Ion pairing in NHC gold(I) olefin complexes: A combined experimental/theoretical study

The relative anion-cation orientation in [(NHC)Au(alkene)]BF₄ ion pairs [NHC = N-Heterocyclic Carbene = 1,3-bis(di-iso-propylphenyl)-imidazol-2-ylidene (IPr) and 4,5-dimethyl-N,N′-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (^MeIPr); alkene = 4-methyl-1-pentene, 2,3-methyl-2-butene and 4-methylstyrene] has been investigated by combining ¹⁹F,¹H-HOESY NMR spectroscopy in CD₂Cl₂ and a detailed analysis of the Coulomb potential of the cationic fragment through DFT calculations. Two main orientations have been found where the anion locates close to the imidazole ring (NHC-side) and close to the olefin (olefin-side). The NHC-side orientation is always predominant (65-83%) while the exact position of the anion in the olefin-side is finely tuned by the nature of olefin substituents. In all cases, the counterion resides far away from the gold site, the latter carrying only a small fraction of the positive charge.