Ion Pairing in Cationic Olefin-Gold(I) Complexes

The relative anion-cation orientation in [(PPh3)Au(4-Me-styrene)]BF4 (1BF(4)) and [(NHC)Au(4-Me-styrene)]BF4 [2BF(4); NHC = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene] has been investigated by combining F-19, H-1-HOESY NMR spectroscopy and Density Functional Theory (DFT) calculations incorporating solvent and relativistic effects. It has been found that BF4- locates on the side of 4-Me-styrene, close to the olefin region that is opposite to the 4-Me-Ph moiety in 1BF(4). In 2BF(4), the counterion approaches the cation from the side of the NHC ligand and is mainly located close to the imidazole ring. In both cases, the counterion resides far away from the gold site, the latter carrying only a small fraction of the positive charge. This indicates that the preferential position of the counterion is tunable through the choice of the ancillary ligand, and this opens the way to greater control over the properties and activity of these catalysts.