Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Arraying Octahedral {Cr2Dy4} Units into 3D Single-Molecule-MagnetLike Inorganic Compounds with Sulfate Bridges Cai-Ming Liu,* De-Qing Zhang, Xiang Hao, and Dao-Ben Zhu Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry, CAS Research/Education Center for Excellence in Molecular Science, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
divergence. Remarkably, their SMM properties are effectively regulated by the arrangement direction of the [Cr2Dy4(μ4O)2(μ3-OH)4]10+ clusters. Both pure inorganic coordination polymers were obtained by hydrothermal reactions at 170 °C, and the starting materials have a decisive influence on the composition of the two phases. The purity of the two compounds was checked by powder X-ray diffraction patterns (Figures S1 and S2). Thermogravimeric analysis revealed that a weight loss of 15.5% at 35−350 °C for 1 and a weight loss of 11.9% at 50−350 °C for 2 (Figure S3) correspond to the loss of all water molecules (calcd 15.9% and 11.9% for 1 and 2, respectively). As shown in Figure 1, both complexes contain a [Cr2Dy4(μ4O)2(μ3-OH)4]10+ core unit, in which two Cr3+ ions at axial positions and four Dy3+ ions on the equatorial plane are bridged by two μ4-O atoms and four μ3-OH− anions, forming an interesting {Cr2Dy4} compressed metallic octahedron, similar to that observed in an organic−inorganic hybrid cluster complex, (pipzH2)[Cr2Dy4(μ4-O)2(μ3-OH)4(H2O)10(μ3-SO4)4(SO4)2]· 2H2O (pipz = piperazine).8 The sulfate anions bridge the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motifs in several modes. For complex 1, there are two types of sulfate bridging ligands (Figure 1a,b): the μ4-SO42− anion (containing a S2, S3, S4, or S5 atom) not only bridges two Dy atoms and one Cr atom from the same [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif through three O atoms but also links to one Dy atom from the adjacent [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif through the fourth O atom; the μ3-SO42− anion (containing a S1 atom) bridges three Dy atoms from two neighboring [Cr2Dy4(μ4O)2(μ3-OH)4]10+ structural motifs through three O atoms, leaving the fourth O atom free. All [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motifs in 1 are parallel to each other (Figure 1c). In other words, they have the same orientation (assumed to be determined by the line connecting two Cr atoms for convenience): first, the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motifs are connected with each other through paired μ4-SO42− bridges along the a-axis and b-axis directions, forming a layer parallel to the ab plane; then, the adjacent [Cr2Dy4(μ4-O)2(μ3OH)4]10+ structural motifs along the c axis are linked to each other through a single μ3-SO42− bridge. Therefore, each [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif in 1 is connected to six neighbors (Figure 1b). Consequently, the layers parallel to the ab plane are connected to each other to generate a 3D
ABSTRACT: Two novel 3D pure inorganic compounds based on [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ cluster units and sulfate anions are presented. Both complexes exhibit single-molecule-magnet (SMM)-like behavior. Permutation of the magnetic moment direction among SMM-like cluster units has a significant effect on the performance of molecular nanomagnets, and directional consistency shows obvious advantages.
