Uranyl Complexes with Hydroxybenzoates: Stability, Structure, and Effects of Hydroxyl Groups Quantity and Position.

Area of Science:

• Inorganic Chemistry
• Coordination Chemistry
• Physical Chemistry

Background:

• Uranyl ions are crucial in nuclear fuel cycles and environmental remediation.
• Understanding uranyl complexation with organic ligands is vital for predicting their behavior in various chemical environments.
• Benzoate and hydroxybenzoates are common organic ligands found in natural and industrial settings.

Purpose of the Study:

• To determine the stability constants of uranyl complexes with benzoate and twelve hydroxybenzoate anions.
• To investigate the relationship between the protolytic properties of hydroxybenzoic acids and the stability of their uranyl complexes.
• To quantify the contribution of hydroxyl group positions to complex stability and identify coordinating functional groups.

Main Methods:

• Potentiometric titration was used to determine stability constants at 298.2 K, 0.1 MPa, and I = 0.05 M (NaClO4).
• Analysis of experimental and literature data established correlations between acid properties and complex stability.
• Carbon-13 Nuclear Magnetic Resonance (13C NMR) spectroscopy identified functional groups involved in complexation.
• X-ray diffraction analysis was performed to determine the crystal structure of a representative uranyl complex.

Main Results:

• Stability constants for uranyl-benzoate and uranyl-hydroxybenzoate complexes were reported.
• A clear relationship was found between the acidity of hydroxybenzoic acids and the stability of their uranyl complexes.
• An incremental approach successfully quantified the impact of hydroxyl group positions (ortho, meta, para) on stability constants.
• 13C NMR confirmed the involvement of carboxylate and hydroxyl groups in uranyl complexation.
• The crystal structure of [UO2(2,6-DHBA)2(H2O)2]·1.5H2O revealed coordination details.

Conclusions:

• The position of hydroxyl groups on the benzoate ring significantly influences uranyl complex stability.
• The protolytic properties of hydroxybenzoic acids are key predictors of uranyl complex formation.
• Spectroscopic and crystallographic methods provide complementary insights into uranyl complexation mechanisms.