Gas-Phase Structures of [Au21(SR)14]- and [Au17(SR)10]- with Eight Electrons: Can They Support an Icosahedral Au13 Core?

A prototypical thiolate (RS)-protected gold cluster [Au25(SR)18]- has high stability due to specific geometric and electronic structures: an icosahedral (Ih) Au13 core with a closed electronic shell containing eight electrons is completely protected by six units of Au2(SR)3. Nevertheless, collisional excitation of [Au25(SR)18]- in a vacuum induces the sequential release of Au4(SR)4 to form [Au21(SR)14]- and [Au17(SR)10]- both containing eight electrons. To answer a naive question of whether these fragments bear an Ih Au13(8e) core, the geometrical structures of [Au21(SC3H7)14]- and [Au17(SC3H7)10]- in the gas phase were examined by the combination of anion photoelectron spectroscopy and density functional theory (DFT) calculation of simplified models of [Au21(SCH3)14]- and [Au17(SCH3)10]-. We concluded that [Au21(SC3H7)14]- retains a slightly distorted Ih Au13(8e) core, while [Au17(SC3H7)10]- has an amorphous Au13 core composed of triangular Au3, tetrahedral Au4, and prolate Au7 units. DFT calculations on putative species [Au19(SCH3)12]- and [Au18(SCH3)11]- suggested that the Ih Au13(8e) core undergoes dramatic structural deformation due to mechanical stress from µ2 ligation of only one RS.

Geometric and Electronic Properties of P Atom-Doped Al Nanoclusters: Alkaline-like Superatom of P@Al12.

The geometric and electronic characteristics of phosphorus-atom doped aluminum nanoclusters, AlnPm (n = 7-17, m = 1 and 2), were investigated through a combination of experiments and theoretical calculations. The size dependences of the ionization energy (Ei) for AlnPm NCs exhibit a local minimum of 5.37 eV at Al12P1, attributed to an endohedral P@Al12 superatom (SA). This SA originates from an excess electron toward the 2P shell closing (40e), coexisting with an exohedral isomer featuring a vertex P atom. The stability of the endohedral P@Al12 is further enhanced in its cationic state compared to the exohedral isomer, when complexed with a fluorine (F) atom, forming an SA salt denoted as P@Al12+F- with an elevated Ei ranging from 6.42 to 7.90 eV. In contrast, for the anionic Al12P1-, the exohedral form is found to be more stable than the endohedral one using anion photoelectron spectroscopy and calculations. The geometric and electronic robustness of neutral P@Al12 SAs against electron donation and acceptance is discussed in comparison to rare-gas-like Si@Al12 SAs.

Revisiting the structure of [PdAu9(PPh3)8(CN)]2+ produced by atmospheric pressure plasma irradiation of [PdAu8(PPh3)8]2+ in methanol.

Some of the authors of the present research group have previously reported mass spectrometric detection of [PdAu9(PPh3)8(CN)]2+ (PdAu9CN) by atmospheric pressure plasma (APP) irradiation of [MAu8(PPh3)8]2+ (PdAu8) in methanol and proposed based on density functional theory (DFT) calculations that PdAu9CN is constructed by inserting a CNAu or NCAu unit into the Au-PPh3 bond of PdAu8 [Emori et al., J. Chem. Phys. 155, 124312 (2021)]. In this follow-up study, we revisited the structure of PdAu9CN by high-resolution ion mobility spectrometry on an isolated sample of PdAu9CN with the help of dispersion-corrected DFT calculation. In contradiction to the previous proposal, we conclude that isomers in which an AuCN unit is directly bonded to the central Pd atom of PdAu8 are better candidates. This assignment was supported by Fourier transform infrared and ultraviolet-visible spectroscopies of isolated PdAu9CN. The simultaneous formation of [Au(PPh3)2]+ and PdAu9CN suggests that the AuCN species are formed by APP irradiation at the expense of a portion of PdAu8. These results indicate that APP may offer a unique method for transforming metal clusters into novel ones by generating in situ active species that were not originally added to the solution.

Gas-Phase Anion Photoelectron Spectroscopy of Alkanethiolate-Protected PtAu12 Superatoms: Charging Energy in Vacuum vs Solution.

