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

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 photocatalysts, 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 C-C 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 ESIMS 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.

Automated summary

Transition metal complexes catalyzed visible-light assisted photoreactions are highly demanded for sustainable processes. In this study, copper-containing photocatalysts were developed using a newly synthesized Schiff base ligand. The catalysts showed efficient cleavage of lignin C-C bonds under visible light irradiation, and could be recovered and recycled multiple times without significant loss of efficiency. The study also explored the mechanistic pathway of the degradation reaction and demonstrated the catalysts' potential for lignin degradation using sunlight.