memory in an Isotopically Enriched and Magnetically Isolated Mononuclear Dysprosium Complex. Angew. Chem., Int. Ed. 2015, 54, 1504−1507. (13) (a) Li, X. L.; Chen, C. L.; Gao, Y. L.; Liu, C. M.; Feng, X. L.; Gui, Y. H.; Fang, S. M. Modulation of Homochiral DyIII Complexes: Single-Molecule Magnets with Ferroelectric Properties. Chem. - Eur. J. 2012, 18, 14632−14637. (b) Menelaou, M.; Ouharrou, F.; Rodríguez, L.; Roubeau, O.; Teat, S. J.; Aliaga-Alcalde, N. DyIII- and YbIIICurcuminoid Compounds: Original Fluorescent Single-Ion Magnet and Magnetic Near-IR Luminescent Species. Chem. - Eur. J. 2012, 18, 11545−11549. (14) (a) Sorace, L.; Benelli, C.; Gatteschi, D. Lanthanides in molecular magnetism: old tools in a new field. Chem. Soc. Rev. 2011, 40, 3092−3104. (b) Ungur, L.; Lin, S. Y.; Tang, J. K.; Chibotaru, L. F. Single-molecule toroics in Ising-type lanthanide molecular clusters. Chem. Soc. Rev. 2014, 43, 6894−6905. (15) (a) Chilton, N. F.; Collison, D. E.; McInnes, J. L.; Winpenny, R. E. P.; Soncini, A. An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes. Nat. Commun. 2013, 4, 2551−2557. (b) Ungur, L.; LeRoy, J. J.; Korobkov, I.; Murugesu, M.; Chibotaru, L. F. An electrostatic model for the determination of magnetic anisotropy in dysprosium complexes. Angew. Chem., Int. Ed. 2014, 53, 4413−4417. (c) Rinehart, J. D.; Long, J. R. Exploiting singleion anisotropy in the design of f-element single-molecule magnets. Chem. Sci. 2011, 2, 2078−2085. (d) Sun, W. B.; Yan, P. F.; Jiang, S. D.; Wang, B. W.; Zhang, Y. Q.; Li, H. F.; Chen, P.; Wang, Z. M.; Gao, S. High symmetry or low symmetry, that is the question−high performance Dy (iii) single-ion magnets by electrostatic potential design. Chem. Sci. 2016, 7, 684−691. (16) (a) Sheldrick, G. M. SADABS, Program for Empirical Absorption Correction; University of Göttingen: Göttingen, Germany, 1996. (b) Sheldrick, G. M. SHELXTL; Bruker Analytical X-ray Instruments Inc.: Madison, WI, 1998. (17) Karlström, G.; Lindh, R.; Malmqvist, P. Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P. O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. MOLCAS: a Program Package for Computational Chemistry. Comput. Mater. Sci. 2003, 28, 222−239. (18) Llunell, M.; Casanova, D.; Cirera, J.; Alemany, P.; Alvarez, S. SHAPE, v2.1; Universitat de Barcelona: Barcelona, 2013. (19) Liu, S. S.; Xu, L.; Jiang, S. D.; Zhang, Y. Q.; Meng, Y. S.; Wang, Z.; Wang, B. W.; Zhang, W. X.; Xi, Z. F.; Gao, S. Half-Sandwich

X.; Cen, P. P.; Chen, X. Y.; Zhou, H. L.; Song, W. M.; Hu, Q. L. Auxiliary ligand-triggered assembly of two dinuclear Cu(II) compounds with a pyridylhydrazone derivative: Synthesis, crystal structure and magnetic property. Inorg. Chim. Acta 2016, 447, 12−17. (4) (a) Skomski, R. Simple Models of Magnetism; Oxford University Press, 2008. (b) Wang, W. M.; Wang, S. Y.; Zhang, H. X.; Shen, H. Y.; Zou, J. Y.; Gao, H. L.; Cui, J. Z.; Zhao, B. Modulating single-molecule magnet behaviour of phenoxo-O bridged lanthanide(iii) dinuclear complexes by using different β-diketonate coligands. Inorg. Chem. Front. 