Influence of Magnetic Interactions and Single-Ion Anisotropy on

Apr 10, 2019 - Influence of Magnetic Interactions and Single-Ion Anisotropy on Magnetic Relaxation within a Family of Tetranuclear Dysprosium Complexe...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Magnetic Interactions and Single-Ion Anisotropy on Magnetic Relaxation within a Family of Tetranuclear Dysprosium Complexes Jingjing Lu,†,⊥ Yi-Quan Zhang,*,‡ Xiao-Lei Li,† Mei Guo,† Jianfeng Wu,† Lang Zhao,† and Jinkui Tang*,†,⊥ Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/10/19. For personal use only.

†

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, P. R. China ‡ School of Physical Science and Technology, Nanjing Normal University, Nanjing, Jiangsu 210023, P. R. China ⊥ School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Six tetranuclear DyIII complexes [Dy4(L)2(CH3OH)3(NO3)3]·3NO3·2H2O (1a), [Dy4(L)2(CH3OH)2(SCN)4(OCH3)2]·2CH3OH·2H2O (1b), {[Dy4(L)2(CH3OH)(SCN)6(CH3CN)]·3CH3OH·4CH3CN}2 (2a), [Dy4(L)2(CH3OH)2(SCN)6]·6CH3OH·2H2O (2b), [Dy4(L)2(CH3OH)2(SCN)4(OCH3)2]·5CH3OH·2H2O (3a), and [Dy4(L)2(CH3OH)(SCN)5(H2O)2]·SCN·4CH3OH·2H2O (3b) were structurally and magnetically characterized. The Dy1/ Dy2 centers in these complexes are eight-coordinate and submitted to pseudo-D4d symmetry environments. It is noteworthy that the modulation of coordination terminal around Dy1/Dy2 centers induces distinct magnetic relaxation processes, switching from single relaxation (1b) to two-step relaxation (2b). All complexes show significant zero-field single-molecule magnet (SMM) properties with the exception of 3b, which only features the slow magnetic relaxation behavior under a zero dc field. Ab initio calculations substantiate that the excellent SMM property of complex 1b should mainly profit from strong ferromagnetic interactions between the individual DyIII ions, while different single-ion magnetism results in better SMM property of complex 3a than that of 3b.

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H15/2,4a,6 has indisputably yielded the largest quality of strongly blocked Ln-SMMs. Notably, the mononuclear Ln-SMMs, usually qualified as single-ion magnets (SIMs),7 have been at the forefront of great progresses in molecular magnetism, which reveal great potential in achieving high-performance SMMs.8 In particular, the dysprosium metallocene cation [(Cp iPr5)Dy(Cp*)]+ (CpiPr5 = penta-iso-propylcyclopentadienyl, Cp* = pentamethylcyclopentadienyl) reported by Layfield and co-workers very recently, established new records of effective energy barrier (Ueff = 2217 K) and blocking temperature (TB = 80 K), which represents the best SMM reported to date.9 Certainly, some of the dinuclear Ln-SMMs with extremely strong intramolecular magnetic interactions also exhibit excellent 6

INTRODUCTION

As a remarkable class of molecule-based materials, singlemolecule magnets (SMMs),1 capable of retaining magnetization below a certain temperature at the molecular level, have attracted great interest in view of their potential applications in ultrahigh-density information storage, quantum computing, and molecular spintronics.2 Since the seminal discovery of the mononuclear double-decker phthalocyanine (Pc) complex (Bu4N)[Tb(Pc)2] showing an SMM behavior in 2003,3 sustained efforts have been focused on developing lanthanide-based SMMs (Ln-SMMs).4 In contrast to transition metal ions, lanthanide ions by virtue of their large magnetic moments and significant intrinsic magnetic anisotropies arising from a strong spin−orbit coupling effect have been considered as the superior spin carriers for the construction of high-performance SMMs.5 Especially the DyIII ion, profiting from its unparalleled single-ion anisotropy with the Kramers ground state of © XXXX American Chemical Society

