Slow Inversion of Coordinated Thioether Groups in SNS-Type Ruthenium Pincer Complexes

The thioether-group-containing SNS-type pincer complex [({EtSCH2CH2}(2)NH)RuCl(H)(PPh3)] (2) exists in three different diastereomers (2 a-c). The molecular structures obtained from single crystal X-Ray diffraction studies of all three isomers reveal a difference in the relative orientation of the respective EtS-groups, while other commonly observed diastereomers as the result of cis-/trans- or fac-/mer-isomerism are not observed. Chemical exchange between the three diastereomers 2 a-c was discovered by phase-sensitive H-1 and P-31 NOESY NMR spectroscopy, and further quantified by line shape analysis of H-1 NMR spectra. The experimentally derived averaged Gibbs energies of activation for the interconversion of the isomers (65-70 kJ/mol) are in good agreement with the results obtained from DFT calculations, which suggest an inversion of the ligating sulfur atoms, although a dissociative pathway for the configurational inversion can be competitive.