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Crystal structure and magnetic dynamics of a novel salen type and β-diketonate Dy2 complex
Inorganic Chemistry Communications 69 (2016) 79–84
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Feature article
Crystal structure and magnetic dynamics of a novel salen type and β-diketonate Dy2 complex Xiaoyan Zou a,b, Guangming Li b,⁎, Fengming Zhang a, Haijun Pang b, Huiyuan Ma a,⁎ a
Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China
b
a r t i c l e
i n f o
Article history: Received 22 March 2016 Received in revised form 21 April 2016 Accepted 25 April 2016 Available online 27 April 2016 Keywords: Magnetic dynamics Salen type β-Diketonate Complex
a b s t r a c t A novel salen type dinuclear dysprosium complex e.g., [Dy2(H2L)(acac)6]·4CH3OH (1) [H2L = N,N′-(1.3propylene)bis(3-methoxysalicylideneimine) has been isolated by reactions of Dy(acac)3·H2O (acac = acetylacetonate) with salen type (H2L), respectively. X-ray crystallographic analyses reveal that asymmetric Dy(III) ions for 1 was bridged by one ligand displaying bicapped trigonal prism coordination geometry. Magnetic measurement shows that complex 1 exhibit slow relaxations under zero dc field. The presence of two magnetic relaxations in complex 1 is associated with the presence of minute differences in bond lengths around Dy(III) center. © 2016 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . 2.1. Materials and instrumentation . . 2.2. Synthesis of complex 1 . . . . . 3. Results and discussion . . . . . . . . . 3.1. Structural description of complex 1 3.2. Magnetic properties . . . . . . . 4. Conclusion . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . Appendix A. Supplementary material . . . References . . . . . . . . . . . . . . . . .
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1. Introduction Single molecule magnets of the homo-multinuclear lanthanide complexes continue to be attractive owing to their potential applications for the uses of high-density magnetic memories, molecular spintronics and quantum computing devices [1]. Particular attention
⁎ Corresponding authors.
http://dx.doi.org/10.1016/j.inoche.2016.04.033 1387-7003/© 2016 Elsevier B.V. All rights reserved.
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has been devoted to Dy(III) ion, attributed to the inherently large magnetic moment with a Kramers ground state of 6 H15/2 and a large Ising-type magnetic anisotropy. It has indisputably led to the largest number of pure 4f SMMs with various nuclear [2]. Among the SMMs of Dy(III) complexes, the recorded effective energy barrier was 528 K reported by Blagg R. J. et al. in 2011 [3]. It demonstrated that the ligands played essential roles to achieve SMMs by way of ligand field and defined geometries [4]. Among numerous ligands, salen type with rich oxygen and nitrogen donors can stabilize different Dy(III) ion in various coordination environments which exhibited unique molecule structure displaying distinct anisotropic centers, such
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X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84 Table 1 Crystal data and structures refinement for complex 1.
Scheme 1. Representation of the hexadentate salen-type and acac ligands.
as mononuclear [5], dinuclear [2e,6], tetranuclear [7], and 1D relatively higher energy barriers. E.g. Long and co-workers have reported a centrosymmetric salen type dinuclear complex [Dy2(valdien)2](NO3)2] (H2valdien = N1,N3-bis(-3-methoxysalicylidene)diethylenetriamine) with a anisotropy barrier of 76 K in 2011 [6d]. Although considerable efforts have been dedicated to understand the correlationship of the magnetism-structure in these salen type Dy(III)-based SMMs [4b], there are still great challenge to tell the origin of the slow magnetic relaxation of salen-type dinuclear Dy2 complexes. In view of the recent important progress on the structure and magnetic of salen type lanthanide complexes [8-9] as well as our long-standing research on this domain [10], attempting to explore the correlationship of magnetismstructure of salen type dinuclear Dy2 complexes, and acetylacetonate were employed to develop salen-type and β-diketonate lanthanide complexes. As a result, a salen-type and β-diketonate dinuclear lanthanide complex, namely, [Dy2(H2L)(acac)6]·4CH3OH (H2L = N,N′bis(2-oxy-3-methoxybenzylidene)-1.2-phenylenediamine; acac = acetylacetonate) has been synthesized. Crystal structure has been determined. 2. Experimental 2.1. Materials and instrumentation All chemicals and solvents except Dy(acac)3·H2O and H2L were obtained from commercial sources and used without further purification. The salen-type ligand H2L (Scheme 1) was prepared according to the literature and lanthanide precursors [11]. Dy(acac)3·H2O was prepared according to a literature procedure previously described [12]. Elemental (C, H and N) analyses were performed on a Perkin–Elmer 2400 analyzer. FT-IR data were collected on a Perkin–Elmer 100 spectrophotometer by using KBr disks in the range of 4000–500 cm− 1. UV spectra (in
