Controlling Cleavage of C _α –C _β Bonds in Lignin Induced by Photo-Driven Iron Salt at Simulated Natural Environment Conditions

Reforming 3d-metal-based visible light catalytic platforms is desirable yet challenging for the selective cleavage of the C–C bond in lignin to value-added biochemicals. Herein, we provide a cost-effective iron-catalyzed photochemical strategy for the selective conversion of lignin to benzaldehyde under simulated natural conditions. Furthermore, the product distribution can be rationally regulated by changing the counteranion of Fe3+. With Fe(NO3)3·9H2O, a 100% conversion of diphenyl ethanol (lignin model) was achieved, affording a 186.0 mol % yield and 93.0% selectivity for benzaldehyde via the Cα–Cβ bond cleavage. By contrast, FeCl3·6H2O predominantly favored the Cα–OH oxidation to form diphenylethanone (72.4% selectivity). The results of the mechanistic study and density functional theory (DFT) calculation unveil that benzaldehyde formation proceeds via β-scission of an FeIII alkoxide intermediate through photodriven ligand-to-metal charge transfer (LMCT), wherein the nitrate counteranion serves as an internal oxidant in the iron nitrate catalytic system. Conversely, FeCl3 generates chlorine radicals via homolytic cleavage, resulting in the hydrogen atom abstraction at Cα–OH which consequently inhibits the breakage of the Cα–Cβ bond. Notably, the Fe(NO3)3 catalytic system also enables efficient C–C bond cleavage in realistic lignin (121.3 mg g–1 yield of monophenols), as evidenced by 2D HSQC NMR and FT-IR. Therefore, the findings in this work advance solar-driven lignin valorization and, more importantly, offer deep insights into the recently reemerging photochemistry of FeIII salts.