- Email: [email protected]
A tetranuclear holmium compound exhibiting single molecule magnet behavior
Inorganic Chemistry Communications 61 (2015) 169–172
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A tetranuclear holmium compound exhibiting single molecule magnet behavior Wei-Wei Kuang, Li-Li Zhu, Yun Xu, Pei-Pei Yang ⁎ College of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China
a r t i c l e
i n f o
Article history: Received 24 July 2015 Received in revised form 9 September 2015 Accepted 15 September 2015 Available online 24 September 2015 Keywords: Tetranuclear holmium cluster Schiff base Magnetic properties Single molecular magnet
a b s t r a c t The synthesis, structure, and magnetic properties of a new tetranuclear holmium cluster [HoIII4(μ3OH)2(L)4(piv)2(DMF)2]·2DMF (1) is reported, where the Schiff base H2L ligand is 2-hydroxy-3-methoxyphenylsalicylaldimine and piv is pivalate. The cluster has a planar “butterfly” Ho4 core, which is bridged by two μ3-hydroxide and six phenoxide oxygen atoms. Magnetically, compound 1 displays field-induced slow relaxation of magnetization. Fitting the dynamic magnetic data to the Arrhenius law gives energy barrier ΔE/kB = 21.49 K and τ0 = 6.97 × 10−7 s. © 2015 Elsevier B.V. All rights reserved.
In the past two decades or so, single-molecule magnets (SMMs), the individual molecules that behave as magnets below a certain blocking temperature (TB), have attracted increasing interest due to their potential applications in high-density information storage, quantum computing, and molecular spintronics [1–3]. The origin of SMM behavior is the existence of an appreciable thermal barrier U for spin-reversal called magnetic anisotropy barrier which depends on the total spin number of a ground state (S) and the zero-field splitting parameter of a molecule (D) [4]. Many attempts have been directed to increase S and D. Some progress has been achieved in this regard, however, a breakthrough essentially needs to be made from a practical point of view. Recently, particular emphasis has been placed on the design of new SMMs applying 4f metal ions, by virtue of their significant magnetic anisotropy arising from the large, unquenched orbital angular momentum which increases the D value for the compound resulting in higher energy barriers [5]. In particular, the DyIII ion has been proved to be one of the excellent lanthanide ions to generate slow relaxation of magnetization when incorporated into molecular aggregates with structures of n nuclearity (n = 1–5) [6–14]. An encouraging output in such system is a square-pyramidal [Dy5O(OiPr)13] molecular nanomagnet which was observed with slow relaxation at temperature as high as 40 K with a very high thermal barrier of 530 K [6]. Compared with DyIII families of various nuclearities, examples of HoIII SMMs are still scarce to date and only three cases with mono- and pentanuclear entities have been structurally and magnetically characterized so far; they include the monometallic [Ho(W5O18)2]9− [15] and (Bu4N)[Ho(Pc)2] [16], and a iso-propoxide bridged holmium square⁎ Corresponding author. E-mail address: [email protected] (P.-P. Yang).
http://dx.doi.org/10.1016/j.inoche.2015.09.007 1387-7003/© 2015 Elsevier B.V. All rights reserved.
based pyramid [Ho5O(OiPr)13] [17]. The development of polymetallic HoIII systems is desirable to advance the understanding of magnetization relaxation with respect to local symmetry and magnetic coupling. Remarkably, the alteration of bridging ligands turns out to be a key factor in constructing lanthanide-based SMMs. The Schiff base ligand, 2-hydroxy-3-methoxy-phenylsalicylaldimine (H 2L), which possesses two different pocket sites: Pocket-I, ONO-donating; Pocket-II, OO-donating (Scheme 1), has been successfully employed in the synthesis of polynuclear 4f [18], or 3d-4f [19–22] clusters before. In this context, we report the preparation, structural description, and magnetic properties of a planar “butterfly” tetranuclear cluster [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2]·2DMF (1). Interestingly, the [HoIII4(μ 3-OH) 2(Ophenol) 6] core in 1 has the same connectivity as for that in the reported [Ho III 4(μ3 -OH) 2 (L) 4(HL)2 ] (2) cluster [18], however, compound 1 shows slow relaxation of magnetization and no out-of-phase alternating current signal is noticed for 2. Notably, compound 1 is the first example of a tetranuclear HoIII 4 system that exhibits field-induced SMM behavior. Compound 1 was synthesized according to the literature [23]. Single crystal X-ray diffraction analyses [24] show that compound 1 crystallizes in the monoclinic space group P21/c. Selected bond lengths and angles for compound 1 are given in Table 1. The core structure of 1 has a precisely coplanar tetranuclear arrangement of HoIII ions with crystallographic inversion symmetry (Fig. 1). This motif is often referred to as a butterfly motif in terms of the positions of the metal centers with Ho1 and Ho1′ defining the wing-tips and Ho2 and Ho2′ the hinge (or body). The two oxygen atoms (O1 and O1′) of the μ3-OH ligands are located on opposite sides of the Ho4 plane and are displaced out of that plane by 0.9627 Å. The hydroxo group forms a fairly symmetrical triple bridge, with the Ho–O1 distances 2.371(3), 2.326(3), and 2.344(3) Å
170
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
Scheme 1. The structure of 2-hydroxy-3-methoxy-phenylsalicylaldimine ligand (H2L). Scheme 2. The coordination modes of the two dianionic ligands in 1. Table 1 Selected bond lengths (Å) and bond angles (deg) for compound 1. Ho(1)–O(1) Ho(1)–O(2) Ho(1)–O(6) Ho(1)–O(9) Ho(2)–O(1) Ho(2)–O(3) Ho(2)–N(3) Ho(2)–O(8) Ho(2)a–O(5) Ho(2)a–O(1) Ho(2)–Ho(2)a Ho(1)–Ho(2)a Ho(1)–O(1)–Ho(2) Ho(2)a–O(1)–Ho(2) Ho(1)–O(5)–Ho(2)a O(9)–Ho(2)–O(1)a O(6)–Ho(1)–O(5) O(9)–Ho(1)–O(2) O(5)a–Ho(2)–O(4)
2.371(3) 2.348(3) 2.192(3) 2.348(3) 2.326(3) 2.378(3) 2.500(3) 2.376(3) 2.311(3) 2.344(3) 3.4480(8) 3.7918(14) 109.69(11) 108.59(11) 96.13(10) 96.51(9) 118.83(10) 101.10(10) 109.09(10)
Ho(1)–N(2) Ho(1)–O(5) Ho(1)–O(8)a Ho(1)–O(10) Ho(2)–O(1)a Ho(2)–O(4) Ho(2)–O(5)a Ho(2)–O(9) Ho(1)a–O(8) Ho(1)–Ho(2) Ho(2)–Ho(1)a O1…Ho4(plane) Ho(2)a–O(1)–Ho(1) Ho(1)a–O(8)–Ho(2) Ho(2)–O(9)–Ho(1) O(1)–Ho(2)–N(3) O(1)–Ho(1)–O(10) O(8)a–Ho(1)–N(2) O(8)–Ho(2)–O(3)
2.437(3) 2.324(3) 2.366(3) 2.596(3) 2.344(3) 2.375(3) 2.311(3) 2.313(3) 2.366(3) 3.8402(8) 3.4480(8) 0.9627 93.99(10) 93.29(9) 110.96(11) 130.48(10) 115.50(9) 113.66(10) 124.34(10)
Symmetry transformations used to generate equivalent atoms: a: −x + 1, −y + 1, −z + 1.
