Hydrothermal Crystallization of Uranyl Coordination Polymers Involving an Imidazolium Dicarboxylate Ligand: Effect of pH on the Nuclearity of Uranyl-Centered Subunits

Four uranyl-bearing coordination polymers (<b>1</b>–<b>4</b>) have been hydrothermally synthesized in the presence of the zwitterionic 1,3-bis­(carboxymethyl)­imidazolium (= <i>imdc</i>) anion as organic linkers after reaction at 150 °C. At low pH (0.8–3.1), the form <b>1</b> ((UO<sub>2</sub>)<sub>2</sub>(<i>imdc</i>)<sub>2</sub>(<i>ox</i>)·3H<sub>2</sub>O; <i>ox</i> stands for oxalate group) has been identified. Its crystal structure (XRD analysis) consists of the 8-fold-coordinated uranyl centers linked to each other through the <i>imdc</i> ligand together with oxalate species coming from the partial decomposition of the <i>imdc</i> molecule. The resulting structure is based on one-dimensional infinite ribbons intercalated by free water molecules. By adding NaOH solution, a second form <b>2</b> is observed for pH 1.9–3.9 but in a mixture with phase <b>1</b>. The pure phase of <b>2</b> is obtained after a hydrothermal treatment at 120 °C. It corresponds to a double-layered network (UO<sub>2</sub>(<i>imdc</i>)<sub>2</sub>) composed of 7-fold-coordinated uranyl cations linked via the <i>imdc</i> ligands. In the same pH range, a third phase ((UO<sub>2</sub>)<sub>3</sub>O<sub>2</sub>(H<sub>2</sub>O)­(<i>imdc</i>)·H<sub>2</sub>O, <b>3</b>) is formed: it is composed of hexanuclear units of 7-fold- and 8-fold-coordinated uranyl cations, connected via the <i>imdc</i> molecules in a layered assembly. At higher pH, the chain-like solid (UO<sub>2</sub>)<sub>3</sub>O­(OH)<sub>3</sub>(<i>imdc</i>)·2H<sub>2</sub>O (<b>4</b>) is observed and composed of the infinite edge-sharing uranyl-centered pentagonal bipyramidal polyhedra. As a function of pH, uranyl nuclearity increases from discrete 8- or 7-fold uranyl centers (<b>1</b>, <b>2</b>) to hexanuclear bricks (<b>3</b>) and then infinite chains in <b>4</b> (built up from the hexameric fragments found in <b>3</b>). This observation emphasized the influence of the hydrolysis reaction occurring between uranyl centers. The compounds have been further characterized by thermogravimetric analysis, infrared, and luminescence spectroscopy.