Assembling Dysprosium Dimer Units into a Novel Chain Featuring

Dec 7, 2016 - In 2, the dinuclear units of 1 are strung into chains by formate anions, in which Dy3+ ions are situated in an octa-coordinated, hula-ho...
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Assembling Dysprosium Dimer Units into a Novel Chain Featuring Slow Magnetic Relaxation via Formate Linker Qi Chen,† Fang Ma,† Yin-Shan Meng,§ Hao-Ling Sun,*,† Yi-Quan Zhang,*,‡ and Song Gao*,§ †

Department of Chemistry and Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University, Beijing 100875, 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, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: A dinuclear complex [DyLClCH3OH)]2 (1) and a onedimensional compound [DyL(HCOO)(CH3OH)]n (2) have been synthesized using an organic ligand of N′-(2-hydroxybenzylidene)picolinohydrazide (H2L). Complex 1 exhibits a symmetric dinuclear structure, in which the Dy3+ centers reside in a pentagonal-bipyramidal coordination environment. In 2, the dinuclear units of 1 are strung into chains by formate anions, in which Dy3+ ions are situated in an octa-coordinated, hula-hoop-like coordination geometry. Magnetic studies reveal that ferromagnetic coupling is found between Dy3+ ions in both compounds. Complexes 1 and 2 exhibit slow magnetic relaxation under zero dc field with effective energy barriers of 88.4 and 175.8 K, respectively. Magnetic study combined with ab initio calculations indicates that the better performance of 2 is related to the unique molecular geometry and relatively stronger Dy3+−Dy3+ magnetic interaction within and/or between the dimer units.

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INTRODUCTION Molecular nanomagnet, exhibiting slow magnetic relaxation and magnetic hysteresis at low temperature, is a revived subject in magnetic materials and has attracted considerable attention in the field of spin-based devices and high-density information storage.1 Within the many characteristics required for chemical design of high performance molecular nanomagnets, magnetic anisotropy is the determinant factor.2 Owing to the strong coupling between the spin and orbit angular moments and the crystal-field splitting effects, lanthanide ions can provide large magnetic moments and significant magnetic anisotropy, making them ideal candidates for the construction of molecular nanomagnets.3 To date, many lanthanide molecular nanomagnets with diverse structural topologies have been reported, including systems featuring the highest relaxation energy barrier and the highest blocking temperature.4 However, the magnetic behaviors of lanthanide molecular nanomagnets remain poorly understood because of their complicated magnetic nature, which can be affected by many factors, such as symmetryrelated single-ion anisotropy, spin−orbit coupling, magnetic interactions, and crystal-field effects.5 Particularly, the ligand can directly influence the coordination environments around lanthanide ions and further affect the magnetic anisotropy of lanthanide ions. Among the numerous organic ligands employed for the construction of lanthanide molecular nanomagnets, pyridyl© XXXX American Chemical Society

hydrazide based Schiff base ligands have become synthetic chemists’ favorite because they can provide multichelating groups that can easily coordinate with lanthanide ions. After the first synthesis of a lanthanide compound containing an isonicotinohydrazide based Schiff base ligand with slow magnetic relaxation,6 a series of lanthanide molecular nanomagnets with dinuclear,7 tetranuclear,8 hexanuclear,9 and octanuclear9a,d structures have been constructed by employing these Schiff base derivatives with additional coordination sites or functional groups. In our previous work, we introduced the pyridine-N-oxide group to this type of ligands in an effort to enhance their coordination capabilities with lanthanide ions, and we successfully constructed a one-dimensional (1D) and a two-dimensional (2D) dysprosium system featuring slow magnetic relaxation.10 More recently, we obtained a superparamagnetic dinuclear dysprosium compound [DyLCl(CH3OH)]2 (1) by reacting the ligand N′-(2-hydroxybenzylidene)picolinohydrazide (H2L) with DyCl3·6H2O. It was interesting to us that one of the coordination sites of the Dy3+ ion in 1 was occupied by a chloride ion. We contemplated whether we could bridge this superparamagnetic dinuclear unit by replacing the chloride ion with carboxylate linkers. We anticipated that this would lead to lanthanide superparamangetic systems with 1D Received: September 24, 2016

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DOI: 10.1021/acs.inorgchem.6b02276 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Dinuclear structure (a) and coordination polyhedral around Dy3+ ions in complex 1 (b).

