Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Modulating the Magnetic Interaction in New Triple-Decker Dysprosium(III) Single-Molecule Magnets Jing-Yuan Ge,†,‡ Hai-Ying Wang,† Jian Su,† Jing Li,† Bao-Lin Wang,§ Yi-Quan Zhang,*,§ and Jing-Lin Zuo*,† †
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, People’s Republic of China ‡ College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, People’s Republic of China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, People’s Republic of China S Supporting Information *
ABSTRACT: A new type of dinuclear dysprosium(III) complex based on phthalocyanine and salicylaldehyde derivatives (HL-R), [Dy2(Pc)2(L-R)2(H2O)]·2THF (R = OCH3 (1), OC2H5 (2); H2Pc = phthalocyanine; HL-OCH3 = 2-hydroxy-3-methoxybenzaldehyde; HL-OC2H5 = 3-ethoxy2-hydroxybenzaldehyde), was successfully synthesized and structurally characterized. Complex 1 features a sandwichtype triple-decker structure, where two coplanar L-OCH3 ligands lie in the middle layer shared by two eight-coordinated DyIII ions and two Pc ligands are located in the outer layer. In 2, the introduction of an ethoxy group generates a noncoordination mode for the Oalkoxy atom. Magnetic studies indicate that complex 1 behaves as a zero-field single-molecule magnet with a higher energy barrier, while 2 exhibits a fast tunneling relaxation process. Theoretical calculations revealed that changes in the ligand field environment around DyIII ions can significantly affect the arrangement of the main magnetic axes and further result in distinct magnetic interactions as well as different relaxation behaviors.
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INTRODUCTION Single-molecule magnets (SMMs) have been investigated extensively over the past few decades due to their promising applications in molecule-based magnetic storage materials, electronic sensors, and molecular spintronics with the characteristic slow relaxation and quantum tunneling of magnetization (QTM).1 Since the discovery of SMM magnetism in [NBu4]+[Tb(Pc)2]− (Pc = phthalocyaninato),2 paramagnetic lanthanide ions have become some of the favorite candidates for constructing novel SMMs. These lanthanide ions with significant intrinsic anisotropy and large ground-state spin can generate high energy barriers, which is necessary for magnetic bistability and slow magnetization relaxation for an SMM.3 Many research efforts have been focused on understanding the mechanism deeply and exploring the factors that influence effective barriers (Ea) and magnetic blocking temperature (TB).4 Recently, a breakthrough has been made by Goodwin and co-workers in mononuclear 4f-SMMs, exhibiting magnetic hysteresis at temperatures up to 60 K and a high effective energy barrier of 1760 K.4d Nevertheless, the magnetic behaviors of 4f-SMMs encapsulating two or more lanthanide ions are still poorly understood because of their complexity.5 A key point is the existence of intramolecular © XXXX American Chemical Society
magnetic interactions in multinuclear 4f-SMMs, which are closely related to the magnetic relaxation behavior. As is wellknown, magnetic interactions can be affected sensitively by any subtle alterations of the local coordination sphere and the ligand field around the lanthanide ion.6 In this regard, continuous efforts should be devoted to modulate the magnetic interaction and further control the magnetic properties. Elucidating the relationship between these two aspects is vital to construct the best SMMs with a high energy barrier.7 Dinuclear complexes are the simplest magnetic entities having an intramolecular coupling interaction. Recently, our group proposed a solvent-responsive phthalocyanine-bridged ErIII-SMM,8 where the exchange of lattice solvents alters the distortion of the bridging Pc ligand, thus resulting in largely distinct coupling interactions. Obviously, modifying the bridging ligand is an effective way to modulate the magnetic interaction.9 To realize the ligand fine-tuning strategy, a second of HL-R ligand (2-hydroxy-3-methoxybenzaldehyde for 1; 3ethoxy-2-hydroxybenzaldehyde for 2) was introduced into the Dy2 system. Unexpectedly, a new type of triple-decker Received: November 5, 2017
A
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry dysprosium(III) complex capped by two Pc ligands and two coplanar bridging L-R ligands was obtained. This modification of the middle layer L-R ligand with different substituents leads to a perturbation of the main magnetic axis, consequently changing the coupling interaction and modulating the magnetic behavior for this Dy2 system.
