NNbissalicylideneethylenediamine

N,N'-bis(salicylidene)ethylenediamine, commonly abbreviated as salen, is a tetradentate Schiff base ligand formed by the condensation of ethylenediamine with two equivalents of salicylaldehyde. The ligand is planar and rigid, providing a defined N2O2 donor set that binds metal centers in a characteristic fashion.

Structurally, the ligand contains two imine nitrogens and two phenolate oxygens. Upon deprotonation of the phenolic groups, salen acts as a dianionic ligand, coordinating to a metal center through its four donor atoms. This arrangement often leads to square-planar or octahedral coordination geometries, depending on the metal and auxiliary ligands present.

Synthesis typically involves a condensation reaction between ethylenediamine and two equivalents of salicylaldehyde under basic or neutral conditions, usually in a polar organic solvent. The reaction is monitored by the formation of the imine linkages and can yield crystalline salen ligands that are isolable in good purity.

In coordination chemistry, salen ligands form stable complexes with a wide range of metals, including Cu(II), Ni(II), Mn(III/II), Fe(III), and Co(III). Mn-salen derivatives, in particular, are well known as catalysts for enantioselective oxidation of olefins, using oxidants such as iodosylbenzene or Oxone. Cu-salen complexes also catalyze various oxidation and cycloaddition reactions. The ligand framework is amenable to chiral modification, enabling enantioselective catalysis and the development of numerous salen-based catalysts and materials.

Overall, N,N'-bis(salicylidene)ethylenediamine is a foundational ligand in organometallic and catalytic chemistry, valued for its tunable steric and electronic properties and its versatility in forming active metal complexes.