Rapid and selective extraction of Pd(II) ions using the SCS type pincer ligand 1,3-bis(dimethylthiocarbamoyloxy)benzene, and its Pd(II) extraction mechanism

We report the salient metal complexation properties of the SCS pincer ligand 1,3-bis(dimethylthiocarbamoyloxy)benzene (1), and its Pd(II) extraction properties in liquid-liquid extraction. Rapid and selective extraction by 1 of >99% Pd(II) ions from an automotive catalyst leach liquor containing Rh, Pd, Pt, Zr, Ce, Ba, Al, La, and Y in Cl- media was observed. The extractability (E%) of 1 toward Pd was higher than that of the analogous compounds 1,4-bis(dimethylthiocarbamoyloxy) benzene (2), 1,4-bis(dimethylthiocarbamoyloxy) 2,5-di-tert-butylbenzene (3), and 1,2-bis(dimethylthiocarbamoyloxy) benzene (4), and the commercial Pd(II) extractant di-n-hexyl sulfide (DHS). In o-dichlorobenzene diluent, 1 showed a high E%(Pd) (99.9-97.0%) in Cl- media (0.1-8.0 M) in 30 min, and excellent Pd(II) extraction and good phase separation were obtained with the various diluents studied. Pd(II) ions were stripped from the extractants using 0.1 M thiourea in 1.0 M HCl, enabling the reuse of 1. Loading capacity measurements revealed a Pd/extractant 1 ratio of 1. Palladation is promoted at the benzene ring and dimethylthiocarbamoyl moieties via formation of stable fused six-membered chelate rings. The extractant 1-Pd(II) complex was characterized using FT-IR spectroscopy, NMR spectroscopy, elemental analysis, single-crystal XRD study, and MALDI-TOF MS analysis. Extractant 1 displayed a high E%(Pd) (99%) even after five extraction cycles, indicating that it is very stable in acidic media and may be useful for the rapid and selective recovery of Pd(II) from catalyst solutions in platinum group metal refineries.