Accurate Prediction of the Magnetic Ordering Temperature of Ultralow-Temperature Magnetic Refrigerants

Adiabatic demagnetization refrigeration is known to be the only cryogenic refrigeration technology that can achieve ultralow temperatures (≪1 K) at gravity-free conditions. The key indexes to evaluate the performance of magnetic refrigerants are their magnetic entropy changes (-ΔSm) and magnetic ordering temperature (T0). Although, based on the factors affecting the -ΔSm of magnetic refrigerants, one has been able to judge if a magnetic refrigerant has a large -ΔSm, how to accurately predict their T0 remains a huge challenge due to the fact that the T0 of magnetic refrigerants is related to not only magnetic exchange but also single-ion anisotropy and magnetic dipole interaction. Here, we, taking GdCO3F (1), Gd(HCOO)F2, Gd2(SO4)3·8H2O, GdF3, Gd(HCOO)3 and Gd(OH)3 as examples, demonstrate that the T0 of magnetic refrigerants with very weak magnetic interactions and small anisotropy can be accurately predicted by integrating mean-field approximation with quantum Monte Carlo simulations, providing an effective method for predicting the T0 of ultralow-temperature magnetic refrigerants. Thus, the present work lays a solid foundation for the rational design and preparation of ultralow-temperature magnetic refrigerants in the future.

Construction of a High Nuclear Gadolinium Cluster with Enhanced Magnetocaloric Effect through Structural Transition

Starting from a dinuclear complex {Gd₂(L)₂(NO₃)₄(H₂O)₂}·2(CH₃CN) (1) based on 2,6-dimethoxyphenol (HL), a nonanuclear cluster {Gd₉(L)₄(μ₄-OH)₂(μ₃-OH)₈(μ₂-OCH₃)₄(NO₃)₈ (H₂O)₈}(OH)·2H₂O (2) was obtained via modulating the amount of the ligand and base. Both of them have been structurally and magnetically characterized. Complex 1 decorates the Gd₂ core bridged by double μ₂-phenoxyl oxygen atoms and coordinated neutral CH₃CN molecules, while 2 features the Gd₉ core with a sandglass-like topology. Magnetic investigations reveal that the weaker antiferromagnetic interactions between adjacent metal ions exist in complex 2 than in 1, which is in agreement with the theoretical results. Meanwhile, the magnetocaloric effect with a maximum -ΔS _m value changes from 27.32 to 40.60 J kg^-1 K^-1 at 2 K and 7 T.

An [FeIII8] molecular oxyhydroxide

An [Fe^III₈] hexagonal bipyramid displays antiferromagnetic exchange between the two capping tetrahedral ions and the six ring octahedral ions resulting in a spin ground state of S = 10.

A Stable 3d-4f Heterometallic Cluster with Magneto-Optical Activity

A stable 3d-4f heterometallic cluster, namely, {Dy₄Ni₅L₁₀(NO₃)₄(CO₃)₄(CH₃OH)₂}·CH₃CN (Dy₄Ni₅, HL = 8-hydroxyquinoline), has been solvothermally synthesized and structurally characterized. The compound exhibits an interesting structure in which a tetrahedron based on 4f ions interpenetrates with a square pyramid based on 3d ions. Besides, a unique intermolecular interaction was found in Dy₄Ni₅, giving rise to its high stability not only when it is in the solid state but also when it dissolves in organic solvents. In addition, the magnetic behavior of solid Dy₄Ni₅ and the magneto-optical activity of the Dy₄Ni₅ solution were also studied.

Synthesis of silica-stabilized Ag44 clusters aided by a designed mercaptosilane ligand

The novel and precise design of both a microscopic ligand and macroscopic structure has been demonstrated to improve the stability and potential optical applications of Ag₄₄ clusters. The ligand with designed silane substituents on its thiophenol enabled the synthesized [Ag₄₄(SPhSi(OEt)₃)₃₀](PPh₄)₄ clusters to maintain UV-vis absorption for 13 h when heated at 60 °C in air and be readily coated with silica shells via a one-pot reverse microemulsion method. This composite structure overcomes the issue that non-luminescent Ag₄₄ clusters cannot be applied in photothermal and photoacoustic imaging due to their instability.

