Employing Cu(II) complexes of N,O-donor ligand for catalysis in visible light driven cleavage of lignin C-C bonds

Abstract

Transition metal complexes catalyzed visible-light assisted photoreactions are recently in huge demand for the development of environmentally friendly sustainable processes. To develop potential copper-containing photo-catalysts, a Schiff base ligand viz. 2-[((3-(dimethylamino)-2,2-dimethyl propyl)imino)(phenyl)methyl]phenol (HL) has been synthesized and characterized. Two binuclear copper complexes [Cu(L)Cl]2 (1) and [CuL(N3)]2 (2) have been obtained by reacting HL with Cu(II) salts. Under visible light irradiation, these catalysts can cleave the CC bonds of lignin in H2O as suggested by HPLC data. After completion of the reaction, the photo-catalysts could be recovered almost quantitatively and recycled for at least six times without much decrease in the catalytic efficiency. A plausible mechanistic pathway for the degradation reaction has been explored through ESI-MS spectrometry, NMR spectroscopy, UV-vis and computational study. The photocatalysts can degrade up to 95% of the authentic extracted lignocellulose sample into valuable chemicals. The catalysts have shown activity with sunlight as well towards degradation of lignin.