Oxidation-Induced Transformation of Eight-Electron Gold Nanoclusters: [Au23(SR)16]- to [Au28(SR)20]0

Gold-Thiolate Nanocluster Dynamics and Intercluster Reactions Enabled by a Machine Learned Interatomic Potential

Monolayer protected metal clusters comprise a rich class of molecular systems and are promising candidate materials for a variety of applications. While a growing number of protected nanoclusters have been synthesized and characterized in crystalline forms, their dynamical behavior in solution, including prenucleation cluster formation, is not well understood due to limitations both in characterization and first-principles modeling techniques. Recent advancements in machine-learned interatomic potentials are rapidly enabling the study of complex interactions such as dynamical behavior and reactivity on the nanoscale. Here, we develop an Au-S-C-H atomic cluster expansion (ACE) interatomic potential for efficient and accurate molecular dynamics simulations of thiolate-protected gold nanoclusters (Aun(SCH3)m). Trained on more than 30,000 density functional theory calculations of gold nanoclusters, the interatomic potential exhibits ab initio level accuracy in energies and forces and replicates nanocluster dynamics including thermal vibration and chiral inversion. Long dynamics simulations (up to 0.1 μs time scale) reveal a mechanism explaining the thermal instability of neutral Au25(SR)18 clusters. Specifically, we observe multiple stages of isomerization of the Au25(SR)18 cluster, including a chiral isomer. Additionally, we simulate coalescence of two Au25(SR)18 clusters and observe series of clusters where the formation mechanisms are critically mediated by ligand exchange in the form of [Au-S]n rings.

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.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

The Pivotal Radical Intermediate [Au21(SR)15]+ in the Ligand-Exchange-Induced Size-Reduction of [Au23(SR)16]- to Au16(SR)12

The atomic precision of sub-nanometer-sized metal nanoclusters makes it possible to elucidate the kinetics of metal nanomaterials from the molecular level. Herein, the size reduction of an atomically precise [Au23(CHT)16]- (HCHT = cyclohexanethiol) cluster upon ligand exchange with HSAdm (1-adamantanethiol) has been reported. During the 16 h conversion of [Au23(CHT)16]- to Au16(SR)12, the neutral 6e Au21(SR)15, and its 1e-reduction state, i.e. the 5e, cationic radical, [Au21(SR)15]+, are active intermediates to account for the formation of thermodynamically stable Au16 products. The combination of spectroscopic monitoring (with UV-vis and ESI-MS) and DFT calculations indicates the preferential size-reduction on the corner Au atoms on the core surface and the terminal Au atoms on longer AunSn+1 staples. This study provides a reassessment on the electronic state of the Au21 structure and highlights the single electron transfer processes in cluster systems and thus the importance of the EPR analysis on the mechanistic issues.

Recent developments in the investigation of driving forces for transforming coinage metal nanoclusters

Metal nanoclusters serve as an emerging class of modular nanomaterials. The transformation of metal nanoclusters has been fully reflected in their studies from every aspect, including the structural evolution analysis, physicochemical property regulation, and practical application promotion. In this review, we highlight the driving forces for transforming atomically precise metal nanoclusters and summarize the related transforming principles and fundamentals. Several driving forces for transforming nanoclusters are meticulously reviewed herein: ligand-exchange-induced transformations, metal-exchange-induced transformations, intercluster reactions, photochemical transformations, oxidation/reduction-induced transformations, and other factors (intrinsic instability, pH, temperature, and metal salts) triggering transformations. The exploitation of transforming principles to customize the preparations, structures, physicochemical properties, and practical applications of metal nanoclusters is also disclosed. At the end of this review, we provide our perspectives and highlight the challenges remaining for future research on the transformation of metal nanoclusters. Our intended audience is the broader scientific community interested in metal nanoclusters, and we believe that this review will provide researchers with a comprehensive synthetic toolbox and insights on the research fundamentals needed to realize more cluster-based nanomaterials with customized compositions, structures, and properties.

