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.

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.

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.

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.

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.

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.

Controlled Dimerization and Bonding Scheme of Icosahedral M@Au12 (M=Pd, Pt) Superatoms.

Targeted syntheses of MM'Au36 (PET)24 (M, M'=Pd, Pt; PET=SC2 H4 Ph) were achieved by hydride-mediated fusion reactions between [MAu8 (PPh3 )8 ]2+ and [M'Au24 (PET)18 ]- . Single-crystal X-ray diffraction analysis indicated that the products have bi-icosahedral MM'Au21 cores composed of M@Au12 and M'@Au12 superatoms. Although the MM'Au21 superatomic molecules correspond to O2 in terms of the number of valence electrons (12 e), the distances between the icosahedrons were larger than that in the bi-icosahedral Au23 core of Au38 (PET)24 corresponding to F2 and the spin state was singlet. These counterintuitive results were explained by a "bent bonding model" based on tilted (non-orthogonal) bonding interaction between the 1P superatomic orbitals of M@Au12 and M'@Au12 superatoms.

Electron-Rich Gold Clusters Stabilized by Poly(vinylpyridines) as Robust and Active Oxidation Catalysts.

In this report, we introduced poly(n-vinylpyridine) (PnVP, n = 2, 4) as an electron-donating stabilizer for small (<2 nm) Au clusters and elucidated how coordinating pyridines affect the physical, optical, chemical, and catalytic properties of Au clusters. Spectroscopic measurements and theoretical calculation suggested the high electron-donating ability of PnVP. PnVP-stabilized Au clusters improved robustness in aerobic oxidation of alcohols compared to poly(N-vinyl-2-pyrrolidone)-stabilized ones, while retaining catalytic activities.

Understanding Doping Effects on Electronic Structures of Gold Superatoms: A Case Study of Diphosphine-Protected M@Au12 (M = Au, Pt, Ir).

Dopants into ligand-protected Au superatoms have been hitherto limited to group X-XII elements (Pt, Pd, Ag, Cu, Hg, and Cd). To expand the scope of the dopants to the group IX elements, we synthesized unprecedented [IrAu12(dppe)5Cl2]+ [IrAu12; dppe = 1,2-bis(diphenylphosphino)ethane] and [PtAu12(dppe)5Cl2]2+ (PtAu12) and compared their electronic structures with that of [Au13(dppe)5Cl2]3+ (Au13). Single-crystal X-ray diffractometry, 31P{1H} NMR, and Ir L3-edge extended X-ray absorption fine structure analysis of IrAu12 revealed that the single Ir atom is located at the center of the icosahedral IrAu12 core. Electrochemical analysis demonstrated that the energy levels of the highest occupied molecular orbitals are upshifted in the order of Au13 < PtAu12 < IrAu12. This trend was qualitatively explained in such a manner that the jellium core potential at the central position becomes shallower by replacing Au+ with Pt0 and further with Ir-. IrAu12 underwent reversible redox reactions between the charge states of 1+ and 2+. The gradual increase of the energy gap between the highest occupied molecular orbital and lowest unoccupied molecular orbital in the order of Au13 < PtAu12 < IrAu12 was observed by electrochemical measurement and optical spectroscopy. This study provides a simple guiding principle to tune the electronic structures of heterometal-doped superatoms.

Efficient and Selective Conversion of Phosphine-Protected (MAu8)2+ (M = Pd, Pt) Superatoms to Thiolate-Protected (MAu12)6+ or Alkynyl-Protected (MAu12)4+ Superatoms via Hydride Doping.

An efficient and selective method was developed for the synthesis of bimetallic clusters, MAu24L18 (M = Pd or Pt; L = thiolates or alkynyls), by the reaction of Au(I)L oligomers with quasi-spherical superatoms [HMAu8(PPh3)8]+ activated by hydride doping. This hydride-mediated conversion afforded previously known clusters MAu24(SC2H4Ph)18 having an icosahedral (M@Au12)6+ core at ∼200 mg scale, with a yield of >50%, and was successfully applied to a variety of primary thiols with good yields. Although the application to secondary and tertiary thiolates was limited, the conversion produced the novel cluster [PdAu23(ScC6H11)17]0 having a poorly symmetrical, flattened (Pd@Au12)6+ core. The conversion produced the new alkynyl-protected clusters [MAu24(C≡CArF)18]2- (ArF = 3,5-(CF3)2C6H3) having an icosahedral (M@Au12)4+ core, with a yield of >50%. The larger number of valence electrons in the M@Au12 core protected by alkynyls is ascribed to an increase in attractive potential of the M@Au12 core owing to the stronger electron-withdrawing nature of alkynyls than thiolates. This simple and versatile bottom-up approach will provide an opportunity to synthesize a variety of superatoms on a large scale for the promotion of materials science based on superatoms as building units.

Stoichiometric Formation of Open-Shell [PtAu24(SC2H4Ph)18]- via Spontaneous Electron Proportionation between [PtAu24(SC2H4Ph)18]2- and [PtAu24(SC2H4Ph)18]0.

[PtAu24(SC2H4Ph)18]0 ([PtAu24]0) was fully and selectively converted to [PtAu24]-, having an open electronic structure with seven valence electrons, upon the addition of an equiamount of NaBH4. Stoichiometric production of [PtAu24]- by the reaction between an equal amount of [PtAu24]0 and [PtAu24]2- revealed that the above reaction proceeds via the spontaneous electron transfer (ET) from [PtAu24]2- nascently reduced by H--mediated reduction to [PtAu24]0 remaining in the solution. Theoretical calculation suggested that the driving force of this novel ET reaction was the larger adiabatic electron affinity of [PtAu24]0 compared to that of [PtAu24]-, partly associated with reduction-induced relief of the Jahn-Teller strain. We propose that ET proceeds via the dimer complex of [PtAu24]2- and [PtAu24]0 formed through the aurophilic interaction between Au(I) sites in the surface layer.

Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes.

Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au13 core surrounded by a symmetrical arrangement of nine NHCs and three chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0%, which is significantly greater than that of the most luminescent ligand-protected Au13 superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to phosphine stabilized clusters.