Syntheses, crystal structures and magnetic properties of sandglass DyIII9 and irregular tetrahedron DyIII4 complexes

Polyhedron 141 (2018) 69–76

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Syntheses, crystal structures and magnetic properties of sandglass DyIII 9 III and irregular tetrahedron Dy4 complexes Hui-Sheng Wang ⇑, Qiao-Qiao Long, Cheng-Ling Yin, Zi-Wei Xu, Zhi-Quan Pan School of Chemistry and Environmental Engineering, Key Laboratory of Green Chemical Process of Ministry of Education, Key Laboratory of Novel Reactor and Green Chemical Technology of Hubei Province, Wuhan Institute of Technology, Wuhan 430074, PR China

a r t i c l e

i n f o

Article history: Received 31 August 2017 Accepted 19 November 2017 Available online 24 November 2017 Keywords: Dysprosium Magnetic anisotropy Nonanuclear compounds Tetranuclear compounds Slow magnetic relaxation

a b s t r a c t The synthesis, crystal structures, and magnetic properties of a nonanuclear dysprosium complex [Dy9(l3-OH)8(l4-OH)2(N3)8{(py)2C(OCH3)O}8](OH)
4H2O (1, (py)2C(OCH3)OH = the hemiacetal form of di-2-pyridyl ketone) and a tetranuclear dysprosium complex [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4] (OH)2
3.5CH3OH (2, HL = 2-(benzothiazol-2-yl-hydrazonomethyl)-6-methoxyphenol) have been prepared and structural characterization. Nine DyIII ions in 1 form a sandglass with the eight DyIII atoms at the apexes and one DyIII ion located at the center of a square antiprism formed by eight DyIII ions, while four DyIII ions in 2 form an irregular tetrahedron. Complexes 1 and 2 have been investigated by direct current (dc) and alternating current (ac) susceptibility measurements. The ac susceptibility studies revealed that complexes 1 and 2 exhibit slow magnetic relaxation. Moreover, complex 2 shows two-step thermal magnetic relaxation with their energy barriers of about 15 K and 2.68 K, respectively. The orientations of the magnetic anisotropy of DyIII ions in 1 and 2 were also estimated by electrostatic calculations. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Single-molecule magnets (SMMs) are a focused material in recent years, mainly due to their potential applications for memory storage devices, molecular spintronics and quantum computation [1]. SMMs are discrete molecular complexes which possess slow relaxation after removing external magnetic field and the origin of molecular magnetism in SMMs is energy barrier (U) of the magnetization reversals between ‘spin up’ state and ‘spin down’ state [2,3]. For the 3d transition metal SMMs complexes, the energy barrier can be improved by increasing the value of spin ground state (S) and Ising-type magnetic anisotropy (negative zero-field splitting parameter, D) [4]. However, in 2008, Ruiz et al. have drawn a conclusion that large magnetic anisotropy is not favored by a high spin state of the ground state through theoretical study of two Mn6 complexes [5]. Indeed, in some highnuclear Mn complexes, the spin ground state can be maximized through ferromagnetic couplings between paramagnetic ions within these complexes, however, their magnetic anisotropy is very low (close to zero), which may be due to mutual perpendicular for the Jahn–Teller axes of MnIII ions within these complexes [6]. Therefore, synthesis of SMMs with a large magnetic anisotropy

⇑ Corresponding author. E-mail address: [email protected] (H.-S. Wang). https://doi.org/10.1016/j.poly.2017.11.025 0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.

