Synthesis and characterisation of cobal (II)- imidazolyl complexes as potential catalysts in the oxidation of phenol and styrene

Abstract

This study deals with the synthesis and characterisation of cobalt complexes of nitrogen-donor imidazolyl- salicylaldimine ligands and their potential catalytic activity in the oxidation reactions of phenol and styrene. Five ligands were used in the study, four of which are new. Compound 2,4-di-tert-butyl-6{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L1), was synthesised according to a literature procedure. The other imidazole-based salicylaldimine compounds 2-ethoxy-6{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L2), 4-methoxy-6{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L3), 1{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-benzene (L4) and 4-methyl-1{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-benzene (L5) were prepared by Schiff base condensation reaction of histamine dihydrochloride with 3-ethoxy-2-hydroxybenzaldehyde, 5-methoxy-2-hydroxybenzaldehyde, benzaldehyde, 4-methylbenzaldehyde respectively. L1-L5 were characterised by 1H and 13C{1H} NMR, IR and UV-vis spectroscopy and high resolution mass spectrometry (HRMS). Reactions of L1-L5 with CoCl2 yielded complexes C1-C5, while complexes C6-C10 were synthesised by reactions of L1-L5 with CoBr2 all in a ratio of 1:1 of ligand to metal precursor. The cobalt(II) complexes were characterised by IR and UV-vis spectroscopy, elemental analysis and high resolution mass spectrometry due to their paramagnetic nature. All the characterisation data point to complexes C1-C10 being formulated as [Co(3-L)X] (where L = ligand and X = halide). However, single crystal X-ray diffraction showed that the prolonged stay of complex C6 in a solution gave a new structure as complex C11. The imidazole-based salicylaldimine cobalt(II) complexes (C1-C10) were tested as catalysts for the oxidation reactions of phenol and styrene with H2O2, O2 and 3-chloroperbenzoic acid as oxidants. The complexes were found not to be active for the oxidation of both substrates (phenol and styrene) with 0% conversion, even when the reactions were left for 1 week. Analysis of the oxidation reactions were performed with GC and 1H NMR spectroscopy. Both analysis methods showed the presence of unreacted substrates at the end of the reaction period. Effects of concentration of pre-catalysts, substrates and oxidants, time and temperature of reaction and nature of solvent on catalytic activity were also investigated, however the conversions remained at 0% conversion.