Received: January 9, 2019

A

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry SMM properties.10 Since the pioneering work in polynuclear Ln-SMMs,2a hundreds of such SMMs with diverse structural topologies have been discovered, and some of them display larger anisotropy barriers for reversing the magnetization,11 such as Dy56d and Ho5.12 However, for the great majority of polynuclear Ln-SMMs, their magnetic properties seem to not be very prominent, with the highest anisotropic energy barrier of 692 K for the reported {Dy4K2} system so far.13 Meanwhile, recent studies have shown that only very few well-behaved polynuclear Ln-SMMs exist, because of the huge difficulty in controlling the coordination geometry and manipulating easyaxis arrangement of spin carriers in such large clusters.2a,6b,14 Thus, the construction of polynuclear complexes with strongly uniaxial magnetic anisotropy still represents a formidable challenge. More importantly, the presence of multiple distinct anisotropic centers in polynuclear systems will generate more complicated magnetic interactions, resulting in the origin of the magnetic relaxation behavior hardly explored and poorly understood. It is therefore of great importance to develop intriguing polynuclear lanthanide-based complexes to demonstrate the magneto-structural correlation for further improvement of their SMM properties. With this in mind, we probe the influence of the local coordination geometry on the magnetic interactions and single-ion magnetism and eventually the relaxation dynamics of polynuclear dysprosium(III) SMMs. Herein, we designed a tripodal Schiff-base ligand [2,4,6-tris(2-hydroxy-3methoxybenzylidene)hydrazono-1,3,5-triazine] (H3L, Scheme 1) from which six tetranuclear dysprosium(III) complexes,

prominent zero-field SMM behavior of these complexes, except the slow relaxation of magnetization behavior for complex 3b. Remarkably, ab initio calculations demonstrate that the modulation of local coordination geometry can dramatically alter the magnetic interactions and single-ion magnetism, which can further modulate the magnetism of Dy4 complexes.

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RESULTS AND DISCUSSION Crystallography. With the combination of ligand H3L and the corresponding dysprosium salts in different solvents under basic conditions and at different crystallization temperatures, yellow crystals of tetranuclear dysprosium complexes of 1a−3a and 1b−3b were obtained (detailed information in the Supporting Information). The main difference in isolating these complexes is probably the different bases employed during the synthesis. Such a tiny change results in the formation of structurally related molecules with different local coordination terminals around the DyIII sites. Single-crystal Xray diffraction studies reveal that complexes 1a−3a and 1b−3b are essentially isoskeletal, with a similar tetranuclear arrangement of DyIII ions, primarily differing in the coordination terminal (solvents or anions) around the DyIII ions and the number of cocrystallized solvent molecules and counteranions. For brevity, the structure of complex 1a will only be described here (Figure 1).

Scheme 1. Schematic Drawing of the Coordination Modes of Ligand H3L Indicated by the Harris Notation15

Figure 1. Molecular structure of complex 1a; H atoms, counteranions, and solvent molecules are omitted for clarity.

Complex 1a belongs to the monoclinic space group P21/c with asymmetric unit containing the whole Dy4 cluster: four independent DyIII ions, two trideprotonated ligands L3−, three coordinated NO3− anions, three coordinated MeOH molecules, and some dissociative H2O molecules and NO3− anions. Within the Dy4 cluster, each of the trideprotonated ligands L3− with a binding mode of 3.1122313223131111121212 coordinates to three dysprosium ions (Scheme 1), giving rise to an unclosed Dy3 core with three Dy centers taking up the N2O, N3O2, and O4 coordination pockets, respectively. The two Dy3 cores share the two terminal DyIII ions (Dy1 and Dy2), forming a regular parallelogram where the four DyIII ions reside in the corners with Dy−Dy−Dy angles of 114° and 63°. However, the four DyIII ions are not coplanar, which should mainly be due to the asymmetric configuration of the parallelogram and relatively rigid Schiff-base ligands. The Dy4 core can also be regarded as the linkup of two binuclear subunits encapsulated by two deprotonated L3− ligands. Each of the subunits is composed of two different types of DyIII ions, which are doubly bridged by the phenol oxygen atoms O (O8, O13, O7, and O15) of two

namely, [Dy 4 (L) 2 (CH 3 OH) 3 (NO 3 ) 3 ]·3NO 3 ·2H2 O (1a), [Dy4(L)2(CH3OH)2(SCN)4(OCH3)2]·2CH3OH·2H2O (1b), {[Dy4(L)2(CH3OH)(SCN)6(CH3CN)]·3CH3OH·4CH3CN}2 (2a), [Dy4(L)2(CH3OH)2(SCN)6]·6CH3OH·2H2O (2b), [Dy4(L)2(CH3OH)2(SCN)4(OCH3)2]·5CH3OH·2H2O (3a), and [Dy 4 (L) 2 (CH 3 OH)(SCN) 5 (H 2 O) 2 ]·SCN·4CH 3 OH· 2H2O (3b) were successfully prepared. Interestingly, the change of coordination terminal around Dy1/Dy2 centers results in a great difference in the coordination environment, which induces a significant modification of the relaxation process, switching from single relaxation (1b) to two-step relaxation (2b). It is worth mentioning that the D4d symmetry environments of the Dy1/Dy2 centers and the relatively strong intramolecular magnetic interactions are responsible for the B

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Crystal structures and orientations of easy axes (green arrows) of the ground states for complexes 1a−3a and 1b−3b. All H atoms, solvent molecules, and counteranions are omitted for clarity. Color code: Dy, purple; O, red; N, blue; C, black; S, yellow.

