We reported the synthesis of the manganese(II) and manganese(III) complexes of a novel hexadentate Schiff base ligand (HBME) (1) obtained from the condensation of triethylenetetramine and 2 equiv. of 1-(biphenyl)-2-hydroxyimino-2-(morpholino)-1-ethanone. Hexadentate Schiff base ligand reacted with manganese(II) perchlorate or manganese(III) acetate and sodium azide, to give rise to a series of mono and dinuclear complexes [Mn(BME)(2)tren)] (2), [Mn-2(BME)(2)tren(N-3)(2)] (3) and [Mn-2(BME)(2)tren(N-3)(4)] (4). Synthesized molecules were characterized by elemental analyses, ICP-MS, spectroscopic studies, thermal analyses, molar conductivity and magnetic susceptibility measurements. The quantum chemical calculations of the HBME and complexes (2-4) were performed in the ground state using the DFT of B3LYP level with 6-311G(d,p) and LanL2DZ basis sets, respectively. H-1 and C-13 NMR chemical shifts were calculated by using the gauge independent atomic orbital (GIAO) method and the electronic properties such as HOMO-LUMO energies were determined by the time-dependent DFT approach. The calculated spectroscopic values for the HBME were in very good agreement with experimental ones. Furthermore, catalytic activity of the complexes for the disproportionation of hydrogen peroxide was also investigated in the presence of 1-methylimidazole. All the complexes displayed catalytic activity for the disproportionation of H2O2 significantly, but the activities of the (3) and (4) were relatively higher than the other molecules. (C) 2019 Elsevier B.V. All rights reserved.