Design and synthesis of ruthenium(II) OCO pincer type NHC complexes and their catalytic role towards the synthesis of amides

The present contribution describes the synthesis and characterization of a family of robust ruthenium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene [ (tBu)(OCO) (2-)] (NHC). Ruthenium(II) complexes (1-3) bearing bis-phenolate-N-heterocyclic carbene ligand were synthesized in good yields by the reaction of imidazolinium proligand (HL) with metal precursors [RuHCl(CO)(EPh3)(2)(B)] (E = P or As; B = PPh3, AsPh3 or Py) by transmetalation from the corresponding silver carbene complex. All the Ru(II)-NHC complexes have been characterized by elemental analyses, spectroscopic methods as well as ESI mass spectrometry. Based on the spectral results, an octahedral geometry was assigned for all the complexes. The tridentate nature of the (tBu)(OCO) (2-) ligand as well as some level of steric protection provided by the (t) Bu groups may rationalize the excellent stability of the Ru-C-carbene bond in the present systems. Moreover, for the explorations of catalytic potential of the synthesized compounds, all the three [Ru-NHC] complexes (1-3) were tested as catalysts for amidation of alcohols with amines. Notably, the complex 1 was found to be very efficient and versatile catalyst towards amidation of a wide range of alcohols with amines.