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

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. The activation step of [Cu(NHC)₂]X precatalysts towards hydrosilylation was investigated by means of ¹H NMR spectroscopy. Notably, it was shown that one of the N,N-bis(2,6-diisopropylphenyl)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.