Constructing [Co^II₆] hexagon-centered heterometallic {Ln₆Co₆} (Ln = Y, Eu and Dy) clusters with a calix[8]arene ligand

Abstract

Three isostructural 3d–4f polynuclear clusters {(Ln^III₆Co^II₆)(C8A)₃} (Ln = Y (CIAC-248), Eu (CIAC-249), Dy (CIAC-250); C8A = p-tert-butylcalix[8]arene) were synthesized under solvothermal conditions. These three compounds feature some tripod-like entities in which three Ln₂-C8A secondary building units acting as the legs bolster a [Co₆] hexagon. Magnetic study indicated that the complex {(Eu^III₆Co^II₆)(C8A)₃} (CIAC-249) exhibits spin glass behaviour at ≈9 K, and the complex {(Dy^III₆Co^II₆)(C8A)₃} (CIAC-250) shows slow relaxation of magnetization.

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Introduction

Heterometallic 3d–4f clusters have attracted considerable attention due to their fascinating structures, interesting magnetic properties such as single-molecule magnet (SMM) behaviour, and potential applications in high-density information storage and quantum computing.^1–3 Commonly, the syntheses of these types of clusters are based on some shielded or multidentate ligands which can act as the magnetic coupling mediator. Calixarenes have been proven to be excellent multidentate ligands to construct single-molecule magnets due to their polyphenolic nature, which can not only induce ferromagnetic or antiferromagnetic coupling efficiently but also assume diverse coordination modes. To date, there have been some reports on the construction of paramagnetic molecular clusters with calixarenes. For example, Dalgarno and Brechin^4–7 applied methylene-bridged calix[4]arene in the construction of SMMs and magnetic coolers. We^8,9 and others^10–13 used thia- and sulfonyl-bridged calix[4]arenes for the preparation of polynuclear clusters possessing interesting magnetic properties. However, to the best of our knowledge, polynuclear clusters based on larger calixarenes such as p-tert-butylcalix[8]arene with SMM properties and ferromagnetic ordering behaviours are rare.^14–16 Compared to calix[4]arene, calix[8]arene, which has more coordination sites, variable conformation and higher flexibility, may generate novel materials with interesting properties.^17,18 In addition, a carbonate anion is also a good bridge due to its various bridging modes, such as μ₂-CO₃²⁻,^19,20 μ₃-CO₃²⁻,^21,22 μ₄-CO₃²⁻,^23,24 and μ₆-CO₃²⁻.^25–27 Furthermore, we found that calix[8]arene adopting a double cone conformation can form a Ln₂-C8A secondary building unit (SBU) when reacting with lanthanide ions and the Ln₂-C8A SBUs can bond some transition metals (TM) to form heterometallic 3d–4f clusters. Here, we present the crystal structures and magnetic properties of three {(Ln^III₆Co^II₆)(C8A)₃} compounds (Ln = Y (CIAC-248), Eu (CIAC-249), Dy (CIAC-250); C8A = p-tert-butylcalix[8]arene) incorporating a central μ₆-carbonate anion. These compounds exhibit ferromagnetic long-range ordering or SMM properties.

Experimental

Starting materials

All starting materials were of analytical reagent grade and used as received without further purification. p-tert-Butylcalix[8]arene was prepared according to the literature method.²⁸

Syntheses of compounds CIAC-248–250

{[(Y₆Co₆)(C8A)₃(CH₃OH)₅(CH₃O⁻)₃(CO₃²⁻)(DMF)₆(CH₃COO⁻)₂]}·2.3H₂O 0.35CH₃OH·[(C₂H₅)₃NH⁺] (CIAC-248). Blue block crystals were obtained by the solvothermal reaction of a mixture of C8A (0.065 g, 0.04 mmol), Co(CH₃COO)₂·4H₂O (0.038 g, 0.15 mmol), Y(CH₃COO)₃·6H₂O (0.04 g, 0.39 mmol), CH₃OH (6 mL), triethylamine (1.1 mL) and several drops of DMF in a 20 mL Teflon-lined autoclave which was kept at 150 °C for 3 days and then slowly cooled to room temperature at about 4 °C h⁻¹. The crystals were isolated by filtration and washed with methanol and dried under air. Yield: 36% with respect to Y. Elemental analysis (%): calcd for [C₂₉₂H₃₉₀Y₆Co₆N₆O₄₅] (excluding disordered solvent molecules): C, 62.73; N, 1.50; H, 6.98. Found: C, 62.23; N, 1.31; H, 6.31. FT-IR (KBr pellet, cm⁻¹): 3527(w), 2955(s), 1650(s), 1569(w), 1454(s), 1387(m), 1360(m), 1293(s), 1212(s), 1125(w), 1044(m), 909(w), 875(w), 822(m), 754(m), 674(w).