Ligand Induced Double-Chair Conformation Ln12 Nanoclusters Showing Multifunctional Magnetic and Proton Conductive Properties

Many methods have been utilized to adjust the size of superatomic metal nanoclusters, while tuning the geometric conformations of specific nanoclusters is rare. Here, we demonstrate that conformation variation can be realized by slightly modifying the ligand under maintaining the nuclei number of metal atoms. A series of novel "double-chair" conformation Ln₁₂ (Ln = Sm (1), Eu (2), Gd (3), Tb (4), and Dy (5)) clusters were generated by replacing 3-formylsalicylic acid with 2,3-dihydroxybenzoic acid in the Ln₁₂ nanocluster. Intriguingly, Dy₁₂ displays slow magnetic relaxation at low temperatures, while Gd₁₂ shows a large magnetocaloric effect (MCE) of 35.63 J kg^-1 K^-1 at 2 K for ΔH = 7 T. Additionally, the introduction of numerous coordination water molecules in these clusters enables Dy₁₂ and Gd₁₂ with high proton conductivity, namely, 2.13 × 10^-4 and 3.62 × 10^-4 S cm^-1 under 358 K and 95% RH humidity conditions.

Construction and structural transformation of two coordination sphere supramolecular isomers based on Co(ii) and 4-(2-pyridyl)-NH-1,2,3-triazole via one-pot synthesis

Two coordination sphere supramolecular isomers based on Co(ii) have been obtained via one-pot synthesis. The structural transformation process has been investigated with the help of mass spectroscopy and theoretical calculations.

Efficiently increasing low-field magnetic entropy by incorporating SO32− into Gd22Ni21 clusters

One distinct nanosized cluster, Gd22Ni21, was isolated by the mixed-anion-templates (SO32- and SO42-), which was firstly reported in 3d-4f metal clusters. Additionally, Gd22Ni21 shows the largest low-field magnetic entropy change...

An uncommon multicentered ZnI-ZnI bond-based MOF for CO2 fixation with aziridines/epoxides

A novel cluster-based MOF with uncommon multicentered Zn^I-Zn^I bonds {[K_1.2Na_2.8Zn^I₈(HL)₁₂]·4H₂O}_n (HL = tetrazole monoanion) (1) was synthesized, which showed higher stability than the reported Zn^I-Zn^I bonded compounds. Moreover, 1 can effectively and circularly catalyze the cyclization of CO₂ and aziridines or epoxides with five substituent groups. Importantly, this is the first time that the catalytic properties of MOFs with multicentered metal-metal bonded clusters as the catalyst have been studied.

Real-time and visual sensing devices based on pH-control assembled lanthanide-barium nano-cluster

Real-time and visual monitoring of pollutants in the air is of great importance since they are usually cannot be seen, smelled, or touched. Lanthanide nano-cluster is a kind of luminescent sensor for various species. However, controlling synthesis of lanthanide nano-cluster remains experimentally challenging. In this work, four series of lanthanide-barium (Ln-Ba) nano-clusters of Dy₂Ba (1), Tb₂Ba₂ (2), Ln₄Ba₃ (Ln = Tb, 3a; Eu, 3b), Tb₄Ba₄ (4) were assembled through precisely controlling the pH of the reactant solutions. The work features the first example that the number of cluster's nuclei changes regularly with the pH. Moreover, investigation reveals that nano-cluster 3a is a highly selective and sensitive sensor towards acetylacetone (acac) and aniline. Interestingly, easy-to-use sensing devices of test paper, agarose gel, and five kinds of film on CaCO₃, polyfoam, coin, mask, and wall that based on 3a were fabricated by facile methods. The seven sensing devices showed remarkable ability to sense aniline and acac vapors with visibility to the naked eyes. This is the first work on multiple real-time and visual sensing devices based on the lanthanide nano-cluster.

Ferromagnetic Archimedean polyhedra {Fe24M18} (M = Fe, Ni, and Mn) with tunable electron configurations

Three symmetric nanocages {Fe24M18} that mimic the Archimedean polyhedra, namely pseudo-rhombicuboctahedron, were synthesized. Their electron configurations depend highly on the changes of metal ions and the deprotonation of auxiliary ligands.

