Effect of the halogen ligand in [Mn(salen)X] complexes on the catalytic styrene epoxidation in scCO2

• scCO2 is reported as good solvent in the [Mn(salen)X]-catalyzed styrene epoxidation.
• Styrene conversion got favored increasing electronegativity of the halogen ligand.
• Iodosylbenzene showed better performance than H2O2 as oxidizing agent in scCO2.
• Presence of water in the styrene epoxidation promoted formation of by-products

[Mn(salen) X] (X = Cl, Br and I) complexes were synthesized with yields ranging 47–56%. The H2-salen ligand and complexes were characterized by elemental analysis, 1H- and 13C NMR, UV/Vis and FTIR spectroscopies. The obtained complexes were tested as active species in the catalytic styrene epoxidation in supercritical carbon dioxide under pretty mild pressure (8.0 MPa) as solvent using either iodosylbenzene or hydrogen peroxide as oxidizing agents. Both, conversion of styrene and yield of styrene epoxide were favored as the electronegativity of the halogen ligand in the complex raised. Benzaldehyde, phenyl methyl ketone, benzoic acid, and 1-phenyl-1,2-ethanediol were other by-products detected by GC/MS together with the targeted styrene epoxide throughout the catalytic experiments. Second stage treatment in the single-pot reaction under 140 °C and 1.0 MPa did not yield the corresponding styrene carbonate, probably because an opposite trend takes place in every one of the two stages of reaction regarding the catalytic activity of the [Mn(salen)X] complex respect to the electronegativity of the halogen ligand.

Figure optionsDownload as PowerPoint slide