Modulating Slow Magnetic Relaxation of Dysprosium Compounds

Oct 25, 2017 - The position change of coordinating nitrate group in two novel dysprosium complexes with chain or dinuclear structures induces distinct...
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Cite This: Inorg. Chem. 2017, 56, 13430-13436

Modulating Slow Magnetic Relaxation of Dysprosium Compounds through the Position of Coordinating Nitrate Group Fang Ma,† Qi Chen,† Jin Xiong,§ Hao-Ling Sun,*,† Yi-Quan Zhang,*,‡ and Song Gao*,§

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Department of Chemistry and Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University, Beijing 100875, P. R. China ‡ Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China § College of Chemistry and Molecular Engineering, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: A chain complex [Dy(L)(NO3)2CH3OH]n (1) and a dinuclear compound [Dy2(L)2(NO3)4(CH3OH)2]·2CH3OH (2) were synthesized by the assembly of a novel pyridine-N-oxide-containing ligand with dysprosium nitrate under different reaction temperatures, where two coordinating nitrates are located in para or ortho position with respect to each other around dysprosium ions. Magnetic studies indicate that the chain complex with two para-coordinating nitrates shows fast quantum tunnelling of the magnetization under zero direct-current field, while the dinuclear complex with two ortho-coordinating nitrates exhibits a thermal-activated process with an effective energy barrier of 51 K. Theoretical and magnetostructural correlation studies indicate that position change of coordinating nitrates can significantly modulate the crystal field around dysprosium ion and further lead to their different relaxation behaviors.

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because they provide main contribution to crystal field thus leading to a different magnetic anisotropy.5 In addition to the above-mentioned factors caused by the ligands around lanthanide ions, stimulation from solvent molecules within the lanthanide crystals and magnetic coupling between magnetic centers can also dictate their magnetic behaviors.6 Therefore, continuous efforts should be devoted to explore effective strategies to modulate coordination environment and further control the magnetic properties. In our previous work, we found that introduction of pyridineN-oxide (PNO) group into pyridine carbohydrazine-based Schiff base ligands can significantly increase their coordination capabilities with lanthanide ions and result in novel compounds with chain or layer structures that show slow magnetic relaxation with relatively high energy barriers.6a,7 Inspired by this finding, we attempted to employ nitrate anion to investigate the influence of different anions on the final crystal structure and magnetic behavior. Herein, we report the synthesis and magnetic study of two novel dysprosium compounds, namely, [Dy(L)(NO 3 ) 2 CH 3 OH] n (1) and [Dy2(L)2(NO3)4(CH3OH)2]·2CH3OH (2) (HL = N′-(2hydroxybenzylidene)pyridine-N-oxide-carbohydrazide)), which feature a chain or dinuclear structure, respectively, and were obtained under different reaction temperatures. The coordina-

INTRODUCTION Molecular nanomagnets have been widely synthesized and magnetically characterized, as they are potentially applicable to high-density information storage and spin-based devices.1 Among all molecular nanomagnets, the lanthanide-based molecule nanomagnets have attracted particular attention due to their strong intrinsic spin−orbit coupling and significant magnetic anisotropy. So far, a variety of lanthanide molecular nanomagnets based on dysprosium, erbium, and terbium have been reported,2,3 and the records of both effective energy barrier (Ueff) and blocking temperature (TB) were repeatedly broken.3 Although many lanthanide molecular nanomagnets have been successfully constructed, the rational designing of lanthanide molecular nanomagnets is still challenging due to numerous factors, where the magneto-structural correlation analysis sometimes must be done on a case-by-case basis. Researchers have found that subtle alteration to the coordination environment on local metal centers can strongly influence the relaxation dynamics of lanthanide molecular nanomagnets. It is also well-known that change in coordination atoms and coordination number and the resulting variation of local coordination geometry play an important role on the relaxation behavior of lanthanide complexes, because they can result in different magneto-crystalline anisotropy of lanthanide ions.4 Furthermore, metal−ligand covalent effects and charge distribution around lanthanide ion are also key factors governing the magnetic behavior of lanthanide compounds, © 2017 American Chemical Society

Received: August 22, 2017 Published: October 25, 2017 13430

DOI: 10.1021/acs.inorgchem.7b02162 Inorg. Chem. 2017, 56, 13430−13436

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Inorganic Chemistry tion species around Dy3+ ions in both complexes are identical; however, the two coordinating nitrates in each compound are in either para or ortho position with respect to each other. Importantly, magnetic studies indicate that the two complexes show different magnetic relaxation behaviors under zero directcurrent (dc) field, indicating the subtle effect of nitrate positioning and potential use of such a structural manipulation to modulate the magnetic properties.

