Effect of coordination environment on the magnetic relaxation of mononuclear DyIII field-induced single molecule magnets

Accepted Manuscript Effect of coordination environment on the magnetic relaxation of mononuclear DyIII field-induced single molecule magnets

Guo Peng, Ying-Ying Zhang, Zhao-Yang Li PII: DOI: Reference:

S1387-7003(16)30645-1 doi: 10.1016/j.inoche.2017.01.031 INOCHE 6545

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

28 December 2016 24 January 2017 26 January 2017

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ACCEPTED MANUSCRIPT Effect of coordination environment on the magnetic relaxation of mononuclear DyIII field-induced single molecule magnets Guo Penga,*, Ying-Ying Zhanga, Zhao-Yang Lib a

Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and

Department of Chemistry and Graduate School of Science, Tohoku University,

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b

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E-mail: [email protected]

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Technology, 210094 Nanjing, P. R. China

6-3 Aramaki-Aza-Aoba, Aoba-ku, 980-8578, Sendai, Japan

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ABSTRACT

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Two mononuclear DyIII complexes [Dy(H2TEG)(NO3)3]·[18-crown-6] (H2TEG= triethylene glycol) (1) and [Dy(H2TEG)Cl3]·[18-crown-6] (2) have been prepared

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using H2TEG as ligands. Magnetic studies revealed that both complexes show

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field-induced double magnetic relaxation behavior with energy barriers of 28K and 54K. Distinct magnetic dynamic properties observed in 1 and 2 are mostly attributed

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complexes.

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to the dissimilar coordination environments around DyIII centers in these two

Keywords: Single molecule magnet, Two-step relaxation, Pentagonal bipyramid, Triethylene glycol

Since the discovery of the first series mononuclear lanthanide single molecule magnets (SMMs) [Bu4N][LnPc2] by Ishikawa et al [1], lanthanide complexes with magnetic relaxation have been investigated intensively [2] because of their potential 1

ACCEPTED MANUSCRIPT applications in high density information storage, quantum computing and spintronic devices [3]. The relaxation of SMMs is strongly dependent on spin ground state and magnetic anisotropy. The large intrinsic magnetic moment and anisotropy of heavy lanthanide ions promise them good candidates to fabricate SMMs. Among the heavy

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lanthanide ions, DyIII ion plays an irreplaceable role in the development of

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lanthanide-based SMMs due to its huge single ion anisotropy and spin parity effect

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for a Kramers ion [4]. The anisotropy of DyIII ion, which is the source of magnetic relaxation, is strongly influenced by ligand field and geometry [5]. Considerable

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efforts have been dedicated to modulate the anisotropy and eventually the magnetic

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relaxation behaviors of DyIII-based SMMs by tuning the ligand field or/and geometry [5]. For example, Murugesu significantly increase the energy barriers of two different

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systems of Dy2 SMMs by introducing electron-withdrawing substituents on the

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terminal ligands [5a]. On the other hand, Tian and Tang reported that the replacement of 1, 10-phenanthroline by its large aromatic derivatives in β-diketone-based DyIII

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complexes can reach high energy barrier [5b]. Interestingly, the anisotropy barrier of a

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single DyIII ion magnet can be switched via changing the coordination geometry from octahedron to pentagonal bipyramid reported by Tong [5c]. Though some progress has been made [2, 5], manipulating the anisotropy and then controlling the relaxation processes of DyIII-based SMMs is still a formidable task for magnetochemists. Herein, in order to investigate the influence of anions on ligand field and geometry and eventually the magnetic relaxation of dysprosium complexes, we decided to choose Dy(NO)3·6H2O and DyCl3·6H2O salts as sources of anions to assemble with 2

ACCEPTED MANUSCRIPT mixed triethylene glycol (H2TEG) and 18-crown-6 ligands. The reason for the selection of this system is manifold. 1) The rich oxygen donors in H2TEG ligand have high affinity to coordinate with oxophilic DyIII ion. 2) The presence of 18-crown-6 ligand not only can enlarge the separation between different DyIII units, but also can

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promote the crystallization of the target complex. 3) The change of the anion from

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NO3- to Cl- can alter the ligand field and coordination geometry, which might affect

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the magnetic relaxation and hence the energy barrier significantly. 4) The use of H2TEG ligand in lanthanide coordination chemistry since 1980s [6], but the magnetic

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result,

two

mononuclear

DyIII

complexes,

namely,

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As

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properties of DyIII complexes based on H2TEG ligands have never been studied before.

[Dy(H2TEG)(NO3)3]·[18-crown-6] (1) and [Dy(H2TEG)Cl3]·[18-crown-6] (2) were

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isolated, and their structural descriptions and magnetic studies are presented herein.