{[(Eu₆Co₆)(C8A)₃(CH₃OH)₅(CH₃O⁻)₃(CO₃²⁻)(DMF)₆(CH₃COO⁻)₂]}·2H₂O·[(C₂H₅)₃NH⁺] (CIAC-249). The procedure was similar to the synthesis of CIAC-249 except that Y(CH₃COO)₃·6H₂O was substituted with Eu(CH₃COO)₃·6H₂O. The crystals were isolated as blue block crystals in 32% yield based on Eu. Elemental analysis (%): calcd [C₂₉₂H₃₉₀Eu₆Co₆N₆O₄₅] (excluding disordered solvent molecules): C, 58.76; N, 1.41; H, 6.54. Found: C, 58.49; N, 1.32; H, 6.65. FT-IR (KBr pellet, cm⁻¹): 3528(w), 2953(s), 1656(s), 1466(s), 1396(m), 1358(m), 1293(s), 1207(s), 1125(w), 1027(m), 908(w), 876(w), 821(m), 745(m), 685(w).

{[(Dy₆Co₆)(C8A)₃(CH₃OH)₅(CH₃O⁻)₃(CO₃²⁻)(DMF)₆(CH₃COO⁻)₂]}·3H₂O·CH₃OH·[(C₂H₅)₃NH⁺] (CIAC-250). The procedure was similar to the synthesis of CIAC-248 except that Y(CH₃COO)₃·6H₂O was substituted by Dy(CH₃COO)₃·6H₂O. The crystals were isolated as blue block crystals in 32% yield based on Dy. Elemental analysis (%): calcd for [C₂₉₂H₃₉₀Dy₆Co₆N₆O₄₅] (excluding disordered solvent molecules): C, 58.14; N, 1.39; H, 6.49. Found: C, 58.02; N, 1.28; H, 6.39. FT-IR (KBr pellet, cm⁻¹): 3528(w), 2953(s), 1656(s), 1467(s), 1396(m), 1358(m), 1293(s), 1207(s), 1119(w), 1038(m), 914(w), 870(w), 815(m), 750(m), 674(w).

X-ray structure determination

The X-ray intensity data for compounds CIAC-248–250 were collected on a Bruker D8 QUEST system with Cu Kα radiation (λ = 0.15418 nm) operated at 50 W (50 kV, 1 mA). The crystal structures were solved by means of direct methods and refined employing full-matrix least squares on F² (SHELXTL-97).²⁹ The large R₁ and wR₂ factors of compounds CIAC-248–250 might be due to the weak high-angle diffractions and the disorder of p-tert-butyl groups. Hydrogen atoms of the organic ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. CCDC 2013506–2013508† contain the supplementary crystallographic data for this paper.

Physical measurements

Elemental analysis of C, N, and H was performed using a Vario EL instrument. TGA measurement was performed using a NETZSCH STA 449F3. FT-IR spectra were obtained with a Bruker Vertex 70 spectrometer. Magnetic susceptibility measurements were carried out with a Quantum Design MPMS-XL7 SQUID magnetometer. Freshly prepared crystalline samples were embedded in eicosane to avoid any field-induced crystal reorientation. Diamagnetic corrections were applied for eicosane and for the molecule, the latter being calculated from the Pascal constants. X-ray powder diffraction (XRD) measurements were performed on a D8 Focus diffractometer (Bruker) ranging from 0°–40° with Cu Kα radiation (λ = 0.15418 nm).