Structural Complexity and Magnetic Orderings in a Large Family of 3d-4f Heterobimetallic Sulfates

The synthesis of a large family of heterobimetallic lanthanide copper sulfates was realized via stoichiometric hydrothermal reactions among Ln₂O₃, CuO, and H₂SO₄, giving rise to four distinct phases, namely Ln₂Cu(SO₄)₂(OH)₄ (Ln = Sm-Ho) (LnCuSO₄-1), Ln₄Cu(SO₄)₂(OH)₁₀ (Ln = Tm-Lu) (LnCuSO₄-2), LnCu(SO₄)(OH)₃ (Ln = Nd-Gd, except Pm) (LnCuSO₄-3), and LnCu(SO₄)(OH)₃ (Ln = Dy-Lu) (LnCuSO₄-4), with completely different topologies. The passage from LnCuSO₄-1 and LnCuSO₄-3 to LnCuSO₄-2 and LnCuSO₄-4 across the 4f series, respectively, can be ascribed to the effect of lanthanide contraction, which progressively induces shrinking of the Ln-O distance, reduction in the Ln coordination number, and eventually structural transitions. The incorporation of identical 3d-4f metal ions into different spin-lattices, in conjunction with substitution of diverse Ln³⁺ cations within the same spin-lattice, gives rise to tunable magnetic properties varying from ferromagnetic ordering in GdCuSO₄-3 and HoCuSO₄-4 to antiferromagnetic ordering in YbCuSO₄-4, and to paramagnetic correlations found in GdCuSO₄-1 and YbCuSO₄-2.

Capturing Lacunary Iron-Oxo Keggin Clusters and Insight Into the Keggin-Fe13 Cluster Rotational Isomerization

The formation mechanism of ferrihydrite is the key to understand its treatment of pollutants in waste water and purification of surface water and groundwater. Although emerging evidence suggests that formation of the ferrihydrite occurs through the aggregation of prenucleation clusters, rather than classical atom-by-atom growth, its formation mechanism remains unclear. Herein, an iron-oxo anionic cluster of [Fe₂₂ (μ₄ -O)₈ (μ₃ -OH)₂₀ (μ₂ -OH)₁₈ (CH₃ COO)₁₆ (H₂ O)₂ ]^4- viewed as a dimer of bivacant β-Keggin-Fe₁₃ clusters was for the first time obtained by using lanthanide ions as stabilizers. Upon dissolution in a mixed solution of isopropanol and water, the lacunary β-Keggin-Fe₁₃ cluster can transform into an α-Keggin-Fe₁₃ cluster, distinctly demonstrating that the Keggin-Fe₁₃ cluster rotational isomerization can be realized through the vacant Keggin-Fe₁₃ cluster.

Double-Propeller-like Heterometallic 3d-4f Clusters Ln18Co7

Two high-nuclearity lanthanide-transition metal clusters with the general formula [Ln₁₈Co^IICo^III₆(OH)₁₄(CO₃)₉(CH₃CH₂COO)₆(dea)₁₂(H₂O)₃₀]·(NO₃)₈·Cl₄·(CH₃CH₂OH)₆·(H₂O)₁₂ (Ln₁₈Co₇, Ln = Gd (1) and Dy (2)) have been obtained by reacting CoCl₂·6H₂O, Ln(NO₃)₃·6H₂O, and a mixture of ligands consisting of propionate and diethanolamine (H₂dea). Crystal structural analysis exhibits two three-blade propellers composed of the Co^III₃Ln₉ units connected by one Co^II ion and three CO₃^2- ions, which assemble into a double-propeller-like structure (Ln₁₈Co^IICo^III₆). Magnetocaloric effect (MCE) studies indicate that Gd₁₈Co₇ exhibits a large entropy change (-ΔS_m) of 36.9 J kg^-1 K^-1.

