Unlocking reactivity: synthetic, structural and catalytic exploration of ruthenium(ii) complexes featuring pdc and NHC ligands

Abstract

Eleven new ruthenium complexes (C2–C12), each incorporating the pyridine-2,6-dicarboxylate (pdc) pincer ligand and a tailored N-heterocyclic carbene (NHC) ligand, were synthesised in moderate to high yields. The feasibility of NHC coordination to the Ru(II) centre was found to depend on both the order of ligand addition to the precursor complex (C1) and the stepwise substitution of the 1,5-cyclooctadiene ligand with N-donor ancillary ligands. Characterisation techniques such as NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and cyclic voltammetry (CV) provided valuable insights into the synergistic roles of the pdc and NHC ligands in determining the structural, electronic, and catalytic properties of the complexes. The complexes exhibited moderate catalytic activity in both the transfer hydrogenation of ketones and the oxidation of alcohols. Among them, complex C2 showed the highest activity for the transfer hydrogenation of benzophenone to benzhydrol, achieving 99% conversion within 1 hour (TON = 99; TOF = 99 h⁻¹). For the oxidation of benzyl alcohol to benzaldehyde, complex C6 reached 99% conversion within 4 hours (TON = 16; TOF = 8 h⁻¹). Complementary density functional theory (DFT) studies supported the experimental results, particularly those relating to the structural and electronic characteristics of the complexes.