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.

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.

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.

Collision-Induced Fission of Oblate Gold Superatom in [Au9(PPh3)8]3+: Deformation-Mediated Mechanism.

Collision-induced dissociation (CID) patterns of the phosphine-protected Au-based clusters [PdAu8(PPh3)8]2+ (PdAu8) and [Au9(PPh3)8]3+ (Au9), featuring crown-shaped M@Au8 (M = Pd, Au) cores, were investigated. For PdAu8, ordinary sequential PPh3 losses (PdAu8 → [PdAu8(PPh3)m]2+ + (8 - m)PPh3 (m = 7, 6, 5)) were observed. In contrast, Au9 underwent cluster-core fission (Au9 → [Au6(PPh3)6]2+ (Au6) + [Au3(PPh3)2]+ (Au3)) upon sufficiently high energy collision, associated with splitting the number of valence electrons in the superatomic orbitals from 6e (Au9) into 4e (Au6) and 2e (Au3). Density functional theory calculations revealed oblate and prolate cores of Au9 and Au6 with semiclosed superatomic electron configurations of (1S)2(1Px)2(1Py)2 and (1S)2(1Pz)2, respectively. This result indicated a significant deformation of the cluster-core motif during the CID process. We attribute the clear difference between PdAu8 and Au9 to the softer Au-Au bond in Au9 and propose that the collision-induced structural deformation plays a critical role in the fission.

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.

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.

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]- (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy.

Au38(PET)24 (PET = SC2H4Ph) is known to have a bi-icosahedral Au23 core consisting of two Au13 icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au23 core is similar to that in the F2 molecule. The SVB model predicted that the electron configuration of the Au23 core with 14 valence electrons is expressed as (1Σ)2(1Σ*)2(1Π)4(2Σ)2(1Π*)4 where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au13 units. Therefore, the bi-icosahedral Au23 can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M1Au37(PET)24]- (M = Pd and Pt), which are isoelectronic and isostructural with Au38(PET)24. To this end, the neutral precursors [M1Au37(PET)24]0 were first synthesized by fusion reactions between hydride-doped clusters [HAu9(PPh3)8]2+ and [M1Au24(PET)18]-. The formation of bi-icosahedral M1Au22 cores with open electronic structure in [M1Au37(PET)24]0 was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M1Au37(PET)24]- were obtained by reducing [M1Au37(PET)24]0 with NaBH4, and isoelectronicity with [Au38(PET)24]0 was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M1Au37(PET)24]- observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

Synergistically Activated Pd Atom in Polymer-Stabilized Au23Pd1 Cluster.

Single Pd atom doped Au23Pd1 clusters stabilized by polyvinylpyrrolidone (Au23Pd1:PVP) were selectively synthesized by kinetically controlled coreduction of the Au and Pd precursor ions. The geometric structure of Au23Pd1:PVP was investigated by density functional theory calculation, aberration-corrected transmission electron microscopy, extended X-ray absorption fine structure analysis, Fourier transform infrared spectroscopy of adsorbed CO, and hydrogenation catalysis. These results showed that Au23Pd1:PVP takes polydisperse but the same atomic arrangements as undoped Au24:PVP while exposing all the atoms including the Pd atom on the surface. Au23Pd1:PVP exhibited a significantly higher catalytic activity than Au24:PVP for the aerobic oxidation of p-substituted benzyl alcohols. The kinetic studies showed that the rate-determining step was the hydride abstraction from the α-carbon of the alkoxides for both systems. The activation energy for hydride abstraction by Au23Pd1:PVP was lower than that by Au24:PVP, indicating that the doped Pd atom acts as the active center.

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.

NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters.

Herein, we describe the synthesis of a toroidal Au10 cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC-Au-X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au25 was observed even upon the reduction of NHC-Au-I. The isolated bromide derivative of the Au25 cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH-π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au10 cluster. 13C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

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.

Synthesis of active, robust and cationic Au25 cluster catalysts on double metal hydroxide by long-term oxidative aging of Au25(SR)18.

Synthesis of an atomically precise Au25 cluster catalyst was attempted by long-term, low-temperature aging of Au25(BaET)18 (BaET-H = 2-(Boc-amino)ethanethiol) on various double metal hydroxide (DMH) supports. X-ray absorption fine structure analysis revealed that bare Au25 clusters with high loading (1 wt%) were successfully generated on the DMH containing Co and Ce (Co3Ce) by oxidative aging in air at 150 °C for >12 h. X-ray absorption near-edge structure and X-ray photoelectron spectroscopies showed that the Au25 clusters on Co3Ce were positively charged. The Au25/Co3Ce catalyst thus synthesized exhibited superior catalytic performance in the aerobic oxidation of benzyl alcohol under ambient conditions (TOF = 1097 h-1 with >97% selectivity to benzoic acid) and high durability owing to a strong anchoring effect. Based on kinetic experiments, we propose that abstraction of hydride from α-carbon of benzyl alkoxide by Au25 is the rate-determining step of benzyl alcohol oxidation by Au25/Co3Ce.

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.

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.

Decorating an anisotropic Au13 core with dendron thiolates: enhancement of optical absorption and photoluminescence.

We successfully introduced up to 12 poly(benzyl ether)dendron-thiols of the second generation (D2SH) into the Au13 core of [Au23(ScC6H11)16]- while retaining the geometric structure. The decoration with D2SH enhanced the optical absorbance in the >2.5 eV region and the quantum yield of photoluminescence at ∼1.6 eV by ∼15 times.

Critical Role of CF3 Groups in the Electronic Stabilization of [PdAu24(C≡CC6H3(CF3)2)18]2- as Revealed by Gas-Phase Anion Photoelectron Spectroscopy.

The role of alkynyl ligands with electron-withdrawing nature in the stability of metal clusters was investigated by gas-phase anion photoelectron spectroscopy (PES) on heteroleptic cluster anions [PdAu24(C≡CArF)18-x(C≡CPh)x]2- (ArF = 3,5-(CF3)2C6H3). Gas-phase PES on the cluster anions with specific x (= 0-6) revealed that electron binding energies decreased linearly with x, indicating that the electron-withdrawing CF3 substituents on the alkynyl ligand played a critical role in the electronic stabilization of [PdAu24(C≡CArF)18]2-. Density functional theory calculations reproduced the decrease of electron binding energies and rationally explained the ligand effect by a mechanism similar to the modulation of the work function of gold films by organic monolayers.

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.

New Magic Au24 Cluster Stabilized by PVP: Selective Formation, Atomic Structure, and Oxidation Catalysis.

An unprecedented magic number cluster, Au24Cl 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 Au24Cl x was investigated by means of aberration-corrected transmission electron microscopy (ACTEM) and density functional theory (DFT) calculations. ACTEM video imaging revealed that the Au24Cl 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 Au24Cl 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 Au24Cl x clusters due to the interaction with PVP. The Au24Cl 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 Au24Cl 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 O2 as a faster process.

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.