S
ingle-molecule magnets (SMMs) are a special class of molecular magnets that exhibit magnetic relaxation and hysteresis loops in low-temperature regions.1 The superparamagnet nature makes them potentially useful in high-density information storage, molecular electronics, and other high-tech fields.1,2 One major difficulty in this field is to keep the magnetic moment direction consistent, which is one of the reasons why it is so hard to obtain a large relaxation energy barrier (Ueff) and high TB in multinuclear cluster complexes.3 To overcome this obstacle, mononuclear complexes are often used to design good-performance SMMs, and significant progress has been made in the single-ion-magnet field recently.4 However, permutation of the magnetic moment direction among SMMs is rarely noticed. We think that coordination polymers behaving as SMMs provide a good platform for studying this issue because the knots in coordination polymers show SMM properties themselves, which are arranged highly ordered in space. Notably, most reported SMMs possess organic ligands or ingredients, pure inorganic SMMs are very rare,5 and highdimensional SMM-like pure inorganic compounds based on 3d− 4f polynuclear clusters have never been documented. We have recently devoted ourselves to the development of highdimensional SMMs6 and hope to extend SMM research to pure inorganic coordination polymer systems. We selected Cr− Dy heterometallic clusters, which often exhibit good SMM behavior,7 as the structural units for building pure inorganic coordination polymers. By changes in the reaction conditions, two 3D pure inorganic coordination polymers based on Cr2Dy4 aggregates and sulfate bridging ligands were successfully obtained, which were formulated as [Cr2Dy4(μ4-O)2(μ3OH)4(H2O)5(μ4-SO4)4(μ3-SO4)]·9H2O (1) and [Cr2Dy4(μ4O)2(μ3-OH)4(H2O)8(μ4-SO4)2(μ3-SO4)(ter-η3-SO4)2]·2H2O (2). The two 3D pure inorganic compounds have similar chemical components; however, the difference in the coordinating and bridging modes of sulfate anions leads to structural © XXXX American Chemical Society
Received: May 3, 2018
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DOI: 10.1021/acs.inorgchem.8b01210 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
S4). For complex 2, the Dy1, Dy2, and Dy3 atoms are eightcoordinated, completed by one μ4-O atom, two hydroxide O atoms, three O atoms from three sulfate anions, and two terminal hydrate O atoms: the Dy1 and Dy2 atoms have a triangular dodecahedron geometry (Tables S5 and S6), the Dy3 atom displays a biaugmented trigonal-prism configuration (Table S7), and the Dy4 atom is nine-coordinated by one μ4-O atom, two hydroxide O atoms, four O atoms from four sulfate anions, and two terminal hydrate O atoms, exhibiting a spherical capped square-antiprism geometry (Table S8). The direct-current (dc) magnetic susceptibilities of the two compounds are somewhat different. The χT values of 1 (60.81 cm3 K mol−1) and 2 (60.67 cm3 K mol−1) at room temperature are close to the expected value of 60.43 cm3 K mol−1 for two noninteracting Cr3+ ions (S = 3/2 and g = 2) and four uncoupled Dy3+ ions (S = 5/2, L = 5, and g = 4/3) (Figure S4). As the temperature decreases, the χT product of 1 only slightly increases; below about 25 K, it rapidly increases until it reaches a maximum value of 173.86 cm3 K mol−1 at 6.0 K and then decreases to 110.87 cm3 K mol−1 at 2 K, suggesting the existence of an obvious ferromagnetic interaction in 1. The χT product of 2 slowly increases when the temperature decreases from 300 K and reaches the maximum value of 78.51 cm3 K mol−1 at 36 K; it then slightly reduces, declining rapidly after 10 K to reach 33.02 cm3 K mol−1 at 2 K. This behavior suggests an overall weak ferromagnetic exchange in 2. The study of field-dependent magnetization reveals that the M−H/T curves measured at 2−6 K are not coincident for both 1 and 2 (Figures S5 and S6), suggesting that magnetic anisotropy exists in the two complexes. The alternating-current (ac) magnetic susceptibility has also been determined to investigate the nature and dynamics of the magnetic properties. The imaginary part (χ′′) of the ac magnetic susceptibility for both compounds shows a strong frequency dependence in zero dc field (Figure 2a,d), which clearly indicates the slow relaxation of magnetization and precludes 3D long-range ordering. Taking into account the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ polynuclear cluster character in both compounds, the observed magnetization dynamics should be due to SMM behavior. Furthermore, the SMM nature of 1 and 2 is also confirmed by analysis of Φ = (ΔTP/T)/Δ(log f),10 both complexes have the same Φ value of 0.19, which is in good agreement with that for an SMM (Φ > 0.1) but obviously larger than that for a typical spin glass (Φ ≈ 0.01).10 The Arrhenius law, τ = τ0 exp(Ueff/kT), was used to fit ac magnetic