Rare-earth chalcogenidotetrachloride clusters (RE₃ECl₄, RE = Dy, Gd, Y; E = S, Se, Te): syntheses and materials properties

Abstract

Rare-earth-containing materials featuring rare-earth–chalcogen (RE–E) coordination are of interest as optical and magnetic materials with enhanced properties over their counterparts with O-coordination. Herein, a series of rare-earth chalcogenidotetrachloride complexes of the general formula [η⁵-Cp*RE(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (RE₃ECl₄: RE = Dy, Gd, Y; E = S, Se, Te; Cp* = pentamethylcyclopentadienide) were reported. Apart from for the identity of the rare-earth and chalcogen elements, these clusters share a common core motif composed of triangularly arranged RE atoms with one chloro ligand bridging each neighboring pair of RE atoms; the trimetallic plane is additionally capped by triply bridging chloro and chalcogenide ligands, one for each kind. Each RE atom is further coordinated with one Cp* and one THF ligand. Comparative magnetic and luminescence studies revealed fine but definitive differences due to using different metal and chalcogen elements. Experimental and computational studies have been used to gain insights into the dipolar and super-exchange interactions for the magnetically active Dy- and Gd-based clusters.

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Introduction

Rare earth (RE) chalcogenides represent a class of advanced functional materials known for their unique optical, electronic, magnetic, and catalytic properties.^1–4 For instance, EuS and Ln₂S₃ (Ln = Gd, Ce) are exemplary magnetic semiconductors and photocatalytic materials, respectively.^5–12 EuSe and LaSe₂ are notable magneto-optical and transparent conductive materials;^13–18 EuTe₂, Pr_2.74Te₄, and RETe₃ (RE = Y, La–Dy) are exemplary quantum materials utilized in magnetic semiconductors, magnetic superconductivity, thermoelectricity, and charge density waves, respectively.^19–26

In this context, atomically precise rare-earth chalcogenide complexes, with their definitive molecular compositions and structures, are propitious for establishing the structure–property relationship. More importantly, they may serve as precursors for the rational design of solid-state rare-earth chalcogenide materials.^27–33 Thus, the diversity in composition, structure, interesting materials properties, and ultimately the potential applications of rare-earth chalcogenides provide a strong impetus for the research of rare-earth chalcogenide complexes.

However, this branch of rare-earth coordination chemistry has lagged far behind the chemistry of O-based ligands. As an interesting and revealing note, a search into the database of current Cambridge Crystallographic Data Centre produced less than 1500 hits of RE–E (E = sulfur (S), selenium (Se), and tellurium (Te)) complexes versus more than 53,283 hits for complexes featuring RE–O bonds.³⁴ On one hand, the mismatch between the hard Lewis acidic RE ions and the soft chalcogenide ligands is primarily responsible for the challenges in the preparation of this particular family of rare-earth complexes. On the other hand, the strong oxophilicity of RE metal ions presents a real technical challenge as any rare-earth chalcogenide complexes are naturally air- and moisture-sensitive. Yet another challenge in making rare-earth chalcogenides is forming the chalcogenide ligands; alkali-metal chalcogenide salts are insoluble in common organic solvents and thus, synthetically inapplicable. The most widely adopted approach to obtaining RE–E complexes is by the reduction of elemental sulfur, selenium, or tellurium with divalent RE complexes.^35–43 However, most divalent RE complexes, with the exception of those of Sm(II), Eu(II), and Yb(II), are highly unstable and hard to be isolated. As such, in general, this method is hard to apply.