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2 were obtained. Yield: 82.5 mg (68.1% based on the metal salt). Anal. Calcd for C30H34Dy2N10O22: C, 29.79, N, 11.58, H, 2.67; found C, 29.72, N, 11.54, H, 2.92%. IR (KBr, cm−1): 3387(br), 1639(s), 1610(s), 1546(m), 1480(s), 1437(m), 1384(s), 1295(s), 1009(m), 765(m).

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EXPERIMENTAL SECTION

X-ray Crystallography and Physical Measurement. Intensity data for crystals of 1 and 2 were collected on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo Kα radiation (0.710 73 Å) at 296 K. The structures were solved by direct methods and refined with the full-matrix least-squares technique based on F2 using the SHELXL program. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed at the calculation positions. The details of crystallographic data and selected bond parameters for 1 and 2 are listed in Tables 1 and S1, respectively.

Table 1. Crystallographic Data and Structure Refinement for 1 and 2 formula Mr crystal system space group A (Å) B (Å) C (Å) Α (deg) Β (deg) Γ (deg) V (Å3) Z Μ (mm−1) F(000) GOF data collected unique Rint R1,wR2[I > 2σ(I)] R1, wR2 [all data]

1

2

C14H13DyN5O10 573.79 orthorhombic Pna21 19.233(1) 12.250(1) 7.962(1) 90 90 90 1876.0(2) 4 4.049 1108 1.052 10 922 3768 0.0172 0.0310, 0.0869 0.0321, 0.0876

C30H34Dy2N10O22 1211.67 monoclinic P21/n 14.244(1) 10.591(1) 15.398(1) 90 116.907(1) 90 2071.2(2) 2 3.677 1184 1.022 10 367 4532 0.0448 0.0364, 0.0919 0.0455, 0.1048

RESULTS AND DISCUSSION

Crystal Structure of 1. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the orthorhombic space group Pna21 and exhibits a chain structure (Table 1 and Figure 1). The asymmetric unit of 1 is composed of one Dy3+ ion, one L− ligand, two coordinating nitrates, and one coordinating methanol molecule. As shown in Figure 1b, each Dy3+ ion is nonacoordinated with a hula-hooplike coordination geometry, in which the equatorial site is defined by one pyridine-N-oxide oxygen atom (O1), three oxygen atoms from two nitrates (O4, O5, O7), and one imine nitrogen atom (N3). Additionally, phenoxide oxygen atom (O3), carbonyl oxygen atom (O2), oxygen atom (O8) from nitrate, and one methanol molecule (O10) are separately located above and below the equatorial plane (Figure 1b). The Dy−O distances range from 2.192 to 2.585 Å (Table S1). It is noteworthy that the two coordinating nitrates lie in a para position with respect to each other with a N−Dy−N angle of 172.67°. In 1, the aroylhydrazone part of the ligand retains the original keto state, and one side of the ligand serves as a tridentate unit and chelates the Dy3+ ion through two oxygen atoms (O2 and O3) and one imine nitrogen atom (N3), while the pyridine-N-oxide part of L− coordinates to a neighboring Dy3+ ion and links them to form a chain structure with an intrachain Dy···Dy distance of 7.517 Å (Figure 1c). The adjacent chains are further linked by van der Waals forces between them, resulting in a three-dimensional supramolecular structure (Figure S1b). Crystal Structure of 2. When the reaction mixture was heated at 60 °C for 72 h, yellow crystals of [Dy2(L)2(NO3)4(CH3OH)2]·2CH3OH (2) were obtained. Complex 2 crystallizes in the monoclinic space group P21/n and exhibits a discrete dinuclear structure (Table 1 and Figure 2a). The asymmetric unit of 2 is composed of one Dy3+ ion, one L− ligand, two coordinating nitrates, one coordinating methanol molecule, and one lattice methanol molecule, which is similar to the overall coordination environment in 1. However, in contrast to 1, the two coordinating nitrates in 2 are located in an ortho configuration around the Dy3+ ion with a N−Dy−N angle of 91.86°. The Dy−O distances cover the range of 2.232−2.569 Å (Table S1). In 2, the aroylhydrazone part of the ligand similarly retains the original keto state, and one side of the ligand is in a tridentate mode and chelates one Dy3+ ion, and the pyridine-N-oxide part of L− coordinates to a neighboring Dy3+ ion, which also resembles that found in 1. The two neighboring Dy3+ ions are linked with each other by two pyridine-N-oxide groups, forming a dinuclear structure with an intradimer Dy···Dy distance of 6.890 Å. Adjacent dinuclear units are further linked by numerous hydrogen bonds to form a three-dimensional supramolecular framework (Figure S2 and Table S2). Powder XRD Pattern and TGA Data. To confirm the phase purity, complexes 1 and 2 were analyzed by powder Xray diffraction (XRD) at room temperature using the same microcrystalline samples from the magnetic measurements (Figure S3). The diffraction peaks of the as-prepared samples are in good agreement with the corresponding simulated

