Insight into the Geometric and Electronic Structures of Gold/Silver Superatomic Clusters Based on Icosahedron M 13 Units and Their Alloys

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.

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.

Superatomic Aromaticity in Cyclic Superatomic Molecules: Ligand-Protected Penta-Icosahedral [M@Au11]5 (M = Au, Pt) Clusters

Pure or doped gold icosahedra, which can be generally viewed as superatoms, are promising candidates for cluster-assembled structures. As the first large-scale ring-like gold cluster, the report of [Au60Se2(Ph3P)10(SeR)15]+ has arisen much interest, where its Au60 core is composed of five vertex-sharing gold icosahedra in a cyclic way. From electronic characters, this Au60 core is a 40e cyclic penta-superatom network formed by five 8e closed-shell superatoms (S2P6). When more valence electrons are introduced into the penta-superatom network by atomic doping, global delocalized bonds are induced in its bonding framework. In the 42e Au60 core of the [Au60Se2Cl15]- cluster, two extra electrons occupy one delocalized π-bonding orbital formed by super D orbitals of five superatoms, resulting in superatomic π aromaticity. In the 46e [Pt@Au11]5 core of [(Pt@Au11)5Ga2Cl15] cluster, three delocalized super-π bonds are formed, which are organized in the similar way as the aromatic C5H5- molecule. The unveiling of superatomic aromaticity promotes our understanding of the stability of cyclic superatom assemblies and extends the family of superatomic bonding patterns.

Total Structure, Structural Transformation and Catalytic Hydrogenation of [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- Constructed from Twisted Cu13 Units

Herein, a remarkable achievement in the synthesis and characterization of an atomically precise copper-hydride nanocluster, [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- via a mild one-pot reaction is presented. Through X-ray crystallography analysis, it is revealed that [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- exhibits a unique shell-core-shell structure. The inner Cu29 kernel is composed of three twisted Cu13 units, connected through Cu4 face sharing. Surrounding the metal core, two Cu6 metal shells, resembling a protective sandwich structure are observed. This arrangement, along with intracluster π···π interactions and intercluster C─H···F─C interactions, contributes to the enhanced stability of [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- . The presence, number, and location of hydrides within the nanocluster are established through a combination of experimental and density functional theory investigations. Notably, the addition of a phosphine ligand triggers a fascinating nanocluster-to-nanocluster transformation in [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- , resulting in the generation of two nanoclusters, [Cu14 (SC6 H3 F2 )3 (PPh3 )8 H10 ]+ and [Cu13 (SC6 H3 F2 )3 (P(PhF)3 )7 H10 ]0 . Furthermore, it is demonstrated that [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- exhibits catalytic activity in the hydrogenation of nitroarenes. This intriguing nanocluster provides a unique opportunity to explore the assembly of M13 units, similar to other coinage metal nanoclusters, and investigate the nanocluster-to-nanocluster transformation in phosphine and thiol ligand co-protected copper nanoclusters.

Effect of Specific Heavy Doping of Silver Atoms into the Icosahedral Au13 on Electronic Structure and Catalytic Performance

The exploration of specific heavy doping of silver atoms into icosahedral Au13 clusters and their electronic structures and properties has been somewhat limited. Herein, we report two heavily Ag doped nanoclusters, [Au7Ag6(C7H4NOS)4(Dppf)3Cl]0 and [Au7Ag6(C7H4NOS)3(Dppf)3Cl](SbF6) (Au7Ag6-0 and Au7Ag6-1, respectively) [C7H4NOSH = 2-mercaptobenzoxazole, and Dppf = 1,1'-bis(diphenylphosphino)ferrocene]. The electronic structures and superatomic orbitals of nanoclusters were determined by density functional theory (DFT) calculations, and the energy degeneracy of the superatomic orbitals of Au7Ag6-1 is higher than that of Au7Ag6-0. Transient absorption spectroscopy was performed, revealing that Au7Ag6-0 significantly extends the excited-state lifetime. Both nanoclusters were supported on activated carbon for the oxygen reduction reaction. DFT calculations confirm that the catalytic activities mainly stem from the carbon atom of ferrocene rather than the iron atom. This study not only sheds light on the preparation of icosahedral alloy clusters but also provides insights into the regulation of icosahedral superatomic structure and electrocatalytic properties.

Elucidation of the electronic structures of thiolate-protected gold nanoclusters by electrochemical measurements

Metal nanoclusters (NCs) with sizes of approximately 2 nm or less have different physical/chemical properties from those of the bulk metals owing to quantum size effects. Metal NCs, which can be size-controlled and heterometal doped at atomic accuracy, are expected to be the next generation of important materials, and new metal NCs are reported regularly. However, compared with conventional materials such as metal complexes and relatively large metal nanoparticles (>2 nm), these metal NCs are still underdeveloped in terms of evaluation and establishment of application methods. Electrochemical measurements are one of the most widely used methods for synthesis, application, and characterisation of metal NCs. This review summarizes the basic knowledge of the electrochemistry and experimental techniques, and provides examples of the reported electronic states of thiolate-protected gold NCs elucidated by electrochemical approaches. It is expected that this review will provide useful information for researchers starting to study metal NCs.