We have recently developed a generally applicable method for the production of rare-earth telluride cluster complexes.⁴⁴ The multifarious tellurido ligands present in the cluster core are produced in situ by the reduction of element Te with KC₈. More recently, we showed that together with NaBH₄, rare-earth pentamethylcyclopentadienyl borohydrides (Cp*RE(BH₄)₂(THF)₂) – produced in situ by the reaction between Cp*RECl₂ and NaBH₄ – is capable of reducing Te powder, and the resulting Te²⁻ ligands organize five Dy(III) ions into a trigonal bipyramidal core of Dy₅Te₆ in the pentanuclear cluster anion of [(η⁵-Cp*Dy)₅(μ₃-Te)₆]²⁻ (Cp* = pentamethylcyclopentadienyl).⁴⁵ In this work, the reduction of elemental S, Se, and Te with NaBH₄ has been extended for the formation of a series of rare-earth chalcogenidotetrachlorides of the general formula [η⁵-Cp*RE(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (RE₃ECl₄: RE = Dy, Gd, Y; E = S, Se, Te) (Scheme 1). All nine cluster complexes were fully characterized including structure determination by crystallographic studies. Comparative studies of these isostructural clusters reveal fine yet definitive differences in their photoluminescence and magnetic properties; such disparities can be correlated with the systematically altered structures arising from the different RE and/or E elements used.

Scheme 1 Syntheses of [η⁵-Cp*RE(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (RE₃ECl₄; Cp* = pentamethylcyclopentadienyl; RE = Dy, Gd, Y; E = S, Se, Te).

Experimental

Materials and physical measurements

All manipulations (unless otherwise stated) were carried out under an atmosphere of argon using standard Schlenk techniques on a dual vacuum/inlet manifold or in a glovebox due to the air and moisture sensitivity of the samples. Glassware was dried overnight at 120 °C before use. Anhydrous RECl₃ (RE = Dy, Gd, Y)⁴⁶ and NaCp*⁴⁷ were synthesized by following previously published procedures. NaBH₄ was purchased from Sigma-Aldrich, while all other reagents, including solvents, were obtained from Energy-Chemical and used as received. Elemental analyses (CHN) were performed on a Carlo Erba EA1110 automatic analyzer. ¹H NMR spectra were obtained in CDCl₃ solutions at room temperature using an AVANCE III HD 400 MHz spectrometer.

