Palladium complexes with tridentate pincer bis-carbene Ligands as efficient catalysts for C-C coupling

The synthesis and X-ray crystallographic characterization of a Pd complex (3) with a rigid C,N,C-tridentate pincer carbene ligand are described. At high temperatures (165 degreesC) it is an active and robust catalyst in the Heck reaction. Complex 3 gives some of the highest turnover numbers yet reported for coupling with aryl chlorides. Time dependence, reuse, and Heck reaction conditions are discussed for 3. Data with F- as base did not support Shaw's Heck mechanism. Suzuki and Sonogashira coupling reactions are also catalyzed by 3. The palladium carbene complex 5, containing an analogous C,N-bidentate ligand, is compared to 3 in terms of stability, catalytic activity, and reaction profile in the Heck reaction.