Syntheses of HL. The Schiff base HL ligand is synthesized by condensation of pyridine-N-oxide-2-carbohydrazide and salicyladehyde in 1:5 ratios in methanol, and then recrystallization in 1, 2dichloroethane solvent. All the starting materials were commercially available reagents for analytical grade and used without further purification. [Dy(L)(NO3)2CH3OH]n (1). HL (0.2 mmol, 48.2 mg) was dissolved in 8 mL of methanol; then, methanolic solution of NaOH (4.0 mL, 0.05 mol·L−1) was added to the solution, and the reaction mixture was stirred for 2 min, after which solid Dy(NO3)3·6H2O (0.2 mmol, 91.3 mg) was added, and the orange solution was continually stirred for 20 min. The mixture was transferred to a 20 mL Teflon reactor, heated to 80 °C for 72 h under autogenous pressure, and then cooled to room temperature at a rate of 5 °C·h−1. The red block crystals of 1 were obtained. Yield: 50.9 mg (44.2% based on the metal salt). Anal. Calcd for C14H13DyN5O10: C, 29.31, N, 12.21, H, 2.28; found C, 29.29, N, 12.17, H, 2.20%. IR (KBr, cm−1): 3425(br), 1635(s), 1608(s), 1541(m), 1492(s), 1384(s), 1286(s), 1031(m), 754(m). [Dy2(L)2(NO3)4(CH3OH)2]·2CH3OH (2). The synthetic procedure is similar to that of 1, but instead the reaction mixture was heated at 60 °C for 72 h under autogenous pressure. The yellowish block crystals of 13431

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Inorganic Chemistry

Figure 1. Local structure (a), coordination polyhedron around Dy3+ ion (b), and chain structure (c) of 1. Hydrogen atoms and solvent molecules are omitted for clarity.

Figure 2. Dinuclear structure (a) and coordination polyhedron around Dy3+ ions (b) of 2.