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RESULTS AND DISCUSSION Crystal Structure of 1. Single-crystal X-ray analysis reveals that complex [DyLCl(CH3OH)]2 (1) crystallizes in the monoclinic space group P21/c and exhibits a symmetric dinuclear structure (Figure 1a and Table 1). The asymmetric

or 2D structures not yet found in the known compounds derived from the H2L ligand. Herein, we report the synthesis of a 1D compound [DyL(HCOO)(CH 3 OH)] n (2) by a coordination-driven self-assembly method using compound 1 as a building unit and formate group as a bridge. In compound 2, two formate groups adopting a syn−anti bridging mode replace the coordinating chloride ion in 1 and link the dimer units to form a 1D chain structure. Magnetic studies indicate that there are ferromagnetic interactions between the Dy3+ ions in complexes 1 and 2. As unambiguously indicated by the welldefined temperature- and frequency-dependent ac signals under zero dc field, they both show typical slow magnetic relaxation behavior featuring a Ueff of 88.4 and 175.8 K, respectively. Theoretical calculation suggests that the larger effective energy barrier obtained for 2 than that observed for 1 is ascribed to its larger Dy3+−Dy3+ magnetic coupling, which can largely suppress the quantum tunneling magnetization at low temperatures.

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Table 1. Crystallographic Data and Structure Refinement for Complexes 1 and 2 formula Mr crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z μ (mm−1) F(000) GOF data collected unique Rint R1, wR2 [I > 2σ(I)] R1, wR2 [all data]

EXPERIMENTAL SECTION

The pyridine-carbohydrazine based Schiff base ligand N′-(2-hydroxybenzylidene)picolinohydrazide (H2L) was synthesized by condensation of picolinoylhydrazide and salicylaldehyde in methanol using the reported procedure.11 Other reagents and solvents employed were commercially available and used as received without further purification. [DyLCl(CH3OH)]2 (1). H2L (0.2 mmol, 48.2 mg) was dissolved in 8 mL of methanol, to which a methanolic solution of Et3N (2.0 mL, 0.2 mol·L−1) was added. The reaction mixture was then stirred for 2 min, after which solid DyCl3·6H2O (0.2 mmol, 75.4 mg) was added and the yellow solution was continually stirred for 20 min. The mixture was transferred to a 20 mL Teflon reactor and heated to 80 °C for 72 h under autogenous pressure, before cooling to room temperature at a rate of 5 °C·h−1. Yellowish block crystals of 1 were obtained. Yield: 57.8 mg (61.7% based on the metal salt). Elemental analysis (%) calcd for C14H13ClDyN3O3: C, 35.84, N, 8.96, H, 2.79; found C, 35.85, N, 8.86, H, 2.90.IR (KBr, cm−1): 3238(br), 1610(s), 1560(m), 1541(m), 1477(m), 1438(w), 1334(m), 1197(m), 1153(m), 1004(m), 894(m), 686(m), 543(m). [DyL(HCOO)(CH3OH)]n (2). H2L (0.02 mmol, 4.8 mg) was dissolved in 8 mL of methanol. A methanolic solution of Et3N (1.0 mL, 0.02 mol·L−1) was added to the mixture. After stirring for 2 min, solid DyCl3·6H2O (0.02 mmol, 7.5 mg) and HCOONa·2H2O (0.06 mmol, 6.2 mg) were added and the yellow solution was continually stirred for 20 min. The mixture was transferred to a 20 mL Teflon reactor and heated to 80 °C for 72 h under autogenous pressure, before cooling to room temperature at a rate of 5 °C·h−1. Yellowish block crystals of 2 were obtained. Yield: 2.6 mg (26.5% based on the metal salt). Elemental analysis (%) calcd for C15H14DyN3O5: C, 37.63, N, 8.78, H, 2.95; found C, 37.56, N, 8.73, H, 2.88. IR (KBr, cm−1): 3425(br), 1606(s), 1560(s), 1475(s), 1440(m), 1336(s), 1195(m), 1014(m), 896(s), 798(m), 758(m), 686(m), 534(m).

