Au13(8e): A secondary block for describing a special group of liganded gold clusters containing icosahedral Au13 motifs

Intermediate Intercluster Bond Orders. Electronic Communication in Au38(SR)24 Superatomic Molecules

Ligand-protected gold clusters remain potential building blocks for envisaged molecular materials. The archetypal Au38(SR)24 cluster can be viewed as a robust template for the fusion of two Au25(SR)18 - cluster units, retaining a bi-icosahedral Au23 core. Via electrochemical properties, the overall charge state can be selectively tuned, enabling the access of 14 valence electron (ve) species featuring a single intercluster bond and nearby charge from -1 to +3, achieving related species bearing 15- to 11-ve with variable intercluster bond orders. Here, we explore the characteristics of intermediate intercluster bond orders in order to provide insights into the plausible electron communication between the constituent building blocks, with Au38(SR)24, as a representative template. Our results denote a small structural variation along -1 to +3 charge states, provided by the core-protecting ligand interaction, which is enhanced towards more oxidized species. The remaining unpaired electron from intermediate intercluster bond orders of 1.5 for Au38(SR)24 1-, 1.5 for Au38(SR)24 1+, and 2.5 for Au38(SR)24 3+, holds delocalized characteristics between the building block units, favoring electron communication for conductive and cooperative cluster aggregates. Such features are relevant for the formation of molecular electronic device applications, favoring the rationalization prior to engaging in explorative synthesis of larger ligand-protected cluster aggregates.

Decoding Chemical Formula to Spatial Conformation: A Structural Study Targeting the [Au25(SR)19]0 Nanocluster

Structural global searches employing highly efficient algorithms have been extensively applied for studying molecules and clusters. However, the code-aided spatial conformational determination of thiolated gold nanoclusters (AuNCs) has not been accomplished because of the complex structural architecture of AuNCs, especially when only the chemical formula of the cluster is known. Experiments have shown that the star [Au25(SR)18]-1 cluster can transform into the [Au25(SR)19]0 cluster. However, the crystal structure of the [Au25(SR)19]0 cluster has not been experimentally determined, and theoretical structural predictions for this cluster are challenging because no template cluster presents for [Au25(SR)19]0. Utilizing the grand unified model, this study succeeded in obtaining the structure of the [Au25(SR)19]0 cluster by using minimal computations, which was verified to be reasonable through stability analysis and experimental absorption spectrum confirmation. Although the predicted [Au25(SR)19]0 cluster has the same number of Au atoms as the [Au25(SR)18]-1 cluster, the structure is considerably altered, owing to the presence of a face-centered cubic kernel. This study provides insights for decoding the chemical formulas of AuNCs to determine their spatial conformations.

Isomerism effects in relaxation dynamics of Au24(SR)16thiolate-protected gold nanoclusters

Understanding the excited state behavior of isomeric structures of thiolate-protected gold nanoclusters is still a challenging task. In this paper, based on grand unified model and ring model for describing thiolate-protected gold nanoclusters, we have predicted four isomers of Au₂₄(SR)₁₆nanoclusters. Density functional theory calculations show that the total energy of one of the predicted isomers is 0.1 eV lower in energy than previously crystallized isomer. The nonradiative relaxation dynamics simulations of Au₂₄(SH)₁₆isomers are performed to reveal the effects of structural isomerism on relaxation process of the lowest energy states, in which that most of the low-excited states consist of core states. In addition, crystallized isomer possesses the shorter e-h recombination time, whereas the most stable isomer has the longer recombination time, which may be attributed to the synergistic effect of nonadiabatic coupling and decoherence time. Our results could provide practical guidance to predict new gold nanoclusters for future experimental synthesis, and stimulate the exploration of atomic structures of same sized gold nanoclusters for photovoltaic and optoelectronic devices.