may be another option [7,8]. Ln-ions are a good candidate to meet this demand, because it have no quenching of the orbital angular momentum in the crystal field, which result in the strong spin orbit coupling interactions and further make 4f ions possess significant magnetic anisotropy [9–12]. In many of the lanthanide metal ions, dysprosium is considered to be the most suitable candidate in the study of 3d-4f and 4f SMMs [13,14]. The reason is that DyIII possesses high anisotropy of the spin–orbit coupled Kramers doublet ground state, and it have a large separation between the doublet ground state and the first excited state or the higher excited states, which leads to the slow spin relaxation phenomenon [15,16]. Up to date, many DyIII SMMs complexes with different structural topologies, such III III III III III as DyIII 3 , Dy6 , Dy8 , Dy10, Dy11, Dy12 et al. [17–22], have been obtained. However, due to the weak magnetic coupling interactions and the obviously quantum tunneling effect, the challenge of improving energy barrier remains [23,24]. Quantum tunneling of magnetization (QTM) is resulted from the mixing of ±MJ level via hyperfine coupling, dipolar spin–spin interactions, or transverse anisotropy (E) [25a]. The latter two factors can be minimized by improving the symmetry of coordination geometry of a Kramers ion [25b]. The DyIII ions with high symmetries like C1v, D1h, S8 (I4), D4d, D5h and D6d can decrease QTM of the SMMs complexes [25c]. Our group has recently been investigating the syntheses, crystal structures and magnetic

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properties of polynuclear 3d and 3d-4f complexes containing two organic ligands to regulate the structural topologies of metal ions [26]. In this paper, we will turn to synthesize DyIII complexes containing two organic ligands with different symmetries and coordination modes, to regulate the topologies of Dy complexes and to make DyIII within complexes possess the high symmetries of coordination geometries as stated above. Reaction between Dy (NO3)3
5H2O, di-2-pyridyl ketone (dpk), NaN3 and 2-(Hydroxymethyl)pyridine (Hhmp) or HL ligands in the methanol solution containing triethylamine, obtained the crystals of [Dy9(l3-OH)8(l4-OH)2(N3)8{(py)2C(OCH3)O}8](OH)
4H2O (1, (py)2C(OCH3)OH = the hemiacetal form of dpk) or [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4] (OH)2
3.5CH3OH (2, HL = 2-(benzothiazol-2-yl-hydrazonomethyl)6-methoxyphenol). It should be noted that complexes 1 and 2 contain only one kind of organic ligand. The reason may be that the coordination ability of dpk is greater than that of Hhmp in compound 1, while the coordination ability of HL is greater than dpk in compound 2, so that resulting in only one organic ligand coordinated to the metal in the process of reaction. Certainly, one organic ligand with large steric hindrance is not easy to coordinate with metal ions in the formation of 1 and 2. Here, we describe the synthesis, crystal structures and magnetic properties of 1 and 2. 2. Experimental 2.1. Materials and physical methods All synthetic procedures were completed under aerobic conditions, and all of the reagents and the chemicals were obtained through commercial sources and were used without further purification. The 2-(benzothiazol-2-yl-hydrazonomethyl)-6methoxyphenol ligand (HL, Scheme 1a) was prepared by the already reported method [27]. Caution! Azide salts is potentially explosive and should be used in small quantities and with utmost care at all times. 2.2. Synthesis of [Dy9(l3-OH)8(l4-OH)2(N3)8{(py)2C(OCH3)O}8](OH)
4H2O (1) A mixture of Dy(NO3)3
5H2O (0.0686 g, 0.15 mmol), di-2-pyridyl ketone (0.0276 g, 0.15 mmol), 2-(Hydroxymethyl) pyridine (0.0164 g, 0.15 mmol), NaN3 (0.0098 g, 0.15 mmol) and triethylamine (0.0455 g, 0.45 mmol) was stirred in methanol (MeOH, 30 mL) for 2 h, resulting in a colorless slightly turbid solution, then the solution was filtered. Colorless crystals were collected by filtration after slow evaporation of the resulting solution for about 20 days. Yield: 15 mg (about 23% based on Dy salt). Elemental analysis (%), calcd for C96H107Dy9N40O31 (Mr = 3779.72): C 30.51, H 2.85, N 14.82; Found: C 30.47, H 2.86, N 14.76. IR data (KBr, v/cm1, s, m and w stand for strong, medium and weak, respectively): 3451 (s), 2954(w), 2821(w), 2075(s), 1601(m), 1571(w), 1508(m), 1472 (m), 1437(s), 1384(s), 1288(m), 1260(m), 1228(m), 1157(w), 1108(s), 1067(s), 1013(m), 987(m), 785(s), 683(s), 634(m), 489(m). 2.3. Synthesis of [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4](OH)2 3.5CH3OH (2) A mixture of Dy(NO3)3
5H2O (0.0343 g, 0.075 mmol), di-2-pyridyl ketone (0.0138 g, 0.075 mmol), HL ligand (0.0112 g, 0.0375 mmol), NaN3 (0.0060 g, 0.092 mmol) and triethylamine (0.0228 g, 0.225 mmol) was stirred in methanol (MeOH, 20 mL) for 2 h, resulting in a bright yellow solution, then the resulting solution was filtered. Yellow crystals were collected by filtration after slow evaporation of the resulting solution for about 15 days. Yield: 7 mg (about 17% based on Dy salt). Elemental analysis (%), calcd for C62H56Dy4N18O13S4 (Mr = 2039.48, 3.5 solvent CH3OH molecules

