Single-Molecule Magnet Behavior Enhanced by Synergic Effect of

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Single-Molecule Magnet Behavior Enhanced by Synergic Effect of Single-Ion Anisotropy and Magnetic Interactions Li Zhang,‡ Yi-Quan Zhang,*,† Peng Zhang,*,§ Lang Zhao,‡ Mei Guo,‡ and Jinkui Tang*,‡ †

Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China § Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569, Stuttgart, Germany S Supporting Information *

ABSTRACT: As the simplest entity carrying intramolecular magnetic interactions, a dinuclear lanthanide complex serves as a model to investigate the effects of magnetic interactions on relaxation of magnetization, and importantly, it proves to be an efficient method to obtain robust singlemolecule magnets via improving the communication between lanthanide centers. Here, three Dy2 complexes (1, 2, 3) with a similar structural motif, namely, [Dy2(HL)2(NO3)2(CH3CN)2]·2CH3CN (1), [Dy2(HL)2(NO3)2(DMF)2]·2H2O (2), and Dy2(HL)2(NO3)2(DMF)4 (3), were successfully assembled. One critical difference found in this series of complexes is that the Dy center in complex 3 is coordinated by one more solvent molecule. Surprisingly, complex 3 exhibits the best magnet-like behavior, as evidenced by the high effective barrier and butterfly-type hysteresis, although the crystal field effect around Dy ions is weakened heavily. Ab initio calculations revealed the crucial reason is the significant synergic effect between single-ion anisotropy and magnetic interactions, i.e., not only the axiality of the Dy ion is improved efficiently but also the exchange magnetic interactions increased to the same order of magnitude to the dipolar interaction in 3. This effect mainly benefits from the elaborate modification of the local coordinate environment around the Dy ion, which results in a special arrangement of anisotropy axes different from the other two complexes. It demonstrates that the magnetic interactions could be effectively enhanced by means of deliberate local structural modulation.

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INTRODUCTION Single-molecule magnets (SMMs) are molecular materials capable of blocking the magnetization for a relatively long time at the molecular level,1 which could make it possible to develop the molecule-based information storage and processing technologies.2 Here, one critical and challenging task is to achieve the SMMs retaining functionality in an operable temperature regime, and chemistry, in particular coordination chemistry, is playing a central role.3 Given the high performance in SMM properties of lanthanide complexes, a sustained interest has been developing in lanthanide coordination chemistry since 2003.4 Recently, SMMs encapsulating only one lanthanide ion, usually referred to as single-ion magnets (SIMs),5 have been used to achieve remarkable feats in increasing the effective barriers (Ueff) for reversing magnetic moments and opening temperature of hysteresis (TB).6 Especially, the TB temperature appears to be closely comparable to, even surmount, the record created by N23−-radical-bridged Tb2 SMM.7 Nevertheless, fast quantum tunneling of magnetization (QTM) in SIMs usually triggers the sudden decline of magnetization at zero field in hysteresis loops, and results in the decrease and even disappearance of remanence and coercivity © 2017 American Chemical Society

representing the typical characteristics of permanent magnets. Here, a key point is the high sensitivity of QTM in SIMs, where any slight changes of environment could generate important perturbation on the axial crystal field, thus accelerating the QTM tunneling relaxation rate.8 In contrast, intramolecular magnetic coupling interactions further impose critical effects on QTM in polymetallic lanthanide systems, some of which thus exhibit rather long relaxation times.9 Typically, the N23−-radicalbridged system still represents the hardest SMM magnet known to date, as evidenced by larger remanence and coercivity when compared to any other molecular species at the same temperature, which mainly benefits from the effective suppression of QTM by the strong magnetic exchange coupling mediated by the N23−-radical ligand.7d Naturally, developing such strongly coupled polymetallic lanthanide systems is extraordinarily promising for achieving SMMs operating at higher temperature, and it is essential to obtain an in-depth understanding of how the nature and strength of interactions Received: March 9, 2017 Published: June 30, 2017 7882