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Table 1. Crystallographic Data for 1 and 2 formula formula wt temp, K cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 ρcalcd, g/cm3 GOF on F2 R1/wR2 (I > 2σ(I))a R1/wR2 (all data)a CCDC no.
EXPERIMENTAL SECTION
Materials and Physical Measurements. All solvents and chemicals were received from commercial sources and used without further purification. The starting material [Dy2(thd)4Pc] was prepared with reference to the literature method.10 Electronic absorption data (Figure S1 in the Supporting Information) were recorded on a PerkinElmer Lambda 35 spectrophotometer. A Bruker Advance D8 Xray powder diffractometer (XRPD) (Cu Kα radiation, λ = 1.5405 Å) was used to collect the experimental XRD patterns. Calculated XRPD patterns were obtained by the Mercury 3.8 program. Elemental analyses were carried out on an Elementar Vario MICRO analyzer. IR spectra (Figure S2 in the Supporting Information) were obtained on a Bruker Vector22 spectrophotometer from 400 to 4000 cm−1 with KBr pellets. The static magnetic measurements on polycrystalline samples for both complexes were performed using a Quantum Design SQUID VSM magnetometer from 1.8 to 300 K with external direct current (dc) fields in the range of 0−70 kOe. Temperature- and frequencydependent alternating current (ac) susceptibilities were measured under an oscillating ac field of 2 Oe. Diamagnetism contributions of the sample holder and the sample were corrected using Pascal’s constants.11 Synthesis of [Dy2(Pc)2(L-OCH3)2(H2O)]·2THF (1). To a solution of [Dy2(thd)4Pc] (16 mg, 0.01 mmol) in 5 mL of anhydrous tetrahydrofuran (THF) was added HL-OCH3 (6 mg, 0.04 mmol). The mixture was heated at 120 °C under autogenous pressure for 48 h and then cooled to room temperature. Green crystals of 1 were obtained. Yield: 41%. Anal. Calcd for C88H64N16O9Dy2: C, 58.25; N, 12.35; H, 3.56. Found: C, 58.59; N, 12.68; H, 3.35. IR (KBr pellet, cm−1): 3438 (br), 1621 (m), 1485 (m), 1406 (w), 1330 (m), 1282 (w), 1216 (w), 1161 (w), 1114 (m), 1077 (m), 1003 (m), 955 (w), 884 (w), 775 (w), 729 (s), 668 (w), 628 (w). Electronic absorption data for 1 in CHCl3/ DMF (9/1): λmax 346 and 676 nm. Synthesis of [Dy2(Pc)2(L-OC2H5)2(H2O)]·2THF (2). Complex 2 was synthesized in a manner similar to that for 1 except that HLOC2H5 was used in place of HL-OCH3. Yield: 36%. Anal. Calcd for C90H68N16O9Dy2: C, 58.66; N, 12.16; H, 3.72. Found: C, 58.75; N, 12.31; H, 3.38. IR (KBr pellet, cm−1): 3439 (br), 1618 (m), 1500 (m), 1436 (m), 1403 (w), 1332 (m), 1276 (w), 1183 (w), 1157 (w), 1117 (m), 1092 (m), 1003 (m), 950 (w), 872 (w), 777 (w), 729 (s), 683 (w), 614 (w). Electronic absorption data for 2 in CHCl3/DMF (9/1): λmax 347 and 676 nm. X-ray Crystallography. X-ray crystal structures of 1 and 2 were collected at 123 K on a Bruker Smart Apex II CCD-based diffractometer with Mo Kα radiation (λ = 0.71073 Å). Data were integrated using SAINT12 and SADABS13 routines. The structures of both complexes were solved from direct method E-maps and refined by the full-matrix least-squares method against F2 utilizing the program SHELXL-2016/6.14 Hydrogen atoms were fixed theoretically in the ideal positions and