Dynamic Metal Nanoclusters: A Review on Accurate Crystal Structures

Dynamic metal nanoclusters have garnered widespread attention due to their unique properties and potential applications in various fields. Researchers have been dedicated to developing new synthesis methods and strategies to control the morphologies, compositions, and structures of metal nanoclusters. Through optimized synthesis methods, it is possible to prepare clusters with precise sizes and shapes, providing a solid foundation for subsequent research. Accurate determination of their crystal structures is crucial for understanding their behavior and designing custom functional materials. Dynamic metal nanoclusters also demonstrate potential applications in catalysis and optoelectronics. By manipulating the sizes, compositions, and surface structures of the clusters, efficient catalysts and optoelectronic materials can be designed and synthesized for various chemical reactions and energy conversion processes. This review summarizes the research progress in the synthesis methods, crystal structure characterization, and potential applications of dynamic metal nanoclusters. Various nanoclusters composed of different metal elements are introduced, and their potential applications in catalysis, optics, electronics, and energy storage are discussed. Additionally, the important role of dynamic metal nanoclusters in materials science and nanotechnology is explored, along with an overview of the future directions and challenges in this field.

Mechanistic Insights into Oxidation-Induced Size Conversion of [Au6(dppp)4]2+ to [Au8(dppp)4Cl2]2

Oxidation-induced conversion of gold nanoclusters is an important strategy for preparing novel atomically precise clusters and elucidating the kinetic correlations of different clusters. Herein, the oxidation-induced growth from [Au6(dppp)4]2+ to [Au8(dppp)4Cl2]2+ (reported by Konishi and co-workers) has been studied by density functional theory calculations. A successive oxidation → Cl- coordination → oxidation → Cl- coordination sequence occurs first to activate the Au6 structure, resulting in the high Au(core)-Au(corner) bond cleavage activity and the subsequent formation of [Au2(dppp)2Cl]+ and [Au4(dppp)2Cl]+ fragments. Then, the dimerization of two Au4 fragments and the rearrangement of the diphosphine coordination occur to generate the thermodynamically stable [Au8(dppp)4Cl2]2+ products. The proposed mechanism agrees with the experimental outcome for the fast reaction rate and the residual of the Au2 components. Specifically, a multivariate linear regression analysis indicates the strong correlation of the oxidation potential of Au6, Au8, Au23, and Au25 clusters with the HOMO energy, the number of Au atoms, and cluster charge state. The main conclusions [e.g., oxidation-induced Au(corner)-Au(core) bond activation, easy 1,2-P transfer steps, etc.] of this study might be widely applicable in improving our understanding of the mechanism of other cluster-conversion reactions.

Insights into mechanisms of diphosphine-mediated controlled surface construction on Au nanoclusters

Unraveling the rules governing the size regulation of nanoclusters is of great importance not only in fundamental research, but also in practical applications because of the high structure-property correlation in nanoclusters. Diphosphine-mediated size tailoring is recognized as a powerful method for modulating the size, configuration, and properties of nanoclusters, but the role of diphosphines in these size-controlled processes is still poorly understood due to a lack of systematic studies. Herein, using Au₂₃(SR)₁₆^- as the template for modification, the factors influencing the size-modulation of nanoclusters by diphosphines were systematically investigated. It is revealed that by controlling the length of the diphosphines (from shorter to longer), Au₂₁(SR)₁₂L₂⁺ (L = diphosphine) and Au₂₂(SR)₁₄L can be produced. Moreover, introducing a rigid group into the diphosphines can twist the structural framework or lead to the formation of a new surface motif configuration in the nanoclusters, forming twisted Au₂₂(SR)₁₄L and Au₂₅(SR)₁₆L₂⁺. The size regulation of these nanoclusters enables fine-tuning of the optical properties, including the absorption wavelengths and photoluminescence emission intensity, affording an avenue for precise control of the physicochemical properties of nanoclusters for practical applications.

