Structure and magnetism of two chair-shaped hexanuclear dysprosium(iii) complexes exhibiting slow magnetic relaxation

Two novel hexanuclear DyIII complexes with polyhydroxy Schiff-base ligands, [Dy6(L1)4(μ3-OH)4(MeOH)4]Cl2·2MeOH·2MeCN (1) and [Dy6(HL2)2(μ3-OH)2(μ3-OCH3)2(piv)10(MeOH)2] (2) (H3L1 = N,N′-bis(3-methoxysalicylidene)(propylene-2-ol)-1,3-diamine, H3L2 = 2,3-dihydroxypropylimino)methyl)-6-methoxyphenol, piv = pivalate), have been prepared under solvothermal conditions and structurally characterized by single-crystal X-ray diffraction, elemental analyses, thermal analyses, and IR spectroscopy. Each of the hexanuclear complexes is constructed with Dy3 triangular motifs as building blocks, and the six DyIII ions are arranged in a chair-shaped conformation. Variable-temperature magnetic susceptibility measurements in the temperature range of 2–300 K indicate dominant ferromagnetic exchange interactions between the DyIII ions in the complexes. Both complexes exhibit slow magnetic relaxation behavior.


Introduction
The synthesis of new single-molecule magnets (SMMs) containing lanthanide metals continues to grow because of their fascinating magnetic behaviours and potential applications in many elds such as high-density information storage, quantum computing, molecular spintronics etc. 1-6 Most of the 4f-based SMMs reported in the literature are derived from Dy III ion due to its unquenched orbital angular momentum and large intrinsic magnetoanisotropy. [7][8][9][10][11] In the past decade, a number of mono-and polynuclear Dy III complexes have been synthesized and characterized, 12 and many of them have been demonstrated to exhibit remarkable properties of singlemolecule magnets. For example, mononuclear Dy III complexes with pentagonal bipyramidal coordination geometry featuring strong axial ligand eld and weak equatorial donors exhibit very high effective energy barriers for magnetic relaxation. 13 A dinuclear Dy III complex bridged by an N 2 3À radical ligand was observed to show magnetic hysteresis up to 8.3 K. 14 A tetranuclear Dy III alkoxide cage complex and a square-based pyramid iso-propoxide-bridged pentanuclear Dy III complex have been reported to possess large thermal energy barrier for the reversal of magnetization. 1b, 15 Among the various Dy III -based SMMs reported, the Dy 3 triangular molecules have been found to be a magnetically interesting system. This system has an essentially nonmagnetic spin ground state, but exhibits SMM behavior of thermally populated excited states, as well as toroidal magnetic moments which are useful in molecule-based multiferroics. 16 Stimulated by the fascinating magnetic behaviors of these intriguing Dy 3 triangles, research on the utilization of theirs as building blocks to construct larger SMMs with enhanced magnetic properties has attracted much attention. It was reported that linking of two such highly anisotropic Dy 3 triangles in different forms led to the creation of hexanuclear dysprosium SMMs, 17 of which enhanced slow magnetic relaxation was observed, 17b and enhanced toroidal magnetisms were obtained by ne-tuning the arrangements of the Dy III ions or by modifying the local ligand-eld around the Dy III ions. 17d,e Combination of two independent Dy 3 SMM-building blocks by a paramagnetic [Dy(m 2 -CH 3 O) 2 Dy] 4+ linker afforded an octanuclear Dy III complexes with SMM behavior inherited from its Dy 3 precursor. 18 An even larger cluster of decanuclear Dy III based on the peculiar Dy 3 triangles was obtained by incorporating two sets of vertex-sharing Dy 3 triangular motifs. 19 These successful examples represent a promising strategy for preparation of novel Dy-based SMMs by using highly anisotropic Dy 3 triangles as building blocks.

