From Bis(silylene) and Bis(germylene) Pincer-Type Nickel(II) Complexes to Isolable Intermediates of the Nickel-Catalyzed Sonogashira Cross-Coupling Reaction

The first [ECE]­Ni­(II) pincer complexes with E = SiII and E = GeII metallylene donor arms were synthesized via C–X (X = H, Br) oxidative addition, starting from the corresponding [EC­(X)­E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]­Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]­Ni acetylide → CuBr} intermediates revealed an unusual E–Cu–Ni three-center–two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.