Syntheses, structure and single-molecule magnet behavior of a rhombus shaped Dy4 cluster

Accepted Manuscript Research paper Syntheses, structure and single-molecule magnet behavior of a rhombus shaped Dy4 cluster Jianwei Chu, Chao Li, Wenjiao Yuan, Peng Liu PII: DOI: Reference:

S0020-1693(18)31091-0 https://doi.org/10.1016/j.ica.2018.11.038 ICA 18653

To appear in:

Inorganica Chimica Acta

Received Date: Revised Date: Accepted Date:

11 June 2018 26 October 2018 24 November 2018

Please cite this article as: J. Chu, C. Li, W. Yuan, P. Liu, Syntheses, structure and single-molecule magnet behavior of a rhombus shaped Dy4 cluster, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.11.038

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Syntheses, structure and single-molecule magnet behavior of a rhombus shaped Dy4 cluster Jianwei Chu a, Chao Li b,*, Wenjiao Yuan a, Peng Liu a a

Tianjin Modern Vocational Technology College, Tianjin 300350, P. R. China

b

College of Food Engineering and Biotechnology, Tianjin University of Science & Technology,

Tianjin 300457, P. R. China *Corresponding

author. E-mail: [email protected]

Abstract A new Dy4 cluster, namely, Dy4(acac)4L6(μ3-OH)2·4CH3CN (1) (HL = 5-(2hydroxy-5-methylbenzylidene)-8-hydroxylquinoline, acac = acetylacetonate), has been synthesized based on a bidentate Schiff base ligand. Furthermore , elemental analysis (EA), powder X-ray diffraction (PXRD), single-crystal X-ray diffraction and magnetic properties of cluster 1 have been characterized. The X-ray structural analysis exhibits that cluster 1 is a tetranuclear dysprosium cluster with rhombusshaped arrangement, and each Dy(III) ion of 1 is eight coordinated, and their coordination polyhedrons can be described as a distorted square-antiprismatic geometry. Magnetic properties measurement indicate that two magnetic relaxation processes phenomenon occurs for cluster 1 under 0 dc filed, with pre-exponential factor τ0 = 8.19 × 10−7 s and ΔE/kB = 37.73 K for the LT relaxation and τ0 = 4.36 × 10−8 s and ΔE/kB = 105.97 K for the HT relaxation. Keywords: Dy4 cluster; bidentate Schiff base ligand; structure; magnetic properties; two magnetic relaxation processes.

1. Introduction Lanthanide based clusters have been extensively studied for their fascinating structures and interesting magnetic properties [1-3]. As one of the hotspots in magnetic study of lanthanide clusters, the lanthanide single molecule magnet (SMM) is particular attractive due to their potential applications in magnetic information 1

storage and processing [4]. Since TbPc2 (Pc = dianion of phthalocyanine) compound, as the first lanthanide-based single molecule magnet (SMM), was reported by Ishikawa in 2003 [5], the heavy lanthanide ions, especially the Dy(III) ion, has become the most optimal candidates for constructing SMMs [6]. Since then lots of lanthanide-based SMMs displaying excellent and interesting magnetic behaviors have been reported [7]. In 2016, a mononuclear Dy(III) compound with excellent magnetic behaviors (Ueff =1025 K) have been reported by Tong group [8]; soon afterwards, the Zheng group reported a nearly perfect pentagonal bipyramidal Dy complex with a largest energy barrier (Ueff = 1815 K) [9]. Up to now, the largest effective energy barrier

is

1837

K,

which

is

from

a

dysprosium

metallocene

SMM

[(Cpttt)2Dy][B(C6F5)4]( Cpttt = 1,2,4-tri(tertbutyl)cyclopentadienide) reported by Richard A. Layfield group [10a]; at the same time, Conrad A. P. Goodwin et al. reported the same SIM [10b]. Due to these interesting and outstanding magnetic behaviors of Ln(III)-based SMMs, an increasing number of researchers focus on the lanthanide compounds and lots of excellent results were appeared. Among them, the polynuclear

