A decanuclear [Dy^III₆Zn^II₄] cluster: a {Zn^II₄} rectangle surrounding an octahedral {Dy^III₆} single molecule magnet

Abstract

An octahedral {Dy^III₆} cage within a diamagnetic {Zn^II₄} rectangle is reported, with magnetic relaxation studies revealing single-molecule magnet behaviour for the complex under zero external dc field with U_eff = 43 K and τ_o = 1 × 10⁻⁵ s.

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The field of lanthanide-based molecular magnetic materials has witnessed a vast growth over the last few years, due to the deeper understanding of the fundamental parameters that govern the magnetic behaviour of the 4f centres. Especially for single molecule magnets (SMMs) or single-ion magnets (SIMs), i.e. molecules that are able to retain their magnetism once magnetized at low temperatures upon removal of the external magnetic field,¹ employment of 4f centres has led to exceptional magnetic properties, due to both the magnitude of the spin, as well as the spin–orbit coupling based magnetic anisotropy that the 4f species possess.² Furthermore, for Dy-based SMMs/SIMs the uniaxial magnetic anisotropy of the Dy atoms plays a crucial role in enhancing the energy barrier, U_eff, needed for the re-orientation of the magnetisation, and thus molecular nanomagnets with U_eff values over 1000 K, and blocking temperatures as high as 60 K have been reported,³ with the best SMM/SIM being the recently reported organometallic complex [(Cp^iPr5)Dy(Cp*)]⁺ (Cp^iPr5 = penta-iso-propylcyclopentadienyl) with an U_eff value of ∼2210 K and a blocking temperature of ∼80 K.⁴ On the other hand, regarding mixed-metal 3d–4f heterometallic complexes as SMM candidates, the situation seems to differ; despite the ongoing research for the construction and characterization of 3d–4f SMMs, the energy barrier for such species remains rather low compared to 4f species, with [Co₂Dy(L^Br)₂(H₂O)]NO₃ (L^Br = 2,2′,2′′-(((nitrilotris(ethane-2,1diyl))tris(azanediyl))tris(methylene))tris-(4-bromophenol)) holding the record of U_eff = 600 K,⁵i.e. ∼1/4 of the highest U_eff reported so far for the best lanthanide SIM/SMM. Such a discrepancy may be attributed to either weak (and most commonly antiferromagnetic) interactions between the 3d and 4f metal centres, leading to low lying split sublevels, or to the perturbation of the 4f local magnetic field due to random transversal secondary magnetic fields created by the nearby paramagnetic metal ions, resulting in quantum tunnelling of the magnetization and, thus, faster magnetic relaxation.⁶

Recently we embarked on a project of constructing heterometallic Zn^II–Ln^III clusters and investigating their magnetic properties, as a means of: (i) protecting the 4f-centres from the influence of 3d paramagnetic centres and (ii) employing a diamagnetic 3d metal atom for structural stability and diversity of the products.⁷ Herein we report our latest finding regarding a decanuclear [Zn^II₄Dy^III₆] complex upon employment of the Schiff-base ligand 2-(β-naphthalideneamino)-2-hydroxymethyl-1-propanol, H₃L (Scheme 1). The reaction of Zn(OAc)₂·2H₂O, Dy(NO₃)₃·5H₂O and H₃L in MeOH in 1 : 1 : 1 ratio under solvothermal conditions, and in the presence of base NEt₃, gave complex [Dy₆Zn₄O₂(L)₂(HL)₂(OAc)₈(CH₃O)₄(H₂O)₂]·4MeOH (1·4MeOH) in good yield, which was characterized by means of X-ray single crystal crystallography.‡

Scheme 1 The ligand and its coordination modes in 1.

Complex 1 crystallizes in the triclinic P1̄ space group (Fig. 1, top). Its metallic core consists of a diamagnetic {Zn^II₄} rectangular unit of ∼6.12 × 6.42 Å dimensions, surrounding a central magnetic {Dy^III₆} slightly “squeezed” octahedron (Fig. 1, bottom). The basal Dy^III ions (Dy2, Dy2′, Dy3, Dy3′) are located ∼3.49 and ∼5.64 Å apart, while the axial Dy^III centres (Dy1, Dy1′) are found ∼1.73 Å above and below the basal plane.