refined with isotropic displacement parameters. All non-hydrogen atoms were refined anisotropically. Crystal parameters are given in Table 1. Important bond distances and angles are supplied in Table S1 in the Supporting Information. Ab Initio Calculations. Complete-active-space self-consistent-field (CASSCF) calculations were performed with the MOLCAS 8.0 program15 on two types of individual DyIII fragments of complexes 1 and 2. The calculation for each DyIII center was carried out on the basis of the experimentally determined dinuclear structures (Figure S18 in the Supporting Information shows the model structure of complex 1), and the adjacent DyIII center was replaced by diamagnetic LuIII. The basis sets used are atomic natural orbitals from the MOLCAS ANO-RCC library for all atoms: ANO-RCC-VTZP for DyIII; VDZ for distant atoms, and VTZ for close O and N. During the
a
1
2
C88H64Dy2N16O9 1814.56 123(2) monoclinic P21/n 12.1972(6) 47.344(2) 13.4811(7) 90 110.597(1) 90 7287.2(6) 1.654 1.177 0.0744/0.1445 0.0950/0.1510 1581129
C90H68Dy2N16O9 1842.61 123(2) monoclinic P21/n 12.1398(13) 47.732(5) 13.6390(15) 90 109.701(3) 90 7440.6(14) 1.645 1.021 0.0755/0.2299 0.1024/0.2494 1581130
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]0.5.
calculations, the second-order Douglas−Kroll−Hess Hamiltonian was employed concerning scalar relativistic contractions in the basis sets and separately handling the spin−orbit coupling in the restricted active space state interaction procedure. For each DyIII fragment in two complexes, active electrons in seven active spaces include all f electrons CAS (9 in 7). The maximum number of spin-free states was also mixed with our hardware (all from 21 sextets, 130 from 490 doublets, 128 from 224 quadruplets for DyIII fragment).
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RESULTS AND DISCUSSION Syntheses and Characterizations. Phthalocyanine (H2Pc) has a high stability and planarity, making decker complexes easier.5c,16 In this study, mixed-ligand triple-decker structures 1 and 2 were isolated using the stepwise synthetic approach. First, [Dy2(thd)4Pc] was prepared from Dy(thd)3· 2H2O (Hthd = 2,2,6,6-tetramethylheptanedione) and Li2Pc following a procedure in our previous method.10 Then, [Dy2(thd)4Pc] and the HL-R ligand were mixed in anhydrous tetrahydrofuran and heated at 120 °C to obtain the interesting neutral triple-decker complexes [Dy2(Pc)2(L-OCH3)2(H2O)]· 2THF (1) and [Dy2(Pc)2(L-OC2H5)2(H2O)]·2THF (2)) (Scheme 1; HL-OCH3 = 2-hydroxy-3-methoxybenzaldehyde, Scheme 1. Ligands Used in This Work
HL-OC2H5 = 3-ethoxy-2-hydroxy-benzaldehyde). XRPD patterns match well with those calculated using the single-crystal X-ray data, which confirms the purity of the bulk synthesized samples (Figure S3 in the Supporting Information). Crystal Structures. Complex 1 features a sandwich-type triple-decker structure, as depicted in Figure 1a. It crystallizes into a monoclinic P21/n space group, and the asymmetric unit B
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) Triple-decker structure of 1 with hydrogen atoms and solvents removed for clarity. (b) Local coordination spheres of DyIII ions. (c) Top view of the inner layer in 1.
