| Salen ligand | |
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| Other names | 2,2'-Ethylenebis(nitrilomethylidene)diphenol, N,N'-Ethylenebis(salicylimine) |
| Identifiers | |
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| Properties | |
| Molecular formula | C16H16N2O2 |
| Molar mass | 268.31 |
| Appearance | yellow solid |
| Melting point |
125-129 °C, 269 K, -75 °F |
| Solubility in water | organic solvents |
 (what is this?) (verify)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
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 ethylenediamine. As an anionic tetradendate ligand, salen2- resembles tetradentate ligands including those that are macrocyclic, such as porphyrinate, corrin, bis(dimethylglyoximate), and some Schiff bases. Analogous ligands are also available by condensation of acetylacetone and ethylenediamine, the so-called acacen ligand.
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Preparation
SalenH2 is commercially available. It 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 by the condensation of ethylenediamine and salicylaldehyde.[2]
Coordination chemistry
In 1938, T. 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). With d8 metal ions, low-spin square planar complexes form, such as Ni(salen).
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A square planar M(salen) complex.
Ligands related to 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. For example, condensation of the C2-symmetric trans-1,2-diaminocyclohexane with 3,5-di-tert-butylsalicylaldehdye gives that forms complexes with Cr, Mn, Co, Al have proven useful for diverse asymmetric transformations. For an example, see the Jacobsen epoxidation.[4]
References
- ^ P. Pfeiffer, E. Breith, E. Lübbe, T. Tsumaki (1933). ""Tricyclische orthokondensierte Nebenvalenzringe". Justus Liebig's Annalen der Chemie 503: 84–130. doi:.
- ^ Harvey Diehl, Clifford C. Hach (1950). "Bis(N,N' - Disalicylalethylenediamine) -μ - Aquodicobalt(II)". Inorg. Synth. 3: 196–201. doi:.
- ^ Tokuichi Tsumaki (1938). "Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine". Bulletin of the Chemical Society of Japan 13: 252–260. doi:.
- ^ 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., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=V75P0001; Coll. Vol. 10: 96
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