Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation

A series of amine (1-6) and imine (5 & PRIME;,6 & PRIME;) 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, 2.4 x 105-6.2 x 106 h- 1) are better than amine-based (kcat, 6.3 x 104-3.9 x 105 h- 1) complexes. The complex-substrate adducts [Cu(L3) (OAP)] (7) and [Cu(L3 & PRIME;)(OAP)] (7 & PRIME;) were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3 & PRIME;) 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 & PRIME; is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7 & PRIME;. Further, the computed HOMO-LUMO energy gap of 7 & PRIME; was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (& UDelta;GR.E = -5.8 kcal/ mol) was found to be thermodynamically more favorable than amine complexes (& UDelta;GR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.