Figure 2. (a) Triple-decker structure of 2 with hydrogen atoms and solvents removed for clarity. (b) Local coordination geometries of DyIII ions. (c) Top view of the inner layer in 2.
cell includes two crystallographic DyIII centers, two Pc ligands, two L-OCH3 ligands, one crystallized water molecule, and two free tetrahydrofuran molecules. The Dy1 ion lies in the eightcoordinated [DyN4O4] coordination sphere, where four N atoms (N9, N11, N13, N15) come from one Pc ligand and four O atoms (O3, O2, O6, O4w) come from two L-OCH3 ligands (one Oaldehydic and two Ophenolic) and one water molecule (Figure 1b). The Dy2 center also has an eight-coordinated geometry composed of four N atoms (N1, N3, N5, N7) from another Pc ligand and four O atoms (O7, O6, O2, O1) from two L-OCH3 ligands (one Oaldehydic, two Ophenolic, and one Oalkoxy). The Dy−N bond lengths range from 2.380 to 2.393 Å; meanwhile, the Dy−O bonds are in the 2.310−2.665 Å range. Among them, the Dy2−O1 bond length (2.665 Å) is slightly longer than other Dy−O bonds. However, all of the Dy−N and Dy−O bond lengths are in the normal range and are comparable to those in previously reported literature (Table S1 in the Supporting Information).17 Two Ophenolic atoms from two L-OCH3 ligands bridge Dy1 and Dy2 centers to construct a Dy2O2 core with a Dy···Dy separation of 3.936 Å and Dy− O−Dy angles of 106.4 and 116.4°. In the dinuclear unit, both Pc and L-OCH3 ligands are arranged in planes to form the triple-decker structure. Two Pc ligands are located in the upper and lower planes, respectively, while two L-OCH3 ligands lie in the middle plane, shared by two DyIII ions (Figure 1c). The exact geometries of DyIII ions were also evaluated by using SHAPE 2.1 software. Calculated SAPR-8 parameters are 1.262 and 1.025 for Dy1 and Dy2, respectively (Table S3 in the Supporting Information), indicating that both Dy1 and Dy2 belong to approximate square-antiprismatic (SAP, D4d) symmetry. To investigate the effect of the bridging ligand on the building of the crystal structure, complex 2 was synthesized in
an approach similar to that employed for 1 but using HLOC2H5 instead of HL-OCH3. Complex 2 features a tripledecker structure similar to that of 1, as shown in Figure 2a. Two DyIII centers are bridged through two Ophenolic atoms of two LOC2H5 ligands to construct a Dy2O2 core, where the Dy···Dy distance is 3.922 Å and two Dy−O−Dy angles are 105.8 and 114.2°. The Dy−O and Dy−N bonds are in the ranges of 2.369−2.431 and 2.313−2.519 Å, respectively. Differing from the case for complex 1, it is worth noting that the Dy2 ion is seven-coordinated in 2 with the uncoordinated Oalkoxy atom (O1) (Figure 2b). In 2, the distance between Dy2 and O1 (2.891 Å) is much longer than that in 1 (2.665 Å), which may be caused by the greater steric hindrance of ethoxy in comparison to methoxy (Figure 2c). Table S3 in the Supporting Information indicates that the coordination sphere of Dy1 is close to a square antiprism (D4d), while Dy2 belongs to an approximate capped trigonal prism (C2v) symmetry. Such structural changes upon modifying the bridging ligands (R group) are most likely responsible for the alteration in the magnetic relaxation behaviors (see below). In these sandwich structures, one tetrahydrofuran molecule resides in the middle plane and links to the coordinated water through a hydrogen bond (O···O = 2.617 Å for 1, 2.647 Å for 2) between the O atom from tetrahydrofuran and the H atom from a water molecule (Figure S4 and Table S2 in the Supporting Information). The other hydrogen atom of a crystallized water molecule was linked to the uncoordinated Oalkoxy atom of the dinuclear unit (O···O = 2.720 Å for 1, 2.760 Å for 2). In the crystal packing diagram (Figure S5 in the Supporting Information), these dinuclear units stack first along the crystallographic a axis, adopting the “end-to-end” method, and then arrange regularly in an ···ABCDA··· fashion along the c axis. The shortest intermolecular Dy···Dy distances are C