Results and discussion

Single-crystal X-ray diffraction analysis reveals that compounds CIAC-248–250 are isostructural and feature a tripod-like unit composed of a wrapped {Co₆} hexagon and three Ln₂-C8A SBUs (Fig. 1). So compound CIAC-250 is described in detail as an example. Complex CIAC-250 crystallizes in the monoclinic system with the space group C2/c. In the structure, three p-tert-butylcalix[8]arene molecules adopt the double cone conformation. Each conic subunit of the calix[8]arene molecule binds a Dy atom through its oxygen atoms at the lower rim so that a Dy₂-C8A secondary building unit (SBU) is formed, which is a common unit found in the compounds of 4f/5f metals and calix[8]arene. Six adjacent Co atoms are bonded to the same μ₆-CO₃²⁻ anion in situ generated from the decomposition of DMF to form a {Co₆} hexagonal plate. Furthermore, this hexagon is bolstered by three Dy₂-C8A SBUs to form a tripod-like {(Ln^III₆Co^II₆)(C8A)₃} entity (Fig. 2 and 3). It should be noted that all three calix[8]arene molecules in an isolated entity are nearly perpendicular to the planar {Co₆} hexagon (Fig. S3†). The extended structure of compound CIAC-250 is stacked by these dodecanuclear heterometallic units through supramolecular interactions (Fig. S4†).

Fig. 1 Molecular structure of compound CIAC-250. Hydrogen atoms and solvent molecules are omitted for clarity. Blue: Co atoms; green: Dy atoms; red: O atoms; gray: C atoms.

Fig. 2 Scheme of the assembly of compound CIAC-250.

Fig. 3 Arrangement of the metal ions of compound CIAC-250.

For the Ln^III₆Co^II₆ core, there are twelve crystallographically independent metal sites which have three types of coordination. Each Co atom is five coordinated in a distorted trigonal bipyramid coordination geometry. The coordination geometries of Dy(III) ions were calculated by SHAPE analysis (Table S2†).^30,31 Site Dy1 is seven coordinated in a capped octahedron with C_3v local symmetry. Sites Dy2, Dy4 and Dy5 are also seven coordinated, but in a capped trigonal prism with C_2v local symmetry. Sites Dy3 and Dy6 are eight coordinated in a biaugmented trigonal prism J50 with C_2v local symmetry. The distances of the Dy–O and Co–O bonds are in the range of 2.18–2.89 Å and 1.93–2.13 Å, respectively. The O–Dy–O and O–Co–O angles fall in the ranges of 52.7–159.7° and 74.8–178.6°. It is noted that the 3d–4f heterometallic compounds having a μ₆-CO₃²⁻ bridging-[Co₆] ring are documented for the first time.

Magnetic properties

The variable-temperature direct current (dc) magnetic susceptibility data for all compounds were collected under a 1 kOe applied field at the temperature range of 2–300 K (Fig. 4), and the field (H) dependence of the magnetizations (M) for these three complexes was measured up to 7 T at low temperatures (Fig. S6–S8†). The χT value for CIAC-248 is 19.88 emu K mol⁻¹ at room temperature, which is larger than the spin-only value (11.25 emu K mol⁻¹) expected for six uncoupled Co(II) ions (S = 3/2, g = 2.0), owing to the contribution from the orbital angular momentum. Upon cooling, the χT value decreases slowly and reaches 5.75 emu K mol⁻¹ at 2 K, presumably corresponding to the depopulation of higher Kramers doublets of the Co(II) centers and possible antiferromagnetic coupling between Co(II) ions. The magnetization for CIAC-248 increases slowly with the field and reaches a maximum value of 6.30 μ_B at 2 K and 7 T.

Fig. 4 The χT versus T plots for CIAC-248, 249 and 250 under a 1 kOe dc field. The solid line is the best fit.

