Oxidant/complexing properties of the methimazole (MeImHS)/iodine system towards palladium and gold metals. Crystal structure of the complex cation [PdII(MeImHS)4]2+ balanced by a tetraiodide/iodide mixture

Abstract

This paper reports on the oxidative dissolution ability in dichloromethane and water of the methimazole–iodine system towards gold and palladium in powder. This research has a potential application in the recovery process of these metals from electrical and electronic waste equipment (WEEE). The iodine-adduct of methimazole (1-methyl-3H-imidazole-2-thione; MeImHS) oxidises gold and palladium powders in dichloromethane to form the complexes [Au^I(MeImHS)₂]I₃ and [Pd^II(MeImHS)₄](I₄)_0.73·2(I)_0.27. The X-ray crystal structure of the palladium complex shows a square-planar Pd(II) ion S-coordinated to four MeImHS units with the charge essentially balanced by an unusual tetraiodide I₄²⁻ with minor amounts of iodide. The oxidative dissolution of palladium in water medium produces the cation [Pd^II(MeImHS)₄]²⁺. Considering the reaction conditions under which the process was performed ([I₂] = 1.14 × 10⁻³ M, T = 20 °C, pH = 5, reaction time 24 h), the obtained value of 64% of oxidation yields from Pd(0) to Pd(II) is to be considered satisfactory. No appreciable oxidation was observed for the gold powder in water. The feasibility of recovering palladium from the triiodide salt of the complex cation [Pd^II(MeImHS)₄]²⁺ was verified by carrying out a two-step reduction process using magnesium powder.