Syntheses of [η⁵-Cp*RE(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (RE₃ECl₄: RE = Dy, Gd, Y; E = S, Se, Te).
Dy₃SCl₄ . In an argon-filled glovebox, solution A containing DyCl₃ (403.3 mg, 1.500 mmol) and NaCp* (237.3 mg, 1.500 mmol) in 2.5 mL of THF was mixed with solution B obtained by dissolving sulfur (16.0 mg, 0.50 mmol) and NaBH₄ (37.8 mg, 1.00 mmol) in 2.5 mL of THF. The resulting mixture was stirred at room temperature for 24 h and then centrifuged (5000 rpm, 5 min) to obtain a bright yellow filtrate. Bright yellow single crystals of Dy₃SCl₄ were obtained upon standing of the concentrated filtrate (ca. 3 mL) at room temperature (80 mg, 12% based on DyCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Dy₃O₃S: C, 39.31; H, 5.42. Found: C, 39.19; H, 5.56.
Dy₃SeCl₄ . The synthesis followed a similar procedure to that of Dy₃SCl₄ with the use of Se powder (39.5 mg, 0.50 mmol) in place of S. The desired product was obtained as a polycrystalline solid (75 mg, 11% based on DyCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Dy₃O₃Se: C, 37.92; H, 5.23. Found: C, 37.48; H, 5.63.
Dy₃TeCl₄ . The synthesis followed a similar procedure to that of Dy₃SCl₄ using Te powder (63.8 mg, 0.50 mmol) instead of S. However, the Te powder did not dissolve like S and Se powder in preparing the S- and Se-containing congeners. The reaction mixture was stirred at 80 °C for 24 hours, during which the color of the mixture changed gradually from silver-gray to black-red. Centrifugation of the cooled reaction mixture followed by the concentration and standing of the filtrate afforded orange single crystals of Dy₃TeCl₄ (65 mg, 9.0% based on DyCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Dy₃O₃Te: C, 36.58; H, 5.04. Found: C, 36.61; H, 5.14.
Gd₃SCl₄ . The synthesis followed a similar procedure to that of Dy₃SCl₄, using GdCl₃ (395.4 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as a colorless polycrystalline solid (78 mg, 11% based on GdCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Gd₃O₃S: C, 39.80; H, 5.49. Found: C, 39.45; H, 5.80.
Gd₃SeCl₄ . The synthesis followed a similar procedure to that of Dy₃SeCl₄, using GdCl₃ (395.4 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as a colorless polycrystalline solid (70 mg, 10% based on GdCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Gd₃O₃Se: C, 38.38; H, 5.29. Found: C, 38.18; H, 5.38.
Gd₃TeCl₄ . The synthesis followed a similar procedure to that of Dy₃TeCl₄, using GdCl₃ (395.4 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as an orange polycrystalline solid (68 mg, 10% based on GdCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Gd₃O₃Te: C, 37.01; H, 5.10. Found: C, 36.98; H, 5.10.
Y₃SCl₄ . The synthesis followed a similar procedure to that of Dy₃SCl₄, using YCl₃ (292.9 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as a colorless polycrystalline solid (76 mg, 14% based on YCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Y₃O₃S: C, 47.47; H, 6.55. Found: C, 47.15; H, 6.88. ¹H NMR: δ 2.05 (s, 45H, C₅Me₅), δ 1.87 (m, 12H, THF), δ 3.87 (m, 12H, THF).
Y₃SeCl₄ . The synthesis followed a similar procedure to that of Dy₃SeCl₄, using YCl₃ (292.9 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as a colorless polycrystalline solid (70 mg, 12% based on YCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Y₃O₃Se: C, 45.47; H, 6.27. Found: C, 45.11; H, 6.67. ¹H NMR: δ 2.05 (s, 45H, C₅Me₅), δ 1.87 (m, 12H, THF), δ 3.87 (m, 12H, THF).
Y₃TeCl₄ . The synthesis followed a similar procedure to that of Dy₃TeCl₄, using YCl₃ (292.9 mg, 1.500 mmol) instead of DyCl₃. The desired product was obtained as an orange polycrystalline solid (60 mg, 10% based on YCl₃). Anal. Calc. for C₄₂H₆₉Cl₄Y₃O₃Te: C, 43.56; H, 6.01. Found: C, 43.10; H, 6.49. ¹H NMR: δ 2.06 (s, 45H, C₅Me₅), δ 1.88 (m, 12H, THF), δ 3.86 (m, 12H, THF).

Structural characterization

Single-crystal X-ray intensity data were collected on a Bruker D8 VENTURE diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 100/200 K. Using Olex2,^48,49 the structures were solved with the SHELXT structure solution program using Intrinsic Phasing and refined with the SHELXL refinement package using least squares minimization.⁵⁰ All hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms, with displacement parameters also dependent on the parent carbon atom U_eq value. Cambridge Crystallographic Data Centre contains the crystal structure with the following CCDC numbers: 2344964 (Dy₃SCl₄), 2344965 (Dy₃SeCl₄), 2344954 (Dy₃TeCl₄), 2344974 (Gd₃SCl₄), 2344970 (Gd₃SeCl₄), 2344975 (Gd₃TeCl₄), 2344958 (Y₃SCl₄), 2344969 (Y₃SeCl₄), and 2344973 (Y₃TeCl₄).†

Optical measurements

The absorption spectra of the title compounds were recorded using a computer-controlled UV-vis-NIR 3600i Plus spectrometer in the wavelength range of 300–2000 nm. Emission spectra were acquired at room temperature by using a steady-state spectrometer (FLS-1000, Edinburgh) with a xenon lamp. Time-resolved photoluminescence decay curves were obtained on the same spectrometer but with a 405 nm band of a laser diode.

