Fine-Tuning Ligand to Modulate the Magnetic ... - ACS Publications

May 17, 2016 - Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R.. China. ...
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Fine-Tuning Ligand to Modulate the Magnetic Anisotropy in a Carboxylate-Bridged Dy2 Single-Molecule Magnet System Yu-Ling Wang,† Chang-Bao Han,† Yi-Quan Zhang,*,‡ Qing-Yan Liu,*,† Cai-Ming Liu,§ and Shun-Gao Yin† †

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China § Beijing National Laboratory for Molecular Sciences, Institution of Chemistry, Chinese Academy of Sciences, Center for Molecular Sciences, Beijing 100190, P. R. China ‡

S Supporting Information *

ABSTRACT: A series of dinuclear Dy(III) compounds with the general formula [Dy2(μ2-anthc)4(anthc)2(L)2] (anthc− = 9-anthracenecarboxylate, L = 2,2′-bipyridyl (1), 1,10-phenanthroline (2), and 4,7-dimethyl-1,10phenanthroline (3)) were synthesized and magnetically characterized. These compounds exhibit single-molecule magnet (SMM) behavior in the absence of the direct-current field, which is rarely observed for carboxylatebridged dinuclear Dy2 system. With the first coordination sphere of Dy(III) centers being fixed, the energy barrier was modulated by sequentially modifying the terminal neutral L ligands in this Dy2 system. Theoretical calculations revealed that the symmetry of the charge distribution surrounding the Dy(III) centers in 1−3 is the decisive factor to determine the relaxation of the SMMs. The combination of the larger charge distribution along the magnetic axis and lower charge distribution in the equatorial plane (hard plane) formed by five coplanar coordination atoms including two N atoms provided by an L ligand led to a strong easy-axis ligand field in these compounds. This work presents a rational method to modulate the dynamic magnetic relaxation of the lanthanide SMMs through fine-tuning electrostatic potential of the atoms on the hard plane.

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modulated by a subtle change of the ligand field.7 Few examples of such strategies are altering the monoanionic ligands in Dy2 system,8 replacing atoms of porphyrin core in mononuclear dysprosium system,9 and exchanging of the guest solvents in three-dimensional dysprosium compounds.10 For rational modulation of the anisotropy and elucidation of the origin of slow relaxation of the SMMs, a systematic approach is required. In this contribution, a carboxylate-bridged dinuclear Dy2 system was selected for systematically studying via fine-tuning the terminal neutral ligand. The 9-anthracenecarboxylic acid ligand (Hanthc) is selected for bridging the Dy(III) ions to form the Dy2 dimer through the single carboxyl group, wherein the central Dy2 core is protected by the peripheral anthracenes with large backbone, isolating the dimers from each other effectively. Two pairs of anthc− ligands bridge two Dy(III) ions to give the central [Dy2(μ2-anthc)4] unit with each Dy(III) ion being capped by an another anthc− ligand. For realizing the ligand fine-tuning strategy, the second ligands of N-containing derivatives L such as 2,2′-bipyridyl (2,2′-bpy), 1,10-phenan-

INTRODUCTION Since the single-molecule magnet (SMM) behavior of the TbPc2 compound reported by Ishikawa,1 the paramagnetic lanthanide ions, especially for the heavy lanthanide ions such as Dy(III), Tb(III), and Er(III), have become the most attractive candidates for constructing novel SMMs. 2 Thus, the lanthanide-based SMMs have been at the forefront of major advances in the field of SMMs in recent years.3 These lanthanide ions with significant intrinsic anisotropy originating from large unquenched orbital angular momentum and significant spin−orbit coupling can generate high barriers to spin reversal, which is necessary for magnetic bistability and slow magnetization relaxation for an SMM.4 Many research efforts have been applied to increase the energy barrier of the SMM having the possible applications in high-density information storage, quantum computing, and spintronics.5 Owing to the strong spin−orbit coupling, the magnetic structure of the lanthanides is more complex than that of the transition metal system. It has been reported that the magnetic anisotropy for a lanthanide SMM is extremely sensitive to the local coordination geometry of the lanthanide ion and the ligand field effects.6 Thus, the anisotropy energy can be © XXXX American Chemical Society

