Syntheses, structures and magnetic properties of chiral lanthanide tetrahedral clusters supported by symmetrical amidate ligands

Inorganica Chimica Acta 464 (2017) 119–124

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Research paper

Syntheses, structures and magnetic properties of chiral lanthanide tetrahedral clusters supported by symmetrical amidate ligands Shuang-Yan Lin ⇑, Baodong Sun, Zhikun Xu ⇑ Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, PR China

a r t i c l e

i n f o

Article history: Received 25 October 2016 Received in revised form 3 May 2017 Accepted 7 May 2017 Available online 8 May 2017 Keywords: Amidate ligand Tetrahedron Cluster Coordination-induced chirality Magnetic property

a b s t r a c t Two clusters with formula [Ln4(l4-O)(HL)3(SCN)4(H2O)2] (Ln = Dy (1), Er (2)) were assembled by the hydrothermal reaction of Ln(SCN)3 hydrate and symmetrical amidate H3L ligand (2-hydroxy-N-[2hydroxy-3-[(2-hydroxybenzoyl)amino]propyl]benzamide) in the presence of triethylamine. X-ray crystallographic analysis reveals that 1 and 2 are isostructural and are tetranuclear with a l4-O centred Ln4 tetrahedron core, in which four crystallographically unequivalent lanthanide ions are all seven-coordinated in distorted capped octahedron geometries. Significantly, both clusters show chirality that is induced by coordination of spirally twisted ligands. The magnetic properties of 1 and 2 have been investigated. Both clusters do not exhibit slow magnetic relaxation that may be ascribed to the effect of the symmetry of ligand field on the oblate Dy3+ and prolate Er3+ ions, as well as the regular tetrahedral core. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction The design and synthesis of polynuclear lanthanides continues to attract attention, due to their aesthetically pleasing structures [1] as well as their potential applications as magnetic [2] and optical materials [3]. Lanthanide compounds, especially of Dy3+, Tb3+ and Er3+, are promising candidates for single-molecule magnets (SMMs) [4]. This is because they possess significant inherent magnetic anisotropy arising from the large, unquenched orbital angular momentum [3d,5]. Since the discovery of Dy3 SMMs with toroidal arrangement of magnetic moments [6], the interest in polymetallic lanthanide SMMs has been developing, and yielded remarkable results including high anisotropy barriers and accessible blocking temperature regime [2c,7]. Dozens of Dy4 SMMs with different structures, such as linear, planar and polyhedral, have played important roles in such breakthrough [8]. Through analysis of the structural-magnetic relationship of these metallic cores, important features relevant to lanthanide SMMs might be revealed that will provide significant insight for structural assembly and magnetic modulations [9]. In particular, only two Dy4 tetrahedral SMMs [10], [Dy4(l4-O)(l-OMe)2(beh)2(esh)4] (3) [10b], [Dy4(l4-O) L2(C6H5COO)6] (4) [10a], have been reported to also show coordination-induced chirality. In reality, the isolation of

⇑ Corresponding authors. E-mail addresses: [email protected] (S.-Y. Lin), [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.ica.2017.05.010 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

lanthanide compounds incorporating SMMs behavior and chirality represents a challenge. Hence, the studies of structure and magnetic properties of such chiral systems could improve the knowledge of the structure-property relationship. The common ligands in the reported lanthanide SMMs are mixed N, O-donors with alkoxo, phenoxo and amino groups [7d,11]. Salen-type ligands have been used in the synthesis of lanthanide SMMs [12]. However, C@N double bonds in Salen-type ligand are rigid and limit the diversified coordination. In order to explore the coordination of a flexible ligand, we select a symmetrical amidate ligand, (2-hydroxy-N-[2-hydroxy-3-[(2hydroxybenzoyl)amino]propyl]benzamide) (H3L, Scheme 1), to construct lanthanide compounds using the alkoxo, carboxido and phenoxo donors. The ligand has only been used to synthesize transition metal compounds so far [13]. Herein, two lanthanide clusters with molecular formula of [Ln4(l4-O)(HL)3(SCN)4(H2O)2] (Ln = Dy (1), Er (2)) were prepared by using the symmetrical amidate ligand H3L. X-ray crystallography shows that the isostructural clusters 1 and 2 consist of four lanthanide ions arranged in a tetrahedral geometry around the central l4-O atom, which shows chirality that is induced by coordination of spirally twisted ligands. The synthesis, structures, magnetic properties of 1 and 2 were investigated.

