Trapping Dynamically Formed Cationic Materials of Amine–Diamide Ligands in the Nitric Acid Media for Technetium-99 Cleanup

Amine-diamides have exhibited good extractability toward radiotoxic pertechnetate (⁹⁹TcO₄⁻) and high acidic resistance. Their starting cationic materials, however, have not been experimentally captured in the nitric acid media, and the enhanced extraction mechanism over triamide analogues remains underexplored. Herein, a series of 2,2'-(alkylimino)bis(N,N-diethylacetamide), marked as RNDA (R = Me, Et, n-Bu and n-Oct), for extracting TcO₄⁻ have been investigated using molecular dynamics and density functional theory calculations. The dynamically formed [(HMeNDA)⁺·NO₃⁻] was trapped for the first time, along with its anion exchange reaction with TcO₄⁻. A binary extracted complex [(HRNDA)⁺·TcO₄⁻] was recognized and stabilized through multiple hydrogen bonds. It contrasts with the ternary complex of N,N,N',N',N'',N''-hexaethylnitrotriacetamide (NTA) that contains an additional water molecule. The difference is because more hydrophobic RNDA, especially with a long-chain alkyl substituent, squeezes the water out. It is found that RNDA shows better extractability than NTA. It is the alkyl introduction that has increased electron density around the central N atom (i.e., raised nucleophilicity and basicity) and concurrently decreased electron density of surrounding carbonic H atoms (raised electrophilicity and acidity). Taken together, the hydrogen-bonding interactions between NDA and TcO₄⁻ are enhanced.