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...

We synthesized a new trimetallic cluster [PtCdAu 23 (SC 2 H 4 Ph) 18 ] − , which has an icosahedral Pt@CdAu 11 superatomic core with eight electrons, and investigated how the electronic structure and optical properties of the Au@Au 12 superatomic core of [Au 25 (SC 2 H 4 Ph) 18 ] − are affected by codoping of group X and XII atoms. Spectroscopic and electrochemical measurements showed that the HOMO−LUMO (HL) gap of Pt@ CdAu 11 is larger than that of Au@Au 12 and that the photoluminescence quantum yield (PLQY) of Pt@CdAu 11 is 46 times larger than that of Au@Au 12 . To shed light on the origin of the codoping effects, a comparison was made with monodoped clusters [CdAu 24 (SC 2 H 4 Ph) 18 ] 0 and [PtAu 24 (SC 2 H 4 Ph) 18 ] 2− having the Au@ CdAu 11 and Pt@Au 12 superatomic cores, respectively. The results revealed that the increment of the HL gap of Pt@CdAu 11 is mainly because of Pt doping, while the doping of Pt and Cd synergistically enhances the PLQY of Pt@CdAu 11 .

Scheme 1. Structures of (a) [Au 25 (SR) 18 ] − and (b) [MAu 24 (SR) 18 ] 0 (M = Pt, Pd) a a R moieties are omitted for clarity. Communication pubs.acs.org/JACS

In this work, we systematically investigated the ligand effects on spontaneous electron transfer (ET) between alkanethiolate-protected metal clusters in solution. The donor and acceptor clusters used were [PtAu 24 (SC n H 2n+1 ) 18 ] 2− (8e(Cn)) and [PtAu 24 (SC m H 2m+1 ) 18 ] 0 (6e(Cm)) (n, m = 2−16), which have icosahedral Pt@Au 12 cores with eight and six valence electrons, respectively. The ET rate constant (k ET ) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the k ET decreased with n + m in the range of 4−12, whereas it monotonically increased with n + m in the range of 12−32. Electrospray ionization mass spectrometry of the mixture of 8e(Cn) and 6e(Cm) detected the dimer complex 8e(Cn)•6e(Cm), the relative population of which increased with n + m. The activation energy (E a ), determined based on the Arrhenius plots for n = m, monotonically decreased with n (≥ 6). Based on these results, we proposed that the promotion of ET by longer alkanethiolates was ascribed to two effects on the key intermediate 8e(Cn)•6e(Cm): (1) elongation of the lifetime and (2) the contraction of the distance between 8e(Cn) and 6e(Cm) due to the stronger van der Waals interaction between the longer alkyl chains. Such alkyl-chain-promoted ET is specific to ultrasmall clusters in solution because a nonuniform ligand layer could be formed due to the large curvature of the cluster core.