were free): C 36.51, H 2.77, N 12.36; Found: C 36.55, H 2.78, N 12.39. IR data (KBr, v/cm1, s, m and w stand for strong, medium and weak, respectively): 3427(s), 2939(w), 2063(s), 1640(s), 1606 (s), 1548(m), 1458(s), 1405(w), 1300(w), 1262(w), 1242(m), 1220 (s), 1168(w), 1065(m), 1012(w), 965(m), 848(m), 783(w), 740(m), 634(w), 604(w), 509(w), 473(w), 444(m). 2.4. Physical measurements Fourier transform infrared spectra (IR) were taken with a VECTOR 22 spectrometer at room temperature using KBr pellets in the range of 4000–400 cm1. Elemental analysis of C, H, N was performed using an Elementary Vario Perkin-Elmer 240C analyzer. The magnetic properties of polycrystalline samples of 1–2 were performed with a Quantum Design MPMS-XL7 superconducting quantum interference device (SQUID) magnetometer. The direct current (dc) magnetic susceptibilities were collected in the 2–300 K temperature range in a dc magnetic field of 1000 Oe and the alternating-current (ac) measurements were performed in a zero-applied dc field or in a 1000 Oe for 1 (500 Oe for 2) dc field with a 5.0 Oe ac field oscillating at frequencies in the range of 1–999 Hz and in the temperature range of 2.0–15 K. The diamagnetic corrections for 1–2 were estimated using Pascal’s constants. 2.5. X-ray crystallographic studies Single-crystal data for complexes 1 and 2 were collected at 173 K on a Bruker Smart CCD diffractometer with graphite-monochromatic Mo Ka radiation (k = 0.71073 Å) by x-scan mode. The collected data were reduced using the software package SAINT [28], and semi-empirical absorption correction was applied to the intensity data using the SADABS program [29]. The structure of 1 and 2 were solved using direct methods, and all non-hydrogen atoms were refined anisotropically by least squares on F2 using the SHELXT-2014

program [30]. Hydrogen atoms were placed in calculated positions and refined isotropically using the riding model. Some solvent molecules in compound 1 and 2 cannot be properly modeled, so the contributions were subtracted by the SQUEEZE command as implemented in PLATON [31]. Unit cell data and structure refinement details are listed in Table 1. Selected bond lengths and angles for 1 and 2 are listed in Tables S1 and S2, respectively.

3. Results and discussion 3.1. Synthesis Previous studies have shown that if two different ligands are selected to react with magnetic spin carrier salts, novel complexes containing mixed organic ligands can be obtained. Moreover, the change of the number of metal nuclearity and coordination geometry of rare earth ions affect the magnetic anisotropy and the energy barriers. According to the rules of the hard soft acid base (HSAB) theory, the hydroxyl/phenoxido O atoms and pyridyl N atoms on the selected 2-(benzothiazol-2-yl-hydrazonomethyl)-6methoxyphenol (HL), 2-(Hydroxymethyl)pyridine (Hhmp) and hemiacetal form of di-2-pyridyl ketone (dpk) (Scheme 1) belong to hard base, and DyIII ion belongs to hard acid. Therefore, dpk and Hhmp (or dpk and HL) may be simultaneously coordinated with DyIII ions to form Dy complexes with mixed organic ligands. In this work, we selected Dy(NO3)3 5H2O, di-2-pyridyl ketone, NaN3 reacting with Hhmp or HL ligands in the presence of MeOH containing triethylamine. However, the obtained Dy9 (1) and Dy4 (2) contain only one organic ligand.