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temperature until it turned to a solution. Then, the solution was transferred to Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 100 °C for two days. After cooling to room temperature, yellow single crystals of cluster 1 were obtained in 50% yield (32 mg, 0.02 mmol) based on the ligand. Elemental analysis (%) calcd for Dy4C55H52N10O18S4: C, 34.42, H, 2.73, N, 7.30; found C, 34.12, H, 2.60, N, 6.95. IR (KBr, cm
1), shown in Fig. S1: 3355 (br), 2361 (m), 2342 (w), 2076 (vs), 1608 (vs), 1584 (s), 1553 (s), 1474 (m), 1443 (m), 1344 (w), 1299 (m), 1252 (m), 1085 (w), 854 (m), 764 (m).

Scheme 1. Ligand H3L and its coordination formation.

2.3.1.2. Synthesis of [Er4(l4-O)(HL)3(SCN)4(H2O)2] (2). This cluster was obtained by a similar procedure as described for 1, substituting Er(SCN)36H2O for Dy(SCN)36H2O. Yield: 40 mg, 0.02 mmol (61%, based on the ligand). Elemental analysis (%) calcd for Er4C55H52N10O18S4: C 34.08, H 2.70, N 7.23; found: C 33.83, H 2.58, N 6.91. IR (KBr, cm
1), shown in Fig. S1: 3335 (br), 2961 (s), 2342 (m), 2079 (s), 1607 (s), 1583 (s), 1550 (s), 1473 (w), 1437(w), 1340 (w), 1250 (m), 1150 (w), 1084 (w), 850 (w), 765 (m), 699 (w).

2. Experimental section 3. Results and discussion 2.1. General 3.1. Crystal structures All chemicals and solvents used for the syntheses were of A.R. Grade and were used without purification. Elemental analyses for C, H and N were carried out on a PerkinElmer 2400 analyzer. IR spectra (4000–300 cm
1) were measured using KBr pellets by a Fourier transform infrared spectrometer Nicolet 6700. All magnetization data were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. The variable-temperature magnetization was measured with an external magnetic field of 1000 Oe in the temperature range of 2–300 K. The experimental magnetic susceptibility data are corrected for the diamagnetism estimated from Pascal’s tables and sample holder calibration. 2.2. X-ray crystallography Suitable single crystals were selected for single-crystal X-ray diffraction analysis. Crystallographic data were collected at 293 (2) K on a Bruker Apex II CCD diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å). The structure was solved by direct methods and refined on F2 with full-matrix least-squares techniques using SHELXS-97 and SHELXL-97 programs [14]. The locations of lanthanide ions were easily determined, and O, N, and C atoms were subsequently determined from the difference Fourier maps. Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The H atoms were introduced in calculated positions and refined with a fixed geometry with respect to their carrier atoms. Crystallographic data and refinement details are given in Table S1. CCDC 1497830–1497831 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

The hydrothermal reaction of symmetrical amidate ligand H3L with Ln(SCN)3 hydrate (Ln = Dy, Er) in a mixed solvent of methanol and acetonitrile in the presence of a base produces tetranuclear clusters 1 and 2, respectively. Single-crystal X-ray studies revealed that 1 and 2 were crystallographically isostructural, with the tetranuclear [Ln4(l4-O)(HL)3 (SCN)4(H2O)2] core, and both crystallizes in the orthorhombic space group Pna21 with Z = 4. The structure of 1 is described as a representative. As shown in Fig. 1, 1 contains four Dy ions, three ligands HL2
, one l4-O2
, four SCN
, and two coordinated H2O. The central l4-O2
ion bridges four Dy ions to produce a l4-O (O1) centred tetrahedral core (Fig. 2a) with Dy–O1 distances of 2.226(11)–2.268(12) Å. All of six edges of the tetrahedron are bridged by singly deprotonated phenol oxygen atoms from HL2– ligands. Within the core, the Dy  Dy distances are in the range 3.5995(14)–3.7140(12) Å, Dy–Dy–Dy angles are 58.32(3)61.20(2)°. The dihedral angles between the bottom (Dy2, Dy3, Dy4) of the tetrahedral Dy4 core and the other three sides are