patterns calculated from the single-crystal XRD data, demonstrating a high phase purity of the experimental samples. To investigate the thermal stability and further affirm the exact molecular formula of the compounds, the experiments of thermogravimetric analysis (TGA) were performed on samples of 1 and 2 in the temperature range of 25−1000 °C under a synthetic air atmosphere with a heating rate of 5 °C min−1 (Figure S4). The TGA data indicate compound 1 is stable up to ca. 200 °C and then suffers a weight loss from 200 to 250 °C. Subsequently, without any platform, the weight keeps losing to reach the final weight of 33.6%, which is consistent with a residue of Dy2O3 (calcd 32.8%). Complex 2 is stable up to 160 °C and then suffers a weight loss from 160 to 200 °C with a short platform, corresponding to the loss of two lattice and two coordinating methanol molecules (obsd 10.5%, calcd 10.6%). The remaining weight of 31.6% indicates that the final product is Dy2O3 (calcd 31.0%). Magnetic Behavior of 1. The dc magnetic susceptibility measurements for 1 were performed in the temperature range of 2−300 K under 1 kOe dc field. As shown in Figure 3, the χMT values of 1 at room temperature is 13.87 cm3 mol−1 K, which is slightly smaller than the theoretical value of 14.17 cm3 mol−1 K for one Dy3+ ion with the ground state 6H15/2 and g = 4/3.8 The χMT values for 1 gradually reduce as the temperature decreases, reaching 9.87 cm3 mol−1 K at 2 K. The thermal evolution of 1 is mainly ascribed to the progressive

Figure 3. Temperature dependence of χMT products for 1 and 2. The black and red line are the simulation from ab initio calculation. (inset) Plots of M vs H/T for 1 and 2 at 2, 5, and 10 K, respectively.

depopulation of the excited Stark sublevels and/or significant magnetic anisotropy in Dy3+ ion systems. Magnetization data for 1 were collected in the 0−70 kOe dc field at 2, 5, and 10 K (inset of Figure 3). The non-superimposition of the M versus H/T plots on a single master curve suggests the presence of significant magnetic anisotropy caused by crystal-field effect. Alternating-current (ac) magnetic susceptibility measurements were also performed for 1 under zero dc field to further explore the dynamics of magnetization. Both in-phase (χM′) 13432

DOI: 10.1021/acs.inorgchem.7b02162 Inorg. Chem. 2017, 56, 13430−13436

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Inorganic Chemistry and out-of-phase (χM″) signals of 1 show temperature and frequency dependency, indicating slow relaxation of the magnetization (Figure 4). With decreasing temperatures, the

Figure 5. Temperature dependence of the in-phase χM′ (top) and outof-phase χM″ (bottom) ac signals under zero dc field for 2.

Ueff of 52 K and a pre-exponential factor of 1.6 × 10−6 s (Figure 6). Furthermore, the Cole−Cole diagram is created based on Figure 4. Temperature dependence of the in-phase χM′ (top) and outof-phase χM″ (bottom) ac signals under zero dc field for 1.

increase of χM′ and χM″ is observed, but the values do not pass through the characteristic maximum at 2 K, which could be due to fast quantum tunnelling of the magnetization. To suppress the quantum tunnelling, 1.5 kOe dc field was applied. As shown in Figure S5, both the χM′ and χM″ signals of ac susceptibility exhibit maximum values. The maxima shift to higher temperatures with an increasing frequency, while low-frequency peaks occur in the lower-temperature region, which is characteristic of a super-paramagnet. The relaxation time (τ) can be extracted by fitting χM″ versus f curves at different temperatures using an extended Debye model (Table S3 and Figures S6 and S7). Fitting ln(τ) versus T −1 plots to the Arrhenius law affords an anisotropy energy barrier of 36 K with a pre-exponential factor of 8.4 × 10−6 s (Figure S8). Magnetic Behavior of 2. The variable-temperature magnetic susceptibility measurement for a collection of crystals 2 was also investigated in the temperature range of 2−300 K under 1 kOe dc field (Figure 3). The experimental χMT value of 13.96 cm3 mol−1 K at room temperature is slightly smaller than the expected value of 14.17 cm3 mol−1 K for one uncoupled dysprosium ion with a ground state 6H15/2 and g = 4/3. Similar to 1, The χMT values for 2 gradually reduce as the temperature decreases, reaching 11.38 cm3 mol−1 K at 2 K, similarly denoting the progressive depopulation of the excited Stark sublevels and/or significant magnetic anisotropy in Dy3+ ion systems. The field dependence of the magnetization of 2 was investigated in the range of 0−70 kOe dc field at 2, 5, and 10 K (inset of Figure 3). The non-superposition of the M versus H/ T plots at higher fields suggest the presence of significant magnetic anisotropy caused by the crystal-field effect in 2. To further probe the dynamics of magnetization, ac susceptibility measurements were also conducted for 2. Under zero dc field, strong temperature and frequency dependencies of χM′ and χM″ signals can be observed (Figure 5). Different from that found in 1, clear maxima of χM′ and χM″ signals can be found in the temperature range of 9−15 K, which suggests the existence of a thermal-activated relaxation process. Below 9 K, the observed increase of χM′ and χM″ is due to the quantum tunnelling process.9 The relaxation time of 2 can also be extracted from the frequency-dependent ac data (Figure S9 and Table S4). In the high-temperature region, the relaxation process of 2 follows the Arrhenius law with an effective barrier