Fig. 1. Crystallographic structure of 1. Dy is shown in violet, O in red, N in blue and C in black. H atoms are omitted for clarity. Symmetry code A: 2-x, y, 0.5-z. 3

ACCEPTED MANUSCRIPT The reaction of H2TEG and 18-crown-6 with Dy(NO3)3·6H2O/ DyCl3·6H2O (1:1:1) in the mixture of CH3CN and CH3OH (3:1) yielded complexes 1 and 2 [7]. Powder X-ray diffraction (PXRD) was used to check the phase purity of both complexes. As illustrated in Fig. S1, the experimental patterns are consistent with that from

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simulation, confirming the purity of the as-synthesized products. Single crystal X-ray

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diffraction analyses revealed that both complexes 1 and 2 crystallize in monoclinic

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space group C2/c with Z=4 [8]. The asymmetric unit of 1 contains one DyIII ion, half H2TEG ligand, one and halves NO3- anions and half 18-crown-6 molecule (Fig. 1).

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The DyIII ion is coordinated by ten oxygen donors from one H2TEG ligand and three

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NO3- anions with the Dy-O bond lengths varying from 2.357(3) to 2.641(4) Å. Continuous shape measures by Shape 2.1 software [9] indicates that the coodination

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geommetry around DyIII ion can be assiged to tetradecahedron with a CShM value of

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3.292 (Table S1). Complex 2 was obtained by changing the raw materials from Dy(NO3)3·6H2O to DyCl3·6H2O. There are one DyIII ion, half H2TEG ligand, two Cl-

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anions and half 18-crown-6 molecule in the asymmetric unit of 2 (Fig. 2). The DyIII

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ion is seven coordinate, surrounded by four oxygen donors from one H2TEG ligand and three Cl- ions. The coordination geometry around DyIII ion can be described as pentagonal bipyramid, which is further proved by Shape 2.1 program [9] with a CShM value of 0.909 for D5h symmetry (Table S1). Four oxygen donors (O1, O2, O2A, O1A) and Cl2 form the equatorial plane, while Cl1 and Cl1A occupy the axial positions with the Cl1-Dy1-Cl1A bond angle of 164.35(6)º. The Dy-O bond lengths vary from 2.324(3) to 2.430(3) Å, whereas the Dy-Cl bond lengths in the range of 4

ACCEPTED MANUSCRIPT 2.5898(17)- 2.6249(12) Å. The nearest Dy···Dy distance is 8.712 Å for complex 1 and 8.294 Å for complex 2, suggesting well isolated between different mononuclear units. The nearest Dy···Dy distance of 2 is longer than that (6.036 Å) of [Dy(H2TEG)Cl3]·CH3CN complex without 18-crown-6 ligand reported by Rogers

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[6b], indicating that the presence of 18-crown-6 ligand can enlarge the separation

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between different DyIII units, which plays an important role in improving SMM

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behavior of the target complex.

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Fig. 2. Crystallographic structure of 2. Dy is shown in violet, O in red, Cl in green and C in black. H atoms are omitted for clarity. Symmetry code A: -x, y, 0.5-z.

Direct current (dc) susceptibility measurements were condcuted on polycrystalline samples of 1 and 2 in the temperature range of 2-300K under an applied dc field of 1000Oe. The experimental χT prodcuts of 1 and 2 at room remperature are 14.09 and 13.76 cm3 K mol-1, which are close to the expected value (14.17 cm3 K mol-1) for one 5

ACCEPTED MANUSCRIPT isolated DyIII ion (J=15/2, g=4/3) (Fig. 3). On lowering the temperature, the χT prodcuts of 1 and 2 decrease with cooling over the whole temperature range, which can be attributed to the thermal depopulation of the Stark levels of DyIII ion. The field dependence of the magnetization for 1 and 2 below 5K increase with rising applied dc

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field without reach saturation even up to 70KOe (Fig. S2), indicating the presence of

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magnetic anisotropy or/and low lying-excited states in these systems. This is further

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confirmed by the M versus HT-1 plots (Fig. S3), which are non-superposition on a

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master curve.

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Fig. 3. Temperature dependence of the χT products for 1 and 2.

