Effect of Ligands on the Reactivity of the Undercoordinated Fragment Ions of Co₆S₈(PEt3–xPhx)₆⁺ (x = 0–3) Clusters on Surfaces

Atomically precise ligated clusters are of interest to a broad range of applications in which their reactivity is often controlled by removing one or more ligands. This process generates under-coordinated species known to be chemically and catalytically active. Despite the power of this approach, selective ligand removal from clusters in the condensed phase is challenging. In this study, we generated well-defined, undercoordinated clusters by selectively removing one ligand in the gas phase. We then examined their reactivity on surfaces using a specially designed ion soft landing instrument equipped with a collision cell. We selected superatomic hexanuclear metal chalcogenide clusters protected with different phosphine ligands, Co6S8L6+ (L = PPh3, PEtPh2, PEt2Ph, or PEt3), as a model system. The loss of one ligand from the cluster in the gas phase generates a fragment ion that contains an under-coordinated metal atom. Collision-induced dissociation (CID) experiments indicate that the ligand binding energy increases gradually as the number of phenyl groups in the ligand decreases. Interestingly, the reactivity of the Co6S8L5+ fragments on a surface is dependent on the ligand binding energy to the core of the precursor Co6S8L6+ cluster. Specifically, the precursor ion, Co6S8(PPh3)6+, which is the least stable species in the gas phase, generates a fragment that remains unreactive on the surface. Meanwhile, fragments of clusters containing PEtPh2, PEt2Ph, or PEt3 ligands undergo selective dimerization, forming Co12S16(PEtPh2)102+, Co12S16(PEt2Ph)102+, and Co12S16(PEt3)102+, respectively. Our results demonstrate that the reactivity of the undercoordinated clusters on surfaces may be tailored using ligands with different affinity to the cluster core. Understanding this reactivity establishes fragment ions as unique building blocks with tunable properties for innovations in materials science, catalysis, and other disciplines.