Figure 6. Plot of ln(τ) vs T−1 for 2 under zero dc field.

the frequency dependencies of the ac susceptibility data, which is fitted to a generalized Debye model, giving rise to a narrow distribution of coefficient α parameters in the range of 0.001− 0.060 (Figure S10 and Table S4). This result is consistent with the presence of a unique coordination sphere of Dy3+ ion in 2. To suppress the quantum tunneling of magnetization (QTM) in 2 at low temperatures, ac susceptibility measurements were performed at various frequencies under 1.5 kOe dc field. As shown in Figure S11, the tail value of χM″ clearly decreases, and the corresponding temperature-dependent out-of-phase maximum signal peaks can be observed for all frequencies used for measurement. The relaxation time deduced from the frequencydependent ac data in the high-temperature range (5−11 K) nicely follow the Arrhenius equation with an effective energy barrier of Ueff = 76 K and a pre-exponential factor of τ0 = 4.1 × 10−7 s (Figures S12−S14 and Table S5). Hysteresis Behaviors of 1 and 2. To further study the dynamic magnetic behavior of 1 and 2, magnetic hysteresis measurements were also performed on microcrystalline samples of 1 and 2 using a SQUID-VSM magnetometer at low temperatures. As expected, 1 does not exhibit clear magnetic hysteresis behavior at 2 K (Figure S15), while 2 displays a clear butterfly-shaped hysteresis loop with a large step around zero field below 4 K under field-sweeping rates of 700 Oe/s (Figure 7). At 2 K, the loops become narrower with the decrease of the field sweeping rate, in agreement with the slow magnetic relaxation found in molecular nanomagnets (Figure S16).1−7 The strong dependence of magnetic hysteresis loops on the 13433

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Inorganic Chemistry

were indicated in Figure S17, where the magnetic axes on Dy3+ ions for 1 and 2 have the opposite direction. For 2, the main magnetic axes on two neighboring Dy3+ ions are completely antiparallel, and those of 1 are parallel, forming an angle of 122.78°. We also listed the exchange energies and the main values of the gz for the lowest four or two exchange doublets of 1 and 2 in Table S8, where the gz values of the ground exchange state for 1 and 2 also confirm that the Dy3+−Dy3+ coupling is anti-ferromagnetic within both complexes. To further investigate the mechanism of magnetic relaxation, the transition moments between two lowest Kramers doublets for the Dy3+ ion fragments of 1 and 2 were computed (Figure 8). In both complexes, the most probable pathways for relaxation were found to go through the first excited state (Table S6), respectively. We note that, compared to 2, 1 has a relatively larger transversal component in the ground state, which suggests stronger QTM within the ground Kramers doublet, thus leading to the absence of thermal-activated relaxation processes of 1 under zero dc field. The different magnetic behaviors of the two complexes can be attributed to the unique structural features of each complex and most likely arise from the positional change of two nitrate groups. As depicted in Figure 9, clearly the magnetic easy axes from ab

Figure 7. Hysteresis loop for 2 measured at different temperatures with sweep rates of 700 Oe/s.

