Efficient and selective recovery of palladium by Thiabenzo-15-crown-5 ether derivatives

In the present study, two crown ether derivatives containing different numbers of sulfur atoms, monothiabenzo-15-crown-5 ether (MTB15C5) and dithiabenzo-15-crown-5 ether (DTB15C5), were designed and synthesized for efficient and selective recovery of palladium. The complexes formed of MTB15C5 and DTB15C5 with Pd(II), [(MTB15C5)₂·PdCl₂] and [(DTB15C5)·PdCl₂], were analyzed by single crystal XRD to clarify the coordination mechanisms. The trans and cis square configured S₂PdCl₂ fragments formed through SPd coordination bonds were observed in the structures of [(MTB15C5)₂·PdCl₂] and [(DTB15C5)·PdCl₂], respectively. Multiple intramolecular [C-H···Cl-Pd] hydrogen bonds were demonstrated to maintain the conformation of the complexes. In the proof-of-principle experiments for Pd(II) recovery through liquid-liquid extraction (LLE), >98 % of Pd(II) can be extracted within a contact time of 15 and 10 min by MTB15C5 and DTB15C5, respectively. The extraction efficiencies of Pd(II) in multi-ions containing solutions by MTB15C5 and DTB15C5 were 98.9 % and 99.7 %, respectively, which were much higher than the competitive ions (generally below 8 %), showing the excellent selective extractability of the two thiacrown ethers toward Pd(II). Job's plot and high-resolution mass spectrometry analysis confirmed that the extraction of palladium is achieved through the coordination of MTB15C5 and DTB15C5 with Pd(II) in the stoichiometries of 2:1 and 1:1, respectively, forming the complexes of [(MTB15C5)₂·PdCl₂] and [(DTB15C5)·PdCl₂] in the organic phase during the LLE process.