N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters.

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

Cyclic ion mobility of doped [MAu24L18]2- superatoms and their fragments (M = Ni, Pd and Pt; L = alkynyl).

Collision-induced dissociation and high-resolution cyclic ion mobility mass spectrometry, along with quantum chemical calculations and trajectory simulations, were used to compare the structures of isolated [MAu24(CCR)18]2-, M = Ni, Pd, or Pt, and their associated fragment ions. The three different alkynyl ligand-stabilized (CCR, R = 3,5-(CF3)2C6H3), transition metal-doped, gold cluster dianions showed mutually resolvable collision cross sections (CCS), which were ordered consistently with their molecular structures from X-ray crystallography. All three [MAu24(CCR)18]2- species fragment by sequential diyne loss to form [MAu24(CCR)18-n]2-, with n up to 12. The resultant fragment isomer distributions are significantly n- and M-dependent, and hint at a process involving concerted elimination of adjacent ligands. In particular [NiAu24(CCR)18]2- also fragments to generate alkyne-oligomers, an inference supported by the parallel observation of precursor dianion isomerization as collision energy is increased.

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.

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.

Diphosphine-Protected IrAu12 Superatom with Open Site(s): Synthesis and Programmed Stepwise Assembly.

One or two phenylacetylide (PA) ligand(s) were successfully removed from the IrAu12 superatomic core of [IrAu12(dppe)5(PA)2]+ (dppe=1,2-bis(diphenylphosphino)ethane) by reaction with controlled amounts of tetrafluoroboric acid. Optical and nuclear magnetic resonance spectroscopies and density functional theory calculations revealed the formation of open Au site(s) on the IrAu12 core of [IrAu12(dppe)5(PA)1]2+ and [IrAu12(dppe)5]3+ with the remaining structure intact. Isocyanide was efficiently trapped at the open electrophilic site on [IrAu12(dppe)5(PA)1]2+, whereas a dimer or trimer of the IrAu12 superatoms was formed using diisocyanide as a linker. These results open the door to designed assembly of chemically modified metal superatoms.

Spontaneous Intercluster Electron Transfer X2- + X0 → 2 X- (X = PtAu24(SCnH2n+1)18) in Solution: Promotion by Long Alkyl Chains.

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 [PtAu24(SCnH2n+1)18]2- (8e(Cn)) and [PtAu24(SCmH2m+1)18]0 (6e(Cm)) (n, m = 2-16), which have icosahedral Pt@Au12 cores with eight and six valence electrons, respectively. The ET rate constant (kET) from 8e(Cn) to 6e(Cm) in benzene exhibited a novel turnover behavior as a function of the total chain length n + m: the kET 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 (Ea), 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.

Clarifying the Electronic Structure of Anion-Templated Silver Nanoclusters by Optical Absorption Spectroscopy and Theoretical Calculation.

Electronic structures of anion-templated silver nanoclusters (Ag NCs) are not well understood compared to conventional, template-free Ag NCs. In this study, we synthesized three new anion-templated Ag NCs, namely [S@Ag17(S-4CBM)15(PPh3)5]0, [S@Ag18(S-4CBM)16(PPh3)8]0, and [Cl@Ag18(S-4CBM)16(PPh3)8][PPh4], where S-4CBM = 4-chlorobenzene methanethiolate, and single-crystal X-ray crystallography revealed that they have S@Ag6, S@Ag10, and Cl@Ag10 cores, respectively. Investigation of their electronic structures by optical spectroscopy and theoretical calculations elucidated the following unique features: (1) their electronic structures are different from those of template-free Ag NCs described by the superatomic concept; (2) optical absorption in the range of 550-400 nm for S2--templated Ag NCs is attributed to the charge transitions from S2--templated Ag-cage orbitals to the s-shaped orbital in the S2- moiety; (3) the Cl--templated Ag NCs can be viewed as [Cl@Ag18(S-4CBM)16(PPh3)8]0[PPh4]0 rather than the ion pair [Cl@Ag18(S-4CBM)16(PPh3)8]-[PPh4]+; and (4) singlet-coupled singly occupied orbitals are involved in the optical absorption of the Cl--templated Ag NC.

