Complexes of salen per se are poorly soluble in organic solvents. Substitution of the organic framework increases the solubility of the complex. 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 adjacent to the phenoxide group can give complexes with enhanced catalytic activity.  These substituents suppress formation of dimers. For these reasons, salen ligands derived from 3,5-di-tert-butylsalicylaldehyde have received particular scrutiny.
Chirality may be introduced into the ligand either via the diamine backbone, via the phenyl ring, or both. For example, condensation of the C-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: