Styrene oxidation catalyzed by Au11(PPh3)7Cl3 and [Au11(PPh3)8Cl2]Cl nanoclusters: Impacts of capping ligands, particle size and charge state

Structurally similar molecule-like Au 11 (PPh 3 ) 7 Cl 3 and [Au 11 (PPh 3 ) 8 Cl 2 ]Cl nanoclusters have been successfully synthesized and employed as catalysts for selective oxidation of styrene. The two clusters exhibited similar but slightly different well-defined optical absorbance, and the subtle structural difference is also reflected in proton nuclear magnetic resonance ( 1 H NMR) studies. In electrochemical measurements, Au 11 (PPh 3 ) 7 Cl 3 and [Au 11 (PPh 3 ) 8 Cl 2 ]Cl show a large electrochemical band gap of 2.15 eV and 2.09 eV, respectively. Styrene oxidation has been employed as a model reaction to investigate the impacts of the ligand shell composition on the catalytic activity and selectivity of the two clusters. Interestingly, the main product is benzaldehyde when oxygen is employed as oxidant, and the catalytic activity of Au 11 (PPh 3 ) 7 Cl 3 is higher than that of [Au 11 (PPh 3 ) 8 Cl 2 ]Cl. Under the same conditions, the two Au 11 clusters exhibit much higher catalytic activity than larger clusters (Au 25 , Au 38 and Au 144 ) capped with phenylethiolates, as a manifestation of both size influence and surface ligand impacts. In addition, the oxidation efficiency of styrene exhibit a sharp increase when tert-butyl hydroperoxide is utilized as the oxidant, where the catalytic activities of Au 11 (PPh 3 ) 7 Cl 3 and [Au 11 (PPh 3 ) 8 Cl 2 ]Cl are comparable with styrene epoxide being the major product. Finally, the twin clusters are either reduced or oxidized to probe the charge effects upon the catalytic reaction, and it was found that the conversion of styrene went up with the charge state from reduced to oxidized for both clusters.