Charge-Transfer-Mediated Mechanism Dominates Oxygen Quenching of Ligand-Protected Noble-Metal Cluster Photoluminescence

Photoluminescence (PL) quenching of ligand-protected noble-metal clusters (NMCs) by molecular oxygen is often used to define whether the PL of NMC is fluorescent or phosphorescent, and only energy transfer has been always considered as the quenching mechanism. Herein, we performed the Rehm-Weller analysis of the O₂-induced PL quenching of 13 different NMCs and found that the charge-transfer (CT)-mediated mechanism dominates the quenching process. The quenching rate constant showed a clear dependence on the CT driving force, varied markedly from 10⁶ to 10⁹ M^-1s^-1. Transient absorption spectroscopy and photon upconversion measurements confirmed the triplet sensitization of aromatic molecules by NMCs regardless of the quenching degree by O₂, establishing that the PL of NMCs under investigation originates from the excited triplet state (i.e., phosphorescence). The results herein provide an essential indicator for correctly determining whether the PL of an NMC is fluorescent or phosphorescent.

Photo-Induced Cluster-to-Cluster Transformation of [Au37-xAgx(PPh3)13Cl10]3+ into [Au25-yAgy(PPh3)10Cl8]+: Fragmentation of a Trimer of 8-Electron Superatoms by Light

We present the photoinduced size/structure transformation of [Au_37-xAg_x(PPh₃)₁₃Cl₁₀]³⁺ (M37) into [Au_25-yAg_y(PPh₃)₁₀Cl₈]⁺ (M25) cluster. Single-crystal X-ray diffraction revealed that M37 has a tri-icosahedron M₃₆ metal core assembled via the fusion of three Au₇Ag₆ icosahedrons in a cyclic fashion and that the M₃₆ core is further protected by phosphine and chloride ligands. The M37 cluster is found to be highly sensitive toward ambient light, and the M37 → M25 transition is observed with 530 nm irradiation, monitored by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and femtosecond transient absorption spectroscopy. Linear-response time-dependent DFT calculations indicated that the strong absorption of the M37 cluster close to 500 nm induces an antibonding-type configuration in the induced electron density within the plane of the three 8-electron systems, possibly promoting dissociation of one of the 8-electron superatoms. This theoretical result supports the experimental observation of the sensitivity of the M37 → M25 transition to 530 nm irradiation.

Anisotropic Evolution of Nanoclusters from Ag40 to Ag45: Halogen- and Defect-Induced Epitaxial Growth in Nanoclusters

Halogens have widely served as handles for regulating the growth of nanoparticles and the control of their physicochemical properties. However, their regulatory mechanism is poorly understood. Nanoclusters are the early morphology of nanoparticles and play an important role in revealing the formation and growth of nanoparticles due to their precise structures. Here, we report that halogens induce the anisotropic growth of Ag₄₀(C₆H₅COO)₁₃(SR)₁₉(CH₃CN) (Ag₄₀-II, where SR = 4-tert-butylbenzylmercaptan) into Ag₄₅(C₆H₅COO)₁₃(SR)₂₂Cl₂ (Ag₄₅), where Ag₄₀-II is converted from Ag₄₀(CH₃COO)₁₀(SR)₂₂ (Ag₄₀-I). Experiments and theoretical simulations showed that halogen ions adsorb at both ends of the cluster, forming defect sites. The -SR-Ag- complexes fill the defects and complete the anisotropic transition from Ag₄₀-II to Ag₄₅. Circular dichroism spectra show that the chirality of Ag₄₅ decreases 15-fold compared with that of Ag₄₀-II. This work provides important insights into the effects of halogens on the growth mechanism and property regulation for nanomaterials at the atomic level and the benefits of further applications of halogen-induced nanomaterials.

