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)]mMe)(2)](PF6)(3) complex 1-3PF(6) (Im = imidazol-2-ylidene) with excess halides affords complexes 1-3X (X = Cl, Br, and I), resulting from counter anion PF6 (-)/ X- exchange. The H-1 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 center dot center dot center dot HC interactions. Complex 1(3+) 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 C-l-, 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 center dot center dot center dot 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 1(3+) are strongly influenced by the nature of the solvent. Formation of the 1 : 1 and 1 : 2 species (1X(2+) and 1X(2+)) is observed in DMSO-d(6), while that of only the 1 : 1 aggregates (1X(2+)) 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-> Iin DMSO-d(6) and I-> Br-> Cl- in D2O. The formation constants of the species 1X2+ and 1X2 + in DMSO and 1X(2+) in water have been determined by fitting the NMR titration curves.