Controlling solid state structure of Uranyl(VI) Complexes: Monomeric Complexes with malonate and malonamate

Previously, the only solid state structures identified for the complexes of UO₂²⁺ with simple dicarboxylic acids were polymeric, unless the ligand:metal ratio was greater than 2:1 or the ligand was exceptionally bulky. Here, we report the synthesis and characterisation by X-ray crystallography of three monomeric 1:2 complexes of UO₂²⁺ with malonic acid or malonamic acid proligands. In the dimalonate hydrate complex in 1 the U centre is 5-coordinate in the equatorial plane, with two 1,5-bidentate ligands and one coordinated water molecule. In the dimalonate fluoride complex in 2, the U centre has a similar coordination environment, with the fluoride occupying the position of the water molecule in 1. Unusually, there are no fluoride bridges in 2. In the dimalonamate complex in 3, the U is 5-coordinate in the equatorial plane, with two 1,5-bidentate ligands and a coordinated water molecules. The occurrence of these structures shows that the structural motif adopted by the uranyl ion in 1:2 complexes with simple carboxylates can be modified through the use of bulky organic countercations and/or changes to ligand functionality.