Synthesis and catalytic evaluation of ruthenium-arene complexes generated using imidazol(in)ium-2-carboxylates and dithiocarboxylates

The ability of five imidazol(in)ium-2-carboxylates and dithiocarboxylates bearing cyclohexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms to act as NHC precursors for in situ catalytic applications was probed in ruthenium-promoted ring-opening metathesis and atom transfer radical polymerizations. Results obtained with 1:2 mixtures of [RuCl₂(p- cymene)]₂ and NHC-CO₂ adducts were in line with those reported previously starting from preformed [RuCl₂(p-cymene)(NHC)] complexes, whereas the NHC·CS₂ zwitterions were almost completely inactive. To account for this dichotomy, the preparation of preformed ruthenium-arene complexes from [RuCl₂(p-cymene)]₂ and NHC·CX₂ inner salts was thoroughly investigated. As expected, imidazolium-2-carboxylates lost their CO₂ moiety and afforded [RuCl₂(p-cymene)(NHC)] complexes in high yields, whereas the NHC·CS₂ betaines retained their zwitterionic nature and led to cationic complexes of the [RuCl(p-cymene)(NHC·CS₂)]PF ₆ type. These stable, 18-electron species are the first examples of well-defined transition-metal complexes bearing chelating NHC·CS ₂ ligands. They were characterized by various analytical techniques, and the molecular structure of [RuCl(p-cymene)(IMes·CS ₂)]PF₆ was determined by X-ray diffraction analysis.