C₂O₄²⁻-templated cage-shaped Ln₂₈ (Ln = Gd, Eu) nanoclusters with magnetocaloric effect and luminescence

Abstract

The assembly of high-nuclearity lanthanide nanoclusters with favourable magnetocaloric effects suffers from a significant challenge. Herein, two outstanding examples of cage-shaped nanoclusters, namely, [Ln₂₈(IN)₂₅(C₂O₄)₆(HCOO)(μ₃-OH)₃₆(μ₂-OH)₂(H₂O)₃₆]·8Br·xH₂O (Ln = Gd and Eu; x = 21, abbreviated as Gd₂₈; x = 17, abbreviated as Eu₂₈, HIN = isonicotinic acid; H₂C₂O₄ = oxalic acid) have been triumphantly designed and constructed under hydrothermal conditions. The crystallographic study revealed that the aesthetically pleasing triangle-shaped Gd₁₂ subunit and Gd₁₆ subunit built separately from four Gd₃ units and four Gd₄ units, respectively, are shaped into an unprecedented cage-shaped configuration. Prominently, six small and simple C₂O₄²⁻ anions generated via the in situ reaction of HIN are faultlessly anchored in the cage-shaped framework. Furthermore, the existence of abundant Gd^III ions impart Gd₂₈ with a captivating magnetic entropy change (−ΔS^max_m = 37.5 J kg⁻¹ K⁻¹) at 2.0 K for ΔH = 7.0 T.

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Introduction

Owing to severe pollution of the environment and deficiencies from energy shortages, research on lanthanide nanoclusters with beneficial magnetocaloric effects (MCEs) has been heralded in materials chemistry and magnetochemistry.^1–7 The MCE, which explains the phenomenon of heat absorption or heat release of a magnetic material in a changing magnetic field, was first discovered in 1881 by Warburg.^8,9 In particular, magnetic cooling materials at ultra-low temperatures play an indispensable role in the development of quantum computers.^7,10–12 As a unique class of energy-efficient and environmentally friendly refrigerants at ultra-low temperatures, lanthanide nanoclusters, especially high-nuclearity Gd^III oxide/hydroxide clusters, are prospective materials featuring decent MCEs.^13–15 At the same time, aesthetically captivating configurations with various conformations and shapes have been designed, such as chair, boat, cage, disk, wheel, nanocapsule, etc.^16–21

However, due to practical obstacles in their sizeable ionic radius and high coordination numbers, as well as the mutual repulsion between lanthanide ions, the assembly of high-nuclearity lanthanide nanoclusters is a great challenge, especially when the number of the lanthanide ions is greater than 20.^15,22,23 It is well known that in situ reactions can benefit the formation of anionic templates through decomposition, rearrangement and other unpredictable side reactions of solvents or ligands.²⁴ Once an in situ reaction occurs, the decomposition of ligands can effectively realize the slow release of anions. The decomposition process is pretty slow, thus avoiding the formation of precipitation caused by the direct introduction of raw materials, which makes the sophisticated coordination mode possible.^25,26 For example, the synthesis of classical cage-like Ln₆₀ is through the in situ reaction of 2,2′-bipyridine-4,4′-dicarboxylic acid, where the small and simple carbonate anion is resoundingly encapsulated in the nanocluster. The synthesis of stable and compact inorganic–organic hybrid Gd₄ with twisted cubic [Gd₄O₄] units is bridged by in situ generated oxalate and sulfate.^27,28 So far, it has been observed that ligands containing carboxylic acid, an N-donor or sulfur tend to react in situ.^29,30 Indeed, pyridinecarboxylate acids, such as isonicotinic acid, may be converted into some small ligands via an in situ reaction to form into exquisite frameworks with large MCEs. As a result, the first mixed-ligand (oxalate and isonicotinic acid) coordination polymer [Zn₂(IN)₂(oxa)] was afforded under hydrothermal conditions from the in situ reaction of HIN to oxalate. Subsequently, a 3D compound of [La₂(C₂O₄)₂] was reported under hydrothermal conditions from the in situ reaction of HIN.^31,32 Although HIN can be chemically rearranged to oxalate under appropriate conditions, it has never been observed in high-nuclearity lanthanide nanoclusters.