Table 1. Summary of Coordination Terminals and Coordination Geometries with CShMs (the Continuous Shape Measures Values) of the Dysprosium Atoms in 1a−3a and 1b−3b complex

Dy center

coordination terminal

Dy1 Dy2 Dy3 Dy4

coordination geometry

Dy1 Dy2 Dy3 Dy4

1a 1 MeOH 1 MeOH 2 NO3− 1 NO3− 1 MeOH D4d (1.436) D4d (1.645) C4v (5.083) D2d (7.245)

1b

2a

2b

3a

3b

1 MeO− 1 MeO− 2 SCN− 1 MeOH 2 SCN− 1 MeOH D4d (1.683) D4d (1.683) C2v (6.399) C2v (6.399)

1 SCN− 1 SCN− 2 SCN− 1 MeOH 2 SCN− 1 MeCN D4d (1.423) D4d (1.325) D2d (6.234) D2d (5.644)

1 SCN− 1 SCN− 2 SCN− 1 MeOH 2 SCN− 1 MeOH D4d (1.382) D4d (1.232) C2v (5.838) D2d (6.670)

1 SCN− 1 MeO− 2 SCN− 1 MeOH 1 SCN− 1 MeO− + 1 MeOH D4d (1.505) D4d (1.530) C2v (7.046) C2v (6.010)

1 MeOH 1 SCN− 2 SCN− 1 H2O 2 SCN− 1 H2O D4d (1.487) D4d (1.347) D2d (5.151) C2v (6.327)

complex 1a, resulting in spherical-relaxed capped cube (C2v) geometries with an O6N2 sphere for Dy3 and Dy4 centers of 1b, which is clearly different from the case of 1a. However, the Dy1 and Dy2 centers display almost identical SAP coordination geometries in the two complexes. Another striking feature is that, for complexes 1b and 2b, the Dy1 and Dy2 centers possess similar D4d coordination geometries but different coordination atoms at the DyIII sites, namely, two MeO− anions and two SCN− anions, respectively, while the coordination environments around Dy3 and Dy4 centers only show a tiny change considering the same coordination numbers and coordination atoms. Significantly, such an elaborate modification at Dy3 and Dy4 sites leads to a minor impact on the magnetic behaviors, in contrast to the modulation at Dy1 and Dy2 sites, which causes great changes of relaxation processes, transforming from single relaxation (1b) to two-step relaxation (2b) (see below). Direct Current (dc) Magnetism. The static dc magnetic susceptibility measurements for these complexes were performed under an applied field of 1000 Oe ranging from 2 to 300 K. As shown in Figure 3, the room-temperature χMT values are 56.82, 56.35, and 58.24 cm3·K·mol−1 for 1a, 2a, and 3a, respectively, in line with the predicted value of 56.68 cm3· K·mol−1 for four noninteracting dysprosium ions.16 When the temperature is lowered, the χMT values exhibit gradual decreases reaching the minimum values of 51.08, 49.20, and 52.33 cm3·K·mol−1 at ca. 18 K for 1a, 2a, and 3a, respectively, probably resulting from the progressive depopulation of excited Stark sublevels.10c Upon further cooling, the χMT

antiparallel H3L ligands with the Dy···Dy distances of 3.802 and 3.810 Å, respectively. Furthermore, both Dy1 and Dy2 are coordinated by one MeOH molecule, while the remaining coordination sites in axial positions of Dy3 and Dy4 are filled by two NO3− anions and one MeOH molecule, and one NO3− anion, respectively. The Dy1···Dy3 and Dy2···Dy4 separations are 6.833 and 6.795 Å, respectively. Consequently, both Dy1 and Dy2 are eight-coordinated, adopting a slightly distorted square-antiprism (SAP) geometry with an O6N2 environment, whereas Dy3 and Dy4 resemble a nine-coordinated sphericalrelaxed capped cube (C4v) and an eight-coordinated Johnson gyrobifastigium J26 (JGBF-8, D2d) geometry, respectively (Table S2). The Dy−O and Dy−N bond distances are in the ranges of 2.176−2.700 and 2.403−2.596 Å, respectively. Note that the Dy−Ophenyl bonds of 2.176 (Dy1−O19) and 2.185 Å (Dy2−O11) are significantly shorter than those of some reported Dy 4 SMMs (Table S5). The nearest intermolecular Dy···Dy distance is 9.506 Å, which is longer than the diagonal Dy···Dy distances of 6.158 and 9.036 Å, indicating relatively weak intermolecular interactions. Structural Comparison. As shown in Figure 2 and Table 1, a critical difference in these complexes is the distinctive coordination terminal around the DyIII site. When going from 1a to 1b (2a to 2b), the coordination environments of Dy3 and Dy4 centers exhibit some great differences, while Dy1 and Dy2 centers maintain almost identical environments (same coordination atoms). For instance, both the axial positions of Dy3 and Dy4 centers in complex 1b are replaced by two SCN− anions and one MeOH molecule compared to those of C