and the Ho–O1–Ho angles 93.99(10), 108.59(11) and 109.69(11)°, respectively. Such a tetranuclear structure is often viewed in terms of two edge-sharing triangles, which in this case are close to scalene triangles, because the distances for Ho1…Ho2, Ho2…Ho2′, and Ho1…Ho2′ equal to 3.8402(8), 3.7918(14), and 3.4480(8) Å, respectively. The shortest edge, Ho1…Ho2′, is that which involves three oxygen bridges rather than just two. There are two different bridging modes of the four deprotonated L2− ligands (η0:η1:η1:η2:μ2 and η1:η2:η1:η2:μ3 Scheme 2). All ligands act in a chelating and bridging mode: the first ligand with Ho1–Ho2′ and the second with Ho1–Ho2–Ho1′. The coordination sphere is completed by a DMF molecule for Ho1 and a chelating piv− ligand for Ho2. Therefore, each eight-coordinate HoIII ion possesses a distorted square-antiprismatic (SAP) geometry with a NO7 coordination environment. The Ho-Dy–O bond lengths are in the range of 2.192(3)–2.596(3) Å and the Ho–N bond lengths are 2.437(3) and 2.500(3) Å. The two square bases of the
Fig. 1. Molecular structure of the neutral [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2] in compound 1 (hydrogen atoms and solvent molecules have been omitted for clarity).
Fig. 2. Coordination polyhedra of distorted square-antiprismatic (SAP) geometry for Ho3+ ion in 1.
square antiprism for Ho1 consist of N2, O5, O6, and O8′ and O1, O2, O9, and O10, respectively, whereas for Ho2, the two square bases are defined by the atoms N3, O1, O1′, and O9 and O3, O4, O5′, and O8, respectively. (Fig. 2). In 2013, Powell et al. reported a tetranuclear Ho4 compound derived from the Schiff base H2L ligand, formulated as [HoIII4(μ3-OH)2(L)4(HL)2] (2). Compound 1 has similar connectivity with 2, the only difference being the replacement of the two HL− ligands in 2 by two piv− and two DMF ligands in 1. The two HL− ligands in 2 adopt a η1:η1:η0:η1:μ2 coordination mode and act in a chelating group (η1:η1:μ1) like the piv− ions in 1 and a terminal group like the DMF molecules in 1. The above-mentioned small differences will be seen to have significant effects on the magnetic behavior of the two Ho4 compounds. The temperature dependence of the direct-current (dc) magnetic susceptibilities (χm) of compound 1 has been measured on polycrystalline samples in the temperature range of 2–300 K at 0.1 T (1000 Oe) dc magnetic field. The result is shown as χmT vs T plots in Fig. 3. For 1, the χmT value at room temperature is 56.56 cm3 K mol−1, which is in line with the expected value of 56.32 cm3 K mol−1 for four uncoupled HoIII ions (5I8, S = 2, L = 6, J = 8, gJ = 5/4, and C = 14.08 cm3 K mol−1). Upon cooling, the χmT value slowly decreases to 14 K with the value of 51.36 cm3 K mol−1, and then increases to 52.65 cm3 K mol− 1 at
Fig. 3. The plots of χmT versus T under 1 kOe for compound 1.
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
Fig. 4. Field-dependent magnetizations for compound 1 at 2.0 K.
4.0 K. On further cooling, the χmT value decreases to 49.75 cm3 K mol−1 at 2.0 K. This behavior indicates that ferromagnetic couplings could counteract the thermal depopulation effect of the Stark sublevels in Ho3+ ion in the higher temperature zone, and the ferromagnetic couplings dominate below 14 K. However, the sharp decrease of χmT values below approximately 4.0 K for compound 1 is probably due to the intermolecule antiferromagnetic interactions and magnetic anisotropy. The isothermal magnetizations of 1 recorded at 2.0 K (Fig. 4) display a rapid increase under low fields and then slowly approach the maxima of 16.71 Nβ at the highest field of 70 kOe. The maximum value is considerably smaller than the theoretical saturation value (40 Nβ) for four magnetically isolated HoIII ions. The unsaturated magnetization suggests the existence of magnetic anisotropy and/or low lying excited states in compound 1 [25].
171
Magnetization dynamics of compound 1 were explored by alternating current (ac) susceptibility under a zero direct current (dc) field with an oscillation of 2.5 Oe. The frequency dependent behavior to some different extent is observed for the imaginary ac signal (χm″) of 1 at the temperature lower than 10.0 K (Fig. 5 (left)), suggesting the appearance of slightly slow relaxation behavior of the magnetization [26]. Notably, the maximum value of the frequency-dependent χm″ signals of 1 cannot be detected probably due to the fast quantum tunneling of magnetization (QTM), which can be essentially suppressed by supplying a dc magnetic field [27]. Therefore, 5 kOe external magnetic field was employed during the ac susceptibility measurements, and a set of obvious frequency-dependent χm′ and χm″ signals are clearly seen in the temperature ranges of 2.8 (100 Hz)–4.0 K (1000 Hz) for 1, shown in Fig. 5 (right), indicating slow field induced relaxation of magnetization. The Mydosh parameters were estimated from this dependence, φ = (ΔTp / Tp) / Δ(logω) = 0.14 for 1 (Tp is defined as the maximum value of χm′ at different temperatures and ω = 2πν), which is in the range of normal superparamagnets and excludes the possibility of a spin glass (0.01 b φ b 0.1) [28–29]. Fitting the χm″ versus T data to Arrhenius law gives effective energy barrier ΔE/kB = 21.49 K and τ0 = 6.97 × 10− 7 s for 1, which were shown in the inset of Fig. 5 (right). The τ0 value of 1 is consistent with the expected characteristic preexponential factor 10−6–10−12 s for typical SMMs [30–31]. As far as we are aware compound 1 is only the fourth holmium singlemolecule magnet reported, after the monometallic [Ho(W5O18)2]9 − (broad shoulder in χm″(T) at 5 K for ν N 10 kHz), (Bu4N)[Ho(Pc)2] (M(H) hysteresis observed at T b 0.5 K; ac data not reported), and [Ho5O(OiPr)13] (a very high thermal energy to magnetization relaxation), and the second polymetallic example. The slow relaxation behaviors seen in 1 and 2 are different probably because the ligand substitution has a larger effect on the geometries of the ligand shell about the HoIII ions, and this has clearly had a significant effect on the
Fig. 5. Temperature dependence of the in-phase (upper) and out-of-phase (lower) ac susceptibilities for 1 measured in a 2.5 ac field without a dc field (left) and with a 5 kOe dc field (right). (inset) The solid lines represent the best fits to the Arrhenius law.