2.3. Synthesis of the clusters 2.3.1. Ligand H3L was synthesized according to a previously published method [13] 2.3.1.1. Synthesis of [Dy4(l4-O)(HL)3(SCN)4(H2O)2] (1). To a solution of H3L (0.1 mmol, 33 mg) in a mixed solvent of methanol (3 mL) and acetonitrile (7 mL), triethylamine (0.3 mmol, 300 lL methanol solution of 1 mol L
1 triethylamine) and Dy(SCN)36H2O (0.2 mmol, 88.9 mg) were added. The mixture was stirred at room

Fig. 1. Structure of 1 with hydrogen atoms omitted for clarity.

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Fig. 2. (a) Tetrahedral metal core; (b) Coordination polyhedra (distorted monocapped octahedral) for four Dy3+ ions in 1.

70.950(28), 71.153(29) and 71.332(25)°. The edges of the tetrahedron, its angles and its dihedral angles have similar values, respectively, which indicate the Dy4 core is close to a regular tetrahedron. This is different from the reported distorted tetrahedron [10]. All four Dy ions are seven-coordinated in distorted capped octahedron geometries (Fig. 2b and Table S2) based on SHAPE 2.1 software [15], which are rarely reported. However, they coordinate differently: the coordination sphere of Dy1 ion is completed by three SCN
anions, forming a [N3O4] coordination environment; the [O7] coordination environments of Dy2 and Dy3 ions are both filled by two carbonyl oxygen atoms from the ligands and one H2O molecule; while the [N1O6] coordination environment of Dy4 ion is completed by two carbonyl oxygen atoms and one SCN
anion. Three HL2– ligands show the same coordination (Scheme 1), i.e. each ligand binds (by phenoxo and amide oxygens) two of the Dy3+ ions at the bottom and a Dy3+ ion at the top, so that one ligand is constrained in the same side in a spiral twist. Therefore, coordination induces chirality and two stereoisomers form a racemic mixture [10] with O1 as a chiral centre (Fig. 3). The Dy–O distances are 2.226(11)–2.415(17) Å, and the Dy–O–Dy angles are 102.1(5)– 112.9(5)°. For 2, shown in Fig. S2, the edges of the Er4 tetrahedron (Er  Er separations) are 3.5587(10)–3.6796(9) Å, the Er–Er–Er angles are 58.054(18)–61.328(18)°, the dihedral angles between the bottom (Er2, Er3, Er4) of the tetrahedron and the other three sides are 71.075(24), 71.147(24), and 71.574(21)°. All of these indicate a regular tetrahedron core in 2. Additionally, the Er–O distances are 2.198(10)–2.431(17) Å, and the Er–O–Er angles are 101.7(4)– 112.1(4)°. The two stereoisomers form a racemic mixture with O1 as a chiral centre, which can be seen in molecular structures and crystal packing (Figs. S3–S4). Examination of the crystal packing reveals that both 1 and 2 interact through hydrogen bonds. Two stereoisomers alternate in the packing along the crystallographic axes. For 1 (Fig. 4 and

Fig. 3. Two stereoisomers of 1.

