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 (H 2 L), has been synthesized and characterized.Two copper complexes [Cu (HL 1 )] ( 1) and [Cu (HL 2 )] (2) have been obtained from H 2 L, 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.

A binuclear Cu(ii) complex as an efficient photocatalyst for N-alkylation of aromatic amines

“…The ligand (H 2 L) was synthesized and characterized by our previously reported methodology. 54 In our previous investigation, we observed an unprecedented transformation of the ligand by the treatment of copper acetate in the presence of methanol/ ethanol as solvents, 54 whereas, in the presence of polar aprotic solvent DCM, no change in the ligand was observed. The polar protic solvents like MeOH/EtOH are capable of forming hydrogen bonds with the substrate molecule, which can help in the oxidation and cleavage of the piperazinyl ring.…”

“…Conversion and selectivity were defined using previously reported methods. 54 Scheme 1 Formation of binuclear copper complexes.…”

“…Based on the above concept, we have already developed and reported two copper( ii ) complexes which can show photocatalytic behaviour in the oxidation of primary alcohols. 54…”

“…Based on the above concept, we have already developed and reported two copper(II) complexes which can show photocatalytic behaviour in the oxidation of primary alcohols. 54 In continuation of our effort to extend the photocatalysis in the realm of N-alkylation of amine hereby we have employed a flexible ligand viz. 2,4-dichloro-6-((4-(2-hydroxyethyl)piperazin-1-yl)methyl)phenol [H 2 L] 54 to synthesize copper(II) complexes which are further investigated for prospective photocatalytic reactions.…”

“…54 In continuation of our effort to extend the photocatalysis in the realm of N-alkylation of amine hereby we have employed a flexible ligand viz. 2,4-dichloro-6-((4-(2-hydroxyethyl)piperazin-1-yl)methyl)phenol [H 2 L] 54 to synthesize copper(II) complexes which are further investigated for prospective photocatalytic reactions. To provide a N 2 O 2 environment around the copper center just like the galactose oxidase active center, we developed a N 2 O 2 -based donor ligand, and to provide hydrogen bonding sites to the incoming substrate/reagent during the catalytic cycle, an alcoholic arm has been purposefully incorporated.…”