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

order to probe the magnetic dynamics. Clearly, all complexes show strong frequency dependence of both in-phase (χ′) and out-of-phase (χ″) susceptibility components, implying the slow magnetic relaxation that is associated with typical SMM behavior (Figures S8−S10). For complexes 1a and 1b, the temperature-dependent ac signals exhibit peaks at 26 and 24 K, respectively (Figures S6 and S7). As the temperature decreases, the sharp increase of the χ″ signal is observed, which is suggestive of the appearance of fast quantum tunneling of the magnetization (QTM) relaxation process, as commonly found in most reported LnSMMs.8f,10d,19 Consistent with this, frequency-dependent ac signals display temperature-independent χ″(v) peaks at the low-temperature regime (Figure S8). The Cole−Cole plots defined as χ″ vs χ′ for 1a and 1b show relatively symmetrical semicircular shapes (Figures S11 and S12), which were nicely fitted with the generalized Debye model, with fitted α values in the ranges of 0.07−0.34 and 0.04−0.11, respectively. This indicates the larger distribution of relaxation time and possible multiple relaxation paths are involved in 1a, as also verified by broader ac peak in 1a than that of 1b (Figure 4). Moreover, we fitted the ln(τ) vs 1/T plots to evaluate the effective barrier of magnetic relaxation. As shown in Figure 5, the fitting of high-temperature range following Arrhenius law (τ = τ0 exp(Ueff/kBT)) affords effective energy barriers (Ueff) of 181 and 172 K with pre-exponential factors (τ0) of 1.52 × 10−7 and 1.70 × 10−7 s for 1a and 1b, respectively, which are comparable to the higher barrier of reported zero-field Dy4 SMMs (Table S5),20 while the linear fitting of the lowtemperature region yields Ueff of 2.1 and 2.0 K with τ0 of 2.23 × 10−3 and 1.15 × 10−3 s for 1a and 1b, respectively. Here, such complicated relaxation processes should be the result of the presence of several weakly coupled spin centers in these Dy4 systems. In stark contrast to complexes 1a and 1b, complex 2a exhibits two well-resolved χ″ peaks ranging from 1.9 to 20 K (Figure 6), demonstrating the occurrence of the two-step thermally activated relaxation processes.20g,h,21 However, above 20 K, only one series of frequency-dependent ac peaks occurs at higher frequencies, illustrating the evolution from fast relaxation (FR) to slow relaxation (SR) within the measured

Figure 3. Plots of χMT vs T for complexes 1a, 2a, and 3a under a 1000 Oe dc field.

values rapidly increase to 58.05, 62.66, and 61.76 cm3·K·mol−1 at 2.0 K, respectively, indicating the dominance of intramolecular ferromagnetic interactions. The χMT values of complexes 1b−3b undergo similar thermal evolution, with χMT values of 55.56, 55.89, and 54.53 cm3·K·mol−1 at 300 K, increasing to 62.34, 68.26, and 54.92 cm3·K·mol−1 at 2.0 K, respectively (Figure S3). The field (H) dependence of magnetization (M) for these complexes was measured from 0 to 70 kOe at 1.9, 3.0, and 5.0 K (Figure S4). The magnetization for each of the complexes displays a precipitous rise below 10 kOe suggesting the presence of ferromagnetic interactions, in agreement with the sharp increase of the χMT value at low temperature, and then is followed by the slowly linear increase at the high-field region, without reaching the saturation value of 40 μB at 70 kOe, which is mainly ascribed to the considerable crystal field effect at the DyIII ions.17 The nonsuperposition of the M vs H/T plots on a single master curve indicates significant magnetic anisotropy and/or low-lying excited states in these systems.10d,18 Additionally, there is no obvious hysteretic behavior found at 1.9 K (Figure S5). Alternating Current (ac) Magnetism. The ac magnetic susceptibility measurements of these complexes were carried out under a zero dc field using a 3.0 Oe oscillating field in