DOI: 10.1021/acs.inorgchem.7b00625 Inorg. Chem. 2017, 56, 7882−7889

Article

Inorganic Chemistry between lanthanide ions as well as possible alignment of anisotropy axes can affect the SMM behavior.10 Dinuclear lanthanide SMMs are the simplest magnetic entities carrying intramolecular magnetic exchange, which have greatly contributed to the exploration into relaxation mechanisms influenced by magnetic interactions.11 The recent study of electron paramagnetic resonance (EPR) spectroscopy for a Dy2 complex provided important experimental validation that parallel alignment of magnetic moments of individual spins could produce an exchange bias effect within multinuclear SMMs, which shifts the zero-field quantum tunneling step to a finite field, thus leading to the possible presence of remanence in hysteresis loops.12 More clearly, ab initio calculations of an asymmetric Dy2 SMM from our group revealed the critical effects of single-ion anisotropy and dipolar magnetic interactions on suppressing zero-field QTM.9b Herein, singleion anisotropy dominates the high-temperature relaxation process and determines the height of effective barriers, while strong magnetic interactions between single ions drive an exchange-blocking regime, thus effectively slowing down the tunneling relaxation rate. Nevertheless, the ferromagnetic coupling comes almost entirely from a dipolar interaction, thus the exchange contribution being negligible, which appears to be against further enhancement of SMM behavior. Given the precedent for SMMs in hydrazone-based Dy2 complexes and high tunability of this ligand platform (hydrazone and terminal ligands),13 we hopefully could tailor chemically those magnetic molecules to increase the exchange magnetic interactions between spin centers and further to enhance the blocking behavior. Here, we employed a hydrazone ligand (3-hydroxy-N′-(2hydroxy-3-methoxybenzylidene)picolinohydrazide, H3L) to successfully construct three novel Dy2 complexes (1, 2, 3), which all hold the typical skeleton of this kind of Dy2 SMMs; i.e., two metal centers are bridged by the alkoxido groups of two “head-to-tail” hydrazone ligands, but different terminal ligands.13 In particular, the coordination of more solvent molecules in complex 3 directly changes the local crystal field of Dy ions and leads to an important distortion of the whole molecular structure, which can be correlated with a special arrangement of anisotropy axes different from the other two complexes. Importantly, the exchange magnetic interaction in 3 is effectively increased to the same order of magnitude to the dipolar interaction, thus resulting in stronger blocking behavior.

Figure 1. Crystal structures of compounds (a) 1, (b) 2, and (c) 3 with lattice solvent molecules and hydrogen atoms omitted for clarity. Color code: Dy, purple; O, red; N, blue; C, gray. Symmetry codes: *, −x + 1, −y + 2, −z + 1 for 1, −x, −y + 1, −z + 1 for 2, and −x + 1, −y + 1, −z + 1 for 3.

coordinated solvents together with the coordinating direction of nitrate. Compounds 1 and 2 are shown to be isomorphous, primarily differing in the coordinated solvents with CH3CN or DMF molecule, respectively. In 1, the DyIII ion has a N3O5 coordination environment and exhibits a distorted hula-hoop (HH-8) geometry with the cyclic ring (hula-hoop) defined by the atoms N3 and N1* from two ligands, atoms O5 and O6 from a bidentated nitrate, as well as atom N5 from the terminal CH3CN molecule (Figure 2a). In contrast, the DyIII ion in 2 adopts a similar HH-8 geometry with an O6N2 sphere except that atom O8 from the DMF molecule replaces N5 in 1 (Figure 2b). However, for 3, further coordination of one additional DMF molecule makes each DyIII ion nine-coordinate (O7N2) in a distorted muffin (MFF-9) geometry described as a 1:5:3 polyhedron (Figure 2d), concomitant with altering the coordinating direction of bidentated nitrate. Therefore, the cyclic set of five vertices is composed of two nitrogen atoms (N1* and N3) from two ligands, two oxygen atoms (O8 and O9) from two different DMF molecules, and the only single atom O6 from the nitrate different from that in 1 and 2 (Figure 2c), leading to lower negative charge present on the equatorial ring in 3. Here, the definition of the five-membered ring around the Dy center seems to be different from that in previous reports,14 where the ring was defined from the coordinating atoms of two hydrazone ligands. The reason is to correspond to the anisotropy axis of the Dy ion, as shown in the following calculation. The comparison of some important bond distances and angles of 1−3 shown in Table 1 reveals the shortest bond

