[(NHC)CuX] complexes: Synthesis, characterization and catalytic activities in reduction reactions and Click Chemistry. On the advantage of using well-defined catalytic systems

The preparation of three series of [(NHC)CuX] complexes (NHC = N-heterocyclic carbene, X = Cl, Br, or I) is reported. These syntheses are high yielding and only use readily available starting materials. The prepared complexes were spectroscopically and structurally characterized. Notably, two of them present a bridging NHC ligand between two copper centers in the solid state, an extremely rare coordination mode for these ligands. These complexes were then applied to two distinct organic reactions: the hydrosilylation of ketones and the 1,3-dipolar cycloaddition of azides and alkynes. In both transformations, outstanding catalytic systems were found for preparing the corresponding products in excellent yields and short reaction times. Most remarkably, the screening of well-defined systems in the hydrosilylation reaction allowed for the identification of a pre-catalyst previously overlooked since, originally, catalytic species were in situ generated. Under such conditions, major formation of [(NHC)₂Cu]⁺ species, inactive in this reduction reaction, occurred instead of the expected copper hydride. These results highlight one of the most important advantages of employing well-defined complexes in catalysis: gaining an improved control of the nature of the catalytically relevant species in the reaction media.