Materials and methods
Metal salts and solvents were purchased from commercially available and used directly without further purication in the preparation of the free ligands and complexes. The Schiff-base ligands H 3 L 1 and H 3 L 2 were prepared as previously described. 21 IR spectra were recorded in the range of 4000-400 cm À1 on Perkin-Elmer Spectrum Two FT/IR spectrometer using a KBr pellet. Elemental analysis (C, H, N) was performed on a Elementar Micro cube CHN elemental analyzer. The thermal analysis was performed on Labsys Evo TG-DTG/DSC. The crushed single-crystal sample was heated up to 1000 C in N 2 (99.99%) at a heating rate of 10 C min À1 . Magnetic susceptibility measurements were performed in the temperature range of 2-300 K, using a Quantum Design MPMS SQUID-XL-5 magnetometer equipped with a 5 T magnet. The diamagnetic corrections for these complexes were estimated using Pascal's constants, and magnetic data were corrected for diamagnetic contributions of the sample holder. 22 Alternating current susceptibility measurements were taken of powdered samples to determine the in-phase and out-of-phase components of the magnetic susceptibility. The data were collected by increasing temperature from 2 K to 10 K, with no applied external dc eld and a drive frequency of 2.5 Oe, with frequencies between 10 and 1000 Hz. In the samples where free movement of crystallites were prevented, silicone grease was employed for the embedding.

Results and discussions
Syntheses Both complexes were synthesized under solvothermal conditions. Reactions of DyCl 3 $6H 2 O with H 3 L 1 in the presence of Et 3 N in 10 : 5 : 2 molar ratio in a mixed solvent of CH 3 OH and CH 3 CN (1 : 1) in Teon-lined autoclave at 80 C for ve days produced yellow crystals of 1 in 42% yield. Utilization of single solvent could not yield the compound. The Schiff base ligand in the complex has a À3 charge with all hydroxyl groups deprotonated. Reactions of Dy(NO 3 ) 3 $6H 2 O, H 3 L 2 , pivalic acid, and Et 3 N in 5 : 5 : 30 : 2 molar ratio in similar solvent as 1 in Teonlined autoclave at 120 C for six days produced yellow crystals of 2 in 31% yield. The ligand in complex 2 has a À2 charge with the Scheme 1 Schiff-base ligands of H 3 L 1 (left) and H 3 L 2 (right).
phenolic hydroxyl group and one alcoholic hydroxyl group deprotonated. Reactions of H 3 L 1 or H 3 L 2 with other Dy(III) sources, such as Dy(NO 3 ) 3 $6H 2 O for H 3 L 1 and DyCl 3 $6H 2 O for H 3 L 2 , have been tested, but no crystals of 1 and 2 were obtained.

