Palladium Nanoparticles from Different Reducing Systems as Heck Catalysts

Abstract

Palladium(0) nanoparticles have been widely used in cross coupling reactions, including Heck reactions. For this study, we synthesized palladium(0) nanoparticles in colloidal suspension using different combinations of solvents and reducing methods under aerobic conditions. The variation in systems used to synthesize palladium(0) nanoparticles resulted in different nanoparticle sizes. To investigate whether the particle size had an effect on catalysis, we first used common Heck C–C cross-coupling reaction conditions (200 °C and 18 h). In addition, we omitted the use of stabilizing agents, other than the solvent and/or the anions in the initial nanoparticle synthesis, since the use of stabilizing agents adds cost and processing time to catalysis. All of the catalysts investigated worked in the cross coupling C–C Heck reaction, but yields did not show appreciable differences, as high temperature and long reaction times promote a high reduction of palladium(II). Therefore, we decided to work with a temperature and reaction time in which conversion would start to be observed (minimum reaction conditions). The experiments to determine minimum reaction conditions showed that this would be 120 °C and 10 h, therefore we used these conditions in Heck C–C cross-coupling reactions and all the palladium nanoparticle systems. The best C–C catalysis conversion was observed when N,N-dimethylformamide was used as solvent in the absence of reducing agent. This catalyst system resulted in the largest possible nanoparticles, which were kept in dispersion (did not precipitate out), showing that size is important in obtaining good yields in C–C Heck catalysis (where cocktail-type catalysis could explain the conversion). Nanoparticles of this size also act as a reservoir of soluble palladium species that behave as the true catalyst. The second best conversion was observed in N,N-dimethylformamide with sodium citrate, where citrate may have added extra protection, and since the palladium(0) nanoparticles were small, cocktail-type catalysis was not involved in obtaining high yields.