Charging behavior of molecular Au clusters protected by alkanethiolate (SCnH2n+1 = SCn) is, under electrochemical conditions, significantly affected by the penetration of solvent and electrolyte into the SCn layer. In this study, we estimated the charging energy EC(n) associated with [PtAu24(SCn)18]- + e → [PtAu24(SCn)18]2- (n = 4, 8, 12, and 16) in vacuum using mass-selected, gas-phase anion photoelectron spectroscopy of [PtAu24(SCn)18]z (z = -1 and -2). The EC(n) values of PtAu24(SCn)18 in vacuum are significantly larger than those in solution and decrease with n in contrast to the behavior reported for Au25(SCn)18 in solution. The effective relative permittivity (εm*) of the SCn layer in vacuum is estimated to be 2.3-2.0 based on the double-concentric-capacitor model. Much smaller εm* values in vacuum than those in solution are explained by the absence of solvent/electrolyte penetration into the monolayer. The gradual decrease of εm* with n is ascribed to the appearance of an exposed surface region due to the bundle formation of long alkyl chains.

Effect of total charge on the electronic structure of thiolate-protected X@Ag12 superatoms (X = Ag, Au).

Electronic structures of chemically synthesized silver-based clusters [XAg16(TBBT)12]3- (X = Ag or Au; TBBT = 4-tert-butylbenzenethiolate) having an icosahedral X@Ag12 superatomic core were studied by gas-phase photoelectron spectroscopy and density functional theory calculations. The electron binding energy of the highest occupied molecular orbital (HOMO) with a 1P superatomic nature was determined to be 0.23 and 0.29 eV for X = Ag or Au, respectively. Resonant tunnelling electron emission through the repulsive Coulomb barrier (RCB) was observed. From the kinetic energy of the tunnelling electrons, it was estimated that the lowest unoccupied molecular orbital (LUMO) was supported at 1.51 and 1.62 eV above the vacuum level by the RCB for X = Ag or Au, respectively. The HOMO of [XAg16(TBBT)12]3- (X = Ag or Au) was destabilized by 3.74 and 3.71 eV, respectively, compared with those of [XAg24(DMBT)18]- (DMBT = 2,4-dimethylbenzenethiolate) having the icosahedral X@Ag12 core due to the larger negative charge imparted by the ligand layers.

Controlled Synthesis of Diphosphine-Protected Gold Cluster Cations Using Magnetron Sputtering Method.

We demonstrated, for the first time, atomically precise synthesis of gold cluster cations by magnetron sputtering of a gold target onto a polyethylene glycol (PEG) solution of 1,3-bis(diphenylphosphino)propane (Ph2PCH2CH2CH2PPh2, dppp). UV-vis absorption spectroscopy and electrospray ionization mass spectrometry revealed the formation of cationic species, such as [Au(dppp)n]+ (n = 1, 2), [Au2(dppp)n]2+ (n = 3, 4), [Au6(dppp)n]2+ (n = 3, 4), and [Au11(dppp)5]3+. The formation of [Au(dppp)2]+ was ascribed to ionization of Au(dppp)2 by the reaction with PEG, based on its low ionization energy, theoretically predicted, mass spectrometric detection of deprotonated anions of PEG. We proposed that [Au(dppp)2]+ cations thus formed are involved as key components in the formation of the cluster cations.

Gas-phase studies of chemically synthesized Au and Ag clusters.

Atomically precise Au and Ag clusters protected by monolayers of organic ligands have attracted growing interests as promising building units of functional materials and ideal platforms to study size-dependent evolution of structures and physicochemical properties. The use of gas-phase methods including mass spectrometry, ion mobility mass spectrometry, collision-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation spectroscopy will reveal novel and complementary information on their intrinsic geometric and electronic structures that cannot be obtained by conventional characterization methods. This Perspective surveys the recent progress and outlook of gas-phase studies on chemically synthesized Au/Ag clusters.

Chemical transformations of [MAu8(PPh3)8]2+ (M = Pt, Pd) and [Au9(PPh3)8]3+ in methanol induced by irradiation of atmospheric pressure plasma.