2016, 3, 133−141. (c) Woodruff, D. N.; Winpenny, R. E.; Layfield, R. A. Lanthanide single-molecule magnets. Chem. Rev. 2013, 113, 5110−5148. (d) Yin, D. D.; Chen, Q.; Meng, Y. S.; Sun, H. L.; Zhang, Y. Q.; Gao, S. Slow magnetic relaxation in a novel carboxylate/ oxalate/hydroxyl bridged dysprosium layer. Chem. Sci. 2015, 6, 3095− 3101. (e) Liu, K.; Zhang, X. J.; Meng, X. X.; Shi, W.; Cheng, P.; Powell, A. K. Constraining the coordination geometries of lanthanide centers and magnetic building blocks in frameworks: a new strategy for molecular nanomagnets. Chem. Soc. Rev. 2016, 45, 2423−2439. (f) Liu, K.; Li, H. H.; Zhang, X. J.; Shi, W.; Cheng, P. Constraining and tuning the coordination geometry of a lanthanide ion in metal−organic frameworks: Approach toward a single-molecule magnet. Inorg. Chem. 2015, 54, 10224−10231. (5) (a) Feltham, H. L. C.; Brooker, S. Review of purely 4f and mixedmetal nd-4f single-molecule magnets containing only one lanthanide ion. Coord. Chem. Rev. 2014, 276, 1−33. (b) Pedersen, K. S.; Bendix, J.; Clerac, R. Single-molecule magnet engineering: building-block approaches. Chem. Commun. 2014, 50, 4396−4415. (c) Jiang, S. D.; Wang, B. W.; Sun, H. L.; Wang, Z. M.; Gao, S. An organometallic single-ion magnet. J. Am. Chem. Soc. 2011, 133, 4730−4733. (d) Guo, Y. N.; Xu, G. F.; Wernsdorfer, W.; Ungur, L.; Guo, Y.; Tang, J. K.; Zhang, H. J.; Chibotaru, L. F.; Powell, A. K. Strong axiality and ising exchange interaction suppress zero-field tunneling of magnetization of an asymmetric Dy2 single-molecule magnet. J. Am. Chem. Soc. 2011, 133, 11948−11951. (6) Bi, Y.; Guo, Y. N.; Zhao, L.; Guo, Y.; Lin, S. Y.; Jiang, S. D.; Tang, J. K.; Wang, B. W.; Gao, S. Capping Ligand Perturbed Slow Magnetic Relaxation in Dysprosium Single-Ion Magnets. Chem. - Eur. J. 2011, 17, 12476−12481. (7) (a) 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. (b) Jiang, S. D.; Wang, B. W.; Su, G.; Wang, Z. M.; Gao, S. A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior. Angew. Chem. 2010, 122, 7610−7613. (c) CardonaSerra, S.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Ariño, A.; Camón, A.; Evangelisti, M.; Luis, F.; Martínez-Pérez, M. J.; Sesé, J. Lanthanoid Single-Ion Magnets Based on Polyoxometalates with a 5-fold Symmetry: The Series [LnP5W30O110] 12−(Ln3+= Tb, Dy, Ho, Er, Tm, and Yb). J. Am. Chem. Soc. 2012, 134, 14982−14990. (d) Le Roy, J. J.; Ungur, L.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. Coupling Strategies to Enhance Single-Molecule Magnet Properties of Erbium− Cyclooctatetraenyl Complexes. J. Am. Chem. Soc. 2014, 136, 8003− 8010. (8) Aravena, D.; Ruiz, E. Shedding light on the single-molecule magnet behavior of mononuclear DyIII complexes. Inorg. Chem. 2013, 52, 13770−13778. (9) (a) Zhang, P.; Guo, Y. N.; Tang, J. K. Recent advances in dysprosium-based single molecule magnets: Structural overview and synthetic strategies. Coord. Chem. Rev. 2013, 257, 1728−1763. (b) Watanabe, A.; Yamashita, A.; Nakano, M.; Yamamura, T.; Kajiwara, T. Multi-Path Magnetic Relaxation of Mono-Dysprosium(III) Single-Molecule Magnet with Extremely High Barrier. Chem. Eur. J. 2011, 17, 7428−7432. (10) (a) Chilton, N. F.; Langley, S. K.; Moubaraki, B.; Soncini, A.; Batten, S. R.; Murray, K. S. Single molecule magnetism in a family of mononuclear β-diketonate lanthanide(III) complexes: rationalization of magnetic anisotropy in complexes of low symmetry. Chem. Sci. 2013, 4, 1719−1730. (b) Liu, C. M.; Zhang, D. Q.; Zhu, D. B. Fieldinduced single-ion magnets based on enantiopure chiral β-diketonate 3654


