Trace Thiol\nMoieties in the Ligand Layer Induce Superior\nCatalytic Gold Nanoclusters

We here show that the typical poison of thiols, if below\na certain\nlevel, promotes rather than suppresses the catalytic activity of gold\nnanoclusters (AuNCs). A few thiol groups functionalized hyperbranched\npolyethylenimine (PEI, <i>M</i><sub>n</sub> = 2000 Da) patched\non a mesoporous polymeric bead aid the direct synthesis of AuNCs.\nThe nucleation efficiency of AuNC is 93-fold favored at a level of\n2 thiols per PEI (0.04 equiv of the amino units) than that by neat\nPEI, and AuNCs (1.3 nm) are obtained up to a gold load of 6.3% on\nthe support. Unexpectedly, the catalytic activity of AuNCs is favored\nby the thiol up to 2 thiols per PEI, as evaluated from the surface-normalized\nrate constant of the model reaction of 4-nitrophenol-reduction. The\ncatalytic promotion by thiols probably stems from optimized electron\ndensity on AuNC. If the residual NH groups of PEI were further fully\ntreated with glycidyltrimethylammonium chloride, the catalytic activity\nis again enhanced, where the accelerated mass transfer is responsible\nfor the promotion. Overall, the catalytic activity reaches an unprecedented\nvalue (metal-normalized rate constant <i>k</i><sub>c</sub> = 29.4 L mmol<sup>–1</sup> s<sup>–1</sup> and turnover\nfrequency = 1623 h<sup>–1</sup>, as evaluated with the model\nreaction of 4-nitrophenol reduction) ever reported for supported AuNCs.\nOur results suggest that orthogonal ligand optimization is an effective\nmanner of triggering the release of the catalytic potential of AuNCs,\namong which thiol is unique.