Uranium(IV) Chloride Complexes: UCl62- and an Unprecedented U(H2O)4Cl4 Structural Unit

The room temperature synthesis and structural characterization of two U(IV) compounds isolated from acidic aqueous solution is reported. Evaporation of a U(IV)/HCl solution containing pyridinium (HPy) yielded (HPy)2UCl6 (1), yet in the presence of an organic carboxylate U(H2O)4Cl4·(HPy·Cl)2 (2) is obtained. The structures have been determined by single crystal X-ray diffraction and characterized by Raman, IR, and optical spectroscopies. The magnetism of both compounds was also investigated. The structure of 1 is built from UCl62- anionic units, pervasive in descriptions of the aqueous chemistry of tetravalent uranium, and is found to undergo a phase transition from C2/m to P1̅ upon cooling. By comparison, the structure of 2 contains a neutral U(IV)-aquo-chloro complex, U(H2O)4Cl4, for which there is no literature precedence. Density functional theory calculations were performed to predict the geometries, vibrational frequencies, and relative energetics of the UCl62- and U(H2O)4Cl4 units. The energetics of the reaction of U(H2O)4Cl4 to form the dianion are predicted to be exothermic in the gas phase and in aqueous solution. The predicted energetics coupled with no previous solid state reports of a U(IV)-aquo-chloro complex may point toward the importance of hydrogen bonding and other supramolecular interactions, prevalent in the structures of 1 and 2, on the stabilization and/or crystallization of the U(H2O)4Cl4 structural unit.