Based on the high-nuclearity lanthanide nanoclusters of Ln₂₆ (Ln = Ho and Er) and Ho₄₈ by employing isonicotinic acid as the ligand, we elaborately designed and isolated two unprecedented cage-shaped nanoclusters under hydrothermal conditions with the formulas of [Gd₂₈(IN)₂₅(C₂O₄)₆(HCOO)(μ₃-OH)₃₆(μ₂-OH)₂(H₂O)₃₆]·8Br·21H₂O and [Eu₂₈(IN)₂₅(C₂O₄)₆(HCOO)(μ₃-OH)₃₆(μ₂-OH)₂(H₂O)₃₆]·8Br·17H₂O.^20,33 Structural analysis showed that the aesthetically elegant cage-shaped Gd₂₈ can be divided into the charming triangle-shaped Gd₁₂ subunit and the triangle-shaped Gd₁₆ subunit that are constructed from four Gd₃ units and four Gd₄ units, respectively. Remarkably, the small and simple C₂O₄²⁻ anion generated via the in situ reaction of HIN is splendidly anchored in the cage-shaped framework, which is the first case in the family of high-nuclearity lanthanide nanoclusters. Besides, magnetocaloric analysis showed that Gd₂₈ exhibited a considerable value of −ΔS^max_m = 37.5 J kg⁻¹ K⁻¹ at 2.0 K for ΔH = 7.0 T.

Experimental section

Materials and physical measurements

Chemical raw solvents and reagents from commercial sources were available for use without further purification. A PerkinElmer 2400 analyzer (USA) was used for the elemental analyses of C, H, and N. A Bruker D8 diffractometer (Germany) was used to collect powder X-ray diffraction (PXRD) data using Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 3–50° at a scanning rate of 3° min⁻¹ at room temperature. IR spectra of the two compounds in KBr pellets were recorded using a Nicolet Impact 410 spectrometer (Thermo Fisher, USA) in the region of 4000–400 cm⁻¹. In order to conduct a thermogravimetric analysis (TGA), a Q50 Thermogravimetric Analyzer (TA Instruments, USA) was used in flowing nitrogen air to record thermogravimetric curves of the two compounds from ambient temperature to 1000 °C at a rate of 10 °C min⁻¹. The energy dispersive spectrometry (EDS) data were acquired with a Hitachi S-4800 (Japan) emission scanning electron microscope with a 20 kV accelerating voltage. Magnetization measurements were performed using an MPMS-XL7 SQUID magnetometer (Quantum Design, USA) at 1.8–300 K and in fields of 0–7 T. The solid-state luminescence properties of Eu₂₈ were measured using an FLS1000 spectrophotometer (Edinburgh Instruments, UK) at room temperature.

X-ray crystallography

Under an optical microscope, single crystals of Gd₂₈ with good crystal quality were selected and coated quickly with epoxy glue in air, and placed on a thin glass fiber for data collection; the crystal data of Eu₂₈ were not good enough to be reported in detail. Single crystal X-ray diffraction (SCXRD) data of Gd₂₈ were collected by employing a Bruker Apex II CCD (Germany) with a sealed-tube X-ray source in the ω–2θ scanning method at room temperature. The SHELX software package was used to resolve the crystal structures with direct methods and refine the crystal structures via full-matrix least-squares approaches. Besides, all the non-hydrogen atoms were refined by using anisotropic thermal parameters according to experience. The H atoms of water were not located. The SQUEEZE command was used to remove the H₂O molecules. Table 1 provides the relevant crystal data. Table S5† shows the selected bond lengths and angles.

Table 1 Crystal data and structure refinements for Gd₂₈

Compound	Gd28
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = ∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]^1/2.
Formula	C163H253Gd28N25O171Br8
Formula weight	10 341
T (K)	296(2)
Crystal system	Triclinic
Space group	P1̄
a (Å)	21.465(5)
b (Å)	24.112(5)
c (Å)	30.492(6)
α (°)	87.190(3)
β (°)	79.559(3)
γ (°)	88.080(3)
V (Å^3)	15 496(6)
Z	2
D c (mg m^−3)	2.216
μ (mm^−1)	7.027
F (000)	9692
θ range (°)	0.680–27.322
Limiting indices	−26 ≤ h ≤ 27, −30 ≤ k ≤ 29, −38 ≤ l ≤ 36
Reflections collected	123 261
R(int)	0.0611
Data/restraints/parameters	61 613/852/3297
GOF	1.013
R 1 , wR2^b [I > 2σ(I)]	R 1 = 0.0496, wR2 = 0.1353
R 1, wR2 (all data)	R 1 = 0.1085, wR2 = 0.1586