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Table S3), the uncoordinated alkoxo-oxygen atoms (O4/O14) of one molecule bind to the amido nitrogen atoms (N3/N1) of the neighboring molecule via intermolecular hydrogen bonds N1H1  O14#2 (symmetry code: #2
x + 1/2, y + 1/2, z
1/2) and N3-H3  O4#1 (symmetry code: #1
x + 1/2, y
1/2, z
1/2), thus resulting in a 2D plane along a axis and a zigzag chain along c axis with alternating stereoisomers. For 2 (Fig. S4 and Table S3), the intermolecular hydrogen bonding between uncoordinated alkoxo-oxygen atoms (O4/O14) and amido nitrogen atoms (N3/N1) from two adjacent Er4 molecules generate a 2D plane along a axis and a zigzag chain along c axis with alternative stereoisomers. A key for achieving coordination-induced chirality is the binding mode of the ligands. In the triple-stranded helicates [Ln2L3]3+ [16], the three Schiff-based ligands are twisted along the central NAN bonds to give stereoisomers, D and K, in the crystal structure. In the first example of a chiral tetrahedral Dy4 SMM [10b], chirality is induced by a twisted diazine bridge from a Schiff base ligand that is locked in a conformation which prevents free rotation. In the two locally chiral dysprosium compounds [10a], the salen-type ligands are constrained in the same side in a spiral twist or bind two Dy ions in a double-stranded spiral twisted fashion, the resulting configuration of the ligands imparts a coordination-induced chirality with the formation of a racemic mixture. In 1 and 2, the symmetrical amidate ligands HL2– coordinate more slick than the Schiff-base ligands, each amidate ligand binds two of the Dy3+ ions at the bottom and a Dy3+ ion at the top, and each one is constrained in the same turn in a spiral twist. Thus, chirality with O1 as a chiral centre is induced upon coordination. The analysis above indicates that designing a ligand is the key to make lanthanide complexes with tailored properties. 3.2. Magnetic properties Direct current (dc) magnetic susceptibilities of 1 and 2 were performed on crystalline samples in the temperature range 300– 2 K under an applied field of 1000 Oe. The plots of vMT vs. T are shown in Fig. 5. The vMT value of 1 is 55.78 cm3 K mol
1 at room temperature (T = 300 K), which is slightly lower than the expected values of 56.68 cm3 K mol
1 for four non-interacting Dy(III) ions (6H15/2, S = 5/2 and g = 4/3). With the decrease of temperature, the vMT value of 1 gradually decreases to 50.19 cm3 K mol
1 at 40 K, and then rapidly decreases to 31.32 cm3 K mol
1 at 2 K. In 2, the vMT value is 45.04 cm3 K mol
1 at 300 K, which is close to the expected value of 45.92 cm3 K mol
1 for four non-interacting Er(III) ions (4I15/2, S = 3/2 and g = 6/5). The vMT product gradually decreases to 38.01 cm3 K mol
1 at 40 K and then drops sharply to reach a minimum of 17.10 cm3 K mol
1 at 2 K. The decrease should be an integrated result of the depopulation of excited Stark sublevels, weak antiferromagnetic interactions, crystal field effect and strong spin-orbit coupling [17]. The field dependence of the magnetization of both clusters was performed below 5 K from 0 to 70 kOe dc field. As shown in Fig. 6, the magnetization (M) vs H/T data do not overlap on a single master curve below 5 K, suggesting the presence of a significant magnetic anisotropy and/ or low-lying excited states. The M values for both clusters show a relatively rapid increase at low fields and then a slow linear increase without saturation through increasing dc field. For 1, M reaches 23.12 lB at 65 kOe and 1.9 K without saturation (the saturation value is equal to 4  10 lB). However, the value is close to the expected value of 20.92 lB (4  5.23 lB), which is due to the anisotropy and the crystal field effect of the Dy(III) ion that eliminates the 16-fold degeneracy of the 6H15/2 ground state [6]. 2 shows the same trend as 1 for the magnetization, i.e. M reaches 19.99 lB at 70 kOe and 1.9 K, and the value is lower than the expected saturation of 36 lB (4  9 lB).

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Fig. 4. Crystal packing of 1 along the crystallographic a axis (top) and c axis (bottom) showing the hydrogen bonding (dashed lines) and the stereoisomers (grey and blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Plots of vMT vs. T for 1 (black square) and 2 (red circle) in a dc field of 1000 Oe (2–300 K).

