Insight Into the Influence of Ligand Conformation on Extraction Behaviour of Uranium: A Combined Theoretical and Experimental Study

Herein, two polymer-based solid-phase extraction materials, loading with urea monodentate ligand (UML) and biuret bidentate ligand (BBL) respectively, were newly synthesized. The influence of ligand conformation on uranium extraction was systematically studied for the first time. Our study indicates that the affinity of ligand for uranyl is determined by not only the coordinating atoms, coordination number and ligand structure, but also its conformation characteristics. The theoretical calculations reveal that (1) All 4 UML conformers and 12 BBL conformers are identified and divided into three types: having no hydrogen bond, having intramolecular CH center dot center dot center dot O=C bond, and having both intramolecular CH center dot center dot center dot O=C and NH center dot center dot center dot O=C bonds; (2) 4 coordinating-inactive BBL conformers are demonstrated with high fraction (> 99%) and lower coordination ability for uranium due to formation of the distinctive NH center dot center dot center dot O=C bond between coordinating oxygen and hydrogen of imino group in the same ligand (inactivation of coordinating atom), which significantly weaken the expected advantage of BBL on its ability to extract uranium; (3) Above inactivation may more easily exist in conformers of the structurally complicated multidentate ligand; and (4) Based on Boltzmann distribution and self-consistent-field calculations of electronic energies, the most probable paths of the two uranium extraction processes for UML- and BBL-based polymers, respectively, were obtained, which indicates that the extraction ability of BBL towards uranyl could not be obviously stronger than that of UML, resulting from nearly equal-values of Delta G (-28 kJ mol(-1) for UML, -30.8 kJ mol(-1) for BBL). The results show good agreement with the experimental results.