Complex
1
Formula Formula weight Color Crystal system Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F (000) R1, [I N 2σ (I)] wR2, [I N 2σ (I)] R1, (all data) wR2, (all date) GOF on F2
C53H78Dy2N2O20 1388.17 buff Triclinic Pī 9.836(5) 17.521(5) 18.143(5) 93.448(5) 99.601(5) 95.672(5) 3058(2) 2 1.507 2.495 1404 0.0313 0.0666 0.0435 0.0721 1.060
methanol) were recorded on a Perkin-Elmer 35 spectrophotometer. Thermal analyses were carried out on a STA-6000 with a heating rate of 10 °C min−1 in a temperature range from 30 °C to 800 °C in atmosphere. CCDC No. 1449107 contain the supplementary crystallographic data for complex 1 respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www. ccdc.cam.ac.uk/data_request/cif 2.2. Synthesis of complex 1 [Dy2(H2L)(acac)6]·4CH3OH (1). A solution of Dy(acac)3·H2O (acac = acetylacetonate) (0.094 g, 0.2 mmol) in MeOH (10 mL) was dropwise added to a solution of H2L (0.068 g, 0.2 mmol) in CH2Cl2 (10 mL). The resulting solution was stored in the dark at ambient temperature. Yellow crystal was obtained in about one week. Yield: 0.104 g (75%). Anal. Calcd for C53H78Dy2N2O20 (1390.37): C, 45.86; H, 5.66; N, 2.02; found: C, 45.80; H, 5.60; N, 2.01%. IR (KBr pellet, cm− 1): 3418 (w), 3104 (w), 1654 (vs), 1453 (s), 1285 (s), 1228 (s), 1099 (m), 1034 (m), 813 (m). UV–VIS [MeOH, λmax]: 340, 265, 222 nm. 3. Results and discussion 3.1. Structural description of complex 1 X-ray crystallographic analysis reveals that complex 1 crystallizes in a Triclinic space group of Pī, possessing a neutral asymmetric structure (Fig. 1, Table 1). Complex 1 consists of two Dy(III) ions, six acac ligands
Fig. 1. Molecular structure of complex 1 (Hydrogen atoms and solvent molecules are omitted).
Fig. 2. The coordination geometry of the Dy(III) ions in complex 1.
X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84
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Table 2 Selected bond lengths (Å) and angles (°) for complex 1. Bonds
Length
Bonds
Length
Dy(1)–O(1) Dy(1)–O(2) Dy(1)–O(3) Dy(1)–O(4) Dy(1)–O(5) Dy(1)–O(6) Dy(1)–O(7) Dy(1)–O(8) O(1)–Dy(1)–O(2) O(3)–Dy(1)–O(5) O(6)–Dy(1)–O(7) O(4)–Dy(1)–O(8)
2.300(3) 2.316(3) 2.370(3) 2.339(2) 2.311(3) 2.301(3) 2.349(3) 2.561(3) 73.45(11) 73.50(13) 72.40(12) 64.14(9)
Dy(2)–O(9) Dy(2)–O(10) Dy(2)–O(11) Dy(2)–O(12) Dy(2)–O(13) Dy(2)–O(14) Dy(2)–O(15) Dy(2)–O(16) O(13)–Dy(2)–O(14) O(11)–Dy(2)–O(12) O(15)–Dy(2)–O(16) O(9)–Dy(2)–O(10)
2.361(3) 2.612(3) 2.335(3) 2.336(3) 2.363(3) 2.310(3) 2.311(3) 2.317(3) 73.08(10) 72.60(10) 72.40(12) 62.33(19)
and one H2L ligand. Both Dy1(III) ion and Dy2(III) ion display a eightcoordinate and is bonded to eight oxygen atoms (six from the two top acac ligands and two from the phenolic oxygen of the salen-type ligand) to form a bicapped trigonal prism coordination polyhedron geometry (Fig. 2). Two Dy(III) ions are bridged by a pair of chelating oxygen atoms from phenoxo and methoxy groups of one ligand (H2L) with Dy1–Dy2 separation of 9.8617(27) Å. The Dy\\O bond lengths are in the range of 2.300(3)–2.612(3) Å. The bond lengths (2.361(3), 2.339(2) Å) from phenoxo are distinctively shorter than those from the methoxy groups (2.612(3) and 2.561(3) Å). The bond angles of O9–Dy2–O10 and O4–Dy1–O8 are 62.33(19)° and 64.14(9)°, respectively (Fig. 1, Table 2). It is distinctively different from the same salentype ligand complex [Dy(H2L)(NO3)3]2·CH2Cl2 in which two crystallographically equivalent Dy(III) ions are bridged by only a pair of ligands in bidentate coordination modes forming a closed ring of Dy2L22 with the Dy⋯Dy distance of 10.098 Å (Fig. 3) [13]. 3.2. Magnetic properties Magnetic measurements were performed on polycrystalline samples of complex 1. The phase purity of the bulk samples was confirmed by XRD analyses (Fig. 4). Direct current (dc) magnetic properties of complex 1 was investigated under 0 Oe field in the temperature range 300–1.8 K (Fig. 5). The values of χmT at room temperature are 24.19 cm3 K mol−1 for complex 1, respectively. These experimental values are smaller than the expected value of 28.34 cm3 K mol−1 for two uncoupled Dy(III) ions (6H15/2, S = 5/2, L = 5, g = 4/3, χT = 14.17 cm3 K mol−1), which high likely results from the single-ion behavior of Dy(III) ions rather than from spin-spin coupling interactions between the Dy(III) ions [14]. For complex 1, followed by a slight decrease on lowering the temperature from 300 to 20 K and then drop sharply to reach a minimum of 18.43 K mol−1 at 1.8 K. Therefore, the decrease is ascribed to the progressive depopulation of excited Stark sublevels, significant magnetic anisotropy or weak antiferromagnetic
Fig. 4. X-ray powder diffraction patterns of simulated and experimental for 1(left) and 2(right).