1

2

C14H13ClDyN3O3 469.22 monoclinic P21/c 9.644(3) 7.117(2) 22.423(6) 90 97.503(4) 90 1525.9(7) 4 5.086 900 1.106 8694 2988 0.0259 0.0226, 0.0488 0.0257, 0.0499

C15H14DyN3O5 478.79 monoclinic P21/n 9.321(2) 8.249(3) 20.125(4) 90 100.547(1) 90 1521.2(7) 4 4.944 924 1.065 7884 2888 0.0573 0.0415, 0.1210 0.0598, 0.1634

unit of 1 is composed of one crystallographically independent Dy3+ ion, one L2− ligand, one chloride ion, and one methanol molecule. As shown in Figure 1, each Dy3+ ion is heptacoordinated with an approximately pentagonal-bipyramidal coordination geometry, in which the equatorial site is defined by one pyridyl nitrogen atom (N1), two alkoxido oxygen atoms (O1 and O1A), one hydrazide nitrogen atom (N3A), and one phenolate oxygen atom (O2A). Additionally, one chloride ion (Cl1) and one methanol molecule (O3) are situated at the axial sites. The Dy−O distances are in the range of 2.177−2.366 Å (Table S1), resembling those found in documented dysprosium complexes constructed by related Schiff base ligands.9−12 It is noteworthy that the Dy1−O2A bond length (2.177 Å) is substantially shorter than those between Dy1 and the other coordination atoms (2.312−2.627 Å), which is likely due to the stronger coordination affinity of the phenolate oxygen atom which has a relatively large negative charge. The shortest Dy1− O2A bond distance may play an important role in mediating the magnetic behavior of complex 1. The dysprosium centers in B

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Figure 2. Dinuclear unit (a), coordination polyhedral around Dy3+ ion (b), and 1D chain in complex 2 (c).

(O1 and O1A) with a Dy···Dy distance of 3.938 Å and a Dy− O−Dy angle of 113.4°. The formate groups adopt the syn−anti bridging mode and connect the adjacent dinuclear subunits to produce a 1D chain structure along the b direction with the shortest interdimer Dy···Dy distance at 5.752 Å (Figure 2c). Adjacent chains along the a-axis are further linked to construct a 2D layer structure through π−π interactions between interdigitating pyridyl (phenyl) and phenyl groups with the shortest interchain Dy···Dy distance of 8.794 Å (Figure S2). To the best of our knowledge, complex 2 is a rare example of a 1D chain derived from the assembly of neighboring dinuclear subunits based on a pyridyl-carbohydrazine Schiff base ligand. In order to confirm the phase purity, compounds 1 and 2 were analyzed by powder X-ray diffraction (XRD) at room temperature using the same microcrystalline samples from the magnetic measurements (Figure S3). The diffraction peaks of the as-prepared samples are in good agreement with the corresponding simulated patterns calculated from the singlecrystal X-ray diffraction data, demonstrating a high phase purity of the experimental samples. Magnetic Behavior of 1. Temperature-dependent magnetic susceptibility measurement was performed on a microcrystalline sample of 1 in the temperature range of 2−300 K at 1 kOe. As shown in Figure 3, at room temperature, the observed χMT value of 13.96 cm3 mol−1 K is slightly smaller than the expected value (14.17 cm3 mol−1 K) for one Dy3+ ion with a ground state 6H15/2 and g = 4/3.12 With decreasing temperatures, the χMT value gradually decreases, reaching a minimum value of 12.46 cm3 mol−1 K at 32 K, which is mainly ascribed to the progressive depopulation of the excited Stark components of Dy3+ ions. Upon further cooling, the χMT value abruptly increases to a maximum of 14.33 cm3 mol−1 K at 3 K. The increase of χMT in the low temperature range clearly implies intramolecular ferromagnetic interactions between Dy3+ ions. The field dependence of the magnetization was also investigated in the range of 0−50 kOe at 2, 3, 5, 8, and 10 K, respectively (Figure S4a). A rapid increase of the magnetization is observed at low magnetic fields, indicating well-separated excited Kramer’s doublets. The nonsaturation and nonsuperposition of the M vs H/T plots at higher fields suggest