Magnetic measurements

Magnetic susceptibility measurements were obtained using a Quantum Design MPMS-3 SQUID magnetometer between 2 and 300 K. A crushed polycrystalline sample was sealed with melted eicosane in an NMR tube under vacuum.

Ab initio calculations

OpenMolcas⁵¹ was used to perform the CASSCF-SO calculation of the electronic structures of Dy₃ECl₄ (E = S, Se, Te) with the molecular geometries taken from the crystallographic analyses; no optimization was made except for taking the largest disorder component. Relativistic effects have been accounted for by using the 2nd order Douglas–Kroll Hess Hamiltonian, and the basis sets from ANO-RCC library^52,53 were accordingly employed, with VTZP quality for Dy atoms, VDZP quality for the coordinating C, S, Se, and Te atoms, and VDZ quality for the remaining atoms. Cholesky decomposition of the two-electron integrals with a threshold of 10⁻⁸ was performed to save disk space and to reduce computational demand. The state-averaged CASSCF orbitals of the sextets, quartets, and doublets were optimized with 21, 224, and 490 states, respectively, with the RASSCF module. 21, 128, and 130 sextets, quartets, and doublets were chosen to construct and diagonalize in spin–orbit (SO) coupling Hamiltonian with the RASSI module. The resulting spin–orbit wavefunctions were decomposed into their CF wavefunctions, and the magnetic susceptibility was calculated using SINGLE_ANISO.⁵⁴ The program POLY_ANISO was used to calculate the dipolar interaction and fit the super-exchange coupling in Dy₃ECl₄ (E = S, Se, Te).^55,56

Results and discussion

Syntheses and structural characterization

Compounds RE₃ECl₄ (RE = Dy, Gd, Y; E = S, Se) were prepared using similar procedures. The pre-formed Cp*LnCl₂ complex was reacted with the formally doubly negatively charged E²⁻ (E = S or Se) ligand generated by the reduction of elemental E (E = S or Se) with NaBH₄; the formation of a bright yellow (S) or an orange-red (Se) solution, accompanied by gas bubbling when NaBH₄ and respectively elemental S and Se were dissolved completely in THF, is a good indication of the formation of E²⁻. However, stirring Te powder and NaBH₄ in THF, even with heating, only yielded a suspension instead of a clear solution as in the case of S or Se. As such, the syntheses of the Te-containing clusters RE₃TeCl₄ (RE = Dy, Gd, Y) were achieved in a one-pot reaction using solution A and the suspension of Te/NaBH₄ in THF. It is believed that Cp*RE(BH₄)₂(THF)₂ was produced in situ by the reaction between the pre-formed Cp*LnCl₂ complex in solution A with NaBH₄; this species is presumably soluble in the reaction mixture, enabling the reduction of Te to afford Te²⁻ for subsequent RE coordination.⁴⁵ All nine clusters are soluble in THF and CHCl₃. Single crystals suitable for X-ray diffraction studies (vide infra) were obtained by slow evaporation of the corresponding saturated THF solution at room temperature in a glovebox.

