Experimental and Theoretical Investigation of Ion Pairing in Gold(III) Catalysts

The ion pairing structure of the possible species present in solution during the gold(III)-catalyzed hydration of alkynes: [(ppy)Au(NHC)Y]X₂ and [(ppy)Au(NHC)X]X [ppy = 2-phenylpyridine, NHC = NHC^iPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene; NHC = NHC^mes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene X = Cl^-, BF₄^-, OTf^-; Y = H₂O and 3-hexyne] are determined. The nuclear overhauser effect nuclear magnetic resonance (NMR) experimental measurements integrated with a theoretical description of the system (full optimization of different ion pairs and calculation of the Coulomb potential surface) indicate that the preferential position of the counterion is tunable through the choice of the ancillary ligands (NHC^iPr, NHC^mes, ppy, and Y) in [(ppy)Au(NHC)(3-hexyne)]X₂ activated complexes that undergo nucleophilic attack. The counterion can approach near NHC, pyridine ring of ppy, and gold atom. From these positions, the anion can act as a template, holding water in the right position for the outer-sphere attack, as observed in gold(I) catalysts.