For CIAC-249, although the Eu(III) ion (J = 0, ⁷F₀) possesses a diamagnetic ground state, the χT value at 300 K (31.10 emu K mol⁻¹) is much larger than that of CIAC-248. Such a large value is due to a certain amount of magnetic moment from the excited states (e.g., ⁷F₁) of the Eu(III) ion. The χT value is about 1.87 emu K mol⁻¹ for each Eu ion at room temperature based on complex CIAC-248, which is close to the reported value.³² The χT values decrease slowly as the temperature decreases to 12.56 emu K mol⁻¹ at 22 K, followed by a sharp increase to a maximum value of 26.81 emu K mol⁻¹ at 8.7 K, which indicates ferromagnetic interactions in the {Eu₆Co₆} cluster, and then drop to 10.03 emu K mol⁻¹ at 2 K. The field dependence of magnetizations for CIAC-249 gives a small value of 5.98 μ_B at 2 K and 7 T, which is close to those of CIAC-248, in accordance with the diamagnetic ground state of Eu(III) ions.

For CIAC-250, the χT value at room temperature (97.56 emu K mol⁻¹) is consistent with the expected value of six uncoupled Dy(III) ions (S = 5/2, L = 5, g = 4/3, ⁶H_15/2) and six isolated Co(II) ions. The χT value is smaller than the expected value with the orbital angular momentum from Co(II) ions, owing to the antiferromagnetic coupling between Dy(III) and Co(II). The χT values decrease gradually from 300 to 50 K and much rapidly below 50 K. At 2 K, the χT value is 54.55 emu K mol⁻¹ and the maximum M value is 34.82 μ_B at 7 T. This unsaturation M value indicates the presence of magnetic anisotropy and/or low-lying excited states in the system. The program PHI was used to fit the magnetic susceptibility of CIAC-248 with a minimal model (eqn (1)),³³ which can reproduce the temperature-dependent susceptibility at the temperature range from 50 K to 300 K (Fig. 4) and provide a reasonable estimate of the intramolecular magnetic exchange coupling (J) between the Co(II) ions. The best simulation gives J = −3.15 cm⁻¹ with g = 2.73. Attempts for fitting the lower temperature data failed, probably due to the interplay and competition of spin-orbital coupling and magnetic interactions. The alternating current (ac) susceptibility measurement of CIAC-248 at the frequency of 1488 Hz with a zero dc field displays a paramagnetic property (Fig. S9†), thus showing neither magnetic ordering nor slow relaxation of the magnetization above 2 K.

As complex CIAC-249 shows a peak at 8.7 K in its χT–T plots, further magnetic investigation was performed. The zero-field cooled (ZFC) and field-cooled (FC) magnetic susceptibility for CIAC-249 was recorded under a field of 50 Oe. Both the peak of the ZFC curve and the divergence of the ZFC and FC curves appear at about 9.2 K (Fig. 5), indicating a transition from a paramagnetic state to either a long-range ordering, a spin-glass state or a superparamagnetic state.³⁴ Magnetic hysteresis measurements show a clear hysteresis loop at 2 K with a coercivity field of 580 Oe and a remnant magnetization of 0.38μ_B, and a closed hysteresis loop at 9 K, indicative of a magnet behavior (inset in Fig. 5 and Fig. S13†).

Fig. 5 χ–T plots of CIAC-249 under field-cooled (FC) and zero-field-cooled (ZFC) conditions with a field of 50 Oe. Inset: Field-dependent magnetizations for complex CIAC-249 from 0 to 50 kOe at 2 K.

Ac magnetic susceptibility measurements for CIAC-249 were further carried out under a zero dc field. Wide peaks of the in-phase (χ′) and out-of-phase (χ′′) signals were observed between 8 K and 10 K with a very weak frequency dependence, supporting the existence of spin glassy behavior (Fig. 6). The parameter φ = (ΔT_p/T_p)/(Δlog f) was estimated as 0.02, which falls into the range of a spin glass and excludes a superparamagnet.^35,36 We tried to fit the relaxation times at various temperatures via the Arrhenius law (τ = τ₀ exp(U_eff/k_BT), where τ₀ is a pre-exponential factor and U_eff is the energy barrier), and obtained ΔE = 1176 K and τ₀ = 2.43 × 10⁻⁵⁷ s (Fig. S10†). This rather fast τ₀ also excludes the possibility of an SMM. Those behaviours are similar to other Co clusters with spin glass behaviours.^37,38 For CIAC-249, the spin glassy behavior probably is attributed to the weak magnetic moment from Eu(III) ions that modified the magnetic coupling between Co(II) ions. Every Eu₂ C8A SUB bridges two Co(II) ions (Co₂ dimer), and three Co₂ dimers result in a triangular structure. Such a triangular geometry often leads to spin frustration with spin-glass ground state.