Phosphinous Acid-Phosphinito Tetra-Icosahedral Au52 Nanoclusters for Electrocatalytic Oxygen Reduction

While the formation of superatomic nanoclusters by the three-dimensional assembly of icosahedral units was predicted in 1987, the synthesis and structural determination of such clusters have proven to be incredibly challenging. Herein, we employ a mixed-ligand strategy to prepare phosphinous acid-phosphinito gold nanocluster Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ with a tetra-icosahedral kernel. Unlike expected, each icosahedral Au₁₃ unit shares one vertex gold atom with two adjacent units, resulting in a "puckered" ring shape with a nuclearity of 48 in the kernel. The phosphinous acid-phosphinito ligand set, which consists of two phosphinous acids and one phosphinito motif, has strong intramolecular hydrogen bonds; the π-π stacking interactions between the phosphorus- and sulfur-based ligands provide additional stabilization to the kernel. Highly stable Au₅₂(HOPPh₂)₈(OPPh₂)₄(TBBT)₁₆ serves as an effective electrocatalyst in the oxygen reduction reaction. Density functional theory calculations suggest that the phosphinous acid-phosphinito ligands provide the most active sites in the electrochemical catalysis, with O* formation being the rate-determining step.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.

Elucidating the stabilities and properties of the thiolate-protected Au nanoclusters with detaching the staple motifs

Thiolate-protected Au nanoclusters (AuNCs) have been widely studied in areas of catalysis, biosensors, and bioengineering. In real applications, e.g., catalytic reactions, the thiolate groups are normally partially detached. However, which of the thiolate groups are easily detached and how the detachment of the ligands affects the geometries and electronic structures of the Au nanoclusters have been rarely studied. In this work, we employed the density functional theory calculations as well as the molecular orbital analysis to explore the detachment effect of the ligands using nine thiolate-protected AuNCs as examples. Our results showed that there existed a nearly linear relationship between the averaged detachment energies and the numbers of Au atoms in the motifs. Detaching longer motifs normally required more energies owing to the stronger aurophilic effects. For detaching a full motif, based on the structure decomposition via the grand unified model, analysis on the inner Au core indicated that the change in Au-Au bond length was more sensitive for the inter-block compared to the intra-block. The detachment of the -SH fragment generally needs less energy and brings less structural deformations when compared to the removal of a full motif. Molecular orbital analysis showed that the relative energies of the HOMO orbitals were elevated, which led to the narrow down of the HOMO-LUMO gap. This work provides a primary description of the correlation of the ligands' detachment with the relative stabilities and structures of the AuNCs, which would be beneficial for establishing the structure-property relationship of AuNCs in real applications.

Ring Model for Understanding How Interfacial Interaction Dictates the Structures of Protection Motifs and Gold Cores in Thiolate-Protected Gold Nanoclusters

Understanding the effect of interfacial interactions between the protection motifs and gold cores on the stabilities of thiolate-protected gold nanoclusters is still a challenging task. Based on analyses of 95 experimentally crystallized and theoretically predicted thiolate-protected gold nanoclusters, we present a ring model to offer a deeper insight into the interfacial interactions for this class of nanoclusters. In the ring model, all the gold nanoclusters can be generically viewed as a fusion or interlocking of several [Au_m(SR)_n] (m = 4-8, 10, and 12 and 0 ≤ n ≤ m) rings. Guided by the ring model and the grand unified model, a new Au₄₂(SR)₂₆ isomer is predicted, whose total energy is lower than those of two previously crystallized isomers. The ring model offers a mechanistic understanding of the interactions between the protection ligands and gold cores and practical guidance on predicting new gold nanoclusters for future experimental synthesis and confirmation.

Modulation of the Double-Helical Cores: A New Strategy for Structural Predictions of Thiolate-Protected Gold Nanoclusters

A fundamental understanding of the structural growth of thiolate-protected gold nanoclusters not only benefits experimental synthesis but also will advance the methodology for structural predictions and for rational design of highly stable nanoclusters. Herein, we report numerous new structures (11 total) of thiolate-protected gold nanoclusters predicted from theoretical modulation of the double-helical cores of experimentally determined nanoclusters. Among these newly predicted structures, Au₃₂(SR)₂₂, Au₄₀(SR)₂₆, and Au₄₈(SR)₃₀ are obtained by adding a defective layer containing 4 gold atoms on a structural sequence of experimentally crystallized nanoclusters, namely, Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, and Au₄₄(SR)₂₈. The generic growth pattern underlying this sequence of nanoclusters can be viewed as adding the highly stable tetrahedral Au₄ unit on the double-helical cores. Likewise, the other eight newly predicted structures, including two groups of isomeric structures corresponding to the sequence of experimentally determined Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, Au₄₄(SR)₂₈, and Au₅₂(SR)₃₂ nanoclusters, are successfully predicted. Density functional theory calculations show that these 11 newly predicted nanoclusters exhibit large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and all-positive harmonic vibrational frequencies, suggesting their high chemical stabilities. Additional analyses on the structures and properties suggest that these newly predicted nanoclusters are very likely to be synthesized in the laboratory. Confirmation by experiments would validate the new strategy for structural prediction of thiolate-protected gold nanoclusters by taking advantage of a large structure database of crystallized ligand-protected gold nanoclusters with a variety of gold cores.

