New class of RSO2-NHC ligands and Pd/RSO2-NHC complexes with tailored electronic properties and high performance in catalytic C–C and C–N bonds formation

Abstract

Imidazolium salts have found ubiquitous applications as N-heterocyclic carbene precursors and metal nanoparticle stabilizers in catalysis and metallodrug research. Substituents directly attached to the imidazole ring can have a significant influence on the electronic, steric, and other properties of NHC-proligands as well as their metal complexes. In the present study, for the first time, a new type of Pd/NHC complex with the RSO₂ group directly attached to the imidazol-2-ylidene ligand core was designed and synthesized. The electronic properties as well as structural features of the new ligands were evaluated by means of experimental and computational methods. Interestingly, the introduction of a 4-aryl(alkyl)sulfonyl group only slightly decreased the electron donation, but it significantly increased the π-acceptance and slightly enhanced the buried volume (%V_bur) of new imidazol-2-ylidenes. New Pd/NHC complexes were obtained through selective C(2)H-palladation of some of the synthesized 4-RSO₂-functionalized imidazolium salts under mild conditions. Several complexes demonstrated good activity in the catalysis of model cross-coupling reactions, outperforming the activity of similar complexes with non-substituted NHC ligands.