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry ∼10.166 and ∼10.107 Å in 1 and 2, respectively. Both separations in 1 and 2 are greater than 10 Å, suggesting weak magnetic dipole interactions between neighboring dinuclear units.18 Static Magnetic Properties. Direct current (dc) magnetic susceptibility measurements are performed at 1 kOe dc field from 2 to 300 K on freshly prepared microcrystalline samples of 1 and 2 (Figure 3). The room-temperature χMT values are
high temperature and then more quickly below 30 K, reaching 21.30 cm3 K mol−1 at 2 K. In contrast to the case for 1, the χMT value for 2 in the low-temperature region has no obvious increase trend, showing quite different magnetic interactions in the two complexes. The magnetization versus field plots for 1 and 2 were further determined in the range of 0−70 kOe at different temperatures. As depicted in the inserts in Figure 3, both complexes behave similarly with an abrupt increase below 10 kOe and then a slight increase to the maximum values without saturation, indicating the existence of great magnetic anisotropy of DyIII ions and/or low-lying excited states. This is also supported by the nonsuperposition of iso-temperature lines in the M versus H/T curves (Figure S6 in the Supporting Information).19 Dynamic Magnetic Properties. To better understand the dynamic magnetic behavior of 1 and 2, the alternating current (ac) magnetic susceptibilities were measured. Under a zero dc field, both χ′ and χ″ signals are strongly temperature dependent below 12 K, which is a positive proof for the SMM nature of 1 (Figure 4a). In χ″ versus T plots, the full peaks with good shape are clearly detected and shift to high temperatures with increasing frequencies. At 999 Hz, two maxima can be easily observed around 5.2 and 8.0 K, indicating the existence of two regimes of relaxation. This scenario may be attributed to the presence of two crystallographic DyIII centers, with the subtle differentiation resulting from the different ligand fields. Additionally, the gradual increase in χ″ at low temperature suggests the intervention of QTM. This is not unfamiliar for the Dy2-SMMs described in the literatures.3c,18b,20 The two relaxation times τ1 (in the low-temperature part) and τ2 (in the high-temperature part) were extracted from the peaks of χ″ signals in Figure 4a. Both relaxations are thermally activated on the basis of Arrhenius fits. The thermal energy barrier (Ea/kB) and the pre-exponential factor (τ0) are 32.5 K and 3.93 × 10−7 s for the low-temperature relaxation process and 59.5 K and 1.59 × 10−7 s for the high-temperature relaxation process (Figure 5), respectively. At the lower temperature, the relaxation time gradually deviates from linearity of the Arrhenius fit, suggesting the intervention of other possible relaxation processes. Then, the τ1 data were also fitted to eq 1, considering multiple
Figure 3. Temperature (T) dependence of χMT for 1 (black) and 2 (red) at 1 kOe dc field. The solid lines correspond to the calculated data with the intermolecular interaction zJ′ of −0.03 for 1 and −0.02 cm−1 for 2, respectively. The insets represent the field-dependent magnetizations for 1 (left) and 2 (right) at 1.8, 3.0, 4.0, and 6.0 K.