Crystal structures
The solid state structures of 1-2 were determined by single crystal X-ray crystallographic studies. Crystallographic data for complexes 1-2 are presented in Table S1, † and selected bond lengths and angles are given in Tables S3-S5 † in the ESI. † Single-crystal X-ray studies revealed that complex 1 crystallized in the triclinic space group P 1, with the asymmetric unit containing the entire molecule as well as a chloride counterion and MeOH and MeCN solvent molecules. A view of the cationic complex 1 is shown in Fig. 1. The X-ray structure of 1 reveals a centrosymmetric core with two equivalent Dy 3 moieties linked by two m 3 -OH À anions. Each m 3 -OH À anion links three Dy ions, and the Dy-Dy separations are similar at 3.521Å, 3.507Å and 3.639Å for Dy1-Dy2, Dy1-Dy3 and Dy2-Dy3, respectively. Dy1 is coordinated with six oxygen atoms and two nitrogen atoms, which originating from two different L 1 ligands, and two m 3 -OH À anions. The Dy2 center is ligated with seven oxygen atoms (Dy-O ¼ 2.047(6)-2.235(5)Å) and one nitrogen atom (Dy-N ¼ 2.235(5)Å) from three different Schiff-base ligands, one m 3 -OH À anion, and one MeOH molecule. Dy3 is coordinated with seven oxygen atoms and one nitrogen atom from two Schiff-base ligands, three m 3 -OH À anions, one MeOH molecule. All Dy ions are 8-coordinate and display distorted bicapped trigonal prism geometry. The hexanuclear Dy(III) complex contain four Dy 3 triangles. The Dy1, Dy3, Dy1A, and Dy3A atoms are in the same plane, forming a parallel quadrilateral, which containing two Dy 3 triangles (Dy1, Dy3, Dy1A, and Dy1A, Dy3A, Dy1). Each triangle link by one m 3 -OH À anion. Dy2 and Dy2A are located at both ends of the parallel quadrilateral, Dy2A lying above the plane and Dy2 below. Therefore, two other Dy 3 triangles (Dy1, Dy2, Dy3, and Dy1A, Dy2A, Dy3A) are formed. The latter two Dy 3 triangles are linked via one m 3 -bridging hydroxyl group of the ligand L 1 and one m 3 -OH À (located on the front and back of the triangle). Obviously, the layout of the six Dy atoms forms a chair-shaped. Notably, such a hexanuclear Dy(III) complex is quite different from other known examples. 24 First, there exit a parallel quadrilateral containing two Dy 3 triangles. Second, all Dy atoms are 8-coordinate with distorted bicapped trigonal prism geometry. Furthermore, the Schiff-base ligand has a À3 charge with two phenolic hydroxyl groups and one backbone hydroxyl group deprotonated in 1. And the ligands adopt two different coordination fashions (Scheme 2). Adjacent molecules are segregated by these ligands, the shortest intermolecular Dy-Dy separation is 8.478Å, indicating that the molecules are quite well isolated. Hydrogen atoms were determined by analyzing the coordination environment of oxygen atoms and hydrogen bonds were found in 1. There is a triply hydrogen-bonded among the chloride ion (Cl1) and the solvent methanol molecule (O15), the coordinated methanol (O13), m 3 -OH (O1) (Fig. S1 †). Meanwhile, there is a strong hydrogen bonds interaction between the solvent methanol molecule (O15) and coordinated oxygen atoms (O2) with the O2-H2/O15 distances of 2.853Å.
A perspective view of 2 is depicted in Fig. 1. Complex 2 crystallises in the monoclinic space group P2 1 /c. The hexanuclear complex surrounded by two Schiff-base ligands and ten pivalate ligands. Only one coordination and bridging mode can be observed for the unique polydentate Schiff-base HL 2 ligand (Scheme 2). Three different coordination modes of pivalate ligands coexist in the crystal structure (Scheme 3). Six of the ten pivalate anions acts as a bridging ligand with two oxygen atoms coordinating separately to two Dy ions. Two pivalate anions coordinate to two Dy ions respectively in m 2 -pivalate-k 3 O 1 O 2 :O 2 fashion. The other two pivalate anions coordinate respectively to Dy ions in monodentate mode. Two methanol molecules occupy the remaining coordination sites of the Dy2 and Dy2A metal centres. The Dy1 is nine-coordinate with distorted capped square antiprism geometries (Fig. S2 †), Dy2 and Dy3 are eightcoordinate, displaying triangular dodecahedron and square antiprism geometry, respectively. In the hexanuclear unit, Dy1, Dy2, Dy1A and Dy2A ions are present in the same plane, forming a parallel quadrilateral, whereas the Dy3 and Dy3A ions are arranged in a trans geometry with respect to each other and are displaced by 1.618Å from the plane. Thus, the hexanuclear core adopts the chair conformation. This Dy 6 complex also contain four Dy 3 triangles. The two Dy 3 triangles (Dy1, Dy2, Dy1A, and Dy1, Dy2A, Dy1A) are linked by one m 3 -OH À anion, respectively. Two other Dy 3 triangles (Dy1, Dy2, Dy3, and Dy1A, Dy2A, Dy3A) are linked via one m 3 -bridging hydroxyl group of the ligand L 2 and one m 3 -OCH 3 . The closest intercluster Dy-Dy distances is 9.49Å, also indicating that the molecules are quite well isolated.
Up to now, many hexanuclear Dy III complexes with different topologies have been reported (Table S2 †). The arrangements of six Dy III ions in the complex with ring (or wheel), 25 hollow, 26 hemi-cubane, 27 trigonal prism, 28 edge-to-edge, 17c,29 etc. have been described. The chair-shaped arrangement described here is newly-reported, and contributes an interesting example to the topology catalogues of the Dy 6 complex.
Metal complexes of ligands H 3 L 1 and H 3 L 2 have been reported in literatures. 24,[30][31][32] For ligand H 3 L 1 , most of reported complexes, with 3d, 4f, 3d-4f, and others as metal centers, are nite multinuclear compounds. 24,30,31a-f Only two examples containing Cu ions are coordination polymers with onedimensional structure which was built by the bridging of alcoholic hydroxyl group of the ligand. 31g,h The coordination modes of the ligand H 3 L 1 in these reported complexes are summarized in Scheme S1. † For ligand H 3 L 2 , 3d and 4f metal complexes are reported, which are all nite multinuclear. 32 The ligand in the complexes is found to show a variety of coordination modes that are listed in Scheme S2. † It is interesting to note that the coordination modes of the ligands H 3 L 1 and H 3 L 2 exhibited in this work (Scheme 1) have never been reported previously.

