Three‐Dimensional MOF‐Type Architectures with Tetravalent Uranium Hexanuclear Motifs (U₆O₈)

Abstract

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 m(2) g(-1) for 1, <7 m(2) g(-1) for 2-4) and showed that the structures are quite unstable upon removal of the encapsulated DMF solvent.

A Mixed‐Valent Uranium Phosphonate Framework Containing UIV, UV, and UVI

“…As ap owerful technique to identifya nd quantify oxidation states of uranium, [11] X-ray photoelectrons pectroscopy (XPS) analysisw as first performed on both compounds.A ss hown in Figure 1, the binding energies( BEs) of the U4f 7/2 peaks in 1,a s referenced to adventitiousC1s (BE = 285.0 eV), are at 380.75, 381.96, and 382.53 eV,i ng ood agreement with the values for the U IV site in [U 6 O 4 (OH) 4 (H 2 O) 6 (L) 6 ]·x DMF, [12] and the U VI site in 2.M oreover, the BE separations of the satellites from the main peaks are in the typical range of U IV ,U V ,a nd U VI components, where the presence of the U V is confirmed by the satellite values of 7.75 and 7.44 eV.T he XPS data also indicatet he presence of U IV /U V /U VI (U 4f 7/2 )w ith ap rimary ratio of 2.02:1:2.18 (Supporting Information, Tables S5). Interestingly,f or comparison, ad ecent fitting for the spectrumo f2 could only be achieved using components of U IV and U VI ,w here the ratio of U IV to U VI is 2.02:1 (Supporting Information, Figure S4, Ta ble S6).…”

Solvothermal Synthesis of Tetravalent Uranium with Isophthalate or Pyromellitate Ligands

“…If we compare the other systems where the role of water was clearly identified in the formation of the final structure [23,26,27] and the reaction medium involving pyromellitic acid, the main difference lies in the acidity strength of the selected carboxylic acid. Indeed, while the pK a values of pyromellitic acid are ranging from 1.92 to 5.63, those of benzoic, isophthalic or terephthalic acid do not exceed 4.82.…”

“…These led to the production of approximately twenty new structures that were isolated by using various organic ligands (formate, [13] acetate, [14][15][16] glycolate, [17] dipicolinate, [18][19][20] malonate, [21] fumarate, [22] benzoate, [23][24][25] terephthalate, [26,27] 4,4Ј-biphenyldicarboxylate, [26] 2,6-naphthalenedicarboxylate, [26] trimesate [28] ). The structural diversity of tetravalent uranium chemistry was also highlighted by the isolation of different nuclearities of inorganic building units, such as monomers, trimers, [28,[17][18][19][20][21] tetramers, [14][15][16]22] hexamers, [13,22,25,26] decamers, [24] hexadecamers, [24] chains [29] or, more recently, the U 38 poly-oxo cluster. [23] In the present contribution, we extended this systematic study to the use of two new carboxylate ligands, such as isophthalic acid (noted 1,3-H 2 bdc) or pyromellitic acid (or 1,2,4,5-benzentetracarboxylic, noted H 4 btec).…”

Influence of oxophilic behavior of UiO‐66(Ce) metal–organic framework with superior catalytic performance in Friedel‐Crafts alkylation reaction

“…As commented earlier, UiO‐66 architecture types of MOFs have been achieved with a wide range of metals ions, such as U(IV), Hf(IV), Th(1V), and Ce(IV) . UiO‐66(Ce) was first reported by Stock and co‐workers in 2015 with UiO‐66 architecture .…”