Complex formation of U(VI) with benzamidoxime in non-aqueous solvents

The structural and kinetic studies of U(VI) complex with benzamidoxime (Hba) as ligand in CD3COCD3 have been studied by means of H-1 and C-13 NMR. The Hba molecule was found to coordinate to UO22+ in the form of anionic benzamidoximate (ba), and the number of ba coordinated to UO22+ was determined to be 3 by analyzing the chemical shift of C-13 NMR signal for Hba in the presence of UO22+. The exchange rate constants (k(ex)) of ba in [UO2(ba)(3)](-) were determined by the NMR line-broadening method. The kinetic parameters were obtained as follows: k(ex)(25 degrees C)=1.3x10(3) s(-1), Delta H-not equal=35.8+/-3.5 kJ.mol(-1), and Delta S-not equal=-65+/-13.7 J.K-1.mol(-1). The UV-visible absorption spectra of solutions containing UO22+ and Hba were also measured. The molar extinction coefficient of the complex was found to be extremely large compared with those of UO2(L)(5)(2+) (L=unidentate oxygen donor ligands) complexes. This is due to the strong electron withdrawing of UO22+ from Hba and suggests that an interaction between UO22+ and Hba, is very strong. Such a high affinity of monomeric amidoxime to UO22+ reasonably explains the high adsorptivity of amidoxime resin to U(VI) species, and is considered to result in the high recovery of U(VI) species from sea water using amidoxime resin.