New polysilyl dendritic precursors of triazolylferrocenyl and triazolylcobalticenium dendrimers - comparative electrochemical study and stabilization of small, catalytically active Pd nanoparticles
Résumé
The 27-ferrocenyl-terminated and the 27-cobalticenium-terminated metallodendrimers. 5 and 6, respectively,,were synthesized using only hydrosilylation reactions and click methodology, These new metallodendrimers were characterized by H-1 and C-13 NMR, infrared, and mass spectroscopy, elemental analysis, and cyclic voltammetty. The dendrimers 5 and 6, being isostructural in their neutral forms, show the same intensities for the redox exchange between their neutral and cationic forms, corresponding approximately to the transfer of 27 electron, as deduced from the Bard-Anson equation. The metallodendrimer 5 was used to form dendrimer-encapsulated palladium nanopartides (PdDENs). TEM images indeed show that the PdDENs are very small (average diameter = 1.0 nm) and relatively mono disperse. Their catalytic activity was evaluated for the Suzuki-Miyaura C-C coupling with various substituted aryl bromides (0.2 mol% of Pd per mole of substrate used). This activity is lower than that of previously studied dendrimers containing longer tethers than in 5, emphasizing the crucial role of the peripheral thickness of dendrimers that encapsulate PdNPs.