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 (1–4) have been hydrothermally
synthesized in the presence of the zwitterionic 1,3-bis­(carboxymethyl)­imidazolium
(= imdc) anion as organic linkers after reaction
at 150 °C. At low pH (0.8–3.1), the form 1 ((UO2)2(imdc)2(ox)·3H2O; ox 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 imdc ligand together with
oxalate species coming from the partial decomposition of the imdc molecule. The resulting structure is based on one-dimensional
infinite ribbons intercalated by free water molecules. By adding NaOH
solution, a second form 2 is observed for pH 1.9–3.9
but in a mixture with phase 1. The pure phase of 2 is obtained after a hydrothermal treatment at 120 °C.
It corresponds to a double-layered network (UO2(imdc)2) composed of 7-fold-coordinated uranyl
cations linked via the imdc ligands. In the same
pH range, a third phase ((UO2)3O2(H2O)­(imdc)·H2O, 3) is formed: it is composed of hexanuclear units of 7-fold-
and 8-fold-coordinated uranyl cations, connected via the imdc molecules in a layered assembly. At higher pH, the chain-like solid
(UO2)3O­(OH)3(imdc)·2H2O (4) 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 (1, 2) to hexanuclear
bricks (3) and then infinite chains in 4 (built up from the hexameric fragments found in 3).
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.