Manganese(III) complexes of N2O2 donor 5-bromosalicylideneimine ligands: Combined effects of electron withdrawing substituents and chelate ring size variations on electrochemical properties

A series of mononuclear manganese(III) complexes of formulae [Mn(L)(X)(H2O)] (1–13) and [Mn(L)(X)] (14–17) (X = ClO4, F, Cl, Br, I, NCS, N3), derived from the Schiff bases of 5-bromosalicylaldehyde and different types of diamine (1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane and 1,4-diaminobutane), have been synthesized and characterized by the combination of IR, UV–Vis spectroscopies, cyclic voltammetry and by X-ray crystallography. The redox properties of all the manganese(III) complexes show grossly identical features consisting of a reversible or quasireversible MnIII/MnII reduction. Besides MnIII/MnII reduction, the complexes 4, 5, 10, 13 and 16 also show reversible or quasireversible MnIII/MnIV oxidation. A linear correlation has been found for the complexes 5, 7, 11 and 13 [Mn(L2)(X)(H2O)] (X = F, Cl, Br, I) when E1/2 [MnIII/MnII] is plotted against Mulliken electronegativities (χM). The effect of the flexibility of the ligand on redox potential has been studied. It has been observed that the manganese(II) state is stabilized with increasing flexibility of the ligand environment. The crystal structure of 6 shows an octahedral geometry.