Article

Inorganic Chemistry Complexes of DyIII: A Janus-Motif with Facile Tunability of Magnetism. Inorg. Chem. 2015, 54, 5162−5168. (20) (a) Kahn, M. L.; Sutter, J. P.; Golhen, S.; Guionneau, P.; Ouahab, L.; Kahn, O.; Chasseau, D. Systematic investigation of the nature of the coupling between a Ln(III) ion (Ln = Ce(III) to Dy(III)) and its aminoxyl radical ligands. structural and magnetic characteristics of a series of {Ln(organic radical)2} compounds and the related {Ln(Nitrone)2} derivatives. J. Am. Chem. Soc. 2000, 122, 3413−3421. (b) Kahn, M. L.; Ballou, R.; Porcher, P.; Kahn, O.; Sutter, J. P. Analytical Determination of the {Ln−Aminoxyl Radical} Exchange Interaction Taking into Account Both the Ligand-Field Effect and the Spin−Orbit Coupling of the Lanthanide Ion (Ln = DyIII and HoIII). Chem. - Eur. J. 2002, 8, 525−531. (c) Guo, Y. N.; Xu, G. F.; Gamez, P.; Zhao, L.; Lin, S. Y.; Deng, R.; Tang, J. K.; Zhang, H. J. Two-step relaxation in a linear tetranuclear dysprosium(III) aggregate showing single-molecule magnet behavior. J. Am. Chem. Soc. 2010, 132, 8538−8539. (d) Zhang, P.; Zhang, L.; Wang, C.; Xue, S. F.; Lin, S. Y.; Tang, J. K. Equatorially coordinated lanthanide single ion magnets. J. Am. Chem. Soc. 2014, 136, 4484−4487. (e) Guo, Y. N.; Ungur, L.; Granroth, G. E.; Powell, A. K.; Wu, C. J.; Nagler, S. E.; Tang, J. K.; Chibotaru, L. F.; Cui, D. M. An NCN-pincer ligand dysprosium single-ion magnet showing magnetic relaxation via the second excited state. Sci. Rep. 2014, 4, 5471−5477. (f) Ke, H. S.; Zhang, S.; Li, X.; Wei, Q.; Xie, G.; Wang, W. Y.; Chen, S. P. A Dy2 single-molecule magnet with benzoate anions and phenol-O− bridging groups. Dalton Trans. 2015, 44, 21025−21031. (21) (a) Osa, S.; Kido, T.; Matsumoto, N.; Re, N.; Pochaba, A.; Mrozinski, J. A tetranuclear 3d-4f single molecule magnet: [CuIILTbIII(hfac)2]2. J. Am. Chem. Soc. 2004, 126, 420−421. (b) Gao, Y.; Xu, G. F.; Zhao, L.; Tang, J.; Liu, Z. Observation of slow magnetic relaxation in discrete dysprosium cubane. Inorg. Chem. 2009, 48, 11495−11497. (22) Abbas, G.; Lan, Y. H.; Kostakis, G. E.; Wernsdorfer, W. C.; Anson, E.; Powell, A. K. Series of isostructural planar lanthanide complexes [LnIII4 (μ3-OH)2 (mdeaH)2 (piv)8] with single molecule magnet behavior for the Dy4 analogue. Inorg. Chem. 2010, 49, 8067− 8072. (23) (a) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S. Y.; Kaizu, Y. Mononuclear lanthanide complexes with a long magnetization relaxation time at high temperatures: A new category of magnets at the single-molecular level. J. Phys. Chem. B 2004, 108, 11265−11271. (b) Dong, Y. P.; Yan, P. F.; Zou, X. Y.; Li, G. M. Azacyclo-auxiliary ligand-tuned SMMs of dibenzoylmethane Dy(III) complexes. Inorg. Chem. Front. 