Properties of aqueous Ce^IVDOTA prepared radiolytically and electrochemically in the presence of halide and azide anions

Ce^IV(DOTA) (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was produced both radiolytically and electrochemically in the presence of halides and azide anions. Only fluoride, the hardest Lewis base studied, ligates to both [Ce^III(DOTA)(H₂O)]⁻ and Ce^IV(DOTA), stabilizing the high oxidation state by two orders of magnitude versus [Ce^IV/III(DOTA)]^0/− in its absence. The Ce^IV(DOTA) complexes are long-lived in the dark (decompose photochemically). The tetravalent complexes decompose mainly via kinetics which obeys a first order rate law. The first step is de-carboxylation of the DOTA ligand followed by the formation of CH₂O. The kinetics of oxidation of [Ce^III(DOTA)(H₂O)]⁻ by Cl₂^[rad]−, Br₂^[rad]− and N₃^[rad] were studied. Cl₂^[rad]−, the strongest oxidizing agent studied, reacts mainly via H atom abstraction from the DOTA ligand. Br₂^[rad]− at both acidic and neutral pH oxidizes [Ce^III(DOTA)(H₂O)]⁻ with rate constants of <3 × 10⁶ M⁻¹s⁻¹, as measured by pulse radiolysis, via an inner sphere mechanism. N₃^[rad] oxidizes [Ce^III(DOTA)(H₂O)]⁻ to its tetravalent analogue as well, as proved by UV–vis data after steady-state radiolysis. The presence of F⁻ increased the yield of the Ce^IV(DOTA) formed. The oxidation rate by N₃^[rad] is probably lower than that measured for Br₂^[rad]−.