interactions present in the systems [15]. The field dependence of the magnetization of both complexes has been investigated in the range of 0 to 40 KOe at the temperatures 1.8, 3 and 5 K (Fig. 5, inset). The magnetization increase rapidly at low field and then increase smoothly but without saturation even at 40 KOe. The magnetization eventually reaches the value of 8.03μB (for 1), this value is lower than theoretical saturation value of 10.46μB (2 × 5.23μB), which high likely result from the crystal field effect around Dy(III) ions [16]. To explore the dynamics of the magnetization, alternating current (ac) susceptibility measurements for complex 1 was carried out in the frequency range of 1–1000 Hz. Complex 1 displays clear frequencydependent out-of-phase (χ″) signals at low temperatures under zero dc magnetic field (Fig. 6), suggesting the existence of slow magnetic relaxation behavior [17]. The absence of any frequency-dependent peaks indicates the quantum tunneling of magnetization (QTM), which reduces the expected energy barrier in complex 1. In order to suppress the possible tunneling effects, an optimum field of 1000 Oe was applied. The slow magnetic relaxation peaks of out-phase signal (χ″) could be observed below 6 K in the frequency range of 3 Hz and 999 Hz for 1 (Fig. 5, right) at 1000 Oe. In addition, a broad shoulder between 6 K (333 Hz) and 12 K (999 Hz) is exhibited at 1000 Oe, which suggests the presence of two relaxation processes for 1. To further confirm the relaxation processes in complex 1, the frequency-dependent ac susceptibilities for 1 were run to further verify the relaxation dynamics under 1000 Oe dc field (Fig. 6). There are two relaxation phases corresponding to the high-frequency signal (fast relaxation phase, FR)
Fig. 3. Left: Molecular structure of 1; Right: Molecular structure of complex [Dy(H2L)(NO3)3]2·CH2Cl2 in ref. [13].
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Fig. 5. Temperature dependence of χMT for 1 at 10 Oe field. Inset: Plot of magnetization as a function of field for 1 at 1.8 K, 3 K and 5 K.
and the low-frequency region (slow relaxation phase, SR) in complex 1 (Fig. 7) [18]. The Cole–Cole plot show fairly symmetrical semicircles and irregular curves in the high-frequency and low-frequency regions, respectively (Fig. 8). On the basis of ac susceptibility data we were able to extract the relaxation time for the temperature peak. Therefore, the energy barriers (Ueff/k B) and the pre-exponential factors (τ 0) can be fitted at 34.5 K and 4.6 × 10 − 6 s from the Arrhenius
relation [τ = τ 0exp(U eff / k BT)](Fig. 7(left), inset).The Cole–Cole plot under additional 2000 Oe dc field was conducted to further confirm the two relaxation processes in complex 1. Strikingly, the presence of two relaxation processes in complex 1 is unexpected as the two Dy(III) ions are of the same geometry. In general, the observation of two relaxation processes in dinuclear Dy(III) complexes were associated with the existence of anisotropic Dy(III) centers [18] and conformers [19]. However, two observed two relaxation processes may ascribed to the minute differences in bond lengths around two Dy(III) center which result in essential changes on the local anisotropy of the Dy(III) ions. The relatively higher energy barrier of complex 1 than that of complex [Dy(H2L)(NO3)3]2·CH2Cl2 in ref. [13] may result from the following reason. The Dy(III) ions are located in the 9coordinated geometry of bicapped trigonal prism coordination with D3h symmetry in complex 1, the high axiality of the Dy(III) ion achieved the efficient blockage of magnetization which enhance the SMMs behavior [16]. In contrast, the Dy(III) ions in complex [Dy(H2L)(NO3)3]2·CH2Cl2 is 10-coordinated with a distorted bi-capped square anti-prismatic geometry. The obvious disparity in magnetic dynamics should mainly result from coordination geometry in the Dy(III) ions, thus leading to the fast quantum tunneling from a more transverse anisotropy and relatively lower energy barrier [6f].
