Reaction Conditions‐Dependent Formation of Catalytically Active Palladium Complexes or Palladium Nanoparticles on a Silica Support

Silica nanoparticles carrying covalently bound dipyridylmethylene (dpm) ligands were treated with Pd(OAc)(2) in acetone in an attempt to prepare a silica surface-immobilized Pd2+-dpm complex. However, instead of the expected tethered complexes, small, monodisperse and evenly distributed palladium nanoparticles (PdNPs) formed. We found that an organic impurity, most likely an alcohol, in reagent-grade acetone led to the reduction of Pd(OAc)(2) to form PdNPs. The size and surface distribution of the PdNPs are affected by temperature, and formation of PdNPs can be inhibited by using high purity solvents and/or low temperature. The PdNPs supported on silica nanoparticles demonstrated catalytic activity in oxidation of benzyl alcohol to benzaldehyde, while silica nanoparticles with surface-immobilized Pd2+-dpm complexes required an induction period in order to reach a similar level of catalytic activity. In all cases, PdNPs were observed after the catalytic reaction, providing evidence that the induction period exhibited by the materials carrying Pd2+-dpm complexes may be related to a time-dependent PdNP formation process, with PdNPs rather than the immobilized complexes playing a central role in the catalytic reaction. Although nanoparticles are sometimes identified after carrying out catalytic reactions with supported molecular Pd complexes, our studies demonstrate the importance of identifying the species present in as-synthesized heterogeneous materials of supported transition metal complexes.