Single and multiple site Cu(II) catalysts for benzyl alcohol and catechol oxidation reactions

Abstract

Cu(II) catalysts both in homogeneous and heterogeneous form were synthesized to bring the distinction between the single and multiple site catalysis. The single site mononuclear complexes with axial occupancy of Cl- and CH3COO- ligand and the acetate, imidazolate bridged dinuclear Cu(II) complexes were tested for selective oxidation of benzyl alcohol and catechol oxidation. The reactivity of the complexes was compared with the multiple site copper oxide (CuO) nanocatalyst derived from the metal complex precursor. The single site Cu(II) catalyst showed much superior activity in the selective oxidation of BA compared to the dinuclear and the CuO-nanocatalyst. The oxidation of catechol to o-benzoquinone was however found to be catalyzed more preferably by the di-nuclear systems. The high lability of Cl- and -OOCH3 groups in the mononuclear complexes allowed the incoming oxidant, substrate as well as the solvent (acetonitrile, CH3CN) molecules to interact with the active Cu-centre and thereby favoured the benzyl alcohol oxidation. Both the experimental and theoretical studies provided a conclusive idea about the reaction mechanism of benzyl alcohol and catechol oxidation. UV–vis, cyclic voltammetric study and density functional theory (DFT) calculations provided the strong evidence for the formation of Cu-CH3CN complex during benzyl alcohol oxidation. FTIR and Raman study substantiated by the theoretical calculations indicated for the formation of Cu- hydroperoxo species during the catalytic studies. The kinetics study revealed for the 1st order kinetic for the catechol oxidation and the DFT study predicted for the exothermic nature of the catechol oxidation process.