Received: March 16, 2016

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DOI: 10.1021/acs.inorgchem.6b00653 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for 1−3a compound formula fw temp (K) cryst syst space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dcalcd (g·cm−3) μ (mm−1) no. of reflns collected independent reflns obsd reflns (I > 2σ(I)) F(000) 2θ range [deg] R [int] R1, wR2 [I > 2σ(I)] R1, wR2 (all data) CCDC number a

1 C110H70N4O12Dy2 1964.70 293(2) triclinic P1̅ 1 11.9907(3) 13.6805(3) 15.2443(4) 109.302(2) 110.826(2) 100.833(2) 2069.43(10) 1.577 1.864 40670 10117 9012 986 3.350 to 29.389 0.0677 0.0354, 0.0666 0.0428, 0.0720 1465206

2 C114H70N4O12Dy2 2012.74 293(2) triclinic P1̅ 1 12.5093(5) 13.3389(6) 15.0003(6) 110.723(4) 102.983(4) 106.928(4) 2083.99(17) 1.604 1.853 18605 9631 7783 1010 3.353 to 29.194 0.0552 0.0499, 0.0671 0.0669, 0.0737 1465207

3 C118H78N4O12Dy2 2068.84 293(2) monoclinic P21/n 2 15.124(3) 21.055(2) 15.431(6) 90 115.15(3) 90 4448(2) 1.545 1.739 49471 11030 9302 2084 3.249 to 29.339 0.0485 0.0307, 0.0647 0.0418, 0.0696 1465208

R1 = ∑∥Fo| − |Fc∥/∑|Fo| and wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. (m), 852 (w), 778 (s), 755 (m), 731 (s), 652 (m), 640 (m), 594 (w), 557 (m), 517 (w). Synthesis of [Dy2(μ2-anthc)4(anthc)2(1,10-phen)2] (2). A mixture of Dy(NO3)2·5H2O (0.088 g, 0.2 mmol), 9-anthracenecarboxylic acid (0.132 g, 0.6 mmol), and 1,10-phenanthroline (0.036 g, 0.2 mmol) in 5 mL of DMF was sealed in a Teflon-lined stainless steel autoclave, which was heated at 85 °C for 72 h under autogenous pressure. The resulting mixture was cooled naturally to obtain the yellow crystals (yield: 0.064 g, 32% on the basis of Dy). Anal. Calcd for C114H70N4O12Dy2 (2012.74): C, 68.03; H, 3.51; N, 2.78%. Found: C, 68.01; H, 3.55; N, 2.79%. Main IR features (KBr pellet, cm−1): 3439 (w), 3045 (w), 1622 (s), 1600 (s), 1590 (s), 1517 (m), 1488 (w), 1439 (m), 1423 (s), 1410 (m), 1389 (m), 1344 (s), 1322 (m), 1279 (w), 1139 (w), 1103 (w), 1014 (w), 894 (m), 863 (s), 840 (m), 778 (m), 755 (s), 737 (s), 727 (s), 652 (w), 594 (w), 558 (m), 515 (w). Synthesis of [Dy 2(μ2-anthc)4(anthc) 2(4,7-dimethyl-1,10phen)2] (3). The preparation method for compound 3 is similar to that of compound 2 with the 1,10-phenanthroline displaced by 4,7dimethyl-1,10′-phenanthroline (yield: 0.048 g, 23% on the basis of Dy). Anal. Calcd for C118H78N4O12Dy2 (2068.84): C, 68.50; H, 3.80; N, 2.71%. Found: C, 68.48; H, 3.76; N, 2.72%. Main IR features (KBr pellet, cm−1): 3438 (w), 3045 (w), 1604 (s), 1524 (w), 1487 (w), 1439 (s), 1387 (m), 1321 (s), 1278 (w), 1246 (w), 1176 (w), 1016 (w), 889 (w), 868 (s), 857 (m), 846 (w), 777 (m), 756 (s), 732 (s), 653 (w), 597 (m), 515 (w). X-ray Crystallography. X-ray diffraction data were collected on a Rigaku Oxford SuperNova Single Source diffractometer with an Eos detector and a Mo Kα radiation (λ = 0.710 73 Å). CrysAlisPro Agilent Technologies software was used for collecting the frames of data, indexing the reflections, determining the lattice constants, absorption correction, and data reduction.12 The structures were solved by the direct methods, successive Fourier difference syntheses, and refined by the full-matrix least-squares method on F2 (SHELXTL-2014).13 All non-hydrogen atoms are refined with anisotropic thermal parameters. Hydrogen atoms were assigned to calculated positions. The R1 values are defined as R1 = ∑∥Fo| − |Fc∥/∑|Fo| and wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]}1/2. Details of the crystal parameters, data collection, and refinement are summarized in Table 1. Important