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H N

S

N N OH O

N

N

(a)

O

(b) OH N

N Dy

Dy

(c) O

III

S N

O O

CH3

H N

II I

N

N

Dy

II I

O

Dy

I II

(e)

(d)

Scheme 1. The structure of HL (a), dpk (b) and Hhmp (c) ligands; coordination modes of (py)2C(OCH3)O (e) and L (d).

3.2. Crystal structures of 1 Single-crystal X-ray diffraction analyses reveal that complex 1 comprises of a nonanuclear DyIII monocation, [Dy9(l3-OH)8(l4OH)2(N3)8{(py)2C(OCH3)O}8]+, a OH counter anion and four solvent water molecules. Complex 1 crystallized in the tetragonal space group I4 with Z = 2 (Table 1). The crystal structure of the nonanuclear DyIII monocation is shown in Fig. 1 and Fig. S1 and the selected bond parameters are summarized in Table S1. As shown in Fig. 1, the topology of the metal ions core in 1 can be viewed as a sandglass, which is similar to several DyIII 9 compounds reported before, as shown in Table S7 in supporting information. The III two DyIII (Dy2) by 4 square planes are further linked the central Dy  eight l3-OH ions, with the two square planes are twisted at about 44.6°. Peripheral ligation about the core is completed by eight (py)2C III (OCH3)O ligands and eight terminal N ions are 3 , and the nine Dy eight-coordinate. As stated above, the central Dy2 ion is surrounded by eight l3-OH, which forms a O8 coordination geometry, with the distances of Dy2-O3 and Dy2-O4 being 2.366(10) Å and 2.381(11) Å, respectively. However, for Dy1 and Dy3, their coordination spheres    are each completed by two l 3 OH , one l4 OH , one N atom from  N3 , two O atoms and two N atoms from two (py)2C(OCH3)O, to form a N3O5 coordination geometry. By carefully checked the geometry of Dy ions on the square plane for the reported four DyIII 9 complexes, the N3O5 geometry in 1 was first reported. The distances of Dy2 Dy1 and Dy2 Dy3 are both 3.7602(14) Å, indicating that two DyIII 4 square plane units are not bilateral symmetry for the central Dy2 ion. Additionally, all of the eight (py)2C(OCH3)O ligands in 1

possess the same coordination mode, in which the two pyridyl N atoms each bonded with two DyIII atoms, and its alkoxido-type O atom also linked with the two Dy atoms, while the O atom from methoxy group not coordinated with metal ions, hence, its coordination mode is g1:g1:g2:l (Scheme 1, e). Moreover, to our knowledge, complex 1 represents the largest nuclear number in rare earth metal compounds with the hemiacetal form or gem-diol form of dpk ligand [32]. In the case of lanthanides, the geometry of the metallic center is strongly correlated to the local anisotropy of the rare earth ions. Thus, performing an analysis of the coordination geometry of rare earth ions is of great importance. For this reason, the coordination geometry of the nine DyIII ions in 1 was determined using the SHAPE software [33]. Based on the calculation results, the coordination geometry of Dy1, Dy2 and Dy3 was distorted biaugmented trigonal prism type (BTPR-8, C2v), square antiprism prism type (SAPR-8, D4d) and snub disphenoid type (J84) (JSD-8, D2d) as the analysis gave minimum values of 25.870, 23.487 and 22.347 for 1, respectively (Tables S3 and S4, Fig. S2). The larger calculated values show that they more deviate from the ideal geometry. It demonstrates that the low symmetry of the coordination geometry of the DyIII may lead to the low energy barrier (Ueff, vide infra). 3.3. Crystal structures of 2 Single-crystal X-ray diffraction analyses reveal that complex 2 consists of a [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4]2+, two OH counter anion and seven CH3OH solvent molecules. Complex 2 crystallized

