Polymer-supported Pd–NHC complexes: Strategies for the development of multifunctional systems

Different strategies to introduce multiple ionic liquid-like functionalities in PS–DVB based polymers in an easy and reproducible way are reported. A basic common structural element has been, in all cases, the presence of covalently attached imidazolium subunits able to form palladium–NHC complexes (Pd–NHC) and to stabilize PdNPs. Several structural alternatives have been studied in order to modify the catalytic behavior of the corresponding N-heterocyclic carbenes (NHC) complexes, in terms of both activity and stability. Thus, two types of supported pincer-like ligands were prepared using different linker/spacer combinations. The microenvironment of those supported NHC-ligands was further modified by the introduction of additional IL-like groups based on ammonium, pyridinium and imidazolium moieties. The presence of the additional functionalities in the support proved to play interesting roles in the catalytic activity of the supported catalysts/precatalysts for the Heck Csingle bondC coupling reaction. In particular, catalytic systems derived from resins containing ditopic groups being able to form stable pincer complexes have shown a very remarkable stability with no sign of deactivation after several cycles.