4. Conclusion Isolation of complex 1 demonstrates that synthesis of salen type dinuclear dysprosium complexes with rigid salen-type (H2L = N,N′bis(2-oxy-3-methoxybenzylidene)-1.2-phenylenediamine) and β-
Fig. 6. Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility of 1 at 0 Oe field (left) and at 1000 Oe field (right).
Fig. 7. Frequency dependence of in-phase (χ′) (left) and out-of-phase (χ″) (right) ac susceptibility of 1 at 1000 Oe field in the temperature range 2–9.5 K. The inset is the Arrhenius fit for the lnτ vs. T−1 plot.
X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84
Fig. 8. Cole–Cole plot for 1 obtained using the ac susceptibility data under 2000 Oe dc field. The inset is the Arrhenius fit for the lnτ vs. T−1 plot.
diketonate (acac = acetylacetonate) ligands are possible, and the structure of the salen type ligand dominate the structures of the complexes and the coordination geometries of the Dy(III) ions. The magnetic analysis suggests the high axial coordination geometry around each Dy(III) ions with a stronger ligand field lead to the typical SMM behavior with a higher energy barrier. The presence of two magnetic relaxations in complex 1 is associated with the presence of differences in bond lengths around Dy(III) center. Acknowledgement This work is financially supported by the National Natural Science Foundation of China (Nos. 51402092, 21471051, 21071038, 21101045 and 21501036). Appendix A. Supplementary material Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2016.04.033. References [1] (a) X. Feng, Y. Li, Z. Zhang, E. Wang, Polyoxometalate-mediated single-molecule magnets, Acta Chim. Sin. 71 (2013) 539–542; (b) Y.N. Guo, G.F. Xu, P. Gamez, L. Zhao, S.Y. Lin, R. Deng, J. Tang, H.J. Zhang, Two-step relaxation in a linear tetranuclear dysprosium(III) aggregate showing singlemolecule magnet behavior, J. Am. Chem. Soc. 132 (2010) 8538–8539; (c) L. Sorace, C. Benelli, D. Gatteschi, Lanthanides in molecular magnetism: old tools in a new field, Chem. Soc. Rev. 40 (2011) 3092–3104; (d) D.N. Woodruff, R.E.P. Winpenny, R.A. Layfield, Lanthanide single-molecule magnets, Chem. Rev. 113 (2013) 5110–5148; (e) P.F. Yan, P.H. Lin, F. Habib, T. Aharen, M. Murugesu, Z.P. Deng, G.M. Li, W.B. Sun, Planar tetranuclear Dy(III) single-molecule magnet and its Sm(III), Gd(III), and Tb(III) analogues encapsulated by salen-type and beta-diketonate ligands, Inorg. Chem. 50 (2011) 7059–7065; (f) P.P. Yang, X.F. Gao, H.B. Song, S. Zhang, X.L. Mei, L.C. Li, D.Z. Liao, Slow magnetic relaxation in novel Dy-4 and Dy-8 compounds, Inorg. Chem. 50 (2011) 720–722; (g) Y.Q. Hu, M.H. Zeng, K. Zhang, S. Hu, F.F. Zhou, M. Kurmoo, Tracking the formation of a polynuclear Co16 complex and its elimination and substitution reactions by mass spectroscopy and crystallography, J. Am. Chem. Soc. 135 (2013) 7901–7908; (H) M.H. Zeng, Z. Yin, Y.X. Tan, W.X. Zhang, Y.P. He, M. Kurmoo, Nanoporous cobalt(II)Mof exhibiting four magnetic ground states and changes in gas sorptior upon post-synthetic modification, J. Am. Chem. Soc. 136 (2014) 4680–4688; (L) Q. Chen, M.H. Zeng, Y.L. Zhou, H.H. Zou, M. Kurmoo, Hydrogen-bonded dicubane CoII7 single-molecul-magnet coordinated by in situ solrothermally generated 1.2-bis(8-hydroxyquinolin-2-yl) ethane-1,2,-diol arranged in a trefoil, Chem. Mater. 22 (2010) 2114–2119.
