Salen ligand
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| Names | |
|---|---|
| Other names
2,2'-Ethylenebis(nitrilomethylidene)diphenol, N,N'-Ethylenebis(salicylimine)
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| Identifiers | |
| ECHA InfoCard | 100.002.161 |
CompTox Dashboard (EPA)
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| Properties | |
| C16H16N2O2 | |
| Molar mass | 268.31 |
| Appearance | yellow solid |
| Melting point | 125–129 °C (257–264 °F; 398–402 K) |
| organic solvents | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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 ethylenediamine. The ligand is a bright yellow micaceous solid that is soluble in polar organic solvents.
Nomenclature
The diphenol H2salen is the conjugate acid of the ligand that logically is salen2-. But the terminology is used loosely. As an anionic tetradendate ligand, salen2- resembles tetradentate ligands including those that are macrocyclic, such as porphyrinate, corrin, bis(dimethylglyoximate), and some Schiff bases.
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, Tsumaki reported that the cobalt(II) complex Co(salen) reversibly bound O2, which led to intensive research on cobalt complexes of salen and related ligands for their capacity for oxygen storage and transport, looking for potential synthetic oxygen carriers.[3] SalenH2 forms complexes with most transition metals. In many cases, a square pyramidal complex with composition M(salen)L or an octahedral coordination sphere with stoichiometry M(salen)L2 is formed. Illustrative examples include VO(salen) and Co(salen)Cl(py). With d8 metal ions, low-spin square planar complexes form, such as Ni(salen).
Ligands related to salen
Numerous variations of salen are derived for salicaldehyde or the diamine.
- The ligand acacen (parent: H2acacen) is derived by condensation of acetylacetone and ethylenediamine.
- The ligand abbreviated "Salph" is derived from the condensation of 1,2-phenylenediamine and salicyaldehyde.
- The ligand "Salqu," derived by condensation of salicylaldehyde and 2-quinoxalinol, is an anionic tetradendate ligand, reminiscent of other macrocyclic ligands. Salqu copper complexes have been investigated as oxidation catalysts.[4]
Salen-type ligands called imino pyridines are also known, and are derived in a similar manner from 2-formyl pyridine and a diamine.[5]
Chiral derivatives
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 a ligand that forms complexes with Cr, Mn, Co, Al, which have proven useful for asymmetric transformations. For an example, see the Jacobsen epoxidation.[6]
References
- ^ P. Pfeiffer, E. Breith, E. Lübbe, T. Tsumaki (1933). ""Tricyclische orthokondensierte Nebenvalenzringe". Justus Liebig's Annalen der Chemie. 503: 84–130. doi:10.1002/jlac.19335030106.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Harvey Diehl, Clifford C. Hach (1950). "Bis(N,N' - Disalicylalethylenediamine) -μ - Aquodicobalt(II)". Inorg. Synth. 3: 196–201. doi:10.1002/9780470132340.ch53.
- ^ Tsumaki, T. (1938). "Nebenvalenzringverbindungen. IV. Über einige innerkomplexe Kobaltsalze der Oxyaldimine". Bull. Chem. Soc. Jap. (in German). 13 (2): 252–260. doi:10.1246/bcsj.13.252.
- ^ Wu, Xianghong, Gorden, A. V. E., (2009). "2-Quinoxalinol Salen Copper Complexes for Oxidation of Aryl Methylenes". Eur. J. Org. Chem. 4: 503–509. doi:10.1002/ejoc.200800928.
{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) - ^ Chatterjeea, D.; Mitraa, A. (2004). "Synthesis, Characterization and reactivities of Schiff-base complexes of
Ruthenium(III)". J. Coord. Chem. 57: 175–182. doi:10.1080/00958970410001662435.
{{cite journal}}: line feed character in|title=at position 73 (help)CS1 maint: multiple names: authors list (link) - ^ Larrow, J. F.; Jacobsen, E. N. (2004). "(R,R)-N,N'-Bis(3,5-Di-tert-Butylsalicylidene)-1,2-Cyclohexanediamino Manganese(III) Chloride, A Highly Enantioselective Epoxidation Catalyst". Organic Syntheses; Collected Volumes, vol. 10, p. 96.