Fluorite-Type Silsesquioxane-Protected Cerium Cluster as Molecular Analog of Highly Reduced Nanoceria Fragment for Efficient Photocatalysis.

The surface oxygen vacancy defects generated from the abstraction of lattice oxygen atoms of bulk ceria accompanied by the reduction of CeIV to CeIII, are considered to play a pivotal role in photocatalysis. As ideal molecular models of nanoceria, atom-precise cerium-oxo clusters (COCs) hold great promise in determining the accurate CeIII/CeIV ratio and disclosing the underlying catalytic mechanism. However, the number of COCs, especially those mixed-valent COCs with high surface CeIII concentrations, is very limited on account of their formidable synthetic challenges. Herein, we report the first silsesquioxane-protected COC, Ce13, featuring a fluorite-type Ce13O8 core, which is structurally reminiscent of bulk ceria. The cluster exhibits a substantially high CeIII/CeIV ratio with one central CeIV encapsulated by twelve surface-exposed CeIII atoms, serving as a molecular analogue of highly reduced ceria surfaces. This cluster shows strong light-harvesting ability that covers the entire visible range and demonstrates high photocatalytic activity and selectivity for oxidative coupling of various amines, where superoxide radical was involved using O2 as the oxidant at room-temperature under visible light. The surface CeIII sites are proposed to significantly enhance the adsorption and activation of O2. These results highlight the potential of Ce13 as an efficient sustainable photocatalyst for organic transformations.