172
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
orientation, and presumably the magnitude, of the HoIII single-ion anisotropy tensors [32–33]. To summarize, a tetradentate Schiff base ligand, 2-hydroxy-3methoxy-phenylsalicylaldimine was successfully used in the synthesis of a tetranuclear compound [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2] (1), possessing a planar “butterfly” core. Alternating current magnetic studies on 1 reveal the presence of SMM behavior. Notably, compound 1 is a unique example of a tetranuclear HoIII cluster exhibiting SMM behavior. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 21101068) and the National Science Foundation of Anhui (1308085MB16), the National Science Foundation of Anhui Educational Bureau (no. KJ2015A066) and the Scientific and Technological Projects (20140219). Appendix A. Supplementary material The powder X-ray diffraction and thermogravimetric analyses, Figs. S1-S2. CCDC (1047490) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/ 10.1016/j.inoche.2015.09.007. References [1] D. Gatteschi, R. Sessoli, Quantum tunneling of magnetization and related phenomena in molecular materials, Angew. Chem. Int. Ed. 42 (2003) 268–297. [2] S. Hill, R.S. Edwards, N. Aliaga-Alcalde, G. Christou, Quantum coherence in an exchange-coupled dimer of single-molecule magnets, Science 302 (2003) 1015–1018. [3] M.N. Leuenberger, D. Loss, Quantum computing in molecular magnets, Nature 410 (2001) 789–793. [4] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, Oxford, U. K., 2006 [5] (a) R. Sessoli, A.K. Powell, Strategies towards single molecule magnets based on lanthanide ions, Coord. Chem. Rev. 253 (2009) 2328–2341; (b) L. Sorace, C. Benelli, D. Gatteschi, Lanthanides in molecular magnetism: old tools in a new field, Chem. Soc. Rev. 40 (2011) 3092–3104; (c) J.D. Rinehart, J.R. Long, Exploiting single-ion anisotropy in the design of felement single-molecule magnets, Chem. Sci. 2 (2011) 2078–2085; (d) A. Palii, B. Tsukerblat, S. Klokishner, K.R. Dunbar, J.M. Clemente-Juan, E. Coronado, Beyond the spin model: exchange coupling in molecular magnets with unquenched orbital angular momenta, Chem. Soc. Rev. 40 (2011) 3130–3156. [6] R.J. Blagg, C.A. Muryn, F. Tuna, E.J.L. McInnes, R.E.P. Winpenny, Single pyramid magnets: Dy5 pyramids with slow magnetic relaxation to 40 K, Angew. Chem. Int. Ed. 50 (2011) 6530–6533. [7] A. Watanabe, A. Yamashita, M. Nakano, T. Yamamura, T. Kajiwara, Multi-path magnetic relaxation of mono-dysprosium(III) single-molecule magnet with extremely high barrier, Chem. Eur. J. 17 (2011) 7428–7432. [8] J.D. Rinehart, M. Fang, W.J. Evans, J.R. Long, Strong exchange and magnetic blocking in N3− 2 -radical-bridged lanthanide complexes, Nat. Chem. 3 (2011) 538–542. [9] Y.-X. Wang, W. Shi, H. Li, Y. Song, L. Fang, Y.-H. Lan, A.K. Powell, W. Wernsdorfer, L. Ungur, L.F. Chibotaru, M. Shen, P. Cheng, A single-molecule magnet assembly exhibiting a dielectric transition at 470 K, Chem. Sci. 3 (2012) 3366–3370. [10] Y.-N. Guo, G.-F. Xu, P. Gamez, L. Zhao, S.-Y. Lin, R. Deng, J.-K. Tang, H.-J. Zhang, Twostep relaxation in a linear tetranuclear dysprosium(III) aggregate showing singlemolecule magnet behavior, J. Am. Chem. Soc. 132 (2010) 8538–8539. [11] P.-F. Shi, G. Xiong, B. Zhao, Z.-Y. Zhang, P. Cheng, Anion-induced changes of structure interpenetration and magnetic properties in 3D Dy-Cu metal–organic frameworks, Chem. Commun. 49 (2013) 2338–2340. [12] Y.-L. Hou, G. Xiong, B. Shen, B. Zhao, Z. Chen, J.-Z. Cui, Structures, luminescent and magnetic properties of six lanthanide-organic frameworks: observation of slow magnetic relaxation behavior in the Dy III compound, Dalton Trans. 42 (2013) 3587–3596. [13] Y.-L. Hou, G. Xiong, P.-F. Shi, R.-R. Cheng, J.-Z. Cui, B. Zhao, Unique (3,12)-connected coordination polymers displaying high stability, large magnetocaloric effect and slow magnetic relaxation, Chem. Commun. 49 (2013) 6066–6068.