Figure 4. Frequency dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility for 1a (left) and 1b (right) under zero dc field. D

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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two modified Debye functions22 (eq 1) affords α parameters in the ranges of 0.09−0.37 and 0.02−0.14 for SR and FR, respectively, indicative of wider distributions of the relaxation times for the SR process. χac (ω) = χS,tot +

Δχ1 (1 − α1)

1 + (iωτ1)

+

Δχ2 1 + (iωτ2)(1 − α2) (1)

Complexes 2b and 3a reveal a similar multiple relaxation behavior compared to that of complex 2a, with one set of broad ac peaks and two isolated χ″(v) peaks below 4.0 K, respectively, presumably as a consequence of the similar coordination environments of DyIII ions of these complexes. As the temperature increases, the χ″(v) peak maxima shifts gradually to the higher frequency, which is also a symbol of transition from the FR to the SR process. Both the Cole−Cole plots of 2b and 3a (Figures S14 and S15) present a distorted semicircular shape, which were simulated to the modified Debye functions (eq1), with the α1 parameters being less than 0.25, but the α2 parameters being as large as 0.62 and 0.55, respectively, implying a relatively narrow distribution of relaxation time of the SR process in the two complexes. Further, we extracted the magnetization relaxation times (τ) of complexes 2a, 2b, and 3a from the χ″(v) data. The ln(τ) vs 1/T plot (Figure 7) exhibits a temperature-dependent linear regime at the high temperature, which is associated with the thermally activated Orbach relaxation mechanism. The data were fitted using the pure Arrhenius law, giving the Ueff = 107 cm−1 (154 K) with τ0 = 1.25 × 10−6 s for complex 2a, Ueff = 75 cm−1 (107 K) with τ0 = 2.57 × 10−6 s for complex 2b, and Ueff = 68 cm−1 (97 K) with τ0 = 2.35 × 10−6 s for complex 3a, respectively. With a decrease of the temperature, the plot exhibits a slight curvature and becomes weakly temperature dependent, which could be attributed to the existence of the Raman and QTM relaxation. With a view to probe the whole relaxation mechanisms, we fitted the ln(τ) vs 1/T plots of 2a, 2b, and 3b over the entire temperature range according to eq 2,23 which has taken a combination of the relaxation processes into account.

Figure 5. Plots of ln(τ) vs T−1 for 1a (top) and 1b (bottom) under zero dc field. The solid lines correspond to the best fit of the experimental data to the Arrhenius law.

frequency window with an improvement in the temperature. Accordingly, the Cole−Cole diagram (Figure S13) for 2a between 1.9 and 20 K displays two well-separated semicircular profiles corresponding to SR and FR phases, respectively, but above 20 K, only a nearly semicircle shape corresponding to SR phase was found. Fitting the data according to the sum of

Figure 6. Frequency dependence of the in-phase (χ′) and out-of-phase (χ″) ac susceptibility for complexes 2a (left), 2b (middle), and 3a (right) under zero dc field. E

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Plots of ln(τ) vs T−1 for complexes 2a (left), 2b (middle), and 3a (right) under zero dc field, where τ corresponds to the slow relaxation time. The red lines represent the best fit of the multiple relaxation processes based on eq 2.

Figure 8. Frequency dependence of χ′ and χ″ ac susceptibility for complex 3b.

ln τ = −ln[AT + B + CT n + τ0−1 exp( −Ueff /kBT )]

MOLCAS 8.225 and SINGLE_ANISO26 programs (see the Supporting Information for details). The energy levels (cm−1), g (gx, gy, gz) tensors, and the predominant mJ values of the lowest two Kramers doublets (KDs) of individual DyIII fragments for these complexes are shown in Table S6, where the energy gap between the lowest two KDs of 3b_Dy1 is the largest (266.3 cm−1), and those of 1a_Dy3, 1a_Dy4, and 2a_Dy4 are similar (42.8, 45.3, and 42.1 cm−1, respectively). The ground KDs of individual DyIII fragments for 1a_Dy3, 1a_Dy4, and 2a_Dy4 are mixed severely with several mJ states, but those for the others are mostly composed of a single mJ = ±15/2 state. Additionally, the first excited states of 1a_Dy1, 1a_Dy2, 2a_Dy1, 2a_Dy2, 2a_Dy3, 2b_Dy1, 2b_Dy2, 2b_Dy4, 3a_Dy1, 3a_Dy4, 3b_Dy1, and 3b_Dy2 are all mostly composed by a single mJ = ±13/2, but those for the others are all usually composed by several mJ states. The magnetization blocking barriers for individual DyIII fragments for these complexes are shown in Figure S20, where the transversal magnetic moments in the ground KDs of individual Dy1 and Dy2 fragments in all structures are close to 10−2 μB, which indicates the QTMs in their ground KDs are all suppressed at low temperature. However, those transversal magnetic moments of individual Dy3 and Dy4 fragments in the ground KDs are all about 10−1 μB, therefore allowing a fast

