Complex Formation of U(VI) with Benzamidoxime in Non-aqueous Solvents

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The structural and kinetic studies of U(VI) complex with benzamidoxime (Hba) as ligand in CD3COCD3 have been studied by means of 1H and 13C 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 13C NMR signal for Hba in the presence of UO22+. The exchange rate constants (kex) of ba in [UO2(ba)3]− were determined by the NMR line-broadening method. The kinetic parameters were obtained as follows: kex(25°C)=1.3×103 s−1, ΔH\\neqq=35.8±3.5 kJ·mol−1, and ΔS\\neqq=−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)52+ (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.
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