[14] G. Xiong, X.-Y. Qin, P.-F. Shi, Y.-L. Hou, J.-Z. Cui, B. Zhao, New strategy to construct single-ion magnets: a unique Dy@Zn 6 cluster exhibiting slow magnetic relaxation, Chem. Commun. 50 (2014) 4255–4257. [15] M.A. AlDamen, J.M. Clemente-Juan, E. Coronado, C. Martí-Gastaldo, A. Gaita-Ariño, Mononuclear lanthanide single-molecule magnets based on polyoxometalates, J. Am. Chem. Soc. 130 (2008) 8874–8875. [16] J.D. Rinehart, M. Fang, W.J. Evans, J.R. Long, Lanthanoid single-ion magnets based on polyoxometalates with a 5-fold symmetry: the series [LnP5W30O110]12− (Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb), J. Am. Chem. Soc. 134 (2012) 14982–14990. [17] R.J. Blagg, F. Tuna, E.J.L. McInnes, R.E.P. Winpenny, Pentametallic lanthanidealkoxide square-based pyramids: high energy barrier for thermal relaxation in a holmium single molecule magnet, Chem. Commun. 47 (2011) 10587–10589. [18] K.C. Mondal, G.E. Kostakis, Y.-H. Lan, A.K. Powell, Magnetic properties of five planar defect dicubanes of [LnIII4(μ3-OH)2(L)4(HL)2]·2THF (Ln = Gd, Tb, Dy, Ho and Er), Polyhedron 66 (2013) 268–273. [19] I. Nemec, M. Machata, R. Herchel, R. Boča, Z. Trávníček, A new family of Fe2Ln complexes built from mononuclear anionic Schiff base subunits, Dalton Trans. 41 (2012) 14603–14610. [20] K.C. Mondal, G.E. Kostakis, Y.-H. Lan, W. Wernsdorfer, C.E. Anson, A.K. Powell, Defectdicubane Ni2Ln2 (Ln = Dy, Tb) single molecule magnets, Inorg. Chem. 50 (2011) 11604–11611. [21] K.C. Mondal, A. Sundt, Y.-H. Lan, G.E. Kostakis, O. Waldmann, L. Ungur, L.F. Chibotaru, C.E. Anson, A.K. Powell, Coexistence of distinct single-ion and exchange-based mechanisms for blocking of magnetization in a CoII2DyIII2 singlemolecule magnet, Angew. Chem. Int. Ed. 51 (2012) 7550–7554. [22] H.-S. Ke, L. Zhao, Y. Guo, J.-K. Tang, Syntheses, structures, and magnetic analyses of a family of heterometallic hexanuclear [Ni4M2] (M = Gd, Dy, Y) compounds: observation of slow magnetic relaxation in the DyIII derivative, Inorg. Chem. 51 (2012) 2699–2705. [23] The Schiff base ligand (H2L) was synthesized according to the reported procedures [C. Boskovic, E. Rusanov, H. Stoeckli-Evans, H. U. Güdel, Inorg. Chem. Commun. 5 (2002) 881–886]. Triethylamine (0.2 mmol) was added to a stirring solution of Ho(NO3)3·6H2O (0.1 mmol, 0.0459 g), H2L (0.1 mmol, 0.0243 g) and pivalate sodium (0.1 mmol, 0.0142 g) in the mixed solvent CH3OH (10 ml) and MeCN (10 ml) at room temperature, resulting in a turbid yellow solution. After stirring for 2 h, the yellow precipitate was collected through filtering and washed with cold MeOH. The residue was recrystallized from N,N′-dimethylformamide (DMF), giving a bright yellow solution, which left undisturbed to concentrate slowly by evaporation. X-ray quality crystals of 1 slowly grew over 4 days, which were collected by filtration, washed with cold DMF, and dried under air. The yellow crystals were produced in 36% yield based on Ho. Anal. Calcd for C78H92N8O22Ho4: C, 43.51; H, 4.31; N, 5.20. Found: C, 43.70, H, 4.40, N, 5.20. Selected IR data (ν/cm−1) using KBr disks: 3626(m), 3445(s), 3049(vw), 2952(w), 1658(vs), 1605(s), 1587(m), 1530(m), 1486(vs), 1387(m), 1329(w), 1277(m), 1250(m), 1220(s), 1174(m), 1107(w), 1035(vw), 973(w), 861(vw), 892(w), 823(w), 744(m), 697(w), 577(w), 501(w). [24] Crystal data for 1: C78H92Ho4N8O22, Mr = 2153.32, monoclinic, P21/c, a = 12.903(3) Å, b = 24.552(5) Å, c = 13.538(3) Å, β = 110.10(3)°, V = 4027.6(14) Å3, Z = 2, ρ = 1.776 mg·cm−3, μ = 3.964 mm−1. R = 0.0506 for 6293 reflections with I N 2σ(I) and 0.0601 for 7080 reflections; wR2 = 0.0745 and S = 1.095. Singlecrystal X-ray diffraction experiments were carried out with a Rigaku Saturn CCD diffractometer equipped with a graphite monochromator utilizing Mo-Kα radiation with radiation wavelength 0.71073 Å by using the ω-scan technique. Empirical absorption corrections were applied using the SADABS program. The structure was solved by the direct method and refined by the full-matrix least-squares method on F2, with all non-hydrogen atoms refined with anisotropic thermal parameters. [25] H. Xiang, Y.-H. Lan, H.-Y. Li, L. Jiang, T.-B. Lu, C.E. Ansona, A.K. Powell, Novel mixedvalent CoII2CoIII4LnIII4 aggregates with ligands derived from tris-(hydroxymethyl) -aminomethane (Tris), Dalton Trans. 39 (2010) 4737–4739. [26] S.K. Langley, B. Moubaraki, K.S. Murray, Magnetic properties of hexanuclear lanthanide(III) clusters incorporating a central μ6-carbonate ligand derived from atmospheric CO2 fixation, Inorg. Chem. 51 (2012) 3947–3949. [27] M.U. Anwar, L.K. Thompson, L.N. Dawe, F. Habib, M. Murugesu, Predictable selfassembled [2 × 2] Ln(III)4 square grids (Ln = Dy, Tb)—SMM behaviour in a new lanthanide cluster motif, Chem. Commun. 48 (2012) 4576–4578. [28] N. Zhou, Y. Ma, C. Wang, G.-F. Xu, J.-K. Tang, J.-X. Xu, S.-P. Yan, P. Cheng, L.-C. Li, D.-Z. Liao, A monometallic tri-spin single-molecule magnet based on rare earth radicals, Dalton Trans. 40 (2009) 8489–8492. [29] P.-F. Shi, Y.-Z. Zheng, X.-Q. Zhao, G. Xiong, B. Zhao, P. Cheng, 3D MOFs containing trigonal bipyramidal Ln5 clusters as nodes: large magnetocaloric effect and slow magnetic relaxation behavior, Chem. Eur. J. 18 (2012) 15086–15091. [30] Y.-L. Miao, J.-L. Liu, J.-Y. Li, J.-D. Leng, Y.-C. Ou, M.-L. Tong, Two novel Dy8 and Dy11 clusters with cubane [Dy4(μ3-OH)4]8+ units exhibiting slow magnetic relaxation behaviour, Dalton Trans. 40 (2011) 10229–10236. [31] S.-Y. Lin, G.-F. Xu, L. Zhao, Y.-N. Guo, Y. Guo, J.-K. Tang, Observation of slow magnetic relaxation in triple-stranded lanthanide helicates, Dalton Trans. 40 (2011) 8213–8217. [32] L.F. Chibotaru, L. Ungur, A. Soncini, The origin of nonmagnetic kramers doublets in the ground state of dysprosium triangles: evidence for a toroidal magnetic moment, Angew. Chem. Int. Ed. 47 (2008) 4126–4129. [33] J. Luzon, K. Bernot, I.J. Hewitt, C.E. Anson, A.K. Powell, R. Sessoli, Spin chirality in a molecular dysprosium triangle: the archetype of the noncollinear ising model, Phys. Rev. Lett. 100 (2008) 247205/1–247205/4.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Short communication
A tetranuclear holmium compound exhibiting single molecule magnet behavior Wei-Wei Kuang, Li-Li Zhu, Yun Xu, Pei-Pei Yang ⁎ College of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China
a r t i c l e
i n f o
Article history: Received 24 July 2015 Received in revised form 9 September 2015 Accepted 15 September 2015 Available online 24 September 2015 Keywords: Tetranuclear holmium cluster Schiff base Magnetic properties Single molecular magnet
a b s t r a c t The synthesis, structure, and magnetic properties of a new tetranuclear holmium cluster [HoIII4(μ3OH)2(L)4(piv)2(DMF)2]·2DMF (1) is reported, where the Schiff base H2L ligand is 2-hydroxy-3-methoxyphenylsalicylaldimine and piv is pivalate. The cluster has a planar “butterfly” Ho4 core, which is bridged by two μ3-hydroxide and six phenoxide oxygen atoms. Magnetically, compound 1 displays field-induced slow relaxation of magnetization. Fitting the dynamic magnetic data to the Arrhenius law gives energy barrier ΔE/kB = 21.49 K and τ0 = 6.97 × 10−7 s. © 2015 Elsevier B.V. All rights reserved.