Redox-Induced Interconversion of Two Au8 Nanoclusters: the Mechanism and the Structure-Bond Dissociation Activity Correlations

The interconversion of atomically precise nanoclusters represents an excellent platform to understand the structural correlations of nanomaterials at the atomic level. Herein, density functional theory calculations were performed to elucidate the mechanism of the redox-induced interconversion of [Au₈(dppp)₄]²⁺ and [Au₈(dppp)₄Cl₂]²⁺ (dppp is short for 1,3-bis(diphenylphosphino)propane) nanoclusters. Reduction is the driving force for the conversion of [Au₈(dppp)₄Cl₂]²⁺ to [Au₈(dppp)₄]²⁺, while the Au-Au and first Au-Cl bond dissociations occur asynchronously on the two different corner Au atoms to avoid the formation of an electron-deficient Au atom. By contrast, the reduced electron density of [Au₈(dppp)₄]²⁺ by oxidation with O₂ weakens the outmost Au-Au bond therein and facilitates the coordination of the electron-rich chloride(s). The reduction- and oxidation-induced activations, respectively, of Au-Cl and Au-Au bonds and the elucidated principles on the structure-activity correlations might also be generalized to other size conversions upon redox treatment.

Carboranealkynyl-Protected Gold Nanoclusters: Size Conversion and UV/Vis-NIR Optical Properties

Structure evolution has become an effective way to assemble novel monolayer-protected metal nanomolecules. However, evolution with alkynyl-stabilized metal clusters still remains rarely explored. Herein, we present a carboranealkynyl-protected gold nanocluster [Au₂₈ (C₄ B₁₀ H₁₁ )₁₂ (tht)₈ ]³⁺ (Au₂₈ , tht=tetrahydrothiophene) possessing an open-shell electronic structure with 13 free electrons, which was isolated by a facile self-reduction method with 9-HC≡C-closo-1,2-C₂ B₁₀ H₁₁ as the two-in-one reducing and protecting agent. Notably, Au₂₈ undergoes a complete transformation in methanol into a stable and smaller-sized nanocluster [Au₂₃ (C₄ B₁₀ H₁₁ )₉ (tht)₆ ]²⁺ (Au₂₃ ) bearing 12 valence electrons and crystal-field-like split superatomic 1D orbitals. The transformation process was systematically monitored with ESI-MS and UV/Vis absorption spectra. Au₂₈ and Au₂₃ both display optical absorption covering the UV/Vis-NIR range and NIR emission, which facilitates their potential application in the biomedical and photocatalytic fields.

Controlling ultrasmall gold nanoparticles with atomic precision

Gold nanoparticles are probably the nanoparticles that have been best studied for the longest time due to their stability, physicochemical properties and applications. Controlling gold nanoparticles with atomic precision is of significance for subsequent research on their structures, properties and applications, which is a dream that has been pursued for many years since ruby gold was first obtained by Faraday in 1857. Fortunately, this dream has recently been partially realized for some ultrasmall gold nanoparticles (nanoclusters). However, rationally designing and synthesizing gold nanoparticles with atomic precision are still distant goals, and this challenge might rely primarily on rich atomically precise gold nanoparticle libraries and the in-depth understanding of metal nanoparticle chemistry. Herein, we review general synthesis strategies and some facile synthesis methods, with an emphasis on the controlling parameters determined from well-documented results, which might have important implications for future nanoparticle synthesis with atomic precision and facilitate related research and applications.

The synthesis strategy, methods and parameters for atomically precise gold nanoclusters were reviewed, and future outlook was also proposed.