lanthanide

clusters

show

outstanding

structural

and

magnetic

characteristics and the most reported ones were Dy3, Dy4 and Dy6 clusters [11]. It's worth mentioning that a coplanar Dy4 cluster with a larger energy barrier (Ueff = 170 K) was reported by Muralee Murugesu group in 2009 [11c], soon afterwards, Jin-Kui Tang group reported a linear-shaped Dy4 cluster with remarkable two-step magnetic relaxation characteristics in 2010 [11d]. In view of this, the goal of our research is that we focus on the magnetic behaviors of polynuclear lanthanide clusters, and further explore and understand the relationship between the structures and magnetic properties. In this paper, we designed and synthesized a bidentate Schiff base ligand (Scheme 1), reacting with the β-diketonate salt (Dy(acac)3·2H2O), a new rhombus shaped Dy4 cluster, Dy4(acac)4L6(μ3OH)2·4CH3CN (1) (HL = 5-(2-hydroxy-5-methylbenzylidene)-8-hydroxylquinoline, acac = acetylacetonate), was synthesized, structurally and magnetically characterized. Magnetic property measurement indicates that 1 shows single-molecule magnet behavior with energy barrier of 38.93 K and τ0 = 9.17 × 10−7 s. 2

Scheme 1 Structures of ligand HL.

2. Experimental section 2.1 Materials and instrumentation All chemicals and solvents used for the syntheses in this work were purchased from suppliers without further purification. The β-diketonate salt (Dy(acac)3·2H2O) and the Schiff base ligand (HL) were synthesized according to the method in the literature and further optimized the reaction conditions [12]. Elemental analyses (EA) for C, H and N were performed on a Perkin-Elmer 240 CHN elemental analyzer. PXRD data was examined on a Rigaku Ultima IV instrument with Cu Kα radiation (λ = 1.54056 Å), with a scan speed of 10° min−1 in the range of 2θ = 5−50°. Luminescence property was recorded on an F-4500 FL spectrophotometer with a xenon arc lamp as the light source. The magnetic measurements were carried out with a Quantum Design MPMS-XL7 and a PPMS-9 ACMS magnetometer. The diamagnetic corrections for the complexes were estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic contributions of the sample holder. 2.2 Synthesis of cluster 1 A solution of Dy(acac)3·2H2O (0.025 mmol) in 25 mL of CH3CN was heated to 70 °C, then a CH2Cl2 solution (3 mL) containing HL (0.025 mmol) was added. The resulting mixture was stirred for 3 h at 70 °C and then cooled to room temperature. After filtration, the resulting solution was concentrated slowly by evaporation at room temperature. After about 5 days, block-shaped and yellow crystals were obtained. Yield: 47% (based on Dy). Anal. Calcd for C130H120Dy4N16O22 (2908.41): C, 53.64; H, 4.13; N, 7.70. Found: C, 53.56; H, 4.25; N, 7.82. 3

2.3 Single-crystal X-ray diffraction measurements Single crystal X-ray diffraction data of cluster 1 was collected on a computercontrolled Rigaku Saturn CCD area detector diffractometer, equipped with confocal monochromatized Mo K radiation with a radiation wavelength of 0.71073 Å using the - scan technique. The structures were solved by direct methods and refined with a full-matrix least-squares technique based on F2 using the SHELXS-97 and SHELXL-97 programs [13]. All non-hydrogen atoms were refined with anisotropic parameters, while hydrogen atoms were placed in calculated positions and refined using a riding model. Crystallographic data and structural refinement parameters are listed in Table 1. CCDC (1848597 for 1) contains the supplementary crystallographic data for this paper. The important bond lengths and angles are listed in Table S1 in the Supporting Information. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. Table 1 Crystallographic Data and Structure Refinements for cluster 1. cluster

1

Formula

C130H120Dy4N16O22

Mr (g mol−1)

2908.41

Cryst syst

Monoclinic

Space group

P21/c

a (Å)

19.1906(8)

b (Å)

25.5347(9)

c (Å)

12.9601(5)

 ()

90

 ()

97.6703(12)

 ()

90

V (Å3)

6294.0(4)

Z

2

Dc (g cm−3)

1.535

μ (mm−1)

2.421

θ/°

2.249 to 26.419 4

F(000)

2904

Reflns collected

89296

Unique reflns

12922

Rint

0.0808

GOF on F2

1.094

R1, wR2 [I >2σ(I)]

0.0423, 0.0987

R1, wR2(all data)