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Feature article
Crystal structure and magnetic dynamics of a novel salen type and β-diketonate Dy2 complex Xiaoyan Zou a,b, Guangming Li b,⁎, Fengming Zhang a, Haijun Pang b, Huiyuan Ma a,⁎ a
Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China
b
a r t i c l e
i n f o
Article history: Received 22 March 2016 Received in revised form 21 April 2016 Accepted 25 April 2016 Available online 27 April 2016 Keywords: Magnetic dynamics Salen type β-Diketonate Complex
a b s t r a c t A novel salen type dinuclear dysprosium complex e.g., [Dy2(H2L)(acac)6]·4CH3OH (1) [H2L = N,N′-(1.3propylene)bis(3-methoxysalicylideneimine) has been isolated by reactions of Dy(acac)3·H2O (acac = acetylacetonate) with salen type (H2L), respectively. X-ray crystallographic analyses reveal that asymmetric Dy(III) ions for 1 was bridged by one ligand displaying bicapped trigonal prism coordination geometry. Magnetic measurement shows that complex 1 exhibit slow relaxations under zero dc field. The presence of two magnetic relaxations in complex 1 is associated with the presence of minute differences in bond lengths around Dy(III) center. © 2016 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . 2.1. Materials and instrumentation . . 2.2. Synthesis of complex 1 . . . . . 3. Results and discussion . . . . . . . . . 3.1. Structural description of complex 1 3.2. Magnetic properties . . . . . . . 4. Conclusion . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . Appendix A. Supplementary material . . . References . . . . . . . . . . . . . . . . .
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1. Introduction Single molecule magnets of the homo-multinuclear lanthanide complexes continue to be attractive owing to their potential applications for the uses of high-density magnetic memories, molecular spintronics and quantum computing devices [1]. Particular attention
⁎ Corresponding authors.
http://dx.doi.org/10.1016/j.inoche.2016.04.033 1387-7003/© 2016 Elsevier B.V. All rights reserved.
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has been devoted to Dy(III) ion, attributed to the inherently large magnetic moment with a Kramers ground state of 6 H15/2 and a large Ising-type magnetic anisotropy. It has indisputably led to the largest number of pure 4f SMMs with various nuclear [2]. Among the SMMs of Dy(III) complexes, the recorded effective energy barrier was 528 K reported by Blagg R. J. et al. in 2011 [3]. It demonstrated that the ligands played essential roles to achieve SMMs by way of ligand field and defined geometries [4]. Among numerous ligands, salen type with rich oxygen and nitrogen donors can stabilize different Dy(III) ion in various coordination environments which exhibited unique molecule structure displaying distinct anisotropic centers, such
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X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84 Table 1 Crystal data and structures refinement for complex 1.
Scheme 1. Representation of the hexadentate salen-type and acac ligands.
as mononuclear [5], dinuclear [2e,6], tetranuclear [7], and 1D relatively higher energy barriers. E.g. Long and co-workers have reported a centrosymmetric salen type dinuclear complex [Dy2(valdien)2](NO3)2] (H2valdien = N1,N3-bis(-3-methoxysalicylidene)diethylenetriamine) with a anisotropy barrier of 76 K in 2011 [6d]. Although considerable efforts have been dedicated to understand the correlationship of the magnetism-structure in these salen type Dy(III)-based SMMs [4b], there are still great challenge to tell the origin of the slow magnetic relaxation of salen-type dinuclear Dy2 complexes. In view of the recent important progress on the structure and magnetic of salen type lanthanide complexes [8-9] as well as our long-standing research on this domain [10], attempting to explore the correlationship of magnetismstructure of salen type dinuclear Dy2 complexes, and acetylacetonate were employed to develop salen-type and β-diketonate lanthanide complexes. As a result, a salen-type and β-diketonate dinuclear lanthanide complex, namely, [Dy2(H2L)(acac)6]·4CH3OH (H2L = N,N′bis(2-oxy-3-methoxybenzylidene)-1.2-phenylenediamine; acac = acetylacetonate) has been synthesized. Crystal structure has been determined. 2. Experimental 2.1. Materials and instrumentation All chemicals and solvents except Dy(acac)3·H2O and H2L were obtained from commercial sources and used without further purification. The salen-type ligand H2L (Scheme 1) was prepared according to the literature and lanthanide precursors [11]. Dy(acac)3·H2O was prepared according to a literature procedure previously described [12]. Elemental (C, H and N) analyses were performed on a Perkin–Elmer 2400 analyzer. FT-IR data were collected on a Perkin–Elmer 100 spectrophotometer by using KBr disks in the range of 4000–500 cm− 1. UV spectra (in