Anion-Dependent Assembly of 3d-4f Heterometallic Clusters Ln5Cr2 and Ln8Cr4

A series of heterometallic Ln-Cr clusters with the formulas [Ln₅Cr₂(H₂L)₂(OAc)₆(μ₃-OH)₆(H₂O)₁₅](ClO₄)₇·xH₂O (Ln = Gd and x = 33 for 1 and Ln = Dy and x = 21 for 2) and [Ln₈Cr₄(H₂L)₄(OAc)₈(μ₃-OH)₁₆(μ₄-O)₁(H₂O)₈](Cl)(ClO₄)₅·10H₂O (Ln = Gd for 3 and Ln = Dy for 4) was obtained through the reaction of the acetate ligands 2,2-dimethylolpropionic acid (H₃L) and Ln(ClO₄)₃ in the presence of chromium salts with different anions under the same high pH conditions. X-ray analysis revealed that compound 1 contained a metal unit [Gd₃Cr₂] displaying the pentagonal bipyramid configuration and that compound 3 was templated by Cl^- and ClO₄^- as a mixed anion template featuring a quadrangular structure. In compound 3, the 12 metal atoms were arranged in a wheel-shaped metal skeleton [Gd₈Cr₄], which was produced by 4 tetrahedral metal units [Gd₃Cr] sharing vertices. The introduction of the mixed anion template increased the number of metal atoms in the Ln-Cr clusters. Magnetic calculations indicated that there was weak antiferromagnetic Gd···Cr coupling and weak ferromagnetic Gd···Gd coupling in 1, whereas both Gd···Cr and Gd···Gd in 3 exhibited weak antiferromagnetic interactions. Magnetothermal studies showed that compounds 1 and 3 displayed magnetic entropy changes of 25.2 J kg^-1 K^-1 at 5 K and 7 T and 33.8 J kg^-1 K^-1 at 2 K and 7 T, respectively.

Atomically Precise Lanthanide-Iron-Oxo Clusters Featuring the ϵ-Keggin Ion

Atomically precise molecular metal-oxo clusters provide ideal models to understand metal oxide surfaces, self-assembly, and form-function relationships. Devising strategies for synthesis and isolation of these molecular forms remains a challenge. Here, the synthesis of four Ln-Fe oxo clusters that feature the ϵ-{Fe₁₃ } Keggin cluster in their core is reported. The {Fe₁₃ } metal-oxo cluster motif is the building block of two important iron oxyhydroxyide phases in nature and technology, ferrihydrite (as the δ-isomer) and magnetite (the ϵ-isomer). The reported ϵ-{Fe₁₃ } Keggin isomer as an isolated molecule provides the opportunity to study the formation of ferrihydrite and magnetite from this building unit. The four currently reported isostructural lanthanide-iron-oxo clusters are fully formulated [Y₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₂₈ (H₂ O)₁₂ ](ClO₄ )₂₃ ⋅50 H₂ O (1, Y₁₂ Fe₃₃ ), [Gd₁₂ Fe₃₃ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₀ (μ₄ -O)₃₂ (H₂ O)₁₂ ](ClO₄ )₁₅ ⋅50 H₂ O (2, Gd₁₂ Fe₃₃ ) and [Ln₁₆ Fe₂₉ (TEOA)₁₂ (Hyp)₆ (μ₃ -OH)₂₄ (μ₄ -O)₂₈ (H₂ O)₁₆ ](ClO₄ )₁₆ (NO₃ )₃ ⋅n H₂ O (Ln=Y for 3, Y₁₆ Fe₂₉ , n=37 and Ln=Gd for 4, Gd₁₆ Fe₂₉ n=25; Hyp=trans-4-Hydroxyl-l-proline and TEOA=triethanolamine). The next metal layer surrounding the ϵ-{Fe₁₃ } core within these clusters exhibits a similar arrangement as the magnetite lattice, and Fe and Ln can occupy the same positions. This provides the opportunity to construct a family of compounds and optimize magnetic exchange in these molecules through composition tuning. Small-angle X-ray scattering (SAXS) and high-resolution electrospray ionization mass spectrometry (HRESI-MS) show that these clusters are stable upon dissolution in both water and organic solvents, as a first step to performing further chemistry towards building magnetic arrays or investigating ferrihydrite and magnetite assembly from pre-nucleation clusters.