0.0706, 0.1145

3. Results and Discussion 3.1 Structure description of cluster 1 X-ray diffraction crystal structure analyses reveal that cluster 1 crystallizes in the monoclinic space group P21/c, with Z = 2 (Table 1). As shown in Fig. 1, the molecular structure of 1 is mainly composed of four Dy(III) ions, six ligands (L-), four acac- and two μ3-OH. In the centrosymmetric unit, both Dy1 and Dy2 ions are eight coordinated, and their coordination polyhedrons can be described as a distorted square-antiprismatic geometry which are confirmed by using the SHAPE 2.0 Soft program (Table S2). The Dy1 ion is connected by six oxygen atoms (O2, O4, O6, O9, O10 and O11) and two nitrogen atoms (N2 and N4), and the Dy2 ion is connected by seven oxygen atoms (O2a, O4a, O6, O7, O8, O11 and O11a) and one nitrogen atoms (N6); the four Dy(III) ions are connected by four μ2-O bridges which are coming from the phenoxo atoms of the deprotonated ligand and the two μ3-O bridges which are from hydroxides (Fig. S1). The four eight coordinated Dy(III) ions are precisely coplanar and display a rhombus-shaped Dy4 core. The lengths of the Dy–O bonds are in the range of 2.302(3)–2.408(4) Å and the Dy–N bond distances are in the range of 2.534(5) -2.561(4) Å, as well as angles of O–Dy–O are in the range of 68.11(13) to 142.45(13)°; which are comparable to other reported dysprosium compounds [14]. Furthermore, in the rhombus-shaped Dy4 core, the shortest intramolecular Dy⋯ Dy distance is 3.5476(3) Å and the bond angles of Dy-O-Dy are in the range of 96.41(19)-108.5(2)°.

5

Fig. 1 Molecular structure for 1 (all hydrogen atoms and CH3CN solvents molecules are omitted for clarity).

3.2 Powder X-ray Diffraction of cluster 1 The polycrystalline product of 1 has been characterized by powder X-ray diffraction (PXRD) at room temperature (Fig. S2). The observed PXRD patterns are in good agreement with the results simulated from the single crystal data, indicating the purity of the crystalline samples. 3.3 Luminescence property of cluster 1 The solid-state luminescence property of cluster 1 was measured at room temperature. As shown in Fig. S3, under the excitation of 296 nm, the emission spectra of 1 exhibits the characteristic emissions of DyIII ion. Two characteristic DyIII bands at ca. 480 and 574 nm are attributed to the 4F9/2→ 6HJ (J = 15/2 and 13/2) transitions of DyIII centers [15]. 3.4 Magnetic Properties cluster 1 Over the temperature range 2.0–300 K, the direct-current (dc) magnetic susceptibility of cluster 1 has been carried out in an applied magnetic field of 1 kOe (Fig. 2). At room temperature, the χMT value of 1 is 56.42 cm3 K mol−1 which is close to the expected value (56.68 cm3 K mol−1) for four isolated DyIII ions (S = 5/2, L = 5, 6H

15/2,

g = 4/3). As the temperature dropping, the χMT values of 1 decrease gradually

in the temperature range 300−20 K and then decline abruptly to a minimum value of 19.50 cm3 K mol−1 at 2.0 K. This behavior is generally due to the thermal 6

depopulation of the DyIII ion Stark sublevels and/or the weak antiferromagnetic interactions between the DyIII ions in 1 [16].

Fig. 2 Temperature dependence of the χMT products at 1000 Oe for cluster 1.

In the range of 0–80 kOe and T = 2.0 K, the variation of the magnetization M with the applied magnetic field H was investigated for 1. As shown in Fig. S4, M value increases rapidly at low field and then grows slowly without complete saturation up to 80 kOe. The M value of 1 is 16.10 Nβ at 80 kOe, which is much lower than the theoretical saturated value of 40 Nβ anticipated for four independent Dy3+ ions. This behavior can be attributed to the ligand-field-induced splitting of the Stark level as well as magnetic anisotropy [17]. Moreover, as shown in Fig. S5, the M vs. HT−1 data at 2.0–9.0 K show non-superimposed magnetization curves for 1, which also suggest the presence of a significant anisotropy and/or low-lying excited states [18]. In order to deeply study and understand the dynamics of the magnetization of 1, the temperature and frequencies dependencies of the alternating-current (ac) susceptibility were measured under zero dc fields with a 3.0 Oe ac magnetic field (Fig. 3 and Fig. 4). Strikingly, both in-phase (χ′) and out of-phase (χ″) signals of ac magnetic susceptibilities are strongly frequency and temperature dependent. It is interesting that two remarkable peaks of the out of-phase (χ″) component of the ac susceptibility are observed which reveal the presence of two magnetic relaxation processes, this behavior was described in recent reported literatrues and may be attributed to the presence of two Dy(III) sites in cluster 1 with two different coordination environment [19]. Moreover, tail of a peaks are observed around 2.0 K in both in-phase (χ′) and out of-phase (χ″) signals, which generally indicate the presence of quantum tunneling of 7

magnetization (QTM) which is usually seen in Ln(III)-based SMMs [20].

