Thioamide-Modified Pincer Extractant for Selective Pd(II) Recovery

Solvent extraction is an important method for recovering Pd(II) from secondary resources; however, conventional Pd(II) extractants often exhibit limited efficiency and selectivity. In this study, a new pincer extractant was synthesized by incorporating two thioamide groups as polar moieties at the 1,3-positions of the benzene ring. Its Pd(II) extraction performance was evaluated as a function of the shaking time and HCl concentration. Optimal stripping reagents for the back-extraction of Pd(II) were also investigated. The extractant effectively extracted Pd(II) from a simulated mixed-metal solution containing platinum group metals and base metals as well as from a leach liquor of a spent automotive catalyst. Mechanistic studies using ¹H nuclear magnetic resonance (¹H NMR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, single-crystal X-ray diffraction (SCXRD), and X-ray absorption spectroscopy (XAS) revealed distinct coordination behaviors: in solution, Pd(II) bound only to the two sulfur atoms of the thioamide groups, behaving like a bidentate ligand, whereas in the crystalline state, the Pd(II) complex displayed typical features of pincer ligands, that is, tridentate binding involving two sulfur atoms and a central donor atom that together "clamp" the Pd center. This study provides a foundation for the molecular design of novel pincer extractants for enhanced Pd(II) recovery and organometallic catalyst development.