Cyclodextrins as multitask agents for metal nano-heterogeneous catalysis: a review

Research in green chemistry has recently focussed on designing reactions in water as a safe solvent, calling for catalysts that operate in aqueous solutions. In particular, supramolecular metal nanoparticles have been synthesized by associating macrocyclic compounds with transition metal nanoparticles, to produce water-dispersible catalytic systems of high performance and molecular recognition ability. Here we review methods for stabilizing catalytically active metal nanoparticles in cyclodextrin-based systems. Indeed, cyclodextrins form inclusion complexes with molecules of appropriate size and shape, and the presence of hydroxyl groups enables coordination with metal ions. Representative examples show increasing stability, recyclability and catalytic activity. We present 1) the synthesis, characterization and catalytic behaviors with focus on the development of metal nanoparticles in solvent-dispersed form or immobilized onto supports in the presence of cyclodextrins or their derivatives; 2) the multi-functional role of cyclodextrins capable of acting as reducing agents, stabilizing/dispersing agents or mass transfer promoters; and 3) the design of more sophisticated catalytic systems, in which the cyclodextrin is a supramolecular host with dynamic equilibrium.