Complexation of Astatine(III) with Ketones: Roles of NO₃^- Counterion and Exploration of Possible Binding Modes

Ketones have been proven effective in extracting astatine(III) from aqueous solvents. Previous theoretical studies suggested a mechanism where the "sp²" lone pair on the carbonyl oxygen donates electron density into the π system of the AtO⁺ molecular cation to form a dative-type bond. In this study, co-extraction of NO₃^- as AtO(NO₃)·(O═CR₁R₂) species into the organic phase appears to be a key factor. Adjusting the electronic properties of the ketone, by having an aryl group instead of an alkyl group in the alpha position of the ketone, increased the electron density on C═O, increased the bond strength between the ketone and AtO⁺, and in turn increased the extraction of ²¹¹At into the organic phase. Extraction with diketones shows dependence on the bridging distance between the two carbonyl moieties, where a C₃ or longer bridge results in a 10-fold increase in extraction into the organic phase. DFT calculations show the longer bridge allows for the chelation of AtO(NO₃) by either the second carbonyl or the phenyl ring.