In the past two decades or so, single-molecule magnets (SMMs), the individual molecules that behave as magnets below a certain blocking temperature (TB), have attracted increasing interest due to their potential applications in high-density information storage, quantum computing, and molecular spintronics [1–3]. The origin of SMM behavior is the existence of an appreciable thermal barrier U for spin-reversal called magnetic anisotropy barrier which depends on the total spin number of a ground state (S) and the zero-field splitting parameter of a molecule (D) [4]. Many attempts have been directed to increase S and D. Some progress has been achieved in this regard, however, a breakthrough essentially needs to be made from a practical point of view. Recently, particular emphasis has been placed on the design of new SMMs applying 4f metal ions, by virtue of their significant magnetic anisotropy arising from the large, unquenched orbital angular momentum which increases the D value for the compound resulting in higher energy barriers [5]. In particular, the DyIII ion has been proved to be one of the excellent lanthanide ions to generate slow relaxation of magnetization when incorporated into molecular aggregates with structures of n nuclearity (n = 1–5) [6–14]. An encouraging output in such system is a square-pyramidal [Dy5O(OiPr)13] molecular nanomagnet which was observed with slow relaxation at temperature as high as 40 K with a very high thermal barrier of 530 K [6]. Compared with DyIII families of various nuclearities, examples of HoIII SMMs are still scarce to date and only three cases with mono- and pentanuclear entities have been structurally and magnetically characterized so far; they include the monometallic [Ho(W5O18)2]9− [15] and (Bu4N)[Ho(Pc)2] [16], and a iso-propoxide bridged holmium square⁎ Corresponding author. E-mail address: [email protected] (P.-P. Yang).
http://dx.doi.org/10.1016/j.inoche.2015.09.007 1387-7003/© 2015 Elsevier B.V. All rights reserved.
based pyramid [Ho5O(OiPr)13] [17]. The development of polymetallic HoIII systems is desirable to advance the understanding of magnetization relaxation with respect to local symmetry and magnetic coupling. Remarkably, the alteration of bridging ligands turns out to be a key factor in constructing lanthanide-based SMMs. The Schiff base ligand, 2-hydroxy-3-methoxy-phenylsalicylaldimine (H 2L), which possesses two different pocket sites: Pocket-I, ONO-donating; Pocket-II, OO-donating (Scheme 1), has been successfully employed in the synthesis of polynuclear 4f [18], or 3d-4f [19–22] clusters before. In this context, we report the preparation, structural description, and magnetic properties of a planar “butterfly” tetranuclear cluster [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2]·2DMF (1). Interestingly, the [HoIII4(μ 3-OH) 2(Ophenol) 6] core in 1 has the same connectivity as for that in the reported [Ho III 4(μ3 -OH) 2 (L) 4(HL)2 ] (2) cluster [18], however, compound 1 shows slow relaxation of magnetization and no out-of-phase alternating current signal is noticed for 2. Notably, compound 1 is the first example of a tetranuclear HoIII 4 system that exhibits field-induced SMM behavior. Compound 1 was synthesized according to the literature [23]. Single crystal X-ray diffraction analyses [24] show that compound 1 crystallizes in the monoclinic space group P21/c. Selected bond lengths and angles for compound 1 are given in Table 1. The core structure of 1 has a precisely coplanar tetranuclear arrangement of HoIII ions with crystallographic inversion symmetry (Fig. 1). This motif is often referred to as a butterfly motif in terms of the positions of the metal centers with Ho1 and Ho1′ defining the wing-tips and Ho2 and Ho2′ the hinge (or body). The two oxygen atoms (O1 and O1′) of the μ3-OH ligands are located on opposite sides of the Ho4 plane and are displaced out of that plane by 0.9627 Å. The hydroxo group forms a fairly symmetrical triple bridge, with the Ho–O1 distances 2.371(3), 2.326(3), and 2.344(3) Å
170
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
Scheme 1. The structure of 2-hydroxy-3-methoxy-phenylsalicylaldimine ligand (H2L). Scheme 2. The coordination modes of the two dianionic ligands in 1. Table 1 Selected bond lengths (Å) and bond angles (deg) for compound 1. Ho(1)–O(1) Ho(1)–O(2) Ho(1)–O(6) Ho(1)–O(9) Ho(2)–O(1) Ho(2)–O(3) Ho(2)–N(3) Ho(2)–O(8) Ho(2)a–O(5) Ho(2)a–O(1) Ho(2)–Ho(2)a Ho(1)–Ho(2)a Ho(1)–O(1)–Ho(2) Ho(2)a–O(1)–Ho(2) Ho(1)–O(5)–Ho(2)a O(9)–Ho(2)–O(1)a O(6)–Ho(1)–O(5) O(9)–Ho(1)–O(2) O(5)a–Ho(2)–O(4)
2.371(3) 2.348(3) 2.192(3) 2.348(3) 2.326(3) 2.378(3) 2.500(3) 2.376(3) 2.311(3) 2.344(3) 3.4480(8) 3.7918(14) 109.69(11) 108.59(11) 96.13(10) 96.51(9) 118.83(10) 101.10(10) 109.09(10)
Ho(1)–N(2) Ho(1)–O(5) Ho(1)–O(8)a Ho(1)–O(10) Ho(2)–O(1)a Ho(2)–O(4) Ho(2)–O(5)a Ho(2)–O(9) Ho(1)a–O(8) Ho(1)–Ho(2) Ho(2)–Ho(1)a O1…Ho4(plane) Ho(2)a–O(1)–Ho(1) Ho(1)a–O(8)–Ho(2) Ho(2)–O(9)–Ho(1) O(1)–Ho(2)–N(3) O(1)–Ho(1)–O(10) O(8)a–Ho(1)–N(2) O(8)–Ho(2)–O(3)
2.437(3) 2.324(3) 2.366(3) 2.596(3) 2.344(3) 2.375(3) 2.311(3) 2.313(3) 2.366(3) 3.8402(8) 3.4480(8) 0.9627 93.99(10) 93.29(9) 110.96(11) 130.48(10) 115.50(9) 113.66(10) 124.34(10)
Symmetry transformations used to generate equivalent atoms: a: −x + 1, −y + 1, −z + 1.
