Heterometallic thiacalix[4]arene-supported Na₂Ni^II₁₂Ln^III₂ clusters with vertex-fused tricubane cores (Ln = Dy and Tb)

Abstract

Two heterometallic thiacalix[4]arene-supported complexes possess a trinary-cubane core composed of one [Ni₂Ln₂] cubane unit and two [NaNi₂Ln] cubane units sharing one Ln^III ion (Ln = Dy and Tb). Only the Dy^III complex exhibits slow magnetic relaxation behaviour of single molecule magnet nature.

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The design and construction of transition-lanthanide (3d–4f) heterometallic complexes has attracted considerable attention in the past two decades because of their fascinating architectures¹ and their potential applications as functional materials in magnetism,^2–4 luminescent materials,⁵ absorption materials⁶ and bimetal catalysts.⁷ As a result of the cooperativity between the 3d–4f metal ions, significant magnetic behaviors have been documented such as single molecule magnets (SMMs),² single chain magnets (SCMs)³ and magnetic refrigerants.⁴ However, the preparation of designed heterometallic aggregates is still a formidable challenge owing to the variable and versatile coordination behaviour of lanthanide ions and other factors. As is widely known, the critical factor for the construction of heterometallic 3d–4f complexes is the rational choice of organic building block.

On the other hand, calix[4]arene derivatives and thiacalix[4]arene derivatives with functional groups and molecular backbones are most pivotal in the structural regulation of resultant crystalline materials.^8–10 In particular, thiacalix[4]arene derivatives with four additional bridging sulfur atoms can bind to up to four metal ions simultaneously forming metal₄-thiacalix[4]arene building blocks to support various polynuclear clusters. To date, several homometallic thiacalix[4]arene-supported complexes have been obtained.^9,10 However, few 3d–4f mixed-metal complexes involving (thia)calix[4]arenes have been reported.^8a–c,9a,b To the best of our knowledge, Mn^III₄Ln^III₄ are the largest 3d–4f aggregates sustained by (thia)calix[4]arenes so far.^8b For the reasons above and along the line of our previous research on creating thiacalix[4]arene-based complexes with interesting magnetic properties, we have tried to expand the uses of carbonato anion in synthesizing thiacalix[4]arene-supported 3d and/or 4f heterometallic clusters employing sodium carbonate as a reactant. In the present work, we have successfully obtained two new high nuclearity 3d–4f heterometallic clusters with vertex-fused tricubane cores: [Na₂Ni^II₁₂Ln^III₂(BTC4A)₃(μ₇-CO₃)₃(μ₃-OH)₄(μ₃-Cl)₂(OAc)₆(dma)₄]·2OAc·0.5dma·3MeCN·8DMA (Ln = Dy for 1, and Tb for 2; H₄BTC4A = p-tert-butylthiacalix[4]arene; dma = dimethylamine and DMA = N,N′-dimethylacetamide). Herein the syntheses, structures and magnetic properties of complexes 1 and 2 are presented.

X-Ray analysis reveals that both 1 and 2 crystallize in the monoclinic system with space group P2₁/c.‡ The two thiacalix[4]arene-supported clusters are isomorphous. Taking complex 1 as representative, the structure possesses a heterometallic Na₂Ni^II₁₂Dy^III₂ core capped by three BTC4A⁴⁻ ligands (Fig. 1). Within complex 1, both sodium ions are five-coordinated in a distorted square-pyramidal geometry, while each Ni^II ion is six-coordinated with distorted octahedral geometry. In addition, Dy1 and Dy2 are ten-coordinated with a distorted bicapped square-antiprism geometry and eight-coordinated with a distorted square-antiprism geometry, respectively. The two Dy^III centers are connected by two carbonato and one acetate anions. Every four Ni^II ions simultaneously bond to the lower-rim phenoxy oxygens and bridge sulfur atoms of one fully deprotonated BTC4A⁴⁻ ligand leading to a shuttlecock-like building block of Ni^II₄-BTC4A, in which one carbonato anion acts as the cork base. Three subunits are linked together in an up-to-up fashion through four cations (two Na⁺ ions and two Dy^III ions) along with other anions (including two Cl⁻ anions, four OH⁻ anions and three OAc⁻ anions), leading to a pseudo-trigonal planar entity with heterometallic Na₂Ni^II₁₂Dy^III₂ cluster. All carbonato anions bind to seven metal centres with the same chelating and bridging configurations, which can be represented as [7.2₁₂3₃₄₅3₅₆₇] according to Harris notation (Fig. 2a),¹¹ different from the reported [6.222]^10b or [5.2₁₂2₃₄2₄₅]^10c within the thiacalix[4]arene-supported complexes. It is noteworthy that this coordination mode has not been reported heretofore.

