Gold(I) Complexes of N-Heterocyclic Carbenes and Pyridines

Overall ten ionic [Au(NHC)(Py)][PF6](-) and [Au-2(NHC)(2)(Py)(2)]-[PF6](2)-type complexes [NHC = N,N'-dialkylbenzimidazol-2-ylidene, denoted as R-2-bimy with alkyl (R) being methyl (Me) or ethyl (Et), and Py = 4-substituted pyridine or 4,4'-bipyridine] have been prepared by incorporating various Pys into the Au-I-NHC core with Au(NHC)Cl as starting material. Their crystal structures characterized by X-ray diffraction indicate a linearly coordinated Au-I center and exhibit secondary forces such as Au center dot center dot center dot Au, Au center dot center dot center dot pi, or pi center dot center dot center dot pi interactions. The luminescent properties of these compounds were studied in the solid state. Density-functional theory calculations on [Au(Me-2-bimy)(4-dmapy)][PF6] [4-dmapy = 4-(dimethylamino)pyridine] predict the lowest electronic transition with nonzero oscillator strength is the fourth HOMO-LUMO transition. Whereas the fourth HOMO is mainly associated with the Py ligands, the LUMO is predominantly aurophilic. Four Au-I-NHC compounds were examined for their catalytic activity towards the oxidation of benzyl alcohol to aldehyde, in which the starting material [Au(Et-2-bimy)Cl] gave the highest yield. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)