Metal extraction by silyl-substituted diphosphonic acids. II. Effect of alkylene bridge length on aggregation and metal ion extraction behavior

Abstract

In an ongoing effort to determine the relationship between structure and function in diphosphonic acids, the aggregation and solvent extraction chemistry of four novel silicon-substituted diphosphonic acids, P,P′-di[3-(trimethylsilyl)-1-propyl] propylene- (H2DTMSP[PrDP]), butylene- (H2DTMSP[BuDP]), pentylene- (H2DTMSP[PDP]), and hexylene diphosphonic acid (H2DTMSP[HDP]), were investigated. Vapor pressure osmometry indicates that H2DTMSP[PrDP] and H2DTMSP[PDP] are dimeric in toluene at 25°C, while H2DTMSP[BuDP] and H2DTMSP[HDP] are primarily trimeric and hexameric, respectively. The solvent extraction of selected alkaline earth cations (Ca, Sr, and Ba) and representative tri-, tetra-, and hexavalent actinides [Am(III), Th(IV), and U(VI)], from nitric acid solutions into o-xylene solutions of the ligands has been studied and compared to results reported previously for some analogous di(2-ethylhexyl)-substituted diphosphonic acids. Differences observed in the magnitude of distribution ratios, based on the number of bridging methylene groups, have been explained considering factors such as ligand aggregation, chelate ring size, P=O basicity (qualitatively determined by 31P nuclear magnetic resonance and infrared spectroscopy), and the relative acidities of the ligands.