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Synthesis, Characterization, and Solid-State Structural Chemistry of Uranium(IV) Aliphatic Dicarboxylates

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journal contribution
posted on 15.03.2021, 16:38 by Aphra V. Murray, Jennifer N. Wacker, Jeffery A. Bertke, Karah E. Knope
Examinations of uranium­(IV) complexation by aliphatic dicarboxylates, ranging from the three-carbon malonate to the six-carbon adipate, yielded six novel phases. Single crystals of the compounds were prepared through either evaporation or solvent layering, and the structures were determined via single-crystal X-ray diffraction. The compounds were further characterized via IR and Raman spectroscopies. For the phases synthesized via solvent layering, correlations between the solution- and solid-state speciation were probed using UV–visible absorption spectroscopy. In the presence of malonate (MA), [U­(MA)2(H2O)3]n (U-1a) was isolated. As the crystals were not suitable for structure determination, reactions of MA with thorium were pursued. These efforts yielded polymorphs, Th-1a and Th-1b; in both An-1a (An = Th, U) and Th-1b, the An­(IV) metal centers are bridged via the MA ligands into 2D sheets. Upon increasing the length of the aliphatic backbone, monodentate and bridging bidentate binding modes are observed. Two distinct ligand-bridged extended networks that vary in metal ion nuclearity were observed for succinate (SA): [UCl2(SA)­(H2O)2]n (U-2) and [U6O4(OH)4(SA)4(H2O)10]·4Cl·mH2O (U-3). Whereas U-2 consists of mononuclear U­(IV) atoms that are bridged through the organic carboxylate into a 3D network, U-3 is built from hexanuclear U­(IV)-hydroxo-oxo clusters that are further bridged into a 3D network. From glutarate (GA) ligand systems, two phases were isolated including [UCl2(HGA)2(H2O)2]n (U-4) that consists of ligand-bridged one-dimensional chains and [U2Cl6(HGA)2(H2O)2]n (U-5), which is built from chloride-bridged {U2Cl2} dimers that are further linked via GA into two-dimensional sheets. Finally, in the presence of adipate (AA), [U6O4(OH)4(AA)4(H2O)8]·4Cl·7­(H2O) (U-6) is observed; the structure consists of hexameric [U6O4(OH)4]12+ clusters that are propagated into one-dimensional chains via the AA and uranium-bound water molecules that directly link adjacent clusters. Taken together, this work provides a systematic examination of the influence of the identity of the dicarboxylate backbones on actinide structural chemistry and highlights the role that the organic ligand plays in stabilizing unique tetravalent actinide structural units.