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Table 1 Crystallographic data for 1 and 2. Compounds

1

2

Formula Formula weighta Crystal color Crystal size (mm) Crystal systema Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Unit cell volume (Å3) T (K) Z Radiation type l (mm1) Dc/g (cm3) Index ranges

C96H107Dy9N40O31 3779.72 colorless 0.23  0.20  0.17 tetragonal I4

C65.5H70Dy4N18O16.5S4 2151.63 yellow 0.23  0.09  0.06 monoclinic P21/n

19.0813(5) 19.0813(5) 19.8885(14) 90.00 90.00 90.00 7241.3(6) 293(2) 2 Mo/Ka 4.656 1.733 22  h  22, 22  k  21, -15  l  23 3609 12866 5248 [0.0650] 4223 R1 = 0.0382, wR2 = 0.0784 R1 = 0.0628, wR2 = 0.0832 1.036

12.8459(8) 21.5069(14) 27.7460(17) 90 90.110(2) 90 7665.5(8) 173(2) 2 Mo/Ka 4.040 1.864 15  h  13, 25  k  25, 32  l  32 4196 67804 13324 [0.0891] 9477 R1 = 0.0529, wR2 = 0.0949 R1 = 0.0912, wR2 = 0.1050 1.086

F(0 0 0) Reflections collected Unique reflections [Rint] Reflections with I > 2r(I) Final R indices (I > 2r(I))b,c Final R indices (all data) S (all data) a b c

The formula and the formula weights include the H2O solvent molecules. R1 = R(||Fo|  |Fc||)/R|Fo|. wR2 = [R[w(F2o  F2c )2]/R[w(F2o)2]]1/2, w = 1/[r2(F2o) + [(ap)2 + bp], where p = [max(F2o, 0) + 2F2c ]/3.

Fig. 1. (left) Molecular structure of compound 1 (the H atoms and solvent molecules are omitted for clarity), DyIII pink, N blue, O red, C gray; (right) the mental core of 1 shows a sandglass-like topology. Symmetry codes: a 1 + y, 1  x, z; b x, 2  y, z; c 1  y, 1 + x, z. (Color online.)

in the monoclinic P21/n space group with Z = 4 (Table 1). The crystal structure of [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4]2+ is shown in Fig. 2, and its selected bond parameters are listed in Table S2. As shown in Fig. 2, the bivalent complex ion contains four eight-coor-

dinated DyIII ions, four L ligands, two CH3O ions and two N 3 ligands. The four DyIII ions are linked by a central l4-O2(O9) ion, with the distances of Dy1-O9, Dy2-O9, Dy3-O9 and Dy4-O9 being 2.184(6) Å, 2.198(6) Å, 2.183(6) Å and 2.181(6) Å, respectively.

H.-S. Wang et al. / Polyhedron 141 (2018) 69–76

73

Fig. 2. (left) Molecular structure of compound 2 (the H atoms and solvent molecules are omitted for clarity), DyIII pink, N blue, O red, C gray, S yellow; (right) the mental central core of 2 shows an irregular tetrahedron. (Color online.)

