Extraction ability and mechanism of a sulfur-bridged dithiophenol-based derivative as a Pd(II) extractant

• Comparison of extraction ability: a sulfide-bridged derivative vs commercial reagent. • The sulfide-bridged derivative has the highest potential for Pd(II) recovery. • Pd(II) extraction mechanism of the derivative was estimated by XAFS and VT NMR. • Formation and breakage of bonds occur between mirror images of the Pd(II) complex. The recovery of Pt-group metals (PGMs) from spent products is vital owing to their essential roles in electronics, automotive catalysts, and other applications. Extractants with high selectivity, effective extraction ability, and high extraction capacity are required for efficient PGM recovery. Our group previously synthesized and evaluated a dithiophenolic ligand with a S-bridged sulfide moiety (extractant 1 ) for Pd(II) extraction. We then synthesized an analogue of 1 , extractant 2 , to evaluate its Pd(II) extraction mechanism. Unfortunately, this mechanism could not be clearly elucidated because of inconsistencies between the structure of the extractant–Pd complex inferred by 1 H nuclear magnetic resonance (NMR) spectroscopy and that determined by single-crystal X-ray diffraction analysis. Hence, seeking to clarify the mechanism of 2 , in this study, we used X-ray absorption spectroscopy in combination with X-ray absorption fine structure (XAFS) analysis to estimate the coordination environment around Pd(II). Firstly, the Pd(II) extraction ability of 2 was evaluated using a single Pd(II) component solution and the leachate of a spent automotive catalyst. Subsequently, the XAFS measurements revealed that the structures of the 2 –Pd(II) complex in the solution and crystalline states were identical. Variable-temperature (VT) NMR spectroscopy was subsequently applied to determine the exchange rate of the mirror-images of the complex at equilibrium. Pd(II) was extracted via bidentate coordination exchange with Pd–S bond formation and cleavage between the Pd(II) center and two thiomethyl groups in solution at room temperature from the results of the XAFS and VT NMR analyses.