Structure and synthesis of copper-based Schiff base and reduced Schiff base complexes: a combined experimental and theoretical investigation of biomimetic catalytic activity

Abstract

Three copper(II) complexes, [Cu(L¹)(NCS)]_n (1), [Cu(L¹)(N₃)]_n (2) and [Cu(L²)(N₃)] (3) were synthesized from one Schiff base ligand and one reduced Schiff base ligand, (E)-4-chloro-2-[(2-propylaminoethylimino)methyl]phenol (HL¹) and 4-chloro-2-[(2-(propylaminoethylamino) methyl]phenol (HL²), respectively. All complexes were characterized by various physicochemical studies, such as FT-IR, UV-Vis, ESI-MS, EPR and single crystal X-ray diffraction. Complexes 1 and 2 have 1D polymeric chain-like structures bridging through thiocyanate and azide anions, whereas complex 3 has a mononuclear structure in the solid state. All the complexes are active towards mimicking two well-known proteins, phosphatase and phenoxazinone synthase, using the disodium salt of 4-nitrophenylphosphate (4-NPP) and 2-aminophenol (OAP) as the substrate in DMF medium. Complexes 2 and 3 show the highest activity towards phosphatase and phenoxazinone synthase activity with k_cat values of 22.6 s⁻¹ and 134.4 h⁻¹, respectively. EPR studies confirmed that for complex 1, the OAP oxidation goes through the generation of an organic radical at g = 1.99, which is due to an imine radical formation, whereas the metal center redox pathway is followed for complex 3. Extensive DFT calculations have been performed for both catalytic studies to put forward the most probable mechanistic pathways.