Recent advances in S-functionalized N-heterocyclic carbene ligands: From the synthesis of azolium salts and metal complexes to applications

This review summarizes the recent progress in the field of S-functionalized N-heterocyclic carbene ligands and their metal complexes. The syntheses of the azolium precursors and of the corresponding metal complexes are discussed as a function of the five different S-containing chemical groups examined here: thiolate, thioether, thiophene, sulfonate and thione. In addition to an example of application of dithienyl-functionalized NHC acting as a switchable organocatalyst, the properties of the S-functionalized NHC metal complexes are discussed for each category of ligand, and are mostly related to applications in homogeneous catalysis.