throline (1,10-phen), and 4,7-dimethyl-1,10-phenanthroline (4,7-dimethyl-1,10-phen), were introduced into the Dy2 system. One L ligand binds to a Dy(III) ion in a chelating manner to give the complete formula of [Dy2(μ2-anthc)4(anthc)2(L)2] for this series compounds. By sequentially modifying the terminal neutral L ligands with substituent, while maintaining the first coordination sphere configuration, the effect of substituent on energy barriers for this Dy2 system is expected to be obtained. Thus, the present Dy2 system provides an elegant model to understand the magnetostructural correlation and a way to optimize the magnetic performance of the SMMs.

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EXPERIMENTAL SECTION

Chemicals and Physical Measurements. All chemicals were of reagent grade and used as commercially obtained. Elemental analyses were performed on an Elementar Vario EL III analyzer, and IR spectra (KBr pellets) were recorded on PerkinElmer Spectrum One. Elemental analyses were performed on Elementar PerkinElmer 2400CHN microanalyzer. Magnetic measurements were performed on crystalline samples with a Quantum Design MPMS-XL5 SQUID magnetometer in the temperature range of 2−300 K. Diamagnetic corrections were estimated from Pascal’s constants for all constituent atoms.11 Synthesis of [Dy2(μ2-anthc)4(anthc)2(2,2′-bpy)2] (1). A mixture of Dy(NO3)2·5H2O (0.176 g, 0.4 mmol), 9-anthracenecarboxylic acid (0.264 g, 1.2 mmol), and 2,2′-bipyridyl (0.062 g, 0.4 mmol) in a 12 mL of dimethylformamide (DMF)/H2O (v/v = 1:2) was sealed in a Teflon-lined stainless steel autoclave, which was heated at 85 °C for 72 h under autogenous pressure. The resulting mixture was cooled naturally to obtain the yellow crystals (yield: 0.059 g, 15% on the basis of Dy). Anal. Calcd for C110H70N4O12Dy2 (1964.70): C, 67.24; H, 3.59; N, 2.85%. Found: C, 67.32; H, 3.63; N, 2.89%. Main IR features (KBr pellet, cm−1): 3435 (w), 3046 (w), 1656 (w), 1602 (s), 1575 (s), 1528 (m), 1487 (w), 1474 (m), 1439 (s), 1410 (s), 1389 (m), 1322 (s), 1279 (m), 1174 (w), 1151 (w), 1066 (w), 1010 (m), 888 (m), 870 B

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Figure 1. Molecular structures of 1−3. The difference between them is in the N-containing ligands.

terminal anthc− ligands in a chelating fashion, and two additional chelating L ligands. The Dy(III) ion in these compounds is nine-coordinated by seven carboxylate O atoms from five anthc− ligands and two N atoms of an L ligand (Figure 1), which generates a distorted monocapped square antiprism geometry with O1A at the capped position (Figure S1 in the Supporting Information). The Dy−O bond distances range from 2.287(3) to 2.6693(19) Å with the longest one associated with the capped O1A atom (Table S1). The anthc− ligand in the dinuclear compound exhibits three different coordination modes bridging two metal ions or chelating a metal ion (Scheme S1). A pair of η2-O (O1 and O1A) atoms bridge two Dy(III) ions, which are also connected by a pair of carboxylate bridges, to form a centrosymmetric [Dy2(μ2-O)2] unit with Dy1···Dy1A distances of 3.9490(2) (1), 3.9176(4) (2), and 3.9224(6) Å (3) and Dy1−O1−Dy1A angles of 105.18(6) (1), 106.20(10) (2), and 105.18(6)° (3; Figure 2).

