Modulating the electronics of orthometalated RuII-NHC complexes via substitution patterns or NHC donors: Studies towards the impacts in catalysis

Abstract

The effect of electronic modulations on a series of C˄C orthometalated RuII-NHC complexes 2-10 was explored in (de)hydrogenation reaction. The electronic tweaking of the complexes was achieved either by varying the carbene donors (ImNHC vs 1,2,4-TzNHC) or the nature and position of the substituents on the N-phenyl wingtip of the NHC ligand. All the synthesized new complexes were fully characterized by using NMR analyses, mass spectrometry as well as X-ray crystallography. The electronic nature of the complexes was ascertained from the electrochemical analyses in addition to 13C{1H} NMR studies. Catalytic activities of the complexes were investigated in the acceptorless dehydrogenation of 1-phenyl-1-propanol to propiophenone, and we observed that RuII-NHC complexes containing ImNHC donors outperformed their TzNHC analogues. Further, the RuII-ImNHC precatalysts with substituents at the para-position with respect to the imidazolium moiety exhibited an activity trend of electron deficient complexes being more active than that of the electron rich complexes. Whereas the analogous precatalysts with the substituents at meta-position exhibited different activity trend (electron rich complex is superior compared to the electron deficient complexes). Additionally, the complexes were also tested in the transfer hydrogenation of acetophenone and surprisingly, in contrast to dehydrogenation reaction, the meta-substituents had no effect on the transfer hydrogenation activities of the complexes.