Porous Uranium Diphosphonate Frameworks with Trinuclear Units Templated by Organic Ammonium Hydrolyzed from Amine Solvents

Bimetallic Uranium Complexes with 2,6-Dipicolinoylbis(N,N-Dialkylthioureas)

2,6-Dipicolinoylbis(N,N-dialkylthioureas), H2LR, readily react with uranyl salts under formation of monomeric or dimeric complexes of the compositions [UO2(LR)(solv)] (solv = donor solvents such as H2O, MeOH or DMF) or [{UO2(LR)(µ-OMe)}2]2- (1). In such complexes, the uranyl ions are exclusively coordinated by the "hard" O,N,O or N,N,N donor atom sets of the central ligand unit and the lateral sulfur donor atoms do not participate in the coordination. Different conformations have been found for the dimeric anions. The bridging methanolato ligands and the four uncoordinated sulfur atoms can adopt different orientations with respect to the equatorial coordination spheres of the uranyl units. The presence of non-coordinated sulfur atoms offers the opportunity for the coordination of additional, preferably "soft" metal ions. Thus, reactions with [AuCl(PPh3)], lead acetate or acetates of transition metal ions such as Ni2+, Co2+, Fe2+, Mn2+, Zn2+, or Cd2+, were considered for the syntheses of bimetallic complexes. Various oligometallic complexes with uranyl units were prepared: [{UO2(LR)(μ-OMe)(Au(PPh3)}2] (2), [(UO2)3Pb2(LR)4(MeOH)2(μ-OMe)2] (3), [M{UO2(LR)(OAc)}2] (M= Zn, Ni, Co, Fe, Mn or Cd) (R = Et: 5, RR = morph: 6), or [(UO2)(NiI)2(LR)2] (7). The products were extensively studied spectroscopically and by X-ray diffraction.

A tetrahedron from homooxacalix[3]arene, the fifth Platonic polyhedron from calixarenes and uranyl

A self-assembled tetrahedral cage results from two C ₃-symmetry building blocks, namely, homooxacalix[3]arene tricarboxylate and uranyl cation, as demonstrated by X-ray crystallography. In the cage, four metals coordinate at the lower rim with the phenolic and ether oxygen atoms to shape the macrocycle with appropriate dihedral angles for tetrahedron formation, whereas four additional uranyl cations further coordinate at the upper-rim carboxylates to finalize the assembly. Counterions dictate the filling and porosity of the aggregates, whereas potassium induces highly porous structures, and tetrabutylammonium yields compact, densely packed frameworks. The tetrahedron metallo-cage complements our previous report (Pasquale et al., Nat. Commun., 2012, 3, 785) on uranyl-organic frameworks (UOFs) from calix[4]arene and calix[5]arene carboxylates (octahedral/cubic and icosahedral/dodecahedral giant cages, respectively) and completes the assembly of all five Platonic solids from just two chemical components.

Layered Uranyl Phosphonates Encapsulating Co(II)/Mn(II)/Zn(II) Ions: Exfoliation into Nanosheets and Its Impact on Magnetic and Luminescent Properties

Layered heterometallic 5f-3d uranyl phosphonates can exhibit unique luminescent and/or magnetic properties, but the fabrication and properties of their 2D counterparts have not been investigated. Herein we report three heterobimetallic uranyl phosphonates, namely, [(UO₂ )₃ M(2-pmbH)₄ (H₂ O)₄ ] ⋅ 2H₂ O [MU, M=Co(II), CoU; Mn(II), MnU; Zn(II), ZnU; 2-pmbH₃ =2-(phosphonomethyl)benzoic acid]. They are isostructural and display two-dimensional layered structures where the M(II) centers are encapsulated inside the windows generated by the diamagnetic uranyl phosphonate layer. Each M(II) has an octahedral geometry filled with four water molecules in the equatorial positions and two phosphonate oxygen atoms in the axial positions. The uranium atoms adopt UO₇ pentagonal bipyramidal and UO₆ square bipyramidal geometries. The lattice and coordination water molecules can be released by thermal treatment and reabsorbed in a reversible manner, accompanied with changes of magnetic dynamics. Interestingly, the bulk samples of MU can be exfoliated in acetone via freezing and thawing processes forming nanosheets with single-layer or two-layer thickness (MU-ns). Magnetic studies revealed that the CoU and MnU systems exhibited field-induced slow magnetization relaxation at low temperature. Compared with crystalline CoU, the magnetic relaxation of the CoU-ns aggregates is significantly accelerated. Moreover, photoluminescence measured at 77 K showed slight red-shift of the five characteristic uranyl emission bands for ZnU-ns in comparison with those of the crystalline ZnU. This work gives the first examples of 2D materials based on 5f-3d heterometallic uranyl phosphonates and illustrates the impact of dimension reduction on their magnetic/optical properties.

Lanthanide-uranyl phosphonates constructed from diethyl ((phenylsulfonyl)methyl)phosphonate

Lanthanide-uranyl phosphonates possess intriguing crystal structures and huge application prospects, but the construction of these materials remains challenging. In this work, we demonstrate that new uranyl and lanthanide-uranyl sulfonylphosphonates with elegant crystal structures and photophysical properties can be assembled hydrothermally by employing the heterofunctional diethyl phosphonate (diethyl ((phenylsulfonyl)methyl)phosphonate, Et₂L) as a precursor ligand, which provides an effective strategy for the construction of lanthanide-uranyl phosphonates.

