Cationic receptors based on five-membered and six-membered aromatic N-heterocycles for pertechnetate anion binding in aqueous solutions: comparative DFT analysis

Extraction of the pertechnetate anion TcO₄^- from spent nuclear fuel is one of the most challenging tasks in modern nuclear energetics. It requires the creation of highly selective receptors for binding this anion in aqueous solutions. In this study, density functional theory was used to perform quantum chemical modeling of the structure and properties of tight ion pairs (TIPs) that TcO₄^- forms in solution with cations of N-methylated six- and five-membered heterocycles. The ab initio GGA functional PBE and the hybrid LPBEVVV functional, which more accurately reproduce the phenomenon of long-range electron correlation responsible for various types of non-covalent interactions, were used in combination with a relativistic all-electron triple-ζ basis set. It was found that the mutual spatial orientation of the cation and anion in such ion pairs is determined by two types of non-covalent interactions: the formation of O₃Tc-O⋯H-C hydrogen bonds and the n → π* interaction, due to which the electron density from the anion oxygen atoms is partially transferred to the lowest unoccupied π-orbitals (LUMOs) of electron-deficient aromatic N-heterocycles. The anion/cation binding energies ΔE and ΔG in TIPs increase with decreasing LUMO energies. LUMO energies decrease with increasing number of nitrogen atoms in the heterocycle and with the introduction of electron-withdrawing substituents into it. The n → π* interaction is the main contributor to ΔE and ΔG for receptors based on six-membered heterocycles (pyridines, pyrimidines, pyrazines, and 1,3,5-triazines). In both these series, a linear correlation between ΔE and ΔG and LUMO energy was demonstrated. In the series of receptors based on five-membered heterocycles (imidazoles, triazoles, and tetrazoles) the contributions of hydrogen bonding and n → π* interactions to ΔG with N-alkylimidazolium and N-alkyltriazolium cations are comparable in magnitude, but for N-alkyltetrazolium-based receptors, the latter contribution is predominant. The new data obtained and the identified patterns open up opportunities for fine-tuning the binding energy of anions to the receptor through relatively simple and straightforward structural modifications. These patterns must be taken into account when developing cationic receptors for TcO₄^- and other tetrahedral oxoanions. In particular, the synthesis and characterization of cationic receptors for the pertechnetate anion TcO₄^- containing triazole, tetrazole, and 1,3,5-triazine cores, which have not yet been described, appear highly promising.