Ruthenium complexes bearing zwitterionic imidazolium-dithiocarboxylate ligands derived from N-heterocyclic olefins

C2-alkylated imidazolium salts reacted with carbon disulfide and cesium carbonate under mild, aerobic conditions to afford imidazolium-dithiocarboxylate zwitterions bearing 2,6-diisopropylphenyl or mesityl substituents on their nitrogen atoms. The addition of CS₂ took place regioselectively at the exocyclic C2α position of the starting materials and four new compounds were isolated in satisfactory to high yields. Two of them featured a chiral center and were obtained as racemic mixtures. Compared to previous strategies that required a strong base to deprotonate the imidazolium salts, followed by the nucleophilic addition of the intermediate N-heterocyclic olefins (NHOs) onto CS₂, our method involved only one step and relied on a weak, innocuous base. Moreover, it did not necessitate the use of dry and degassed solvents under an inert atmosphere, making it more environmentally friendly and easier to implement. The four NHO·CS₂ zwitterions served as ligands to prepare a small library of heteroleptic ruthenium-arene complexes with the generic formula [RuCl(p-cymene)(S₂C·NHO)](PF₆) and homoleptic dicationic complexes of the type [Ru(S₂C·NHO)₃](PF₆)₂. These chelates are the first examples of coordination compounds based on NHO·CS₂ inner salts. They were fully characterized using various analytical techniques and the molecular structures of three of them were determined. We also synthesized the IMesCH₂-d₂·CS₂ betaine in three steps from 1,3-dimesitylimidazolium chloride (IMes·HCl) using a straightforward procedure, and we used it as an isotopically labeled ligand to form the stable [RuCl(p-cymene)(S₂C·CH₂IMes-d₂)](PF₆) complex in 95% yield. This experiment served as a proof of concept to demonstrate the potential of our methodology for the design of new deuterated metallodrugs. Lastly, the catalytic activity of the eight Ru(S₂C·NHO) chelates was probed in the transfer hydrogenation of acetophenone with isopropanol and potassium hydroxide. Gratifyingly, they proved superior to their Ru(S₂C·NHC) cousins featuring xanthinium-8-dithiocarboxylate ligands derived from caffeine and theophylline.