Phosphane-Free Palladium-Catalyzed Coupling Reactions: The Decisive Role of Pd Nanoparticles

Abstract

Nanosized palladium colloids, generated in situ by reduction of Pd(II) to Pd(0) [Eq. (a)], are involved in the catalysis of phosphane-free Heck and Suzuki reactions with simple palladium salts such as PdCl(2) or Pd(OAc)(2), as demonstrated by transmission electron microscopic investigations.

Hydrogenation of benzene and toluene over size controlled Pt/SBA-15 catalysts: Elucidation of the Pt particle size effect on reaction kinetics

“…35 Leached anionic palladium species resulting from interactions with organic iodide have been also isolated 27 and observed to change oxidation state over the reaction; possibly indicative of their role in the catalytic cycle. 36 If all the aryl halide is converted to PhPdI type species before the addition of alkene the reaction also proceeds readily, 37 pointing to the fact such species may well offer a pathway by which these reactions can proceed. Filtration 38,39 and poisoning 39,40 experiments also suggest the reaction proceeds via monometallic species in solution.…”

Palladium Nanoscale Catalysts in Ionic Liquids: Coupling and Hydrogenation Reactions

“…Reduction with dihydrogen or hydrides (D-E; X = OAc, halide) or reductive elimination of a palladacycle by reaction with a diene (F). Adapted from references (Takahashi et al, 1970;Reetz & Westermann, 2000;Xu et al, 2000;Deshmukh et al, 2001;Cassol et al, 2005;Umpierre et al, 2005).…”

“…Pointing out, that often the active species is not the palladium complex but molecular Pd species derived from Pd-NPs stabilised by the IL serving as reaction medium. (Reetz & Westermann, 2000;Astruc, 2007) These Pd-NPs are resulting from the reduction of the Pd(II) species to Pd(0) in the presence of bases employed in the reaction. In the last two decades, classical solvents have been substituted by ILs in chemical reactions.…”

“…(Cassol et al, 2005;Migowski & Dupont, 2007) This indicates, at least for ligand-free Pd sources, that soluble Pd-NPs are involved as a reservoir of active species. (Reetz & Westermann, 2000;Rocaboy & Gladysz, 2003;Tromp et al, 2003) The first article of a zero-valent Pd complex suitable for the formation of Pd-NPs, by Takahashi and co-workers was published in 1970, used Pd(dba) 2 (dba = dibenzylideneacetone) under thermal decomposition conditions forming metallic palladium and dba in solution. (Takahashi et al, 1970) More studies of Pd-NP synthesis and its applications followed in the 1980s and 1990s by, for example Bönnemann, Reetz and their respective co-workers.…”

“…Synthetic approaches using controlled thermal decomposition for the generation of Pd-NPs in ILs use for example: Pd(dba) 2 , (Takahashi et al, 1970) Pd(OAc) 2 or palladium carbene complexes (Scheme 3). (Reetz & Westermann, 2000;Xu et al, 2000;Deshmukh et al, 2001; Furthermore, palladium salts such as PdCl 2 , Na 2 PdCl 4 can be reduced with metal hydrides or simply dihydrogen. Pd-NPs are also formed starting from palladacycles by reaction with dienes (Scheme 3).…”