Moreover, the distances of Dy Dy are ranging from 3.4206(7) Å to 3.6834(6) Å, and the angles of Dy-O9-Dy are in the range of 102.7(2)° to 114.5(3)°. All of the four DyIII ions in 2 adopt the N3O5 coordination geometry, in which one N atom from a N 3, two N atoms from an L ligand, three phenoxido oxygen atoms from two L ligands, a l4-O2 and an oxygen atom from a CH3O ion. Moreover, the distances of Dy-O and Dy-N are similar. Taking Dy1 as an example, the Dy1 the distances of Dy-O and Dy-N are in the range of 2.184(6)-2.562(6) Å and 2.339(7)–2.519(8) Å, respectively, which are in agreement with +3 for Dy ions. All of the four L ligands in 2 possessed the same g2:g1:g1:g1: l coordination mode (Scheme 1, d), in which a phenoxido oxygen atom bonded with two DyIII ions, a methoxy group linked with one DyIII ion, and two N atoms each bonded with one DyIII ion. The coordination geometry of the four DyIII ions in 2 was also calculated by using SHAPE software [33], the results showed that the Dy1 is square antiprism (SAPR-8, D4d), Dy2 and Dy4 are JohnsonBiaugmentedtrigonal prism (J50) (JBTP-8, C2v), Dy3 is snub disphenoid (J84) (JSD-8, D2d) with their minimum CShM values of 25.198, 22.985, 23.585 and 24.206, respectively (Tables S3 and S4, Fig. S3). The larger calculated values show that they more deviate from the ideal geometry. The reason for the different coordination geometries of the four DyIII is that the bond lengths of Dy-N and Dy-O for four DyIII are different from each other. The differences of Dy-O9 (l4-O2) bond lengths for each DyIII also indicate that four DyIII ions form an irregular tetrahedron. 3.4. Magnetic properties Direct current (dc) magnetic susceptibility measurements were performed on polycrystalline samples of 1 and 2 under 1 kOe at 2– 300 K. The plots of vMT versus T, where vM is the molar magnetic susceptibility, are shown in Fig. 3. The vMT value at 300 K is 127.71 cm3 K mol1, which is slightly higher than the expected value of 127.53 cm3 K mol1 for nine DyIII ions (6H15/2, S = 5/2, L = 5, g = 4/3 and C = 14.17 cm3 K mol1). Upon cooling temperature from 300 to about 100 K, the vMT values of 1 decrease slowly and then drop more steeply to a minimum of 66.97 cm3 K mol1 at 2 K. The vMT value for 2 at 300 K is 56.66 cm3 K mol1, which is close to

Fig. 3. Plot of the temperature dependence of vMT vs. T for 1 and 2.

the expected value of 56.68 cm3 K mol1 for four DyIII ions. The vMT values for 2 also decrease smoothly in the temperature range of 300–100 K, and drop more rapidly to a minimum of 27.28 cm3 K mol1 at 2 K. The smooth drop of vMT for 1 and 2 in the high-temperature region mainly results from the thermal depopulation of the excited stark sub-levels of the DyIII ions, while the sharp decrease in the low-temperature region may be assigned to the presence of the intramolecular antiferromagnetic coupling interactions, the large magnetic anisotropy of anisotropic DyIII ions, and/ or intermolecular antiferromagnetic interactions. The field dependence of the magnetization (M) for 1 and 2 has been measured in the range of 0–70 kOe at 2.0 K and the isothermal magnetization (M) curves are shown in Fig. S4. The magnetization values of both complexes show a rapidly increase for fields below20 kOe followed by a slower increase up to 70 kOe. The magnetization values of 50.29 lB for 1 and 21.85 lB for 2 at 70 kOe are far from reaching the theoretical saturation values of 90 lB and 40 lB (using the equation of n  gDy(III)JDy(III)) [34], where n is the

