Synthesis of Cu(II) complexes by N,O-donor ligand transformation and their catalytic role in visible-light-driven alcohol oxidation

Abstract

Visible-light-driven photoreactions using metal complexes as catalysts are currently a research hotspot in terms of the development of environmentally friendly sustainable processes. To develop potential copper-based photocatalysts, a Mannich base ligand, namely, 2,4-dichloro-6-((4-(2-hydroxyethyl)piperazin-1-yl)methyl)phenol (H2L), has been synthesized and characterized. Two copper complexes [Cu (HL1)] (1) and [Cu (HL2)] (2) have been obtained from H2L, where the ligand undergoes an unprecedented transformation plausibly via oxidation of piperazine ring to ketone, subsequent oxidation of enol and nucleophilic attack of methanol followed by hydrolysis of amide bond, resulting piperazine ring cleavage. Under the irradiation of visible light, these catalysts can oxidize primary alcohols into corresponding aldehydes with very good conversion and high selectivity in the presence of molecular oxygen. The photocatalysts could be recovered almost quantitatively after completion of the catalytic cycle and recycled at least four times without much depreciation of catalytic activity. A plausible mechanistic pathway for alcohol oxidation has been explored through electrospray ionization mass spectrometry (ESI-MS) spectrometric, cyclic voltammetry, UV–vis, and computational study. © 2021 John Wiley & Sons, Ltd.