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RESULTS AND DISCUSSION Structure. The reactions of dysprosium nitrate with H3L in a 1:1 ratio in CH3OH/CH3CN, treated with Et3N (0.1 mmol), LiOH·H2O (0.1 mmol), and Et3N (0.2 mmol), respectively, using different methods (slow evaporation of solvent or slow diffusing of diethyl ether) produce yellow crystals of 1−3 (detailed information in the SI). Single-crystal X-ray diffraction studies reveal that three centrosymmetric dinuclear complexes crystallize in triclinic space group P1̅ with Z = 1. As depicted in Figure 1, all of them possess the similar skeleton of Dy2(HL)2 in which a pair of DyIII ions are doubly bridged by the carbonyl oxygen atoms (O1 and O1*) of two antiparallel HL2− ligands in the enolate form with the similar Dy1−O1−Dy1* angles of approximately 114° and the two centers take up the O2N (O1, O2, and N3) and ON (O1 and N1) coordination pockets, respectively. The crucial difference for these complexes is the distinct terminal ligands including the number and species of 7883

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Figure 3. Temperature dependence of the χMT values at 1000 Oe for Dy2-CH3CN (1), Dy2-DMF (2), and Dy2-2DMF (3). The solid lines correspond to calculated fits to the data.

theoretical value of 28.34 cm3 K mol−1 for two noninteracting DyIII ions (6H15/2, S = 5/2, L = 5, g = 4/3, and C = 14.17 cm3 K mol−1).15 For 1 and 2, the temperature dependence of the χMT product exhibits similar thermal evolutions over the whole temperature range of 300−2 K. Their χMT values gradually decrease with lowering the temperature to a minimum of 25.09 and 25.40 cm3 K mol−1 at ca. 40 K, followed by a rapid increase to reach the values of 35.12 and 36.25 cm3 K mol−1 at 2 K for 1 and 2. The decrease in χMT value for both compounds results from the progressive depopulation of the mJ sublevels of the ground-state multiplet, while the sharp increases of χMT values at low temperature are indicative of the existence of intramolecular ferromagnetic coupling between the Dy spin carriers. In contrast, for 3, the χMT values do not show a clear decline, but remain relatively constant until 30 K, then sharply increase to 41.63 cm3 K mol−1 at 2.0 K, suggesting possible stronger intramolecular ferromagnetic interactions in 3. Magnetization data for 1−3 at fields of 0−70 kOe below 5 K (Figure S4) reveal a rapid increase in the magnetization at low magnetic fields, followed by a slow linear increase at high fields without saturation even at 7 T, most possibly as a result of the local crystal field effect at the DyIII ion and the strong ferromagnetic interactions between them. Furthermore, M versus H/T curves (inset of Figure S4) are not superimposed on a single master curve at varying temperatures, suggesting the presence of a non-negligible magnetic anisotropy and/or lowlying excited states in all systems.7d Importantly, magnetic hysteresis measurements were performed on complexes 1−3 on a sweep rate accessible with a conventional magnetometer, and thus only complex 3 displays a butterfly-shaped hysteresis loop (Figure 4). Such a behavior is likely correlated with the stronger ferromagnetic interaction between Dy centers than those in complexes 1 and 2.9a In fact, most of polynuclear lanthanide SMMs reported to date experience an antiferromagnetic interaction between metal centers, which is unfavorable to improve coercivity given the diamagnetic ground state for an ideal antiferromagnetic system.9c,16 Therefore, such an exploration into an enhanced ferromagnetic system is of great interest for future information storage. Dynamic Susceptibility Studies. In order to gain insight into the dynamics of magnetization relaxation for complexes 1−3, ac susceptibility data were collected in the absence of an external magnetic field at frequencies between 0.05 and 1500

Figure 2. Coordination polyhedra observed in 1−3.