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number of DyIII ions (gDy(III) = 4/3 and JDy(III) = 15/2) in 1 or 2 for nine and four independent DyIII ions in 1 and 2, respectively, which may be due to the presence of magnetic anisotropy and/or the effects of low-lying excited states in the both complexes. In order to investigate the relaxation dynamics of 1 and 2, alternating current (ac) magnetic susceptibility were first measured at 2–15 K under zero dc field with a 5.0 Oe ac field oscillating in the frequency range of 1–1000 Hz. The in-phase (vM0 ) and outof-phase (vM00 ) ac magnetic susceptibility signals are shown in Fig. S5 and Fig. 4, respectively. Both compounds show a frequency-dependent character for the vM00 signals, indicating the onset of slow magnetic relaxation behavior. However, no obvious peaks of vM00 were observed. These results may be due to fast zero-field quantum tunneling of magnetization (QTM). To reduce the QTM effect [35], ac susceptibility were measured under the optimized dc fields of 1000 Oe for 1 and 500 Oe for 2, with the in-phase (vM0 ) and out-of-phase (vM00 ) ac magnetic susceptibility signals shown in Figs. S6 and S7, respectively. Unfortunately, the applied dc fields seem not to have an impact on the vM0 and vM00 of 1 and 2, with no maxima still appearing. This indicates that the QTM cannot be effectively suppressed by the applied fields. Due to no peaks presented for the both complexes, the effective energy barrier (Ea) and the pre-exponential factor s0 cannot be calculated using the Arrhenius’ law. To estimate the values of Ea and s0, a new method based on the equation of ln(vM00 /vM0 ) = ln(xs0) + Ea/(kBT) [36], which was discovered by Bartolomé, was employed. As shown in Fig. S8, whether dc field added or not, the plots of ln(vM00 /vM0 ) versus 1/T are not linear, therefore, the parameters of Ea and s0 for 1 cannot be obtained by the method stated above. However, the plots of ln(vM00 /vM0 ) versus 1/T without dc field and with 500 Oe dc field are linear at the temperatures above 8 K (1/T < 0.125) and below 5 K (1/T > 0.2), while they are not linear in the temperature range of 5–8 K (Fig. 5, Figs. S9 and S10). The linear variation at high- and low-temperature region indicates that 2 shows two-step thermal magnetic relaxation [37], and nonlinear change at medium temperatures shows other relaxation processes such as Raman, Orbach, and temperature-independent processes present in 2. It should be noted that two thermal processes may be resulted from the slight differences of the local coordination spheres at the DyIII centers in 2. The data of ln(vM00 /vM0 ) versus 1/T with no dc field or 500 Oe dc field (Fig. 5) were fitted using the above method. The two average values for the energy barrier (Ea) and pre-exponential factor (s0) were obtained by a linear fit of the low temperature (LT) data and high temperature (HT) data, respectively, which are listed in Table 2,

Fig. 5. Linear approximation plots of ln(v00 /v0 ) vs. 1/T at different frequencies for compound 2, resulting in a calculated Ea value of 15.45 K.

Table 2 Characteristic dynamic parameters for 2 extracted from a new method discovered by Bartolomé. Hdc/Oe

Ea/K (LT)

s0/106 s (LT)

Ea/K (HT)

s0/106 s (HT)

0 500

2.02 2.27

17.40 15.20

15.07 15.60

2.68 2.25

Tables S5 and S6, the dynamic parameters obtained the data without dc field are well consistent with that obtained the data with 500 Oe dc field, which further indicate that the applied dc field have no impact on the relaxation behavior of 2, in other words, the QTM of 2 cannot be suppressed by the dc field. III Recently, four sandglass-like DyIII 9 complexes and many Dy4 complexes possessing different metal arrangements have been reported with their dynamic parameters listed in Table S7 in supporting information. As shown in Table S7, there are no sandglass00 like DyIII 9 complexes exhibiting peaks for the vM signals in spite of III only one sandglass-like Dy9 complex possessing energy barrier (Ueff) of 15.3 K, which indicate that this kinds of complexes may possess low energy barrier or possess fast QTM. For DyIII 4 complexes, there

Fig. 4. Temperature dependence of the out-of-phase (vM00 ) for 1 (left) and 2 (right) in a 5 Oe ac field oscillating at 1–1000 Hz with a zero applied dc field.

