Enlarging the ring by incorporating a phosphonate coligand: from the cyclic hexanuclear to octanuclear dysprosium clusters

Self-assembly of octanuclear Ln(III)-based clusters: their large magnetocaloric effects and highly efficient conversion of CO2

The design and construction of high-nuclear lanthanide clusters with fascinating topology and functional properties have been an active area of research, however, the development of an effective approach for obtaining high-nuclear lanthanide clusters with multifunctional properties is still extremely difficult. Up to now, a systematic approach for guiding the further expansion of Ln(III)-based clusters showing good functional properties is lacking. Herein, we design and synthesize a polydentate Schiff base ligand (HL), which reacts with β-diketonate salts Ln(acac)₃·2H₂O, and a series of Ln₈ clusters [Ln₈(acac)₆(L)₂(μ₃-O)₆(μ₂-C₂H₅O)₄(μ₂-Hacac)₂]·2CH₃CN (Ln(III) = Gd (1), Dy (2), and Ho (3); HL = pyridine-2-carboxylic acid (5-hydroxymethyl-furan-2-ylmethylene)-hydrazide, Hacac = acetylacetone) have been successfully synthesized. Single-crystal X-ray diffraction studies reveal that clusters 1-3 are isostructural and can be viewed as a Ln₈ core bridged by eighteen μ₂-O atoms, six μ₃-O atoms and two μ₄-O atoms. Magnetic studies show that cluster 1-Gd8 displays a large magnetocaloric effect with -ΔS_m = 46.14 J kg^-1 K^-1 (T = 2.0 K and ΔH = 7.0 T); cluster 2-Dy8 exhibits single-molecule magnet behavior under zero-field conditions. It is worth mentioning that the -ΔS_m of cluster 1-Gd8 is larger than that of most reported polynuclear Gd(III)-based clusters; the 2-Dy8 cluster is one of the rare polynuclear Ln_n SMMs (n ≥ 8) under zero dc field. Importantly, these Ln(III)-based clusters (1-3) can catalyze the cycloaddition of CO₂ with epoxides with high efficiency under mild conditions; and cluster 1-Gd8 as a catalyst could be reused at least three times without obvious loss of catalytic performance.

Ring-forming transformation associated with hydrazone changes of hexadecanuclear dysprosium phosphonates

{Dy16(μ6-C10H7PO3)2(μ5-C10H7PO3)8(spch)8(μ3-OH)2(μ2-OH)2(μ2-AcO)6(μ3-COO)2(DMF)2(H2O)6}·0.5CH3OH·4.5H2O (1) and {Dy16(μ5-C10H7PO3)4(μ3-C10H7PO3)12(μ2-C10H7PO3H)8(opch)4(DMF)8(MeOH)4}·2.5CH3OH·3H2O (2), where H2spch is ((E)-N'-(2-hydroxybenzylidene)pyrazine-2-carbohydrazide, C10H7PO3H2 is 1-naphthylphosphonic acid and H2opch is (E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide, were successfully synthesized by varying the hydrazone ligands in the Dy-phosphonate system. It is important that the ellipsoidal core experiences a ring forming structural transformation to the supramolecular square motif upon the incorporation of an ortho-methoxy substituent into the hydrazone. Alternating-current (ac) magnetic susceptibility studies of 1 and 2 suggest that similar single molecule magnet behaviors occur for these two complexes. The result represents an effective molecular assembly tactic to develop highly complicated lanthanide coordination clusters through the multicomponent self-assembly of the coalescence of phosphonate- and hydrazone-based ligands and metal salts.

Reticular Chemistry of Uranyl Phosphonates: Sterically Hindered Phosphonate Ligand Method is Significant for Constructing Zero-Dimensional Secondary Building Units

Designability is an attractive feature for metal-organic frameworks (MOFs) and essential for reticular chemistry, and many ideas are significantly useful in the carboxylate system. Bi-, tri-, and tetra-topic phosphonate ligands are used to achieve framework structures. However, an efficient method for designing phosphonate MOFs is still on the way, especially for uranyl phosphonates, owing to the complicated coordination modes of the phosphonate group. Uranyl phosphonates prefer layer or pillar-layered structures as the topology extension for uranyl units occurs in the plane perpendicular to the linear uranium-oxo bonds and phosphonate ligands favor the formation of compact structures. Therefore, an approach that can construct three-dimensional (3D) uranyl phosphonate MOFs is desired. In this paper, a sterically hindered phosphonate ligand method (SHPL) is described and is successfully used to achieve 3D framework structures of uranyl phosphonates. Four MOF compounds ([AMIM]₂ (UO₂ )(TppmH₄ )⋅H₂ O (UPF-101), [BMMIM]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅H₂ O (UPF-102), [Py14]₂ (UO₂ )₃ (TppmH₄ )₂ ⋅3 H₂ O (UPF-103), and [BMIM](UO₂ )₃ (TppmH₃ )F₂ ⋅2 H₂ O (UPF-104); [AMIM]=1-allyl-3-methylimidazolium, [BMMIM]=1-butyl-2,3-dimethylimidazolium, [Py14]=N-butyl-N-methylpyrrolidinium, and [BMIM]=1-butyl-3-methylimidazolium) are obtained by ionothermal synthesis, with zero-dimensional nodes of uranyl phosphonates linked by steric tetra-topic ligands, namely tetrakis[4-(dihyroxyphosphoryl)phenyl]methane (TppmH₈ ), to give 3D framework structures. Characterization by PXRD, UV/Vis, IR, Raman spectroscopy, and thermogravimetry (TG) were also performed.