the dinuclear core are bridged by two alkoxido groups (O1 and O1A) from two antiparallel ligands adopting the conjugate deprotonated enol form with a Dy···Dy distance of 3.900 Å and a Dy−O−Dy angle of 113.7°. Two ligands bind to two Dy3+ ions with the tridentate (O1, N3, O2) and bidentate (N1, O1) parts, as observed in most reported complexes containing a pyridyl-carbohydrazide moiety.6−9 Adjacent dimer units are connected via Cl1···H3A−O3 hydrogen bonds between coordinated methanol molecules (O3) and neighboring chloride ions (Cl1···O3 = 3.117 Å, ∠O3−H3A···Cl1 = 136.82°), forming a 1D supramolecular chain with the shortest interdimer Dy···Dy distance at 7.117 Å (Figure S1a). Neighboring chains along the a-axis are further linked to produce a 2D layer through π−π interactions between interdigitating pyridyl (phenyl) and phenyl with the shortest interchain Dy···Dy distance of 8.272 Å (Figure S1b). Crystal Structure of 2. It is interesting to note that, when sodium formate was added to the reaction system of 1, the formate group replaced the coordinated chloride ion and connected the dinuclear units of 1, forming a distinct 1D chain complex of [DyL(HCOO)(CH3OH)]n (2) (Figure 2). Compound 2 crystallizes in the monoclinic space group P21/ n, the asymmetric unit of which consists of one Dy3+ ion, one L2− ligand, one formate group, and one coordinated methanol molecule. As shown in Figure 2, each Dy3+ ion in 2 is octacoordinated with an approximately hula-hoop-like coordination geometry. The circle of the hula-hoop resembles that found in 1, which is defined by one pyridyl nitrogen atom (N1), two alkoxido oxygen atoms (O1 and O1A), one hydrazide nitrogen atom (N3A), and one phenolate oxygen atom (O2A); however, the axial coordination groups are quite distinct, occupied by two formate groups (O3 and O4) and one methanol molecule (O5). The Dy−O distances are between 2.181 and 2.419 Å (Table S1), slightly longer than that of 1 (2.179−2.366 Å). Nevertheless, the bond length of Dy−OHCOO− (2.373 and 2.404 Å) in 2 is obviously shorter than that of Dy−Cl (2.627 Å) in 1, indicating a stronger coordination strength in the axial direction, which might have a great impact on the magnetic behavior of 2. Similar to 1, two neighboring Dy3+ ions in the dinuclear subunits of 2 are also bridged by two alkoxido groups C

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Figure 5. Magnetic relaxation dynamics for 1 under zero dc field.

Figure 3. Temperature dependence of χMT products for 1 and 2. The black and red lines are the simulation from ab initio calculation.