bond lengths are listed in Table S1. More details on the crystallographic data are given in the X-ray crystallographic files in CIF format. Theoretical Calculations. The dinuclear compounds 1−3 have an inversion center; thus, only one magnetic center is calculated. Complete-active-space self-consistent field (CASSCF) calculations on individual lanthanide Dy(III) fragment of the model structure extracted from each compound on the basis of single-crystal X-ray determined geometry were performed with MOLCAS 8.0 program package.14 Each dysprosium center was calculated keeping the experimentally determined structure of the corresponding compound while replacing the neighboring Dy(III) ion by diamagnetic Lu. The basis sets for all atoms are atomic natural orbitals from the MOLCAS ANO-RCC library: ANO-RCC-VTZP for Dy(III); VTZ for close O and N; VDZ for distant atoms. The calculations employed the second-order Douglas−Kroll−Hess Hamiltonian, where scalar relativistic contractions were taken into account in the basis set and the spin−orbit couplings were handled separately in the restricted active space state interaction (RASSI-SO) procedure. For the fragment of Dy(III), active electrons in seven active spaces include all f electrons (CAS(9 in 7) in the CASSCF calculation. We mixed the maximum number of spin-free state, which was possible with our hardware (all from 21 sextets, 128 from 224 quadruplets, 130 from 490 doublets for the Dy(III) fragment). To fit the exchange interactions in these compounds, we took two steps to obtain them. First, we calculated one Dy(III) fragment using CASSCF to obtain the corresponding magnetic properties. Then, the exchange interaction between the magnetic centers is considered within the Lines model,15 while the account of the dipole−dipole magnetic coupling is treated exactly. The Lines model is effective and has been successfully used widely in the research field of f-element SMMs.16 For each of compounds 1−3, there is only one type of J. The exchange Hamiltonian is

Figure 2. Core structure for 1−3 showing the five-membered ring (light orange lines). The outer backbone of the ligands is omitted for clarity.

Looking carefully at the crystal packing indicates the shortest Dy···Dy separations between the dinuclear molecules were found to be 9.318, 9.798, and 11.748 Å for 1, 2, and 3, respectively (Figure S2), indicative of a well-isolated dinuclear units. As shown in Figure 1, if we take compound 1 as the parent structure, compounds 2 and 3 can be obtained just by sequentially modifying the terminal L ligands with substituent groups. Thus, these compounds have the similar [Dy2(μ2anth)4(anthc)2(2,2′-bpy)2] molecular structures wherein the Dy(III) ions have a very similar first coordination sphere, only differing in the outer 2,2′-bpy moiety. As mentioned above, the coordination geometry of the central Dy(III) ion is best described as a distorted capped square antiprism. Interestingly, careful inspection of the coordination environment of the Dy(III) center in compounds 1−3 reveals that there is a five-membered ring formed by two N atoms and three carboxylate oxygen atoms (O3, O1A, and

The Jtotal is the parameter of the total magnetic interaction (Jtotal = ∧ ∼ Jdiploar + Jexchange) between magnetic center ions. The SDy = ±1/2 are the ground pseudospin on the Dy(III) sites. The dipolar magnetic coupling can be calculated exactly, while the exchange coupling constants were fitted through comparison of the computed and measured magnetic susceptibility and molar magnetization using the POLY_ANISO program.17

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RESULTS AND DISCUSSION Reaction of Dy(NO3)3·5H2O with Hanthc in the presence of the appropriate N-containing ligand under solvothermal conditions afforded compounds 1−3 with the general formula [Dy2(μ2-anthc)4(anthc)2(L)2], where the N-containing ligand L is 2,2′-bpy for 1, 1,10-phen for 2, and 4,7-dimethyl-1,10-phen for 3. The isostructural compounds 1−3 have a centrosymmetric dinuclear core with four bridging anthc− ligands, two C

DOI: 10.1021/acs.inorgchem.6b00653 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. χMT vs T plots of 1−3. The solid lines correspond to calculated fits to the data. The intermolecular interactions zJ′ of 1−3 all are 0.00 cm−1 from the fitting results. (inset) Plots of magnetization vs applied field at 2, 3, 4, and 5 K.

Figure 4. Temperature dependence of out-of-phase ac susceptibility signals under zero dc field for 1−3. (inset) Magnetization relaxation time, ln τ vs T−1 plots. The solid line is fitted with the Arrhenius law.

