Semiring chemistry of au₂₅(sr)₁₈: Fragmentation pathway and catalytic active site

The semiring chemistry of the Au₂₅(SR)₁₈, particularly its fragmentation mechanism and catalytic active site, is explored using density functional theory (DFT) calculations. Our calculations show that the magically stable fragmental cluster, Au₂₁(SR)₁₄ ^-, as detected in several mass spectrometry (MS) measurements of fragmentation of the Au₂₅(SR)₁₈^-, contains a quasi-icosahedral Au₁₃-core fully protected by four -SR-Au-SR- and two -SR-Au-SR-Au-SR- staple motifs. A stepwise fragmentation mechanism of the semiring staple motifs on the surface of Au₂₅(SR)₁₈ ^- is proposed for the first time. Initially, the Au ₂₅(SR)₁₈^- transforms into a metastable structure with all staple motifs binding with two neighboring vertex Au-atoms of the Au-core upon energy uptake. Subsequently, a 'step-by-step' detachment and transfer of [Au(SR)]_x (x = 1-4) units occurs, which leads to the formation of highly stable products including Au₂₁(SR) ₁₄^- and a cyclic [Au(SR)]₄ unit. The continued fragmentation of Au₂₁(SR)₁₄^- to Au ₁₇(SR)₁₀^- is observed as well, which shows same stepwise fragmentation mechanism. The proposed mechanism well explains the favorable formation of Au₂₁(SR)₁₄^- and Au ₁₇(SR)₁₀^- from Au₂₅(SR) ₁₈^- as observed from experimental abundance. Taking the Au₂₁(SR)₁₄ and its parent cluster Au₂₅(SR) ₁₈ as the benchmark model systems, the catalytic active site of the thiolate protected gold clusters toward the styrene oxidation and the associated reaction mechanism are further investigated. We show that the Au atom in the staple motifs is the major active site for the styrene oxidation in presence of TBHP as oxidant or initiator. The Au atom in the staple motifs can change from Au(I) (bicoordinated) to Au(III) (tetracoordinated). The O₂ activation is achieved during this process.