Structure determination of a metastable Au22(SAdm)16 nanocluster and its spontaneous transformation into Au21(SAdm)15

It is of great interest to investigate the evolution pattern of gold nanoclusters (Au NCs) due to its significance in understanding the growth mechanism and origin of Au NCs. Capture of metastable cluster intermediates is an effective way to meet this demand since they provide valuable information for understanding the conversion pathway of Au NCs. However, it is still challenging to obtain metastable Au NCs, especially thiol-protected Au NCs, and solve their structures. In this work, a metastable thiol-protected gold nanocluster, Au22(SAdm)16 (Au22), was synthesized and its structure was determined by single crystal X-ray diffraction. Au22 shows a close structure-evolution correlation with Au21(SAdm)15 (Au21). The symmetric Au10 kernel of Au21 is twisted by the insertion of an additional Au-SR unit on the motif during its structure evolution into Au22. The distortion in structures results in significantly distinguishing absorption and emission spectra between Au22 and Au21. Noteworthily, the structure correlation between Au22 and Au21 was also found experimentally that Au22 can spontaneously transform into Au21 due to the metastability of Au22 in solution. This size conversion process was monitored by time-dependent UV-vis spectroscopy and ESI-MS. Furthermore, the solvent effect on the size conversion process was also investigated. This transformation from Au22 to Au21 provides a unique platform for studies on the evolution pattern of gold nanoclusters at the single atom level.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

Isomerization-induced enhancement of luminescence in Au28(SR)20 nanoclusters

Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core (i.e. the inner kernel) and the exterior shell (i.e. surface Au(i)-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au₂₈(SR)₂₀ nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au₂₈(CHT)₂₀ (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au₂₈(TBBT)₂₀ nanocluster (TBBT: p-tert-butylbenzenethiolate). Such an enhancement is found to originate from the different structural arrangement of the staple motifs in the shell, which modifies the electron relaxation dynamics in the inner core to different extents for the two nanoclusters. The emergence of a long PL lifetime component in the more emissive Au₂₈(CHT)₂₀ nanocluster reveals that its PL is enhanced by suppressing the nonradiative pathway. The presence of long, interlocked staple motifs is further identified as a key structural parameter that favors the luminescence. Overall, this work offers structural insights into the PL origin in Au₂₈(SR)₂₀ nanoclusters and provides some guidelines for designing luminescent metal nanoclusters for sensing and optoelectronic applications.

Two Au₂₈(SR)₂₀ nanoclusters with an identical core but different shells exhibit a ∼15-fold difference in photoluminescence.

Size Transformation of the Au22(SG)18 Nanocluster and Its Surface-Sensitive Kinetics

For many applications of well-defined gold nanoclusters, it is desirable to understand their structural evolution behavior under working conditions with molecular precision. Here we report the first systematic investigation of the size transformation products of the Au₂₂(SG)₁₈ nanocluster under representative working conditions and highlight the surface effect on the transformation kinetics. Under thermal and aerobic conditions, the consecutive and pH-dependent transformation from Au₂₂ to both well-defined clusters and small Au(I)SR species was identified by ESI-MS and UV-vis spectroscopy. By introducing a perturbation onto the Au₂₂ surface, significant changes in the activation parameters were determined from the kinetic study of the Au₂₂ transformation. This indicates the sensitivity of the nanocluster transformation pathway to the cluster surface. The systematic study of cluster transformation and the sensitivity of cluster transformation to the surface revealed herein has significant implications for future attempts to design gold nanoparticles with adaptation to the working environment and the regeneration of active nanoparticles.

Light-Induced Size-Growth of Atomically Precise Nanoclusters

A photo-induced transformation from [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ to Au₂₈(S-c-C₆)₂₀ nanocluster was first reported in this work. The [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ nanocluster is first excited to [Au₂₃(S-c-C₆)₁₆]^•-(TOA)⁺ by photons with energy higher than its E_g (E_g = HOMO - LUMO energy gap), and then, the negatively charged [Au₂₃(S-c-C₆)₁₆]^•- nanocluster was oxidized to the neutral state by transfering one electron to O₂. The unstable neutral cluster [Au₂₃(S-c-C₆)₁₆]⁰ obtained was decomposed into smaller nanocluster and finally reassembled into the Au₂₈(S-c-C₆)₂₀ nanocluster. Time-dependent UV-vis, matrix-assisted laser desorption/ionization time of flight mass spectrometry, electron paramagnetic resonance, and electrospray ionization mass spectrometry characterizations were performed to monitor the nanocluster size transformation.

