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

Area of Science:

• Inorganic Chemistry
• Uranium Chemistry
• Solid-State Chemistry

Background:

• Tetravalent uranium (U(IV)) chemistry is predominantly described through the UCl6(2-) anionic unit in aqueous solutions.
• The isolation and structural characterization of novel U(IV) solid-state complexes are crucial for understanding its diverse coordination environments.
• Previous literature lacks reports on neutral U(IV)-aquo-chloro complexes in the solid state.

Purpose of the Study:

• To synthesize and structurally characterize new U(IV) compounds from acidic aqueous solutions.
• To investigate the structural differences and properties of complexes formed with and without organic carboxylates.
• To explore the stability and potential supramolecular interactions influencing the formation of U(IV) complexes.

Main Methods:

• Room temperature synthesis of U(IV) compounds from acidic aqueous solutions.
• Single crystal X-ray diffraction for detailed structural determination.
• Raman, IR, and optical spectroscopies for characterization.
• Magnetic susceptibility measurements.
• Density functional theory (DFT) calculations for energetic and geometric predictions.

Main Results:

• Two U(IV) compounds were synthesized: (HPy)2UCl6 (1) and U(H2O)4Cl4·(HPy·Cl)2 (2).
• Compound 1 exhibits a UCl6(2-) anionic unit and undergoes a phase transition upon cooling.
• Compound 2 contains a novel neutral U(IV)-aquo-chloro complex, U(H2O)4Cl4, stabilized by hydrogen bonding and supramolecular interactions.
• DFT calculations predict exothermic formation of the U(H2O)4Cl4 unit from its dianionic form.

Conclusions:

• The presence of organic carboxylates can lead to the isolation of unprecedented neutral U(IV)-aquo-chloro complexes.
• Supramolecular interactions play a significant role in the stabilization and crystallization of U(H2O)4Cl4.
• This study expands the known structural diversity of U(IV) coordination compounds.