Formation and stabilization of nanosized Pd particles in catalytic systems: Ionic nitrogen compounds as catalytic promoters and stabilizers of nanoparticles

Abstract

• Pd catalytic systems usually contain nanoparticles regardless of the type of pre-catalyst and require special stabilizers. • Ionic nitrogen compounds stabilize Pd nanoparticles and open up new fields of application in organic synthesis. • The structure of the ionic compound affects the stabilization mechanism and the efficiency of the catalysts. • The catalytic action of the ionic nitrogen compound can outweigh the action of the ligands. • The overall effect of stabilization can be highly dependent on the nature of catalysis and reaction conditions. Actual palladium catalysts in synthetic transformations in reaction mixtures are usually represented by dynamic catalytic systems that contain various interconvertible forms of metal particles, including molecular complexes, metal clusters, and nanoparticles. The low thermodynamic stability of Pd nanoparticles can lead to their aggregation and, as a consequence, to the deactivation of the catalytic systems. Therefore, stabilization of nanosized Pd particles is of key importance to ensure efficient catalysis. This review discusses the main pathways for the formation of Pd nanoparticles and clusters from various precatalysts in catalytic systems, as well as current views on the mechanisms of stabilization of these nanosized Pd particles using various types of ionic nitrogen compounds, such as ammonium, amidinium, azolium, and pyridinium salts. The use of ionic nitrogen compounds as specially added or in situ formed stabilizers, ligands, catalytic promoters, heterogenized catalysts (supported ionic liquid phase, SILP) and reaction media (ionic liquids) is exemplified by several important catalytic reactions. The main effects of ionic nitrogen compounds on catalytic processes are also discussed, including possible involvement in catalytic cycles and unwanted side reactions.