Synthesis, characterization, catalytic and biological application of half-sandwich ruthenium complexes bearing hemilabile (κ2-C,S)-thioether-functionalised NHC ligands

Abstract

A series of cationic Ru(II)(η⁶-p-cymene) complexes with thioether-functionalised N-heterocyclic carbene ligands have been prepared and fully characterized. Steric and electronic influence of the R thioether substituent on the coordination of the sulfur atom was investigated. The molecular structure of three of them has been determined by means of X-ray diffractrometry and confirmed the bidentate (κ²-C,S) coordination mode of the ligand. Interestingly, only a single diastereomer, as an enantiomeric couple, was observed in the solid state for complexes 1c, 1i and 1j. DFT calculations established a low energy inversion barrier between the two diastereomers through a sulfur pyramidal inversion pathway with R donating group while a dissociative/associative mechanism is more likely with R substituents that contain electron withdrawing group, thus suggesting that the only species observed by the ¹H-NMR correspond to an average resonance position of a fluxional mixtures of isomers. All these complexes were found to catalyse the oxydant-free double dehydrogenation of primary amine into nitrile. Ru complex bearing NHC-functionalised S-tBu group was further investigated in a wide range of amines and was found more selective for alkyl amine substrates than for benzylamine derivatives. Finally, preliminary results of the biological effects on various human cancer cells of four selected Ru complexes are reported.