2,2′-Bipyridine and 1,10-Phenanthroline as Coligands or Structure-Directing Agents in Uranyl–Organic Assemblies with Polycarboxylic Acids

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Abstract

Seven uranyl complexes with polycarboxylic acids were synthesized under hydrothermal conditions in the presence of 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen), and their crystal structures were determined. The bipy and phen molecules are uncoordinated in the two oxalate complexes [Hbipy]2[(UO2)2(C2O4)(OH)3]2·bipy (1) and [Hphen]2[(UO2)2(C2O4)3] (2), which provide two new two-dimensional architectures in the uranyl oxalate family. The first uranyl complexes with citraconic acid (H2citcn) could also be obtained in these conditions: [UO2(citcn)(bipy)] (3) and [UO2(citcn)(phen)] (4) are isomorphous one-dimensional, ribbonlike polymers in which bipy and phen are terminal coligands. The complex formed with the trans,trans,trans isomer of 1,2,3,4-cyclobutanetetracarboxylic acid (H4cbtc), [Hbipy][UO2(Hcbtc)]·1.5H2O (5), displays a two-dimensional arrangement. One of the carboxylic groups retains its proton and is uncoordinated, in contrast to the bonding scheme in the previously reported framework with the same ligand. Bipy is a coligand in the complex with citric acid (H4cit), [(UO2)2(cit)(bipy)]·0.5H2O (6), which contains both the {[UO2(cit)]2}4– dimeric subunits frequently encountered in uranyl citrates and a third uranium centre bound to three carboxylate groups and chelated by bipy. The closely related D-(–)-citramalic acid (H3citml) gives the complex [Hphen]2[(UO2)5(citml)4(H2O)]·3H2O (7), a three-dimensional framework comprising {[UO2(citml)]2}2– dimeric subunits assembled into chains, and other uranium atoms connecting four such chains. Large channels are occupied by columns of Hphen+ counterions held together by π-stacking interactions. In these complexes, bipy and phen are either present as coligands or as counterions. In the latter case, they act as organic templates that lead to structures different from those previously known with the same carboxylate ligands.

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