(2)

In this equation, AT + B, CTn, and τ0−1exp(Ueff/kBT) account for direct, Raman, and Orbach relaxation processes, respectively. The best fits yield Ueff of 118 cm−1 (169 K), 96 cm−1 (137 K), and 78 cm−1 (120 K), for complexes 2a, 2b, and 3a, respectively, in accord with the ones extracted from the high-temperature range. The other parameters are collected in Table S3. For complex 3b, both in-phase (χ′) and out-of-phase (χ″) susceptibility components show temperature-dependent (Figure S17) and frequency-dependent (Figure 8) signals below 30 K under a zero dc field, suggesting the onset of slow relaxation behavior. Unfortunately, the well-resolved out-of-phase peaks are absent until 1.9 K due to more pronounced QTM relaxation, which hampers the accurate evaluation of the effective energy barrier. Thus, we applied another method based on the relationship24 ln(χ″/χ′) = ln (ωτ0) + Ueff/kT to roughly evaluate the energy barrier. The linear fitting of ln(χ″/ χ′) vs 1/T plots gives the energy barrier Ueff of ∼1.3 K and the characteristic time τ0 of 1.40 × 10−4 s (Figure S18). Theoretical Calculations. Complete-active-space selfconsistent field (CASSCF) calculations on individual DyIII fragments of complexes 1a, 1b, 2a, 2b, 3a, and 3b on the basis of X-ray determined geometries have been carried out with F

DOI: 10.1021/acs.inorgchem.9b00067 Inorg. Chem. XXXX, XXX, XXX−XXX

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magnetic axes on four DyIII ions for these complexes are indicated in Figure 2. Complexes 1a and 1b. On the basis of single-ion analysis, the calculated ground state gz values of Dy1/Dy2 centers for 1a and 1b are close to 20 (Table S6), confirming the strongly axial nature of the ground state, in coincident with the significant SMM feature of the two complexes without the application of external dc field, as corroborated by ac magnetic susceptibility measurements. The ground states of individual Dy1/Dy2 centers of two complexes are mostly comprised of the mJ = ±15/2 state, but those of 1a are purer. As can be seen from Figure S20, the transversal magnetic moments of the individual Dy1/Dy2 fragments for 1a are smaller thanks to its purer ground state, but those of 1b are very large, thus facilitating the fast QTM relaxation of the ground state in complex 1b. Besides, the energy separation between the two lowest KDs for Dy1/Dy2 centers in 1a (ca. 235 cm−1) is more than two times larger than that of 1b (ca. 127 cm−1), suggesting stronger crystal field splitting of DyIII ions in 1a. Therefore, we can theoretically predict that the individual Dy1/Dy2 centers in 1b cannot show zero-field slow magnetic relaxation because of prominent QTM effects. Furthermore, the fitted DyIII···DyIII interactions (J2 and J4) of complexes 1a and 1b within the Lines model28 are ferromagnetic with similar values, primarily due to predominant dipole−dipole interactions (Jdip). In addition, the gz values of the ground exchange state for 1a and 1b are 64.666 and 74.366, respectively, demonstrating all DyIII···DyIII interactions in 1a and 1b are ferromagnetic. As a consequence, the strong ferromagnetic coupling between the individual DyIII ions may significantly reduce the zero-field QTM process, leading to stronger SMM behavior in complex 1b. Complexes 2a and 2b. For complexes 2a and 2b, the Dy1/ Dy2 centers both display an almost pure ground state (98%| ±15/2⟩) with an Ising type, strongly axial anisotropy (gz around 19.7). However, the Dy4 center of 2a shows a more mixed ground state (33%|±13/2⟩+43%|±11/2⟩+11%|±7/2⟩) and slightly smaller axial anisotropy (gz = 13.845) compared with those of 2b, characterizing a poor magnetic anisotropy on the Dy4 center in 2a. Correspondingly, the transversal magnetic moment in the ground state of the Dy4 center of 2a is the largest because of its most mixed ground state, but that of the Dy3 center of 2a is 9.4 × 10−3 μB, which is smaller than that of Dy3/Dy4 centers of 2b by an order of magnitude. Complexes 2a and 2b show very similar crystal structures but different magnetic properties, which should be attributed to different coordinate solvent molecules for the Dy4 center, namely, MeCN for 2a and MeOH for 2b, modulating the local ligand-field around the DyIII ions. Such structural differences lead to changes of electronic distributions around Dy4 centers. Consequently, the orientation of the magnetic anisotropy axes of the Dy4 center of complex 2a deviates largely from the Dy− Dy vector compared with that of 2b, thus resulting in distinct magnetic interactions. Apparently, the total interactions J4 of complexes 2a and 2b are antiferromagnetic and ferromagnetic, respectively, which can be mainly attributed to distinctive dipole−dipole interactions Jdip of −1.13 for 2a and 4.89 for 2b, respectively. Once again, the larger gz values of ground-state exchanges reveal that the DyIII···DyIII couplings of the two complexes are all ferromagnetic. Complexes 3a and 3b. For complex 3a, the Dy1/Dy4 centers possess an almost pure (98%)|±15/2⟩ ground state with the calculated gz values approaching the Ising-limit value

