Metal nanoclusters stabilized by pH-responsive microgels: Preparation and evaluation of their catalytic potential

Unstabilized, pH-responsive soluble crosslinked polymers (microgels) bearing pendant trialkylamino or pyridyl groups and containing size-controlled Au or Pd nanoclusters have been prepared by radical copolymerization in dilute solution, followed by loading with HAuCl4 or Pd(OAc)2 and chemical reduction. The hydrodynamic volume, the solubility and the partition between immiscible solvents of the microgels and of the resulting microgel-metal nanocomposites have been investigated, together with the variation of these parameters with the solution pH: separation of our microgel-containing metal nanoclusters from aqueous solutions can be accomplished by precipitation or by extraction into an organic solvent phase upon pH change, thus enabling their potential recovery. The catalytic performance of the microgels in the aerobic oxidation of benzyl alcohol (Au) and in Sonogashira coupling reactions (Pd) has been determined. The microgel-Au nanocomposites exhibit poor catalytic activity, whereas better results have been obtained in the copper-free Sonogashira reaction with microgel-Pd nanocomposites as precatalysts.