Salen refers to a tetradentate C2-symmetric ligand synthesized from salicylaldehyde (sal) and ethylenediamine (en). 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.[1] For this reason salen-type compounds are used as metal deactivators. Metal salen complexes also find use as catalysts.[2]
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Other names
2,2′-Ethylenebis(nitrilomethylidene)diphenol, N,N′-Ethylenebis(salicylimine)
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3D model (JSmol)
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ECHA InfoCard | 100.002.161 |
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CompTox Dashboard (EPA)
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Properties | |
C16H16N2O2 | |
Molar mass | 268.32 |
Melting point | 126 °C (259 °F; 399 K) |
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H315, H319, H335 | |
P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501 | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
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H2salen may be synthesized by the condensation of ethylenediamine and salicylaldehyde.[3]
Complexes of salen with metal cations may be made without isolating it from the reaction mixture.[4][5] This is possible because the stability constant for the formation of the metal complexes are very high, due to the chelate effect.
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.[6][7]
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.[8] 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.[5] 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:[9][10]
A class of tetradentate ligands with the generic name acacen are obtained by the condensation of derivatives of acetylacetone and ethylenediamine.[11] Cobalt complexes [Co(acacen)L2]+, selectively inhibit the activities of histidine-containing proteins through exchange of the axial ligands. These compounds show promise for the inhibition of oncogenesis.[12]
The salan and salalen ligands are similar in structure to salen ligands, but have one or two saturated nitrogen-aryl bonds (amines rather than imines). They tend to be less rigid and more electron rich at the metal center than the corresponding salen complexes.[13][14] Salans can be synthesized by the alkylation of an appropriate amine with a phenolic alkyl halide. The “half-salen” ligands have only one salicylimine group. They are prepared from a salicylaldehyde and a monoamine.[15]
The name “salen” or “salen-type” may be used for other ligands that have similar environment around the chelating site, namely two acidic hydroxyls and two Schiff base (aryl-imine) groups. These include the ligands abbreviated as salph, from the condensation of 1,2-phenylenediamine and salicylaldehyde, and salqu, from the condensation of salicylaldehyde and 2-quinoxalinol.[16]
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