Nanoparticles can serve as semi-heterogeneous supports since they readily disperse in common solvents and combine high surface area with excellent accessibility. Reversible agglomeration through solvent changes and magnetic separation provide technically attractive alternatives to classical catalyst filtration. This account places emphasis on recent developments in this emerging area.

Boomerang catalysis: A catalyst catch–release system is established by the noncovalent attachment of a Pd N‐heterocyclic carbene complex to graphene‐coated magnetic Co nanoparticles. The immobilization by pyrene tags (see scheme; blue) is reversible at elevated temperatures, releasing the homogeneous catalyst. The hydroxycarbonylation of aryl halides is performed in 16 iterative reactions with this highly active catalyst.

TEMPO was grafted on graphene-coated nanobeads with a magnetic cobalt core by using a general applicable "click"-chemistry protocol. The new heterogeneous CoNP-TEMPO emerged as a highly active catalyst for the chemoselective oxidation of primary and secondary alcohols using bleach as terminal oxidant. The outstanding stability of the C/Co nanoparticles enables the nanopowder to tolerate several TEMPO-mediated iterative oxidation reactions without any significant loss in catalyst activity. Furthermore, the excellent magnetic properties enable the rapid separation and quantitative recycling of CoNP-TEMPO out of the reaction mixture by simple magnetic decantation. The recovered nanoparticles can be subsequently reused without any further purification.

Two different types of azide functionalized magnetite@silica nanoparticles are synthesized, which are ideally suited as inexpensive supports for catalysts and reagents as demonstrated with the grafting of copper(II)‐azabis(oxazoline) complexes via a copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) reaction. The potential of the immobilized complexes as catalysts is tested in the desymmetrization of racemic 1,2‐diols through asymmetric benzoylation. Compared to azabis(oxazolines) “clicked” to common polymeric supports such as MeOPEG or Merrifield resin, Fe3O4@SiO2 proves to be superior with respect to activity and selectivity, as exemplified by employing the catalysts in up to five runs with consistent high activity and selectivity. Recycling of the catalysts is achieved quantitatively by magnetic decantation.