Uranyl-Polyoxometalate as a Heterogeneous Catalyst for Visible-Light-Driven Oxidation Coupling of Amines

Abstract

Developing efficient heterogeneous visible-light-responsive photocatalysts to achieve organic oxidation reactions is essential. We synthesized the uranyl-polyoxometalate (POM) compound Na₃[H₁₉(UO₂)₂(μ₂-O)(Se₂W₁₄O₅₂)₂]·41H₂O (denoted as U2) by incorporating uranyl (VI) ions (UO₂²⁺) into the POM framework and conducted the photocatalytic oxidation reaction of the uranyl-POM compounds. Results revealed that the introduction of uranyl groups not only extended the light absorption range of U2 to 500 nm but also promoted its carrier separation efficiency and charge transfer rate remarkably. Meanwhile, the synergistic effect among the components enhanced the catalytic activity notably. The U2-catalyzed oxidative coupling of the benzylamine reaction was completed within 6 h under a green solvent-free system with a high yield of 98.7%. The turnover number and turnover frequency reached 1974 and 329 h^–1, respectively, far exceeding the activity of uranyl or POM used alone. Furthermore, U2 was demonstrated to exhibit excellent recyclability, maintaining its activity over five consecutive cycles without significant degradation. Incorporating uranyl into POMs to form U2 achieves excellent catalytic performance in the heterogeneous photocatalytic oxidation coupling of benzylamine to N-benzylidenebenzylamine under solvent-free conditions (yield = 98.7%; turnover number = 1974; turnover frequency = 329 h^–1).