coefficient of the Raman process) and the quantum tunnelling process (τQTM−1). The fitting of the ln τ vs 1/T plots with the consideration of all relaxation processes gives a good agreement with the data over the whole temperature range, generating the following parameters: Ueff = 88.4 K, τ0 = 2.65 × 10−7 s, C = 0.35 s−1 K−3.18, n = 3.18, and τQTM = 0.028 s. The values of τ0 are of the correct order of magnitude expected for an Orbach relaxation process, and the values of C and n are as expected for Raman process. Furthermore, the Cole−Cole diagram is created based on the frequency dependencies of the ac susceptibility data, which is fitted to a generalized Debye model, giving rise to a narrow distribution of coefficient α parameters in the range of 0.015−0.26 (Tables S3 and S4). This is consistent with the presence of a unique coordination sphere of Dy3+ ion in 1. Since the external field can suppress the QTM process, we applied an optimized dc field of 2 kOe. As a consequence, the out-of-phase signal is decreased below 5 K compared to the zero field case (Figure S6). In the high temperature region, the magnetic relaxation is similar to that of the zero field case, since both of them originate from the single lanthanide center. There is no observable evidence of the existence of the direct process. Magnetic Behavior of 2. The variable-temperature magnetic susceptibility measurement for polycrystals 2 was also investigated in the temperature range of 2−300 K under 1 kOe dc field (Figure 3). The experimental χMT value of 13.79 cm3 mol−1 K at room temperature is slightly smaller than the expected value of 14.17 cm3 mol−1 K for one uncoupled dysprosium ion with a ground state 6H15/2 and g = 4/3. Similar to 1, the χMT value of 2 gradually decreases in a wide temperature range from 300 to 24 K, reaching a minimum value of 12.64 cm3 mol−1 K at 24 K, which is mainly ascribed to the depopulation of the excited Stark sublevels. Below 24 K, the χMT value goes up to a maximum value of 15.37 cm3 mol−1 K at 2 K, similarly denoting the presence of dominant ferromagnetic interactions between the dysprosium ions within and/or between the dinuclear subunits. The field dependence of the magnetization of 2 was investigated in the range of 0−50 kOe at 2, 3, 5, 8, and 10 K, respectively (Figure S4b). A rapid increase of the magnetization is observed at low magnetic fields, followed by a linear increase up to 5.5 Nβ at 2 K. The lack of saturation of magnetization at 50 kOe or superposition of the M vs H/T plots suggests the presence of a significant magnetic anisotropy and/or low-lying excited states in 2. Ac susceptibility measurements were also conducted for 2 under zero dc field to further explore the dynamics of magnetization. Similar to 1, both the real (χM′) and imaginary

the presence of significant magnetic anisotropy caused by the crystal-field effect in 1. To further probe the dynamics of magnetization, alternating current (ac) susceptibility measurements were performed for 1 in the frequency range of 100−10 000 Hz under zero dc field. As shown in Figure 4, both the in-phase (χM′) and out-of-phase

Figure 4. Temperature dependence of the in-phase χM′ (top) and outof-phase χM″ (bottom) ac signals under zero dc field for 1.

(χM″) signals of ac susceptibilities are strongly temperaturedependent below 28 K, which is a positive proof for the slow magnetic relaxation and superparamagnetic behavior in 1. In the χac−T plots, good peak shapes are clearly observed, which shift to high temperatures with increasing frequencies. Upon decreasing the temperature, the upturns of χM′ and χM″ are observed below 6 K, implying that the quantum tunneling mechanism, which occurs between the degenerated ground states, may play a dominant role at low temperatures.13 The relaxation time can be deduced from the frequency-dependent ac data at different temperatures using the generalized Debye model and is plotted as ln τ vs 1/T as shown in Figure 5 (also see Figure S5; Tables S3 and S4). Generally speaking, the relaxation process of a lanthanide molecular nanomagnet is a combination of the Orbach process, Raman process, and quantum tunneling process.14 However, different relaxation processes dominate in different temperature ranges. For example, at high temperatures, the relaxation is dominated by the Orbach process (τOrbach−1 ∼ τ0−1 exp(−Ueff/kBT)), whereas, at low temperatures, gradual transitions are observed as a result of the Raman process (τRaman−1 ∼ CTn, where C is the D

DOI: 10.1021/acs.inorgchem.6b02276 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (χM″) part of the ac susceptibility for 2 show strong temperature dependence, with a clear evidence of the peaks covering the range of 100−10 000 Hz (Figure 6), suggesting

