Synthesis, Characterization, and Density Functional Theory Investigation of the Solid-State [UO2Cl4(H2O)]2– Complex

A significant number of solid-state [UO2Cl4]2– coordination compounds have been synthesized and structurally characterized. Yet, despite their purposive relative abundance in aqueous solutions, characterization of aquachlorouranium(VI) complexes remain rare. In the current study, a solid-state uranyl aqua chloro complex ((C4H12N2)2[UO2Cl4(H2O)]Cl2) was synthesized using piperazinium as a charge-balancing ligand, and the structure was determined using single-crystal X-ray diffraction. Using periodic density functional theory, the electronic structure of the [UO2Cl4(H2O)]2– complex was compared to [UO2Cl4]2– to uncover the strengthening of the UO bond in [UO2Cl4(H2O)]2–. Changes in the strength of the UO bond were validated further with Raman and IR spectroscopy, where uranyl symmetrical (ν1) and asymmetrical (ν3) stretches were blue-shifted compared to the reference [UO2Cl4]2– complex. Furthermore, the formation energy of the solid-state (C4H12N2)2[UO2Cl4(H2O)]Cl2 complex was calculated to be −287.60 ± 1.75 kJ mol–1 using isothermal acid calorimetry. The demonstrated higher stability relative to the related [UO2Cl4]2– complex was related to the relative stoichiometry of the counterions.