page_title: Salen ligand
section_title: Synthesis and properties
hierarchical_section_title: Salen ligand / Synthesis and properties
caption_attribution_description: chemical structure of salcomine (Co(SALEN)2, Ethylenebis(salicylimine) cobalt(II) salt)
context_page_description: Salen refers to a tetradentate C₂-symmetric ligand synthesized from salicylaldehyde and ethylenediamine. It may also refer to a class of compounds, which are structurally related to the classical salen ligand, primarily bis-Schiff bases. Salen ligands are notable for coordinating a wide range of different metals, which they can often stabilise in various oxidation states. These metal salen complexes primarily find use as catalysts.
context_section_description: H₂salen may be synthesized by the condensation of ethylenediamine and salicylaldehyde.
  Complexes of salen with metal cations may be made without isolating it from the reaction mixture. This is possible because the stability constant for the formation of the metal complexes are very high, due to the chelate effect.
  H₂L + Mⁿ⁺ → ML + 2 H⁺
  where L stands for the ligand. The pyridine adduct of the cobalt(II) complex Co(salen)(py) (salcomine) has a square-pyramidal structure; it can act as a dioxygen carrier by forming a labile, octahedral O₂ complex.
  The name “salen ligands” is used for tetradentate ligands which have similar structures. For example, in salpn there is a methyl substituent on the bridge. It is used as a metal deactivation additive in fuels. The presence of bulky groups near the coordination site may enhance the catalytic activity of a metal complex and prevent its dimerization. Salen ligands derived from 3,5-di-tert-butylsalicylaldehyde fulfill these roles, and also increase the solubility of the complexes in non-polar solvents like pentane. Chiral “salen” ligands may be created by proper substitution of the diamine backbone, the phenyl ring, or both. An example is the ligand obtained by condensation of the C₂-symmetric trans-1,2-diaminocyclohexane with 3,5-di-tert-butylsalicylaldehyde. Chiral ligands may be used in asymmetric synthesis reactions, such as the Jacobsen epoxidation:
caption: Salcomine, a complex of salen with cobalt
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