Semiring Chemistry of Au25(SR)18: Fragmentation Pathway and Catalytic Active site

The semiring chemistry of the Au-25(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, Au-21(SR)(14)(-), as detected in several mass spectrometry (MS) measurements of fragmentation of the Au-25(SR)(18)(-), contains a quasi-icosahedral Au-13-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 Au-25(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 Au-21(SR)(14)(-) and a cyclic [Au(SR)](4), unit. The continued fragmentation of Au-21(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 Au-21(SR)(14)(-) and Au-17(SR)(10)(-) from Au-25(SR)(18)(-) as observed from experimental abundance. Taking the Au-21(SR)(14) and its parent cluster Au-25(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 O-2 activation is achieved during this process.