Hourglass-Shaped Homo- and Heteronuclear Nonanuclear Lanthanide Clusters: Structures, Magnetism, Photoluminescence, and Theoretical Analysis

Synthesis of nonameric cationic clusters [Dy9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (1), [Dy8Tb (acac)16(μ3-OH)8(μ4-OH)2]OH·2H2O (2), and [Gd9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (3) (acac = acetylacetonate) is reported. The emission spectrum of 1 shows Dy(III) ion characteristic bands assignable to the 4F9/2 → 6HJ (J = 15/2 to 9/2) transitions. Emission due to both Dy(III) and Tb(III) ions is observed for 2 in the visible range, with Tb(III) specific bands appearing due to the 5D4 → 7FJ (J = 6, 4, and 3) transitions. Cluster 3 exhibits a significant magnetocaloric effect (MCE), with -ΔSm values increasing with decrease in temperature and increase in field, reaching -ΔSmmax = 20.98 J kg-1 K-1 at 2 K and 9 T. Isotropic magnetic coupling constants (Js) in 3 derived from density functional theory (DFT) calculations reveal that the exchange interactions are antiferromagnetic and weak. Compound 3 possesses S = 7/2 ground state arising from the central Gd(III) ion along with several nested excited states due to competing antiferromagnetic interactions that yield reasonably large MCE values. Utilizing computed exchange coupling interactions, we have performed ab initio CASSCF/RASSI-SO/POL_ANISO calculations on antiferromagnetic 1 and 2 to estimate the exchange interactions using the Lines model. For 2, Dy(III)···Tb(III) exchange interactions were extracted for the first time and were found to be weakly antiferromagnetically coupled.

Thiacalix[4]arene-Sandwiched Sandglass-like Ln9 Clusters (Ln = Tb and Eu): Insights into the Selective Luminescence Quenching Properties by p-Nitrobenzene Derivatives

Nonanuclear lanthanide clusters Ln₉ (Ln = Tb and Eu) based on p-tert-butylthiacalix[4]arene (H₄TC4A) have been synthesized by the solvothermal reaction and were structurally determined by single-crystal X-ray diffraction. The framework of Ln₉ can be termed as a sandglass-like structure whose two Ln₄-TC4A polynuclear secondary building units are bridged by one octa-coordinate {Ln(μ₃-O)₈} unit. Efficient TC4A-to-Ln energy transfer was observed for Tb₉ but not for Eu₉. The luminescence quantum yield (QY) of Tb₉ in the solid state at room temperature was determined to be 17.6%, while its highest QY in a methanolic solution (2 × 10^-5 mol/L) is 59.2% upon excitation at 318 nm. The luminescence of Tb₉ was quenched selectively by derivatives of p-nitrobenzene, as demonstrated by the results of photoluminescence and UV-vis titration experiments and supported by density functional theory calculations. We believe that the interactions between the analyte molecules and the pocket of Tb₉ are primarily responsible for the observed quenching. As such, this work represents one of the few examples of utilizing structurally interesting lanthanide cluster complexes as a sensory platform for the recognition of meaningful analytes and portends the further development of lanthanide-calixarene-complex-based functional materials.

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.

A mixed-valence [CoII4CoIII2] cluster with defect disk-shaped topology

The employment of the new Schiff base ligand 2-[(4-chloro-2-hydroxybenzylideneamino)methyl]phenol (H2L) bearing O2N donors for the preparation of a novel Co6 cluster is reported. The hexanuclear cobalt complex, namely, di-μ2-acetatotetrakis{μ2-2-[(4-chloro-2-oxidobenzylideneamino)methyl]phenolato}tetra-μ3-methanolato-tetracobalt(II)dicobalt(III), [CoII4CoIII2(C14H10ClNO2)4(CH3COO)2(CH3O)4], was obtained using Co(CH3COO)2·4H2O and H2L as starting materials in MeOH under solvothermal conditions. The six metal ions are linked together by the μ3-O atoms of four deprotonated MeOH molecules, two CH3COO- units and six phenolate O atoms of four L2- ligands to form a defect disk-shaped topology. DC magnetic susceptibility investigations revealed the existence of antiferromagnetic interactions in the Co6 cluster.

Information encryption, highly sensitive detection of nitrobenzene, tetracycline based on a stable luminescent Cd-MOF

A stable luminescent Cd-MOF, formulated as [Cd(L)_0.5(4, 4'-bpy)_0.5]·H₂O (1), (H₄L = 1, 1'-ethylbiphenyl -3, 3', 5, 5'- tetracarboxylic acid, 4, 4' -bpy = 4, 4'-bipyridine), is acquired under solvothermal conditions. 1 exhibits stability in the pH range from 1.5 to 12.2 and in different organic solvents. 1 can detect tetracycline and nitrobenzene by fluorescence quenching with high sensitivity and selectivity. The detection limits are 0.14 μM and 14 nM, respectively. Interestingly, 1 can encapsulate Tb³⁺ and sensitize its characteristic peaks. Moreover, the fluorescent ink is prepared by using the luminescent properties of the Tb³⁺@Cd-MOF. The light of the fluorescent ink disappears in an acid gas HCl atmosphere and then reappears in an alkaline gas ammonia atmosphere. This phenomenon can be repeated and the reason for this phenomenon is also explained in the article. Therefore, Tb³⁺@Cd-MOF has huge application potential in information encryption.

Modulating the relaxation dynamics via structural transition from a dinuclear dysprosium cluster to a nonanuclear cluster

A dinuclear dysprosium cluster [Dy₂(NO₃)₄(H₂O)₂(L)₂]·2CH₃CN was successfully prepared by employing HL (HL = 2,6-dimethoxyphenol) and Dy(NO₃)₃·6H₂O in a mixture of CH₃OH and CH₃CN. The conversion of this Dy₂ compound by reaction with additional deprotonated ligand generated a Dy₉ cluster [Dy₉(μ₄-OH)₂(μ₃-OH)₈(μ₂-OCH₃)₄(NO₃)₈(H₂O)₈(L)₄](OH)·2H₂O with the well-known "diabolo" topology. Magnetic investigation revealed that both of the clusters exhibit typical SMM characteristics, and variable magnetic relaxation with the energy barrier changing from 217.87 K to 9.24 K along with the transition from a dinuclear dysprosium cluster to a nonanuclear one. Ab initio calculations further confirm the corresponding structure-activity relationships that originate the different magnetic behaviours. This design may afford a feasible strategy for modulating the magnetic relaxation dynamics of polynuclear systems.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.

Two hexanuclear lanthanide Ln6III clusters featuring remarkable magnetocaloric effect and slow magnetic relaxation behavior

Two novel hexanuclear lanthanide clusters (1 and 2) were successfully prepared. Magnetic investigations reveal that 1 exhibits remarkable magnetocaloric effect and 2 displays interesting slow magnetic relaxation behaviors.