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

Reforming 3 d -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 Fe 3+ . With Fe(NO 3 ) 3 ·9H 2 O, a 100% conversion of diphenyl ethanol (lignin model) was achieved, affording a 186.0 mol % yield and 93.0% selectivity for benzaldehyde via the C α –C β bond cleavage. By contrast, FeCl 3 ·6H 2 O predominantly favored the C α –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 Fe III 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, FeCl 3 generates chlorine radicals via homolytic cleavage, resulting in the hydrogen atom abstraction at C α –OH which consequently inhibits the breakage of the C α –C β bond. Notably, the Fe(NO 3 ) 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 Fe III salts.