Metal extraction by silyl-substituted diphosphonic acids. III. Ester group substituent effects on phosphoryl oxygen basicity

Partial esters of alkylenediphosphonic acids are powerful metal ion extractants for actinides in all oxidation states. A study of americium extraction by silyl-substituted alkylenediphosphonic acids revealed higher extraction efficiency for these ligands compared to their 2-ethylhexyl analogs. The increased efficiency of silyl extractants could arise from differences in phosphoryl oxygen basicity and/or metal complex aqueous phase solubility. Substituent effects for alkyl-, silyl-, and fluoroalkyl-substituted tetraesters of methylenediphosphonic acid were investigated to gain insight into these differences. Tetraesters were used to avoid potential complications from the strong hydrogen bonding and aggregation of the partial esters. Nitric acid and Am(III) extraction by the 2-ethylhexyl, 3-(trimethylsilyl)propyl, and 3-(perfluorohexyl)propyl tetraesters of methylenediphosphonic acid were measured. The shift in the phosphoryl stretching band that accompanies nitric acid extraction was followed by FTIR spectroscopy. This study demonstrates the influence of remotely positioned ester substituents on the basicity of the phosphoryl oxygens and the extraction efficiency of these ligands.