Fig. 3 Temperature dependence of the in-phase (left) and out-of-phase (right) components of the ac magnetic susceptibility for 1 in zero dc fields with an oscillation of 3.0 Oe.

Fig. 4 Frequency dependence of the in-phase (left) and out-of-phase (right) ac susceptibility for 1 under zero dc field.

Due to two relaxation processes taking place in 1, an evaluation of the effective energy barriers (ΔE/kB) and relaxation times associated to the lower (LT) and higher temperature (HT) signals have been deduced from the plot of lnτ = f(1/TB). The relaxation time τ data of 1 derived from the χ″ peaks follow the Arrhenius law (lnτ = lnτ0 + ΔE/kBT) [21]. As show in Fig. 5, the obtained pre-exponential factor τ0 is 8.19 × 10−7 s and the effective barrier (ΔE/kB) is 37.73 K for the LT relaxation and τ0 = 4.36 × 10−8 s and ΔE/kB = 105.97 K for the HT relaxation. The anisotropic energy barrier of 1 is comparable to those previously reported for Dy-based clusters [22]. It is worth noting that lnτ of 1 become weakly dependent on T as the temperature decreases. The Cole−Cole plots of χ″ vs χ′ for 1 were fitted to the generalized Debye model to obtain α values (Fig. 6) [23]. The parameter α = 0.18−0.50 was obtained in the temperature 8

range of 2.0−14.0 K; indicating the presence of a larger distribution of relaxation times in the Dy4 cluster. The larger α values also indicate that multiple magnetic relaxation processes occur in 1 [24]. In fact, two semicircles were clearly observed in the temperature 8.0 K, 10.0 K and 12.0 K of the Cole−Cole curves which also reveal two or more magnetic relaxation processes occur in cluster 1 [25]. In order to check the QTM effect in cluster 1, the variable-temperature ac susceptibility was determined under a small dc field of 500 Oe. As shown in Fig. S6, three peaks were clearly observed in both in-phase (χ′) and out of-phase (χ″) signals components, and remarkable peak shapes were observed around 2.0 K; while it was not clearly observed when under zero dc field. This can prove that the QTM in cluster 1 is pronounced and the QTM effect is basically suppressed when it was under an external a 500 Oe dc field.

Fig. 5 Plots of ln(τ) versus T-1 and best fit of the Arrhenius law for 1 under zero dc field.

9

Fig. 6 Cole–Cole plots for cluster 1 measured in zero-dc field. The red solid lines are the best fit to the experimental data, obtained with the generalized Debye model with α = 0.18–0.50 for 1.

4. Conclusion In summary, in our work, the structure and magnetic property of a rhombus-shaped Dy4 cluster based on Schiff-base ligand have been reported. Cluster 1 is consist of tetranuclear dysprosium plane, each center Dy(III) ion is 8-coordinated and their coordination polyhedrons can be described as a distorted square-antiprismatic geometry. The magnetic investigation indicates that 1 exhibits two magnetic relaxation processes with ΔE/kB of 37.73 K and τ0 = 8.19 × 10−7 s for LT relaxation and τ0 = 4.36 × 10−8 s and ΔE/kB = 105.97 K for the HT relaxation under zero dc filed.

Acknowledgements This work was supported financially by the Science and Technology Program of Tianjin, China (No. 16YFFCZC00070 and No. 17YFFCZC00220).

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The structure and magnetic property of a rhombus-shaped Dy4 cluster, Dy4(acac)4L6(μ3OH)2·4CH3CN (1), have been reported. Magnetic property measurement indicates that 1 shows single-molecule magnet behavior.

(1) A new Dy4 cluster with rhombus-shaped structure have been synthesized, structurally and magnetically characterized. (2) Magnetic property measurement indicates that two magnetic relaxation processes phenomenon occur for cluster 1 under dc = 0 Oe.

14