atures. As shown in Figures S9 and S10, magnetic memory effects are clearly visible below 5 K at a field sweep rate of 500 Oe/s for both complexes, which is characteristic of lanthanide molecular nanomagnets. When the temperature drops down to 2 K, the butterfly-shaped hysteresis loops for both complexes become much larger and wider. As expected for a molecular nanomagnet, upon decreasing the sweeping rate to 300 or 100 Oe/s, the loops become narrower, also indicating the dynamic behavior of 1 and 2. The strong dependence of hysteresis loops on the temperature and field sweep rates is clearly in agreement with the slow magnetic relaxation suggested by the ac susceptibility data. Theoretical Calculations. Ab initio calculation has proved to be an efficient method for probing the insight of magnetic anisotropy in lanthanide-containing compounds. In particular, MOLCAS 7.815 and SINGLE_ANISO16 programs with the complete-active-space self-consistent field (CASSCF) method have been used to probe individual Dy3+ fragments of complexes 1 and 2 on the basis of geometries determined by X-ray diffraction analysis (see the Supporting Information for details). The results provide the lowest spin−orbit energies and the corresponding g tensors of one dysprosium fragment extracted from 1 and 2. Table S7 shows that the energy separations between the two lowest KDs for the Dy3+ fragments of 1 and 2 are 184.6 and 152.0 cm−1, respectively, and the gz values of both fragments are 19.487 and 19.474, close to 20. Thus, the Dy3+−Dy3+ magnetic interactions in both complexes can be approximately regarded as the Ising type. The main magnetic axes on dysprosium ions of two complexes are indicated in Figure 8, where the magnetic axes on magnetic centers for each complex are parallel to each other and pointing to the direction of Dy−Ophenoxide.

Figure 6. Temperature dependence of the in-phase χM′ (top) and outof-phase χM″ (bottom) ac signals under zero dc field for 2.

the existence of slow magnetic relaxation expected for molecular nanomagnets.1−10 The imaginary frequency peaks gradually shift to the high temperature region with increasing ac frequency, and the peak maxima fall in the range of 8.99−19.02 K, more concentrated compared with that of 1 (8.49−20.02 K). The relaxation time can be extracted from the frequencydependent ac data at different temperatures (Figure S7; Tables S5 and S6). The plot of ln τ vs T−1 was also fitted with the consideration of all relaxation processes, yielding Ueff = 175.8 K, τ0 = 2.1 × 10−9 s, C = 0.0153 s−1 K−4.62, n = 4.62, τQTM = 0.079 s. The obtained τQTM value for 2 at zero dc field is notably larger than that found for 1, indicating the slowing down of the zero-field quantum tunneling relaxation rate in 2, which is also supported by the smaller tail value in the out-of-phase signal of the ac magnetic data (Figure 7). Similar to the case of 1, when

Figure 8. Orientations of the local main magnetic axes of the ground doublets on magnetic centers of 1 (a) and 2 (b).

The magnetic susceptibilities of complexes 1 and 2 were simulated with the program POLY_ANISO16 (see Figure 3) using the exchange parameters from Table S8. We note that all of the fitted Dy3+−Dy3+ coupling constants in Table S8 were calculated with respect to the pseudospin S̃ = 1/2 of the Dy3+ ions. For complex 1, there is only one type of J constant, which is mediated by two alkoxido oxygen atoms within the dinuclear core. However, for complex 2, two possible exchange pathways could be considered: one pathway involves the double alkoxido bridges within the dinuclear core that connect the Dy3+ ions at a distance of 3.938 Å, while the other, a much longer pathway, concerns with the formate groups in a syn−anti bridging mode that links the Dy3+ ions at a distance of 5.752 Å. Therefore, for complex 2, we consider two types of J values (Figure S11). The

Figure 7. Magnetic relaxation dynamics for 2 under zero dc field.

an optimized dc field of 1.5 kOe was applied, the QTM process was suppressed (Figure S8). Compared with compound 1, which has a dimer structure, the 1D complex 2 shows higher Uef f and τQTM values at zero dc field, probably due to the distinct coordination environment around Dy3+ ions and/or the different Dy3+−Dy3+ magnetic coupling existing in 2. Hysteresis Behaviors of Complexes 1 and 2. To further study the dynamic magnetic behavior of complexes 1 and 2, the hysteresis loops measurement was done on microcrystalline samples using a SQUID-VSM magnetometer at low temperE

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Our finding thus provides a promising strategy for the rational design of novel lanthanide-based magnetic materials.

