A square planar gold(iii) bis-(1,1′-dimethyl-3,3′-methylene-diimidazol-2,2′-diylidene) trication as an efficient and selective receptor towards halogen anions: the cooperative effect of Au⋯X and X⋯HC interactions

Abstract

Treatment of the tricationic gold(III) [Au(MeImCH₂ImMe)₂](PF₆)₃ complex 1-3PF₆ (Im = imidazol-2-ylidene) with excess halides affords complexes 1-3X (X = Cl, Br, and I), resulting from counter anion PF₆⁻/X⁻ exchange. The ¹H chemical shift of the CH₃ groups and particularly that of the CH₂ linker in DMSO-d₆ are different in the three complexes, thus suggesting selective X⋯HC interactions. Complex 1³⁺ can therefore be used as a halide sensor in DMSO and water. The host–guest interaction between the tricationic gold(III) complex and the halides Cl⁻, Br⁻ and I⁻ in solution and in the solid state has been investigated by means of NMR titration experiments, DFT calculations and X-ray structure analysis. The electrostatic interaction between the halides and the triple formal positive charge on the metal centre, together with the CH⋯X hydrogen bonding between the NHC ligand and halides, contributes to the formation of stable supramolecular aggregates in solution and in the solid state. The complexing properties of 1³⁺ are strongly influenced by the nature of the solvent. Formation of the 1 : 1 and 1 : 2 species (1X²⁺ and 1X₂⁺) is observed in DMSO-d₆, while that of only the 1 : 1 aggregates (1X²⁺) is observed in D₂O (X = Cl, Br, and I). Moreover, the selectivity towards the various halides is reversed in the two solvents, being in the order Cl⁻ > Br⁻ > I⁻ in DMSO-d₆ and I⁻ > Br⁻ > Cl⁻ in D₂O. The formation constants of the species 1X²⁺ and 1X₂⁺ in DMSO and 1X²⁺ in water have been determined by fitting the NMR titration curves.