SOLVENT-EXTRACTION OF TRANSITION-METAL CATIONS BY CALIXARENE-BASED CYCLIC LIGANDS

Calix[n]arenes (n = 4 and 6) bearing carboxyl groups (1n), hydroxamate groups (2n), and dimethylamino groups (3n) on the lower rim and their monomeric analogs (1(1), 2(1), and 3(1)) were synthesized to estimate selective extraction of transition metal cations from the aqueous phase to the organic (chloroform) phase. 1(4) and 1(6) showed the selectivity toward Fe3+, Cu2+ Zn2+, and Pd2+. In particular, 1(6) showed the unusually high extractability toward Fe3+. 2(4) and 2(6) showed the selectivity toward Fe3+, Cu2+, and Pd2+, but only Fe3+ was extracted to a significant extent at pH 2.2, the order of the extractability being 2(6) > 2(4) > 2(1). 3(4) and 3(6) showed the selectivity toward Pd2+ and Pt4+. The detailed examination of the extraction mechanism established that the ion-pair extraction mechanism is operative in Pt4+ (i.e., extracted as [PtCl6]2-) whereas both the ion-pair extraction mechanism and the chelate-complex extraction mechanism are operative in Pd2+ (i.e., extracted as [PdCl4]2- in the ion-pair extraction mechanism). The results indicate that the ligand groups circularly arranged on the lower rim of the calixarene cavity form novel binding sites for transition metal cations.