Photoinduced Hydrogen Atom Transfer Catalysis with Ruthenium Polypyridyl Coated-TiO₂ Nanoparticles for Selective C–C Bond Cleavage in a Lignin Model Compound

Ruthenium(II) polypyridine complexes have been widely studied for applications in dye-sensitized solar cells and artificial photosynthesis. Here, this work reports studies of intermolecular charge transfer from metal-to-ligand charge transfer (MLCT) excitation of Ru(II)-based chromophores onto TiO2 nanoparticles to a hydrogen atom transfer (HAT) mediator and photocatalytic activation of HAT for cleavage of the C–C bond in the aryl ether linkages at room temperature. Initial studies demonstrated intermolecular energy/electron transfer among the photoexcited photocatalyst, bis(2,2′-bipyridine)(4,4′-dicarboxy-2,2′-bipyridine)Ru(II) (RuC) coated TiO2 nanoparticles (RuC/TiO2 NPs), the N-hydroxy-phthalimide (NHPI) HAT mediator, and the diol substrate 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (LMC-ol). Photophysical studies of RuC/TiO2 NPs in the presence of NHPI/2,6-lutidine and LMC-ol exhibited a highly effective quenching for photoexcited RuC*/TiO2 with KSV = 7.21 × 103 M–1 compared to the lower quenching constant (KSV ∼ 0.11 × 103 M–1) observed for RuC*/TiO2 with only LMC-ol in acetonitrile solution. Under ambient temperature and aerobic conditions, the oxidative cleavage products of 3,4-dimethoxybenzaldehyde and 2-(2-methoxyphenoxy)-acetaldehyde from LMC-ol were obtained with the oxidized ketone product. This result indicates that the solar driven HAT-mediated process allows catalysis of the oxidative cleavage of C–C bonds in the aryl ether linkages via a one-pot cleavage reaction at room temperature. This photocatalytic system presents a possible approach to support lignin depolymerization targeting value-added aromatic chemicals and developing polymer degradation.