Fig. 6 Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibility signals under a zero dc field at the indicated frequencies for CIAC-249.

The magnetic dynamics for CIAC-250 was studied using ac magnetic susceptibility measurements under a zero dc field. As shown in Fig. 7, χ′ signals diverge at nearly 30 K without reaching the maximum (except for high frequencies) and splitting tails of χ′′ signals were observed above 2 K, indicating the slow relaxation of magnetization, a characteristic SMM behaviour, but with the existence of fast quantum tunneling of magnetization (QTM). The parameter φ is calculated as 0.51, which is larger than the value for a spin glass, but close to a superparamagnetic behaviour. The relaxation times (τ) at different temperatures for CIAC-250 are evaluated from variable-frequency data via a generalized Debye model (Fig. S11 and S12†).³⁹ The obtained ln(τ) values remain linear between 5.4 and 3.5 K, showing a thermally activated process, which can be fitted by the Arrhenius law, giving U_eff = 41.5 K and τ₀ = 8.50 × 10⁻⁸ s (inset in Fig. 7). A slight deviation occurs as the temperature further decreases, which could be attributed to strong QTM at low temperature.

Fig. 7 Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibility signals under a zero dc field at the indicated frequencies for CIAC-250. The lines are visual guides only. Inset: Plots of ln(τ) versus 1/T for CIAC-250. The solid line is the best fit.

To suppress the QTM, ac magnetic susceptibilities were measured under a dc field of 500 Oe from 2 K to 12 K (Fig. 8), which was fitted with a generalized Debye model (Fig. 9).³⁵ The α parameters range from 0.50 to 0.64, indicating a wide contribution for the relaxation times. The obtained ln(τ) values remain linear above 6 K and with slight curvatures at low temperature, but do not become temperature independent. The linear regime at high temperature is often known as the Orbach relaxation process, while for low temperature it can be regarded as the Raman relaxation process because it is not temperature independent. So the Orbach process was combined with the Raman process to fit the relaxation times, giving C = 3.0(5) s⁻¹ Kⁿ, n = 2.6(1), τ₀ = 3(2) × 10⁻⁶ s and U_eff = 56(9) K (Fig. 9).

Fig. 8 Frequency dependence of χ′ and χ′′ ac susceptibility signals under a 500 Oe dc field at the indicated temperatures for CIAC-250. The solid lines are the best fits.

Fig. 9 Plots of ln(τ) versus 1/T for CIAC-250 under a 500 Oe dc field. The solid line is the best fit.

Conclusions

In conclusion, we have synthesized {(Ln^III₆Co^II₆) (C8A)₃} clusters featuring a wrapped [Co₆] hexagon linked to three calix[8]arene molecules in a double-cone conformation. Interestingly, the {Eu₆Co₆} cluster shows spin glass behaviours below ≈ 9 K, while the {Dy₆Co₆} cluster exhibits SMM behaviour with U_eff of 41.5 K under a zero dc field (56(9) K under a 500 Oe dc field). This result emphasized that larger calix[n]arene (n = 6, 7, 8) with more coordination sites and larger molecules will provide further opportunities for controlling the self-assembly and tuning of the magnetic properties. Exploration of another transition metal, Ln, and mixed transition-metal/Ln clusters supported by methylene-bridged calix[8]arenes is underway to generate other novel functional materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (no. 21971233, 21620102002, 21503155 and 21473129), the Science and Technology Development Plan of Jilin Province (20150520009JH), the “National 1000 Young Talents” Program, and the Wuhan National High Magnetic Field Center (2015KF06).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Additional figures, UV-vis spectra, TG/DSC curves and FT-IR spectra. CCDC 2013506–2013508. For ESI and crystallographic data in CIF or other electronic format, see DOI: 10.1039/d0qi00792g

‡ These authors contributed equally.

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