Leaching of palladium species from Pd nanoparticles under C--C coupling conditions was observed for both Heck and Suzuki reactions by using a special membrane reactor. The membrane allows the passage of palladium atoms and ions, but not of species larger than 5 nm. Three possible mechanistic scenarios for palladium leaching were investigated with the aim of identifying the true catalytic species. Firstly, we examined whether or not palladium(0) atoms could leach from clusters under non-oxidising conditions. By using our membrane reactor, we proved that this indeed happens. We then investigated whether or not small palladium(0) clusters could in fact be the active catalytic species by analysing the reaction composition and the palladium species that diffused through the membrane. Neither TEM nor ICP analysis supported this scenario. Finally, we tested whether or not palladium(II) ions could be leached in the presence of PhI by oxidative addition and the formation of [Pd(II)ArI] complexes. Using mass spectrometry, UV-visible spectroscopy and 13C NMR spectroscopy, we observed and monitored the formation and diffusion of these complexes, which showed that the first and the third mechanistic scenarios were both possible, and were likely to occur simultaneously. Based on these findings, we maintain that palladium nanoparticles are not the true catalysts in C--C coupling reactions. Instead, catalysis is carried out by either palladium(0) atoms or palladium(II) ions that leach into solution.

A small library of copper and noble metal nanoclusters is designed and synthesized. These clusters are tested as catalysts in the Suzuki cross-coupling of various aryl halides with phenylboronic acid. It is found that copper and copper/noble metal combination nanoclusters are active catalysts for this reaction, the most active being the combined copper/palladium clusters. Iodo-, bromo-, and chloroarenes can be used. In the case of p-nitrobromobenzene, a one-pot cross-coupling and selective hydrogenation is achieved.

A simple experiment based on exclusion of Pd nanoclusters was used to identify the true catalytic species in cluster‐catalyzed Heck cross‐couplings. A special reactor was used in which two compartments are separated by a nanoporous membrane that allows the passage of Pd atoms and ions but not of Pd nanoclusters (see picture). The results show that the real catalysts in the Heck coupling of n‐butyl acrylate with iodobenzene are species leached from the Pd nanoclusters.

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The importance of small metal clusters in catalysis and the problems in understanding the clustering process in solution are outlined. A new analysis method for UV/Vis spectra is presented and applied to monitor the kinetics of ion reduction and cluster formation in situ. This method, which is based on a combination of two chemometric techniques, takes into account the entire UV/Vis spectrum and offers better interpretation possibilities than the traditional "band-assignment" approach. This is particularly true for nanoclusters because these have significant spectral contributions also outside the broad plasmon band that is usually associated with them. The reduction of palladium, gold, and silver ions and the formation of the corresponding clusters is monitored in the presence of two different reducing agents, sodium borohydride and tetraoctylammonium acetate. While Pd2+ is found to reduce and cluster directly, the spectral decomposition of the Au3+ reduction profiles shows two species corresponding to the Au+ intermediate and the Au0 clusters. The rates of reduction and clustering for Pd, Au, and Ag are compared and the possibilities of synthesising multimetallic clusters of these metals by coreduction are discussed.

The question of whether palladium nanoclusters are the actual catalysts in the so-called cluster -catalyzed Sonogashira cross-coupling is investigated, using the coupling of phenylacetylene with 4-bromobenzonitrile as a model reaction. By combining a detailed kinetic analysis with transmission electron microscopy (TEM), we show that a soluble species must be present in the system when Pd nanoclusters are used as catalysts. Various Pd clusters show similar kinetic profiles to that of a homogeneous Pd(dba) 2 complex. Most importantly, TEM analysis of samples taken before, during, and after the reaction shows that the cluster size decreases during the reaction. Based on these findings, we present a possible two-path mechanism for Sonogashira cross-coupling reactions in the presence of Pd nanoclusters.

The advantages of combining heterogeneous catalysis and aryl chloride substrates for cross-coupling are introduced. A heterogeneous Pd/C catalyst is used for activating aryl bromides and electron withdrawing aryl chlorides via a one-pot 'domino' HALEX-Sonogashira reaction. No ligand or co-catalyst is required, and the cross-coupling products are obtained in moderate to good yields. The influence of the solvent, base, iodide source and catalyst is evaluated. The catalyst is reusable for at least six consecutive reaction cycles. A variation on this reaction using catalytic amounts of KI is also proposed.