Spectroscopic and thermal analyses of complexes 1-2
The infrared (IR) spectra of complexes 1-2 have been recorded between 4000 and 400 cm À1 with KBr pellets (Fig. S3-4 †). In the low-wavenumber region, the spectra exhibit the characteristic Dy-O and Dy-N vibration bands. The characteristic strong absorptions at $1642 cm À1 were ascribed to the vibration of C]N bond, indicating the formation of the Schiff-base ligand. 33 Furthermore, the resonances at 2967 and 2940 cm À1 are assigned to the n(CH 2 ) stretching vibrations, and the signals at 1482-1421 cm À1 and 1467-1385 cm À1 correspond to the d(CH 2 ) bending vibrations, respectively. 34 The peaks appearing in the range of 1640-1390 cm À1 are assigned to the stretch of benzene ring from Schiff-base ligands. The n(O-H) vibration at around 3400 cm À1 can also be observed in complexes 1-2, respectively. In summary, the results of the IR spectras are consistent with the single-crystal structural analyses.
Thermal analyses have been carried out to examine the thermal stability of the complexes 1-2. The crushed singlecrystal samples were heated up to 1000 C in N 2 at a heating rate of 10 C min À1 . The TG curves of 1 (Fig. S5 †) show that the rst weight loss of 7.6% between 30 and 155 C corresponds to the loss of solvent MeCN, MeOH and chloride counterion (calc. 7.8%). The weight gradually decrease 4.3% with temperature increasing from 155 and 203 C corresponds to the loss of terminal MeOH molecules (calc. 4.5%). The residue is stable up to 213 C, where aer pyrolysis of Schiff-base ligand and then ends at 710 C (weight loss: 45.3%; calc. 45.7%). The nal residual weight of 42.5% (calc. 46.7%) corresponds to that of Dy 2 O 3 . The above thermal analysis results basically agree with the formula of 1. The thermal properties of the complex 2 was also investigated. The weight loss of 4.9% is observed at 200 C which maybe corresponding to the coordinated MeOH molecules and part of pivalate ligands. When the temperature continues rising, the Schiff-base ligands and other pivalate ligands begin to decompose in the 200 to 600 C temperature range. Finally, the remaining weight of 43.8% is in agreement with the proportional weight (calc. 43.3%) of Dy 2 O 3 .

