ABSTRACT
A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4â¯×â¯105-6.2â¯×â¯106â¯h-1) are better than amine-based (kcat, 6.3â¯×â¯104-3.9â¯×â¯105â¯h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8â¯eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.Eâ¯=â¯-5.8â¯kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.Eâ¯=â¯+3.3â¯kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.