Construction of high-nuclear 4p–4f heterometallic {Ln11Ge12} cluster-organic frameworks with high-sensitivity luminescence sensing of Fe3+ in aqueous solution

The [Pr₁₁Ge₁₂] 4p–4f cluster-organic framework shows highly selective and sensitive sensing of Fe³⁺ in aqueous solution.

A 2D layer network assembled from an open dendritic silver cluster Cl@Ag11N24 and an N-donor ligand

A 2D layer network was synthesized from an N-donor ligand and a new silver cluster featuring an open dendritic structure with a high coordination ratio of N atom. Both the assembly process and luminescence properties of this complex was studied.

Syntheses, crystal-solution structures and magnetic properties of a series of decanuclear heterometallic [LnCoCo] (Ln = Eu, Gd, Tb, Dy) clusters

Four unprecedented decanuclear heterometallic [Ln2CoII4CoIII4] clusters based on a diethanolamine ligand (H2dea), namely [Eu2CoII4CoIII4(dea)8(HCOO)4(OH)2(Cl)2(CH3OH)2]Cl2·4CH3OH·2H2O (1), [Gd2CoII4CoIII4(dea)8(HCOO)4(OH)2(Cl)2(CH3OH)2]Cl2·4CH3OH·2H2O (2), [Tb2CoII4CoIII4(dea)8(HCOO)4(OH)2(Cl)2(CH3OH)2]Cl2·2CH3OH·4H2O (3) and [Dy2CoII4CoIII4(dea)8(HCOO)4(OH)2(Cl)2(CH3OH)2]Cl2·2CH3OH·4H2O (4) were synthesized through a facile solution method. Single-crystal X-ray diffraction analyses reveal that complexes 1-4 consist of a [Ln2CoII4CoIII4] core, which is constructed by bridging a quasi-double cuboidal [Ln2CoII2CoIII2] core with two [CoIICoIII] units. Electrospray ionization mass spectrometry (ESI-MS) using methanol solution reveals that complexes 1-4 are stable in the solution, and the clusters undergo three different substitution reactions (Cl- replaced by OH-, OH- replaced by CH3O- and HCOO- replaced by OH-/CH3O-) at the same time in the ionization state. Magnetic susceptibilities reveal ferromagnetic couplings within complexes 3 and 4, and the magnetocaloric effect (MCE) for 2 was also evaluated and the maximum entropy change (-ΔSm) value reaches 16.3 J kg-1 K-1 at about 3 K and 5 T.

Polymer-Encapsulated Lanthanide-Containing Clusters as Platforms for Fabricating Magnetic Soft Materials

Although many high-nuclearity lanthanide-containing clusters with aesthetical topological nanoarchitectures and unique magnetic properties have been synthesized, there was a big time lag to develop functional soft materials and devices on the basis of these clusters, because of their stability and processability. Herein, we report a universal strategy to fabricate lanthanide cluster based magnetic soft materials under ambient conditions. The prototypical cluster [Gd₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]¹⁸⁺ was encapsulated as an inorganic core by polymeric shells of sulfonate end functionalized poly(ethylene glycol) monoalkyl ethers through electrostatic interaction. The thickness of the shell was readily controlled by precisely tuning length of the polymer chain, leading to controllably reduced antiferromagnetic interactions between the clusters. The encapsulated hybrids can self-assemble to form vesicles in solution and can be used as an excellent agent for in vivo magnetic resonance imaging.

Magnetic Phase Transition in Spark-Produced Ternary LaFeSi Nanoalloys

Using the magnetocaloric effect in nanoparticles holds great potential for efficient refrigeration and energy conversion. The most promising candidate materials for tailoring the Curie temperature to room temperature are rare-earth-based magnetic nanoalloys. However, only few high-nuclearity lanthanide/transition-metal nanoalloys have been produced so far. Here we report, for the first time, the observation of magnetic response in spark-produced LaFeSi nanoalloys. The results suggest that these nanoalloys can be used to exploit the magnetocaloric effect near room temperature; such a finding can lead to the creation of unique multicomponent materials for energy conversion, thus helping toward the realization of a sustainable energy economy.