QTM in their ground KDs. Although their magnetic anisotropies mainly come from individual DyIII fragments, the DyIII···DyIII interactions have some influences on their slow magnetic relaxation processes. The calculated ground gz values of individual DyIII fragments for these complexes are all close to 20, and thus, the DyIII··· DyIII interactions can be approximately regarded as the Ising type during the fitting. The program POLY_ANISO26 was used to fit the magnetic susceptibilities of these complexes using the exchange parameters from Table 2. Table 2. Fitted Exchange Coupling Constant Jexch, the Calculated Dipole−Dipole Interaction Jdip, and the Total J between Magnetic Centers in Complexes 1a, 1b, 2a, 2b, 3a, and 3b (cm−1)a J1

J2

J3

J4

Jexch Jdip J Jexch Jdip J Jexch Jdip J Jexch Jdip J

1a

1b

2a

2b

3a

3b

0.41 −0.03 0.38 0.86 3.88 4.74 0.52 0.38 0.90 1.15 3.34 4.49

0.23 −0.16 0.07 0.67 5.19 5.86 0.23 −0.16 0.07 0.68 5.20 5.88

0.98 −0.01 0.97 1.74 4.86 6.60 0.01 0.21 0.22 0.18 −1.13 −0.95

0.24 −0.14 0.10 0.72 5.58 6.30 0.24 −0.05 0.19 0.98 4.89 5.87

0.22 −0.17 0.05 0.24 5.40 5.64 0.23 −0.23 0.00 0.25 5.48 5.73

−0.14 −0.36 −0.50 0.13 4.45 4.58 −0.24 −0.07 −0.31 0.23 5.40 5.63

The intermolecular interactions zJ′ of these complexes were all fitted to −0.01 cm−1. a

All parameters from Table 2 were calculated with respect to the pseudospin S̃ = 1/2 of the DyIII ions. For all complexes, the total coupling parameter J (dipolar and exchange) was included to fit the magnetic susceptibilities (Scheme 2). The Scheme 2. J-Coupling Scheme Employed for the Elucidation of the Magnetic Exchange Interactions in Complexes 1a−3a and 1b−3b

calculated and experimental χMT vs T plots of these complexes are shown in Figure S21, where the fits for 1a, 1b, 2a, and 2b well reproduce the experimental results,27 but those for 3a and 3b have some differences from the corresponding experiment. As shown in Table 2, the J4 values in complex 2a (or J1 and J3 in 3b) within the Lines model28 are antiferromagnetic, and those for the others are ferromagnetic. The exchange energies, the energy difference between each exchange doublets Δt, and the main values of the gz for the lowest eight exchange doublets of these complexes are summarized in Table S7, while the main G

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Dy1/Dy2 centers result in the prominent transition of the relaxation process from single relaxation to multiple relaxation. However, the local modification of the coordination environments of Dy3/Dy4 centers only induces a minor change on the magnetic behavior, such as 1a and 1b (2a and 2b). Ab initio calculations substantiate the magneto-structural relationship in these complexes. Complex 1b displays typical zero-field SMM behavior with an effective energy barrier of 172 K, which can be ascribed to the strong ferromagnetic coupling between the individual DyIII ions, while complex 3b shows a SMM property inferior to that of 3a because of different single-ion origin, which reveals that single-ion magnetism is a key factor in influencing the SMM performance of polynuclear complexes. These results demonstrate the importance of the magnetic interaction and single-ion magnetism in controlling the quantum tunneling and enhancing magnetic properties, further shedding light on a promising synthetic strategy to obtain highperformance polynuclear Ln-SMMs by the delicate modification of the local coordination geometry.

