Synergistic Solvent Extraction of Alkaline Earth Cations by Mixtures of Di-n-octylphosphoric Acid and Stereoisomers of Dicyclohexano-18-crown-6

The partitioning of Ca(II), Sr(II), and Ba(II) in solvent extraction systems comprising di-n-octylphosphoric acid (HDOP) and individual stereoisomers of dicyclohexano- 18-crown-6 (DCH18C6) in toluene has been investigated at 23(2) °C. Results of vapor pressure osmometric experiments, continuous variation studies, single- and double-extractant dependencies, and acid dependencies have been used to determine the stoichiometries of the extracted complexes. Extraction of Ca(II) by HDOP alone affords Ca(H(DOP)2)2·HDOP as the extracted species, and no synergism is observed in the presence of DCH18C6, presumably due to the strong complexation of Ca(II) with HDOP and the strained complexes this cation forms with DCH18C6. In the absence of DCH18C6, Sr(II) and Ba(II) are extracted as the M(H(DOP)2)2·2HDOP (M = Sr or Ba) complexes. Upon addition of DCH18C6, the extracted complexes are formulated as M(DCH18C6)(H(DOP)2)2 (M = Sr or Ba), with Ba(II) showing greater synergistic effects than Sr(II). The effectiveness of the DCH18C6 stereoisomers as synergists decreases in the following order: cis−syn−cis > cis−anti−cis > cis−trans > trans−syn−trans > trans−anti−trans. This sequence has been explained by correlating the logarithm of the synergistic adduct formation constants with the ligand strain energies of the DCH18C6 stereoisomers calculated using molecular mechanics methods. Those stereoisomers having the largest strain energies afford the lowest extraction and synergistic adduct formation constants.