The reaction processes of ligand-protected metal clusters induced by irradiating atmospheric pressure plasma (APP) were investigated using optical spectroscopy, mass spectrometry, and density functional theory (DFT) calculations. The target clusters were phosphine-protected gold-based clusters [MAu8(PPh3)8]2+ (M = Pt, Pd) and [Au9(PPh3)8]3+, which have a crown-shaped M@Au8 (M = Pt, Pd, Au) core with an unligated M site at the central position. The APP irradiation of [MAu8(PPh3)8]2+ (M = Pt, Pd) in methanol resulted in the selective formation of [PtAu8(PPh3)8CO]2+ and [PdAu9(PPh3)8CN]2+ via the addition of a CO molecule and AuCN unit, respectively, generated in situ by the APP irradiation. In contrast, the APP irradiation of [Au9(PPh3)8]3+ in methanol yielded [Au9(PPh3)7(CN)1]2+ and [Au10(PPh3)7(CN)2]2+ as the main products, which were produced by sequential addition of AuCN to reactive [Au8(PPh3)7]2+ formed by dissociation equilibrium of [Au9(PPh3)8]3+. DFT calculations predicted that a unique chain-like {-(CNAu)n-PPh3} (n = 1, 2) ligand was formed via the sequential insertion of -CNAu- units into the Au-PPh3 bond of [PdAu8(PPh3)8]2+ and [Au8(PPh3)7]2+. These findings open up a new avenue for developing novel metal clusters via the chemical transformation of atomically defined metal clusters by APP irradiation.

Sequential growth of iridium cluster anions based on simple cubic packing.

Geometric structures of free iridium cluster anions, Irn-, were examined by means of ion mobility mass spectrometry and density functional theory calculation for n = 3-15 with the additional help of photoelectron spectroscopy for n = 4-10. It has been revealed that Irn- clusters with n ≥ 5 favor a square facet and take a cubic motif in contrast to the face-centered cubic structures in the corresponding nanoparticles and bulk. A growth sequence of Irn- for n = 5-15 is proposed: single Ir atoms are sequentially attached to one side of the square plane of Ir4- to form a cubic Ir8-, and are then continuously attached on one of the square facets of Ir8- for n = 9-12 and Ir12- for n = 13-15.

Au3Si4- and Au4Si4: Electronically Equivalent but Different Polarity Superatoms.

A series of AuxSi4- cluster anions (x = 1-4) were generated most abundantly by laser ablation of a Au4Si alloy target. Photoelectron spectroscopy and density functional theory (DFT) calculation of AuxSi4- (x = 1-4) revealed that Au3Si4- can be viewed as an electronically closed superatom and is composed of a Si4 unit whose three adjacent edges of a single facet are bridged by three Au atoms. Such phase-segregated structure is facilitated by aurophilic interaction between the three Au atoms and results in a large permanent dipole moment (4.43 D). DFT calculations on an electronically equivalent superatom Au4Si4 predicted a new structure in which the uncoordinated Si atom of Au3Si4- is bonded by Au+. This Au4Si4 is much more stable than a cubic structure previously reported and has a large HOMO-LUMO gap (1.68 eV) and a small permanent dipole moment (0.41 D).

Characterization of chemically modified gold and silver clusters in gas phase.

Atomically precise Au and Ag clusters protected by organic ligands can be viewed as chemically modified Au/Ag superatoms and have attracted interest as promising building units of functional materials and ideal platforms for studying the size-dependent evolution of structures and properties. Their structures, stability, and physicochemical properties have been characterized in solution and solid (or crystalline) phases by various methods conventionally used in materials science. However, novel and complementary information on their intrinsic stability and structures can be obtained by applying a variety of gas-phase methods, including mass spectrometry, ion mobility mass spectrometry, collision- or surface-induced dissociation mass spectrometry, photoelectron spectroscopy, and photodissociation mass spectrometry, to the chemically modified Au/Ag superatoms isolated in the gas phase. This perspective describes our recent efforts in the gas-phase studies on chemically synthesized Au/Ag superatoms.

Elucidating the Doping Effect on the Electronic Structure of Thiolate-Protected Silver Superatoms by Photoelectron Spectroscopy.

