Exploration of catecholase-like activity of a series of magnetically coupled transition metal complexes of Mn, Co and Ni: new insights into the solution state behavior of Mn complexes

A series of tri-nuclear complexes of general formula [M3L2(OAc)(4)], where M = Mn (1), Co (2) and Ni (3), (HL = (E)-4-bromo-2-(((2-morpholinoethyl)imino)methyl)phenol), have been synthesized. Single crystal X-ray crystallography reveals that each molecule contains three metal ions which are bridged by four acetate moieties. In the solution phase, the complexes are present as mononuclear species. Amongst them, the manganese atom of complex 1 switches its oxidation state from +ii to +iii with time, as confirmed by time dependent UV-Vis and EPR spectroscopic techniques. Furthermore, complex 1 with Mn in both oxidation states can oxidise 3,5-DTBC to 3,5-DTBQ through the ligand centred radical formation pathway. It is remarkable that complex 1 in the Mn-II oxidation state shows an abnormally high rate constant value in the oxidation of 3,5-DTBC to 3,5-DTBQ. This difference in rate constant values for catechol oxidation reaction by complex 1 can be explained by considering the binding constant value of catechol with Mn-II and Mn-III respectively from experimental and theoretical aspects. Similar to complex 1, complexes 2 and 3 also catalyse catechol oxidation following ligand centred imine radical formation pathways. Furthermore, magnetic properties of all the complexes were explored. DC magnetic susceptibility studies of complexes 1 and 2 revealed that in both the complexes the metal centres are antiferromagnetically coupled with adjacent metal centres, whereas in the case of complex 3, weak ferromagnetic interaction occurs between the neighbouring Ni-II centres at low temperature.