MnIIIsalen

MnIIIsalen, often written Mn(III) salen, denotes a class of manganese(III) complexes of the salen ligand, N,N'-bis(salicylidene)ethylenediamine. The salen ligand is formed by condensation of salicylaldehyde derivatives with ethylenediamine and binds to Mn in a tetradentate fashion. In typical complexes the manganese center adopts a five- or six-coordinate geometry, with an axial ligand such as chloride, acetate, or a solvent molecule, and the complex is often chiral if the substituents on the salicylaldehyde units are chosen accordingly.

These complexes are stable in the +3 oxidation state and are widely studied as organometallic oxidation catalysts. Mn(III) salen catalysts are well known for enantioselective epoxidation of alkenes, using oxidants such as mCPBA or Oxone, and for other oxygen-transfer reactions. The stereochemical outcome is governed by the chiral environment of the salen ligand, and numerous derivatives have been developed to improve activity and selectivity.

Structural features include a d4 electronic configuration (high-spin in many cases), paramagnetism, and characteristic UV–visible absorption bands. The geometry is typically described as a distorted octahedron or square-based pyramid depending on axial ligation. Mn(III) salen systems can be prepared from Mn(II) salen precursors by oxidation with chemical oxidants.

Because Mn(III) salen complexes are synthetic catalysts, they are not naturally occurring minerals. They have influenced the field of asymmetric catalysis and serve as a model system for studying structure–activity relationships in chiral oxidation catalysts, with numerous reviews and research articles summarizing their scope and mechanism.