Complexation of Cyclic Glutarimidedioxime with Cerium: Surrogating for Redox Behavior of Plutonium

The complexation of cerium with glutarimidedioxime (H₂L) was studied by potentiometry, ESI-mass spectrometry, and cyclic voltammetry. Crystallization of [Ce^IV(HL)₃]⁺ from Ce³⁺ starting reactant indicated spontaneous complexation-driven oxidation. In aqueous solution, Ce³⁺ ions form three successive complexes, Ce(HL)²⁺, Ce(HL)₂⁺, and Ce(HL)₃ (where HL^- stands for the singly deprotonated ligand). The interactions of glutarimidedioxime with metal ions are dominantly electrostatic in nature, and the stability constants of the complexes are correlated to the charge density of metal ions. Extrapolations of predicted stability constant (log β) values were made from plotting effective charge and the ionic radius of the metal ion for Pu³⁺ and Pu⁴⁺. The stability constants of Pu^IV(HL)³⁺ and Pu^III(HL)²⁺ are estimated to be 27.74 and 19.75, respectively. The differences of stability constants mean that glutarimidedioxime selectively binds Pu⁴⁺ over Pu³⁺ by a factor of about 8 orders of magnitude, suggesting Pu⁴⁺ would be stabilized by chelation with glutarimidedioxime. The mechanism of reduction of Pu⁴⁺ to Pu³⁺ in acidic solution is explained by decomposition of glutarimidedioxime through acid hydrolysis rather than a chelation-driven mechanism.