Bioconjugation of Au25 Nanocluster to Monoclonal Antibody at Tryptophan.

We report the first bioconjugation of Au25 nanocluster to a monoclonal antibody at scarcely exposed tryptophan (Trp) residues toward the development of high-resolution probes for cryogenic electron microscopy (cryo-EM) and tomography (cryo-ET). To achieve this, we improved the Trp-selective bioconjugation using hydroxylamine (ABNOH) reagents instead of previously developed N-oxyl radicals (ABNO). This new protocol allowed for the application of Trp-selective bioconjugation to acid-sensitive proteins such as antibodies. We found that a two-step procedure utilizing first Trp-selective bioconjugation for the introduction of azide groups to the protein and then strain-promoted azide-alkyne cycloaddition (SPAAC) to attach a bicyclononyne (BCN)-presenting redox-sensitive Au25 nanocluster was essential for a scalable procedure. Covalent labeling of the antibody with gold nanoclusters was confirmed by various analytical methods, including cryo-EM analysis of the Au25 nanocluster conjugates.

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.

Synthesis and Characterization of Enantiopure Chiral Bis NHC-Stabilized Edge-Shared Au10 Nanocluster with Unique Prolate Shape.

Herein we report the first chiral Au10 nanoclusters stabilized by chiral bis N-heterocyclic carbene (bisNHC) ligands. ESI-MS and single-crystal X-ray crystallography confirmed the molecular formula to be [Au10(bisNHC)4Br2](O2CCF3)2. The chiral Au10 nanocluster adopts a linear edge-shared tetrahedral geometry with a prolate shape. DFT calculations provide insight into the electronic structure, optical absorption, and circular dichroism (CD) characteristics of this unique Au10 nanocluster. CD spectra demonstrate chirality transfer from the chiral bisNHC ligand to the inner Au10 nanocluster core. Examination of ESI-MS and UV-vis spectra show that cluster [Au9(bisNHC)4Br]Br2 is formed initially and then transformed into the Au10 nanocluster in solution.

N-Heterocyclic Carbene-Stabilized Hydrido Au24 Nanoclusters: Synthesis, Structure, and Electrocatalytic Reduction of CO2.

Atomically precise hydrido gold nanoclusters are extremely rare but interesting due to their potential applications in catalysis. By optimization of molecular precursors, we have prepared an unprecedented N-heterocyclic carbene-stabilized hydrido gold nanocluster, [Au24(NHC)14Cl2H3]3+. This cluster comprises a dimer of two Au12 kernels, each adopting an icosahedral shape with one missing vertex. The two kernels are joined through triangular faces, which are capped with a total of three hydrides. The hydrides are detected by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy, with density functional theory calculations supporting their position bridging the six uncoordinated gold sites. The reactivity of this Au24H3 cluster in the electrocatalytic reduction of CO2 is demonstrated and benchmarked against related catalysts.

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.

A Face-to-Face Dimer of Au3 Superatoms Supported by Interlocked Tridentate Scaffolds Formed in Au18 S2 (SR)12.

A new sulfur-containing gold cluster, Au18 S2 (STipb)12 , was serendipitously obtained using the bulky thiol, 2,4,6-triisopropylbenzyl mercaptan (TipbSH), as protecting ligands. Single-crystal X-ray diffraction analysis revealed that Au18 S2 (STipb)12 has a deformed octahedral Au6 core clutched by two tridentate S[Au2 (STipb)2 ]3 units in an interlocked manner. Based on density functional theory calculations, we propose that the Au6 core with two electrons is better viewed as a face-to-face dimer of Au3 (1e) superatoms rather than an electronically closed Au6 (2e) superatom. In situ formation of the sulfide anions (S2- ) via C-S bond breakage is ascribed to the steric repulsion between the TipbS ligands.

Doping-Mediated Energy-Level Engineering of M@Au12 Superatoms (M=Pd, Pt, Rh, Ir) for Efficient Photoluminescence and Photocatalysis.

