Epichlorohydrin-based CuAAC dendrimers with a calix[4]arene core and polar hydroxyl/oxyethyl terminal groups: synthesis, aggregation and use in catalysis

Abstract

1st generation dendrimers containing polar hydroxyl and both 1st and 2nd generation dendrimers with oxyethyl groups were obtained using a convergent approach and CuAAC reaction. Three types of dendrimer nuclei were utilized: the macrocyclic nucleus of calix[4]arene bearing 2/4 dendron and 2/4 alkyl fragments and the non-macrocyclic analog of calixarene bearing a one dendron/alkyl fragment. Nanoparticle tracking analysis reveals self-organization of the obtained dendrimers in aqueous solution into submicron/nanoparticles with sizes of 80–250 nm. The lowest CAC values in the series are possessed by tetra-alkyl substituted calix[4]arenes (1.6–1.8 μM), while the low molecular weight analog of calixarenes possesses a CAC one to two orders of magnitude larger (44–350 μM). When the obtained dendrimers were used as stabilizers of palladium nanoparticles, the formation of spherical palladium nanodendrites (50–60 nm) consisting of metal Pd and PdO was determined using TEM and SAED. The micellar effect of using stabilized palladium particles with dendrimers at concentrations higher than their CAC in the catalysis of Suzuki reactions and nitro aromatics reduction is shown. In the p-nitrophenol reduction reaction, the catalytic activity of the obtained systems (the maximum k_app in series was 0.47 min⁻¹) exceeds many known Pd-based systems.