The mechanism insight into the cooperative coordination of Aza-18-crown-6 ether with diglycolamide by p-benzyl as linkers enhancing the selectivity for heavy REEs(III).

[Display omitted] • The free rotation of p-benzyl could make Cr6-Bz-DGA better match to REEs(III) of different ion radii with different binding mode. • The aza-crown ether of ligand provided 3 coordination atoms for smaller ionic radii Y(III)/Lu(III) and 6 atoms for larger ionic radii La(III). • The binding capacity of complexes with smaller ionic radii REEs(III) were stronger than complexes with larger ionic radii REEs(III) due to larger charge density. • The NMR titration experiment further proved M/L = 2:2 (aza-crown ether-REEs(III)-DGA connection) was the thermodynamic product. The development of new ligands is crucial for the functional application of REEs(III), because the coordination compounds of rare-earth elements (REEs) have many important applications in pharmaceuticals and nuclear medicine. In this paper, a novel ligand (Cr6-Bz-DGA) with the aza-crown ether and diglycolamide (DGA) units was designed to coordinate with light, medium, and heavy REEs(III). It was found that M/L = 2:2 species (aza-crown ether-REEs(III)-DGA connection) was the thermodynamically stable complex. The stability constants of Nd(III), Eu(III), and Ho(III) complexes were determined by UV–Vis and fluorescence titration, respectively, which were log β Nd = 7.49 ± 0.019, log β Eu = 7.10 ± 0.013 and log β Ho = 8.27 ± 0.032. The variation of fluorescence lifetime suggested that the Eu(III) ion was exclusively coordinated by the ligand with coordination number 9 in the inner sphere of the Eu(III) ion. The shifts of 1H NMR revealed that there are different coordination between the ligand and REEs(III). For larger ionic radii La(III), three O atoms of the DGA unit and five O and one N atoms of the aza-crown ether unit were supplied. While, for Y(III)/Lu(III) with smaller ion radii, the ligand supplied three O atoms from the DGA and one N atom and only two O atoms from the aza-crown ether. The displacement experiment of REEs(III) combined with ligand and the fitted stability constants prove that the smaller ionic radii REEs(III) more strongly bind with ligand than larger ionic radius REEs(III). These results may help understand the coordination behavior of REEs(III) and expand their potential applications. [ABSTRACT FROM AUTHOR]