Bioinspired copper(II) complexes catalyzed oxidative coupling of aminophenols with broader substrate scope

Abstract

The strategic selection of ligand systems in metal complexes has demonstrated a profound impact on the efficiency and specificity of biomimetic reactions. In this work, we introduce a series of aminoquinoline-based copper(II) complexes (1–4) distinguished by systematic variation in terminal amine substituents: di-n-methyl (L1(H)), di-n-ethyl (L2(H)), di-n-propyl (L3(H)), and di-n-butyl (L4(H)). These complexes are synthesized, characterized, and evaluated as the catalyst for the oxidative coupling of different aminophenol derivatives. Remarkably, complex 1, featuring a methyl substituent, exhibited unparalleled catalytic performance, achieving an 86 % (K_cat - 9.7 × 10⁴ h⁻¹) conversion of o-aminophenol to the desired product, 2-amino-phenoxazin-3-one, alongside water and hydrogen peroxide as byproducts. Notably, complex 1 demonstrated exceptional versatility, extending its catalytic activity to other substrates with remarkable activity. Mechanistic investigations, supported by mass-spectrometric analysis, revealed the formation of a complex-substrate adduct with all substrates, enabling us to propose a detailed reaction pathway. The work highlights the benefits of ligand design in improving catalytic performance and sets a new standard for aminoquinoline-based copper(II) complexes in oxidative coupling reactions. To the best of our knowledge, this work is the first to report a wider substrate scope for PHS activity with copper(II) complexes.