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Salen 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. The ligands consist of phenol and Schiff base (aryl-imine groups.
Contents
Preparation of the ligands
Salen ligands are prepared by the condensation of a salicylaldehyde with an amine. For example, the preparation of salen:
The reactants are in principle in equilibrium with the product, and water may be removed via an added drying agent or by azeotropic distillation. In practice, the reaction of the salicylaldehyde with the amine in alcoholic solvent usually goes to completion.
Preparation of complexes
Free salen ligand is often written as SalenH2 to emphasize its diprotic character. Its conjugate base forms complexes with most transition metals. These complexes are usually prepared by the reaction of the diprotic pro-ligand with metal precursors containing built-in bases, such as alkoxides, metal amides, or metal acetate. The pro-ligand may also be treated 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. Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate:
Structures
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).
Enzyme mimics
Tsumaki described the first metal-salen complexes in 1938. He found that the cobalt(II) complex Co(salen) reversibly binds 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. Cobalt salen complexes also replicate certain aspects of vitamin B12.
Homogeneous catalysis
The manganese-containing catalyst is used for the asymmetric epoxidation of olefins. In the hydrolytic kinetic resolution technique, racemic mixture of epoxides may be separated by selectively hydrolyzing one enantiomer, catalyzed by the analogous cobalt(III) complex. In subsequent work, chromium(III) and cobalt(III) salen complexes were found to be good catalysts for preparing poly(carbonates) from carbon dioxide and epoxides.
Ligand design
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. 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.
Salen-type ligands
Numerous types of salen are derived from salicylaldehyde or the diamine.
Salen-like ligands and complexes
The similar salan 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. 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.