Selective oxidation of alcohols with molecular oxygen catalyzed by RuCl₃ · 3H₂O in P- and N-containing ligand functionalized ionic liquids

The application of functionalized ionic liquids (ILs) in homogeneous catalysis facilitates the reaction efficiency in homogeneous phase, liquid-liquid separation, and IL solvent-catalyst recycling. In this work, a series of P- or N-containing ligand functionalized imidazolium- based ILs were synthesized and used in conjunction with common ambient imidazolium-based ILs to afford the IL solution, which acted as both the ligand and the solvent. In such functionalized IL compositions, RuCl₃ · 3H₂O catalyzed oxidation of various alcohols using O₂ to aldehydes and ketones was achieved with high selectivity (most above 95%) in the absence of co-oxidants. The catalytic performance of the RuCl₃ · 3H₂O catalyst was greatly influenced by the solvents and ligands. The N-containing ligand functionalized IL with weaker ligation to Ru center exhibited better activity and much higher selectivity. Whereas, the N,P-hybrid ligand functionalized IL with bidentate chelating ability to Ru center showed relatively low activity. However, the recyclability of RuCl₃ · 3H₂O in such functionalized IL compositions was unavailable. The in situ ³¹P NMR spectroscopy study indicated that the oxidative degradation of the ligand itself was responsible for the deactivation of the Ru catalyst.