High-Nuclearity Lanthanide-Containing Clusters as Potential Molecular Magnetic Coolers

High-nuclearity cluster-type metal complexes are a unique class of compounds, many of which have aesthetically pleasing molecular structures. Their interesting physical and chemical properties arise primarily from the electronic and/or magnetic interplay between the component metal ions. Among the extensive studies in the past two decades, those on lanthanide-containing clusters, lanthanide-exclusive or heterometallic with transition metal elements, are most notable. The research was driven by both the synthetic challenges for these generally elusive species and their intriguing magnetic properties, which are useful for the development of energy-efficient and environmentally friendly magnetic cooling technologies. Our efforts in this vein have been concentrated on developing rational synthetic methods for high-nuclearity lanthanide-containing clusters. By means of the now widely adopted approach of "ligand-controlled hydrolysis" of lanthanide ions, a great variety of cluster-type lanthanide hydroxide complexes had been prepared in the first half of this developing period (1999-2006). In this Account, our efforts since 2007 are summarized. These include (1) further development of synthetic strategies in order to expand the ligand scope and/or to increase the nuclearity (>25) of the cluster species and (2) magnetic studies pertinent to the pursuit of materials with a large magnetocaloric effect (MCE). Specifically, with the hope of expanding the family of ligands and producing clusters of previously unknown structures, we tested under hydrothermal or solvothermal conditions the use of readily available yet not commonly used ligands for controlling lanthanide hydrolysis; such ligands, carboxylates as mundane examples, tend to form insoluble complexes prior to any possible hydrolysis. We have also validated the use of preformed transition metal complexes as metalloligands for subsequent control of lanthanide hydrolysis toward heterometallic 3d-4f clusters. Furthermore, we demonstrated using ample examples that the presence of small anions as templates is essential to the assembly of high-nuclearity lanthanide-containing clusters and that maintaining a low concentration of the anion template(s) is a key to such success. It has been found that slow production/release of such anion templates by in situ ligand decomposition or absorption of atmospheric CO₂ is effective in preventing precipitation of their lanthanide salts, allowing not only controllable lanthanide hydrolysis but also gradual and modular assembly of the giant cluster species. Magnetic studies targeting potential applications of such clusters as molecular magnetic coolers have also been conducted. The results are summarized in the second portion of this Account in an effort to establish a certain magneto-structure relationship. Of particular relevance is the possible correlation between MCE (evaluated using the isothermal magnetic entropy change, -ΔS_M) and magnetic density, and the intracluster antiferromagnetic exchange coupling. We have also made some preliminary attempts at preparing processable and practically useful materials in the form of a monodisperse core-shell nanostructure. We succeeded in encapsulating a single nanosized heterometallic molecular cluster in a nanoshell of silica. It was found that such passivation not only helped stabilize the cluster but also reduced the magnetic interactions between individual clusters. These effects are reflected in the slightly enhanced value of -ΔS_M for the core-shell composite over the parent unprotected cluster.

Control of nuclearity in heterometallic CuII–MnIIcomplexes derived from asymmetric Schiff bases: structures and magnetic properties

Among three asymmetric Schiff bases used for the synthesis of heterometallic Cu^II–Mn^IIcomplexes, the one with N₂O₂donor atoms yielded a tetranuclear and a trinuclear complex whereas two N₂O₃donor ligands produced solely dinuclear complexes. The results of magnetic measurements have been justified by DFT study.

Four 3d–4f heterometallic Ln45M7 clusters protected by mixed ligands

A series of high-nuclearity 3d–4f clusters Ln₄₅M₇ were obtained based on acetate and propionate as mixed protecting ligands and ClO₄⁻ and CO₃²⁻ as mixed anion templates.

Ferromagnetic coupling in copper benzimidazole chloride: structural, mass spectrometry, magnetism, and DFT studies

The square planar Cu(Hmbm)Cl₂ form pseudo-hexagonal magnetic layers with ferromagnetic couplings of J(Cu–Cl2a⋯Cu1) = +0.99(30) cm⁻¹, J′(π⋯π) = +0.35(16) cm⁻¹, and g = 2.38(2).