A comprehensive overview on the synthesis, structures and applications of mono-nuclear transition metal complexes with asymmetrically substituted ‘salen-type’ Schiff bases

Salen-type ligands, prepared by the 1:2 condensation of a diamine and a salicylaldehyde derivative, represent a well known class of symmetrical tetra-dentate ligands, which have been used in the synthesis of a variety of complexes with several transition metals since long. Syntheses of 'salen type' asymmetric Schiff base ligands containing two different salicylaldehyde moieties, on the other hand, are tricky. This review focuses light on the synthesis of different 'salen type' asymmetric Schiff base ligands, and their mono-nuclear transition metal complexes. The X-ray structures and application of these complexes are also discussed.