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.