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

Treatment of the tricationic gold( iii ) [Au(MeImCH 2 ImMe) 2 ](PF 6 ) 3 complex 1-3PF6 (Im = imidazol-2-ylidene) with excess halides affords complexes 1-3X (X = Cl, Br, and I), resulting from counter anion PF 6 − /X − exchange. The 1 H chemical shift of the CH 3 groups and particularly that of the CH 2 linker in DMSO-d 6 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-d 6 , while that of only the 1 : 1 aggregates ( 1X2+ ) is observed in D 2 O (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-d 6 and I − > Br − > Cl − in D 2 O. The formation constants of the species 1X2+ and 1X2+ in DMSO and 1X2+ in water have been determined by fitting the NMR titration curves.