Amide-functionalised phosphonium-based ionic liquids as ligands for rhodium(iii) extraction

Seven new amide-functionalised phosphonium-based ionic liquids (APILs) with chloride anions are synthesised and applied to extraction of rhodium(iii) from HCl solution. The effects of structural modification of the APILs on the extraction performance are examined by liquid-liquid extraction using toluene as a diluent and the results compared with those for trihexyltetradecylphosphonium chloride ([P₆₆₆₁₄][Cl]), a typical commercial extractant. The performance of the APILs as rhodium(iii) extractants is influenced by three main factors: (1) the length of the alkyl chains attached to the P atom; (2) the length of the linker between the amide and phosphonium moiety; and (3) the type of amide group. A novel ligand, [3°C₂P₄₄₄][Cl], had outstanding performance in the effective recovery of rhodium(iii). Extraction of rhodium(iii) from a 1.0 mol dm^-3 HCl solution with 0.5 mol dm^-3 [3°C₂P₄₄₄][Cl] proceeded quantitatively (>98%) and the extraction efficiency was higher than that of the commercial extractant [P₆₆₆₁₄][Cl]. The mechanism of rhodium(iii) extraction by [3°C₂P₄₄₄][Cl] was investigated by slope analysis, UV-vis, and FT-IR spectroscopy. These results indicate that [RhCl₄(H₂O)₂]^- in aqueous solution is extracted by [3°C₂P₄₄₄][Cl] through an anion-exchange mechanism and slowly converted into a dimer, [Rh₂Cl₉]^3-, in the organic phase.