of 20, reflecting considerable uniaxial magnetic anisotropy of the DyIII ions. The transversal magnetic moments of the ground state for individual Dy2/Dy3 centers are larger than those for Dy1/Dy4 centers by an order of magnitude, suggesting the pronounced QTM relaxation in individual Dy2/Dy3 centers compared with Dy1/Dy4 centers, which may preclude any SMM characteristic in individual Dy2/Dy3 centers. Therefore, the observed SMM behavior in 3a should be heavily dominated by Dy1/Dy4 centers. Similarly, Dy1/ Dy2 centers play a decisive role in determining the SMM property of complex 3b. Complex 3a exhibits an energy barrier of 120 K, while complex 3b only shows slow magnetic relaxation behavior in zero dc field, which is mainly attributable to the different single-ion origin of the magnetic relaxation process, in light of the almost identical ferromagnetic interactions J4 and J2 for the two complexes. For 3a, the strong Ising-type anisotropy of the Dy1 center as well as the ferromagnetic interaction between Dy1 and Dy4 centers may favor an axial nature of the ligand field and efficient suppression of the QTM process, respectively, which could be responsible for the strong SMM behavior found in 3a. For 3b, the longer intramolecular Dy1··· Dy2 distance of 11.928 Å is unfavorable to reducing the tunneling relaxation process and thus enhancing SMM properties. Therefore, although complexes 3a and 3b both possess stronger magnetic interactions, complex 3a shows a better SMM property when compared with 3b. All complexes presented herein have the eight-coordinate Dy1/Dy2 centers located in unique D4d symmetry environments, which is an important factor for obtaining better SMM performance. The typical SMM behavior observed in 1b is mainly attributable to the presence of strong ferromagnetic interaction between the individual DyIII ions. In contrast, complex 3b only shows slow relaxation behavior, which is most likely due to the absence of efficient magnetic interaction in view of the large magnetic anisotropy of individual Dy1/Dy2 centers. However, for complexes 2a and 2b, the different coordination terminals around Dy4 center probably influence the local ligand field of the DyIII ions and, thus, affect the orientations of the easy axes on each dysprosium site. Here, the combination of single-ion anisotropy and magnetic interaction is probably responsible for this distinct dynamic behavior observed. Besides, different single-ion origin explains why complex 3a shows better SMM property than 3b. It is worth mentioning that complexes 1a and 3b possess similar singleion origin, namely, Dy1/Dy2 centers, with very different magnetic properties, possibly resulting from the different magnetic anisotropy of individual DyIII ions induced by different coordination terminals. These findings indicate that both the magnetic coupling and single-ion magnetic anisotropy have a significant impact on the magnetic behaviors of polynuclear complexes because they are very sensitive to a slight modification of the coordination environments.

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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.9b00067. Experimental section, magnetic measurements, crystallographic data, thermogravimetric analysis, powder XRD analyses, field dependences of magnetization, magnetic hysteresis loop, temperature dependences of the (χ′′) and out-of-phase ac susceptibilities, frequency dependence of the ac magnetic susceptibility, Cole−Cole plots, plots of ln(τ) vs T−1, plots of ln(χ′′/χ′) vs T−1, computational details, magnetization blocking barriers, calculated and experimental data of magnetic susceptibilities, SHAPE analysis, parameters of Arrhenius plot fitting, and ab initio details (PDF) Accession Codes

CCDC 1854363−1854368 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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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.T.). *E-mail: [email protected] (Y.-Q.Z.). ORCID

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Yi-Quan Zhang: 0000-0003-1818-0612 Jinkui Tang: 0000-0002-8600-7718

CONCLUSION In summary, we have successfully synthesized and characterized six structurally related tetranuclear dysprosium complexes, which exhibit discriminative dynamic magnetic behaviors. The D4d symmetry environment around Dy1/Dy2 centers, in conjunction with strong ferromagnetic coupling, should account for the robust SMM performance in these complexes except for the complex 3b. For complexes 1b and 2b, the subtle evolutions of coordination environments around

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

We thank the National Natural Science Foundation of China (Grants 21525103, 21871247, and 11774178) for financial support. J.T. gratefully acknowledges support of the Royal Society-Newton Advanced Fellowship (NA160075). H

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