Uranium(IV) Chloride Complexes: UCl₆^2– and an Unprecedented U(H₂O)₄Cl₄ Structural Unit

Abstract

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 UCl₆^2- 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 UCl₆^2- 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.