Complex
1
Formula Formula weight Color Crystal system Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) μ (mm−1) F (000) R1, [I N 2σ (I)] wR2, [I N 2σ (I)] R1, (all data) wR2, (all date) GOF on F2
C53H78Dy2N2O20 1388.17 buff Triclinic Pī 9.836(5) 17.521(5) 18.143(5) 93.448(5) 99.601(5) 95.672(5) 3058(2) 2 1.507 2.495 1404 0.0313 0.0666 0.0435 0.0721 1.060
methanol) were recorded on a Perkin-Elmer 35 spectrophotometer. Thermal analyses were carried out on a STA-6000 with a heating rate of 10 °C min−1 in a temperature range from 30 °C to 800 °C in atmosphere. CCDC No. 1449107 contain the supplementary crystallographic data for complex 1 respectively. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via www. ccdc.cam.ac.uk/data_request/cif 2.2. Synthesis of complex 1 [Dy2(H2L)(acac)6]·4CH3OH (1). A solution of Dy(acac)3·H2O (acac = acetylacetonate) (0.094 g, 0.2 mmol) in MeOH (10 mL) was dropwise added to a solution of H2L (0.068 g, 0.2 mmol) in CH2Cl2 (10 mL). The resulting solution was stored in the dark at ambient temperature. Yellow crystal was obtained in about one week. Yield: 0.104 g (75%). Anal. Calcd for C53H78Dy2N2O20 (1390.37): C, 45.86; H, 5.66; N, 2.02; found: C, 45.80; H, 5.60; N, 2.01%. IR (KBr pellet, cm− 1): 3418 (w), 3104 (w), 1654 (vs), 1453 (s), 1285 (s), 1228 (s), 1099 (m), 1034 (m), 813 (m). UV–VIS [MeOH, λmax]: 340, 265, 222 nm. 3. Results and discussion 3.1. Structural description of complex 1 X-ray crystallographic analysis reveals that complex 1 crystallizes in a Triclinic space group of Pī, possessing a neutral asymmetric structure (Fig. 1, Table 1). Complex 1 consists of two Dy(III) ions, six acac ligands
Fig. 1. Molecular structure of complex 1 (Hydrogen atoms and solvent molecules are omitted).
Fig. 2. The coordination geometry of the Dy(III) ions in complex 1.
X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84
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Table 2 Selected bond lengths (Å) and angles (°) for complex 1. Bonds
Length
Bonds
Length
Dy(1)–O(1) Dy(1)–O(2) Dy(1)–O(3) Dy(1)–O(4) Dy(1)–O(5) Dy(1)–O(6) Dy(1)–O(7) Dy(1)–O(8) O(1)–Dy(1)–O(2) O(3)–Dy(1)–O(5) O(6)–Dy(1)–O(7) O(4)–Dy(1)–O(8)
2.300(3) 2.316(3) 2.370(3) 2.339(2) 2.311(3) 2.301(3) 2.349(3) 2.561(3) 73.45(11) 73.50(13) 72.40(12) 64.14(9)
Dy(2)–O(9) Dy(2)–O(10) Dy(2)–O(11) Dy(2)–O(12) Dy(2)–O(13) Dy(2)–O(14) Dy(2)–O(15) Dy(2)–O(16) O(13)–Dy(2)–O(14) O(11)–Dy(2)–O(12) O(15)–Dy(2)–O(16) O(9)–Dy(2)–O(10)
2.361(3) 2.612(3) 2.335(3) 2.336(3) 2.363(3) 2.310(3) 2.311(3) 2.317(3) 73.08(10) 72.60(10) 72.40(12) 62.33(19)
and one H2L ligand. Both Dy1(III) ion and Dy2(III) ion display a eightcoordinate and is bonded to eight oxygen atoms (six from the two top acac ligands and two from the phenolic oxygen of the salen-type ligand) to form a bicapped trigonal prism coordination polyhedron geometry (Fig. 2). Two Dy(III) ions are bridged by a pair of chelating oxygen atoms from phenoxo and methoxy groups of one ligand (H2L) with Dy1–Dy2 separation of 9.8617(27) Å. The Dy\\O bond lengths are in the range of 2.300(3)–2.612(3) Å. The bond lengths (2.361(3), 2.339(2) Å) from phenoxo are distinctively shorter than those from the methoxy groups (2.612(3) and 2.561(3) Å). The bond angles of O9–Dy2–O10 and O4–Dy1–O8 are 62.33(19)° and 64.14(9)°, respectively (Fig. 1, Table 2). It is distinctively different from the same salentype ligand complex [Dy(H2L)(NO3)3]2·CH2Cl2 in which two crystallographically equivalent Dy(III) ions are bridged by only a pair of ligands in bidentate coordination modes forming a closed ring of Dy2L22 with the Dy⋯Dy distance of 10.098 Å (Fig. 3) [13]. 3.2. Magnetic properties Magnetic measurements were performed on polycrystalline samples of complex 1. The phase purity of the bulk samples was confirmed by XRD analyses (Fig. 4). Direct current (dc) magnetic properties of complex 1 was investigated under 0 Oe field in the temperature range 300–1.8 K (Fig. 5). The values of χmT at room temperature are 24.19 cm3 K mol−1 for complex 1, respectively. These experimental values are smaller than the expected value of 28.34 cm3 K mol−1 for two uncoupled Dy(III) ions (6H15/2, S = 5/2, L = 5, g = 4/3, χT = 14.17 cm3 K mol−1), which high likely results from the single-ion behavior of Dy(III) ions rather than from spin-spin coupling interactions between the Dy(III) ions [14]. For complex 1, followed by a slight decrease on lowering the temperature from 300 to 20 K and then drop sharply to reach a minimum of 18.43 K mol−1 at 1.8 K. Therefore, the decrease is ascribed to the progressive depopulation of excited Stark sublevels, significant magnetic anisotropy or weak antiferromagnetic
Fig. 4. X-ray powder diffraction patterns of simulated and experimental for 1(left) and 2(right).