Structural predictions of thiolate-protected gold nanoclusters via the redistribution of Au–S “staple” motifs on known cores

Four structures of gold nanoclusters were predicted via the redistribution of Au–S motifs on known cores.

Application of Electronic Counting Rules for Ligand-Protected Gold Nanoclusters

Understanding special stability of numerous ligand-protected gold nanoclusters has always been an active area of research. In the past few decades, several theoretical models, including the polyhedral skeletal electron pair theory (PSEPT), superatom complex (SAC), and superatom network (SAN), among others, have been developed for better understanding the stabilities and structures of selected ligand-protected gold nanoclusters. This Account overviews the recently proposed grand unified model (GUM) to offer some new insights into the structures and growth mechanism of nearly all crystallized and predicted ligand-protected gold nanoclusters. The main conceptual advancement of the GUM is identification of the duet and octet rules on the basis of the "big data" of 70+ reported ligand-protected gold nanoclusters. According to the two empirical rules, the cores of the gold nanoclusters can be regarded as being composed of two kinds of elementary blocks (namely, triangle Au₃ and tetrahedron Au₄), each having 2 e closed-shell valence electrons (referred as Au₃(2 e) and Au₄(2 e)), as well as the secondary block (icosahedron Au₁₃) with 8 e closed-shell valence electrons (referred as Au₁₃(8 e)). The two elementary blocks (Au₃(2 e) and Au₄(2 e)) and the secondary block (Au₁₃(8 e)), from electron counting point of view, can be regarded as an analogy of the highly stable noble-gas atoms of He and Ne, respectively. In each elementary block, the Au atoms exhibit three different valence-electron states (i.e., 1 e, 0.5 e, and 0 e), depending on the type of ligands bonded with these Au atoms. Such three valence-electron states are coined as three "flavors" of gold (namely, bottom, middle, and top "flavor"), a term borrowed from the quark model in the particle physics. Upon application of the duet and octet rules with accounting the three valence states of gold atoms, the Au₃(2 e), Au₄(2 e), and Au₁₃(8 e) blocks can exhibit 10 (denoted as Δ₁-Δ₁₀), 15 (denoted as T₁-T₁₅), and 91 (denoted as I₁-I₉₁) variants of valence states, respectively. When packing these blocks (with distinct electronic states) together, it forms the gold core of ligand-protected gold nanocluster. As such, the special stabilities of the ligand-protected gold nanoclusters are explained based on the local stability of each block. With GUM, rich and complex structures of ligand-protected gold nanoclusters have been analyzed through structure anatomy. Moreover, the growth of these clusters can be simply viewed as sequential addition of the blocks, rather than as addition of the gold atoms. Another useful application of the GUM is to analyze the structural isomerism. The three types of isomerism for the gold nanoclusters, i.e., core, staple, and complex isomerism, can be considered as an analogy of chain, point, and functional isomerism (known in organic chemistry), respectively. GUM can be applied to predict new clusters, thereby guiding experimental synthesis. Indeed, a number of ligand-protected gold nanoclusters with high stabilities were rationally designed based on the GUM.

