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.
Cheng et al. (Fri,) studied this question.