To further investigate these two complexes, altering current (ac) susceptibility measurements were performed under zero dc field, however, no out-of-phase component was observed above 2K. This is probably due to fast relaxation of magnetization of DyIII ion, which generally can be slowed down by a small dc field. Therefore, further ac susceptibility measurements were carried out under different dc 6

ACCEPTED MANUSCRIPT fields. Well-defined peaks were detected for 1 by driving the dc fields from 500 to1500Oe, whereas clear maximum was observed for 2 under 1500Oe dc field, indicating the relaxation can be slowed down by the application of a dc field (Fig. S4 and Fig. S5). Thus, the dc fields of 1000Oe and 1500Oe were selected to study the

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frequency dependence of ac signals at varies temperatures for 1 and 2 (Fig S6), which

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leads to strong out-of-phase components with clear maxima (Fig. 4 and Fig. 5). This

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suggests that 1 and 2 are field-induced SMMs. The frequency dependence of out-of-phase ac signals for 1 show two maxima at low temperature, indicating the

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presence of two relaxation processes in 1 (Fig. 4). This statement is further verified by

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the two-semicircular-shape Cole-Cole curves. Fitting the Cole-Cole plots by the sum of two modified Debye functions using CC-FIT program [10] (Fig. S7) results in α1=

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0.21-0.42 and α2= 0.11-0.38 (Table S2). The large α values indicate wide distribution

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of relaxation time. Analyzing the relaxation times of 1 extracted from Cole-Cole plots fitting using Arrhenius law produces an energy barrier of 28K with pre-exponential

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factor of 2.49×10-9s (Fig. 6). As shown in Fig. 5, the frequency dependence of

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out-of-phase ac signals for 2 are also indicative of two relaxation processes. The hook-shape Cole-Cole curves of 2 from 2.5-2.9K can be fitted by the sum of two modified Debye functions using CC-FIT program [10] (Fig. S8), leading to α1= 0.29-0.45 and α2= 0.25-0.32 (Table S3), which imply no narrow relaxation time distribution for both processes. Analyzing lnτ versus T-1 plots of 2 by Arrhenius law results in an energy barrier of 54K with pre-exponential factor of 1.44×10-10s (Fig. 6). The magnetic hysteresis properties of complexes 1 and 2 were measured at 2K using a 7

ACCEPTED MANUSCRIPT field weep rate of 50Oe/s, but no hysteresis loops were observed (Fig. S9), confirming

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fast relaxation of the magnetization.

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Fig. 4. Frequency dependence of out-of-phase ac susceptibility signals under 1000Oe

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dc field for 1.

Fig. 5. Frequency dependence of out-of-phase ac susceptibility signals under 1500Oe dc field for 2.

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Fig. 6. Temperature dependence of the relaxation times under 1000Oe and 1500Oe

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dc fields for 1 and 2. The lines are the best fits to Arrhenius law.

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Complexes 1 and 2 show similar static magnetic behavior, but their dynamic properties are drastically different. The energy barrier of 2 is almost twice larger than

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that of 1. Different magnetic dynamic behaviors observed in 1 and 2 can be attributed

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to the dissimilar coordination environments of the DyIII centers in these two complexes. Different coordination environments lead to distinct symmetries and

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ligand fields around DyIII centers, which may strongly affect the magnetic anisotropy,

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causing different dynamic properties. To further understand the magnetic anisotropy, the orientation of magnetic easy axes for 1 and 2 were determined by electrostatic model using MAGELLAN program [11]. As shown in Fig. S10, the O4 and O5 atoms from NO3- group deviate from the magnetic easy axis of 1 24.753º and 27.177º, whereas the Cl1 anion deviates from the magnetic easy axis of 2 7.828º; on the other hand, O2 from NO3- group in 1 deviates from the equatorial plane constructed by four oxygen donors from H2TEG ligand 25.96º, while the Cl2 anion in 2 is almost coplanar 9

ACCEPTED MANUSCRIPT with the equatorial plane. The larger deviation of the coordination atoms in 1 from the easy axis and equatorial plane could induce much more transversal anisotropic component, causing larger reduction of energy barrier. That is the possible reason for the observation of different energy barriers in 1 and 2. Complex 2 with low

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coordination number and high symmetry but still lack of SMM behavior under zero

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dc field, which is quite different from the other systems with pentagonal bipyramidal

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geometry [12]. This might be ascribed to the week ligand field producing by H2TEG and Cl- ligands, which is further confirmed by the long Dy-O and Dy-Cl distances.

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From this phenomenon, it could be deduced that the strength of ligand field is more

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vital to construct Dy-based SMMs than geometric symmetry, which is also proved by other magnetochemists recently [13].

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In conclusion, two mononuclear DyIII complexes based on H2TEG ligand have been

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prepared and characterized. The DyIII ion in 1 coordinates to ten oxygen donors with a tetradecahedron geometry, whereas the DyIII center in 2 is seven coordinate

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possessing a pentagonal bipyramidal configuration. Both complexes show

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field-induced two-step magnetic relaxation behavior with energy barriers of 28K and 54K, respectively. The distinct dynamic properties of these two complexes could be associated with the different anisotropy derived from the change of coordination environment. This result provides another example to prove that the anisotropy of DyIII complexes can be tuned by coordination environments around DyIII centers, which offers an avenue to manipulate the relaxation processes of SMMs. The substitution of the coordination anions by other groups with strong ligand field is 10

ACCEPTED MANUSCRIPT underway in our lab.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21501093) and the Natural Science Foundation of Jiangsu Province (BK20150768).

