Lipidic Mesophase-embedded Palladium Nanoparticles: Synthesis and Tunable Catalysts in Suzuki-Miyaura Cross Coupling Reactions.

Lipidic cubic phases (LCPs) can reduce Pd2+-salts to palladium nanoparticles (PdNPs) of ~ 5 nm size in their confined water channels under mild conditions. The resulting PdNP-containing LCPs were used as nanoreactor scaffolds to catalyze Suzuki-Miyaura cross coupling reactions in the aqueous channels of the mesophase. To turn on catalysis, PdNP-containing LCPs are activated by swelling the aqueous channels of the lipidic framework, thereby enabling diffusion of the water-soluble substrates to the catalysts. The mesophases play a threefold role: they act as the reducing agent for Pd2+, as a limiting template for their growth, and as a support. The system was characterized and investigated by small-angle X-ray scattering (SAXS), cryo-transmission electron microscopy (cryo-TEM), dynamic light scattering (DLS) and nuclear magnetic resonance (NMR). Bulk LCPs as well as three dispersed palladium/lipid hybrid nanoparticle types were applied in catalysis. The latter - liposomes, hexosomes and cubosomes - can be obtained by design through combination of lipid and additive. The Suzuki-Miyaura cross coupling of 5-iodo-2'-deoxyuridine and phenylboronic acid was used as a model reaction to study these systems. Bulk Pd-LCPs deliver the Suzuki-Miyaura product in 24 h in conversions up to 98 % at room temperature, whereas with palladium/lipid dispersions at 40 °C 68 % of the starting material was transformed to the product after 72 h.