The title clusters are isostructural and crystallize in the monoclinic space group P2₁/n with comparable unit cells (Tables S1–S3, ESI†). The core motif features triangularly disposed RE atoms with one chloro ligand bridging two neighboring RE atoms, resulting in a crown-like structure (Fig. S1, ESI†). The trimetallic plane is capped on one side by a μ₃-Cl⁻ and a μ₃-E²⁻ on the other side. Each of the RE atoms is terminally coordinated by one Cp* and one THF. The metal center is formally deca-coordinate but can be viewed as situated in a distorted octahedron if the η⁵-Cp* is treated as a single-site ligand. It is interesting to note that all the η⁵-Cp* ligands are on the same side of the triply bridging E²⁻ ligand, while the THF molecules are coordinated on the other side along with the μ₃-Cl⁻ ligand. The μ₃-E²⁻ are essentially equidistant to the three RE atoms, with an average Dy–E length of 2.688(6) Å, 2.769(39) Å, and 3.037(11) Å for the S-, Se-, and Te-containing clusters, respectively (Fig. 1); the Dy–E distances fall in the range reported for compounds containing the same Dy–E bonds,^57–60 and the differences correspond to the different ionic radius of the E²⁻ ions (S²⁻, 1.84 Å; Se²⁻, 1.98 Å; Te²⁻, 2.21 Å).⁶¹ It is also of note that the different chalcogenide ligands have a small yet definitive impact on the cluster core structures. Specifically, the average Dy⋯Dy separations increase from 3.8033(11) Å in Dy₃SCl₄ to 3.8450(9) Å in Dy₃SeCl₄, and to 3.9287(9) Å in Dy₃TeCl₄ (Table 1 and Fig. S2a, ESI†). Furthermore, the E²⁻ ions shift progressively away from the Dy₃ plane with the use of heavier chalcogenide anions (Fig. S2b, ESI†), with the distance between the E²⁻ ion and the center of the Dy₃ plane increasing from 1.552 Å (Dy₃SCl₄) to 1.722 Å (Dy₃SeCl₄), and to 2.019 Å (Dy₃TeCl₄) (Table 1). It is interesting to note that the μ-Cl–Dy bond lengths range from 2.739(3) to 2.775 (5) Å and the μ₃-Cl–Dy bond lengths are between 2.821(4) and 2.874(3) Å (Tables S6–S8, ESI†); these metric values are in the range reported for Dy(III) complexes with bridging chloro ligands.^62–64 With the gradual size expansion of the trimetallic plane from the S- to Te-containing cluster, the μ₃-Cl⁻ ions are “pressed” to be closer to the plane, with the distance between the μ₃-Cl⁻ ion and the center of the triangular plane being reduced from 1.838 Å (Dy₃SCl₄) to 1.795 Å (Dy₃SeCl₄), and then to 1.704 Å (Dy₃TeCl₄).

Fig. 1 Ball-and-stick depictions of [η⁵-Cp*Dy(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (E = S, Se, and Te) (a) and the local coordination environment for the Dy(III) ion in Dy₃ECl₄ (E = S (b), Se (c), Te (d)). Color code: Violet, Dy; yellow, S, Se, Te; green, Cl; grey, C; red, O. H atoms are omitted for clarity.

Table 1 Selected bond distances/atomic separations (Å) for Dy₃SCl₄, Dy₃SeCl₄, and Dy₃TeCl₄

Bond distance (Å)	Dy3SCl4	Dy3SeCl4	Dy3TeCl4
Dy1–S/Se/Te	2.682(3)	2.731(19)	3.0479(15)
Dy2–S/Se/Te	2.691(3)	2.808(2)	3.0346(15)
Dy3–S/Se/Te	2.692(3)	2.767(9)	3.0271(14)
Average	2.688(6)	2.769(39)	3.037(11)
Dy1–Dy2	3.7937(8)	3.8440(14)	3.9332(12)
Dy2–Dy3	3.8147(9)	3.8550(13)	3.9174(12)
Dy3–Dy1	3.8006(9)	3.8360(13)	3.9354(12)
Average	3.8030(11)	3.8450(9)	3.9287(9)
E^2−-Dy3 plane	1.552	1.722	2.019
μ3-Cl^−-Dy3 plane	1.838	1.795	1.704
Cl3 plane -Dy3 plane	0.368	0.363	0.353

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The nine isostructural clusters with different metal ions and/or E²⁻ ligands form a valuable set of molecules for meaningful comparative studies aiming at the understanding of how the identity of the chalcogen elements and the structural variations due to their size difference may lead to the different photophysical and magnetic properties observed for the clusters of a particular RE element in this series of novel clusters.^65–67

