Salen ligand
From Wikipedia, the free encyclopedia
| Salen ligand | |
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| Other names | 2,2'-Ethylenebis(nitrilomethylidene)diphenol, N,N'-Ethylenebis(salicylimine) |
| Identifiers | |
| CAS number | [94-93-9] |
| Properties | |
| Molecular formula | C16H16N2O2 |
| Molar mass | 268.31 |
| Appearance | yellow solid |
| Melting point |
127-128 °C, 272 K, -71 °F |
| Solubility in water | organic solvents |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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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.
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Salen ligand with M as a metal
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]
[edit] Coordination chemistry
In 1938, Tsumaki reported that the cobalt(II) salen reversibly bound O2, and this finding led to intensive research on salen and related ligands for the storage or transport of oxygen.[3] SalenH2 forms complexes with most transition metals. In many cases, the metal adopts square pyramidal or octahedral coordination sphere with the stoichiometry 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).
[edit] 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.[4]
Analogues of the salenH2 are also derivable by condensation of acetylacetone and ethylenediamine, which yields the so-called acacen ligands.
[edit] References
- ^ 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 10.1002/jlac.19335030106
- ^ Harvey Diehl, Clifford C. Hach "Bis(N,N' - Disalicylalethylenediamine) - mu - Aquodicobalt(II)" Inorganic Syntheses 1950, volume III, 196-201.
- ^ Tokuichi Tsumaki "Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine" Bulletin of the Chemical Society of Japan 1938 Volume 13, pp.252-260. doi:10.1246/bcsj.13.252
- ^ 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.