Toward the Tailoring Chemistry of Metal Nanoclusters for Enhancing Functionalities

Ultrasmall metal nanoparticles (often called nanoclusters) possess unique geometrical structures and novel functionalities that are not accessible in conventional nanoparticles. Recent progress in their synthesis and structural determination by X-ray crystallography has led to deep understanding of the structural evolution, structure-property correlation, and growth modes, such as the layer-by-layer growth in face-centered cubic (fcc)-type nanoclusters, linear assembly of vertex-shared icosahedral units, and other unique modes. The enriched knowledge on the correlation between the structure and the properties has rendered metal nanoclusters a new class of functional nanomaterials. Despite the significant achievements in structural determinations, mapping out the structure-property correlation is still very challenging because of the core-shell structures of nanoclusters (e.g., Au _n(SR) _m protected by thiolate ligands) with metal atoms partitioned between the core and the shell. In such structures, the core and the surface are entangled and cannot be separately studied because changing the core structure would inevitably change the surface (or vice versa). Thus, it is of great importance to develop the "tailoring" chemistry for structural modification of the core (or surface) while retaining the other parts, in order to achieve fundamental understanding of what part of the nanocluster structure plays what role in the functionalities. In this Account, we summarize some recent work on the strategies to control the atomic structures of metal nanoclusters for tuning their properties, such as stability, optical absorption, excited-state electron dynamics, and photoluminescence, as well as their catalytic reactivity. The development of a ligand-based strategy has permitted the synthesis of structural isomers of nanoclusters with the same size but different functionalities. Successful modification of the core (or surface) structure while maintaining the other components has led us to gain some fundamental understanding of the respective roles of the core and the surface in the nanocluster functionalities. Such "tailoring" chemistry on metal nanoclusters can provide a strong basis for functional nanomaterials consisting of nanocluster components with desired properties. Further development of the tailoring chemistry will guide materials chemists to new directions and tailor-made functional nanomaterials for specific applications.

Gold Fusion: From Au25(SR)18 to Au38(SR)24, the Most Unexpected Transformation of a Very Stable Nanocluster

The study of the molecular cluster Au₂₅(SR)₁₈ has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au₂₅(SR)₁₈⁰ undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au₃₈(SR)₂₄. We tested this reaction with a series of Au₂₅ clusters. The fusion was confirmed and characterized by UV-vis absorption spectroscopy, ESI mass spectrometry, ¹H and ¹³C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au₂₅(SR)₁₈⁰ appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au₁₃ building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters.

The structural isomerism in gold nanoclusters

The isomerism in thiolate-protected gold (Au) nanoclusters is important for the understanding of structure-property correlations and the design of Au nanoclusters with specific structures and properties. Although recent studies have identified stereoisomerism, the understanding of structural isomerism is still lacking. Herein, we identified three distinct mechanisms of structural isomerism (i.e., core isomerism, staple isomerism, and complex isomerism) based on the crystallized isomers of thiolate-protected Au nanoclusters, and these mechanisms can be viewed as analogous to those of the structural isomerism in organic molecules (i.e., chain isomerism, point isomerism, and functional isomerism). Using the discovered core isomerism and staple isomerism, two Au28(SR)20 isomers are predicted and their synthesis feasibilities are illuminated. These new insights into the structural isomerism can facilitate rational design of new isomers of thiolate-protected Au nanoclusters and guide future experimental synthesis.

Relativistic DFT investigation of electronic structure effects arising from doping the Au25 nanocluster with transition metals

We perform a theoretical investigation using density functional theory (DFT) and time-dependent DFT (TDDFT) on the doping of the Au₂₅(SR)₁₈^-1 nanocluster with group IX transition metals (M = cobalt, rhodium and iridium). Different doping motifs, charge states and spin multiplicities were considered for the single-atom doped nanoclusters. Our results show that the interaction (or the lack of interaction) between the d-type energy levels that mainly originate from the dopant atom and the super-atomic levels plays an important role in the energetics, the electronic structure and the optical properties of the doped systems. The evaluated MAu₂₄(SR)₁₈^q (q = -1, -3) systems favor an endohedral disposition of the doping atom typically in a singlet ground state, with either a 6- or 8-valence electron icosahedral core. For the sake of comparison, the role of the d energy levels in the electronic structure of a variety of doped Au₂₅(SR)₁₈^-1 nanoclusters was investigated for dopant atoms from other families such as Cd, Ag and Pd. Finally, the effect of spin-orbit coupling (SOC) on the electronic structure and absorption spectra was determined. The information in this study regarding the relative energetics of the d-based and super-atom energy levels can be useful to extend our understanding of the preferred doping modes of different transition metals in protected gold nanoclusters.