Synthesis, structural characterization, theoretical calculations and catecholase mimetic activity of manganese-Schiff base complexes

Potentially hexadentate N2O4 donor Schiff base ligands N,N0-bis(3-ethoxysalicylidenimino)-1,2-diaminoethane (L1H2) and N,N0-bis(3-methoxysalicylidenimino)-1,3-diaminopropane (L2H2) have been used to synthesize two manganese(III) compounds [MnL1(SCN)(H2O)].[MnL1(H2O)2](ClO4) CH3CN (1) and [MnL2(SCN)(H2O)] (2). All the ligands and the complexes have been characterized by microanalytical and spectroscopic techniques. The X-ray structural characterization of all the complexes shows a tetragonally elongated octahedral geometry for the manganese coordination sphere. The octahedron entities are further connected in pairs through intermolecular hydrogen bonding to form l-aquo dimers, which are further stabilized by p–p interactions occurring between the aryl rings of the Schiff base ligands. These noncovalent interactions have been investigated in terms of energies and geometries using theoretical calculations. The catalytic activity of compounds 1 and 2 on the oxidation of 3,5-di-tert-butylcatechol was determined spectrophotometrically by monitoring the increase of the 3,5-di-tert-butyl-obenzoquinone characteristic absorption band at about 400 nm over time in methanol saturated with O2. The complexes were able to oxidize 3,5-di-tert-butylcatechol to the corresponding o-quinone with distinct catalytic activity.