Oxidative Dehydrogenation of Coordinated Amines Assisted by Metal Ions

Abstract

Oxidative dehydrogenation (OD) of amines involves the elimination of two protons and two electrons ─(2H⁺ + 2e) that results in the transformation of ─CH─NH─ to ─C═N─ moiety. A variety of amines coordinated to the transition metal ion, including iron, cobalt, nickel, copper, ruthenium, rhodium, and iridium, also rare earth metals undergo OD reaction. The reaction occurs mainly under a basic medium in the presence of air/O₂, which is a green oxidant. The metal ions play a significant role in the OD reaction. They can participate in redox reactions as an internal oxidant and can also influence the deprotonation of the ─NH function to form an amido intermediate, crucial for the OD reaction of amines. Not only that, the photosensitized metal ion or metal-stabilized ligand-centered radical activates O₂ to promote OD reaction. This review presents the OD reaction of amines that occurred in air/O₂ to explore the role of metal ions and fates of O₂. The study reveals that along with the oxidation state of the metal ion, other factors like the redox potential of Mⁿ⁺/M⁽ⁿ⁻¹⁾⁺, the geometry of the complex, the coordination mode of the ligand, substituents on the ligand, co-ligands, and steric crowding also influence OD. The different spectroscopies, including single-crystal X-ray diffraction, which is employed to study the OD reactions, are also interpreted.