Photophysical properties

The solid-state UV-vis-NIR adsorption spectra of RE₃ECl₄ (RE = Dy, Gd, Y, E = S, Se, Te) were recorded at room temperature from 300 nm to 2000 nm (Fig. S3 and S4, ESI†). The three clusters exhibit essentially the same profile, with six distinct absorption peaks in the visible and near-infrared regions, characteristic of Dy(III), in addition to the absorption in the ultraviolet region (Fig. S3, ESI†).⁶⁸ Room-temperature solid-state photoluminescence studies were also conducted. As shown in Fig. 2, the excitation spectra indicate that effective energy adsorption occurs in the ultraviolet region and extends slightly into the visible window at 310–420 nm. The emission spectra show two intense emission bands under excitation of 396 nm. The emission peaks at 487, 577, and 587 nm are attributed to transitions from ⁴F_9/2 to ⁶H_15/2 (487 nm) and ⁶H_13/2 (577–587 nm) states of Dy(III), respectively.^69,70 The luminescence lifetimes (λ_em = 577 nm) were obtained by fitting the decay curves with the single exponential mode (Fig. S5, ESI†), producing 69.19 ns, 77.03 ns, and 70.25 ns for the S-. Se-, and Te-containing clusters, respectively.

Fig. 2 Room-temperature excitation (λ_ex = 396 nm) and emission (λ_em = 577 nm) spectra of Dy₃ECl₄ (E = S, Se, Te).

Magnetic properties

Using polycrystalline samples wrapped in eicosane, the static-field magnetic susceptibilities of RE₃ECl₄ (RE = Dy, Gd; E = S, Se, Te) were measured under an applied DC field of 1000 Oe and in the 2–300 K temperature range. At 300 K, the χT values for Gd₃SCl₄, Gd₃SeCl₄, and Gd₃TeCl₄ were 23.80, 23.55, and 23.83 cm³ K mol⁻¹, respectively; these values are in good agreement with the expected value of 23.61 cm³ K mol⁻¹ (S = 7/2, g = 2.0) for three uncorrelated Gd(III) ions (Fig. 3).^71–73 As the temperature was lowered, the χT values decreased gradually to ca. 20 K, below which an abrupt decrease occurred, reaching 14.14, 14.28, and 13.55 cm³ K mol⁻¹ at 2 K for Gd₃SCl₄, Gd₃SeCl₄, and Gd₃TeCl₄, respectively. The magnetic susceptibilities at low temperatures increased slowly upon the increase of the magnetic field, producing magnetic moments of 20.41, 20.79, and 20.68μ_B at 2 K and 7 T for the S-, Se-, and Te-containing clusters, respectively; these values are in good agreement with the saturation value of 21.00μ_B calculated for three Gd(III) ions.⁷³ Both the temperature- and field-dependent magnetic susceptibilities were fitted using the PHI program⁷⁴ with the equilateral Hamiltonian (H = −2J(S_Gd1 × S_Gd2+ S_Gd2 × S_Gd3+ S_Gd1 × S_Gd3)) in order to evaluate any intramolecular magnetic interactions in these clusters. For all three clusters, the obtained g values (Table 2) are all close to the value of 2 for isotropic ions, and the super-exchange constants (J) are all around −0.04 cm⁻¹. The weak antiferromagnetic interactions are not too surprising as superexchange interactions between 4f ions mediated by a chloro bridging ligand are generally weak as previously observed.^75,76

Fig. 3 The χT versus T plot for (a) Gd₃SCl₄, (b) Gd₃SeCl₄, (c) Gd₃TeCl₄, (d) Dy₃SCl₄, (e) Dy₃SeCl₄, and (f) Dy₃TeCl₄ under 1000 Oe dc field. Inset: The field-dependent magnetization plots at indicated temperatures.