H.-S. Wang et al. / Polyhedron 141 (2018) 69–76

75

Fig. 6. Ground-state magnetic anisotropy of complex 1 (left) and 2 (right) calculated by Magellan Software. The thick lines with different color represent the orientations of the anisotropy axes for different type of DyIII in 1 and 2 for observing easily.

are diverse structural topologies reported up to date. For tetrahedron DyIII 4 complexes, the largest value of energy barriers is 23.42 K, which is obviously lower than the highest Ueff of some defect dicubane-, butterfly-, parallelogram- and square-like DyIII complexes, but 4 higher than the highest Ueff of linear- and hemicubane-like DyIII 4 complexes. 3.5. Estimation of the anisotropic directions of the Dy

III

In order to get insights on the ground state magnetic directions of the DyIII in complexes 1 and 2, preliminary investigations by electrostatic calculations using the Magellan software were performed. The method of electrostatic calculation in Magellan [38], which was proposed by Chilton et al., assuming that MJ = ±15/2 is the ground state, has been tested by many polynuclear Dy complexes and the directions of the obtained gz coincide well with ab initiog-tensor orientation calculations [39]. Therefore, this method was widely employed to estimate the orientation of the magnetic anisotropy of DyIII in many research works. The calculated results for 1 and 2 are shown in Fig. 6. For 1, the easy magnetization axes for eight DyIII located on the apexes are roughly aligned along the Dy-N (from N 3 ions) with angles ranging from 13.751° to 15.735°. However, the easy magnetization axis of the central Dy2 is not aligned through the centers of these two perfectly parallel Dy4 square planes (Fig. 6, left). The nonparallel arrangements of the easy axes of nine DyIII in 1 may result in the low energy barriers for this kind of complexes. For 2, the easy magnetization axes for Dy1 and Dy4 are roughly aligned along the Dy1-O9 and Dy4-O9 with the angles being 3.543° and 1.524°, respectively. It should be noted that the distances of the Dy1-O9 and Dy4-O9 are obviously shorter than those of other Dy-O and Dy-N for Dy1 and Dy4, which can be observed in other complexes. However, the main magnetic anisotropy axes for Dy2 and Dy3 are almost parallel to the Dy-O (O10, O11, from CH3O ions) with the angles being about 14°. The reason for the magnetic axes for four DyIII not all directing to central O9 may be due to two CH3O III and N ions in 2, which 3 two ions presented between the four Dy result in the molecule not possessing Td symmetry. Finally, based on the above analysis, the arrangements of the easy anisotropy

axes for DyIII in 2 are also not parallel, which does not favor the SMMs properties for tetrahedron Dy4 complexes, as stated above. 4. Conclusions In conclusion, we have successfully obtained two Dy complexes, [Dy9(l3-OH)8(l4-OH)2(N3)8{(py)2C(OCH3)O}8](OH)
4H2O and [Dy4(l4-O)(l-N3)2(l-OCH3)2(L)4](OH)2
3.5CH3OH,which are based on the hemiacetal form of di-2-pyridyl ketone and 2-(benzothiazol-2-yl-hydrazonomethyl)-6-methoxyphenol ligands, III respectively. The DyIII 9 is a sandglass type cluster, while the Dy4 is a tetrahedron-type cluster. Magnetic investigations for 1 and 2 show that both complexes exhibit slow magnetic relaxation behavior, but no peaks value of out-of-phase (v00 ) were observed. Additionally, complex 2 shows two-step thermal magnetic relaxation, with the energy barriers of about 15 K and 2.68 K, respectively. The directions of anisotropic axes for DyIII in 1 and 2 obtained through electrostatic calculations are not parallel, which may be responsible for the low energy barriers for both complexes. Although complexes 1 and 2 containing only one kind of organic ligand, this work provides a thought for constructing 4f complexes containing mixed organic ligands. In future, our laboratory will mix other organic ligands reacting with 4f rare earth salts to synthesize 4f complexes containing mixed organic ligands and to modulate the directions of anisotropic axes of DyIII and adjust the symmetry of coordination of DyIII ions for improving the energy barriers of 4f complexes. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21201136), Science Research Foundation of Wuhan Institute of Technology (No. K201447), and Graduate Innovative Fund of Wuhan Institute of Technology (CX2016169). Appendix A. Supplementary data CCDC nos. 1564336 and 1564335 contains the supplementary III crystallographic data for DyIII 9 and Dy4 . These data can be obtained

76

H.-S. Wang et al. / Polyhedron 141 (2018) 69–76

free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10. 1016/j.poly.2017.11.025.

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