High-nuclearity metal complexes have flourished in an increasing number of fields and inspired researchers to assemble plentiful polymers or give an insight into the extensive properties, due to the aggregation of abundant metal ions resulting in the multiple pleasing structures and diversified properties [1-10]. As one kind of significant metal clusters, heterometallic transition-lanthanide (3d-4f) high-nuclearity clusters have acquired comprehensive attention on account of multiple structures, and smart applications, such as photocatalytic CO₂ reduction, gas separation and magnetocaloric effect (MCE) [11-15].

More specially, numerous efforts have been encouraged by 3d-4f compounds as the promising molecule-based magnetic cooling materials which have the chance to replace rare and costly He-3 in the ultra-low-temperature scope [16]. And compared with pure 3d- or 4f-based compounds, 3d-4f mixed metal complexes with the virtues of different spin centers can provide more opportunities to build magnetic materials [16-23]. In order to achieve the remarkable MCE, high-nuclearity 3d-Gd compounds are chosen as the ideal magnetic materials, because a high-spin ground state, isotropic electron orbit and negligible magnetic anisotropy of Gd³⁺ ions are beneficial to the outstanding MCE and the large low-field magnetic entropy change [15, 24-29]. Moreover, it is generally believed that the correlation of structure-property is presented in nanoscale materials [21, 30]. For example, a certain number of explorations demonstrate that multidimensional metal polymers are powerful to the brilliant MCE with large magnetic density [3, 31]. Nevertheless, it is worth paying consideration to the explorations demonstrate that multidimensional metal polymers challenge of assembling 3d-4f complexes as distinguished magnetic coolants, resulting from the different coordination modes of transition and lanthanide ions and the indissolubility engendered by the over-aggregation of metal ions [32-34]. So far, although a host of high-nuclearity 3d-4f heterometallic metal clusters have been assembled and published [15, 35, 36], most of them presented isolated structure [27, 37]. In other words, constructing multidimensional 3d-4f mixed-metal high-nuclearity clusters manifesting favorable MCE is more challenging but pregnant.

Based on reasonable experimental design, if the isolated structures could be bridged by ligands to form the multidimensional structures? With the purpose of obtaining objective 3d-4f clusters coupled with aesthetically enjoyable structures and large MCE, anionic templates have been efficiently utilized and widely accepted in designing of fabricating metal clusters [12, 30, 35]. Additionally, the CO₃^2- anions as the common template have been successfully used to induce 3d and 4f ions into pure metal clusters or mixed-metal compounds because of its versatile coordination modes and the relatively strong coordination bonding between CO₃^2- and metal ions [29]. However, among publicly reported high-nuclearity clusters, most of CO₃^2- anionic templates came from the adsorption of carbon dioxide in the atmosphere or the decomposition of ligands, which were all of high serendipity [30, 32]. Therefore, we look forward to building novel and tempting 3d-4f high-nuclearity clusters via quantitatively introducing CO₃^2- anions from carbonate.

To put the above considerations into practice, two captivating chain-like coordination polymers based on Ln₁₈Ni_24(23.5) high-nuclearity 3d-4f clusters, 1-(Gd₁₈Ni₂₄)_n and 2-(Eu₁₈Ni_23.5)_n, are isolated through CO₃^2- anionic templates and solvothermal treatment. In terms of the structures, compounds 1-(Gd₁₈Ni₂₄)_n and 2-(Eu₁₈Ni_23.5)_n are both linked by IDA^2- ligands to constitute the scarce 3d-4f chain-shaped metal clusters up to now. Additionally, products in this work are both templated by CO₃^2- anions that are quantitatively introduced by the adding of carbonate. More importantly, compound 1-(Gd₁₈Ni₂₄)_n makes a new breakthrough in MCE, with -∆S_m = 35.30 J kg^-1 K^-1 (at 3.0 K with ∆H = 7.0 T) and -∆S_m = 20.95 J kg^-1 K^-1 (at 2.0 K with ∆H = 2.0 T).

