Liquid‐phase oxidation of olefins with rare hydronium ion salt of dinuclear dioxido‐vanadium(V) complexes and comparative catalytic studies with analogous copper complexes

Abstract

Homogeneous liquid-phase oxidation of a number of aromatic and aliphatic olefins was examined using dinuclear anionic vanadium dioxido complexes [(VO 2 ) 2 ( sal LH)] − (1) and [(VO 2 ) 2 ( Nsal LH)] − (2) and dinuclear copper complexes [(CuCl) 2 ( sal LH)] − (3) and [(CuCl) 2 ( Nsal LH)] − (4) (reaction of carbohydrazide with salicylaldehyde and 4-diethylamino salicylaldehyde afforded Schiff-base ligands [ sal LH 4 ] and [ Nsal LH 4 ], respectively). Anionic vanadium and copper complexes 1, 2, 3, and 4 were isolated in the form of their hydronium ion salt, which is rare. The molecular structure of the hydronium ion salt of anionic dinuclear vanadium dioxido complex [(VO 2 ) 2 ( sal LH)] − (1) was established through single-crystal X-ray analysis. The chemical and structural properties were studied using Fourier transform infrared (FT-IR), ultraviolet-visible (UV-Vis), 1 H and 13 C nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR) spectroscopy, and thermogravimetric analysis (TGA). In the presence of hydrogen peroxide, both dinuclear vanadium dioxido complexes were applied for the oxidation of a series of aromatic and aliphatic alkenes. High catalytic activity and efficiency were achieved using catalysts 1 and 2 in the oxidation of olefins. Alkenes with electron-donating groups make the oxidation processes easy. Thus, in general, aromatic olefins show better substrate conversion in comparison to the aliphatic olefins. Under optimized reaction conditions, both copper catalysts 3 and 4 fail to compete with the activity shown by their vanadium counterparts. Irrespective of olefins, metal (vanadium or copper) complexes of the ligand [ sal LH 4 ] (I) show better substrate conversion(%) compared with the metal complexes of the ligand [ Nsal LH 4 ] (II).