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Four metal-organic frameworks (MOF) with tetravalent uranium have been solvothermally synthesized by treating UCl4 with rigid dicarboxylate linkers in N, N-dimethylfomamide (DMF). The use of the ditopic ligands 4,4′-biphenyldicarboxylate ( 1), 2,6-naphthalenedicarboxylate ( 2), terephthalate ( 3), and fumarate ( 4) resulted in the formation of three-dimensional networks based on the hexanuclear uranium-centered motif [U6O4(OH)4(H2O)6]. This motif corresponds to an octahedral configuration of uranium nodes and is also known for thorium in crystalline solids. The atomic arrangement of this specific building unit with organic linkers is similar to that found in the zirconium-based porous compounds of the UiO-66/67 series. The structure of [U6O4(OH)4(H2O)6(L)6] ⋅X (L=dicarboxylate ligand; X=DMF) shows the inorganic hexamers connected in a face-centered cubic manner through the ditopic linkers to build up a three-dimensional framework that delimits octahedral (from 5.4 Å for 4 up to 14.0 Å for 1) and tetrahedral cavities. The four compounds have been characterized by using single-crystal X-ray diffraction analysis (or powder diffraction analysis for 4). The tetravalent state of uranium has been examined by using XPS and solid-state UV/Vis analyses. The measurement of the Brunauer-Emmett-Teller surface area indicated very low values (Langmuir <300 m2 g−1 for 1, <7 m2 g−1 for 2- 4) and showed that the structures are quite unstable upon removal of the encapsulated DMF solvent. [ABSTRACT FROM AUTHOR]

Crystallized powder of dihydroxide zirconium oxalate Zr(OH) 2 (C 2 O 4 ) ( ZrOx ) was obtained by precipitation and the structure determined from powder X-ray data. The three-dimensional (3D) framework observed in ( ZrOx ) results from the interconnection of zirconium hydroxide chains 1 ∞ [Zr(OH) 2 ] 2+ and zirconium oxalate chains 1 ∞ [{Zr(C 2 O 4 )} 2+ ]. Single crystals of (H 11 O 5 ) 2 [Zr 2 (C 2 O 4 ) 5 (H 2 O) 4 ] ( H2Zr2O5 ) were obtained by evaporation. The structure contains dimeric anions [Zr 2 (C 2 O 4 ) 5 (H 2 O) 4 ] 2- connected through hydrogen bonds to hydroxonium ions (H 11 O 5 ) + to create a 3D supramolecular framework. The addition of ammonium or alkali nitrate led to the formation of single crystals of Na 2 [Zr(C 2 O 4 ) 3 ]·2H 2 O ( Na2ZrOx3 ), M(H 7 O 3 )[Zr(C 2 O 4 ) 3 ]·H 2 O, M = K ( KHZrOx3 ), M = NH 4 ( NH4HZrOx3 ), M(H 5 O 2 ) 0.5 (H 9 O 4 ) 0.5 [Zr(C 2 O 4 ) 3 ], M = Rb ( RbHZrOx3 ), and M = Cs ( CsHZrOx3 ). For the five compounds, the structure contains ribbons 1 ∞ [{ZrOx 3 } 2- ] formed by entities Zr(C 2 O 4 ) 4 sharing two oxalates. In ( Na2ZrOx3 ), the shared oxalates are in cis positions and the chain 1 ∞ [Zr-Ox] is stepped with a Zr-Zr-Zr angle of 98.27(1)°. In the other compounds, the shared oxalates are in trans positions and the chains 1 ∞ [Zr-Ox] are corrugated with Zr-Zr-Zr angles in the range 140.34(1)-141.07(1)°. In the compounds ( MHZrOx3 ), the cohesion between the ribbons is ensured by the alkaline or ammonium cations and the hydroxonium ions (H 7 O 3 ) + for M = K, NH 4 , (H 5 O 2 ) + , and (H 9 O 4 ) + for M = Rb and Cs. During the thermal decomposition of the alkaline-free zirconium oxalates ( ZrOx ), ( H2Zr2Ox5 ), and ( NH4HZrOx3 ), the formed amorphous zirconia is accompanied by carbon; the oxidation of carbon at about 540 °C to carbon dioxide is concomitant with the crystallization of the stabilized tetragonal zirconia., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

A new strategy for the synthesis of large poly-oxo clusters bearing 38 tetravalent uranium atoms {U 38 } has been developed by controlling the water release from the esterification reaction between a carboxylic acid and an alcohol. The molecular entity [U 38 O 56 Cl 40 (H 2 O) 2 (ipa) 20 ]·(ipa) x (ipa = isopropanol) was crystallized from the solvothermal reaction of a mixture of UCl 4 and benzoic acid in isopropanol at temperature ranging from 70 to 130 °C. Its crystal structure reveals the molecular assembly of the UO 2 fluorite-like inner core {U 14 } with oxo groups bridging the uranium centers. The {U 14 } core is further surrounded by six tetrameric sub-units of {U 4 } to form the {U 38 } cluster. Its surface is decorated by either bridging- and terminal chloride anions or terminal isopropanol molecules. Another synthesis using the same reactant mixture at room temperature resulted in the crystallization of a discrete dinuclear complex [U 2 Cl 4 (bz) 4 (ipa) 4 ]·(ipa) 0.5 (bz = benzoate), in which each uranium center is coordinated by two chlorine atoms, four oxygen atoms from carboxylate groups and two additional oxygen atoms from isopropanol. The slow production of water released from the esterification of isopropanol allows the formation of the giant cluster with oxo bridges linking the uranium atoms at a temperature above 70 °C, whereas no such oxo groups are present in the dinuclear complex formed at room temperature. The kinetics of {U 38 } crystallization as well as the ester formation are analyzed and discussed. SAXS experiments indicate that the {U 38 } species are not dominant in the supernatant, but hexanuclear entities which are closely related to the [U 6 O 8 ] type are formed.