Polyaromatic N-heterocyclic carbene ligands and π-stacking. Catalytic consequences

摘要

In the course of our most recent research, we demonstrated how homogeneous catalysts withpolyaromatic functionalities possess properties that clearly differ from those shown by analogues lackingthese polyaromatic systems. The differences arise from the ability of the polyaromatic groups to affordnon-covalent interactions with aromatic molecules, which can either be substrates in a homogeneouscatalysed reaction, or the same catalysts to afford self-assembled systems. This article summarizes allour efforts toward understanding the fundamental effects of p-stacking interactions in homogenouscatalysis, particularly in those cases where catalysts bearing polyaromatic functionalities are used. Thestudy reveals several important implications regarding the influence of ligand–ligand interactions,ligand–additive interactions, and ligand–substrate interactions, in the performance of the catalysts used.In particular, the electronic properties of ligands with fused polyconjugated systems, are modified ifmolecules with p-stacking abilities are added, via a ligand–additive interaction. Also, the kinetics of thereactions in which aromatic substrates and catalysts with polyaromatic ligands are used, are stronglyinfluenced by the self-association of the catalysts and by the non-covalent interaction between thecatalyst and the aromatic substrates. The nature and the magnitude of these supramolecular interactionswere unveiled by using host–guest chemistry methods applied to organometallic catalysis. Finally, noncovalentinteractions afford a very convenient approach for the immobilization of catalysts decoratedwith polyaromatic systems onto the surfaces of graphene derivatives, hence affording an easy yetextremely effective way to support catalysts and facilitate recycling. The results given have fundamentalimplications in the design of future catalysts containing rigid polyaromatic systems, and may inspirefuture researchers in the design of improved homogeneous catalysts, by taking into account that theactivities of the metal complexes are strongly modified by supramolecular interactions.