A density functional theory study on the interaction between UO_2^(2+) and the carbamoylphosphoramidic acid ligand for uranium extraction from seawater

Phosphorylurea molecules, which contain both phosphoryl and carbonyl groups, are considered efficient extractants for UO_2^(2+). This study aims to explain the complexation of UO_2^(2+)with carbamoylphosphoramidic acid(CPO), a simple model for phosphorylurea, for ligand design for uranium recovery from seawater using density functional theory calculations, natural bond order analysis,and the quantum theory of atoms in molecules. The results showed that, when CPO acts as a monodentate ligand, the affinity of phosphoryl for UO_2^(2+)is stronger than that of carbonyl, and CPO coordinates with UO_2^(2+)through the phosphoryl oxygen atom. When CPO serves as a bidentate ligand, both the phosphoryl and carbonyl oxygen atoms connect to UO_2^(2+), and the U–O(carbonyl) bond plays a more important role than the U–O(phosphoryl) bond in the interaction between UO_2^(2+)and CPO. This paradox may be caused by the significant charge transfer from the U–O(carbonyl) p bond orbital to the C–N σ antibond orbital of the bidentate CPO. The NH spacer between the phosphoryl and carbonyl groups could ensure the delocalization of the electron system of the molecule. The bidentate binding motif is favored by entropy and opposed by enthalpy, while the monodentate binding motif is favored by enthalpy and opposed by entropy. Ultimately, the bidentate binding motif is more favorable than the monodentate one. As expected, the interaction between UO_2^(2+)and the deprotonated CPO is stronger than that between UO_2^(2+)and the neutral CPO. Comparing the interaction between UO_2^(2+)and CPO with that between UO_2^(2+)and Nphenylcarbamoylphosphoramidic acid(Ph CPO), formed by replacing one hydrogen atom from the terminal nitrogen atom of CPO with a phenyl group, the phenyl substituent at the terminal nitrogen atom of Ph CPO shows a slightly negative effect on the interaction between UO_2^(2+)and Ph CPO.

Phosphorylurea molecules, which contain both phosphoryl and carbonyl groups, axe considered efficient extractants for UO22＋. This study aims to explain the complexation of UO22＋ with carbamoylphosphoramidic acid （CPO）, a simple model for phosphorylurea, for ligand design for uranium recovery from seawater using density functional theory calculations, natural bond order analysis, and the quantum theory of atoms in molecules. The results showed that, when CPO acts as a monodentate ligand, the affinity of phosphoryl for UO22＋ is stronger than that of carbonyl, and CPO coordinates with UO22＋ through the phosphoryl oxygen atom. When CPO serves as a bidentate ligand, both the phosphoryl and carbonyl oxygen atoms connect to UO22＋, and the U-O（carbonyl） bond plays a more important role than the U-O（phosphoryl） bond in the interaction between UO22＋ and CPO. This paradox may be caused by the significant charge transfer from the U-O（carbonyl） · bond orbital of the bidentate CPO. The phoryl and carbonyl groups to the C-N cy antibond orbital NH spacer between the phos- could ensure the delocalization of the electron system of the molecule. The bidentate binding motif is favored by entropy and opposed by enthalpy, while the monodentate binding motif is favored by enthalpy and opposed by entropy. Ultimately, the bidentate binding motif is more favorable than the mon- odentate one. As expected, the interaction between U022＋ and the deprotonated CPO is stronger than that between UO22＋ and the neutral CPO. Comparing the interaction between U022＋ and CPO with that between U022＋ and N- phenylcarbamoylphosphoramidic acid （PhCPO）, formed by replacing one hydrogen atom from the terminal nitrogen atom of CPO with a phenyl group, the phenyl substituent at the terminal nitrogen atom of PhCPO shows a slightly negative effect on the interaction between U022＋ and PhCPO.