Solvent extraction of gold and platinum-group metals using 2-nonylpyridine 1-oxide, and the crystal and molecular structure of bis(2-nonylpyridine 1-oxide)hydrogen(1 +) tetrachloroaurate(III)

Abstract

2-Nonylpyridine 1-oxide (npyo) is a versatile solvent-extraction reagent when dissolved in chloroform. Adjustment of the acid strength (HCl) and reagent concentration can be used to bring about a range of separations. For example, npyo is effective at separating the [AuCl₄]^– ion from other chloro-anions of the platinum-group metals. If required, the [FeCl₄]^– anion may be coextracted at high acid concentration (6 mol dm^–3 HCl). Chloro-anions of Pt^IV can be 90% extracted at npyo concentrations greater than 0.2 mol dm^–3 in chloroform in a single batch operation. Chloro-anions of Ir^IV and Rh^III may also be separated. The crystal structure of [H(npyo)₂][AuCl₄] indicates that there are discrete [AuCl₄]^– ions with weak O ⋯ Au interactions. Crystals are triclinic, space group P, Z= 4, with a= 17.351(11), b= 10.381(9), c= 20.373(8)Å, α= 88.5(1), β= 101.8(1), and γ= 99.6(1)°. 4 846 Independent data above background [I > 3σ(I)] were collected on a diffractometer and the structure refined to R 0.0585. The structure contains two independent [H(npyo)₂]⁺ cations and one [AuCl₄]^– in general positions and two [AuCl₄]^– ions in special positions with the gold atoms on centres of symmetry. Each [H (npyo)₂]⁺ cation consists of pairs of npyo ligands associated via a hydrogen atom [O ⋯ O 2.412(19) and 2.391(16)Å]. The [AuCl₄]^– anions are square planar but two oxygen atoms approach one of the gold atoms in axial positions [3.544(16) and 3.263(15)Å], to provide some weak interaction which may provide a clue to the effectiveness of npyo in the extraction of [AuCl₄]^–.