Controlling magnetism of Au133(TBBT)52 nanoclusters at single electron level and implication for nonmetal to metal transition

The [Au₁₃₃(SR)₅₂]^q nanocluster is discovered to possess one spin per particle when q = 0, but no unpaired electron when q = +1.

The transition from the discrete, excitonic state to the continuous, metallic state in thiolate-protected gold nanoclusters is of fundamental interest and has attracted significant efforts in recent research. Compared with optical and electronic transition behavior, the transition in magnetism from the atomic gold paramagnetism (Au 6s¹) to the band behavior is less studied. In this work, the magnetic properties of 1.7 nm [Au₁₃₃(TBBT)₅₂]⁰ nanoclusters (where TBBT = 4-tert-butylbenzenethiolate) with 81 nominal “valence electrons” are investigated by electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR analysis shows that each cluster possesses one unpaired electron (spin), indicating that the electrons fill into discrete orbitals instead of a continuous band, for that one electron in the band would give a much smaller magnetic moment. Therefore, [Au₁₃₃(TBBT)₅₂]⁰ possesses a nonmetallic electronic structure. Furthermore, we demonstrate that the unpaired spin can be removed by oxidizing [Au₁₃₃(TBBT)₅₂]⁰ to [Au₁₃₃(TBBT)₅₂]⁺ and the nanocluster transforms from paramagnetism to diamagnetism accordingly. The UV-vis absorption spectra remain the same in the process of single-electron loss or addition. Nuclear magnetic resonance (NMR) is applied to probe the charge and magnetic states of Au₁₃₃(TBBT)₅₂, and the chemical shifts of 52 surface TBBT ligands are found to be affected by the spin in the gold core. The NMR spectrum of Au₁₃₃(TBBT)₅₂ shows a 13-fold splitting with 4-fold degeneracy of 52 TBBT ligands, which are correlated to the quasi-D₂ symmetry of the ligand shell. Overall, this work provides important insights into the electronic structure of Au₁₃₃(TBBT)₅₂ by combining EPR, optical and NMR studies, which will pave the way for further understanding of the transition behavior in metal nanoclusters.

Stibine-protected Au13 nanoclusters: syntheses, properties and facile conversion to GSH-protected Au25 nanocluster

Monostibine-protected ionic Au13 nanoclusters, namely, [Au13(L)8(Cl)4][Cl] (L= SbPh3, 2a·Cl; Sb(p-tolyl)3, 2b·Cl) were prepared by the direct reduction of Au(L)Cl with NaBH4 in dichloromethane. Anion exchange with 2a·Cl afforded [Au13(SbPh3)8(Cl)4][X] (X = PF6, 2a·PF6; BPh4, 2a·BPh4). All these have been characterized by multinuclear NMR, ESI-MS and UV-Vis spectroscopy. Crystallographic analysis of 2a·BPh4 reveals that the cation possesses C 2v symmetry and the tridecagold core adopts a closed icosahedron configuration. The weaker coordinating ability of the stibine ligands leads to the ready reaction of 2b·Cl with PPh3 or glutathione (GSH) to form the smaller phosphine-protected cluster [Au11(PPh3)8Cl2][Cl] or larger thiolate-protected cluster Au25(SG)18, respectively. In the latter reaction, the addition of a small amount (0.5 to 3.5 equivalents) of a suitable oxidant such as K3(Fe(CN)6 accelerates the conversion rate significantly.