Table 1. Selected Bond Lengths [Å] and Angles [deg] for Compounds 1−3 compound 1 Dy1−O1 Dy1−O2 Dy1−O1* Dy1−N1* Dy1−N3 Dy1−Onitrate

2

Dy1−O(N)solvent

2.324(3) 2.144(3) 2.346(3) 2.544(3) 2.477(3) 2.436(4) 2.419(4) 2.501(5)

2.347(4) 2.149(4) 2.341(4) 2.559(5) 2.475(5) 2.443(6) 2.426(6) 2.312(5)

Dy1−Dy1* Dy1−O1−Dy1* O1−Dy1−O1*

3.9192(13) 114.11(10) 65.89(10)

3.9422(6) 114.47(16) 65.53(16)

3 2.330(7) 2.190(7) 2.437(7) 2.581(9) 2.513(9) 2.489(9) 2.580(10) 2.329(8) (O9) 2.508(8) (O8) 4.0151(8) 114.7(3) 65.3(3)

distance occurring at the axial phenoxo-O atom (d(Dy1−O2) = 2.1 Å) showing larger negative charge (Table S2), indicating the strongest electrostatic interaction, which favors an axially enhanced ligand field. Furthermore, the presence of the second DMF molecule in 3 results in the marked increase of all bond distances associated with the DyIII ion, such as the Dy−Onitrate distance elongated from 2.419(4) to 2.580(10) Å and the average Dy−N distances from 2.507 to 2.547 Å, which leads to an expanded coordination sphere and unexpected weakened ligand field in 3. Notably, the changes of local coordination geometry in complex 3 lead to an important distortion of whole molecular structure relative to complexes 1 and 2, where two hydrazone ligands are almost parallel. Examination of the packing diagrams of 1−3 (Figures S2 and S3) reveals that the closest intermolecular Dy···Dy distances are 8.546, 9.257, and 9.864 Å, suggestive of negligible intermolecular exchange interactions. Static Magnetic Studies and Theoretical Calculations. The static magnetic measurements for compounds 1−3 were carried out under an applied field of 1000 Oe from 300 to 2 K using a Quantum-Design MPMS magnetometer. As depicted in Figure 3, the values of χMT product at room temperature are 27.48, 26.90, and 29.50 cm3 K mol−1, which are close to the 7884

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to 0.1 Hz in frequency-dependent measurements, the peaks of χ″ signals were detected below 5 K for complexes 1 and 2 (Figures S6 and S7), and the extracted relaxation times heavily deviate from the thermally activated Arrhenius region. However, such peaks cannot be observed even at 4.5 K in complex 3. Thus, the limited range of frequencies available to us prevented further quantitive measurements, but this evidenced the extremely slow tunneling relaxation rate with τQTM larger than 1592 ms (υ = 0.1 Hz) at least. Furthermore, when compared to the asymmetrical Dy2 SMM reported by some of us (Figure S12),9b the trend of the ln(τ/s) vs 1/T curve reveals the possibly longer tunneling relaxation time present in complex 3 (τQTM > 35 s), which is consistent with the occurrence of coercive field. Here, the strong suppression of zero-field QTM is due to the critical synergic effect between high single-ion anisotropy and strong magnetic interactions in 3 which forces the thermally activated relaxation process over a much broader temperature range (vide inf ra), while the diluted studies and ab initio calculations will give further evidence. For 1−3, the thermally activated relaxation in high-temperature domains obeys an Arrhenius-like behavior (τ = τ0 exp(Ueff/ kBT)), where the energy barriers to magnetization reversal and pre-exponential factors are fitted to be 72 K (50 cm−1, τ0 = 2.4 × 10−6 s), 78 K (54 cm−1, τ0 = 3.3 × 10−7 s), and 155 K (108 cm−1, τ0 = 3.5 × 10−8 s), respectively (Figure S13). The Cole−Cole plots of χ″ versus χ′ for 1−3 fitted to a generalized Debye model are shown in Figures S14−S16. For 1 and 2, nearly symmetric semicircles are observed above 1.9 K with α parameters below 0.16 and 0.08, respectively, implying a narrow distribution of relaxation times, and the increasing α values upon decreasing temperature are most likely due to the coexistence of fast QTM and thermally induced relaxation pathways. In contrast, the smaller parameters α in the range of 0.003−0.033 between 8 and 19 K suggest a much narrower

Figure 4. Magnetic hysteresis loops for Dy2-2DMF (3).

