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

Area of Science:

• Inorganic Chemistry
• Coordination Chemistry
• Uranium Chemistry

Background:

• Solid-state uranyl tetrachlorido complexes ([UO₂Cl₄]^2-) are well-documented.
• Aquachlorouranium(VI) complexes are less characterized, despite their prevalence in aqueous solutions.

Purpose of the Study:

• To synthesize and characterize a novel solid-state uranyl aqua chloro complex.
• To investigate the electronic structure and bonding properties of the synthesized complex.
• To compare the stability of the uranyl aqua chloro complex with its non-aqua counterpart.

Main Methods:

• Single-crystal X-ray diffraction for structural determination.
• Periodic density functional theory (DFT) for electronic structure analysis.
• Raman and Infrared (IR) spectroscopy to study U═O bond vibrations.
• Isothermal acid calorimetry to determine formation energy.

Main Results:

• A new solid-state uranyl aqua chloro complex, (C₄H₁₂N₂)₂[UO₂Cl₄(H₂O)]Cl₂, was successfully synthesized and structurally characterized.
• DFT calculations indicated a strengthened U═O bond in the [UO₂Cl₄(H₂O)]^2- complex compared to [UO₂Cl₄]^2-.
• Spectroscopic analysis showed blue-shifted uranyl symmetrical (ν₁) and asymmetrical (ν₃) stretches, confirming the strengthened U═O bond.
• Isothermal acid calorimetry yielded a formation energy of -287.60 ± 1.75 kJ mol^-1 for the synthesized complex.
• The synthesized complex exhibited higher stability, attributed to counterion stoichiometry.

Conclusions:

• The synthesis and characterization of the uranyl aqua chloro complex provide valuable data on aquachlorouranium(VI) species.
• The strengthened U═O bond and increased stability are key features of this novel complex.
• The findings contribute to a deeper understanding of uranium coordination chemistry and stability in solid-state environments.