total coupling parameters J (dipolar and exchange) were included in the fitting of the magnetic susceptibilities for both compounds. Plots of calculated and experimental χMT versus T for both complexes are shown in Figure 3, where the calculated data are reasonably consistent with the corresponding experimental data in the entire temperature range. Details of the fitting results are shown in Table S8, which reveals that the Dy3+ ions in the dinuclear core of both complexes are all ferromagnetically coupled but the overall interdimer magnetic interactions in 2 are weakly antiferromagnetic. The exchange states splitting for the lowest KD of both complexes are shown in Table S9, where the high excited states are much smaller than their energy barriers. Thus, the relaxations of two complexes are all through excited KDs of the magnetic centers. The theoretical calculation of the anisotropy of individual Dy3+ ions and of the magnetic coupling between them provide us with a good platform to understand the different magnetic behaviors of the two complexes. It is interesting to note, as revealed by Table S7, that the energy difference between the two lowest KDs of individual Dy3+ fragments in 2 is smaller than that in 1 (Table S7), but the corresponding experimental energy barrier associated with the slow relaxation is larger than that of 1. The larger separation between the two lowest KDs in 1 is due to the stronger interaction between the Dy3+ ion and phenolate and alkoxido oxygen atoms along the magnetic axis (as evidenced by the shorter Dy−O bond lengths) and weaker interaction perpendicular to the magnetic axis. The higher energy barrier found in 2 might arise from the difference within the core dimer structure and the connection between them, since these structural factors can affect the relaxation process of individual Dy3+ ions. Within the dimer core, a smaller Dy−O− Dy angle and a longer Dy···Dy distance result in stronger intradimer Dy3+−Dy3+ magnetic interaction, as indicated by the significantly larger coupling constant (6.31 cm−1) than that found for 1 (3.82 cm−1). Additionally, while the overall interdimer magnetic coupling appears small (J = −0.04 cm−1), its exchange component J2 (0.75 cm−1) is considerably larger, indicating the existence of substantial magnetic interaction between the dimer cores through syn−anti formate bridges, which likely imposes an effect on the slow magnetic relaxation. The larger intra- and interdimer magnetic interactions can effectively quench the quantum tunneling at low temperatures, thus affording a larger energy barrier in 2. Another reason for the energy barriers differences may be attributed to the different molecular geometries and molecular rigidity. The latter can affect the spin−phonon coupling, which has been recently evidenced in the spin−phonon bottleneck mechanism study.17

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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.6b02276. Some experimental and computational details; selected bond lengths and angles for 1 and 2; relaxation fitting parameters from least-squares fitting of χ( f) data; crystal structure of 1 and 2; XRD spectra of 1 and 2; and magnetic data for 1 and 2 (PDF) X-ray crystallographic data of compound 1 (TXT) X-ray crystallographic data of compound 2 (TXT)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-L.S.). *E-mail: [email protected] (Y.-Q.Z.). *E-mail: [email protected] (S.G.). 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 (No. 21171023), the National Key Basic Research Program of China (2013CB933402), the Natural Science Foundation of Jiangsu Province of China (BK20151542), the Beijing Higher Education Young Elite Teacher Project, and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We thank Prof. Zhenqiang Wang of the University of South Dakota for helpful comments.

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REFERENCES

(1) (a) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, U.K., 2006. (b) Clérac, R.; Winpenny, R. E. P. Single-Molecule Magnets and Related Phenomena. Struct. Bonding (Berlin, Ger.) 2006, 172, 35. (c) Mannini, M.; Pineider, F.; Danieli, C.; Totti, F.; Sorace, L.; Sainctavit, P.; Arrio, M. A.; Otero, E.; Joly, L.; Cezar, J. C.; Cornia, A.; Sessoli, R. Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets. Nature 2010, 468, 417. (d) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Magnetic memory of a singlemolecule quantum magnet wired to a gold surface. Nat. Mater. 2009, 8, 194. (2) (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Magnetic bistability in a metal-ion cluster. Nature 1993, 365, 141. (b) Fittipaldi, M.; Sorace, L.; Barra, A. L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Molecular nanomagnets and magnetic nanoparticles: the EMR contribution to a common approach. Phys. Chem. Chem. Phys. 2009, 11, 6555. (c) Waldmann, O. A Criterion for the Anisotropy Barrier in Single-Molecule Magnets. Inorg. Chem. 2007, 46, 10035. (3) (a) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694. (b) Jiang, S. D.; Wang, B. W.; Su, G.; Wang, Z. M.; Gao, S. A Mononuclear Dysprosium Complex Featuring Single-Molecule-Magnet Behavior. Angew. Chem., Int. Ed. 2010, 49, 7448. (c) Jiang, S. D.; Wang, B. W.; Sun, H. L.; Wang, Z. M.; Gao, S. An Organometallic Single-Ion Magnet. J. Am. Chem. Soc. 2011, 133, 4730. (d) AlDamen, M. A.; Clemente-Juan, J. M.; Coronado, E.; Marti-Gastaldo, C.; Gaita-Arino,