Figure 5. Temperature dependence of in-phase ac susceptibility signals under zero dc field for 1−3.

mol−1 (3) at 18, 16, and 16 K, respectively. χMT then increases sharply to maximum values of 33.13 (1), 30.87 (2), and 31.90 cm3 K mol−1 (3) at 2 K. The decrease of the χMT products is mainly attributed to the progressive depopulation of excited Stark sublevels. The increase of the χMT products at low temperature suggests that the intramolecular ferromagnetic coupling starts to dominate the magnetic behavior for the ground states of 1−3, as observed in other dinuclear dysprosium compounds.18 Field dependence of the magnetization for 1−3 between 2 and 5 K showed a rapid increase of the magnetization at low field followed by a slow linear increase at high field (Figure 3). The presence of a significant magnetic anisotropy and/or low-lying excited states in this system is supported by the high-field variation and the lack of superposition on a single master curve of the M versus H/T data. Additionally, the M versus H data for these compounds do not display a hysteresis effect at 1.9 K with sweep rates used in a traditional dc magnetic susceptibility measurement SQUID magnetometer (Figure S3).

O4A; Figure 2), which define a least-squares plane with small deviations for the individual atoms (Table S2). From this point of view, the structural difference of this series compounds is along this pentagonal plane. Natural bond order (NBO) analysis was performed to obtain the coordinating atom charges around the Dy(III) center using CASSCF (Table S3). The calculation results showed that a lower charge density distribution on the pentagonal plane is observed in these compounds, while a larger negative charge is distributed on O1, O5, O6, and O2A atoms (Figure 2), resulting in an axially enhanced ligand field. Direct current (dc) magnetic susceptibilities of 1−3 were collected in the temperature range of 300−2 K under an applied magnetic field of 1 kOe. The χMT values of the dinuclear species versus T are depicted in Figure 3. The roomtemperature χMT values of 28.19 (1), 28.04 (2), and 28.17 cm3 K mol−1 (3) are consistent with the expected value of two isolated Dy(III) ions (6H15/2, g = 4/3, and C = 14.17 cm3 K mol−1). As the temperature lowered, the χMT products decrease gradually to minima of 26.96 (1), 26.59 (2), and 26.62 cm3 K D

DOI: 10.1021/acs.inorgchem.6b00653 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. Calculated Energy Levels and mJ Values of the Lowest Kramers Doublets of a Dy Center for 1−3

To better understand the magnetic dynamic behavior of this system, the alternating-current (ac) magnetic susceptibility measurements were performed on the parent compound 1 first. Under a zero-dc field with an ac field of 2.5 Oe, the temperature and frequency dependences of the in-phase (χ′) and out-ofphase (χ″) susceptibilities below 9 K are observed for 1 (Figures 4 and 5), which suggests a slow relaxation of the magnetization and the SMM nature of 1. The full peaks with good shape can be detected when the frequency is higher than 10 Hz in both χ′ and χ″ versus T plots. Analysis of the χ″ peaks with Arrhenius law gave the effective energy barrier Ueff of 51.2(3) K with a pre-exponential factor value τ0 of 3.2(1) × 10−8 s (Figure 4). It should be mentioned that very few examples of the carboxylate bridged lanthanide compounds exhibiting SMM behavior have been documented.19 However, almost of them are filed-induced SMMs. Thus, the zero-field slow magnetization relaxation of the present case is a rare example of the carboxylate-based dysprosium SMM. Inspired by the results of the ac susceptibility measurements of 1 and to detect the effects of ligand-tuning to the terminal L ligands on the slow magnetization relaxation, ac magnetic susceptibilities data for 2 and 3 were collected under zero dc field. For both compounds, frequency-dependent full peaks with one maximum in χ′ and χ″ versus T plots can be found below 8 K and in the frequency ranges 10−1399 Hz for 2 and 250−1399 Hz for 3, confirming the zero-field slow magnetization relaxation and SMM nature of 2 and 3. Thus, this Dy2 series of compounds exhibit SMM behavior even in the absence of an applied field, which is not observed in carboxylate-bridged Dy2 compounds previously. The effective energy barriers Ueff of 49.4(2) K with a pre-exponential factor value τ0 of 4.6(2) × 10−9 s for 2 and 31.6(1) K with a pre-exponential factor value τ0 of 3.4(2) × 10−8 s for 3 are extracted from the out-of-phase ac data. For these compounds, the peaks in χ′ and χ″ versus T plots are frequency dependent, and the maxima are shifted into the higher temperature region over the entire available frequency range (Figures 4 and 5). The Cole−Cole plots of χ″ versus χ′ for 1−3 are depicted in Figure S4, which display semicircle shapes and were fitted to the generalized Debye model (Table S4).20 The α values for 1 are 0.121 and 0.143 and for 3 are 0.157 and 0.166. The smaller α values for 2 of 0.06 and 0.09 were obtained. These small distribution coefficient α values suggest that there is a narrow distribution of relaxation time in these Dy2 system. To obtain deeper insights into the magnetization reversal in these compounds, ab initio calculations were performed at the CASSCF/SORASSI/SINGLE_ANISO level using the MOLCAS 8.0 program (Figure S5). The calculated effective gz values for 1, 2, and 3 are 19.654, 19.629, and 19.672 (Table 2 and S5), respectively, which are very close to the Ising-limit value of 20, indicating the dysprosium sites are strongly axial and the magnetic interaction within the these dimers will be mostly of the Ising type. The energies of the lowest Kramers doublets of the individual Dy(III) ions in 1−3 are listed in Table 3. The