Synthesis of luminescent thorium-based metal-organic frameworks with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene

Three new thorium-based MOFs based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under a similar reaction system (metal salt, ligand, solvent, and acid are the same). Th(iv) forms the central unit of the MOFs in mononuclear and binuclear clusters, respectively. All the MOFs show blue ligand-based luminescence under an ultraviolet environment. This is the first time that multiple thorium-based MOFs with luminescence have been found with the same ligand.

Three new thorium-based MOFs based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (H4TCPB) were obtained under a similar reaction system (metal salt, ligand, solvent, and acid are the same).

Construction of Hybrid Bimetallic Uranyl Compounds Based on a Preassembled Terpyridine Metalloligand

Six hybrid uranyl-transition metal compounds [UO₂ Ni(cptpy)₂ (HCOO)₂ (DMF)(H₂ O)] (1), [UO₂ Ni(cptpy)₂ (BTPA)₂ ] (2), [UO₂ Fe(cptpy)₂ (HCOO)₂ (DMF)(H₂ O)] (3), [UO₂ Fe(cptpy)₂ (BTPA)₂ ] (4), [UO₂ Co(cptpy)₂ (HCOO)₂ (DMF)(H₂ O)] (5), and [UO₂ Co(cptpy)₂ (BTPA)₂ ] (6), based on bifunctional ligand 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine (Hcptpy) are reported (H₂ BTPA = 4,4'-biphenyldicarboxylic acid). Single-crystal XRD revealed that all six compounds feature similar metalloligands, which consist of two cptpy^- anions and one transition metal cation. The metalloligand M(cptpy)₂ can be considered to be an extended linear dicarboxylic ligand with length of 22.12 Å. Compounds 1, 3, and 5 are isomers, and all of them feature 1D chain structures. The adjacent 1D chains are connected together by hydrogen bonds and π-π interactions to form a 3D porous structure, which is filled with solvent molecules and can be exchanged with I₂ . Compounds 2, 4, and 6 are also isomers, and all of them feature 2D honeycomb (6,3) networks with hexagonal units of dimensions 41.91×26.89 Å, which are the largest among uranyl compounds with honeycomb networks. The large aperture allows two sets of equivalent networks to be entangled together to result in a 2D+2D→3D polycatenated framework. Remarkably, these uranyl compounds exhibit high catalytic activity for cycloaddition of carbon dioxide. Moreover, the geometric and electronic structures of compounds 1 and 2 are systematically discussed on the basis of DFT calculations.

Potassium Ions Induced Framework Interpenetration for Enhancing the Stability of Uranium-Based Porphyrin MOF with Visible-Light-Driven Photocatalytic Activity

The stability of many MOFs is not satisfactory, which severely limits the exploration of their potential applications. Given this, we have proposed a strategy to improve the stability of MOFs by introducing alkali metal K⁺ capable of coordinating with metal nodes, which finally induces the interpenetrating uranyl-porphyrin framework to connect as a whole (IHEP-9). The stability experiments reveal that the IHEP-9 has good thermal stability up to 400 °C and can maintain its crystalline state in the aqueous solution with pH ranging from 2 to 11. The catalytic activity of IHEP-9 as a heterogeneous photocatalyst for CO₂ cycloaddition under the driving of visible light at room temperature is also demonstrated. This induced interpenetration and fixation method may be promising for the fabrication of more functional MOFs with improved structural stability.

Auxiliary Ligand-Dependent Adaptive Regulation of Uranyl Coordination in Mixed-Ligand Uranyl Compounds of Flexible Biphenyltetracarboxylic Acid

The mixed-ligand strategy is one of the important methods for preparing new materials and regulating the properties of materials. In this work, by introducing different auxiliary ligands (ALs), we have obtained a series of mixed-ligand uranyl complexes (1-6) from a flexible biphenyltetracarboxylic acid (H₄bptc) with an adjustable orthogonal conformation and studied the influence of different organic base molecules on the coordination and assembly of H₄bptc with a uranyl cation. It is found that the coordinated ALs, including 4,4'-bipyridine-1,1'-dioxide and 1,10-phenanthroline, partially occupy the coordination sites of the uranyl center and directly affect the molecular conformations and uranyl coordination of flexible bptc linkers. On the other hand, noncoordinated ALs such as protonated 4,4'-bipyridine ([H₂(4,4'-bpy)]²⁺) or dimethylammonium, which work as counterions in the form of encapsulated guests or hydrogen-bonded templates, also have a nonnegligible impact on the conformation and coordination of bptc linkers. Most interestingly, the AL-mediated evolution of uranyl coordination by the bptc linker and coordination geometry of the uranyl center is clearly observed, which suggests the adaptability of flexible bptc linkers to take suitable molecular configurations and uranyl coordination modes so as to adapt to the external regulator agents and varying environment. The physicochemical characterization of these uranyl compounds, especially photoluminescence, is addressed and discussed, and the results reveal that compound 5 has the potential to serve as a multifunctional radiation detection material for UV light and X-ray radiation.

Breaking a Paradigm: Observation of Magnetic Order in the Purple U(IV) Phosphite: U(HPO3)2

The hydrothermal crystal growth of a new uranium phosphite, U(HPO₃)₂, is reported. This material was found to exhibit optical and magnetic properties not commonly observed in U(IV) containing materials, specifically purple coloration and low temperature magnetism. We discuss the synthesis, structure determination, and characterization of the title compound and comment on its optical, thermal, and magnetic properties. The magnetic behavior is consistent with frustrated antiferromagnetism that arises from the hexagonal honeycomb lattice of U(IV) ions. This material gives rare evidence for U(IV) ions participating in magnetic order.