Table 2 Parameters for Gd₃ECl₄ (E= S, Se, Te) obtained by fitting the temperature- and field-dependent magnetic susceptibilities using the PHI program with the equilateral Hamiltonian

Cluster	g	J/cm^−1
Gd3SCl4	1.9857(9)	−0.0403(4)
Gd3SeCl4	1.9799(5)	−0.0357(1)
Gd3TeCl4	1.9731(8)	−0.0391(1)

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For Dy₃ECl₄ (E = S, Se, Te), the χT values at 300 K were found to be 42.51, 42.91, and 42.27 cm³ K mol⁻¹ for the S-, Se-, and Te-containing clusters, respectively; these values are in close agreement with the value of 42.51 cm³ K mol⁻¹ (⁶H_15/2, S = 5/2, g = 4/3, L = 5) expected for three uncorrelated Dy(III) ions (Fig. 3).^77–79 The field-dependent magnetization at 2, 3, and 5 K showed a rapid increase under a low magnetic field, followed by a more gradual increase, producing maximum values at 2 K and 7 T for the three clusters at 15.57, 14.72, and 14.63μ_B, respectively. These values are significantly smaller than the expected saturation value of 30μ_B, indicating the presence of a sizable magnetic anisotropy and/or low-lying excited states.^80–83 In contrast to the temperature-dependent changes of magnetic susceptibilities of Gd₃ECl₄ (E = S, Se, Te), the χT values of Dy₃SeCl₄ and Dy₃TeCl₄ decreased only very slowly upon lowering of temperature, to 33.81 and 35.45 cm³ K mol⁻¹ at 2 K, respectively. Interestingly for Dy₃SCl₄, the χT values decreased first to 35.42 cm³ K mol⁻¹ at 16 K, but then increased to 36.45 cm³ K mol⁻¹ with further lowering of temperature to 2 K. These observations indicate the presence of ferromagnetic coupling between the Dy(III) ions, which can attribute to dipolar interactions.^84,85

Ab initio calculations

The dipolar interactions in Dy₃ECl₄ (E = S, Se, Te) were studied by using complete active space self-consistent field spin–orbit (CASSCF-SO) calculations performed using OpenMolcas.⁵¹ It has been found that the ground states of the Dy(III) ions are dominated by the most magnetic component of m_J = ±15/2 (>85%), whereas the excited states are mixed with a combination of different magnetic components (Tables S15–S23, ESI†). Such electronic structures are believed to be responsible for the absence of slow magnetic relaxation above 2 K in these clusters. The principal magnetic axes of the ground Kramers’ doublets obtained are shown in Fig. 4; these magnetic axes are different in the orientation of their magnetic axes (either out-of-plane or in-plane type) when compared with the reported Dy₃ triangles that display characteristic spin frustration or single-molecule toroidal behaviors.^86–89 As indicated by the angles between the axes and the trimetallic plane are comparable (Table 3), the individual axes in the newly formed cluster complexes are essentially in line with the connection between the centroid – of a Cp* ring and the specific Dy atom it coordinates. In comparison, the magnetic axes of the literature precedents are in a toroidal arrangement.^86–89

Fig. 4 The principal magnetic axes of the ground doublets were obtained from the CASSCF-SO calculations for (a) Dy₃SCl₄, (b) Dy₃SeCl₄, and (c) Dy₃TeCl₄. Other atoms are omitted for clarity (Color code: Violet, Dy; yellow, S, Se, Te; green, Cl; grey, C; red, O).

Table 3 The angles (deg) between the principal magnetic axes of each Dy(III) to the trimetallic plane in Dy₃ECl₄ (E = S, Se, Te) and the calculated dipolar coupling parameters (J_dip)

	Angles between the principal magnetic axes and the Dy3 planes/deg		J dip/cm^−1
Dy3SCl4	Dy1	37.38	Dy1–Dy2	1.1281
	Dy2	20.20	Dy1–Dy3	1.1840
	Dy3	27.44	Dy2–Dy3	1.1901
Average		28.34		1.1674
Dy3SeCl4	Dy1	25.51	Dy1–Dy2	1.1603
	Dy2	30.46	Dy1–Dy3	1.1853
	Dy3	31.15	Dy2–Dy3	1.1233
Average		29.04		1.1563
Dy3TeCl4	Dy1	28.83	Dy1–Dy2	1.1345
	Dy2	29.74	Dy1–Dy3	1.0866
	Dy3	30.94	Dy2–Dy3	1.0690
Average		29.84		1.0967