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Synthesis of Gd₂₈

The reactants of HIN (0.25 g, 2.00 mmol), Gd₂O₃ (0.20 g, 0.55 mmol), Al₂O₃ (0.04 g, 0.39 mmol) and KBr (0.01 g, 0.08 mmol) were placed in a 25 mL Teflon-lined autoclave, to which were added HCOOH (0.04 g, 0.87 mmol) and 8 mL of distilled water. Immediately after, the mixture was magnetically stirred at room temperature for 12 hours, and then the pH was adjusted from 4.8 to 2.0 with HNO₃ (50%) under stirring conditions. Subsequently, the mixture was maintained at 170 °C for 7 days. After cooling to about room temperature, light-yellow quadrilateral crystals were harvested (as shown in Fig. S1†), which were washed with distilled water (37.0% yield based on Gd). Anal. calcd (%) for C₁₆₃H₂₅₃Gd₂₈N₂₅O₁₇₁Br₈ (formula weight = 10 341): C, 18.93; H, 2.45; N, 3.39. Found: C, 18.90; H, 2.41; N, 3.40.

Synthesis of Eu₂₈

The synthesis method is the same as the method used for Gd₂₈ except for Gd₂O₃ being replaced with Eu₂O₃. The light-yellow quadrilateral crystals were obtained (as shown in Fig. S1†) (35% yield based on Eu). Anal. calcd (%) for C₁₆₃H₂₄₅Eu₂₈N₂₅O₁₆₇Br₈ (formula weight = 10 269): C, 19.07; H, 2.40; N, 3.41. Found: C, 19.09; H, 2.37; N, 3.40.

Results and discussion

Synthetic description

The nearly light-yellow quadrilateral crystals of Gd₂₈ were synthesized using the hydrothermal reaction of Gd₂O₃, Al₂O₃, HIN and KBr in the presence of HCOOH at 170 °C for seven days. Notably, the reagent isonicotinic acid with both an O-donor and an N-donor on opposite sides is exceptionally significant in the self-assembly of the Gd₂₈ nanocluster. Furthermore, the target product could not be captured when we tried to add oxalic acid directly to the reaction. Due to the slow production of C₂O₄²⁻ anions in the in situ reaction of isonicotinic acid, the environment can self-adjust to realize the balance between the Gd^III ions, C₂O₄²⁻ anions and HIN ligands, resulting in the assembly of the Gd₂₈ nanocluster in the method of slow crystallization.¹⁵ According to the literature, formic acid (HCOOH), as the smallest carboxylate ligand, has been favourably used to prepare lanthanide clusters with great MCEs. Gd₂O₃ is a reactant which not only can slowly release Gd^III ions but also regulates the pH of the reaction system. Although Al^III ions are not involved in the final construction, we cannot obtain the target product without them. We replaced Al₂O₃ with other metallic oxides, such as Fe₂O₃ or Cr₂O₃, but we did not get the target crystal. At the same time, we tried using Eu₂O₃, Tb₂O₃ or Dy₂O₃ instead of Gd₂O₃; only from the environment of Eu₂O₃ were light-yellow quadrilateral crystals of Eu₂₈ isolated, but unfortunately, the crystal data were not good enough to be reported in detail. On the other hand, the synthetic process of high-nuclearity Ln-exclusive nanoclusters is problematic and heavily relies on certain reaction factors, such as reaction conditions (duration, temperature, pH, cooling rate) and reactants (solvents, ligands, and the kinds of anions). For Gd₂₈, a temperature of 170 °C plays a remarkable role in the formation of the framework; no crystals can be obtained at 160 °C or 140 °C. Most importantly, an absolutely acidic environment (pH = 2) is quite prominent in the self-assembly of the structure, where no crystals can be obtained at pH = 3, pH = 4 or pH = 5. The high temperature and acidic conditions make it possible for the slow release or production of C₂O₄²⁻ ions from HIN.