28.22 and 28.19 cm3 K mol−1 for 1 and 2, and both of them are close to the expected value of two isolated DyIII (28.34 cm3 K mol−1; 6H15/2, J = 15/2, g = 4/3) ions.18 With the lowering of temperature, the χMT value for complex 1 decreases gradually until 8 K, which is most likely (especially in the lowtemperature region) attributed to the thermal depopulation of the Stark sublevels under the ligand field effect.5c On further cooling, a sudden increase in χMT is observed, reaching 24.99 cm3 K mol−1 at 2 K, which indicates the existence of intramolecular ferromagnetic interactions between two DyIII centers. For complex 2, the χMT value decreases first slowly at
Figure 4. Temperature-dependent in-phase (χ′) and out-of-phase (χ″) ac susceptibilities for 1 (a) and 2 (b) under zero dc field. D
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
relaxation processes could not be observed obviously in the Cole−Cole plots, which give nearly semicircular shapes. The ac susceptibilities for 1 were further studied in the existence of an optimized dc field of 600 Oe (Figures S9 and S10 in the Supporting Information). In χ″ versus T plots, the obvious increasing trend of χ″ disappears at lower temperature, implying that the presence of QTM can be significantly reduced by the applied field in this system. Furthermore, at 999 Hz, two χ″ signal peaks are observed clearly at the same temperatures as those that appear at zero dc field. Fitting the relaxation time to the Arrhenius law affords Ea/kB = 55.2 K and τ0 = 1.89 × 10−8 s for the low-temperature relaxation and Ea/kB = 67.3 K and τ0 = 6.11 × 10−8 s for the high-temperature relaxation (Figure S11 in the Supporting Information). The Cole−Cole plots (Figure S12 in the Supporting Information) are fitted to a generalized Debye model,23 affording α parameters (Table S6 in the Supporting Information). Below 6 K, the α value is greater than 0.3, indicating the existence of other possible relaxations. The fit of the τ1 data concerning multiple relaxation pathways demonstrates that the relaxation arises through the temperature-dependent Orbach and Raman mechanisms (Figure S11, dashed line) with the satisfied parameters C = 0.044 s−1 K−5.44, n = 5.44, Ea/kB = 58.8 K, and τ0 = 1.12 × 10−8 s. Apparently, under 600 Oe dc field, the QTM process is suppressed and both relaxations become slower with higher energy barriers in terms of the same processes at zero dc field, especially the lowtemperature relaxation. In addition, the ac measurement on powder samples of 1 also shows the existence of two relaxation processes (Figure S13 in the Supporting Information). Concerning those facts, we think that the dual magnetic relaxation of 1 might be attributed to the intramolecular Dy1 and Dy2 ions, not dominated by the intermolecular interactions. In the case of 2, a greatly different relaxation behavior was observed. Under zero field, the χ″(T) signal has a tendency to peak at around 4.2 K at 999 Hz; however, then it increases rapidly with a lowering of the temperature, implying the existence of QTM (Figure 4b).3b,20 There also no obvious frequency-dependent χ″ peaks were detected even at very low temperature, which is different from the case for complex 1
Figure 5. ln τ vs T−1 plot for 1 under zero dc field. The solid and dashed lines are the best fits to the Arrhenius law and eq 1, respectively.
relaxation pathways (Orbach, Raman, QTM).21 The extracted parameters are τQTM = 0.004 s, C = 0.0041 s−1 K−8.27, n = 8.27, Ea/kB = 33.1 K, and τ0 = 1.72 × 10−7 s, all of which are consistent with the expected values for DyIII-SMMs.18a,21 Note that the zero-field SMM behavior of the present case is a rare example for Pc-supported triple-decker dysprosium(III) SMMs and the Ea/kB value for the high-temperature relaxation phase is among the highest values for this system.5c,22 τ −1 = τQTM −1 + CT n + τ0−1 exp( −Ea /kBT )
(1)
For further investigation of the nature of the magnetic dynamics, the frequency-dependent χ′ and χ″ signals were also collected at zero dc field. The good peaks shift gradually toward lower frequency with the temperature down to 1.8 K with overlapping maxima (Figure 6a). The same characteristic splitting from a one-component system into a two-component system was also supported by the Cole−Cole diagrams (Figures S7 and S8 in the Supporting Information). Fitting the data to the generalized Debye model23 afforded an α value of 0.3 for the low-temperature relaxation and 0.2 for the high-temperature relaxation, which supports the presence of multiple relaxation processes for the former and a relatively narrow distribution of the relaxation time for the latter.17a Herein, we also noted that two relaxation times in low- and hightemperature regions are close. As proposed by Tang et al.,24 when the ratio of the relaxation times is small, the double-