Gas-phase photoelectron spectroscopy (PES) was conducted on [XAg24 (SPhMe2 )18 ]- (X=Ag, Au) and [YAg24 (SPhMe2 )18 ]2- (Y=Pd, Pt), which have a formal superatomic core (X@Ag12 )5+ or (Y@Ag12 )4+ with icosahedral symmetry. PES results show that superatomic orbitals in the (Au@Ag12 )5+ core remain unshifted with respect to those in the (Ag@Ag12 )5+ core, whereas the orbitals in the (Y@Ag12 )4+ (Y = Pd, Pt) core shift up in energy by about 1.4 eV. The remarkable doping effect of a single Y atom (Y=Pd, Pt) on the electronic structure of the chemically modified (Ag@Ag12 )5+ superatom was reproduced by theoretical calculations on simplified model systems and was ascribed to 1) the weaker binding of valence electrons in Y@(Ag+ )12 compared to Ag+ @(Ag+ )12 due to the reduction in formal charge of the core potential, and 2) the upward shift of the apparent vacuum level due to the presence of a repulsive Coulomb barrier between [YAg24 (SPhMe2 )18 ]- and electron.

Gold Ultrathin Nanorods with Controlled Aspect Ratios and Surface Modifications: Formation Mechanism and Localized Surface Plasmon Resonance.

We synthesized gold ultrathin nanorods (AuUNRs) by slow reductions of gold(I) in the presence of oleylamine (OA) as a surfactant. Transmission electron microscopy revealed that the lengths of AuUNRs were tuned in the range of 5-20 nm while keeping the diameter constant (∼2 nm) by changing the relative concentration of OA and Au(I). It is proposed on the basis of time-resolved optical spectroscopy that AuUNRs are formed via the formation of small (<2 nm) Au spherical clusters followed by their one-dimensional attachment in OA micelles. The surfactant OA on AuUNRs was successfully replaced with glutathionate or dodecanethiolate by the ligand exchange approach. Optical extinction spectroscopy on a series of AuUNRs with different aspect ratios (ARs) revealed a single intense extinction band in the near-IR (NIR) region due to the longitudinal localized surface plasmon resonance (LSPR), the peak position of which is red-shifted with the AR. The NIR bands of AuUNRs with AR < 5 were blue-shifted upon the ligand exchange from OA to thiolates, in sharp contrast to the red shift observed in the conventional Au nanorods and nanospheres (diameter >10 nm). This behavior suggests that the NIR bands of thiolate-protected AuUNRs with AR < 5 are not plasmonic in nature, but are associated with a single-electron excitation between quantized states. The LSPR band was attenuated by thiolate passivation that can be explained by the direct decay of plasmons into an interfacial charge transfer state (chemical interface damping). The LSPR wavelengths of AuUNRs are remarkably longer than those of the conventional AuNRs with the same AR, demonstrating that the miniaturization of the diameter to below ∼2 nm significantly affects the optical response. The red shift of the LSPR band can be ascribed to the increase in the effective mass of electrons in AuUNRs.

Geometric and electronic properties of Si-atom doped Al clusters: robustness of binary superatoms against charging.

The geometric and electronic properties of silicon-atom-doped aluminum clusters, AlnSim (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of AlnSim show that the stability of AlnSim is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al10Si and Al12Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al10 and Si@Al12. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al11Si2. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al11Si2 is characterized to be, Si@Al11Si as an alkaline-like binary superatom. For Al21Si2, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.

Oxidative Addition of CH3I to Au(-) in the Gas Phase.

Reaction of the atomic gold anion (Au(-)) with CH3I under high-pressure helium gas affords the adduct AuCH3I(-). Photoelectron spectroscopy and density functional theory calculations reveal that in the AuCH3I(-) structure the I and CH3 fragments of CH3I are bonded to Au in a linear configuration, which can be viewed as an oxidative addition product. Theoretical studies indicate that oxidative addition proceeds in two steps: nucleophilic attack of Au(-) on CH3I, followed by migration of the leaving I(-) to Au. This mechanism is supported by the formation of an ion-neutral complex, [Au(-)···t-C4H9I], in the reaction of Au(-) with t-C4H9I because of the activation barrier along the SN2 pathway resulting from steric effects. Theoretical studies are conducted for the formation mechanism of AuI2(-), which is observed as a major product. From the thermodynamic and kinetic viewpoints, we propose that AuI2(-) is formed via sequential oxidative addition of two CH3I molecules to Au(-), followed by reductive elimination of C2H6. The results suggest that Au(-) acts as a nucleophile to activate C(sp(3))-I bond of CH3I and induces the C-C coupling reaction of CH3I.