interactions present in the systems [15]. The field dependence of the magnetization of both complexes has been investigated in the range of 0 to 40 KOe at the temperatures 1.8, 3 and 5 K (Fig. 5, inset). The magnetization increase rapidly at low field and then increase smoothly but without saturation even at 40 KOe. The magnetization eventually reaches the value of 8.03μB (for 1), this value is lower than theoretical saturation value of 10.46μB (2 × 5.23μB), which high likely result from the crystal field effect around Dy(III) ions [16]. To explore the dynamics of the magnetization, alternating current (ac) susceptibility measurements for complex 1 was carried out in the frequency range of 1–1000 Hz. Complex 1 displays clear frequencydependent out-of-phase (χ″) signals at low temperatures under zero dc magnetic field (Fig. 6), suggesting the existence of slow magnetic relaxation behavior [17]. The absence of any frequency-dependent peaks indicates the quantum tunneling of magnetization (QTM), which reduces the expected energy barrier in complex 1. In order to suppress the possible tunneling effects, an optimum field of 1000 Oe was applied. The slow magnetic relaxation peaks of out-phase signal (χ″) could be observed below 6 K in the frequency range of 3 Hz and 999 Hz for 1 (Fig. 5, right) at 1000 Oe. In addition, a broad shoulder between 6 K (333 Hz) and 12 K (999 Hz) is exhibited at 1000 Oe, which suggests the presence of two relaxation processes for 1. To further confirm the relaxation processes in complex 1, the frequency-dependent ac susceptibilities for 1 were run to further verify the relaxation dynamics under 1000 Oe dc field (Fig. 6). There are two relaxation phases corresponding to the high-frequency signal (fast relaxation phase, FR)
Fig. 3. Left: Molecular structure of 1; Right: Molecular structure of complex [Dy(H2L)(NO3)3]2·CH2Cl2 in ref. [13].
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X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84
Fig. 5. Temperature dependence of χMT for 1 at 10 Oe field. Inset: Plot of magnetization as a function of field for 1 at 1.8 K, 3 K and 5 K.
and the low-frequency region (slow relaxation phase, SR) in complex 1 (Fig. 7) [18]. The Cole–Cole plot show fairly symmetrical semicircles and irregular curves in the high-frequency and low-frequency regions, respectively (Fig. 8). On the basis of ac susceptibility data we were able to extract the relaxation time for the temperature peak. Therefore, the energy barriers (Ueff/k B) and the pre-exponential factors (τ 0) can be fitted at 34.5 K and 4.6 × 10 − 6 s from the Arrhenius
relation [τ = τ 0exp(U eff / k BT)](Fig. 7(left), inset).The Cole–Cole plot under additional 2000 Oe dc field was conducted to further confirm the two relaxation processes in complex 1. Strikingly, the presence of two relaxation processes in complex 1 is unexpected as the two Dy(III) ions are of the same geometry. In general, the observation of two relaxation processes in dinuclear Dy(III) complexes were associated with the existence of anisotropic Dy(III) centers [18] and conformers [19]. However, two observed two relaxation processes may ascribed to the minute differences in bond lengths around two Dy(III) center which result in essential changes on the local anisotropy of the Dy(III) ions. The relatively higher energy barrier of complex 1 than that of complex [Dy(H2L)(NO3)3]2·CH2Cl2 in ref. [13] may result from the following reason. The Dy(III) ions are located in the 9coordinated geometry of bicapped trigonal prism coordination with D3h symmetry in complex 1, the high axiality of the Dy(III) ion achieved the efficient blockage of magnetization which enhance the SMMs behavior [16]. In contrast, the Dy(III) ions in complex [Dy(H2L)(NO3)3]2·CH2Cl2 is 10-coordinated with a distorted bi-capped square anti-prismatic geometry. The obvious disparity in magnetic dynamics should mainly result from coordination geometry in the Dy(III) ions, thus leading to the fast quantum tunneling from a more transverse anisotropy and relatively lower energy barrier [6f].