Figure 6. Frequency-dependent in-phase (χ′) and out-of-phase (χ″) ac susceptibilities for 1 (a) and 2 (b) under zero dc field. E
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (Figure 6b). Fitting to ln(χ″/χ′) = ln(ωτ0) + Ea/kBT allows us to evaluate a negligible energy barrier Ea/kB of 0.5 K (Figure S14 in the Supporting Information).25 A series of external dc fields were applied for 2 to elucidate the nature of the relaxation dynamics (Figure S15 in the Supporting Information). Under an optimized field of 1 kOe, the QTM effect was weakened effectively and the clear peaks of the χ″ signal can be observed in χ″ versus T plots (Figure S16a in the Supporting Information). Moreover, the frequency-dependent χ″ data exhibit a gradual shift toward lower frequency with a decrease in temperature to 1.8 K (Figure S16b), which indicates a slow relaxation of the magnetization. The relaxation times τ extracted from the χ″(T) peaks were fitted with the Arrhenius law to give the satisfied parameters Ea/kB = 38.7 K and τ0 = 5.32 × 10−8 s (Figure S17a in the Supporting Information). The Cole−Cole plots are fitted using the generalized Debye model,23 as shown in Figure S17b. The relatively large α values imply the existence of multiple relaxation processes (Table S7 in the Supporting Information). Ab Initio Calculations. To further elucidate the striking difference in magnetic properties of 1 and 2, CASSCF calculations were performed using MOLCAS 8.015 and SINGLE_ANISO26 programs. The lowest Kramers doublets and g tensors of complexes 1 and 2 are given in Table S8 in the Supporting Information. Herein, the gz values for all DyIII fragments in 1 and 2 are both close to 20, which show that the DyIII−DyIII magnetic interactions in these two complexes can be approximated as the Ising type. According to previous reports, the magnetic axiality of the ground state has a significant influence on the performance of lanthanide SMMs.27 The close comparison of g tensor components (gx, gy, gz) of the ground Kramers doublet between 1 and 2 reveals that both Dy1 and Dy2 fragments in 1 possess a larger axial component gz and smaller transverse components gx,y in comparison to those in 2, suggesting a higher degree of axiality and a weaker quantum tunneling of magnetization for the former. For both complexes, the total coupling parameters (J = Jexch + Jdipolar) were included into fitting the susceptibility data. Herein, the dipolar interactions (Jdipolar) were calculated exactly according to the directions of local magnetic axes and g tensors, concerning the pseudospin S̃ = 1/2 of the DyIII ions. The exchange parameters (Jexch) could be obtained through comparison of the calculated and experimental static magnetic data with the POLY_ANISO26 program. As shown in Figure 3, both fits are close to the experimental values.28 From Table 2,
energies of the ground exchange states on the basis of the above interaction parameters and the corresponding gz values are given in Table S9 in the Supporting Information; the gz values for complexes 1 and 2 are 37.001 and 36.840, confirming the ferromagnetic DyIII−DyIII couplings. The enhanced magnetic interaction in complex 1 generates a larger exchange state (1.5 cm−1 for 1 and 0.4 cm−1 for 2), which benefits the suppression of the tunneling relaxation process in complex 1.6b,c To get a deeper understanding of the enhanced magnetic interaction in complex 1, the anisotropy axes on two DyIII ions are provided for both complexes. As shown in Figure 7, the magnetic axes on two Dy1 ions for 1 and 2 are in similar directions and the included angles, θ1, between the magnetic axis on Dy1 and the vector connecting two DyIII ions for these complexes are 32.4 and 35.3°, respectively (Table S10 in the Supporting Information). Specifically, the angle (θ2) of the anisotropy axis on Dy2 with DyIII···DyIII linkage for 1 is only 14.6°, which is about half of that in 2 (30.2°). The smaller value in 1 leads to a weaker influence on the tunneling gap of the individual DyIII ion by the dipolar field and significantly reduces the efficiency of the tunneling relaxations.3c,30 In addition, according to the EPR studies of the Dy2 complex from the van Slageren group, the magnetic interaction between anisotropic lanthanide centers is extremely sensitive to the arrangement of anisotropy axes.6b The drastic change in the arrangement of anisotropy axes most