1 KDs 1 2 3 4 5 6 7 8

1

2

3

0.018 0.040 19.654

0.018 0.043 19.629

0.044 0.063 19.672

0.0 61.9 110.5 160.7 212.4 315.8 358.0 436.7

2 −1

mJ

E, cm

±15/2 ±13/2 ±11/2 ±3/2 ±1/2 ±7/2 ±5/2 ±9/2

0.0 52.1 98.3 156.0 206.8 317.3 368.3 410.5

3 mJ ±15/2 ±13/2 ±11/2 ±7/2 ±5/2 ±9/2 ±1/2 ±3/2

−1

E, cm

mJ ±15/2 ±13/2 ±11/2 ±9/2 ±1/2 ±5/2 ±3/2 ±7/2

0.0 43.8 95.9 151.6 191.5 276.0 306.2 374.8

calculated energy of the first excited state on Dy(III) sites are 61.9 cm−1 (1), 52.1 cm−1 (2), and 43.8 cm−1 (3), which are slightly higher than the corresponding Ueff values extracted from the out-of-phase ac susceptibilities. Another increase of the ac response in both χ′ and χ″ versus T plots with the temperature close to 2 K and in the low-frequency regions was observed for these compounds (Figures 4 and 5). This hints though the thermally activated (Orbach) mechanism via the first excited state is dominated, a direct mechanism via quantum tunneling of the magnetization (QTM) occurred at low temperatures, which accounts for the smaller experimental barriers compared with the calculated energies. The tunneling effect was usually reflected in the transverse anisotropy component. The calculated values of gx and gy values all are slightly larger than 0 (Table 2), suggesting a small amount of the transverse anisotropy components in these compounds and confirming the presence of the QTM. Furthermore, the used geometry for calculations is the geometry at room temperature, which also leads to the discrepancies between experimental barriers and the calculated energies. Within the dimer, the dipolar interaction between the Dy(III) centers was calculated with respect to the pseudospin of 1/2 for the Dy(III) ions (Tables 4). For these compounds, Table 4. Fitted Exchange Coupling Constants Jexch, the Calculated Dipole−Dipole Interactions Jdipolar, and the Total Jtotal (cm−1) between Dy(III) Ions in 1−3 Jdipolar Jexch Jtotal

1

2

3

2.78 −0.50 2.28

3.21 −1.25 1.96

2.67 −0.75 1.92

the total coupling parameters Jtotal (dipolar and exchange) were included to fit the magnetic susceptibilities. The calculated and experimental χMT versus T plots of 1−3 are shown in Figure 3, where the fits are close to the experimental data.16 From Table 4, the Dy···Dy interactions in these compounds within Lines model15 all are ferromagnetic. Thus, the magnetic axes on Dy(III) for each compound have the same direction. Because of the presence of an inversion center in these dinuclear structures, the local anisotropy axes of the Dy(III) centers are aligned in a parallel fashion (Figures 6 and S6). As shown in Table 4, the dipolar interaction is stronger than the exchange interaction and stabilizes the parallel alignment of the local magnetic moments of the Dy(III) centers in the ground exchange doublet. We also gave the exchange energies and the main values of the gz for the lowest two exchange doublets of these compounds in Table S6. The gz values of the ground

Table 2. Principal g Values of the Ground Kramers Doublets of a Dy Center in 1−3

gx gy gz

E, cm

−1

E

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around the hard plane, it will strongly stabilize the maximum |Jz| value of 15/2 of ground state sublevel due to the decreased electron repulsion on the hard plane and lead to easy axial anisotropic ground states.7a The electronic distribution on the hard plane decreases in the order of 3 < 2 < 1, which is reflected in their anisotropy energy barriers (Ueff(1) > Ueff(2) > Ueff(3)).

