The K+/Y3+ cationic exchange in a C5-symmetric metallamacrocyclic scaffold: DFT examination with QTAIM, ELF, MEP approaches and XRD study of the final alaninehydroximate complex.

Context: Participation of the heterobimetallic potassium-copper metallamacrocycle in synthesis of the yttrium-copper C₅-symmetric complex based on the L-α-alaninehydroximate (Alaha) ligands was investigated by DFT. Two possible conformations of the K(I)-Cu(II) intermediate which can realize in solution were simulated. Theoretical estimations of an activation barrier for the conformational conversion, ∆G^≠_sol(298.15) = 2.7 and 1.0 kcal/mol, indicate a structural inversion of the encapsulated K⁺ ion relative to the copper-containing metallamacrocyclic environment (scaffold). The K(I)-Cu(II) intermediate is considered as a handy molecular platform for facile chelation of the Y³⁺ ions in water solutions that can be used in the nuclear medicine. Replacement of the K⁺ central ion by Y³⁺ is driven by the higher positive charge of the latter which makes it a stronger acceptor of the metallamacrocyclic electron density. Corresponding changes in the electron structure were revealed and quantified by the DFT calculations. Also, the theoretical thermodynamic estimations predict enhanced stability of the final Y(III)-Cu(II) metallamacrocycle. Accordingly, the water-soluble Y(III)-Cu(II) alaninehydroximate complex was prepared in the presence of K₂CO₃ as one of starting reagents.

Methods: The quantum chemical calculations were performed at the M06/def2-TZVP and TPSS-D3/def2-TZVP levels of DFT with use of the polarizable continuum model. The differences in the electron structures were investigated in detail by the quantum theory of atoms in molecules, electron localization function, and molecular electrostatic potential. The final product of the Y(CO₃)(H₂O)[15-MC_Cu(II)Alaha-5](I)∙11H₂O composition was characterized by the X-ray diffraction.