To probe the dynamic magnetic behaviors of 1 and 2, alternating current (ac) magnetic susceptibility measurements were performed under zero dc field. As shown in Fig. S5, 1 and 2 do

not exhibit an appreciable frequency-dependent out-of-phase signal (v00 ) at frequencies of up to 997 Hz and at temperatures down to 2.0 K, which indicates the absence of slow magnetic relaxation associated with SMM behavior. This may be attributed to a fast zero-field quantum tunnelling of magnetizations (QTM), which obliterates the advantage of anisotropic lanthanide ions. In order to find an optimal dc field to suppress the QTM, preliminary field-dependent ac measurements were performed [18], which did not show any clear field-dependent v00 signals at 997 Hz and 1.9 K for 1 and 2 (Fig. S6). This may suggest that zerofield quantum relaxation above 1.9 K is not dominating. The absence of slow magnetic relaxation for 1 and 2 can be ascribed to the effect of the symmetry of ligand field on the oblate Dy3+ and prolate Er3+ ions in 1 and 2, as well as the regular tetrahedral core. A simple model developed by Long et al. [5c] predicts the ligand architectures for generating strong magnetic anisotropy for 4f-ions. Two examples are the oblate Dy3+ (6H15/2) and prolate Er3+ (4I15/2). The oblate Dy3+ ion should be located in sandwichtype ligand geometry to maximize the anisotropy; while an equatorial geometry is predicted to be preferable for the prolate Er3+ ion. However, the seven-coordinated capped octahedron does not fit these geometries, thus resulting in the absence of an SMM

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[Ln4(l4-O)]10+ tetrahedral core with three flexible symmetrical amidate ligands coordinated in a spiral twist. Thus, both clusters show coordination-induced chirality. Both clusters have four crystallographically unequivalent lanthanide ions that are all sevencoordinated in distorted capped octahedron geometries based on SHAPE 2.1 software. Magnetic properties have been investigated, and both clusters do not exhibit SMM behavior, which can be ascribed to the effect of the symmetry of ligand field on the oblate Dy3+ and prolate Er3+, as well as the regular tetrahedral core that cancels the possible magnetic anisotropy axes. The results demonstrate the synthesis of chiral clusters by using flexible amidate ligands. Acknowledgements We thank Dr. Lin Li for helpful discussions and support of Foundation in this work. This work was partially supported by the National Natural Science Foundation of China – China (no. 21401034 and 61605036), the Heilongjiang Province Foundation for Returned Chinese Scholars – China (LC201401), Natural Science Foundation of Heilongjiang Province – China (QC2015008), and the Postdoctoral Foundation of Hei Long Jiang Province – China (301000095). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.05.010. References

Fig. 6. M versus H/T plots for 1 (top) and 2 (bottom) at different applied fields below 5 K.

behavior for 1 and 2. In addition, the regular tetrahedral core can cancel the possible magnetic anisotropy axes [1b], which could result in the absence of slow magnetic relaxation. As far as we known, there are three examples of l4-O centred Dy4 tetrahedral cluster with coordination-induced chirality, [Dy4(l4-O)(l-OMe)2(beh)2(esh)4] (3) [10b], [Dy4(l4-O) L2(C6H5COO)6] (4) [10a] and cluster 1 (Table S4). However, they display different magnetic properties: for [Dy4(l4-O)(lOMe)2(beh)2(esh)4] [10b], a frequency-dependent v00 was observed below 15 K, which indicates SMM behavior with anisotropic barriers of 23.42 K; for [Dy4(l4-O)L2(C6H5COO)6] [10a], the frequency-dependent v00 was observed below 6 K, indicating the onset of slow magnetic relaxation behavior with activation energy of ca. 2.3 K. A strongly enhanced v00 signals were observed under an optimal dc field of 1000 Oe. In contrast, 1 does not exhibit an appreciable frequency-dependent v00 and field-dependent v00 at temperatures down to 2.0 K. Analysis of the structure data (Table S4) show that Dy3+ ions are 8coordinated for 3 and 4, 7-coordinated for 1, and that 1 is the regular tetrahedral core rather than 3 and 4. Thus, the absence of slow magnetic relaxation behavior for 1 is due to the low symmetry on local Dy sites and an effective compensation of magnetic contributions in the tetrahedral arrangement. 4. Conclusions In conclusion, two tetranuclear lanthanide clusters [Ln4(l4-O) (HL)3(SCN)4(H2O)2] (Ln = Dy (1), Er (2)) have been assembled by the hydrothermal reaction. X-ray single-crystal analysis indicate that 1 and 2 are isostructural and possess a l4-O centred

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