and the Ho–O1–Ho angles 93.99(10), 108.59(11) and 109.69(11)°, respectively. Such a tetranuclear structure is often viewed in terms of two edge-sharing triangles, which in this case are close to scalene triangles, because the distances for Ho1…Ho2, Ho2…Ho2′, and Ho1…Ho2′ equal to 3.8402(8), 3.7918(14), and 3.4480(8) Å, respectively. The shortest edge, Ho1…Ho2′, is that which involves three oxygen bridges rather than just two. There are two different bridging modes of the four deprotonated L2− ligands (η0:η1:η1:η2:μ2 and η1:η2:η1:η2:μ3 Scheme 2). All ligands act in a chelating and bridging mode: the first ligand with Ho1–Ho2′ and the second with Ho1–Ho2–Ho1′. The coordination sphere is completed by a DMF molecule for Ho1 and a chelating piv− ligand for Ho2. Therefore, each eight-coordinate HoIII ion possesses a distorted square-antiprismatic (SAP) geometry with a NO7 coordination environment. The Ho-Dy–O bond lengths are in the range of 2.192(3)–2.596(3) Å and the Ho–N bond lengths are 2.437(3) and 2.500(3) Å. The two square bases of the
Fig. 1. Molecular structure of the neutral [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2] in compound 1 (hydrogen atoms and solvent molecules have been omitted for clarity).
Fig. 2. Coordination polyhedra of distorted square-antiprismatic (SAP) geometry for Ho3+ ion in 1.
square antiprism for Ho1 consist of N2, O5, O6, and O8′ and O1, O2, O9, and O10, respectively, whereas for Ho2, the two square bases are defined by the atoms N3, O1, O1′, and O9 and O3, O4, O5′, and O8, respectively. (Fig. 2). In 2013, Powell et al. reported a tetranuclear Ho4 compound derived from the Schiff base H2L ligand, formulated as [HoIII4(μ3-OH)2(L)4(HL)2] (2). Compound 1 has similar connectivity with 2, the only difference being the replacement of the two HL− ligands in 2 by two piv− and two DMF ligands in 1. The two HL− ligands in 2 adopt a η1:η1:η0:η1:μ2 coordination mode and act in a chelating group (η1:η1:μ1) like the piv− ions in 1 and a terminal group like the DMF molecules in 1. The above-mentioned small differences will be seen to have significant effects on the magnetic behavior of the two Ho4 compounds. The temperature dependence of the direct-current (dc) magnetic susceptibilities (χm) of compound 1 has been measured on polycrystalline samples in the temperature range of 2–300 K at 0.1 T (1000 Oe) dc magnetic field. The result is shown as χmT vs T plots in Fig. 3. For 1, the χmT value at room temperature is 56.56 cm3 K mol−1, which is in line with the expected value of 56.32 cm3 K mol−1 for four uncoupled HoIII ions (5I8, S = 2, L = 6, J = 8, gJ = 5/4, and C = 14.08 cm3 K mol−1). Upon cooling, the χmT value slowly decreases to 14 K with the value of 51.36 cm3 K mol−1, and then increases to 52.65 cm3 K mol− 1 at
Fig. 3. The plots of χmT versus T under 1 kOe for compound 1.
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
Fig. 4. Field-dependent magnetizations for compound 1 at 2.0 K.
4.0 K. On further cooling, the χmT value decreases to 49.75 cm3 K mol−1 at 2.0 K. This behavior indicates that ferromagnetic couplings could counteract the thermal depopulation effect of the Stark sublevels in Ho3+ ion in the higher temperature zone, and the ferromagnetic couplings dominate below 14 K. However, the sharp decrease of χmT values below approximately 4.0 K for compound 1 is probably due to the intermolecule antiferromagnetic interactions and magnetic anisotropy. The isothermal magnetizations of 1 recorded at 2.0 K (Fig. 4) display a rapid increase under low fields and then slowly approach the maxima of 16.71 Nβ at the highest field of 70 kOe. The maximum value is considerably smaller than the theoretical saturation value (40 Nβ) for four magnetically isolated HoIII ions. The unsaturated magnetization suggests the existence of magnetic anisotropy and/or low lying excited states in compound 1 [25].
171
Magnetization dynamics of compound 1 were explored by alternating current (ac) susceptibility under a zero direct current (dc) field with an oscillation of 2.5 Oe. The frequency dependent behavior to some different extent is observed for the imaginary ac signal (χm″) of 1 at the temperature lower than 10.0 K (Fig. 5 (left)), suggesting the appearance of slightly slow relaxation behavior of the magnetization [26]. Notably, the maximum value of the frequency-dependent χm″ signals of 1 cannot be detected probably due to the fast quantum tunneling of magnetization (QTM), which can be essentially suppressed by supplying a dc magnetic field [27]. Therefore, 5 kOe external magnetic field was employed during the ac susceptibility measurements, and a set of obvious frequency-dependent χm′ and χm″ signals are clearly seen in the temperature ranges of 2.8 (100 Hz)–4.0 K (1000 Hz) for 1, shown in Fig. 5 (right), indicating slow field induced relaxation of magnetization. The Mydosh parameters were estimated from this dependence, φ = (ΔTp / Tp) / Δ(logω) = 0.14 for 1 (Tp is defined as the maximum value of χm′ at different temperatures and ω = 2πν), which is in the range of normal superparamagnets and excludes the possibility of a spin glass (0.01 b φ b 0.1) [28–29]. Fitting the χm″ versus T data to Arrhenius law gives effective energy barrier ΔE/kB = 21.49 K and τ0 = 6.97 × 10− 7 s for 1, which were shown in the inset of Fig. 5 (right). The τ0 value of 1 is consistent with the expected characteristic preexponential factor 10−6–10−12 s for typical SMMs [30–31]. As far as we are aware compound 1 is only the fourth holmium singlemolecule magnet reported, after the monometallic [Ho(W5O18)2]9 − (broad shoulder in χm″(T) at 5 K for ν N 10 kHz), (Bu4N)[Ho(Pc)2] (M(H) hysteresis observed at T b 0.5 K; ac data not reported), and [Ho5O(OiPr)13] (a very high thermal energy to magnetization relaxation), and the second polymetallic example. The slow relaxation behaviors seen in 1 and 2 are different probably because the ligand substitution has a larger effect on the geometries of the ligand shell about the HoIII ions, and this has clearly had a significant effect on the
Fig. 5. Temperature dependence of the in-phase (upper) and out-of-phase (lower) ac susceptibilities for 1 measured in a 2.5 ac field without a dc field (left) and with a 5 kOe dc field (right). (inset) The solid lines represent the best fits to the Arrhenius law.
