Synthesis and characterization of mono- and bi-metallic Mn(III) complexes containing salen type ligands

Abstract

A mononuclear complex 1, of Mn(III) with a Schiff-base ligand L [L = N,N′-ethylenebis(3-formyl-5-methylsalicylaldimine)] has been synthesized and structurally characterized. A series of binuclear complexes, MnML′Cl_xsolv_y [M = Zn(II), Cu(II), Ni(II), Co(II), Fe(III), and Mn(III); x = 3 or 4 depending on the oxidation state of M; L′ is the aniline condensation product of L; y = 1, 2 or 3 and solv = H₂O or CH₃OH] have been prepared by the reaction of 1, aniline and M(II/III) chloride with a 1∶3∶1.2 molar ratio in methanolic media. These bimetallic complexes were characterized by elemental analyses, UV-Vis and IR spectroscopies, magnetic susceptibilities and thermogravimetric analyses. The results of magnetic susceptibility measurements at 300 K indicate that all the metal ions are in their high spin states. The catalytic properties of these complexes for epoxidation of alkenes have been investigated using PhIO as a terminal oxidant. The binuclear Mn(III) complexes are less efficient epoxidation catalysts than the mononuclear Mn(III) complex, 1. The reduced activities of the bimetallic complexes may be due to both the low rates of formation of a Mn(V)O intermediate and the transfer of oxygen from this intermediate to the alkene. Furthermore, the bimetallic complexes also transformed into a catalytically inactive species in the presence of PhIO as indicated by UV-Vis spectroscopic studies.