Metal Ion Extraction by Silyl-Substituted Diphosphonic Acids. I. P,p′-Di-[3-(Trimethylsilyl)-1-Propylene] Methylene- And Ethylenediphosphonic Acids

In conjunction with efforts to develop novel actinide extractants that exhibit solubility in supercritical carbon dioxide (SCCO2), the effect of adding silicon-based functionalities to diphosphonic acids on their aggregation and solvent extraction chemistry was investigated. Two silyl-derivatized diphosphonic acids, P,P′-di[3-(trimethylsilyl)-1-propylene] methylenediphosphonic acid (H2DTMSP[MDP]) and P,P′-di[3-(trimethylsilyl)-1-propylene] ethylenediphosphonic acid (H2DTMSP[EDP]), were prepared and their aggregation and metal ion extraction properties compared to those of the previously studied P,P′-di(2-ethylhexyl) alkylenediphosphonic acids. Vapor pressure osmometry of H2DTMSP[MDP] and H2DTMSP[EDP] in toluene (25°C) indicates that, as is the case for the 2-ethylhexyl-substituted alkylenediphosphonic acid analogs, the compounds are dimeric and (primarily) hexameric, respectively, in the concentration range investigated. Distribution ratio measurements for the alkaline earth cations Ca2+, Sr2+, Ba2+, and Ra2+ as well as the representative tri-, tetra-, and hexavalent actinides Am3+, Th4+, and UO2+ 2 between solutions of H2DTMSP[MDP] and H2DTMSP[EDP] in o-xylene and nitric acid indicate that the behavior of these silyl-derivatized compounds closely mimics that of the analogous P,P′-di(2-ethylhexyl) alkylenediphosphonic acids, indicating that incorporation of a silyl functionality has no adverse impact on the metal ion extraction properties of diphosphonic acids.