Crystal structure

Counterions were confirmed via EDS analyses and charge balance studies, while the guest molecules were determined by elemental analysis, and TG and IR measurements. Data from SCXRD analysis revealed that Gd₂₈ crystallized in a triclinic crystal system, with P1̄ space group. As shown in Fig. 1a, the isolated nanocluster with formula [Gd₂₈(IN)₂₅(C₂O₄)₆(HCOO)(μ₃-OH)₃₆(μ₂-OH)₂(H₂O)₃₆] can be viewed as being constructed from a 28-metal nanocage of [Gd₂₈(C₂O₄)₆(μ₃-OH)₃₆(μ₂-OH)₂]³⁴⁺ protected and stabilized by 25 IN⁻ groups, one HCOO⁻ group and about 36 guest water molecules. Broadly, six small and simple C₂O₄²⁻ anions generated via the in situ reaction of HIN are splendidly anchored in the cage-shaped metal–organic framework to balance the high positive charges and stabilize its construction (Fig. 1b).

Fig. 1 (a) Plot of the whole molecular structure and (b) the C₂O₄²⁻ anion template in the metal skeleton of Gd₂₈. Purple, Ln; blue, N; red, O; and black, C.

For the sake of facilitating the description and understanding of the structural skeleton, the cage-like cationic core of Gd₂₈ can be subdivided into two different types of primary units, i.e., a triangular Gd₃ unit formulated as [Gd₃(μ₃-OH)₂]⁷⁺ and a cubane-like Gd₄ unit formulated as [Gd₄(μ₃-OH)₈]⁴⁺. A Gd_3-type unit, as shown in Fig. 2a, can be considered a triangular construction connected by two μ₃-OH groups and four Gd₃ units are linked by six C₂O₄²⁻ anions and formed into a [Gd₁₂(μ₃-OH)₈(C₂O₄)₆]¹⁶⁺ (Gd₁₂) subunit. For clarity, each of the Gd₃ units, with a shell-shaped construction, is combined with six C₂O₄²⁻ anions, and formed into a delicate triangle-like structure (Fig. 2d). A Gd₄-typeunit is a cubane-like configuration formed by the banding of four μ₃-OH groups, which is extremely common in other lanthanide clusters (Fig. 2c). Four Gd₄ units are combined with the same six C₂O₄²⁻ anions forming a [Gd₁₆(μ₃-OH)₁₆(C₂O₄)₆]²⁰⁺ (Gd₁₆) subunit. For clarity, each tetrahedron is connected by six C₂O₄²⁻ anions forming a fancy triangle-shaped framework (Fig. 2e). As shown in Fig. 2f, the frog-shaped Gd₁₂ subunit and the frog-shaped Gd₁₆ subunit are bonded together into an individual cage-shaped framework templated by six C₂O₄²⁻ anions. The cage-like nanocluster is further stabilized by the 25 IN⁻ groups and one HCOO⁻ group. Additionally, the IN⁻ ligands in Gd₂₈ display three different types of coordination modes, μ₂-η¹:η⁰:η¹, μ₁-η⁰:η⁰:η¹ and μ₁-η¹:η⁰:η¹ (Fig. 3a), and the six C₂O₄²⁻ anions exhibit only one coordination mode, μ₆-η²:η²:η²:η² (Fig. 3b).

Fig. 2 Ball-and-stick views of (a) the triangular [Gd₃(μ₃-OH)₂]⁷⁺ unit; (b) the C₂O₄²⁻ anion; (c) the cubane-like [Gd₄(μ₃-OH)₄]⁸⁺ unit; (d) the [Gd₁₂(μ₃-OH)₈(C₂O₄)₆]¹⁶⁺ subunit; (e) the [Gd₁₆(μ₃-OH)₁₆(C₂O₄)₆]²⁰⁺ subunit; (f) the [Gd₂₈(C₂O₄)₆(μ₃-OH)₃₆(μ₂-OH)₂]³⁴⁺ nanocluster; and (g) the subunits representing Gd₂₈.

Fig. 3 Coordination modes of (a) IN⁻ ligands and (b) C₂O₄²⁻ in Gd₂₈.

The metal framework of Gd₂₈ is shown in Fig. 1b; four tetrahedra (16 Gd^III) and four triangles (12 Gd^III) were joined together to complete the nano-caged framework. All Gd^III ions are located in eight-coordinated or nine-coordinated environments with different geometries (Fig. S5†). Gd3, Gd5, Gd6, Gd8, Gd11, Gd12, Gd14, Gd16, Gd25, Gd26, Gd27 and Gd28 represent eight coordination modes in the [GdO₈] polyhedra; Gd1, Gd2, Gd4, Gd7, Gd9, Gd10, Gd13, Gd15, Gd17, Gd18, Gd19, Gd20, Gd21, Gd22, Gd23 and Gd24 display nine coordination modes in the [GdO₉] polyhedra. As shown in Fig. S5,† 28 Gd^III ions have different coordination environments. For example, Gd1 is coordinated with one HIN, one C₂O₄²⁻ anion, four μ₃-OH groups and two coordinated water molecules. According to the continuous shape measurement (CShM) analysis, the 28 Gd^III ions have seven different configurations. For instance, Gd28 exhibits a twisted octa-coordinated square antiprism, while Gd10 shows a distorted nine-coordinated spherical capped square antiprism (Tables S1 and 2†). The bond lengths between two neighboring ions (Gd–O = 2.3–2.9 Å) and the bond angles between the neighboring ions (O–Gd–O = 58.8–155.3 Å) are equivalent to the previously reported Gd-based nanocluster (Table S5†).

