Salen-type ligand

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.

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.

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:

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).

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