Hz. As shown in Figure 5 and Figures S6−S11, strong frequency- and temperature-dependent in-phase (χ′) and outof-phase (χ″) magnetic susceptibility signals with peak maxima were observed at the corresponding frequency and temperature range for all three complexes, as is characteristic of typical SMM behavior. A close comparison of the χ″(T) plots between these complexes reveals the significant difference happening in the low-temperature region (below 5 K), in which out-of-phase (χ″) components of 1 and 2 gradually increase upon decreasing the frequency, indicating the onset of fast tunneling of magnetization (QTM) at zero dc field; i.e. the magnetic moments are not frozen completely. It is noted that both χ′ and χ″ components for 3 vanish completely even at the frequency as low as 10 Hz, which is rare in lanthanide SMMs, suggesting the highly efficient suppression of zero-field QTM in complex 3. More obviously, when the ac frequency range was extended

Figure 5. Temperature (top) and frequency (bottom) dependence of the out-of-phase ac susceptibility under zero dc field for 1 (left), 2 (middle), and 3 (right). 7885

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2 to 3 seem to be consistent with the above structural analyses, where the coordination sphere is expanded due to the second coordinating DMF molecule, thus weakening the crystal field splitting of Dy ions in complex 3. Surprisingly, complex 3 functions as the best SMM with the highest effective barrier among them. The close comparison of transverse g components (gx, gy) of the ground state between them reveals that the corresponding values for complex 3 are smaller than those of complexes 1 and 2 by an order of magnitude, suggesting a higher degree of axiality present in complex 3. As a result, the tunneling relaxation process is suppressed more effectively in complex 3, as seen in diluted samples where quantum tunneling time τQTM of complex 3 is close to 20 times as long as that of complex 2. Important for the above analysis is the fact that, instead of attempting to increase crystal field splitting, the expansion of the coordination sphere in complex 3 leads to the overall decline of crystal field splitting, but surprisingly decreases the transverse components of the crystal field efficiently, thus greatly improving its SMM properties. The main reason is that the values of gx and gy are far smaller than gz in a strongly unaxial system; thus, the expansion of coordination sphere has a more pronounced impact on transverse anisotropy of the ground state over axial anisotropy. In addition, the comparison of calculated energy separations with the above experimental effective barriers indicates that the Orbach process is possible only in complex 3, because the effective barriers in complexes 1 and 2 are far smaller than the corresponding energy separations between the ground and the first-excited doublets.19 Therefore, for complexes 1 and 2, the Orbach process should be negligible. We further plotted the ln(τ/s) vs ln(T/K) curves, revealing two different linear regions at low and high temperature, respectively, for both complexes (Figure S21). Their linear fitting gave the critical n values for τ−1 ∼ Tn (n = 1.62 and 5.40 for 1, n = 1.68 and 5.38 for 2), indicating the presence of direct and Raman processes at low and high temperature, respectively. Importantly, we refitted the ln(τ/s) vs 1/T plots through a sum of two relaxation processes (τ−1 = AT + BTn) for complexes 1 and 2, and hence a good fitting gave A, B, and n values (Table S5). In comparison, the ln(τ/s) vs ln(T/K) plots of complex 3 present a single linear region with n = 7.13 (τ−1 ∼ Tn), but the plots at high temperature were not fitted very well, indicating the presence of Raman and possible Orbach processes. We fitted the ln(τ/s) vs 1/T plots of complex 3 through a sum of Raman and Orbach processes (τ−1 = BTn + τ0−1 exp(−Ueff/kBT)) and fixed the Ueff to be 200 K (139 cm−1) according to the above calculations, thus giving B = 6.39 × 10−6 and τ0 = 1.21 × 10−8 s, which are well in the corresponding reasonable region (Figure 6). For the diluted samples, the data can be fitted well under zero dc field with the addition of the QTM process, with the parameters shown in Table S5.4b Here, all ln(τ/s) vs 1/T plots were well fitted, and the obtained parameters are all in the reasonable range. In summary, although the energy separations between the ground and the first-excited doublets in complexes 1 and 2 seem to be rather large, the Orbach process indicating the critical magnetic bistability has no contribution to relaxation during the measured range of frequency. In contrast, the Orbach process gets to function in magnetization relaxation of complex 3, leading to the stronger SMM behavior, which greatly benefits from the key modulation of the crystal field in complex 3. The magnetic susceptibilities of complexes 1−3 were simulated with the program POLY_ANISO20 using the