Remarkably, cage-shaped high-nuclearity Gd-based nanoscale clusters, where the number of lanthanide ions is more than 20, are still underdeveloped. To date, only cage-shaped Gd₂₀, Gd₂₆, Gd₂₇, Gd₃₂, Gd₃₈, Gd₅₀, Gd₆₀ and Gd₁₀₄ nanoclusters have been reported.^18,34–37 The Gd₂₈ nanocluster reported in this work is extraordinarily different from other reported cage-shaped nanoclusters. For example, the spherical Gd₂₆ nanocluster consists of five cubane-like Gd₄ units and two triangular Gd₃ units templated by nine CO₃²⁻ anions. Nicotinic acid is the organic linker and the protective ligand in the formation of Gd₂₆.³⁵ Other cage-shaped Gd₂₇ nanoclusters have been isolated by employing sodium propionate as the ligand. The metal skeleton of Gd₂₇ is achieved by five tetrahedral Gd₄ units and one crown-shaped Gd₇ unit templated by eight CO₃²⁻ anions.¹⁸ Explicitly, the Gd₂₈ nanocluster exhibits an unprecedented configuration in the family of high-nuclearity cage-shaped nanoclusters.

It is worth noting that the structure of the Eu₂₈ nanocluster is roughly the same as that of Gd₂₈, according to the main characteristic diffraction peaks of the PXRD (Fig. 4). Meanwhile, the free H₂O molecules of the two nanoclusters were determined by TG analysis, as shown in Fig. S9 and 10.† In the temperature range of 25–265 °C, the weight loss of 3.67% (calcd 3.66%) was attributed to the loss of 21 free water molecules for Gd₂₈, while the weight loss of 2.96% (calcd 2.98%) was attributed to the loss of 17 free water molecules for Eu₂₈.

Fig. 4 PXRD patterns of Gd₂₈ and Eu₂₈.

Luminescence of Eu₂₈

One of the most meaningful characteristics of lanthanide ions is their picturesque photoluminescence properties derived from f–f intraconfigurational transitions.¹³ Additionally, Eu^III is most commonly used in sensing applications due to its strong visible light emission in the red region.³⁸ Accordingly, the luminescence properties of Eu₂₈ with 28 Eu^III ions were measured at room temperature, as shown in Fig. 5. Under UV lamp irradiation, we observed the solid-state luminescence of Eu₂₈. Besides, under maximum excitation at 394 nm, the electronic transitions of Eu^III from ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3 and 4) were observed. The relatively weak emission bands at 581, 653 and 700 nm are attributed to the electronic transitions of ⁵D₀ → ⁷F₀, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄, respectively. In comparison, the two sharp lines at 593 and 616 nm are assigned to the dominant electronic transitions of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂, respectively. Notably, the absence of a strong and broad peak from the IN⁻ ligand indicates the high energy transfer efficiency to the Eu^III center emission, known as the “antenna effect”.³⁹ Furthermore, under the maximum excitation at 394 nm, the luminescence lifetime and the solid-state photoluminescence quantum yield (PLQY) of Eu₂₈ were 309.8 μs and 4.18%, respectively (Fig. 5b). To further learn about the solid-state luminescence of Eu₂₈, we drew a simple Jablonsky level diagram to explain the luminescence properties (Fig. 5c).^40–43 These findings are of significant value for understanding the optical properties of the lanthanide clusters and their applications in optoelectronic devices, among others.⁴⁴

Fig. 5 (a) Excitation and emission spectra; (b) luminescence lifetime; and (c) Jablonsky level diagrams of Eu₂₈.

