Ru(II) Polypyridyl-Modified TiO₂ Nanoparticles for Photocatalytic C–C/C–O Bond Cleavage at Room Temperature

Bond cleavage reactions including that of C–C and C–O bonds are important to the chemical industry and organic chemistry. Performing this chemical transformation under mild conditions (e.g., room temperature, solar light) can benefit both the selectivity and yield of the targeted products. This manuscript describes a simple one-pot approach used to carry out Cα–Cβ/Cβ–O σ-bond cleavage using photocatalytic nanoparticles that afford the cleavage products in excellent yields at room temperature with visible-light illumination. We synthesized a carboxylic acid-functionalized RuII polypyridyl complex (RuC) and TiO2 nanoparticles (TiO2 NPs) with the average dimensions of 6.6 nm width and 14.7 nm length using a hydrothermal method. The photocatalyst RuC was immobilized onto TiO2 NPs (RuC-TiO2 NPs) to perform a photocatalytic cleavage reaction with a nonphenolic lignin model compound, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (DMP-2ol), under simulated solar illumination. Photophysical studies of RuC and TiO2 NPs reveal that intermolecular energy/electron transfer from the photoexcited RuC to TiO2 NPs occurs in acetonitrile solution. Under ambient temperature and aerobic conditions, the photocatalytic reaction with RuC-TiO2 NPs and DMP-2ol generates the main Cα–Cβ/Cβ–O bond cleavage products of 3,4-dimethoxybenzaldehyde (1, 82%) and 2-methoxyphenol (2, 90%) in excellent yields. This study successfully performed the C–C/C–O bond cleavage reaction using a homogeneously dispersed photocatalytic system at room temperature, under solar illumination, and without the need for additional mediators or oxidizing/reducing agents. This system presents a possible approach to support light-driven lignin depolymerization under mild conditions, which is a target of future work.