Conspectus Atomically size-selected gold (Au) clusters protected by organic ligands or stabilized by polymers provide an ideal platform to test fundamental concepts and size-specific phenomena, such as the superatomic concept and metal-to-nonmetal transition. Recent studies revealed that these stabilized Au clusters take atomlike quantized electronic structures and can be viewed as chemically modified Au superatoms. An analogy between Au and hydrogen (H) atoms is an interesting proposal made for bare Au clusters: a Au atom at a low-coordination site of a Au cluster can be replaced with a H atom while retaining the structural motif and electronic structure. However, this proposal has not been experimentally proved in chemically modified Au superatoms while a recent theoretical study predicted the formation of [HAu25(SR)18]0 (RS = thiolate). This Account summarizes our recent studies on the interaction of hydride(s) with two types of chemically modified Au-based superatoms: (1) the Au cores of [Au9(PPh3)8]3+ and [PdAu8(PPh3)8]2+ formally described as (Au9)3+ and (PdAu8)2+, respectively, and (2) Au34 cluster stabilized by poly­(N-vinyl-2-pyrrolidone) (PVP). The (Au9)3+ and (PdAu8)2+ cores correspond to oblate-shaped superatoms with six electrons and a coordinatively unsaturated site at the center, whereas the Au34 cluster in PVP is viewed as a nearly spherical superatom having a closed electronic structure with 34 electrons and multiple uncoordinated sites on the surface. Through this study, we aimed to deepen our understanding on the role of a hydride in the formation processes of Au superatoms, the effect of adsorbed hydride(s) on the electronic structure of Au superatoms, and the activity of adsorbed hydrogen species for hydrogenation catalysis. Mass spectrometry and nuclear magnetic resonance spectroscopy demonstrated that a single hydride (H–) was selectively doped to (Au9)3+ and (PdAu8)2+ upon reactions with BH4 – to form (HAu9)2+ and (HPdAu8)+, respectively. Density functional theory (DFT) calculations showed that (HAu9)2+ and (HPdAu8)+ were more spherical than the original superatoms and had a closed electronic structure with eight electrons. The hydride-doped (HAu9)2+ was selectively converted to the well-known (Au11)3+ by electrophilic addition of two Au­(I) units whereas (HPdAu8)+ was converted to a new hydride-doped (HPdAu10)3+. A two-step mechanism was proposed for hydride-mediated growth of Au-based superatoms: closure of the electronic structures by adsorption of a hydride, followed by the addition of two Au­(I) units. The selective formation of Au34 superatoms in PVP is also explained by assuming that hydride-doped Au clusters with 34 electrons were involved as key intermediates. The Au34 superatom exhibited the localized surface plasmon resonance (LSPR) band by reacting with BH4 – due to the electron donation by multiply adsorbed hydrides. The LSPR band disappeared by exposing hydride-doped Au34 to dissolved O2, but reappeared by reaction with BH4 –. Catalysis for hydrogenation of CC bonds was gene...

An unprecedented magic number cluster, Au 24 Cl x ( x = 0–3), was selectively synthesized by the kinetically controlled reduction of the Au precursor ions in a microfluidic mixer in the presence of a large excess of poly( N -vinyl-2-pyrrolidone) (PVP). The atomic structure of the PVP-stabilized Au 24 Cl x was investigated by means of aberration-corrected transmission electron microscopy (ACTEM) and density functional theory (DFT) calculations. ACTEM video imaging revealed that the Au 24 Cl x clusters were stable against dissociation but fluctuated during the observation period. Some of the high-resolution ACTEM snapshots were explained by DFT-optimized isomeric structures in which all the constituent atoms were located on the surface. This observation suggests that the featureless optical spectrum of Au 24 Cl x is associated with the coexistence of distinctive isomers. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy of CO adsorbates revealed the electron-rich nature of Au 24 Cl x clusters due to the interaction with PVP. The Au 24 Cl x :PVP clusters catalyzed the aerobic oxidation of benzyl alcohol derivatives without degradation. Hammett analysis and the kinetic isotope effect indicated that the hydride elimination by Au 24 Cl x was the rate-limiting step with an apparent activation energy of 56 ± 3 kJ/mol, whereas the oxygen pressure dependence of the reaction kinetics suggested the involvement of hydrogen abstraction by coadsorbed O 2 as a faster process.

A single rhodium atom was precisely doped into a gold cluster Au34 stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) as revealed by mass spectrometry. The Rh-atom-doped Au:PVP exhibited remarkable catalytic activity for hydrogenation reactions of olefins, which was much higher than that of recently reported Pd-atom-doped Au:PVP.

No abstract