Hydride-induced ligand dynamic and structural transformation of gold nanoclusters during a catalytic reaction

Quasi-homogeneous ligand-protected gold nanoclusters (Au NCs) with atomic precision and well-defined structure offer great opportunity for exploring the catalytic nature of nanogold catalysts at a molecular level. Herein, using real-time electrospray ionization mass spectrometry (ESI-MS), we have successfully identified the desorption and re-adsorption of p-mercaptobenzoic acid (p-MBA) ligands from Au25(p-MBA)18 NC catalysts during the hydrogenation of 4-nitrophenol in solution. This ligand dynamic (desorption and re-adsorption) would initiate structural transformation of Au25(p-MBA)18 NC catalysts during the reaction, forming a mixture of smaller Au NCs (Au23(p-MBA)16 as the major species) at the beginning of catalytic reaction, which could further be transformed into larger Au NCs (Au26(p-MBA)19 as the major species). The adsorption of hydrides (from NaBH4) is identified as the determining factor that could induce the ligand dynamic and structural transformation of NC catalysts. This study provides fundamental insights into the catalytic nature of Au NCs, including catalytic mechanism, active species and stability of Au NC catalysts during a catalytic reaction.

Engineering Functional Metal Materials at the Atomic Level

With continuous research efforts devoted into synthesis and characterization chemistry of functional nanomaterials in the past decades, the development of metal materials is stepping into a new era, where atom-by-atom customization of property-dictating structural attributes is expected. Herein, the state-of-the-art modulation of functional metal nanomaterials at the atomic level, by size- and structure-controlled synthesis of thiolate-protected metal (e.g., Au and Ag) nanoclusters (NCs), is exemplified. Metal NCs are ultrasmall (<3 nm) particles with hierarchical primary, secondary, and tertiary structures, reminiscent of natural proteins or enzymes. Given the proven dependence of their physicochemical properties on their size and structure, documented synthetic methodologies delivering NCs with atomic-level monodispersity and tailorable size and structural attributes at individual hierarchical levels are categorized and discussed. Such assured atomic-level modulation could confer metal NCs with novel application opportunities in diverse fields, which are also exemplified by their size- and structure-dictated catalytic and biomedical performance. The precise synthesis and application chemistry developed based on the hierarchical structure scheme of metal NCs could increase the acceptance of metal NCs as a new family of functional materials.

Revealing isoelectronic size conversion dynamics of metal nanoclusters by a noncrystallization approach

Atom-by-atom engineering of nanomaterials requires atomic-level knowledge of the size evolution mechanism of nanoparticles, which remains one of the greatest mysteries in nanochemistry. Here we reveal atomic-level dynamics of size evolution reaction of molecular-like nanoparticles, i.e., nanoclusters (NCs) by delicate mass spectrometry (MS) analyses. The model size-conversion reaction is [Au₂₃(SR)₁₆]⁻ → [Au₂₅(SR)₁₈]⁻ (SR = thiolate ligand). We demonstrate that such isoelectronic (valence electron count is 8 in both NCs) size-conversion occurs by a surface-motif-exchange-induced symmetry-breaking core structure transformation mechanism, surfacing as a definitive reaction of [Au₂₃(SR)₁₆]⁻ + 2 [Au₂(SR)₃]⁻ → [Au₂₅(SR)₁₈]⁻ + 2 [Au(SR)₂]⁻. The detailed tandem MS analyses further suggest the bond susceptibility hierarchies in feed and final Au NCs, shedding mechanistic light on cluster reaction dynamics at atomic level. The MS-based mechanistic approach developed in this study also opens a complementary avenue to X-ray crystallography to reveal size evolution kinetics and dynamics.

How metal nanoclusters evolve in size is poorly understood, particularly at the atomic level. Here, the authors use mass spectrometry to study the size conversion dynamics between two isoelectronic gold nanoclusters with atomic resolution, revealing that the growth reaction proceeds through a distinct balanced equation.

Cluster-to-cluster transformation among Au6, Au8 and Au11 nanoclusters

This manuscript demonstrates the cluster-to-cluster transformations among three gold nanoclusters, which were all monitored and corroborated using UV-Vis spectroscopy and ESI-MS.