172
W.-W. Kuang et al. / Inorganic Chemistry Communications 61 (2015) 169–172
orientation, and presumably the magnitude, of the HoIII single-ion anisotropy tensors [32–33]. To summarize, a tetradentate Schiff base ligand, 2-hydroxy-3methoxy-phenylsalicylaldimine was successfully used in the synthesis of a tetranuclear compound [HoIII4(μ3-OH)2(L)4(piv)2(DMF)2] (1), possessing a planar “butterfly” core. Alternating current magnetic studies on 1 reveal the presence of SMM behavior. Notably, compound 1 is a unique example of a tetranuclear HoIII cluster exhibiting SMM behavior. Acknowledgments This work was supported by the National Natural Science Foundation of China (no. 21101068) and the National Science Foundation of Anhui (1308085MB16), the National Science Foundation of Anhui Educational Bureau (no. KJ2015A066) and the Scientific and Technological Projects (20140219). Appendix A. Supplementary material The powder X-ray diffraction and thermogravimetric analyses, Figs. S1-S2. CCDC (1047490) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/ 10.1016/j.inoche.2015.09.007. References [1] D. Gatteschi, R. Sessoli, Quantum tunneling of magnetization and related phenomena in molecular materials, Angew. Chem. Int. Ed. 42 (2003) 268–297. [2] S. Hill, R.S. Edwards, N. Aliaga-Alcalde, G. Christou, Quantum coherence in an exchange-coupled dimer of single-molecule magnets, Science 302 (2003) 1015–1018. [3] M.N. Leuenberger, D. Loss, Quantum computing in molecular magnets, Nature 410 (2001) 789–793. [4] D. Gatteschi, R. Sessoli, J. Villain, Molecular Nanomagnets, Oxford University Press, Oxford, U. K., 2006 [5] (a) R. Sessoli, A.K. Powell, Strategies towards single molecule magnets based on lanthanide ions, Coord. Chem. Rev. 253 (2009) 2328–2341; (b) L. Sorace, C. Benelli, D. Gatteschi, Lanthanides in molecular magnetism: old tools in a new field, Chem. Soc. Rev. 40 (2011) 3092–3104; (c) J.D. Rinehart, J.R. Long, Exploiting single-ion anisotropy in the design of felement single-molecule magnets, Chem. Sci. 2 (2011) 2078–2085; (d) A. Palii, B. Tsukerblat, S. Klokishner, K.R. Dunbar, J.M. Clemente-Juan, E. Coronado, Beyond the spin model: exchange coupling in molecular magnets with unquenched orbital angular momenta, Chem. Soc. Rev. 40 (2011) 3130–3156. [6] R.J. Blagg, C.A. Muryn, F. Tuna, E.J.L. McInnes, R.E.P. Winpenny, Single pyramid magnets: Dy5 pyramids with slow magnetic relaxation to 40 K, Angew. Chem. Int. Ed. 50 (2011) 6530–6533. [7] A. Watanabe, A. Yamashita, M. Nakano, T. Yamamura, T. Kajiwara, Multi-path magnetic relaxation of mono-dysprosium(III) single-molecule magnet with extremely high barrier, Chem. Eur. J. 17 (2011) 7428–7432. [8] J.D. Rinehart, M. Fang, W.J. Evans, J.R. Long, Strong exchange and magnetic blocking in N3− 2 -radical-bridged lanthanide complexes, Nat. Chem. 3 (2011) 538–542. [9] Y.-X. Wang, W. Shi, H. Li, Y. Song, L. Fang, Y.-H. Lan, A.K. Powell, W. Wernsdorfer, L. Ungur, L.F. Chibotaru, M. Shen, P. Cheng, A single-molecule magnet assembly exhibiting a dielectric transition at 470 K, Chem. Sci. 3 (2012) 3366–3370. [10] Y.-N. Guo, G.-F. Xu, P. Gamez, L. Zhao, S.-Y. Lin, R. Deng, J.-K. Tang, H.-J. Zhang, Twostep relaxation in a linear tetranuclear dysprosium(III) aggregate showing singlemolecule magnet behavior, J. Am. Chem. Soc. 132 (2010) 8538–8539. [11] P.-F. Shi, G. Xiong, B. Zhao, Z.-Y. Zhang, P. Cheng, Anion-induced changes of structure interpenetration and magnetic properties in 3D Dy-Cu metal–organic frameworks, Chem. Commun. 49 (2013) 2338–2340. [12] Y.-L. Hou, G. Xiong, B. Shen, B. Zhao, Z. Chen, J.-Z. Cui, Structures, luminescent and magnetic properties of six lanthanide-organic frameworks: observation of slow magnetic relaxation behavior in the Dy III compound, Dalton Trans. 42 (2013) 3587–3596. [13] Y.-L. Hou, G. Xiong, P.-F. Shi, R.-R. Cheng, J.-Z. Cui, B. Zhao, Unique (3,12)-connected coordination polymers displaying high stability, large magnetocaloric effect and slow magnetic relaxation, Chem. Commun. 49 (2013) 6066–6068.
