Homoleptic ruthenium(ii) complexes bearing imidazol(in)ium-2-dithiocarboxylate ligands: synthesis, characterization, and redox properties

Abstract

Five cationic ruthenium(II) chelates with the generic formula [Ru(S₂C·NHC)₃](PF₆)₂ were readily obtained upon cleavage of the [RuCl₂(p-cymene)]₂ dimer with representative imidazol(in)ium-2-dithiocarboxylate zwitterions (NHC·CS₂) in the presence of KPF₆. The homoleptic complexes were fully characterized by various analytical techniques and the molecular structure of one of them was determined by single-crystal X-ray diffraction analysis. As expected, it featured an octahedral RuS₆ core surrounded by three imidazol(in)ium rings and their nitrogen substituents. The robustness of the Ru–S bonds combined with the chelate effect of the κ²-S,S′-dithiocarboxylate units and the steric protection imparted by bulky 1,3-diarylimidazol(in)ium groups most likely accounted for the outstanding stability of these species in solution. Cyclic voltammetry showed that the five homoleptic complexes featured characteristic waves for a monoelectronic redox process corresponding to the Ru^III/Ru^II couple with E_1/2 values ranging between 0.97 and 1.27 V vs. Ag/AgCl. This half-wave potential was clearly dependent on the nature of their ancillary ligands as the evolution of the E_p,ox values roughly paralleled the basicity sequence of the NHCs used to prepare them, in line with the trends observed when monitoring the chemical shifts of the CS₂⁻ unit on ¹³C NMR spectroscopy and the CS₂⁻ asymmetric stretching vibration wavenumbers on IR spectroscopy.