Synthesis,\nCharacterization, and Catalytic Evaluation\nof Ruthenium Complexes Bearing Xanthinium-8-dithiocarboxylate Ligands\nDerived from Caffeine and Theophylline

Various experimental procedures and\npurification techniques\nwere\napplied to alkylate or arylate the N7 and N9 positions of caffeine\nand theophylline into xanthinium salts. These N-heterocyclic carbene\n(NHC) precursors were converted into xanthinium-8-dithiocarboxylate\nzwitterions using CS<sub>2</sub> and either Cs<sub>2</sub>CO<sub>3</sub> or NaO<i>t</i>Bu. The NHC·CS<sub>2</sub> betaines\nwere employed as chelating ligands to prepare a wide variety of [RuX(<i>p</i>-cymene)(S<sub>2</sub>C·NHC)]Y (X = Cl, SAc; Y = Cl,\nPF<sub>6</sub>, [RuCl<sub>3</sub>(<i>p</i>-cymene)]) and\n[Ru(S<sub>2</sub>C·NHC)<sub>3</sub>]X<sub>2</sub> (X = Cl, PF<sub>6</sub>) complexes that were characterized by NMR and HRMS. Moreover,\nthe molecular structures of three betaines, one hetero-, and one homoleptic\ncomplex were determined by XRD. The catalytic potentials of all these\ncomplexes were investigated in the transfer hydrogenation of ketones\nwith isopropanol, the synthesis of vinyl esters from benzoic acid\nand 1-hexyne, and the cyclopropanation of styrene with ethyl diazoacetate.\nThe reduction of acetophenone into 1-phenylethanol was chosen as a\nmodel reaction for the former application. Monitoring the time course\nof this transformation showed that chelates bearing a NHC·CS<sub>2</sub> ligand displayed an initial activity slightly higher than\nthe analogous [RuCl<sub>2</sub>(<i>p</i>-cymene)(NHC)] complex.\nContrastingly, for the last two catalytic processes, the Ru(S<sub>2</sub>C·NHC) chelates did not outperform their Ru–NHC\ncounterparts.