Size Dependence of Atomically Precise Gold Nanoclusters in Chemoselective Hydrogenation and Active Site Structure

We investigate the catalytic properties of water-soluble Au n (SG) m nanocluster catalysts (H-SG = glutathione) of different sizes, including Au 15 (SG) 13, Au 18 (SG) 14, Au 25 (SG) 18, Au 38 (SG) 24, and captopril-capped Au 25 (Capt) 18 nanoclusters. These Au n (SR) m nanoclusters (SR represents thiolate generally) are used as homogeneous catalysts (i.e., without supports) in the chemoselective hydrogenation of 4-nitrobenzaldehyde (4-NO 2 PhCHO) to 4-nitrobenzyl alcohol (4-NO 2 PhCH 2 OH) with ∼100% selectivity in water using H 2 gas (20 bar) as the hydrogen source. These nanocluster catalysts, except Au 18 (SG) 14, remain intact after the catalytic reaction, evidenced by UV–vis spectra, which are characteristic of nanoclusters of each size and thus serve as spectroscopic “fingerprints”. We observe a drastic size dependence and steric effect of protecting ligands on the gold nanocluster catalysts in the hydrogenation reaction. Density functional theory (DFT) modeling of the 4-nitrobenzaldehyde adsorption shows that both the -CHO and -NO 2 groups closely interact with the S-Au-S staples on the gold nanocluster surface. The adsorptions of the 4-nitrobenzaldehyde molecule on the four different sized Au n (SR) m nanoclusters are moderately strong and similar in strength. The DFT results suggest that the catalytic activity of the Au n (SR) m nanoclusters is primarily determined by the surface area of the Au nanocluster, consistent with the observed trend of the conversion of 4-nitrobenzaldehyde versus the cluster size. Overall, this work offers molecular insight into the hydrogenation of 4-nitrobenzaldehyde and the catalytically active site structure on gold nanocluster catalysts.