temperature and field sweep rates are obviously in accordance with the slow magnetic relaxation suggested by the ac susceptibility data. Theoretical Calculations. To gain more insight into the magnetic anisotropy of Dy3+ ions, complete-active-space selfconsistent field (CASSCF) calculations on one type of individual Dy3+ ion fragment on the basis of the geometries determined by XRD analysis were performed for 1 and 2 using MOLCAS 8.0 and SINGLE_ANISO programs (see Supporting Information for details).10,11 The results provided the lowest spin−orbit energies and corresponding g tensors for 1 and 2 (Table S6). The calculated local g tensors for both complexes on the Dy3+ sites are strongly axial with a gz value of 19.54 and 19.64 for 1 and 2, respectively, close to the Ising limit condition. Therefore, Dy3+−Dy3+ interactions for 1 and 2 can be approximately regarded as the Ising type. The program POLY_ANISO11 was used to simulate the magnetic susceptibilities of 1 and 2. All parameters from Table S7 were calculated with respect to the pseudospin S̃ = 1/2 of the Dy3+ ion. The obtained J values (dipolar and exchange) for 1 and 2 (Table S7) appear negligibly small, suggesting that the slow magnetic relaxation of both complexes originates from individual Dy3+ ion. The calculated and experimental χMT versus T plots of 1 and 2 are shown in Figure 3, where the calculated χMT values match reasonably well with the experimental results. As shown in Table S7, the Dy3+−Dy3+ interactions in 1−2 within Lines model12 are all antiferromagnetic. The main magnetic axes on two Dy3+ ions

Figure 9. Orientations of the local main magnetic axes of the ground doublets on magnetic centers and natural bond order charges per atoms in the ground state of the first coordination sphere of 1 (a) and 2 (b) calculated within CASSCF.

initio calculation on Dy3+ ion for 1 and 2 are both very close to the direction of the Dy−O3 bond, since two O3 possess the largest negative charge, and the Dy−O3 bond features the shortest coordination bond length in two molecules. According to the orientations of the magnetic easy axes, in 1 and 2, the

Figure 8. Magnetization blocking barriers of the individual Dy3+ fragments in complexes 1 (a) and 2 (b). The thick black lines represent the Kramers doublets as a function of their magnetic moment along the magnetic axis. The green lines correspond to diagonal QTM; the blue line represent offdiagonal relaxation process. The numbers at each arrow stand for the mean absolute value of the corresponding matrix element of transition magnetic moment. 13434

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Inorganic Chemistry equatorial sites around Dy3+ ion are occupied by O1, O4, O5, O7, and N3 atoms, and the axial coordination atoms include O2, O3, O8, and O10. In the equatorial positions, the average coordination bond lengths around Dy3+ ion and the average charges of the five atoms are nearly the same (2.463 and 2.483 Å for 1 and 2; −0.595 and −0.600 for 1 and 2), which indicate these atoms offer almost equivalent equatorial ligand field strength in two complexes. Among the axial sites, the largest difference in coordination bonds around the metal centers in 1 and 2 is the one between Dy3+ ion and coordinating methanol. The Dy−O10 distance in 2 is nearly 0.1 Å shorter than that found in 1, which indicates a stronger axial ligand field for 2. The longer bond length of Dy−O10 in 1 may be attributed to the different arrangement of nitrates around the Dy3+ center, which causes stronger repulsive force between the nitrates and O10 with a shorter O5···O10 distance of 2.769 Å and a smaller N4−Dy−O10 angle of 97.5° compared with the corresponding values of 3.247 Å and 102.5° for 2. Furthermore, the different arrangement of the nitrate anions induces distinct angle between the Dy−O8 bond and the magnetic easy axis (123.3° and 128.4° for 1 and 2). The relatively larger angle of 2 makes O8 of nitrate contribute more negative charge to the axial ligand field. Therefore, stronger uniaxial ligand field in 2 can impel the |±15/2⟩ Kramers doublet to be more stable, thus leading to a stronger magnetic anisotropy in 2.

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]. (H.-L.S.) *E-mail: [email protected]. (Y.-Q.Z.) *E-mail: [email protected]. (S.G.) ORCID

Hao-Ling Sun: 0000-0002-2112-4331 Yi-Quan Zhang: 0000-0003-1818-0612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21671024 and 11774178), National Key Basic Research Program of China (2013CB933402), and Beijing Higher Education Young Elite Teacher Project.