distribution of relaxation times and the presence of a single relaxation mechanism. Diluted Studies. The magnetically diluted investigations into complexes 2 and 3 with 95:5 of Y:Dy percentage ratios further elucidate the mechanism of the slow relaxation of the magnetization in such Dy2 SMMs, especially the effects of exchange interactions on the dynamic magnetic properties.17 As shown in Figure S17, ac susceptibility measurements gave similar out-of-phase signals to those of undiluted samples at the high-temperature region (>5 K), while the χ″ values obviously increase with the temperature further decreasing, indicating the onset of the QTM process concomitant with the disappearance of magnetic interactions between Dy centers. Furthermore, the relaxation time can be extracted from the χ″ vs υ plots (Figure S18) and thus Arrhenius fitting gave their effective barriers, Ueff = 49 and 96 cm−1, for the diluted samples of complexes 2 and 3, respectively, which are comparable to the corresponding undiluted samples (Figure S19). Critically, the temperatureindependent regime at low temperature reveals the quantum tunneling time τQTM of complex 3 (763 ms) almost 20 times as large as that of complex 2 (39 ms), indicating the significant contribution of enhanced single-ion anisotropy in suppressing the QTM process in complex 3. The clearer and more important difference occurs in the M (H) hysteresis measurements for the undiluted and diluted systems. Both diluted samples exhibit butterfly-shaped hysteresis, similar to what was observed for some SIMs, and the opening becomes clearer in complex 3, but the remanence is absent anyway (Figure S20). When it comes to the undiluted samples, only complex 3 succeeds to the hysteretic nature of magnetization, highlighting the critical effects of enhanced ferromagnetic interaction on slowing down under barrier relaxation processes, which are further elucidated in the following calculations. Quantum Chemical Calculations. Ab initio CASSCF/ RASSI/SINGLE_ANISO calculations have been carried out with MOLCAS 7.818 on individual DyIII fragments of complexes 1−3 on the basis of X-ray determined structures (see the Supporting Information for details). The lowest Kramers doublets and the g tensors corresponding to the pseudo spin S = 1/2 of DyIII ions for three complexes are shown in Tables 2 and S4, where the energy separations Table 2. Energies (cm−1) of the Lowest Kramers Doublets and Their g (gx, gy, gz) Tensors on Individual DyIII Fragments for Complexes 1−3 1 E 0.00

213.11

2

g (S̃ = 1/2) 0.0033 0.0062 19.6665 0.0995 0.1199 17.0032

E 0.00

203.84

3

g (S̃ = 1/2) 0.0071 0.0139 19.6121 0.0774 0.0963 16.9938

E 0.00

139.49

g (S̃ = 1/2) 0.0001 0.0024 19.6859 0.0495 0.0665 17.2874

between the ground and the first-excited doublets for the DyIII fragments of 1−3 are 213.1, 203.8, and 139.5 cm −1, respectively. Herein, the calculating gz values for all three complexes approach the limiting value of 20 expected for a ground Kramers doublets with the pure mJ = 15/2, confirming strong unaxial anisotropy of the DyIII ion and supporting their typical SMM behavior.8a Furthermore, the decreasing energy separations between the ground and excited doublets from 1 or 7886

