Directing structural transformation in gold(i)-carborane nanoclusters to unlock ultrafast hypergolic ignition

The construction of structurally transformable nanoclusters and the elucidation of their transformation mechanisms remain challenging. Furthermore, uncovering how structural changes govern their properties is even more elusive. Herein, a novel "cation-dissociation & anion-reassembly" structural transformation strategy is investigated. Specifically, [Au₁₈S₈(m-P^P)₆][Cl]₂ (Au₁₈) can transform into [Au₈S₄(m-P^P)₂] (Au₈) through the synergistic effects of cations and anions: cations act as Lewis acids to dissociate Au₁₈, while anions direct the reassembly of fragments into Au₈. Moreover, a novel bis-tetrahedral cluster [Au₇(m-P^P)₂(o- ^nido P^P)₂] (o- ^nido P^P = 7,8-bis(diphenylphosphine)-nido-carborane, Au₇) was synthesized by a ligand exchange strategy with Au₁₈. Concerning high energy densities of carborane backbones, a droplet test was conducted, and the results showed that both Au₁₈ and Au₈ were not able to be ignited by white fuming nitric acid, whereas Au₇ exhibited a very fast ignition delay time of 16(2) ms with high energy densities (heat of combustion, 25.3 kJ g^-1). Theoretical calculations reveal that the bis-tetrahedral metal kernel in Au₇ modifies the electronic structure, lowering the activation barrier for oxidation and thereby shortening the ignition delay time. This work not only provides fundamental insights into the structural transformation pathways of nanoclusters, but also establishes relationships between structures and hypergolic properties, which are meaningful for the design of nanoclusters with tunable properties.