A Six-Coordinate Dysprosium Single-Ion Magnet with Trigonal

Jun 21, 2017 - (1) With the development of studies on molecular magnetism, lanthanide-based SMMs have attracted wide attention thanks to the strong an...
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A Six-Coordinate Dysprosium Single-Ion Magnet with TrigonalPrismatic Geometry Shan-Shan Liu,*,†,‡ Yin-Shan Meng,‡ Yi-Quan Zhang,§ Zhao-Sha Meng,‡ Ke Lang,‡ Zhen-Liang Zhu,† Chang-Fang Shang,† Bing-Wu Wang,*,‡ and Song Gao*,‡ †

Beijing Key Laboratory of Fuels Cleaning and Advanced Catalytic Emission Reduction Technology, College of Chemical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China S Supporting Information *

analogue, it is reasonable to conjecture that [(LCO)Dy(N*)2] probably has a similar ligand field, which leads to strong singleaxial anisotropy. Therefore, we synthesized and structurally characterized the complex [(LCO)Dy(N*)2] and studied its magnetic relaxation as well as anisotropy. The complex [(LCO)Dy(N*)2] was synthesized according to the literature method10 under an argon atmosphere without coordinating solvents. Single crystals suitable for X-ray diffraction were grown under −20 °C over several weeks. The single-crystal structural analysis shows that [(LCO)Dy(N*)2] crystallizes in the monoclinic space group C 2/c. The molecular structure consists of one DyIII ion, one [LCO]− ligand, and two [N*]− ligands (Figure 1a). The DyIII ion is coordinated by six atoms, which include two N atoms from two [N*]− ligands, two O atoms, and the two other N atoms from the [LCO]− ligand. The bond lengths of Dy−N([N*]−), Dy−N([LCO]−), and Dy− O([L CO ]−) are 2.296(2), 2.394(2), and 2.4680(17) Å, respectively. The angles of N1−Dy−O1 and N1−Dy1−N2 are 90.43(7)° and 93.96(8)°, respectively. The coordination

ABSTRACT: A mononuclar six-coordinate dysprosium complex was synthesized and structurally and magnetically characterized. X-ray structural analyses show trigonalprismatic coordination geometry of the DyIII center. Slow relaxation of magnetization in the absence of a directcurrent field and magnetic hysteresis up to 3.0 K could be observed, indicating its single-ion-magnet behavior. Arrhenius fitting and ab initio calculations suggest that the magnetic relaxation process may not occur through the Orbach process at high temperatures under the experimental conditions.

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ingle-molecule magnets (SMMs) could maintain a spin orientation below a certain temperature for a long time at the molecular scale, providing the potential to be applied in spintronics devices and high-density information storage.1 With the development of studies on molecular magnetism, lanthanide-based SMMs have attracted wide attention thanks to the strong anisotropy of 4f ions.2 Among them, SMMs with only one paramagnetic ion are also named single-ion magnets (SIMs), where the exchange interactions between paramagnetic ions are generally insignificant.2 In the previously reported lanthanidebased SIMs (Ln-SIMs), the six-coordinate lanthanide-based complexes are relatively rare in that they hardly exhibit slow relaxation under a zero direct-current (dc) field, such as [Dy(H2BPzMe22)3],3 [Zn2DyL2]NO3·H2O,4 [Dy(AlMe4)3],5 and [Yb(H3L)2]3+,6 with the exception of the dysprosium bis(methanediide)7 and {(H3O)[Dy(NA)2]·H2O}n8 complexes. Related theoretical research has provided perspectives that might be of assistance in the design of superior Dy-SMMs. For example, an axial ligand field and short Dy−L bond length are in favor of stabilizing the oblate-shaped |±15/2⟩ Kramers doublet, thereby yielding strong anisotropy.9 Recently, Růzǐ čka et al. reported sixcoordinate complex [(LCO)Y(N*)2]10 (LCOH = {N-[(2-MeO)C6H5]}NC(Me)CHC(Me)N(H){N′-[(2-MeO)C6H5]}, HN* = HN(SiMe3)2), where the negative charges mainly locate on two [N*]− ligands in lieu of the [LCO]− ligand, forming an axial ligand field. Additionally, the bond length of Y−N(N*) is much shorter than those of Y−N(LCO) and Y−O(LCO). As its © 2017 American Chemical Society