We synthesized a series of MAu12 (dppe)5 Cl2 (MAu12 ; M=Au, Pd, Pt, Rh, or Ir; dppe=1,2-bis(diphenylphosphino)ethane), which have icosahedral M@Au12 superatomic cores, and systematically investigated their electronic structures, photoluminescence (PL) and photocatalytic properties. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was expanded when doping an M element positioned at the lower left of the periodic table. The PL quantum yield was enhanced with an increase in the HOMO-LUMO gap and reached 0.46-0.67 for MAu12 (M=Pt, Rh, or Ir) under deaerated conditions. The bright PLs from MAu12 (M=Pt, Rh, or Ir) were assigned to phosphorescence based on quenching by O2 . MAu12 (M=Pt, Rh, or Ir) acted as a more efficient and stable photocatalyst than Au13 for intramolecular [2+2] cycloaddition of bisenone via the oxidative quenching cycle. This study provides rational guides for designing photoluminescent and photocatalytic gold superatoms by the doping of heterometal elements.

A Unified View on Varied Ultrafast Dynamics of the Primary Process in Microbial Rhodopsins.

All-trans to 13-cis photoisomerization of the protonated retinal Schiff base (PRSB) chromophore is the primary step that triggers various biological functions of microbial rhodopsins. While this ultrafast primary process has been extensively studied, it has been recognized that the relevant excited-state relaxation dynamics differ significantly from one rhodopsin to another. To elucidate the origin of the complicated ultrafast dynamics of the primary process in microbial rhodopsins, we studied the excited-state dynamics of proteorhodopsin, its D97N mutant, and bacteriorhodopsin by femtosecond time-resolved absorption (TA) spectroscopy in a wide pH range. The TA data showed that their excited-state relaxation dynamics drastically change when pH approaches the pKa of the counterion residue of the PRSB chromophore in the ground state. This result reveals that the varied excited-state relaxation dynamics in different rhodopsins mainly originate from the difference of the ground-state heterogeneity (i.e., protonation/deprotonation of the PRSB counterion).

Chemically Modified Gold/Silver Superatoms as Artificial Elements at Nanoscale: Design Principles and Synthesis Challenges.

Pure and doped gold/silver clusters protected by monolayers of organic ligands have attracted much interest as novel functional materials owing to their nonbulk-like, size-specific properties. They can be viewed as chemically modified superatoms because their stabilities and properties are governed by the electron shell configurations of the Au/Ag cores. Chemically modified superatoms are unique from conventional atoms in that they have additional control parameters such as surface modification, compositions, atomic packing, and size, although both of them follow similar Aufbau principles. Atomically precise synthesis and structure determination by X-ray crystallography have deepened our understanding of the correlation between the structures and fundamental properties of the superatoms. However, remaining challenges for the exploration of novel materials using superatoms as artificial elements at the nanoscale include (1) establishment of guiding principles of the electronic structures and (2) development of efficient, targeted synthesis according to rational design guidelines for functionalities. To address the first task, we herein propose and rationalize empirical guiding principles of electronic structures using icosahedral Au13/Ag13 superatoms with the closed electron configuration as platforms. The second task is addressed by proposing design guidelines for functionalities and hydride-mediated transformation processes for efficient, targeted synthesis. These efforts will lead to the construction of a new periodic table of chemically modified superatoms and open up a materials world of quasi-molecules made of superatoms. We hope that this Perspective will contribute to the creation of a new paradigm based on superatoms, which parallels the matured world of molecular science.

Photoluminescence of Doped Superatoms M@Au12 (M = Ru, Rh, Ir) Homoleptically Capped by (Ph2)PCH2P(Ph2): Efficient Room-Temperature Phosphorescence from Ru@Au12.

A series of doped gold superatoms M@Au12 (M = Ru, Rh, Ir) was synthesized by capping with the bidentate ligand (Ph2)PCH2P(Ph2). A single-crystal X-ray diffraction analysis showed that all the M@Au12 superatoms had icosahedral motifs with a significantly higher symmetry than that of the pure Au13 counterpart due to different coordination geometries. The Ru@Au12 superatom exhibited a room-temperature phosphorescence with the highest quantum yield of 0.37 in deaerated dichloromethane. Density functional theory calculations suggested that the efficient phosphorescence is ascribed to a rapid intersystem crossing due to the similarity between the singlet and triplet excited states in terms of structure and energy.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver.

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)x Ly ]z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)x Ly ]z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)x Ly ]z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)x Ly ]z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

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.