Magnetic properties of Gd₂₈

At an applied direct current (dc) magnetic field of 1.0 kOe from 1.8 K to 300 K, temperature-dependent magnetic susceptibilities of Gd₂₈ were characterized. The observed χ_MT value (225.0 cm³ K mol⁻¹) at 300 K is slightly larger than the expected value (220.5 cm³ K mol⁻¹) based on 28 independent Gd^III ions (S = 7/2, g = 2) with the Lande formula.⁶ As shown in Fig. 6a, as the temperature drops from 300 K to 1.8 K, the χ_MT value gradually decreases to a minimum of 94.4 cm³ K mol⁻¹ at 1.8 K.⁴⁵ The overall trend of χ_MT vs. T data indicated that Gd₂₈ exhibited antiferromagnetic interaction between these metal cations. At the same time, fitting the χ_M⁻¹vs. T data with the Curie–Weiss law from 1.8 K to 300 K resulted in the Weiss constant θ = −5.7 K and Curie constant C = 226.5 cm³ K mol⁻¹. All the above discussions demonstrate the existence of weak antiferromagnetic interactions among metal ions (Fig. 6b).^46–49

Fig. 6 (a) Plot of experimental magnetic susceptibility (χ_MT) versus T; (b) χ_M⁻¹versus T plot; (c) plot of M–H under different temperatures; and (d) plot of experimental magnetic entropy change (−ΔS_m) versus T for Gd₂₈.

A large number of Gd^III ions and fairly weak magnetic interactions between Gd^III ions make Gd₂₈ nanoclusters a valid class of materials for magnetic cooling applications. Hence, it encouraged us to conduct research of the magnetocaloric effect of Gd₂₈. Here, the magnetization (M) and magnetic field (H) at low temperature (1.8 K–10 K) was measured (Fig. 6c). With the increase of magnetic field (H), the magnetization (M) also increased steadily to the maximum of 180.3Nβ under 1.8 K and 7 T; the discrepancy between the two magnetization values (calculated value is 196.0Nβ based on 28 uncorrelated metal ions) resulted from the weak antiferromagnetic interaction in the nanocluster. Subsequently, we investigated the magnetocaloric effects of Gd₂₈ by employing the Maxwell equation of (Fig. 6d).⁵⁰ With an increased H and a reduced T, the experimental entropy changes gradually increased, and the −ΔS^max_m is 37.5 J kg⁻¹ K⁻¹ for ΔH = 7 T at 2 K. By using the equation of −ΔS_m = nR ln(2S + 1), the 28 uncorrelated Gd^III ions were calculated and resulted in a theoretical value of 46.8 J kg⁻¹ K⁻¹; the discrepancy between the two ΔS_m values might be due to the existence of antiferromagnetic exchanges among the metal centers.⁵¹ As far as we know, the Gd₂₈ nanocluster exhibits a decent magnetocaloric effect value, comparable to those of the reported high-nuclearity nanoscale clusters Gd₂₇ (−ΔS_m = 41.8 J kg⁻¹ K⁻¹), Gd₃₈ (−ΔS_m = 37.9 J kg⁻¹ K⁻¹) and Gd₃₆ (−ΔS_m = 39.7 J kg⁻¹ K⁻¹), validating its excellent potential application in magnetic cooling (Table S3†).^18,36,52

Conclusions

In summary, we triumphantly obtained two novel high-nuclearity lanthanide nanoclusters Gd₂₈ and Eu₂₈ in the presence of HIN under hydrothermal conditions. Subsequently, through structural characterization studies, we observed that the charming triangle-shaped Gd₁₂ subunit and Gd₁₆ subunit assembled by four Gd₃ units and four Gd₄ units, respectively, were arranged into an unprecedented cage-shaped structure featuring six C₂O₄²⁻ anions. Notably, this is the first case of an in situ hydrolysis of isonicotinic acid to C₂O₄²⁻ ions in constructing high-nuclearity lanthanide nanoclusters. Magnetization analysis revealed that Gd₂₈ exhibits a decent magnetocaloric effect of 37.5 J kg⁻¹ K⁻¹ at 2.0 K for ΔH = 7.0 T. This paper has given a new idea for introducing C₂O₄²⁻ from the in situ reaction of HIN to assemble high-nuclearity lanthanide clusters with large magnetic entropy changes. Next, we will continue to isolate molecular magnetocaloric materials based on Gd nanoclusters through the in situ hydrolysis of HIN and probe more excellent properties and potential applications for high-nuclearity lanthanide clusters.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant 92161109).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2257351. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00743j

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