Counterion-Controlled Formation of an Octanuclear Uranyl Cage with cis-1,2-Cyclohexanedicarboxylate Ligands.

cis-1,2-Cyclohexanedicarboxylic acid ( c-chdcH2) was reacted with uranyl nitrate under (solvo-)hydrothermal conditions in the presence of different possible counterions. Two neutral complexes of 1:1 stoichiometry were obtained, [UO2( c-chdc)(DMF)] (1) and [UO2( c-chdc)(H2O)] (2), which crystallize as two-dimensional coordination polymers and do not include the additional cations present in solution. In contrast, the complex [NH4][PPh4][(UO2)8( c-chdc)9(H2O)6]·3H2O (3) crystallized in the presence of PPh4Br, ammonium cations being generated in situ from acetonitrile hydrolysis. This complex of 8:9 uranium:ligand stoichiometry contains an octanuclear anionic cage of D3 symmetry with a pseudo-cubic arrangement of uranium atoms. The ammonium cation is held within the cage through four hydrogen bonds with uranyl oxo groups directed inward. This cage complex is luminescent, although with a low quantum yield of 0.06, indicating limited potential as a photo-oxidant of included species.