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)]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 [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 (nu(1)) and asymmetrical (nu(3)) stretches were blue-shifted compared to the reference [UO2Cl4](2-) complex. Furthermore, the formation energy of the solid-state (C4H12N2)(2)[UO2Cl4(H2O)]-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 [UO2Cl4](2-) complex was related to the relative stoichiometry of the counterions.

Automated summary

A solid-state uranyl aqua chloro complex was synthesized and characterized, and its electronic structure and stability were investigated. The results showed that the U=O bond in the aqua chloro complex was strengthened and had higher stability compared to the reference complex.