Triazine-based mononuclear copper(ii) cis-dichloro and dibromo complexes as functional biomimetic model systems for phenoxazinone synthase and catecholase activities

Abstract

In recent years, the global perspective of small molecule activation has directed the focus towards biomimetic oxidations. Phenoxazinone synthase and catechol oxidase, two vital metalloenzymes, have drawn the attention of the scientific community towards their catalytic performances. Different transition metal-based complexes have been investigated to mimic their activities, yet many factors related to their mechanistic aspects and structure–activity relationship still lack detailed investigation. To get better insights into these factors, two penta-coordinated cis-dichloro and dibromo Cu(II) complexes [Cu(L)Cl₂] 1 and [Cu(L)Br₂] 2 (where L = N,N-dimethyl-4,6-di(pyridin-2-yl)-1,3,5-triazin-2-amine) were synthesized and comprehensively characterized through various spectroscopic and analytical tools such as UV-vis, FT-IR, NMR, EPR and ESI-MS. The redox behaviour of ligands as well as both the copper complexes were analysed. The molecular structure of 1 and 2 was elucidated via single-crystal X-ray diffraction analysis and a distorted square pyramidal geometry was determined. The halogen atoms (Cl for 1 and Br for 2) are found to be coordinated in a cis fashion for better substrate attachment. Catalytic performances towards the oxidation of o-aminophenol (OAPH) and 3,5-di-tert-butylcatechol (3,5-DTBC) were investigated using UV-vis spectrophotometry. Significant kinetic parameters towards the catalytic activities for 1 and 2 were determined. In terms of OAPH oxidation ability, both complexes were found to be quite comparable, with 2 having greater catalytic performance (K_cat = 156.7 h⁻¹) than 1 (K_cat = 124.5 h⁻¹). However, 2 shows remarkably superior activity for 3,5-DTBC oxidation (K_cat = 367.8 h⁻¹, ∼4.5 times) than 1 (K_cat = 82.2 h⁻¹). The mass spectrometric investigation provides information about the probable mechanistic path towards the oxidation of both the substrates. Iodometric titration experiments were performed to identify H₂O₂ formation, which proves the involvement of molecular O₂ during the catalytic oxidation. Overall, this study offers a detailed investigation on the significance of structural lability and their profound catalytic activities for biomimetic applications, which may open a new direction toward future advancements.