Fig. 1 The crystal structure of 1 (top) and its {Dy^III₆Zn^II₄} metallic core (bottom) highlighting the {Dy^III₆} octahedron. Colour code: Zn^II = yellow, Dy^III = dark-green, O = red, N = blue, C = grey. H-Atoms and solvate molecules are omitted for clarity.

The six metallic centres of the octahedron are held by a combination of two central μ₄-O²⁻ bridges, four monoatomic methoxide bridges, four μ–κ¹O:κ¹O′ acetates and four ligands adopting two coordination modes; two of them are found in the doubly deprotonated form, HL²⁻, with a μ₃–κ³O:κ¹N:κ¹O′:κ¹O′′ coordination mode, while the remaining two are fully deprotonated, L³⁻, adopting a μ₄–κ³O:κ³O′:κ¹N:κ¹O′′ coordination fashion. The linkage of the two metallic sub-units occurs via (i) the deprotonated ligands found in the molecule, (ii) the four methoxide groups, and (iii) two μ–κ¹O:κ¹O′ acetate groups. Finally, two chelate acetates and two terminal water molecules fill the coordination environment of the metallic centres. All Zn centres are five-coordinate adopting square pyramidal geometry (for Zn2/Zn2′, τ = 0.160), and severely twisted square pyramidal/trigonal bipyramidal geometry (for Zn1/Zn1′, τ = 0.546). Regarding the 4f centers, their ideal geometries were found upon performing SHAPE analysis:⁸ Dy2 and Dy3 are eight-coordinate with square antiprismatic (D_4d) geometry (SAPR-8: S_(δi,θi) = 0.958 and S_(δi,θi) = 1.056, for Dy2 and Dy3, respectively), while Dy1 is seven-coordinate with distorted capped trigonal prismatic geometry (C_2ν) (S_(δi,θi) = 11.476 for CTP) (Fig. S1 and S2†). Finally, the {Dy^III₆} octahedron deviates slightly from the ideal octahedral geometry (CShM = 10.649). In the crystal lattice, the molecules pack forming layers stabilized mainly by intermolecular H-bonds between the decanuclear species and the solvate methanol molecules (Fig. 2).

Fig. 2 Crystal packing of 1, highlighting the intermolecular H-bonds (black bold dotted lines). Each colour corresponds to an individual molecule of 1.

DC magnetic susceptibility measurements were performed on 1 in the 2–300 K temperature range under an applied magnetic field of 0.1 T, and the results are plotted as χ_MT vs. T in Fig. 3, with the isothermal magnetisation (M vs. H) curves shown in the inset. The room-temperature χ_MT value of 81.2 cm³ mol⁻¹ K is slightly smaller than the theoretical value of 85.0 cm³ mol⁻¹ K expected for six Dy^III ions (S = 5/2, L = 5, J = 15/2, ⁶H_15/2, g_J = 4/3). Upon cooling, the χ_MT product remains practically unchanged until ∼150 K, below which steadily decreases reaching 41.05 cm³ mol⁻¹ K at 2 K, possibly suggesting the presence of weak antiferromagnetic interactions within the cluster (a Curie–Weiss analysis gave θ = −2.9 K) and/or depopulation of the Dy^III Stark sub-levels. The isothermal magnetization versus B plots increase rapidly, reaching values of 29.2 and 28.5N_μB at 5 T, for T = 2 and 5 K, respectively, significantly lower than the theoretical value of ∼60N_μB for a hexanuclear [Dy^III₆] complex; this is mainly attributed to the presence of magnetic anisotropy and to the depopulation of the Stark sublevels.⁹

Fig. 3 Plot of χ_MT vs. T for 1 in the 2–300 K temperature range, under an applied field of 0.1 T. Inset: M vs. B for 1 in the 0–5 T and 2.0–20.0 K field and temperature ranges.

Given: (i) the large remaining magnetic moment, even at 2 K, and (ii) the square antiprismatic geometry of the Dy2 and Dy3 centres (and their symmetry related) that promotes suitable charge distribution with axial anisotropy on the Dy^III centres,¹⁰ we investigated the AC dynamic magnetic properties of 1. The measurements show temperature and frequency (f) dependent fully-formed in-phase, χ′_M, and out-of-phase, χ′′_M, signals, under zero-applied DC field and a 3.5 AC field oscillating at various frequencies, in the temperature range between ca. 3 and 15 K (Fig. 4). This behaviour seems to indicate slow relaxation of the magnetization, which is typical for a SMM.

