Genesis of coordinatively unsaturated palladium complexes dissolved from solid precursors during Heck coupling reactions and their role as catalytically active species.

The Forum Article critically summarizes investigations and discussions on the nature and role of potential active species in C-C coupling reactions of the Heck type using catalyst systems with "ligand-free" inorganic salts, simple inorganic complexes, and supported and nonsupported (colloidal) Pd particles. From a series of experiments and reports, it can be concluded that the "active species" is generated in situ in catalytic systems at higher temperature conditions (>100 degrees C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalyst surface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the aryl halide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redeposited onto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution-redeposition processes correlate with the reaction rate and are strongly influenced by the reaction conditions. Skilled preparation of the catalyst and careful adjustment of the reaction conditions allowed the development of highly active heterogeneous catalysts (Pd/C, Pd/metal oxide, and Pd/zeolite), converting aryl bromides and aryl chlorides in high yields and short reaction times. Reaction conditions have been developed allowing the conversion of bromobenzene with turnover numbers (TONs) of 10(7) and even of unreactive aryl chlorides (chlorobenzene and chlorotoluene) in high yields with simple "ligand-free" Pd catalyst systems like PdCl2 or Pd(OH)2 in the absence of any organic ligand. Simple coordinatively unsaturated anionic palladium halide (in particular, bromo) complexes [PdXn](m-) play a crucial role as precursor and active species in all ligand-free and heterogeneous catalyst systems and possibly in Heck reactions at all.