Comparison of uranium complexes with methyl-glutarimidedioxime vs. glutarimidedioxime: A methyl change generates different consequences in thermodynamic equilibria and coordination modes

A methyl group addition to the piperidine ring of glutarimidedioxime generates different coordination mode with uranyl and lowers the binding strength. [Display omitted] •A methyl group addition to the piperidine ring of glutarimidedioxime generates different coordination mode with uranyl and lowers the binding strength.•Thermodynamic and structural studies reveal that glutarimidedioxime and methyl- glutarimidedioxime exhibit distinct coordination behaviors, despite both being expected to form tridentate complexes with uranyl.•Methyl-glutarimidedioxime is shown to be a weaker ligand compared to glutarimidedioxime in terms of competition with carbonate for uranyl complexation.•The design and optimization of molecules for extract uranium from seawater is challenging, even a single methyl change can lead to significant consequences. Nuclear power serves as an important source of alternative energy. However, land uranium resources are limited. In contrast, seawater contains an estimated total of 4.5 billion metric tons of uranium, which is more than a thousand times as much as that found in terrestrial ores. Therefore, developing techniques for uranium extraction from seawater is attracting research attention. Amidoxime-functionalized polymer fibers are the most widely utilized sorbents for this purpose. On the surface of amidoxime-based sorbents, cyclic glutarimidedioxime (H2A) is the preferred configuration for sequestration of uranium from seawater. Methyl-glutarimidedioxime (H2Q) forms if a methyl group attaches to the piperidine ring of H2A. The binding strength of uranyl complexes with H2Q and the enthalpy of complexation were investigated via potentiometry and microcalorimetry, respectively. The coordination mode was further revealed by single-crystal X-ray diffraction. It was expected that H2A and H2Q should be identical in terms of coordination as tridentate ligands. However, our investigation revealed that a methyl group attached to the piperidine ring not only altered the chelating mode but also lowered the binding strength of oxime groups. These findings show that developing a mechanism for optimizing molecules that would be formed on fiber surfaces remains challenging, even a methyl structural change would result in major consequences.