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

Herein, 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 summarized. In particular, the discussion is focussed on the use of metal–organic frameworks (MOFs) – an emerging type of porous functional materials capable of acting as vessels or as chemical nanoreactors to template the growth of atomically precise nanoclusters – for application in heterogeneous catalysts, 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. Compared to regular nanoparticles, which are somewhat polydisperse, atomically precise nanoclusters are molecularly pure, ultra small nanoparticles (from sub-nanometre to 2 nm) and exhibit strong quantum confinement effects due to ultra small sizes. We will show that, despite the increasing number of reports on MNCs, their crystallographic characterization is still highly challenging. Indeed, cases of crystallographically solved atomic structures – which are of critical importance in order to shed light on supramolecular host–guest interactions – are still scarce. Structural awareness is mandatory to further enhance the efficiency of current heterogeneous catalysts and pave the way for future design of more advanced catalysts with mechanistic understanding of catalytic reactions, especially at the atomic scale.