Fig. 1 Molecular structure of complexes 1 (Ln = Dy) and 2 (Ln = Tb); hydrogen atoms are omitted for clarity.

Fig. 2 (a) Coordination modes of carbonato anions within 1 and 2 indicated by the Harris notation [7.2₁₂3₃₄₅3₅₆₇]. (b) [NaNi₂Ln] cubane unit and [Ni₂Ln₂] cubane unit of 1 and 2. (c) The heterometallic Na₂Ni^II₁₂Ln^III₂ cluster possessing vertex-fused tricubane core and six peripheral Ni^II ions connected through three μ₇-carbonato anions.

Further analysis shows that there is an unprecedented trinary-cubane core composed of one [Ni₂Dy₂] cubane unit and two [NaNi₂Dy] cubane units, which share one Dy^III ion at the center of the Na₂Ni^II₁₂Dy^III₂ cluster (Fig. 2b and c). To the best of our knowledge, the discrete cubane-like heterometallic systems reported so far are restricted to a few 3d–4f systems. This [NaNi₂Dy] cubane core presents the first example which consists of more than two metal elements. In addition, the vertex-fused tricubane topology is a very rare structural type. There is no previous example of such a discrete unit in heterometallic chemistry, but two 3d–4f examples with tricubane cores have been reported: three [Ce₂Mn₂] cubanes sharing a trigonal-bipyramidal unit {Ce₂(OH)₃} in the centre¹² or three [Gd₂Cu₂] cubanes sharing three Gd^III ions in a triangular way.¹³ The heterometallic cluster of 1 can be viewed as a tricubane connects to six peripheral Ni^II ions through three μ₇-carbonato anions (Fig. 2c). Three BTC4A⁴⁻ ligands are located on the trigonal plane of the tricubane core. Although there has been a report with three calixarenes bonded to one Cu^II₉ cluster, those three p-tert-butylcalix[4]arene ligands lie on the trigonal plane.¹⁴

All the Ni–N, Ni–O, Ni–Cl, Ni-S, Na–N, Na–O, Na–Cl and Dy–O bond distances are located in the normal bond length range.^1d,10b,15 The dma molecules were generated through the decarbonylation of DMA.^10b The Ni⋯Ni distances are 3.07–3.73 Å, while the Ni⋯Dy distances are 3.46–3.69 Å within 1. Analysis of the bond lengths, charge balance and bond valence sum calculations (BVS) suggests all Ni and Ln ions of complexes 1 and 2 to be Ni^II and Ln^III.¹⁶ Two CH₃CN and one DMA molecules penetrate slantwise into three thiacalix[4]arene cavities stabilized by C–H⋯π interactions. In addition, one acetate counter anion locates above the aforementioned tricubane core via hydrogen bonds (Fig. S1†). Upon crystal packing, complex 1 exhibits a bilayer structure in which the pseudo-trigonal planar entities sit in an up–down fashion. The interstices of the lattice are occupied by the solvent molecules and acetate counter anions (Fig. S2†).

The temperature dependent of magnetic susceptibilities measured on the polycrystalline samples of 1 and 2 under H_dc = 1000 Oe were displayed in Fig. 3a. At 300 K, the χ_mT values (40.53 and 36.48 cm³ K mol⁻¹ for 1 and 2) are in good agreement with the theoretical values of 40.34 and 35.64 cm³ K mol⁻¹ for the two non-interacting Ln^III ions (Dy: ⁶H_15/2, g_J = 4/3; Tb: ⁷F₆, g_J = 3/2) and twelve Ni^II ions (S = 1, g = 2).¹⁷ As the temperature is lowered, the χ_mT value decreases continuously and falls rapidly in the lower temperature region, reaching 16.89 and 13.54 cm³ K mol⁻¹ for 1 and 2 at 2 K, respectively. The Curie–Weiss fit of the data above 50 K give the Weiss constants θ = −13.52 and −16.09 K for complexes 1 and 2, respectively (Fig. 3a). The negative θ values and the decrease of the χ_mT values at high temperature could be ascribed to two sources, the antiferromagnetic interaction between the spin carriers and the thermal depopulation of the Stark levels of the Dy^III and Tb^III centers.

