Synthesis and characterization of mixed valence cobalt(III)/cobalt(II) complexes with N,O-donor Schiff base ligands

The review is focused to discuss the synthetic strategy and properties of mixed valence cobalt(III)/cobalt(II) complexes with N,O-donor Schiff base ligands. Use of compartmental ligand is a good approach to prepare such complexes. Polydentate hydroxy-rich Schiff base ligands have also been used by various groups to prepare mixed valence cobalt complexes. Salen types Schiff bases have also been used by many synthetic inorganic chemists to prepare this type of complexes. The oxidation states of cobalt centers in these complexes may be assigned from BVS (Bond valence sum) calculation for each centre. Different oxidation states (i.e. +2 and +3) of two cobalt centers in the complexes could also be inferred comparing the relative bond lengths involving the two metal centers, e.g. shorter cobalt(III)–oxygen distances compared to cobalt(II)–oxygen distances. Same has also been true for cobalt-nitrogen bond lengths. Cobalt(II) is paramagnetic in nature whereas cobalt(III) is usually diamagnetic. The magnetic properties of mixed valence cobalt complexes are therefore important. Only a few complexes are showing SMM (single molecule magnet) behavior. Catalytic activities of few such complexes have also been explored and included in this review.