likely affects the overlap between dysprosium orbitals and valency orbitals of the bridging ligands, thus influencing the magnetic interactions in this system. From the above analysis, it is reasonable that 1 features a zero-field SMM with higher performance. Basically, the differences in the structure and magnetic characteristics correspond to the change in terminal R substituents from methoxy in 1 to ethoxy in 2. Providing a structural comparison of the coordination geometries is necessary in these two complexes. From a structural point of view, the twist angles φ for three DyIII ions (Dy1 and Dy2 ions in 1, Dy1 ion in 2) in an SAP environment show subtle differences (Table S4 in the Supporting Information), and the φ values in 1 are much closer to the ideal angle in SAP symmetry of 45°.5c On the other hand, it is also the most important aspect that the changes of ligand field alter the coordination environment around DyIII ions. The Dy2···O1 distance in 2 is nearly 0.23 Å longer than that found in 1. In 2, the lack of coordination of the O1 atom leads to the sevencoordinated sphere of the Dy2 ion. As shown in Figure 7, the four atoms O2, O6, N13, and N15 with higher negative charge distribution are located near the easy axis of the Dy1 ion (O2, O6, N1, and N3 for Dy2 ions), while the O3, O4, N9, and N11 atoms having lower negative charge distribution constitute the hard plane (O4, O7, N5, and N7 for Dy2 ions) (Table S11 in the Supporting Information). Within the two complexes, these atoms around Dy1 ions offer almost equivalent equatorial and axial charges. However, the different coordination mode of the Dy2 center generates a quite different charge density distribution in the hard plane, which was considered to be a decisive factor in tuning the whole molecular magnetic anisotropy and magnetic interaction in this Dy2 system.
Table 2. Dipolar, Exchange, and Total Coupling Constants (J = Jexch + Jdipolar) (cm−1) between Dy1 and Dy2 Ions in 1 and 2 1
2
Jdipolar
Jexch
J
Jdipolar
Jexch
J
3.92
−1.23
2.69
2.86
−2.63
0.23
the Jexch parameters are negative with signs opposite to the Jdipolar values, and the dipolar interactions between two DyIII ions are stronger than the exchange interactions. In complexes 1 and 2, the DyIII−DyIII interactions within the Lines model29 are both ferromagnetic, but the total interaction J of 1 is more than 10 times larger than that of 2 (2.69 cm−1 for 1 and 0.23 cm−1 for 2). In addition, the best fit gives zJ′ values of −0.03 cm−1 for 1 and −0.02 cm−1 for 2, respectively, suggesting very weak intermolecular interactions. Furthermore, the calculated
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CONCLUSION In conclusion, a new type of dysprosium heteroleptic tripledecker complex has been prepared and characterized, which further enriches the coordination chemistry of sandwich complexes. In the triple-decker motif, two DyIII ions are F
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 7. Main magnetic axes of the ground doublets on Dy1 and Dy2 ions for 1 (a) and 2 (b).
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bridged by two coplanar “head-to-tail” HL-R ligands and capped by two Pc ligands. Under zero dc field, complex 1 shows a thermal-activated process with a high energy barrier of 59.5 K, while 2 exhibits fast quantum tunneling of the magnetization. Theoretical calculations revealed that the crucial reason is the key effect of magnetic interaction J. This effect mainly benefits from changes in the ligand fields around the DyIII ions, which alters the arrangement of the main magnetic axes surrounding two DyIII centers. This work presents a rational method to modulate magnetic relaxation behaviors in DyIII-based dinuclear complexes through elaborate modification of the ligand field.
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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.7b02824. Selected bond lengths, bond angles and structure parameters, electronic absorption spectra, IR spectra, XRPD patterns, details of magnetic characterizations, and calculations for 1 and 2 (PDF) Accession Codes
CCDC 1581129−1581130 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for J.-L.Z.:
[email protected]. *E-mail for Y.-Q.Z.:
[email protected]. ORCID
Yi-Quan Zhang: 0000-0003-1818-0612 Jing-Lin Zuo: 0000-0003-1219-8926 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (2013CB922101), NSFC (91433113, 21631006, 11774178), and NSF of Jiangsu Province of China (BK20151542, BK20130054). G
DOI: 10.1021/acs.inorgchem.7b02824 Inorg. Chem. XXXX, XXX, XXX−XXX
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