Atomically Precise Metal Clusters in Confined Spaces of Metal–Organic Frameworks

Herein, the experimental developments in the construction of various well-defined small aggregations of metal atoms, close to or within the sub-nanometric regime, so-called metal nanoclusters (MNCs) are summarised. In particular, the discussion is focused on the use of Metal-Organic Frameworks –an emerging type of porous functional materials capable to act as vessels or as chemical nano-reactor to template the growth of atomically precise nanoclusters, for application in heterogeneous catalysts. Compared to regular nanoparticles, which are somehow polydispersed, atomically precise nanoclusters are molecularly pure, ultrasmall nanoparticles (from subnanometer to 2 nm) and exhibit strong quantum confinement effects due to ultrasmall sizes. All that with a particular focus on how the MOFs surface engineering, morphology control, composition manipulation and support effect can tune their nuclearity/size and catalytic performance. We will show that, despite the increasing number of reports on MNCs, their crystallographic characterization is still highly challenging. Indeed, the cases of crystallographically solved atomic structures –which are of critical importance in order to shed light on supramolecular host-guest interactions– are still scarce in number. Structural awareness is mandatory to further enhance the efficiency of current heterogeneous catalysts and pave the way for future design of more advanced catalysts and mechanistic understanding of catalytic reactions, especially at the atomic scale.