Substituted salen complexes 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. 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, which is catalyzed by a chiral manganese-salen complex: Complexes with salen-type ligands "Salen-type" metal complexes are formed with ligands with similar chelating groups, such as acacen, salph, and salqu. Salqu copper complexes have been investigated as oxidation catalysts. Complexes with salan ligands Complexes with the similar salan or salalen ligands, with one or two saturated nitrogen–aryl bonds (amines rather than imines) tend to be less rigid and more electron-rich at the metal center than the corresponding salen complexes. Further reading References Coordination chemistry Organometallic chemistry Ligands