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

The relative anion-cation orientation in [(NHC) Au(alkene)] BF(4) 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-diiso-propylphenyl)- imidazol-2-ylidene ((Me)IPr); alkene 4-methyl-1-pentene, 2,3-methyl-2-butene and 4-methylstyrene] has been investigated by combining (19)F, (1)H-HOESY NMR spectroscopy in CD(2)Cl(2) 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.