Reversible ON-OFF switching of single-molecule-magnetism associated with single-crystal-to-single-crystal structural transformation of a decanuclear dysprosium phosphonate

A decanuclear dysprosium phosphonate (I) undergoes two consecutive SC–SC transformations, associated with OFF–ON–OFF switching of its single-molecule-magnetism.

{Dy₅(EDDC)₂(μ₃-AcO)₂(μ₅-C₁₅H₁₁PO₃)(μ₄-C₁₅H₁₁PO₃)(μ₂-AcO)₂(AcO)₂(H₂O)(CH₃OH)₂}₂(μ₄-C₂O₄)·xH₂O (I), where H₂EDDC is N′,N′′,E,N′,N′′,E-N′,N′′-(ethane-1,2-diylidene)dipyrazine-2-carbohydrazide and C₁₅H₁₁PO₃H₂ is 9-anthrylmethylphosphonic acid, is found to undergo two consecutive single-crystal-to-single-crystal transformations. The first is under UV irradiation (λ = 365 nm for 3 d in air) to {Dy₅(EDDC)₂(μ₃-AcO)₂(μ₅-C₁₅H₁₁PO₃)₂(μ₂-AcO)₂(AcO)₂(H₂O)₃}₂(μ₄-C₂O₄)·xH₂O (I-UV) where the two CH₃OH are replaced by two H₂O and the second by annealing under N₂ at 100 °C on a diffractometer or under Ar in a glovebox to {Dy₅(EDDC)₂(μ₃-AcO)₂(μ₅-C₁₅H₁₁PO₃)₂(μ₂-AcO)₄(H₂O)}₂(μ₄-C₂O₄) (I-A-N₂ or I-A-Ar) where it has lost two H₂O molecules. The second transformation is reversible by exposure to air at room temperature (I-A-N₂-cool). While the overall structures are the same (retaining the space group P2₁/c), there is a considerable expansion of the unit cell from I (8171 ų) to I-UV (8609 ų) and I-A-N₂ (8610 ų) and the coordinations of the Dy atoms undergo major reconstructions. This is associated with switching the single-molecule-magnetism (SMM) from OFF for I to ON for I-UV and to OFF again for I-A-Ar in air. Such a switching mechanism associated with the retention of crystallinity is unique in the chemistry of dysprosium. The structure of the molecule is formed from two symmetry related pentamers joined by an oxalate. A related compound containing two isolated neutral pentamers {Dy₅(EDDC)₂(μ₃-AcO)₂(μ₅-C₁₅H₁₁PO₃)₂(μ₂-AcO)₃(AcO)₂(H₂O)₂}{Dy₅(EDDC)₂(μ₃-AcO)₂(μ₅-C₁₅H₁₁PO₃)(μ₄-C₁₅H₁₁PO₃)(μ₂-AcO)₃(AcO)₂(CH₃OH)₂}·2CHCl₃ (II) has also been isolated with closely related Dy coordination and it exhibits similar SMM behaviour in zero field.

A series of six-membered lanthanide rings based on 2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol: synthesis, crystal structures and magnetic properties

A series of six-membered ring cyclic lanthanide(iii) clusters have been constructed by utilizing the pentol ligand.

Di- and octa-nuclear dysprosium clusters derived from pyridyl-triazole based ligand: {Dy2} showing single molecule magnetic behaviour

Dy^III-based two complexes, {Dy₂} (1) and {Dy₈} (2), were successfully isolated with H₂bpte ligand. In 1, H₂bpte bears trans,trans- while in 2 has cis,cis- coordination modes. {Dy₂} displays the single-molecule magnet behaviour with an energy barrier U_eff = 59(4) K.

Cyclic single-molecule magnets: from the odd-numbered heptanuclear to a dimer of heptanuclear dysprosium clusters

A heptanuclear compound and a dimer of heptanuclear dysprosium clusters (Dy₇ and Dy₁₄) have been successfully synthesized, which feature the largest odd-numbered cyclic lanthanide clusters reported thus far. Both exhibit single molecule magnet behaviours at low temperature.