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CONCLUSION In summary, using a coordination-driven self-assembly method, we have synthesized a novel 1D dysprosium chain compound, 2, which can be regarded as a supramolecular assembly resulting from the linking of adjacent dinuclear subunits 1 by a formate group. Both complexes exhibit strong temperatureand frequency-dependent ac signals under zero dc field, showing prominent slow magnetic relaxation properties expected for molecular nanomagnets and featuring effective energy barriers of 88.4 and 175.8 K, respectively. Ab initio calculation demonstrates that the larger energy barrier obtained for 2 is related to its stronger Dy3+−Dy3+ coupling. The different behaviors of 1 and 2 are both affected by the geometry of single lanthanide centers and linkers, while the latter can provide an alternative to manipulate the exchange interactions. F

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b02276 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry dynamics in a series of two-coordinate iron(II) complexes. Chem. Sci. 2013, 4, 125. (b) Liu, J. L.; Yuan, K.; Leng, J. D.; Ungur, L.; Wernsdorfer, W.; Guo, F. S.; Chibotaru, L. F.; Tong, M. L. A SixCoordinate Ytterbium Complex Exhibiting Easy-Plane Anisotropy and Field-Induced Single-Ion Magnet Behavior. Inorg. Chem. 2012, 51, 8538. (15) Karlström, G.; Lindh, R.; Malmqvist, P. Å.; Roos, B. O.; Ryde, U.; Veryazov, V.; Widmark, P. O.; Cossi, M.; Schimmelpfennig, B.; Neogrady, P.; Seijo, L. MOLCAS: a program package for computational chemistry. Comput. Mater. Sci. 2003, 28, 222. (16) (a) Chibotaru, L. F.; Ungur, L.; Soncini, A. The Origin of Nonmagnetic Kramers Doublets in the Ground State of Dysprosium Triangles: Evidence for a Toroidal Magnetic Moment. Angew. Chem., Int. Ed. 2008, 47, 4126. (b) Zhu, Y.; Sun, Z.; Zhao, Y.; Zhang, J.; Lu, X.; Zhang, N.; Liu, L.; Tong, F. Synthesis, crystal structures and luminescence properties of lanthanide oxalatophosphonates with a three-dimensional framework structure. New. J. Chem. 2009, 33, 119. (c) Chibotaru, L. F.; Ungur, L.; Aronica, C.; Elmoll, H.; Pilet, G.; Luneau, D. Structure, Magnetism, and Theoretical Study of a MixedValence CoII3CoIII4 Heptanuclear Wheel: Lack of SMM Behavior despite Negative Magnetic Anisotropy. J. Am. Chem. Soc. 2008, 130, 12445. (17) (a) Lunghi, A.; Totti, F. Inorganics 2016, 4, 28. (b) Tesi, L.; Lunghi, A.; Atzori, M.; Lucaccini, E.; Sorace, L.; Totti, F.; Sessoli, R. Giant spin−phonon bottleneck effects in evaporable vanadyl-based molecules with long spin coherence. Dalton Trans. 2016, 45, 16635.

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DOI: 10.1021/acs.inorgchem.6b02276 Inorg. Chem. XXXX, XXX, XXX−XXX