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Table 4. Calculated Energies (cm−1), the Corresponding Tunneling Gaps (cm−1), and the gz Values of the Low-Lying Exchange Doublet States of Complexes 1−3 Δtun

E 1 2 3

0.00 2.094 0.00 2.303 0.00 4.474

1.4 2.6 6.3 1.0 4.3 1.9

× × × × × ×

gz −07

10 10−07 10−07 10−06 10−09 10−08

39.335 0.000 39.226 0.000 39.226 0.000

complexes 1 and 2, suggesting the extremely efficient suppression of the tunneling relaxation process in complex 3.22 To gain a deeper understanding into the enhanced exchange interaction Jexch in complex 3, the easy-axis anisotropic directions (gz) of Dy centers are given from two different directions in Figure 7 for all complexes. Herein, the gz direction

Figure 6. Plots of ln τ versus T−1 for 1−3 and the diluted samples of 2 and 3 under zero dc field. The solid lines correspond to the best fit of the experimental data to equation, τ−1 = AT + BTn + τ0−1 exp(−Ueff/ kBT) + τQTM−1 (the right terms correspond to direct, Raman, Orbach, and tunneling processes).

exchange parameters from Table 3. All parameters were calculated with respect to the pseudospin S = 1/2 of the Dy Table 3. Fitted Coupling Parameters (J = Jexch + Jdip) (cm−1) within the Lines Model of Complexes 1−3 1 2 3

Jexch

Jdip

J

−0.75 −0.03 5.00

5.22 5.01 4.69

4.47 4.98 9.69

ions. A great agreement was reached between the calculated and experimental χMT versus T plots of three complexes, as shown in Figure 3. From Table 3, the fitted DyIII-DyIII couplings of three complexes within the Lines model21 are all ferromagnetic, and the total coupling parameters J (dipolar and exchange) were included to fit the magnetic susceptibilities. Herein, the ferromagnetic dipolar interactions, Jdip, could be calculated exactly based on the calculated orientations of local anisotropy axes and g tensors, while the exchange interactions, Jexch, were obtained by fitting the static magnetic data. Therefore, the Jdip of three complexes show the similar values given that the main magnetic axes on two DyIII ions of 1−3 (Figure S22) are parallel to each other and deviate from the Dy-Dy vector by a small and similar angle. Critically, the pure exchange contribution (Jexch) to the interactions is distinct in complex 3 from that in complexes 1 and 2, where Jexch is almost negligible. In complex 3, the exchange interactions are effectively increased to the same order of magnitude and with the same sign to the dipolar interactions, thus resulting in the total interaction J being 2 times larger than that in complexes 1 and 2. Furthermore, the spectrum of the lowest exchange multiplets, the corresponding tunneling gaps (Δtun) and gz values are given in Table 4 based on the above interaction parameters. Naturally, the enhanced magnetic interactions in complex 3 relative to complexes 1 and 2 generate a larger energy gap between the two lowest exchange Ising doublets, which greatly increases the difficulty of spin reversal via excited doublets at the low-temperature exchange region. As a step further, all Dy2 complexes exhibit small tunneling gaps (Δtun) for both lowest exchange Ising doublets, but the Δtun values in complex 3 are 2 orders of magnitude lower than that in

Figure 7. Orientations of the local main magnetic axes of the ground doublets on DyIII ions for compounds (a) 1, (b) 2, and (c) 3.