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Appendix A. Supplementary material

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Supplementary information available: Additional figures and tables for complexes

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1 and 2. CCDC 1515849 and 1162041 contain the supplementary crystallographic data for 1 and 2. These data can be obtained free of charge from the Cambridge Data

Centre

via

https://www.ccdc.cam.ac.uk/structures-beta/.

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Crystallographic

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Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.inoche.xxxx.xx.xx.

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ACCEPTED MANUSCRIPT [7] Triethylene glycol (0.075g, 0.5mmol), 18-crown-6 (0.132g, 0.5mmol) and Dy(NO3)3·6H2O (0.228g, 0.5mmol) was dissolved in a mixtures of CH3CN (15mL) and CH3OH (5mL). The resulting mixture was heated at 55ºC for 3h and then filtered when the solution was cooled. Slow evaporation of the filtrate

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at room temperature gave colorless crystals of 1 after one day. Yield: 162mg

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(42% based on Dy). Calc. (%) for C18H38DyN3O19: C 28.33, H 5.02; N 5.51;

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found: C 28.32, H 4.95, N 5.51. Selected IR data (cm-1): 3170(br), 2903(w), 1494(m), 1465(m), 1384(m), 1353(w), 1306(w), 1253(w), 1106(s), 1066(w),

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1039(w), 959(w), 939(w), 839(w), 815(w). Similar procedure with 1 was

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employed to prepare complex 2 except that Dy(NO3)3·6H2O was replaced by DyCl3·6H2O (0.189g, 0.5mmol). The crystals of 2 are easy to deliquesce in the

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air and should be kept in the drier. Yield: 137mg (40% based on Dy). Calc. (%)

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for C18H38Cl3DyO10: C 31.64, H 5.61; found: C 31.74, H 5.54. Selected IR data (cm-1): 3382(br), 2912(w), 1636(w), 1472(w), 1353(w), 1287(w), 1252(w),

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1107(s), 960(w), 837(w).

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[8] The data of 1 and 2 were collected on a Bruker SMART APEX II diffractometer using monochromated Mo-Kα radiation (λ=0.71073Å) at 173(2)K. The crystals of 1 are non-merohedral twins. The sorption corrections were performed using TWINABS for 1 and ASDABS for 2 supplied by Bruker. Both structures were solved by direct methods and refined by full-matrix least squares analysis on F2, using the SHELXTL program package [14]. Ordered non-H atoms were refined anisotropically, H-atoms were placed in calculated 15

ACCEPTED MANUSCRIPT positions and refined using a riding model. Crystal data for 1: C18H38DyN3O19, M = 763.01, Monoclinic, C2/c, a = 18.9159(9) Å, b = 10.4955(4) Å, c = 15.1171(7) Å, β = 110.478(2) º, V = 2811.6(2) Å3, Z = 4, F(000) = 1540, GOF = 1.062, R1 (I > 2σ) = 0.0275, wR2 (all data) = 0.0717. Crystal data for 2:

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C18H38Cl3DyO10, M = 683.33, Monoclinic, C2/c, a = 16.4287(13) Å, b =

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13.6300(10) Å, c = 11.9091(10) Å, β = 92.442(3) º, V = 2664.3(4) Å3, Z = 4,

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F(000) = 1372, GOF = 1.025, R1 (I > 2σ) = 0.0343, wR2 (all data) = 0.0659. The structure of 2 was reported by Rogers in 1988 [6a, 6f], but its magnetic

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properties have never been studied before.

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[14] G. M. Sheldrick, A short history of SHELX, Acta Cryst. A64 (2008) 112.

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ACCEPTED MANUSCRIPT Graphical abstract The preparation and magnetic studies of two mononuclear DyIII complexes based on triethylene glycol ligand are presented. Both complexes show field-induced single molecule magnet behavior with double magnetic relaxation processes. Distinct

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magnetic dynamic properties observed in these two complexes are mostly attributed

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to the dissimilar coordination environments around DyIII centers.

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ACCEPTED MANUSCRIPT

Highlights 

Two mononuclear DyIII complexes with different coordination environments were prepared. Both complexes exhibit single molecule magnet behavior with two-step magnetic

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relaxation.

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The influence of the coordination environment on magnetic relaxation was

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studied.

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