[14] G. Xiong, X.-Y. Qin, P.-F. Shi, Y.-L. Hou, J.-Z. Cui, B. Zhao, New strategy to construct single-ion magnets: a unique Dy@Zn 6 cluster exhibiting slow magnetic relaxation, Chem. Commun. 50 (2014) 4255–4257. [15] M.A. AlDamen, J.M. Clemente-Juan, E. Coronado, C. Martí-Gastaldo, A. Gaita-Ariño, Mononuclear lanthanide single-molecule magnets based on polyoxometalates, J. Am. Chem. Soc. 130 (2008) 8874–8875. [16] J.D. Rinehart, M. Fang, W.J. Evans, J.R. Long, Lanthanoid single-ion magnets based on polyoxometalates with a 5-fold symmetry: the series [LnP5W30O110]12− (Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb), J. Am. Chem. Soc. 134 (2012) 14982–14990. [17] R.J. Blagg, F. Tuna, E.J.L. McInnes, R.E.P. Winpenny, Pentametallic lanthanidealkoxide square-based pyramids: high energy barrier for thermal relaxation in a holmium single molecule magnet, Chem. Commun. 47 (2011) 10587–10589. [18] K.C. Mondal, G.E. Kostakis, Y.-H. Lan, A.K. Powell, Magnetic properties of five planar defect dicubanes of [LnIII4(μ3-OH)2(L)4(HL)2]·2THF (Ln = Gd, Tb, Dy, Ho and Er), Polyhedron 66 (2013) 268–273. [19] I. Nemec, M. Machata, R. Herchel, R. Boča, Z. Trávníček, A new family of Fe2Ln complexes built from mononuclear anionic Schiff base subunits, Dalton Trans. 41 (2012) 14603–14610. [20] K.C. Mondal, G.E. Kostakis, Y.-H. Lan, W. Wernsdorfer, C.E. Anson, A.K. Powell, Defectdicubane Ni2Ln2 (Ln = Dy, Tb) single molecule magnets, Inorg. Chem. 50 (2011) 11604–11611. [21] K.C. Mondal, A. Sundt, Y.-H. Lan, G.E. Kostakis, O. Waldmann, L. Ungur, L.F. Chibotaru, C.E. Anson, A.K. Powell, Coexistence of distinct single-ion and exchange-based mechanisms for blocking of magnetization in a CoII2DyIII2 singlemolecule magnet, Angew. Chem. Int. Ed. 51 (2012) 7550–7554. [22] H.-S. Ke, L. Zhao, Y. Guo, J.-K. Tang, Syntheses, structures, and magnetic analyses of a family of heterometallic hexanuclear [Ni4M2] (M = Gd, Dy, Y) compounds: observation of slow magnetic relaxation in the DyIII derivative, Inorg. Chem. 51 (2012) 2699–2705. [23] The Schiff base ligand (H2L) was synthesized according to the reported procedures [C. Boskovic, E. Rusanov, H. Stoeckli-Evans, H. U. Güdel, Inorg. Chem. Commun. 5 (2002) 881–886]. Triethylamine (0.2 mmol) was added to a stirring solution of Ho(NO3)3·6H2O (0.1 mmol, 0.0459 g), H2L (0.1 mmol, 0.0243 g) and pivalate sodium (0.1 mmol, 0.0142 g) in the mixed solvent CH3OH (10 ml) and MeCN (10 ml) at room temperature, resulting in a turbid yellow solution. After stirring for 2 h, the yellow precipitate was collected through filtering and washed with cold MeOH. The residue was recrystallized from N,N′-dimethylformamide (DMF), giving a bright yellow solution, which left undisturbed to concentrate slowly by evaporation. X-ray quality crystals of 1 slowly grew over 4 days, which were collected by filtration, washed with cold DMF, and dried under air. The yellow crystals were produced in 36% yield based on Ho. Anal. Calcd for C78H92N8O22Ho4: C, 43.51; H, 4.31; N, 5.20. Found: C, 43.70, H, 4.40, N, 5.20. Selected IR data (ν/cm−1) using KBr disks: 3626(m), 3445(s), 3049(vw), 2952(w), 1658(vs), 1605(s), 1587(m), 1530(m), 1486(vs), 1387(m), 1329(w), 1277(m), 1250(m), 1220(s), 1174(m), 1107(w), 1035(vw), 973(w), 861(vw), 892(w), 823(w), 744(m), 697(w), 577(w), 501(w). [24] Crystal data for 1: C78H92Ho4N8O22, Mr = 2153.32, monoclinic, P21/c, a = 12.903(3) Å, b = 24.552(5) Å, c = 13.538(3) Å, β = 110.10(3)°, V = 4027.6(14) Å3, Z = 2, ρ = 1.776 mg·cm−3, μ = 3.964 mm−1. R = 0.0506 for 6293 reflections with I N 2σ(I) and 0.0601 for 7080 reflections; wR2 = 0.0745 and S = 1.095. Singlecrystal X-ray diffraction experiments were carried out with a Rigaku Saturn CCD diffractometer equipped with a graphite monochromator utilizing Mo-Kα radiation with radiation wavelength 0.71073 Å by using the ω-scan technique. Empirical absorption corrections were applied using the SADABS program. The structure was solved by the direct method and refined by the full-matrix least-squares method on F2, with all non-hydrogen atoms refined with anisotropic thermal parameters. [25] H. Xiang, Y.-H. Lan, H.-Y. Li, L. Jiang, T.-B. Lu, C.E. Ansona, A.K. Powell, Novel mixedvalent CoII2CoIII4LnIII4 aggregates with ligands derived from tris-(hydroxymethyl) -aminomethane (Tris), Dalton Trans. 39 (2010) 4737–4739. [26] S.K. Langley, B. Moubaraki, K.S. Murray, Magnetic properties of hexanuclear lanthanide(III) clusters incorporating a central μ6-carbonate ligand derived from atmospheric CO2 fixation, Inorg. Chem. 51 (2012) 3947–3949. [27] M.U. Anwar, L.K. Thompson, L.N. Dawe, F. Habib, M. Murugesu, Predictable selfassembled [2 × 2] Ln(III)4 square grids (Ln = Dy, Tb)—SMM behaviour in a new lanthanide cluster motif, Chem. Commun. 48 (2012) 4576–4578. [28] N. Zhou, Y. Ma, C. Wang, G.-F. Xu, J.-K. Tang, J.-X. Xu, S.-P. Yan, P. Cheng, L.-C. Li, D.-Z. Liao, A monometallic tri-spin single-molecule magnet based on rare earth radicals, Dalton Trans. 40 (2009) 8489–8492. [29] P.-F. Shi, Y.-Z. Zheng, X.-Q. Zhao, G. Xiong, B. Zhao, P. Cheng, 3D MOFs containing trigonal bipyramidal Ln5 clusters as nodes: large magnetocaloric effect and slow magnetic relaxation behavior, Chem. Eur. J. 18 (2012) 15086–15091. [30] Y.-L. Miao, J.-L. Liu, J.-Y. Li, J.-D. Leng, Y.-C. Ou, M.-L. Tong, Two novel Dy8 and Dy11 clusters with cubane [Dy4(μ3-OH)4]8+ units exhibiting slow magnetic relaxation behaviour, Dalton Trans. 40 (2011) 10229–10236. [31] S.-Y. Lin, G.-F. Xu, L. Zhao, Y.-N. Guo, Y. Guo, J.-K. Tang, Observation of slow magnetic relaxation in triple-stranded lanthanide helicates, Dalton Trans. 40 (2011) 8213–8217. [32] L.F. Chibotaru, L. Ungur, A. Soncini, The origin of nonmagnetic kramers doublets in the ground state of dysprosium triangles: evidence for a toroidal magnetic moment, Angew. Chem. Int. Ed. 47 (2008) 4126–4129. [33] J. Luzon, K. Bernot, I.J. Hewitt, C.E. Anson, A.K. Powell, R. Sessoli, Spin chirality in a molecular dysprosium triangle: the archetype of the noncollinear ising model, Phys. Rev. Lett. 100 (2008) 247205/1–247205/4.