Magnetic properties
The direct-current (dc) magnetic susceptibility of 1-2 were measured in the range of 300-2 K with an applied magnetic eld of 1000 Oe and can be seen plotted as c M T vs. T in Fig. 2. The room temperature c M T values of 84.95 and 84.57 cm 3 K mol À1 for 1-2, respectively, which is in agreement with the expected value of 85.02 cm 3 K mol À1 for six uncoupled Dy III ions (Dy III : 6 H 15/2 , S ¼ 5/2, L ¼ 5, J ¼ 15/2, g ¼ 4/3). The c M T values of 1-2 gradually increase with decreasing temperature to reach a maximum of 92.39 cm 3 K mol À1 at 50 K and 87.81 cm 3 K mol À1 Fig. 4 Temperature dependent ac susceptibility for 1 in the absence of a dc field (H ac ¼ 2.5 Oe) (left); plots of ln(c 00 /c 0 ) vs. 1/T for 1. The solid lines represent the fitting results over the temperature range of 2-5 K (right). Paper RSC Advances at 60 K, before decreasing to 39.12 cm 3 K mol À1 and 49.01 cm 3 K mol À1 at 2 K, respectively. The rst increase of the c M T values suggests the presence of a dominant intramolecular ferromagnetic interactions between the Dy III ions. The low temperature decrease may be due to a combination of intermolecular antiferromagnetic interactions, large magnetic anisotropy and thermal population of the excited states of the Dy III ions (Stark sublevels of the 6 H 15/2 state). 35 The eld dependence of the magnetization (M) for 1-2 were performed in the eld (H) range of 0-5 T at 2-5 K. As expected for ferromagnetically coupled spins, the M vs. H data below 5 K reveal a rapid increase of the magnetization at low magnetic elds (Fig. S6 †). At high elds, M increases rapidly to reach 45.76 Nb and 36.38 Nb at 2 K and under 5 T, respectively. These values are all lower than the corresponding theoretical saturated values. The non-superposition of the M vs. H/T data (Fig. 3) on a single master curve and the higheld nonsaturation suggests the presence of signicant magnetic anisotropy and/or low-lying excited states in complexes 1-2. 36,37 The temperature-dependent alternating-current (ac) susceptibility of complexes 1-2 were measured under zero-dc eld and 2.5 Oe ac eld ( Fig. 4 and S7 †). Both the in-phase (c 0 ) and out-ofphase (c 00 ) signals of 1-2 show frequency dependence at low temperatures 2-10 K. The shape and the frequency dependence of the ac susceptibility signal indicate the slow magnetic relaxation behavior of 1-2. Although such ac signals were observed above 2 K, no obvious hysteresis was detected in the M vs. H data obtained using a traditional SQUID magnetometer ( Fig. S8-9 †).
The above magnetic determination indicates that complexes 1-2 display a clear signature of slow magnetic relaxation behavior. Unfortunately, the expected maximum due to the blocking could not be observed above 2 K, even by applying a dc magnetic eld up to 1000 and 2000 Oe (Fig. S10 †). Under the assumption that the SMM relaxation has just one characteristic time, we could obtain the energy barriers and s 0 values by tting the ac susceptibility data from adopt the Debye model. 37 This gave s 0 ¼ 9.03 Â 10 À5 s and U eff ¼ 1.03 K for 1 and s 0 ¼ 2.36 Â 10 À5 s and U eff ¼ 4.21 K for 2 ( Fig. 4 and S7 †). The Cole-Cole diagram can be used to study the distribution of the relaxation process, which is frequently characterized and discussed for SMMs. The data of 1-2 plotted as Cole-Cole diagrams are shown in Fig. S11. † The shape of the Cole-Cole plot of 1-2 are relatively symmetrical and can be tted to the generalized Debye model with a parameter a ranging from 0.11 to 0.15 for 1, 0.07-0.13 for 2, respectively, the relatively small a value indicates that a single relaxation time is mainly involved in the present relaxation process independently of the temperature.

Conclusions
In summary, we have synthesized two hexanuclear Dy III complexes based on two polyhydroxy Schiff-base ligands. It is interested that the complexes can be regarded as the result of the construction with Dy 3 triangular motifs as building blocks, and the six Dy III ions in the complex are arranged in a newlyreported chair-shaped geometry. The measurements of the dc susceptibility and of the eld dependence of magnetization indicated the existence of the intramolecular ferromagnetic interactions. The frequency-dependent ac susceptibility signals revealed by the ac susceptibility investigations evidenced the SMM behaviors under zero dc eld for two compounds. The lanthanide SMMs exhibiting intramolecular ferromagnetic interactions are rarely reported, and the synthesis of such a lanthanide SMM is believed to be of great challenges. The results of this work might imply an effective approach for the rational design and construction of lanthanide SMMs with novel magnetic behaviors by means of the lanthanide building blocks.

Conflicts of interest
There are no conicts to declare.