of an individual Dy ion in three complexes seems to be similar, that is, the direction almost parallel to the shortest Dy−O bonds (Dy−O2), which is also consistent with the structural analysis of hula-hoop-like geometry (Figure S22) and the theoretical prediction of electrostatic model.3c−e However, the molecular structure distortion due to one more coordinating DMF results in a different arranged mode of anisotropy axes in complex 3 relative to the other two complexes. As is shown in Figure 7, the anisotropy axes of Dy centers in complexes 1 and 2 almost completely lie within the Dy2O2 plane with a small angle with a Dy-Dy link, whereas it is obvious that, in complex 3, the axes deviate out of the Dy2O2 plane. Here, although we cannot give a clear relationship between magnetic interactions and structure details, such a special arrangement most likely generates a suitable overlap between lanthanide orbitals and valency orbitals of the bridging atoms to enhance the superexchange interactions between lanthanide ions in complex 3. In fact, the magnetic interactions between anisotropic lanthanide ions are proven to be extremely anisotropic and very sensitive to the arrangement of anisotropy axes in the EPR studies of the Dy2 complex from the van Slageren group.12 The earlier theoretical investigations by Mironov on superexchange interaction between lanthanide ions also revealed that the LnIII−L−LnIII angles and good overlap between the lanthanide’s and the ligand’s orbitals are even more important than the LnIII···LnIII distance in deciding the superexchange 7887

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Inorganic Chemistry mechanism.23 Therefore, it should be reasonable for the above analysis that the arranged mode of anisotropy axes in complex 3 plays a decisive role in enhancing the exchange interaction between Dy ions. As a result, the critical synergic effect between enhanced single-ion anisotropy and strong magnetic interactions is achieved perfectly on suppressing zero-field QTM in complex 3.

Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Corresponding Authors

*E-mail: [email protected] (J.T.). *E-mail: [email protected] (Y.-Q.Z.). *E-mail: [email protected] (P.Z.).

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CONCLUSION The high tunability of hydrazone-based lanthanide complexes and a careful engineering of experiment conditions allow the isolations of three typical Dy2 SMMs (1, 2, 3), which exhibit similar motifs where two metal centers are bridged by the alkoxido groups of two “head-to-tail” hydrazone ligands. Nevertheless, in spite of the seemingly structural similarity, the existing discrepancy in terminal ligands (solvents and NO3−) not only alters the local crystal field around Dy centers but also indirectly affects the consistency of whole molecular structures between them. Specifically, ab initio calculations reveal that one more coordinating DMF molecule in complex 3 leads to obviously decreasing gx,y values of Dy anisotropy and, more importantly, the resulting distortion of molecular structure generates a different arrangement between Dy anisotropy axes and the Dy2O2 plane when compared with complexes 1 and 2. Such a special arrangement most likely enables a suitable overlap between lanthanide orbitals and valency orbitals of the bridging atoms to enhance the superexchange interactions (Jexch) between Dy ions, hence the overall interaction J = Jexch + Jdip in complex 3 being almost twice as large as that in complexes 1 and 2. Finally, the synergistic play of strong single-ion anisotropy and enhanced magnetic interactions on suppressing QTM drives complex 3 to be the best SMM. This study demonstrated that the exchange magnetic interactions (Jexch) between lanthanide ions could be effectively enhanced by means of the intelligent distortion of molecular structure based on a deliberate local structural modulation, which is closely correlated to the alignment of anisotropy axes and resulting orbital overlap between lanthanide and bridging atoms. Thus, it is proved to be a viable means to improve SMM behavior of lanthanide complexes. Of course, in order to achieve the final target, extra efforts to increase the blocking temperature are required, especially the combination with other ideas that are already being applied with success, such as using radical and large aromatic bridges.9a,c

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AUTHOR INFORMATION

ORCID

Yi-Quan Zhang: 0000-0003-1818-0612 Jinkui Tang: 0000-0002-8600-7718 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21525103, 21371166, 21331003, and 21521092) and the Natural Science Foundation of Jiangsu Province of China (Grant BK20151542) for financial support. J.T. gratefully acknowledges support of the Royal Society-Newton Advanced Fellowship (NA160075).

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REFERENCES

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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.7b00625. Experimental procedures, IR spectra (Figure S1), crystallography (Figures S2 and S3 and Table S1), magnetic properties measurements (Figures S4−S21 and Table S3), and computational details (Tables S2, S4, and S5 and Figure S22) (PDF) Accession Codes

CCDC 1507443−1507447 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The 7888

DOI: 10.1021/acs.inorgchem.7b00625 Inorg. Chem. 2017, 56, 7882−7889

Article

Inorganic Chemistry

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