Imidazole-modified Merrifield resin supported oxidovanadium(IV) complexes of Schiff-base-ether-based mixed functionality ligands for the catalytic oxidation of light aliphatic alcohols

Abstract

Three Schiff-base-ether mixed functionality ligands H2ap-Cl (I), H2ap-Br (II), and H2ap-Nt (III) were prepared by reacting precursor 3-(2-aminophenoxy)propan-1-ol with Cl, Br, and –NO2 derivatives of salicylaldehyde, respectively. Single crystal XRD analysis and theoretical calculations suggest the stabilization of azomethine -NH of the ligand III over its conventional phenolic-OH form. However, Schiff-base ligands I and II prefer the conventional phenolic -OH form over the azomethine -NH form. Oxidovanadium(IV) complexes [VO(ap-Cl)] (1), [VO(ap-Br)] (2), and [VO(ap-Nt)] (3) were prepared by reacting VO(acac)2 with the ligands (I), (II), and (III), respectively and grafted on the imidazole-modified Merrifield resin. Polymer-grafted complexes Ps-Im-[VO(ap-Cl)] (4), Ps-Im-[VO(ap-Br)] (5), and Ps-Im-[VO(ap-Nt)] (6) behave as good heterogeneous catalyst and show excellent substrate conversion (~50–96% for listed straight chain aliphatic alcohols) with high TOF values towards the oxidation of unfunctionalized aliphatic alcohols. As the carbon number increases in the aliphatic alcohol, substrate conversion reduce; hence, ethanol displays a maximum of 96% substrate conversion in the presence of catalyst 6, while acetic acid produces selectively. DFT calculations propose that the presence of a strong electron-withdrawing group such as -NO2 in the 3-position of salicylaldehyde of the Schiff-base ligands increases the ionization potential of the complex 6, which causes a marginal reduction in its catalytic activity than 4 and 5.