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CONCLUSIONS In conclusion, a series of dinuclear Dy(III) compounds that displays SMM behavior under zero dc field was synthesized. The magnetic anisotropy of these compounds can be modulated by sequentially modifying the terminal N-containing ligands, while keeping the first coordination sphere of Dy(III) centers. The experimental magnetic studies and theoretical calculations revealed that the symmetry of electron density distribution around the Dy(III) center is the decisive factor to determine the slow relaxation of these molecules. This work presents an important strategy to rationally modulate the dynamic magnetic relaxation of the lanthanide SMMs via finetuning electrostatic potential of the atoms on the hard plane.

Figure 6. Orientation of the main magnetic axes in the ground Kramers doublets of the Dy(III) ions in the core structures of 1−3 (bright green arrows) showing the hard plane (light orange lines).

exchange states for these compounds all are close to 40, which further confirm that the magnetic couplings within the dimers are ferromagnetic. In additional, the total magnetic interaction within the dimer of 1 is larger than those of 2 and 3 (Table 4), which is verified by the M versus H/T data at low temperatures (Figure 3 inset). For 1, the field dependence of magnetization M saturated quickly due to the strong magnetic interaction, while the M values of 2 and 3 with the smaller Jtotal values increase slowly compared with that of 1. To understand the origin of the magnetization dynamics, it is necessary to provide a structural comparison of the coordination spheres in these isostructural compounds. Basically, the difference in the structures corresponds to the terminal L ligands from the nearly coplanar 2,2′-bpy ligand (the dihedral angle of 4.87° between the pyridyl planes) in 1 to the completely coplanar 1,10-phen and 4,7-dimethyl-1,10-phen ligand in 2 and 3. The size of the neutral L ligands increases from compound 1 to 3, which slightly changes the ligand field around the primary coordination sphere of the Dy(III) ions. For example, the N1−Dy−N2 angles are increased from 1 to 3 (Table S1), while the average Dy−N bond distances are decreased from 1 to 3. The results of ab initio calculations showed that an increase in electron density on N1 and N2 atoms from 1 to 3 is observed as the phenyl and methyl substituent are added sequentially (Table S3), which leads to the slight decrease of the energy barrier in this Dy2 system. Additionally, the calculated values of gx and gy increase in the order of 1 < 2 < 3 (Table 2), corresponding to their trends of the transverse anisotropy component associated with the QTM. The calculations based on the X-ray determined geometry also afforded the different values for the dipolar coupling of 2.78, 3.21, and 2.67 cm−1 for 1, 2, and 3, respectively (Table 4). Finally, as shown in Figure 6, the magnetic axis is approximately extended along the Dy1−O1 bond with angles of 9.1 (1), 12.3 (2), and 14.1° (3) between them and nearly perpendicular to the above-mentioned five-atom plane. Therefore, the five atoms of N1, N2, O3, O1A, and O4A with lower negative charge distribution constitute the hard plane surrounding the Dy(III) center (Table S3), while the O1, O5, O6, and O2A atoms having higher negative charge distribution are located at two sides of the hard plane and nearby the easy axis. This type of the charge density distribution in 1−3 generates a strong easyaxis ligand field. For structures 1−3, the difference is only in the L ligand providing the N atoms for constructing the hard plane. Therefore, the charge density distribution in the hard plane was considered as an important factor in modulating the whole molecular magnetic anisotropy in this Dy2 system. For Dy(III) ion, if coordination atoms with lower electronic distribution

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00653. • Structural and magnetic characterization, theoretical calculation, and X-ray crystallographic files in CIF format. (PDF) • X-ray crystallographic information. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-791-88336372. (Q.-Y.L.) *E-mail: [email protected]. (Y.-Q.Z.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21561015 and 21361011), the Natural Science Foundation of Jiangxi Province (Grant No. 20151BAB203002), the Young Scientist Training Project of Jiangxi Province (Grant No. 20153BCB23017), and Natural Science Foundation of Jiangsu Province (BK20151542). C.B.H. acknowledges the innovation fund for the graduate student of Jiangxi Normal University (YJS2015013).

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DOI: 10.1021/acs.inorgchem.6b00653 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00653 Inorg. Chem. XXXX, XXX, XXX−XXX