Solvent effects in solvo-hydrothermal synthesis of uranyl ion complexes with 1,3-adamantanediacetate

Abstract

1,3-Adamantanediacetic acid (H₂ADA) was reacted with uranyl nitrate under solvo-hydrothermal conditions using different organic solvents, giving five complexes which were characterized by their crystal structures and, in most cases, their luminescence properties. The complexes [UO₂(ADA)(H₂O)] (1) and [UO₂(ADA)(NMP)] (2), where NMP is N-methyl-2-pyrrolidone, are ribbon-like 1D coordination polymers including doubly carboxylate-bridged uranyl dimers, with the carboxylate groups being either chelating or bridging bidentate. The denticity of the ADA²⁻ ligand is greater in complex 3, [UO₂(ADA)]·0.5CH₃CN, which crystallizes as a planar 2D network with no coordinated solvent. Two complexes were obtained in water/N,N-dimethylformamide (DMF), which both display 2D arrangements. The topology in [UO₂(ADA)(DMF)] (4) appears to stem from that of the ribbons in 1 and 2 through diversion of the bonds of every other dimer toward adjacent ribbons. The complex [H₂NMe₂]₂[(UO₂)₂(ADA)₃]·3H₂O (5) includes dimethylammonium counter-ions formed in situ from DMF hydrolysis. All ligands in 5 are bis-chelating and the anionic 2D assembly formed, which comprises 8- and 16-membered rings, is ~15 Å thick, half the uranyl ions having their equatorial plane approximately parallel and the other half perpendicular to the sheet plane; adjacent sheets are linked by hydrogen bonded cyclic water hexamers. Emission spectra measured in the solid state show the usual vibronic fine structure, the positions of the maxima reflecting the differences in the number of equatorial donors.