Single-crystal X-ray diffraction studies expose that compounds 1-(Gd₁₈Ni₂₄)_n and 2-(Eu₁₈Ni_23.5)_n both crystallize in the monoclinic space group P2₁/c. As shown in Fig. 1, in compound 2-(Eu₁₈Ni_23.5)_n, [Eu₁₈Ni_23.5(IDA)₂₂(IN)(CH₃COO)₂(NH₂CH₂COO)(μ₃-OH)₃₂(CO₃)₇(H₂O)₅Cl]·Cl₆·(H₂O)₁₇, contains 18 Eu³⁺ ions, 24 Ni²⁺ ions (Ni24 with occupancy factor of 0.5), 7 CO₃^2- ions, 22 IDA^2- ligands, one glycine ligand (NH₂CH₂COO^-) from the decomposition of H₂IDA [18], one IN^- ligand, two CH₃COO^-, 7 Cl^- ions, 32 μ₃-OH^-, 5 coordination water molecules and 17 lattice water molecules. And compound 1-(Gd₁₈Ni₂₄)_n, [Gd₁₈Ni₂₄(CO₃)₇(IDA)₂₂(μ₂-OH)₃(μ₃-OH)₃₂(H₂O)₅Cl]·Cl₈·(H₂O)₁₄, contains 18 Gd³⁺ ions, 24 Ni²⁺ ions, 7 CO₃^2- ions, 22 IDA^2- ligands, 9 Cl^- ions, 32 μ₃-OH^-, 3 μ₂-OH^-, 5 coordination water molecules and 14 lattice water molecules (Figs. S6 and S7 in Supporting information).

In regard to structures of two complexes, here only compound 2-(Eu₁₈Ni_23.5)_n as the example is discussed in detail due to the similarity of two compounds. {[Eu₁₈Ni_23.5(IDA)₂₂(CO₃)₇(μ₃-OH)₃₂(H₂O)₅(IN)(CH₃COO)₂(NH₂CH₂COO)]Cl}_n⁶ⁿ⁺ cation cluster (Fig. 2) of compound 2-(Eu₁₈Ni_23.5)_n can be viewed as the core-shell configuration and can be divided into two parts: inter {Eu₁₈Ni} core (Ni with occupancy factor of 0.5, Fig. 2b) and outer {Ni₂₃} shell (Fig. 2c). Twenty-three Ni²⁺ ions connect with 21 IDA^2- ligands and one CH₃COO^-, leading to a bowl-like {Ni₂₃} shell. In addition, one IN^- as the terminal ligand connects with one Ni²⁺ ion. As shown in Fig. 2b, inter {Eu₁₈Ni} layer can be divided into {Eu₁₁Ni} (top) and {Eu₇} (bottom): in the top, 11 Eu³⁺ ions are linked through μ₃-OH^-; in the bottom, 7 Eu³⁺ ions are interconnected by μ₃-OH^-. Then {Eu₁₁Ni} and {Eu₇} are further connected by 7 CO₃^2-, forming a unique {Eu₁₈Ni} core (Fig. 2b). Interestingly, Ni₂₄ exists in the core of {Eu₁₁Ni}, as shown in Fig. 2b. The Ni₂₄ ion is connected to adjacent six Eu³⁺ ions by oxygen atoms.

The inter {Eu₁₈Ni} core links with outer {Ni₂₃} transition metal shell through μ₃-OH^-, CO₃^2- (Fig. S4 in Supporting information), glycine (Fig. S5c in Supporting information) and IDA^2- ligands (Fig. S3 in Supporting information), forming the core-shell {Eu₁₈Ni₂₄} cluster (Fig. 2a). The adjacent {Eu₁₈Ni₂₄} clusters interact with each other based on IDA^2- ligands (Fig. 3b), forming the one-dimensional chain (Fig. 3).

Although structure of 1-(Gd₁₈Ni₂₄)_n is very similar to 2-(Eu₁₈Ni_23.5)_n, there are still some distinctions between two products: (ⅰ) the occupancy factors of all Ni²⁺ ions in 1-(Gd₁₈Ni₂₄)_n are one, but Ni₂₄ in 2-(Eu₁₈Ni_23.5)_n is 0.5; (ⅱ) μ₂-OH^- groups are only connected with metal ions in 1-(Gd₁₈Ni₂₄)_n; (ⅲ) IN^-, CH₃COO^- and NH₂CH₂COO^- groups are only coordinated with metal ions in 2-(Eu₁₈Ni_23.5)_n; (ⅳ) 9 Cl^- and 14 free H₂O groups are in 1-(Gd₁₈Ni₂₄)_n, but 7 Cl^- and 17 free H₂O groups are in 2-(Eu₁₈Ni_23.5)_n; (ⅴ) the chain-like structures of compounds 1-(Gd₁₈Ni₂₄)_n and 2-(Eu₁₈Ni_23.5)_n are both constructed by IDA^2- ligand, but the coordination modes of IDA^2- ligands are different (as depicted in Figs. 3 and 4)

Meanwhile, CO₃^2- anions act not only as the counter anions to balance the valence, but also as the anion templates to induce the synthesis of metal skeletons. As shown in Figs. S8 and S9 (Supporting information), CO₃^2- anions are distributed in the middle of {Eu₁₈Ni₂₄} and {Gd₁₈Ni₂₄} building blocks as well display multidentate coordination patterns to link the neighbor subunits ({Eu₁₁Ni} and {Eu₇}, {Gd₁₁Ni} and {Gd₇}).

