Salen refers to a

tetradentate In chemistry, tetradentate ligands are ligands that bind four donor atoms to a central atom to form a coordination complex. This number of donor atoms that bind is called denticity and is a method of classifying ligands. Tetradentate ligands are ...

C₂-symmetric ligand synthesized from

salicylaldehyde Salicylic aldehyde (2-hydroxybenzaldehyde) is the organic compound with the formula (C7 H6 O2) C6H4CHO-2-OH. Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde. This colorless oil ...

(sal) and

ethylenediamine Ethylenediamine (abbreviated as en when a ligand) is the organic compound with the formula C2H4(NH2)2. This colorless liquid with an ammonia-like odor is a basic amine. It is a widely used building block in chemical synthesis, with approximately ...

(en). It may also refer to a class of compounds, which are structurally related to the classical salen ligand, primarily bis-

Schiff bases In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimines ...

. Salen ligands are notable for coordinating a wide range of different metals, which they can often stabilise in various oxidation states. For this reason salen-type compounds are used as

metal deactivator Metal deactivators, or metal deactivating agents (MDA) are fuel additives and oil additives used to stabilize fluids by deactivating (usually by sequestrant, sequestering) metal ions, mostly introduced by the action of naturally occurring acids in ...

Metal salen complexes A metal salen complex is a coordination compound between a metal cation and a ligand derived from ''N'',''N''′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt , usually ...

also find use as

catalysts Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

Synthesis and properties

H₂salen may be synthesized by the

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

of ethylenediamine and salicylaldehyde. : Preparation of salen

Complexes of salen with metal cations may be made without isolating it from the reaction mixture. This is possible because the stability constant for the formation of the metal complexes are very high, due to the

chelate effect Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...

. :H₂L + Mⁿ⁺ → ML⁽ⁿ⁻²⁾⁺ + 2 H⁺ where L stands for the ligand. The

pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...

adduct of the cobalt(II) complex Co(salen)(py) (

salcomine Salcomine is a coordination complex derived from the salen ligand and cobalt. The complex, which is planar, and a variety of its derivatives are carriers for O2 as well as oxidation catalysts.Shoichiro Yamada “Advancement in stereochemical aspe ...

) has a square-pyramidal structure; it can act as a dioxygen carrier by forming a labile, octahedral O₂ complex. The name “salen ligands” is used for

ligands which have similar structures. For example, in

salpn } Salpn is the common name for a chelating ligand, properly called ''N'',''N''-bis(salicylidene)-1,2-propanediamine, used as a motor oil additive. The molecular structure of pure (metal-free) salpn, sometimes denoted H2(salpn) or salpnH2, can be ...

there is a methyl substituent on the bridge. It is used as a metal deactivation additive in fuels. The presence of bulky groups near the coordination site may enhance the catalytic activity of a metal complex and prevent its dimerization. Salen ligands derived from 3,5-di-''tert''-butylsalicylaldehyde fulfill these roles, and also increase the solubility of the complexes in non-polar solvents like

pentane Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the ' ...

. Chiral “salen” ligands may be created by proper substitution of the diamine backbone, the phenyl ring, or both. An example is the ligand obtained by

of the C₂-symmetric ''trans''-1,2-

diaminocyclohexane ''trans''-1,2-Diaminocyclohexane is an organic compound with the formula C6H10(NH2)2. This diamine is a building block for ''C''2-symmetric ligands that are useful in asymmetric catalysis. A mixture of all three stereoisomer In stereochemi ...

with 3,5-di-''tert''-butylsalicylaldehyde. Chiral ligands may be used in

asymmetric synthesis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate molecul ...

reactions, such as the

Jacobsen epoxidation The Jacobsen epoxidation, sometimes also referred to as Jacobsen-Katsuki epoxidation is a chemical reaction which allows enantioselective epoxidation of unfunctionalized alkyl- and aryl- substituted alkenes. It is complementary to the Sharpless e ...

Related ligands

A class of tetradentate ligands with the generic name ''acacen'' are obtained by the condensation of derivatives of

acetylacetone Acetylacetone is an organic compound with the chemical formula . It is a colorless liquid, classified as a 1,3-diketone. It exists in equilibrium with a tautomer . These tautomers interconvert so rapidly under most conditions that they are tre ...

and

. Cobalt complexes o(acacen)L₂sup>+, selectively inhibit the activities of histidine-containing proteins through exchange of the axial ligands. These compounds show promise for the inhibition of

oncogenesis Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abnor ...

. The ''salan'' and ''salalen'' ligands are similar in structure to salen ligands, but have one or two saturated nitrogen-aryl bonds (

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituen ...

s rather than

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bo ...

s). They tend to be less rigid and more electron rich at the metal center than the corresponding salen complexes. Salans can be synthesized by the alkylation of an appropriate amine with a phenolic

alkyl halide The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely ...

. The “half-salen” ligands have only one salicylimine group. They are prepared from a salicylaldehyde and a monoamine. The name “salen” or “salen-type” may be used for other ligands that have similar environment around the chelating site, namely two acidic hydroxyls and two

Schiff base In organic chemistry, a Schiff base (named after Hugo Schiff) is a compound with the general structure ( = alkyl or aryl, but not hydrogen). They can be considered a sub-class of imines, being either secondary ketimines or secondary aldimine ...

(aryl-

) groups. These include the ligands abbreviated as salph, from the condensation of 1,2-phenylenediamine and salicylaldehyde, and ''salqu'', from the condensation of salicylaldehyde and ''2-quinoxalinol''.{{cite journal , author = Wu, Xianghong, Gorden, A. V. E., title = 2-Quinoxalinol Salen Copper Complexes for Oxidation of Aryl Methylenes, journal = Eur. J. Org. Chem. , volume = 2009 , pages = 503–509 , year = 2009 , doi = 10.1002/ejoc.200800928 , issue = 4

References

Organometallic chemistry Schiff bases Tetradentate ligands

Synthesis and properties

Related ligands

See also

References