Synthesis and characterization of [Cu(NHC)(2)]X complexes: Catalytic and mechanistic studies of hydrosilylation reactions

The preparation of two series of [Cu(NHC)(2)]X complexes (NHC=N-heterocyclic carbene, X=PF6 or BF4) in high yields from readily available materials is reported. These complexes have been spectroscopically and structurally characterized. The activity of these cationic bis-NHC complexes in the hydrosilylation of ketones was examined, and both the ligand and the counterion showed a significant influence on the catalytic performance. Moreover, when compared with related [Cu(NHC)]-based systems, these cationic complexes proved to be more efficient under similar reaction conditions. ne activation step of [Cu(NHC),]X precatalysts towards hydrosilylation was investigated by means of H-1 NMR spectroscopy. Notably, it was shown that one of the N,N'-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene (IPr) ligands in [Cu(IPr)(2)]BF4 is displaced by tBuO(-) in the presence of NaOtBu, producing the neutral [Cu-(IPr)(OtBu)]. This copper alkoxide is known to be a direct precursor of an NHC-copper hydride, the actual active species in this transformation. Furthermore, reagent loading and counterion effects have been rationalized in light of the species formed during the reaction.