Communication pubs.acs.org/IC
Slow Magnetization Relaxation in NiIIDyIIIFeIII Molecular Cycles Kong-Qiu Hu,† Xiang Jiang,† Shu-Qi Wu,† Cai-Ming Liu,‡ Ai-Li Cui,† and Hui-Zhong Kou*,† †
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China Beijing National Laboratory for Molecular Sciences, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
‡
S Supporting Information *
Scheme 1. Structures of the NiDy Moieties in Complexes 1 and 2
ABSTRACT: Two cyano- and phenoxo-bridged hexanuclear NiII2DyIII2FeIII2 (1) and octanuclear NiII4DyIII2FeIII2 (2) trimetallic cyclic complexes have been obtained. They are the first trimetallic metallocycles. Magnetic studies reveal that 1 and 2 exhibit single-molecule-magnet behavior with an energy barrier of 17.9 K for complex 1 in a 2000 Oe static field and 25.0 K for complex 2 in a zero static field.
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nvestigations on molecule-based magnets are an active research field, involving chemistry, physics, and material science.1 The 3d−4f heterobimetallic complexes, some of which exhibit single-molecule magnets (SMMs) and single-chain magnets (SCMs), have attracted more and more attention.2−6 The incorporation of lanthanide ions in heterobimetallic complexes is due to their large magnetic momentum and high magnetic anisotropy (D), which are necessary for SMMs and SCMs.1−6 It is well documented that the magnetic performance can be improved by introducing anisotropic paramagnetic ions and increasing the spin ground state, ST. This approach stimulates the development of heterotrimetallic magnetic materials. Besides the aesthetical structures of the trimetallic complexes, some exhibit interesting long-range magnetic ordering, SMM or SCM properties. However, the number of trimetallic complexes is still limited,7−27 which is partly due to the fact that the metal ions and ligands tend to form different kinds of byproducts in the presence of several constituents during the reaction. Until now, the highest reversal energy barrier (Δ = S T 2 |D|) for heterotrimetallic SMMs is as low as 20 K.19 Therefore, it is a challenge to synthesize new heterotrimetallic SMMs with different structures and improved magnetic properties. Interested in the trimetallic magnets based on anisotropic [Fe(CN)6]3−, we studied the one-pot reaction of [Ni(valpn)] (Scheme 1) and Dy3+ with [Fe(CN)6]3− in H2O−acetonitrile (MeCN). A cyclic hexanuclear complex, [Ni(valpn)(H2O)Dy(H2O) 3Fe(CN)6]2·8H2O (1), was obtained. When [Ni(Me2valpn)] (Scheme 1) and Dy3+ in a molar ratio of 2:1 were used to react with [Fe(CN)6]3− in MeCN−H2O−N,Ndimethylformamide (DMF), a cyclic octanuclear complex, {[Ni(Me2valpn)]2Dy(H2O)Fe(CN)6}2·14H2O·4DMF (2), was obtained. Both complexes show frequency dependence of alternating-current (ac) out-of-phase magnetic susceptibility, typical of SMMs. Single-crystal X-ray diffraction measurement of complex 1 reveals an alternate alignment of [Ni(valpn)(H2O)Dy(H2O)3]3+ © XXXX American Chemical Society
and [Fe(CN)6]3− connected by two cis-cyano groups of [Fe(CN)6]3− anions (Figure 1). The [Ni(valpn)(H2O)Dy-
Figure 1. Hexanuclear cyclic structure of complex 1. H atoms and solvents have been omitted for clarity.
(H2O)3]3+ moiety has the usual dinuclear structure bridged by four O atoms of [Ni(valpn)]. The Dy atom is eight-coordinate with three H2O molecules and one cyano N atom around it. The coordination sphere of Dy is close to a biaugmented trigonal prism (BTPR-8) or a square antiprism (SAPR-8) calculated using the SHAPE software (Table S1 in the Supporting Information, SI).28 The octahedral Ni atom is axially coordinated by a water O atom and a cyano N atom, and the equatorial positions are occupied by the Schiff base valpn2−. Each [Fe(CN)6]3− anion links to Ni at one side and to Dy at the other in a bent fashion, with bridging angles in the range of 155.9(3)−164.4(3)°. The hydrogen bonding between the coordinating H2O molecules and the nonbridging cyano N Received: December 2, 2014
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DOI: 10.1021/ic502874n Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
when the ac susceptibility was measured under zero directcurrent (dc) field, which should be due to a fast relaxation process via quantum tunneling frequently observed in lanthanide SMMs. The out-of-phase magnetic susceptibility (χm″) of complex 1 exhibits frequency-dependent peaks under an external dc field of 2 kOe (Figure 3, top). The fitting to the Arrhenius law,
atom links the adjacent molecular cycles into a 2D layer (Figure S2 in the SI). As shown in Figure 2, two {[Ni(Me2valpn)]2Dy(H2O)}3+ moieties in complex 2 are connected by two [Fe(CN)6]3− anions
Figure 2. Octanuclear cyclic structure of complex 2. H atoms and solvents have been omitted for clarity.
