Salens are a class of organic compounds used as ligands in coordination chemistry. This class of compounds are named after the simplest example, N,N'-bis(salicylidene)ethylenediamine, more commonly referred to as salen.
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Salen ligands are Schiff bases, usually prepared by the condensation of a salicylaldehyde with an amine. For example, the preparation of salen:[1]
The reactants are in principle in equilibrium with the product, and water may be removed via an added drying agent or by azeotropic distillation.[2] In practice, the reaction of the salicylaldehyde with the amine in alcoholic solvent usually goes to completion.
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).
Salen complexes are prepared by reacting the diprotic pro-ligand with metal precursors containing built-in bases, such as alkoxides, metal amides, or metal alkyls. The pro-ligand may also be reacted with a metal halide, with or without an added base. Lastly, the pro-ligand may be deprotonated by a non-nucleophilic base, e.g. sodium hydride, before treatment with the metal halide. While alkyllithiums are strong bases, they may react with the imine group, and are thus not appropriate.[2] In this example, Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate:[4]
Unsubstituted salen complexes are poorly soluble in organic solvent. The presence of bulky groups near the coordination site is generally desirable, as it enhances catalytic activity and prevents dimerization. Salen ligands derived from 3,5-di-tert-butylsalicylaldehyde are popular because they fulfill both criteria; such complexes tend to be soluble even in non-polar solvents like pentane.
Chirality may be introduced into the ligand either via the diamine backbone, via the phenyl ring, or both.[2] 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.[5]
Numerous types of salen are derived from salicylaldehyde or the diamine.
Salen-type ligands called imino pyridines are also known, and are derived in a similar manner from 2-formylpyridine and a diamine.[7]
Salan[8] ligands are saturated at the nitrogen; these compounds are amines rather than imines. Salalens are intermediate between salan and salens: one side of the compound bears an imine, while the other side bears a free amine.[9] These complexes tend to be less rigid and more electron rich at the metal center than the corresponding salen complexes. Salan pro-ligands may be synthesized by reduction of salen pro-ligands. They may also be synthesized by the alkylation of an appropriate amine with a phenolic alkyl halide. Like the name suggests, half-salen compounds are not symmetrical. They are prepared from a salicylaldehyde and a monoamine.