Figure 1. (a) Crystal structure and magnetic easy-axis orientation (red arrow) of [(LCO)Dy(N*)2]. Color code: Dy, teal; O, red; N, blue; Si, purple; C, gray. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Dy−N([N*]−) 2.296(2), Dy−N([LCO]−) 2.394(2), Dy−O([LCO]−) 2.4680(17), ([N*]−)N−Dy−N([N*]−) 133.66(11). (b) Coordination polyhedron of the DyIII ion. Received: April 12, 2017 Published: June 21, 2017 7320

DOI: 10.1021/acs.inorgchem.7b00952 Inorg. Chem. 2017, 56, 7320−7323

Communication

Inorganic Chemistry geometry of the DyIII ion deviates from octahedral remarkably, and the continuous-shape-measure method shows that it is close to trigonal-prismatic, with the deviation parameter being 2.361.11 A perspective view of the coordination polyhedron of the DyIII ion is shown in Figure 1b. The shortest distance between two DyIII ions is 9.6215 Å. More detailed crystallographic information is available in Tables S1 and S2. The temperature dependence of the magnetic susceptibility χmT was measured under a 1 kOe applied dc field, with the temperature ranging from 2 to 300 K (Figure 2). The value of

Figure 3. Frequency dependence of the out-of-phase magnetic susceptibility (χ″) in the absence of a static field (left) and under a 1 kOe applied dc field (right) for [(LCO)Dy(N*)2].

Figure 2. Temperature dependence of χmT and χm−1 for [(LCO)Dy(N*)2] under a 1 kOe applied dc field by experimental measurements (circle) and ab initio calculations (line). Inset: FC (filled circles) and ZFC (empty circles) under a 1 kOe applied dc field for [(LCO)Dy(N*)2].

Figure 4. ln τ versus 1/T plots under 0 Oe (left) and 1 kOe (right) dc fields for [(LCO)Dy(N*)2].

⎛ −U ⎞ τ −1 = τ0−1 exp⎜ eff ⎟ + CT n + τQTM −1 ⎝ kBT ⎠

χmT at 300 K was 13.84 emu K mol−1, which approached the value of one free DyIII ion (14.17 emu K mol−1, 6H15/2). Upon lowering of the temperature, the value of χmT decreased gradually to 11.14 emu K mol−1 at 2 K, mainly because of depopulation of the excited Stark sublevels.12 A further measurement of the zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility data shows that, as the temperature decreases, the two data sets start to separate at 3.0 K (inset of Figure 2), indicating the presence of magnetic blocking. The dependence of the alternating-current (ac) magnetic susceptibilities on various temperatures (T, 2−50 K) and frequencies (ν, 100−10000 Hz) was studied to gain insight into its dynamic magnetic behavior. In the absence of an applied dc field, a strong frequency dependence of out-of-phase ac susceptibilities (χ″) can be clearly observed. The peak temperature in the plot χ″ versus T is 32.5 K at 10000 Hz, confirming slow relaxation of magnetization of the complex [(LCO)Dy(N*)2] (Figure 3). In addition, the strong quantum tunneling of magnetization (QTM) dominates at low temperatures (Figures 3 and S1). When a 1 kOe dc field was applied, all peaks of the corresponding out-of-phase signals clearly appear from 13 to 32 K (Figures 3 and S2), indicating that the QTM is suppressed effectively. The temperature dependence of the magnetic relaxation time (ln τ) was analyzed to investigate the effective energy barrier and better understand the relaxation process. In the absence of a static field, the independence of the relaxation time at low temperatures is indicative of a QTM relaxation process. The nonlinear dependence at high temperatures suggests the possible presence of multiple relaxation processes (Figure 4). Therefore, the data were fitted using eq 1, which contains the Orbach (first term), Raman (second term), and QTM (third term) processes

(1)