Slow-Reduction Synthesis of a Thiolate-Protected One-Dimensional Gold Cluster Showing an Intense Near-Infrared Absorption.

Slow reduction of Au ions in the presence of 4-(2-mercaptoethyl)benzoic acid (4-MEBA) gave Au76(4-MEBA)44 clusters that exhibited a strong (3 × 10(5) M(-1) cm(-1)) near-infrared absorption band at 1340 nm. Powder X-ray diffraction studies indicated that the Au core has a one-dimensional fcc structure that is elongated along the {100} direction.

Nonscalable oxidation catalysis of gold clusters.

Small, negatively charged gold clusters isolated in vacuum can oxidize CO via electron-transfer-mediated activation of O2. This suggests that Au clusters can act as aerobic oxidation catalysts in the real world when their structure parameters satisfy given required conditions. However, there is a technical challenge for the development of Au cluster oxidation catalysts; the structural parameters of the Au clusters, such as size and composition, must be precisely controlled because the intrinsic chemical properties of the clusters are strongly dependent on these parameters. This Account describes our efforts to achieve precision synthesis of small (diameter <2 nm) Au clusters, stabilized by polymers and immobilized on supports, for a variety of catalytic applications. Since we aim to develop Au cluster catalysts by taking full advantage of their intrinsic, size-specific chemical nature, we chose chemically inert materials for the stabilizers and supports. We began by preparing small Au clusters weakly stabilized by polyvinylpyrrolidone (PVP) to test the hypothesis that small Au clusters in the real world will also show size-specific oxidation catalysis. The size of Au:PVP was controlled using a microfluidic device and monitored by mass spectrometry. We found that only Au clusters smaller than a certain critical size show a variety of aerobic oxidation reactions and proposed that the reactions proceed via catalytic activation of O2 by negatively charged Au clusters. We also developed a method to precisely control the size and composition of supported Au clusters using ligand-protected Au and Au-based bimetallic clusters as precursors. These small Au clusters immobilized on mesoporous silica, hydroxyapatite, and carbon nanotubes acted as oxidation catalysts. We have demonstrated for the first time an optimal Au cluster size for the oxidation of cyclohexane and a remarkable improvement in the oxidation catalysis of Au25 clusters by single-atom Pd doping. The non-scalable catalysis of Au clusters that we reported here points to the possibility that novel catalysis beyond that expected from bulk counterparts can be developed simply by reducing the catalyst size to the sub-2 nm regime.

Surface plasmon resonance in gold ultrathin nanorods and nanowires.

We synthesized and measured optical extinction spectra of Au ultrathin (diameter: ∼1.6 nm) nanowires (UNWs) and nanorods (UNRs) with controlled lengths in the range 20-400 nm. The Au UNWs and UNRs exhibited a broad band in the IR region whose peak position was red-shifted with the length. Polarized extinction spectroscopy for the aligned Au UNWs indicated that the IR band is assigned to the longitudinal mode of the surface plasmon resonance.

Chemically modified gold superatoms and superatomic molecules.

Clusters of gold atoms can be viewed as superatoms, in which valence electrons confined in the particles occupy atomic-like, discrete electronic levels. Chemical modification of the gold superatoms and their aggregated molecules (superatomic molecules) with organic ligands is a promising approach for their application as the building units of new functional materials. This account surveys the present status of the rapidly growing field of gold superatoms and superatomic molecules protected by thiolates and phosphines. The major aim of this article is to provide a simple picture for the structure, stability and bonding scheme of chemically modified superatoms and superatomic molecules for the development of a new class of hierarchical materials.

Structural transition of zinc oxide cluster cations: smallest tube like structure at (ZnO)6(+).

Zinc oxide cluster cations have been analyzed by ion mobility spectrometry using a home-made drift cell combined with a time-of-flight reflectron mass spectrometer. Structural changes from cyclic to tube like structures were observed around n = 8, corresponding to predictions by theoretical calculations. The structures were assigned by comparing with the arrival time simulation using MOBCAL software. We have also observed ion-injection energy dependence of the structures of (ZnO)n(+). The smallest tube structure of (ZnO)6(+) has predominantly been observed at an injection energy of 200 eV. The extraordinary stability of the compact structure at this size has been observed for the first time.