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 Au25(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 Au17(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 Au17(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.