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 Co2+, usually denoted as Co(salen). 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 Co2+, usually denoted as Co(salen). The metal-free salen compound (H2salen or salenH2) has two phenolic hydroxyl groups. The 'salen ligand' is usually its conjugate base (salen2-), resulting from the loss of protons from those hydroxyl groups. The metal atom usually makes four coordination bonds to the oxygen and nitrogen atoms. The salen anion forms complexes with most transition metals. These complexes are usually prepared by the reaction of H2salen ('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. For example, Jacobsen's catalyst is prepared from the salen ligand precursor with manganese acetate: Salen complexes with d8 metal ions, such as Ni(salen), typically have a low-spin square planar molecular geometry in the coordination sphere. Other metal-salen complexes may have additional ligands above the salen nitrogen-oxygen plane. Complexes with one extra ligand, such as VO(salen), may have a square pyramidal geometry. Coplexes with two extra ligands, such as Co(salen)Cl(py), may have octahedral geometry. 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. 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. Unsubstituted salen complexes are poorly soluble in organic solvents. Side chains attached to the ethylene bridge or the benzene rings may increase the solubility. An example is the salpn ligand, derived from 1,2-diaminopropane instead of ethylenediamine, which is used as a metal deactivating additive in motor oils and motor fuel. 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, and tend to be soluble even in non-polar solvents like pentane.