Genesis of Coordinatively Unsaturated Palladium Complexes Dissolved from Solid Precursors during Heck Coupling Reactions and Their Role as Catalytically Active Species
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- 作者：Klaus Kö ; hler ; Wolfgang Kleist ; Sandra S. Prö ; ckl
- 刊名：Inorganic Chemistry
- 出版年：2007
- 出版时间：March 19, 2007
- 年：2007
- 卷：46
- 期：6
- 页码：1876 - 1883
- 全文大小：90K
- 年卷期：v.46,no.6(March 19, 2007)
- ISSN：1520-510X
文摘
The Forum Article critically summarizes investigations and discussions on the nature and role of potential activespecies in C-C coupling reactions of the Heck type using catalyst systems with "ligand-free" inorganic salts, simpleinorganic complexes, and supported and nonsupported (colloidal) Pd particles. From a series of experiments andreports, it can be concluded that the "active species" is generated in situ in catalytic systems at higher temperatureconditions (>100 
C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalystsurface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the arylhalide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redepositedonto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution-redepositionprocesses correlate with the reaction rate and are strongly influenced by the reaction conditions. Skilled preparationof the catalyst and careful adjustment of the reaction conditions allowed the development of highly activeheterogeneous catalysts (Pd/C, Pd/metal oxide, and Pd/zeolite), converting aryl bromides and aryl chlorides in highyields and short reaction times. Reaction conditions have been developed allowing the conversion of bromobenzenewith turnover numbers (TONs) of 107 and even of unreactive aryl chlorides (chlorobenzene and chlorotoluene) inhigh yields with simple "ligand-free" Pd catalyst systems like PdCl2 or Pd(OH)2 in the absence of any organicligand. Simple coordinatively unsaturated anionic palladium halide (in particular, bromo) complexes [PdXn]m- playa crucial role as precursor and active species in all ligand-free and heterogeneous catalyst systems and possiblyin Heck reactions at all.
C). In all heterogeneous systems with solid Pd catalysts, Pd is dissolved from the solid catalystsurface under reaction conditions by a chemical reaction (complex formation and/or oxidative addition of the arylhalide), forming extremely active coordinatively unsaturated Pd species. Pd is partially or completely redepositedonto the support at the end of the reaction when the aryl halide is used up. The Pd dissolution-redepositionprocesses correlate with the reaction rate and are strongly influenced by the reaction conditions. Skilled preparationof the catalyst and careful adjustment of the reaction conditions allowed the development of highly activeheterogeneous catalysts (Pd/C, Pd/metal oxide, and Pd/zeolite), converting aryl bromides and aryl chlorides in highyields and short reaction times. Reaction conditions have been developed allowing the conversion of bromobenzenewith turnover numbers (TONs) of 107 and even of unreactive aryl chlorides (chlorobenzene and chlorotoluene) inhigh yields with simple "ligand-free" Pd catalyst systems like PdCl2 or Pd(OH)2 in the absence of any organicligand. Simple coordinatively unsaturated anionic palladium halide (in particular, bromo) complexes [PdXn]m- playa crucial role as precursor and active species in all ligand-free and heterogeneous catalyst systems and possiblyin Heck reactions at all.