A great number of Gd³⁺ ions in compound 1-(Gd₁₈Ni₂₄)_n encourage us to investigate the MCE. The plot of χ_MT-T for 1-(Gd₁₈Ni₂₄)_n was studied with the scope of 1.8–300 K and under the appropriate magnetic field of 1000 Oe (Fig. 5). The value of χ_MT was 162.31 cm³ K/mol at room temperature, which was slightly inferior to the calculated result (165.75 cm³ K/mol; for 18 Gd³⁺ ions, S = 7/2, g = 2; and 24 Ni²⁺ ions S = 1, g = 2) [16]. Along with the cooling, χ_MT decreased in the scale of T = 1.8–300 K, and attained the minimum (131.06 cm³ K/mol). This behavior implied the existence of antiferromagnetic interactions in 1-(Gd₁₈Ni₂₄)_n. The Curie–Weiss Law was studied to fit the χ_M^-1 vs. T between 1.8 and 300 K, with two parameters C = 162.14 cm³ K/mol and θ = -4.10 K (Fig. S18 in Supporting information) [28]. The negative of θ indicated that weak antiferromagnetic interactions are presented in compound 1-(Gd₁₈Ni₂₄)_n [30]. The research of M-H for 1-(Gd₁₈Ni₂₄)_n was studied (T = 1.8–15 K, H = 0–7 T; Fig. S19 in Supporting information). As H increased, the M gradually rose, and attained the saturation value of 142.07 Nµ_B at 7.0 T. The experimental value of M was smaller than the calculated result (174 Nµ_B), which can be attributed to the antiferromagnetic exchanges of the metal ions [38, 39].

The magnetization studies of 1-(Gd₁₈Ni₂₄)_n were taken based on the scope of temperature (2.0–14.0 K) and the field (0.5–7.0 T), which were performed by the Maxwell relation for ∆S_m(T) = ∫[∂M (T, H)/∂T] [12]. The largest value of -∆S_m for 1-(Gd₁₈Ni₂₄)_n was 35.3 J kg^-1 K^-1 at roughly 3.0 K with a field change of ∆H = 7.0 T (as exhibited in Fig. 6), which was inferior to the theoretical result (60.27 J kg^-1 K^-1). And the theoretical calculation formula is -∆S_m = n_NiR ln (2S_Ni +1)/M_r+n_Gd R ln (2S_Gd +1)/M_r [35]. The discrepancy between theoretical value and experimental value may be ascribed to the antiferromagnetic behavior in 1-(Gd₁₈Ni₂₄)_n [31]. Besides, the value of -∆S_m is relatively large in reported 3d-4f clusters, manifesting that 1-(Gd₁₈Ni₂₄)_n is the potential magnetic cooling material (Table S4 in Supporting information). It is worth noting that the large values of -∆S_m at low fields come up to 12.25 J kg^-1 K^-1 (∆H = 1.0 T) and 20.95 J kg^-1 K^-1 (∆H = 2.0 T) at 2.0 K (Table S5 in Supporting information).

In summary, two attracting high-nuclearity 3d-4f mixed-metal clusters [1-(Gd₁₈Ni₂₄)_n and 2-(Eu₁₈Ni_23.5)_n] were isolated in the existence of CO₃^2- anionic templates and solvothermal treatment. Single-crystal X-ray diffraction studies suggested that products in this work are both bridged via IDA^2- ligands to display the infrequent one-dimensional chain shapes. In addition, magnetic investigations indicated that 1-(Gd₁₈Ni₂₄)_n is the potential magnetic coolant with -∆S_m = 35.3 J kg^-1 K^-1 at 3.0 K and ∆H = 7.0 T. Especially, the low-field magnetic entropy change of compound 1-(Gd₁₈Ni₂₄)_n is remarkable (-∆S_m = 20.95 J kg^-1 K^-1 at 2.0 T and 2.0 K). This work offers a consultant for assembling novel metal clusters together with the large MCE.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Natural Science Foundation of China (No. 21571103) and Jiangsu (No. BK20191359); the Project of Natural Science Foundation of the Higher Education Institutions of Anhui Province, China (No. KJ2019A0350).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.04.042.