susceptibility data in the form of ln(τ) versus 1/T plots, giving Ueff/k values of 37.5(0.9) K for 1 and 25.9(0.7) K for 2 with τ0 values of 6.7(0.3) × 10−8 s for 1 and 7.3(0.3) × 10−9 s for 2 (Figures S7 and S8). The plots of ln(τ) versus 1/T for both complexes remain linearly at all temperatures, suggesting that an Orbach process is dominant for the slow magnet relaxation. The two τ0 values are physically reasonable for an SMM (10−5−10−11 s), and the energy barrier value of 1 is obviously larger than that of 2. Moreover, the frequency-dependent ac susceptibility was also measured to check the SMM behavior. As shown in Figure 2b,e, the maximum values in the χ′′ versus ν curves are temperaturedependent in zero dc field, verifying the slow magnet relaxation of both complexes. Cole−Cole plots of χ′′ versus χ′ for complex 1 show somewhat deformed semicirclar profiles at 2.0−4.5 K, which seem to have some characteristics of the two-step relaxation process (Figure S9); this phenomenon may be caused by the sample taking off a small amount of the solvent water molecule in an extremely dry environment during magnetic measurement because desolvation in 3D SMMs may induce the
Figure 1. Asymmetric structural units of 1 (a) and 2 (d), the connection among structural units of 1 (b) and 2 (e), and views of the 3D network structures of 1 (c) and 2 (f) along the a axis. All lattice water molecules are omitted for clarity.
extended network structure with a 6-connected pcu topology (Figure 1c). For complex 2, there also exist the μ4-SO42− anion (containing a S2 or S4 atom) and the μ3-SO42− anion (containing a S5 atom), which have bridging modes similar to those of 1 (Figure 1d,e). Interestingly, a new type of sulfate bridging ligand, namely, ter-η3SO42−(containing a S1 or S3 atom), is observed in 2, which bridges two Dy atoms and one Cr atom from the same [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif through three O atoms, but the fourth O atom is free too. Unlike 1, the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motifs in 2 have two orientations (Figure 1e,f); they are connected with each other through the μ4-SO42− anions and the μ3-SO42− anions, whereas the ter-η3-SO42− anions just work as terminal ligands to coordinate the same [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif. Each [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif in 2 is also connected to six neighbors, two of which have the same orientation, while the other four have the other orientation (Figure 1e). Hence, a 3D 6-connected pcu topology network structure is produced (Figure 1f), in which the μ4-SO42− bridge always appears as a single rather than as a pair, similar to the μ3SO42− bridge. The Cr atoms in both complexes have a slightly distorted octahedral geometry, coordinated by two μ4-O atoms, two hydroxide O atoms, and two O atoms from two sulfate anions. For complex 1, Dy1, Dy2, and Dy3 atoms have a similar triangular dodecahedron coordination sphere (analyzed by the Shape software;9 Tables S1−S3), which is coordinated by one μ4-O atom, two hydroxide O atoms, four O atoms from four sulfate anions, and one terminal hydrate O atom, while the Dy4 atom is coordinated by one μ4-O atom, two hydroxide O atoms, three O atoms from three sulfate anions, and two terminal hydrate O atoms, also showing a triangular dodecahedron geometry (Table B
DOI: 10.1021/acs.inorgchem.8b01210 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Two orientations of the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ clusters exist in complex 2, similar to (pipzH2)[Cr2Dy4(μ4-O)2(μ3OH)4(H2O)10(μ3-SO4)4(SO4)2]·2H2O (Figure S14),8 while only one cluster orientation is observed in 1, clearly suggesting that the same orientation of the SMM-like cluster units can promote a good SMM performance. This should be the main reason why complex 1 can but complex 2 and (pipzH2)[Cr2 Dy4 (μ4-O)2(μ3-OH)4(H2O)10(μ3-SO4) 4(SO4) 2]·2H2O 8 cannot exhibit a magnetic hysteresis loop at 1.9 K. Interestingly, a quite recent research about manipulating the permutation of 0D SMMs indicated that the quasi-1D arrangement of single-ion magnets may enhance the magnetic-dipole interactions between SMMs and can also significantly improve the energy barrier value and coercivity.15 In summary, two 3D pure inorganic coordination polymers composed of the [Cr2Dy4(μ4-O)2(μ3-OH)4]10+ structural motif and sulfate anions have been successfully synthesized. They are the first examples of 3D SMM-like pure inorganic complexes based on a 3d−4f polynuclear cluster unit. Such 3D pure inorganic SMMs are promising because of their stronger chemical and mechanical durability.5a Our research demonstrates that a consistent orientation of the SMM-like cluster units in coordination polymers can significantly improve the SMM performance, which opens up a new approach to highperformance molecular nanomagnets.