through the cyano bridges. The two [Ni(Me2valpn)] around Dy3+ are not perpendicular with the dihedral angle of 71.9° between the two O4 coordination planes of Me2valpn2−. The Dy3+ ion is nine-coordinate, and the coordination configuration is close to a Muffin (MFF-9) or a spherical-capped square antiprism (CSAPR-9) (Table S1 in the SI). The Ni2+ ion is fivecoordinate, and the cyano N atom is situated at the apical position of the square pyramid. [Fe(CN)6]3− uses two cis-cyano ligands to coordinate to two NiII ions with CN−Ni bond angles in the range of 162.4(6)−164.2(6)°. The coordinating H2O molecules (O1W) are involved in hydrogen-bonding interaction with the O atoms of two DMF molecules, forming a parallelogram (Figure S3 in the SI). The nearest intermolecular metal−metal distance is 9.047 Å for Fe−Ni. The temperature dependence of χmT for complexes 1 and 2 in the range of 2−300 K (Figure S4 in the SI) shows that all complexes display overall ferromagnetic properties, with the χmT value increasing with a decrease of the temperature. The χmT values at room temperature are close to the theoretical ones for the high-spin Ni2+ (S = 1), Dy3+ (6H15/2), and low-spin Fe3+ (S = 1 /2) (Table S2 in the SI). It is well documented that the magnetic coupling between the cyano-bridged Fe−CN−Ni and the phenoxo-bridged Ni−(O)2−Dy is usually ferromagnetic, and both ferromagnetic and antiferromagnetic coupling between Fe3+ and Dy3+ via the cyano bridge are observed.2,29 Therefore, it is not easy to determine the overall magnetic coupling character in complex 1, where the Fe−CN−Dy linkages are present. The field dependence of magnetization at 2 K for complexes 1 and 2 (Figure S7 in the SI) shows that magnetization reaches a value of 20.5 and 25.8 Nβ at 50 kOe, lower than the theoretical saturation values of 26 Nβ (1) and 28 Nβ (2). The unsaturation should be due to the zero-field-splitting effect. For complex 2, magnetization shows a platform with a value of 15 Nβ, indicating the presence of an intermediate-spin state at 2 K. The dynamic magnetic susceptibility measurements were conducted to investigate the SMM behavior of complexes 1 and 2. No obvious out-of-phase peak was observed for complex 1
Figure 3. Temperature dependence of the out-of-phase (χm″) ac magnetic susceptibilities for complexes 1 and 2.
τ = τ0 exp(Δ/kT), gives an effective energy barrier Δ = 17.9 K and τ0 = 8.2 × 10−7 s (Figure S7 in the SI), falling within the range of 10−6−10−11 s for SMMs.30 Complex 2 shows a strong frequency dependence of the out-of-phase χm″ signals under zero dc field (Figure 3, bottom), typical of SMM behavior. The fitting to the ln(τ) versus 1/T plot based on the Arrhenius relationship τ = τ0 exp(Δ/kT) leads to a relaxation time of τ0 = 1.6 × 10−7 s and an effective energy barrier of Δ = 25.0 K (Figure S5 in the SI). The parameter ψ for complexes 1 and 2 can be estimated to be 0.30 (1) and 0.25 (2) for the main peaks by using ψ = (ΔTp/Tp)/ Δ(log f), which is in agreement with the expected value for a superparamagnet (0.1 ≤ ψ ≤ 0.3).31 The Cole−Cole curves for complexes 1 and 2 are asymmetric, showing the presence of two relaxation processes (Figure S8 in the SI), and are well fitted by two derivations of the Debye model (Tables S3 and S4 in the SI).32 Considering that 4f electrons are shielded by the outer-shell electrons,33 the Fe3+−Dy3+ magnetic coupling via the cyano bridges is weak.29 That is why the 3D cyano-bridged complex Dy(H2O)2Fe(CN)6·2H2O possesses a critical temperature as low as TC = 2.8 K.34 Therefore, the small relaxation energy barrier for complex 1 might be tentatively related to the presence of weak Fe3+−Dy3+ magnetic coupling, either antiferromagnetic or ferromagnetic.2,29 Table S5 in the SI collects all cyano- and phenoxo-bridged trimetallic complexes, and it can be seen that the trimetallic complexes without FeIII−CN−DyIII generally possess good magnetic properties. Moreover, there are no B
DOI: 10.1021/ic502874n Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry reports on cyano-bridged FeIIIDyIII bimetallic complexes that exhibit SMM or SCM behavior. These results imply that the FeIII−CN−DyIII linkage has a negative effect in achieving SMMs or SCMs. In this case, it might be a promising way of improving the magnetic performance to synthesize trimetallic or bimetallic complexes without FeIII−CN−DyIII linkages. In complex 2, there are no FeIII−CN−DyIII linkages, and the larger energy barrier validates this assumption. Besides, the FeIII− CN−NiII magnetic coupling and the single-ion magnetic anisotropy of DyIII should contribute to the magnetism as well. In conclusion, novel metallocycles have been obtained via control of the Ni/Ln molar ratio. Complexes 1 and 2 are the first cyclic trimetallic complexes; both of them behave as SMMs. Among them, the Ni4Dy2Fe2 complex (2) has the highest energy barrier of 25.0 K among trimetallic SMMs, which is also higher than that (18.4 K) of the corresponding NiIIDyIII bimetallic SMMs.35 Further work on new heterotrimetallic M′IILnIIIMIII (M′ = Ni, Zn; Ln = Dy, Tb; M = Fe, Cr, Co) SMMs is in progress in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis and characterization of complexes 1 and 2, crystallographic data (CIF files), spectroscopic data, experimental procedures, structural diagrams, and additional magnetic data (Figures S1−S9). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant 2013CB933403) and the National Natural Science Foundation of China (Projects 91222104, 21171103, and 20121318518).
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REFERENCES
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DOI: 10.1021/ic502874n Inorg. Chem. XXXX, XXX, XXX−XXX
