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Salen is the abbreviation for a popular chelating ligand used in coordination chemistry and homogeneous catalysis. The name salen is a contraction for salicylic aldehyde and ethylene diamine, which are the precursors to the ligand. As an anionic tetradendate ligand, salen2- resembles other quasi-planar ligands including those that are macrocyclic, such as porphyrinate, corrin, and bis(dimethylglyoximate. Salen together is a preeminent example of a Schiff base.
Additional recommended knowledge
The ligand salenH2 was first prepared by Pfeiffer. It is often generated in situ followed by the addition of the metal salt, but the ligand is also easily prepared as a pure organic compound, as well as being commercially available.
SalenH2 forms complexes with most transition metals. In most cases, the metal adopts square pyramidal or octahedral coordination, M(salen)L and M(salen)L2. Illustrative examples include VO(salen) and Co(salen)Cl(pyridine). Low-spin d8 metal ions form square planar complexes (see picture at lower right), such as Ni(salen).
Numerous variations of salen are known with diverse substituents. For example the ligand abbreviated "Salph" is derived from the condensation of 1,2-phenylenediamine and salicyaldehyde. Chiral versions the salen motif are derived from chiral 1,2-diamines, such as the C2-symmetric 1,2-aminocyclohexane. When the aldehyde is 3,5-di-tert-butylsalicylaldehdye one obtains a ligand that was popularized by Eric Jacobsen. Complexes of this ligand with Cr, Mn, Co, Al have proven useful for diverse asymmetric transformations. For example, see the Jacobsen epoxidation.
Analogues of the salenH2 are also derivable by condensation of acetylacetone and ethylene diamine.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Salen_ligand". A list of authors is available in Wikipedia.|