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 AuX and XHC interactions

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 XHC 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 CHX 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.