Cobalt(iii) sepulchrate complexes: application as sustainable oxidative catalysts

Abstract

The application of cobalt sepulchrate (sep) complexes as active and robust homogeneous catalysts is reported here for the first time, as well as the crystal structure of the [Co(sep)]₂(SO₄)₃·10H₂O compound. The reaction chosen to investigate the catalytic performance of these catalysts was the oxidation of styrene due to the importance of its products in various industrial applications. Sustainable experimental conditions were selected, using H₂O₂ as the oxidant and ionic liquids as solvents. A similar catalytic activity was found using complexes containing different anions: [Co(sep)]Cl₃ and [Co(sep)]₂(SO₄)₃. Different kinetic profiles were found using different ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate or tetrafluoroborate) or acetonitrile as solvents. Complete conversions were achieved after 24 h of reaction. Benzaldehyde was the main product formed and in some cases the only one observed. The homogeneous cobalt catalysts using an ionic liquid as solvent could be recycled for consecutive cycles without significant loss of activity. In addition, the stability of the cobalt sepulchrate complex was confirmed by ¹H NMR after catalytic use.