Extraction of Pd(ii) and Pt(ii) from aqueous hydrochloric acid with 1,3-diaminocalix[4]arene: switching of the extraction selectivity by using different extraction modes

The extraction ability of 1,3-diaminocalix[4]arene (2: H(2)L) toward platinum group metals (PGMs) has been investigated, which revealed that 2 is able to extract Pd(ii) and Pt(ii) from hydrochloric acid via different extraction modes. The extraction species for Pd(ii) and Pt(ii) are [PdL] and [Pt(2)Cl(6)(H(3)L)(2)], respectively, as evidenced by equilibrium analysis and X-ray crystallography. In [PdL], the two phenoxide oxygens and two amino nitrogens of L(2−) coordinate to the Pd ion. On the other hand, in [Pt(2)Cl(6)(H(3)L)(2)], two anionic trichloro complexes PtCl(3)(−) are sandwiched between two H(3)L(+), in which one amino nitrogen directly coordinates to a PtCl(3)(−) species and another protonated amino group forms an ion pair with another PtCl(3)(−). Utilizing the different extraction modes, switching of the extraction selectivity has been achieved in the competitive extraction between Pd(ii) and Pt(ii) by varying the concentrations of H(+) and Cl(−) in the aqueous phase. Finally, from the extracted organic phase, back-extraction of Pd(ii) and Pt(ii) was easily performed, respectively.