Synthesis, Characterization, and Density Functional Theory Investigation of the Solid-State [UO 2 Cl 4 (H 2 O)] 2- Complex.

A significant number of solid-state [UO 2 Cl 4 ] 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 ((C 4 H 12 N 2 ) 2 [UO 2 Cl 4 (H 2 O)]Cl 2 ) 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 [UO 2 Cl 4 (H 2 O)] 2- complex was compared to [UO 2 Cl 4 ] 2- to uncover the strengthening of the U═O bond in [UO 2 Cl 4 (H 2 O)] 2- . Changes in the strength of the U═O bond were validated further with Raman and IR spectroscopy, where uranyl symmetrical (ν 1 ) and asymmetrical (ν 3 ) stretches were blue-shifted compared to the reference [UO 2 Cl 4 ] 2- complex. Furthermore, the formation energy of the solid-state (C 4 H 12 N 2 ) 2 [UO 2 Cl 4 (H 2 O)]Cl 2 complex was calculated to be -287.60 ± 1.75 kJ mol -1 using isothermal acid calorimetry. The demonstrated higher stability relative to the related [UO 2 Cl 4 ] 2- complex was related to the relative stoichiometry of the counterions.