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2 2009

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Nanocząstki metali przejściowych - synteza i aktywność katalityczna

Gniewek A., Trzeciak A. M.

Wiadomości Chemiczne

2009

[Z] 63, 11-12

953-984

Transition metal nanoparticles (also called metal colloids or nanoclusters) are ordered multi-atom sets characterized by a very small size, generally less than 20 nm. They are intermediate species between single atoms and crystals of macroscopic dimensions [1.8]. Nanoparticles have been synthesized by a variety of methods. The most common synthetic procedures involve chemical reduction of transition metal salts or complexes. By the choice of reduction conditions (kind of the reducing agent, type of the stabilizing agent and the elementary reaction parameters like temperature and concentration) it is possible to obtain colloids showing different particle sizes and morphologies. These two factors play a decisive role from the point of view of catalytic activity of nanoclusters. Palladium nanoparticles have been obtained by chemical reduction of PdCl2 aqueous solution using pyrogallol, hydrazine or chromium(II) acetate as the reducing agent. All these systems have demonstrated a very high catalytic activity in important carbon-carbon bond forming reactions (methoxycarbonylation, Heck and Suzuki processes) carried out under mild conditions [48, 49]. Some of the C.C coupling reactions have also been successfully catalyzed by nickel nanoparticles, however they require more harsh conditions [65, 66]. Cobalt and iron nanoparticles present magnetic properties attractive for application of these materials as removable electronic media of high capacity, as well as biosensors or magnetic probes for biological imaging and therapeutic use [9, 18]. The extreme reactivity of nanoparticles, specifically towards oxygen and water, complicates their synthesis, however it is also beneficial in catalytic applications [16, 17]. Conversion of CO/H2 mixture to hydrocarbons, known as Fischer-Tropsch synthesis, representing one of the most important routes to fuels production, is catalyzed very effectively by iron and cobalt nanoparticles. Monometallic and bimetallic cobalt nanoparticles are excellent catalysts of Pauson-Khand type couplings leading to cyclopentanones [17].

Nanocząsteczkowe katalizatory palladowe w reakcjach tworzenia wiązań C-C

Trzeciak A. M., Ziółkowski J. J.

Wiadomości Chemiczne

2009

[Z] 63, 1-2

1-15

The important role of palladium nanoparticles has been recently demonstrated in many catalytic systems designed for C-C bond forming reactions [1-4]. There are examples of catalytic systems described earlier as homogeneous in which Pd(0) nanoparticles were now identified. In the article three different palladium catalytic systems are discussed. In the first one, Pd(0) nanoparticles, obtained by chemical reduction of PdCl2 and stabilized by polyvinylpyrrolidone, were used for Heck coupling in [Bu4N]Br medium. Decrease of nanoparticles size in reaction conditions was explained as a result of dissolution of Pd(0) colloid and simultaneous formation of catalytically active monomolecular anionic palladium complexes [33]. The second example presents application of Pd(II) and Pd(0) supported on alumina-based oxides in Suzuki-Miyaura reaction [36]. Reduction of Pd(II) to Pd(0) nanoparticles under reaction conditions was confirmed. In contrast to the first described case, in Suzuki-Miyaura reaction the size of Pd(0) nanoparticles was the same before and after the catalytic cycle. The catalytic activity of both palladium forms was quite high, however Pd(0) formed in situ was slightly more efficient as catalyst. In the third part of the article studies of palladium reduction in anionic complexes of [IL]2[PdX4] type are shown, where IL = imidazolium cation [37]. These complexes catalyzed well Suzuki-Miyaura cross-coupling, but they were not stable under reaction conditions and decomposed to Pd(0) nanoparticles and Pd black. Using ESI-MS method it was possible to identify polynuclear (Pd3, Pd5) intermediate forms, stabilized with imidazolium cations or N-heterocyclic carbenes. In all systems discussed in the article co-existence of Pd(0) nanoparticles and monomolecular complexes was observed. That is important for understanding of the nature of catalytically active forms in C-C bond forming reactions.