Fig. 4 (Top) In-phase, χ′_M, and out-of-phase (bottom), χ′′_M, signals for 1 at various frequencies in the 17–1488 Hz range. Inset: Arrhenius fit of the effective time constants.

The main feature is a cusp in the in-phase component that occurs at approximately 10.0 K for f = 17 Hz, accompanied by a cusp in the out-of-phase component at somewhat lower temperature (Fig. 4). Effective time constants can be obtained from the reciprocal angular frequency at maximum absorption. As typical of a thermal activation process, the zero-field relaxation times so obtained can be approximated by the Arrhenius law τ = τ_o exp(U_eff/k_BT), where τ = (2πf)⁻¹, τ_o is an attempt frequency and U_eff an effective energy barrier. The results are presented in the inset of Fig. 4, affording U_eff = 43 K and τ_o = 1 × 10⁻⁵ s. The AC susceptibility reveals that the magnetic relaxation is particularly complex in 1, since at least another (secondary) relaxation pathway can be spotted at somewhat lower temperature than that of the main feature. In order to visualize the orientation of the anisotropy axis for each Dy^III ion present in 1, we employed the electrostatic model reported by Chilton et al., which is based on electrostatic energy minimization for the prediction of the ground state magnetic anisotropy axis,¹¹ assuming that in the absence of high symmetry the ground-state of Dy^III ions is a doublet along the anisotropy axis with m_J = ± 15/2.¹² Following this approach, the ground state magnetic anisotropy axes in 1 were found to form three pairs from the symmetry-related Dy centres (Fig. 5); for Dy1(Dy1′) the axis is tilted towards the μ₄-O1 oxide atom, and towards O2 belonging to a monoatomic methoxide bridge and O2F belonging to bridging acetate group. For Dy2(Dy2′) the axis is pointing towards the μ₄-O1 oxide atom and towards O2B(O2B′) from a deprotonated aromatic hydroxyl group, while for Dy3(Dy3′) the axis is tilted towards the μ₄-O1′oxide atom and the O2D′ atom of a bridging acetate group.

Fig. 5 Ground state magnetic anisotropy axis for the Dy centers present in 1.

Conclusions

In conclusion, in this work we present the synthesis and characterisation of a novel decanuclear [Dy^III₆Zn^II₄] cluster, upon employment of the Schiff-base ligand 2-(β-naphthalideneamino)-2-hydroxymethyl-1-propanol, H₃L, with the cluster's topology describing a {Zn^II₄} rectangle surrounding a {Dy^III₆} octahedron. Furthermore, investigation of the magnetic properties, revealed possible SMM behaviour with U_eff = 43 K and τ_o = 1 × 10⁻⁵ s. To the best of our knowledge, complex 1 represents the first example of a decanuclear [Dy^III₆Zn₄] single molecule magnet.

Synthetic efforts are currently underway in order to isolate more Zn-4f clusters, as a means of investigating the effect of the diamagnetic ions on the magnetic behaviour of the 4f centres.

Conflicts of interest

The authors wish to declare no conflicts of interest.

Acknowledgements

GL and ME thank MINECO for funding (MAT2015-68204-R).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Full details of the experimental microanalyses and crystallographic data. CCDC 1892887. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt00440h

‡ Crystal data for 1·4MeOH: C₈₃H₁₁₂Dy₆N₄O₄₀Zn₄, M = 3042.24, triclinic, space group P1̄, a = 13.591 (3) Å, b = 13.807 (3) Å, c = 14.560 (3) Å, α = 80.22 (2)°, β = 69.83 (2)°, γ = 77.36 (2)°, V = 2489.2 (10) ų, Z = 1, T = 80 K, R₁ (I > 2σ) = 0.037 and wR₂ (all data) = 0.074 for 37 200 reflections collected, 10 607 observed reflections (I > 2σ(I)) of 14 033 (R_int = 0.039) unique reflections and 634 parameters, GOF = 1.02. CCDC reference number: 1892887.†

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