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 benza midoximate (ba), and the number of ba coordinated to UO22+ was determined t o be 3 by analyzing the chemical shift of C-13 NMR signal for Hba in the pr esence of UO22+. The exchange rate constants (k(ex)) of ba in [UO2(ba)(3)]( -) were determined by the NMR line-broadening method. The kinetic parameter s were obtained as follows: k(ex)(25 degrees C)=1.3x10(3) s(-1), Delta H-no t 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 fo und to be extremely large compared with those of UO2(L)(5)(2+) (L=unidentat e oxygen donor ligands) complexes. This is due to the strong electron withd rawing of UO22+ from Hba and suggests that an interaction between UO22+ and Hba, is very strong. Such a high affinity of monomeric amidoxime to UO22reasonably explains the high adsorptivity of amidoxime resin to U(VI) speci es, and is considered to result in the high recovery of U(VI) species from sea water using amidoxime resin.