Chemical transformations of [MAu₈(PPh₃)₈]²⁺ (M = Pt, Pd) and [Au₉(PPh₃)₈]³⁺ in methanol induced by irradiation of atmospheric pressure plasma

The reaction processes of ligand-protected metal clusters induced by irradiating atmospheric pressure plasma (APP) were investigated using optical spectroscopy, mass spectrometry, and density functional theory (DFT) calculations. The target clusters were phosphine-protected gold-based clusters [MAu₈(PPh₃)₈]²⁺ (M = Pt, Pd) and [Au₉(PPh₃)₈]³⁺, which have a crown-shaped M@Au₈ (M = Pt, Pd, Au) core with an unligated M site at the central position. The APP irradiation of [MAu₈(PPh₃)₈]²⁺ (M = Pt, Pd) in methanol resulted in the selective formation of [PtAu₈(PPh₃)₈CO]²⁺ and [PdAu₉(PPh₃)₈CN]²⁺ via the addition of a CO molecule and AuCN unit, respectively, generated in situ by the APP irradiation. In contrast, the APP irradiation of [Au₉(PPh₃)₈]³⁺ in methanol yielded [Au₉(PPh₃)₇(CN)₁]²⁺ and [Au₁₀(PPh₃)₇(CN)₂]²⁺ as the main products, which were produced by sequential addition of AuCN to reactive [Au₈(PPh₃)₇]²⁺ formed by dissociation equilibrium of [Au₉(PPh₃)₈]³⁺. DFT calculations predicted that a unique chain-like {–(CNAu)_n–PPh₃} (n = 1, 2) ligand was formed via the sequential insertion of –CNAu– units into the Au–PPh₃ bond of [PdAu₈(PPh₃)₈]²⁺ and [Au₈(PPh₃)₇]²⁺. These findings open up a new avenue for developing novel metal clusters via the chemical transformation of atomically defined metal clusters by APP irradiation.