Atomically precise Cu41 clusters as model catalysts: Open metal sites matter

Although the significance of open metal sites for controlling metal nanoparticle catalysis have been well-documented, the direct identification of authentic active sites remains a grand challenge. In this work, we report a new couple of “isostructural” copper nanoclusters, namely, [Cu41Cl2(2-F-C6H4S)12(CF3COO)6(PPh3)6H19]2- (1H) and [Cu41(2,5-di-Methyl-C6H3S)12(BO3)3Cl3(PPh3)6H19] (2H), from which heavy dependence of catalytic activity on coordinatively unsaturated metal sites is directly visualized. Both clusters have been characterized by various advanced techniques including high resolution mass spectroscopy (MS), ultra-violet visible spectroscopy (UV-Vis), nuclear magnetic resonance (NMR), density functional theory (DFT) calculations and X-ray single crystal diffraction. The clusters adopt a core-shell arrangement, in which the Cu29 core is grown via the face-fusion of three Cu13 units. Notably, such Cu13-based growth mode is observed for the first time for copper nanoclusters. The main difference of the two Cu41 clusters lies in the presence or absence of two additional chlorides on the surface, which in turn heavily control the exposure of metal sites. 2H with more open metal sites exhibits, as envisioned, much higher activity than 1H in catalytic hydrogenation reaction (6-fold increase in rate constant). This work thus not only represent a mode system to directly visualize the true active sites of metal nanoclusters, but also provides the atomistic evidence on highlighting the importance of exposed metal sites for controlling performance of metal nanocatalysts.