Supramolecular nanocapsules as two-fold stabilizers of outer-cavity sub-nanometric Ru NPs and inner-cavity ultra-small Ru clusters

Abstract

The synthesis of metallic nanoparticles (MNP) with high surface area and controlled shape is of paramount importance to increase their catalytic performance. The detailed growing process of NP is mostly unknown and understanding the specific steps would pave the way for a rational synthesis of the desired MNP. Here we take advantage of the stabilization properties exerted by the tetragonal prismatic supramolecular nanocapsule 8·(BArF)₈ to develop a synthetic methodology for sub-nanometric RuNP (0.6–0.7 nm). The catalytic properties of these sub-nanometric nanoparticles were tested on the hydrogenation of styrene, obtaining excellent selectivity for the hydrogenation of the alkene moiety. In addition, the encapsulation of [Ru₅] clusters inside the nanocapsule is strikingly observed in most of the experimental conditions, as ascertained by HR-MS. Moreover, a thorough DFT study enlightens the nature of the [Ru₅] clusters as tb-Ru₅H₂(η⁶-PhH)₂(η⁶-pyz)₃ (2) trapped by two arene moieties of the clip, or as tb-Ru₅H₂(η¹-pyz)₆(η⁶-pyz)₃ (3) trapped between the two Zn–porphyrin units of the nanocapsule. Both options fulfill the Wade-Mingos counting rules, i.e. 72 CVEs for the closotb. The trapped [Ru₅] metallic clusters are proposed to be the first-grown seeds of subsequent formation of the subnanometric RuNP. Moreover, the double role of the nanocapsule in stabilising ∼0.7 nm NPs and also in hosting ultra-small Ru clusters, is unprecedented and may pave the way towards the synthesis of ultra-small metallic clusters for catalytic purposes.