Mononuclear copper(ii) Schiff base complexes as effective models for phenoxazinone synthase

Abstract

A series of structurally related mononuclear copper(II) complexes of type [Cu(L)(SCN)] 1–3 and [Cu(L3)(N₃)] 4 containing tridentate Schiff base (N₂O) ligands (L1(H)–L3(H)), such as (E)-2-(((2-(benzylamino)ethyl)imino)methyl)phenol (L1(H)), (E)-2-(((2-(benzylamino)ethyl)imino)methyl)-4-methylphenol (L2(H)), and (E)-2-(((2-(benzylamino)ethyl)imino)methyl)-4-bromophenol (L3(H)) have been synthesized and characterized using modern analytical techniques such as UV-vis, IR, EPR, mass and single X-ray diffraction analyses. For instance, the band around 640 nm corresponds to the d–d transitions of 1–4 complexes and their EPR parameters g_|| > 2.1 > g_⊥ > 2.0 reveal a square planar geometry that encompasses the copper(II) centre in the solution. The structural characterization of 3 by a single-crystal X-ray diffraction study reveals a distortion in the square planar geometry encircling the copper(II) centre. Furthermore, complexes 1–4 were subjected to phenoxazinone synthase mimicking activity in methanol, and it was found to promote the conversion of o-aminophenol (OAP) into 2-aminophenoxazin-3-one (APX) effectively under a saturated dioxygen environment. Interestingly, this reaction follows the Michaelis–Menten model of enzyme kinetics and provides significant turnover numbers in the range of 2.4 × 10⁵–6.2 × 10⁶ h⁻¹. More importantly, we have also identified the complex–substrate adduct [Cu(L3)(OAP)] (5) using mass spectrometry. Furthermore, the detailed spectroscopic and kinetic studies point to the enzyme-like activity of 1–4.