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The dipolar interaction parameters were calculated by using the program POLY_ANISO.^55,56 As expected, all dipolar interactions are ferromagnetic in nature, with those in Dy₃SCl₄ being the strongest among the three congener clusters, as revealed by its largest average value (1.1674 cm⁻¹) of the coupling parameters (Table 3); the corresponding values are 1.1563 cm⁻¹ and 1.0967 cm⁻¹ for Dy₃SeCl₄ and Dy₃TeCl₄, respectively. This difference in dipolar interactions, albeit small, is consistent with the difference in the Dy⋯Dy distances (Table 1) as the dipolar interactions vary inversely with the cube of the separation between the Dy ions.^90,91 The stronger ferromagnetic coupling in Dy₃SCl₄ led to the above-mentioned increase of the temperature-dependent magnetic susceptibilities at low temperatures, while a similar observation was not made above 2 K in the case of Dy₃SeCl₄ or Dy₃TeCl₄ due to the presence of antiferromagnetic super-exchange and intermolecular interactions.^92,93

Conclusions

A series of novel rare-earth chalcogenidotetrachloride clusters of the general formula [η⁵-Cp*RE(μ-Cl)(THF)]₃(μ₃-Cl)(μ₃-E) (RE₃ECl₄: RE = Dy, Gd, Y; E = S, Se, Te) were synthesized and fully characterized, including structural determinations. Magnetic and photophysical properties of these isostructural clusters produced results that can be correlated with the structural variations due to the use of different rare-earth and/or chalcogen elements. Of note are the super-exchange interactions in clusters, antiferromagnetic in nature, being comparable in Gd₃ECl₄, but the dipolar interactions in these clusters are invariably ferromagnetic and the dipolar coupling parameters decrease with the use of heavier chalcogenide ligands. In addition, ferromagnetic behavior was observed in Dy₃SCl₄ due to the strongest dipolar interactions among the three Dy₃ECl₄ (E = S, Se, Te) congeners. The clusters presented in this work provide a family of new rare-earth-containing cluster compounds featuring the coordination with chalcogenido ligands – a class of substances with important magnetic and optical applications but more difficult to make than their counterparts with O-based ligands. More importantly, the chalcogen-dependent properties exhibited by the title clusters are expected to stimulate more future research on the preparation and property studies of this type of rare-earth-containing materials, phosphors with much-enhanced luminescence (due to the suppression of vibronic luminescence by virtue of RE–E coordination) and novel magneto-optic materials, for example.

Data availability

Cambridge Crystallographic Data Centre contains the crystal structure with the following CCDC numbers: 2344964 (Dy₃SCl₄), 2344965 (Dy₃SeCl₄), 2344954 (Dy₃TeCl₄), 2344974 (Gd₃SCl₄), 2344970 (Gd₃SeCl₄), 2344975 (Gd₃TeCl₄), 2344958 (Y₃SCl₄), 2344969 (Y₃SeCl₄), and 2344973 (Y₃TeCl₄). Other data supporting the findings of this study are available within the article and its ESI.† Raw experimental and computational data are available upon request to the corresponding author.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support of this work by the National Natural Science Foundation of China (92261203, 22101116, and 21971106), the Key Laboratory of Rare Earth Chemistry of Guangdong Higher Education Institutes (2022KSYS006), the Stable Support Plan Program of Shenzhen Natural Science Fund (20200925161141006), and the Shenzhen Fundamental Research Program (JCYJ20220530115001002 and JCYJ20220818100417037).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2344964, 2344965, 2344954, 2344974, 2344970, 2344975, 2344958, 2344969 and 2344973. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc02778g

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