2015, 2, 827−836. (c) Dong, Y. P.; Yan, P. F.; Zou, X. Y.; Liu, T. Q.; Li, G. M. Exploiting single-molecule magnets of β-diketone dysprosium complexes with C3v symmetry: suppression of quantum tunneling of magnetization. J. Mater. Chem. C 2015, 3, 4407−4415. (24) (a) Zhang, S.; Ke, H. S.; Sun, L.; Li, X.; Shi, Q.; Xie, G.; Wei, Q.; Yang, D. S.; Wang, W. Y.; Chen, S. P. Magnetization Dynamics Changes of Dysprosium(III) Single-Ion Magnets Associated with Guest Molecules. Inorg. Chem. 2016, 55, 3865−3871. (b) Huang, X. C.; Zhang, M.; Wu, D. Y.; Shao, D.; Zhao, X. H.; Huang, W.; Wang, X. Y. Single molecule magnet behavior observed in a 1-D dysprosium chain with quasi-D 5h symmetry. Dalton Trans. 2015, 44, 20834− 20838. (c) Tong, Y. Z.; Gao, C.; Wang, Q. L.; Wang, B. W.; Gao, S.; Cheng, P.; Liao, D. Z. Two mononuclear single molecule magnets derived from dysprosium(III) and tmphen (tmphen = 3, 4, 7, 8tetramethyl-1, 10-phenanthroline). Dalton Trans. 2015, 44, 9020− 9026. (25) Zhang, X. J.; Vieru, V.; Feng, X. W.; Liu, J. L.; Zhang, Z. J.; Na, B.; Shi, W.; Wang, B. W.; Powell, A. K.; Chibotaru, L. F.; Gao, S.; Cheng, P.; Long, J. R. Influence of Guest Exchange on the Magnetization Dynamics of Dilanthanide Single-Molecule-Magnet Nodes within a Metal−Organic Framework. Angew. Chem., Int. Ed. 2015, 54, 9861−9865. (26) Aubin, S. M. J.; Sun, Z.; Pardi, L.; Krzystek, J.; Folting, K.; Brunel, L. C.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N.

Reduced anionic Mn12 molecules with half-integer ground states as single-molecule magnets. Inorg. Chem. 1999, 38, 5329−5340. (27) (a) Zhang, H. X.; Lin, S. Y.; Xue, S. F.; Wang, C.; Tang, J. Acetato-bridged dinuclear lanthanide complexes with single molecule magnet behaviour for the Dy2 species. Dalton Trans. 2014, 43, 6262− 6268. (b) Cole, K. S.; Cole, R. H. Dispersion and absorption in dielectrics I. Alternating current characteristics. J. Chem. Phys. 1941, 9, 341−351. (c) Mori, F.; Nyui, T.; Ishida, T.; Nogami, T.; Choi, K. Y.; Nojiri, H. Oximate-bridged trinuclear Dy-Cu-Dy complex behaving as a single-molecule magnet and its mechanistic investigation. J. Am. Chem. Soc. 2006, 128, 1440−1441. (28) (a) Rinehart, J. D.; Meihaus, K. R.; Long, J. R. Observation of a secondary slow relaxation process for the field-induced single-molecule magnet U (H2BPz2)3. J. Am. Chem. Soc. 2010, 132, 7572−7573. (b) Wang, Y. L.; Ma, Y.; Yang, X.; Tang, J. K.; Cheng, P.; Wang, Q. L.; Li, L. C.; Liao, D. Z. Syntheses, Structures, and Magnetic and Luminescence Properties of a New DyIII-Based Single-Ion Magnet. Inorg. Chem. 2013, 52, 7380−7386. (29) Zhu, J.; Wang, C. Z.; Luan, F.; Liu, T. Q.; Yan, P. F.; Li, G. M. Local Coordination Geometry Perturbed β-Diketone Dysprosium Single-Ion Magnets. Inorg. Chem. 2014, 53, 8895−8901. (30) Gao, F.; Yao, M. X.; Li, Y. Y.; Li, Y. Z.; Song, Y.; Zuo, J. L. Syntheses, Structures, and Magnetic Properties of seven-coordinate Lanthanide Porphyrinate or Phthalocyaninate Complexes with Kläui’s Tripodal Ligand. Inorg. Chem. 2013, 52, 6407−6416. (31) (a) Shrivastava, K. N. Theory of spin−lattice relaxation. Phys. Status Solidi B 1983, 117, 437−458. (b) Brya, W. J.; Wagner, P. E. Direct, Orbach, and Raman Relaxation in Dilute Cerous Magnesium Nitrate. Phys. Rev. 1966, 147, 