yielding the effective energy barriers of 190 K (0 Oe, τ0 = 1.7 × 10−7 s, C = 0.19 s−1 K−3.46, n = 3.46, and τQTM = 4.4 ms). In the case of an applied dc field of 1 kOe, the QTM was suppressed, thereby eliminating the third term in eq 1. The fitting affords an effective energy barrier of 262 K, and the parameters are τ0 = 1.5 × 10−8 s, C = 0.048 s−1 K−3.71, and n = 3.71. Further analyses of the individual contribution of each process (Figure S3) indicate that the Raman process dominates in a high-temperature domain. In addition, we found that the data (Figure 4) can also be well fitted by only considering the Raman (high temperatures) and QTM (low temperatures) processes, and the parameters are C = 0.039 s−1 K−4.02 and n = 4.02 (0 Oe, τQTM = 4.3 ms) and C = 0.0012 s−1 K−4.96 and n = 4.96 (1 kOe). To confirm the magnetic blocking observed in the ZFC−FC plot, we measured the magnetic hysteresis loop of [(LCO)Dy(N*)2] at different temperatures with an average 10 Oe s−1 sweep rate of the magnetic field (Figure 5). A butterfly-shaped loop can be clearly observed at 2.0 K and nearly disappears at 3.0 K. The close-up of the hysteresis at zero field arises from a strong quantum tunneling process. To investigate the magnetic anisotropy, we performed ab initio calculations of the CASSCF/RASSI/SINGLE_ANISO type on the electronic structure of [(LCO)Dy(N*)2] using MOLCAS7.813 (Table S3). The calculated χmT versus T plot is nearly identical with the experimental results (Figure 2). Also, the calculated effective gz (19.7396) is very close to the value of 20 expected for a pure |±15/2⟩ Kramers doublet, and the ground-state wave function shows a dominant 98% |±15/2⟩ contribution (1.3% | ±11/2⟩ + 0.6% |±9/2⟩), attesting to the strong magnetic anisotropy in the present complex. However, the transverse components gx (0.0046) and gy (0.0054) are not sufficiently small 7321

DOI: 10.1021/acs.inorgchem.7b00952 Inorg. Chem. 2017, 56, 7320−7323

Inorganic Chemistry

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Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00952. Details of the experiments and ab initio calculations, tables, and magnetic plots (PDF) Accession Codes

CCDC 1529445 contains 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 5. Magnetic hysteresis measured at 2.0 K (left) and 3.0 K (right) at an average 10 Oe s−1 sweep rate of the magnetic field for [(LCO)Dy(N*)2].

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to be ignored because of the strong QTM at low temperature. The magnetic easy axis is approximately parallel with the direction of ([N*]−)N−N([N*]−) (5.9°; Figure 1a), as a consequence of the strong axial-ligand field featured by considerably short bond lengths of Dy−N([N*]−) and the significant charge accumulation of N([N*]−) (LoProp charge 1.28− calculated by natural bond orbital analyses; Table S4). The energy gap between the first and ground-state Kramers doublets is 241.5 cm−1 (345 K), which is significantly larger than the experimentally fitted energy barrier (190 K). This disagreement might be ascribed to the fact that, under our experimental conditions, the temperature is not sufficiently high and the Orbach process cannot dominate in a high-temperature regime, which leads to a large deviation of the fitted energy barrier. Our work exemplifies the capability of trigonal-prismatic lanthanide-based complexes in terms of magnetic properties for the first time (the reported trigonal-prismatic complexes3,14 only exhibit slow relaxation of magnetization under an applied dc field). However, it is far from reaching the limit of their potentials, and there is still plenty of room for improvement. In this molecule, the negatively charged [LCO]− ligand located on the equatorial plane, along with the departure of ([N*]−)N− Dy−N([N*]−) (angle, 133.66°) from the easy axis, introduces an undesired transverse anisotropy, which, in turn, diminishes the axial anisotropy. Therefore, as long as the transverse anisotropy is minimized, we can further improve the magnetic anisotropy. With this in mind, we reason that the molecules with a linear ([N*]−)N−Dy−N([N*]−) axis and/or a higher differential in the bond lengths between Dy−N([N*]−) and Dy−LCO have a great chance of being a superior Dy-SIM. Efforts are underway to maximize the magnetic anisotropy in this manner by systematically tuning the ligands. In summary, we synthesized and structurally characterized a trigonal-prismatic dysprosium-based monometallic complex, [(LCO)Dy(N*)2], and magnetic analysis reveals that it is a SIM. Theoretical study proves the strong magnetic anisotropy and presents the easy-axis orientation. In addition, the relaxation process at high temperatures (under our experimental conditions) is likely not an Orbach process based on the analysis of the experimental results and ab initio calculations. Moreover, our work encourages researchers to further explore similar structures by modifying ligands and to investigate the magnetostructural relationship in depth.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yi-Quan Zhang: 0000-0003-1818-0612 Bing-Wu Wang: 0000-0001-8092-5959 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grants 21602013, 21621061, 91422302, and 21571008), Beijing Natural Science Foundation (Grant 2174072), General Project of Science and Technology Plan of Beijing Municipal Education Commission (Grant KM201710017001), Outstanding Young Talents from Beijing Party Committee (Grant 2016000020124G060), and Beijing Level College Students Innovation Training Project (Grant 2016J00092). We thank Dr. Ling Xu for help with the synthesis.

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