The concentration and selective extraction of copper(II) rhodium(III), and iridium(III) using a copolymer functionalised with dithiocarbamate groups. Spectroscopic evidence for the nature of the binding sites

Abstract

The use of the water-insoluble dithiocarbamate copolymer (1)(‘PIED’) as a reagent for the concentration and separation of rhodium, iridium, and copper has been investigated. The copolymer is able to separate iridium and rhodium, giving a separation ratio of up to 80 in unacidified solutions, and 5 in strong acid (4 mol dm^–3) solutions and to concentrate both rhodium(III) and copper(II). The selectivity ratio (Rh/Cu) of up to 9.8 is much less than expected and both metals are partially eluted by water. Spectroscopic properties of the metal–polymer complexes have been monitored and compared with those of model complexes and with those of a Cu^II-exchanged copolymer (2)(‘IR-45CS₂H’) which has a CuS₄chromophore. At low loading (1%) the preferred site is five-co-ordinate but at higher loading the co-ordination appears to be CuS₂N₂ and is a model for ‘blue’ copper proteins. The ligand-field strength of the CS₂^– groups in (1) is significantly lower than expected, which is ascribed to strain in the polymer backbone causing weaker M–S bonds and accounts for the ease of elution of the metals. The poor selectivity of (1) for Ru^III over Cu^II is due to two factors: the rhodium and copper co-ordinate to different sites, and the co-ordination of rhodium limits the sites available to copper.