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REFERENCES

(1) (a) Clérac, R.; Winpenny, R. E. P. Single-Molecule Magnets and Related Phenomena. Structure and Bonding; Springer, 2016; Vol. 172, pp 35−48.10.1007/430_2015_198 (b) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnet; Oxford University Press, 2006. (c) Sun, H. L.; Wang, Z. M.; Gao, S. Strategies towards single-chain magnets. Coord. Chem. Rev. 2010, 254, 1081−1100. (d) Mannini, M.; Pineider, F.; Danieli, C.; Totti, F.; Sorace, L.; Sainctavit, P.; Arrio, M. A.; Otero, E.; Joly, L.; Cezar, J. C.; Cornia, A.; Sessoli, R. Quantum tunnelling of the magnetization in a monolayer of oriented singlemolecule magnets. Nature 2010, 468, 417−421. (e) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Magnetic memory of a single-molecule quantum magnet wired to a gold surface. Nat. Mater. 2009, 8, 194−197. (f) Yao, X. N.; Du, J. Z.; Zhang, Y. Q.; Leng, X. B.; Yang, M. W.; Jiang, S. D.; Wang, Z. X.; Ouyang, Z. W.; Deng, L.; Wang, B. W.; Gao, S. Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets. J. Am. Chem. Soc.2017, 139, 373−380.10.1021/jacs.6b11043 (2) (a) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (b) AlDamen, M. A.; Clemente-Juan, J. M.; Coronado, E.; MartiGastaldo, C.; Gaita-Arino, A. Mononuclear Lanthanide SingleMolecule Magnets Based on Polyoxometalates. J. Am. Chem. Soc. 2008, 130, 8874−8875. (c) Jiang, S. D.; Wang, B. W.; Su, G.; Wang, Z. M.; Gao, S. A Mononuclear Dysprosium Complex Featuring SingleMolecule-Magnet Behavior. Angew. Chem., Int. Ed. 2010, 49, 7448− 7451. (d) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013, 113, 5110− 5148. (e) Luzon, J.; Sessoli, R. Lanthanides in molecular magnetism: so fascinating, so challenging. Dalton Trans. 2012, 41, 13556−13567. (f) Zhang, P.; Guo, Y. N.; Tang, J. K. Recent advances in dysprosiumbased single molecule magnets: Structural overview and synthetic strategies. Coord. Chem. Rev. 2013, 257, 1728−1763. (g) 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. (h) 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. (i) 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. 2015, 4, 547−554. (j) Wang, H. L.;

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CONCLUSION In summary, the reaction of a pyridine-N-oxide containing Schiff base ligand with dysprosium nitrate under different reaction temperatures produces two dysprosium complexes, in which two coordinating nitrates exhibit a different configuration (i.e., ortho vs para) with respect to each other. Magnetic studies indicate that the chain dysprosium complex with two paracoordinating nitrates shows fast quantum tunneling of the magnetization under zero dc field, while the dinuclear complex with two ortho-coordinating nitrates exhibits a thermalactivated process with an effective energy barrier of 51 K under zero dc field. Further theoretical and magneto-structural relationship studies indicate that the relative position of the two coordinating nitrates can significantly influence the crystal field around Dy3+ ion and result in distinct magnetic anisotropy of Dy3+ ions as well as their different relaxation behaviors.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02162. Experimental and computational detail; selected bond lengths and angles for 1 and 2; relaxation fitting parameters from least-squares fitting of χ(f) data; crystal structure of 1 and 2; XRD spectra of 1 and 2; magnetic data for 1 and 2; the position change of coordinating nitrate group in two novel dysprosium complexes induces distinct magnetic relaxation behaviors, offering a new way to facilitate magnetic relaxation (PDF) Accession Codes

CCDC 1567313−1567314 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 13435

DOI: 10.1021/acs.inorgchem.7b02162 Inorg. Chem. 2017, 56, 13430−13436

Article

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DOI: 10.1021/acs.inorgchem.7b02162 Inorg. Chem. 2017, 56, 13430−13436