Salen ligand

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, salen^2- 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.

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The ligand salenH₂ was first prepared by Pfeiffer.^[1] 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.^[2]

Coordination chemistry

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).

Salen-related ligands

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.^[3]

Analogues of the salenH₂ are also derivable by condensation of acetylacetone and ethylene diamine.

References

1. ^ P. Pfeiffer, E. Breith, E. Lübbe, T. Tsumaki "Tricyclische orthokondensierte Nebenvalenzringe" Justus Liebig's Annalen der Chemie volume 503, pp. 84-130 (1933). DOI: 10.1002/jlac.19335030106
2. ^ Harvey Diehl, Clifford C. Hach "Bis(N,N' - Disalicylalethylenediamine) - mu - Aquodicobalt(II)" Inorganic Syntheses 1950, volume III, 196-201.
3. ^ Jay F. Larrow and Eric N. Jacobsen (2004). "(R,R)-N,N'-Bis(3,5-Di-tert-Butylsalicylidene)-1,2-Cyclohexanediamino Manganese(III) Chloride, A Highly Enantioselective Epoxidation Catalyst". Org. Synth.; Coll. Vol. 10: 96.