4. Conclusion Isolation of complex 1 demonstrates that synthesis of salen type dinuclear dysprosium complexes with rigid salen-type (H2L = N,N′bis(2-oxy-3-methoxybenzylidene)-1.2-phenylenediamine) and β-
Fig. 6. Temperature dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility of 1 at 0 Oe field (left) and at 1000 Oe field (right).
Fig. 7. Frequency dependence of in-phase (χ′) (left) and out-of-phase (χ″) (right) ac susceptibility of 1 at 1000 Oe field in the temperature range 2–9.5 K. The inset is the Arrhenius fit for the lnτ vs. T−1 plot.
X. Zou et al. / Inorganic Chemistry Communications 69 (2016) 79–84
Fig. 8. Cole–Cole plot for 1 obtained using the ac susceptibility data under 2000 Oe dc field. The inset is the Arrhenius fit for the lnτ vs. T−1 plot.
diketonate (acac = acetylacetonate) ligands are possible, and the structure of the salen type ligand dominate the structures of the complexes and the coordination geometries of the Dy(III) ions. The magnetic analysis suggests the high axial coordination geometry around each Dy(III) ions with a stronger ligand field lead to the typical SMM behavior with a higher energy barrier. The presence of two magnetic relaxations in complex 1 is associated with the presence of differences in bond lengths around Dy(III) center. Acknowledgement This work is financially supported by the National Natural Science Foundation of China (Nos. 51402092, 21471051, 21071038, 21101045 and 21501036). Appendix A. Supplementary material Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.inoche.2016.04.033. References [1] (a) X. Feng, Y. Li, Z. Zhang, E. Wang, Polyoxometalate-mediated single-molecule magnets, Acta Chim. Sin. 71 (2013) 539–542; (b) Y.N. Guo, G.F. Xu, P. Gamez, L. Zhao, S.Y. Lin, R. Deng, J. Tang, H.J. Zhang, Two-step relaxation in a linear tetranuclear dysprosium(III) aggregate showing singlemolecule magnet behavior, J. Am. Chem. Soc. 132 (2010) 8538–8539; (c) L. Sorace, C. Benelli, D. Gatteschi, Lanthanides in molecular magnetism: old tools in a new field, Chem. Soc. Rev. 40 (2011) 3092–3104; (d) D.N. Woodruff, R.E.P. Winpenny, R.A. Layfield, Lanthanide single-molecule magnets, Chem. Rev. 113 (2013) 5110–5148; (e) P.F. Yan, P.H. Lin, F. Habib, T. Aharen, M. Murugesu, Z.P. Deng, G.M. Li, W.B. Sun, Planar tetranuclear Dy(III) single-molecule magnet and its Sm(III), Gd(III), and Tb(III) analogues encapsulated by salen-type and beta-diketonate ligands, Inorg. Chem. 50 (2011) 7059–7065; (f) P.P. Yang, X.F. Gao, H.B. Song, S. Zhang, X.L. Mei, L.C. Li, D.Z. Liao, Slow magnetic relaxation in novel Dy-4 and Dy-8 compounds, Inorg. Chem. 50 (2011) 720–722; (g) Y.Q. Hu, M.H. Zeng, K. Zhang, S. Hu, F.F. Zhou, M. Kurmoo, Tracking the formation of a polynuclear Co16 complex and its elimination and substitution reactions by mass spectroscopy and crystallography, J. Am. Chem. Soc. 135 (2013) 7901–7908; (H) M.H. Zeng, Z. Yin, Y.X. Tan, W.X. Zhang, Y.P. He, M. Kurmoo, Nanoporous cobalt(II)Mof exhibiting four magnetic ground states and changes in gas sorptior upon post-synthetic modification, J. Am. Chem. Soc. 136 (2014) 4680–4688; (L) Q. Chen, M.H. Zeng, Y.L. Zhou, H.H. Zou, M. Kurmoo, Hydrogen-bonded dicubane CoII7 single-molecul-magnet coordinated by in situ solrothermally generated 1.2-bis(8-hydroxyquinolin-2-yl) ethane-1,2,-diol arranged in a trefoil, Chem. Mater. 22 (2010) 2114–2119.
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