Fig. 3 (a) Temperature dependence of magnetic susceptibilities of 1 (Dy) and 2 (Tb) in a 1000 Oe field. The solid lines are the best fitting to the Curie–Weiss Law. (b) Temperature dependence of the in-phase (top) and out-of phase (bottom) components of the ac magnetic susceptibility for complex 1 in a static applied dc field of 2000 Oe and an ac field of 3 Oe.

Upon increasing the applied external magnetic field, the magnetizations of 1 and 2 increase to 15.7 and 16.0 Nβ at 60 kOe, far below the saturation values of 44 and 42 Nβ (Fig. S3†).^17a No obvious hysteresis loop could be observed for either complex at 2 K (Fig. S3†). In addition, temperature dependent ac susceptibilities were measured under zero dc field for 1 and 2; and slow relaxation of magnetization was observed only in 1 (Fig. S4 and S5†). The frequency dependent ac susceptibilities under zero dc field were also measured for 1 (Fig. S6†), from which the relaxation time at different temperatures were evaluated. An estimation of the energy barrier U_eff = 8.4 K and the pre-exponential factor τ₀ = 6.8 × 10⁻⁷ s can be obtained from the Arrhenius fit of the τ values (Fig. S6† inset). Furthermore, Cole–Cole plots (Fig. S7†) have also been obtained. The analyses of the plots according to the generalized Debye functions give α values of 0.18–0.31 above 2.2 K, indicating the presence of a relatively narrow width of the distribution of slow relaxation. However, the α value at 2 K increases sharply to about 0.75, indicating the presence of quantum tunnelling at lower temperature. The quantum tunnelling could be partly suppressed by the application of an external dc field, as can be seen from the shift of the peaks of the ac signals toward higher temperatures measured under a 2000 Oe dc field (Fig. 3b). All these magnetic parameters clearly evidence the SMM nature of this Dy-containing complex 1. Given that 1 and 2 are isomorphous, it appears that the necessary magnetic anisotropy is contributed by the Dy^III ions.^17b One possible reason might be the fact that the ground state of the Kramers ion Dy^III is always degenerate while this is not the case for Tb^III ion.

In conclusion, two isomorphous heterometallic complexes 1 and 2 have been synthesized and structurally characterized. In the presented structures, three shuttlecock-like Ni^II₄-BTC4A subunits are linked together in an up-to-up fashion through two Na⁺ ions and two Ln^III ions, along with other anions, leading to a pseudo-trigonal planar entity. Within this entity, there is a trinary-cubane core composed of one [Ni₂Ln₂] cubane unit and two [NaNi₂Ln] cubane units sharing one Ln^III ion. It should be noted that a cubane unit possessing more than two metal elements has not been reported to date. Magnetic studies reveal that only the Dy^III complex shows the slow relaxation of the magnetization expected for SMM behaviour. This work shows that thiacalix[4]arenes can indeed lead to high-nuclearity heterometallic clusters with intriguing structure and interesting magnetic properties in the presence of ancillary anions. Our efforts to prepare isotypic heterometallic complexes are ongoing.

We thank 973 Program (2011CBA00507, 2011CB932504), National Natural Science Foundation of China (21131006, 20231020, 20971121, 21101093) and the Natural Science Foundation of Fujian Province for funding this research. The authors are grateful to Prof. Xiaoying Huang for assistance with the crystallographic studies and Prof. Zhangzhen He for valuable advice and discussions.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Syntheses, crystallographic information, supplementary figures, PXRD of complexes 1 and 2. CCDC 874210 and 874211. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc32360e

‡ Crystal data for complexes 1/2: C₁₈₆H_272.5N_15.5O₄₉Cl₂S₁₂Na₂Ni₁₂Dy₂/Tb₂, M = 5040.73/5033.58, monoclinic, space group P2₁/c, a = 21.546(2)/21.608(5), b = 30.412(3)/30.507(7), c = 36.028(4)/36.110(9) Å, β = 99.774(2)/99.870(5)°, V = 23265(4)/23451(10) ų, Z = 4, D_c = 1.439/1.437 g cm⁻³, F₀₀₀ = 9496/9488, λ = 0.71073 Å, T = 120(2) K, 2θ_max = 52.0/52.0°, 191383/183986 reflections collected, 45039/45032 unique (R_int = 0.0420/0.0604). Final GooF = 1.076/1.077, R₁ = 0.0814/0.0938, wR₂ = 0.2348/0.2446, R indices based on 41840/37462 reflections with I > 2σ(I) (refinement on F²). The diffraction data were treated by the ‘‘SQUEEZE’’ method as implemented in PLATON¹⁸ to remove diffuse electron density associated with the badly disordered solvent molecules. This had the effect of dramatically improving the agreement indices.

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