239−241. (c) Hoffmann, S. K.; Hilczer, W.; Goslar, J.; Augustyniak-Jablokow, M. A. Raman spin-lattice relaxation, Debye temperature and disorder effects studied with electron spin echo of Cu2+ in Tutton salt crystals. J. Phys.: Condens. Matter 2001, 13, 7443−7457. (32) (a) Gonidec, M.; Luis, F.; Vilchez, A.; Esquena, J.; Amabilino, D. B.; Veciana, J. A Liquid-Crystalline Single-Molecule Magnet with Variable Magnetic Properties. Angew. Chem., Int. Ed. 2010, 49, 1623− 1626. (b) Boulon, M. E.; Cucinotta, G.; Luzon, J.; Degl’Innocenti, C.; Perfetti, M.; Bernot, K.; Calvez, G.; Caneschi, A.; Sessoli, R. Magnetic Anisotropy and Spin-Parity Effect Along the Series of Lanthanide Complexes with DOTA. Angew. Chem., Int. Ed. 2013, 52, 350−354. (c) Boulon, M. E.; Cucinotta, G.; Liu, S. S.; Jiang, S. D.; Ungur, L.; Chibotaru, L. F.; Gao, S.; Sessoli, R. Angular-Resolved Magnetometry Beyond Triclinic Crystals: Out-of-Equilibrium Studies of Cp* ErCOT Single-Molecule Magnet. Chem. - Eur. J. 2013, 19, 13726−13731. (33) (a) Horii, Y.; Katoh, K.; Cosquer, G.; Breedlove, B. K.; Yamashita, M. Weak DyIII−DyIII Interactions in DyIII−Phthalocyaninato Multiple-Decker Single-Molecule Magnets Effectively Suppress Magnetic Relaxation. Inorg. Chem. 2016, 55, 11782−11790. (b) Wernsdorfer, W.; Aliaga-Alcalde, N.; Hendrickson, D. N.; Christou, G. Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets. Nature 2002, 416 (6879), 406−409. (34) (a) Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C. Y.; Rekken, B. D.; Power, P. P.; Neese, F.; Long, J. R. Slow magnetization dynamics in a series of two-coordinate iron(II) complexes. Chem. Sci. 2013, 4, 125−138. (b) Atanasov, M.; Zadrozny, J. M.; Long, J. R.; Neese, F. A theoretical analysis of chemical bonding, vibronic coupling, and magnetic anisotropy in linear iron(II) complexes with single-molecule magnet behavior. Chem. Sci. 2013, 4, 139−156. (35) Oyarzabal, I.; Ruiz, J.; Seco, J. M.; Evangelisti, M.; Camόn, A.; Ruiz, E.; Aravena, D.; Colacio, E. Rational Electrostatic Design of EasyAxis Magnetic Anisotropy in a ZnII−DyIII−ZnII Single-Molecule Magnet with a High Energy Barrier. Chem. - Eur. J. 2014, 20, 14262−14269. (36) Langley, S. K.; Wielechowski, D. P.; Vieru, V.; Chilton, N. F.; Moubaraki, B.; Chibotaru, L. F.; Murray, K. S. Modulation of slow magnetic relaxation by tuning magnetic exchange in {Cr2 Dy2} single molecule magnets. Chem. Sci. 2014, 5, 3246−3256. (37) (a) Ungur, L.; Thewissen, M.; Costes, J. P.; Wernsdorfer, W.; Chibotaru, L. F. Interplay of strongly anisotropic metal ions in 3655


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Inorganic Chemistry magnetic blocking of complexes. Inorg. Chem. 2013, 52, 6328−6337. (b) Singh, S. K.; Gupta, T.; Shanmugam, M.; Rajaraman, G. Unprecedented magnetic relaxation via the fourth excited state in low-coordinate lanthanide single-ion magnets: a theoretical perspective. Chem. Commun. 2014, 50, 15513−15516. (c) Chilton, N. F.; Goodwin, C. A. P.; Mills, D. P.; Winpenny, R. E. P. The first nearlinear bis (amide) f-block complex: a blueprint for a high temperature single molecule magnet. Chem. Commun. 2015, 51, 101−103.

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