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Figure 2. Temperature dependence of χ′′ for 1 (a) and 2 (d) in zero dc field, frequency dependence of χ′′ for 1 (b) and 2 (e) in zero dc field, and magnetic hysteresis at 1.9 K for 1 (c) and 2 (f).
ASSOCIATED CONTENT
S Supporting Information *
two-step relaxation process. The plots could be fitted to the sum of two modified Debye functions,12 giving the α1 value of 0.11− 0.25 and the α2 value of 0.08−0.32, suggesting a narrow distribution of relaxation times for the two-step relaxation processes. For complex 2, classical semicirclar profiles appear in Cole−Cole diagrams at 2.0−2.6 K (Figure S10), and the fitting to a generalized Debye model13 leads to the α value of 0.18−0.22, indicating the narrow distribution of relaxation times for the single relaxation process. Another important magnetic bistability feature for an SMM, the magnetic hysteresis loop, was also investigated. An obvious hysteresis loop is observed for 1 at 1.9 K with a coercive field of 370 Oe and a remanence of 3.8 Nβ (Figure 2c), and this loop is compressed dramatically with increasing temperature but still can be detected at 2.2 K with a coercive field of 40 Oe and a remanence of 0.4 N β (Figure S11). Notably, there is no butterfly or step feature in these loops, suggesting that the quantumtunneling effect is very weak because of the ferromagnetic exchange of 3d−4f metal ions, which may quench the tunnel splitting effectively.14 Comparably, no hysteresis loop can be discovered at 1.9 K for 2 (Figure 2f). These illustrate that the SMM performance of 1 is better than that of 2. Additionally, the curves of field-cooled (FC) and zero-field-cooled (ZFC) susceptibility split at 2.20 K for 1 (Figure S12), suggesting that magnetization blocking commences at this temperature, while the curves of FC and ZFC susceptibility are overlapped above 2 K for 2 (Figure S13), indicating that the blocking temperature is below 2 K. As mentioned before, the energy barrier value of 1 is higher than that of 2, also indicating that 1 has a better SMM performance than 2. The orientation of the [Cr2Dy4(μ4-O)2(μ3OH)4]10+ clusters, which is closely related to the permutation of the magnetic moment direction among SMM-like cluster units, has an important influence on the spin dynamics and coercivity.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01210.
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Experimental procedures, crystal structure determination details, and more structural and magnetic pictures and tables (Figures S1−S14 and Tables S1−S8) (PDF) Accession Codes
CCDC 1841968−1841969 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 Author
*E-mail:
[email protected]. ORCID
Cai-Ming Liu: 0000-0001-7184-6693 De-Qing Zhang: 0000-0002-5709-6088 Dao-Ben Zhu: 0000-0002-6354-940X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research Program of China (Grant 2013CB933403), the National Natural Science Foundation of China (Grants 21471154 and 91022014), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB12010103). C
DOI: 10.1021/acs.inorgchem.8b01210 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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