A heteroleptic fused bi-cuboctahedral Cu21S2 cluster

A new dicationic cluster, [Cu₂₁S₂{S₂CNⁿBu₂}₉(C₂Ph)₆]²⁺, where the Cu21S2 kernel consists of two S@Cu₁₂ cuboctahedra sharing a triangular Cu₃ face is reported. Its waist part is bridged by three dithiocarbamate ligands, each in a hexaconnective, hexametallic (μ₃, μ₃) coordination pattern, an unprecedented feature in Cu nanocluster chemistry.

Recent progress in dichalcophosphate coinage metal clusters and superatoms

Atomically precise clusters of group 11 metals (Cu, Ag, and Au) attract considerable attention owing to their remarkable structure and fascinating properties. One of the unique subclasses of these clusters is based on dichalcophosphate ligands of [(RO)₂PE₂]^- type (E = S or Se, and R = alkyl). These ligands successfully stabilise the most diverse Cu, Ag, and Au clusters and superatoms, spanning from simple ones to amazing assemblies featuring unusual structural and bonding patterns. It is noteworthy that such complicated clusters are assembled directly from cheap and simple reagents, metal(I) salts and dichalcophosphate anions. This reaction, when performed in the presence of a hydride or other anion sources, or foreign metal ions, results in hydrido- or anion-templated homo- or heteronuclear structures. In this feature article, we survey the recent advances in this exciting field, highlighting the powerful synthetic capabilities of the system "a metal(I) salt - [(RO)₂PX₂]^- ligands - a templating anion or borohydride" as an inexhaustible platform for the creation of new atomically precise clusters, superatoms, and nanoalloys.

Effect of total charge on the electronic structure of thiolate-protected X@Ag12 superatoms (X = Ag, Au)

Electronic structures of chemically synthesized silver-based clusters [XAg₁₆(TBBT)₁₂]^3- (X = Ag or Au; TBBT = 4-tert-butylbenzenethiolate) having an icosahedral X@Ag₁₂ superatomic core were studied by gas-phase photoelectron spectroscopy and density functional theory calculations. The electron binding energy of the highest occupied molecular orbital (HOMO) with a 1P superatomic nature was determined to be 0.23 and 0.29 eV for X = Ag or Au, respectively. Resonant tunnelling electron emission through the repulsive Coulomb barrier (RCB) was observed. From the kinetic energy of the tunnelling electrons, it was estimated that the lowest unoccupied molecular orbital (LUMO) was supported at 1.51 and 1.62 eV above the vacuum level by the RCB for X = Ag or Au, respectively. The HOMO of [XAg₁₆(TBBT)₁₂]^3- (X = Ag or Au) was destabilized by 3.74 and 3.71 eV, respectively, compared with those of [XAg₂₄(DMBT)₁₈]^- (DMBT = 2,4-dimethylbenzenethiolate) having the icosahedral X@Ag₁₂ core due to the larger negative charge imparted by the ligand layers.

Composition-Dependent Enzyme Mimicking Activity and Radiosensitizing Effect of Bimetallic Clusters to Modulate Tumor Hypoxia for Enhanced Cancer Therapy

Alloying is an efficient chemistry to tailor the properties of metal clusters. As a class of promising radiosensitizers, most previously reported metal clusters exhibit unitary function and cannot overcome radioresistance of hypoxic tumors. Here, atomically precise alloy clusters Pt₂ M₄ (M = Au, Ag, Cu) are synthesized with bright luminescence and adequate biocompatibility, and their composition-dependent enzyme mimicking activity and radiosensitizing effect is explored. Specifically, only the Pt₂ Au₄ cluster displays catalase-like activity, while the others do not have clusterzyme properties, and its radiosensitizing effect is the highest among all the alloy clusters tested. By taking advantage of the sustainable production of O₂ via the decomposition of endogenous H₂ O₂ , the Pt₂ Au₄ cluster modulates tumor hypoxia as well as increases the efficacy of radiotherapy. This work thus advances the cluster alloying strategy to produce multifunctional therapeutic agents for improving hypoxic tumor therapy.

Evaluation of ultrasmall coinage metal M13(dppe)6 M = Cu, Ag, and Au clusters. Bonding, structural and optical properties from relativistic DFT calculations

Ultrasmall ligand-protected clusters are prototypical species for evaluating the variation at the bottom of the nanoscale range. Here we explored the ultrasmall gold-phosphine M₁₃(dppe)₆ cluster, as a prototypical framework to gain insights into the fundamental similarities and differences between Au, Ag, and Cu, in the 1-3 nm size range, via relativistic DFT calculations. Different charge states involving 8- and 10-cluster electron (ce) species with a 1S²1P⁶ and 1S²1P⁶1D² configuration, leading to structural modification in the Au species between Au₁₃(dppm)₆⁵⁺ and Au₁₃(dppm)₆³⁺, respectively. Furthermore, this structural distortion of the M₁₃ core is found to occur to a lower degree for the calculated Ag and Cu counterparts. Interestingly, optical properties exhibit similar main patterns along with the series, inducing a blue-shift for silver and copper, in comparison to the gold parent cluster. For 10-ce species, the main features of 8-ce are retained with the appearance of several weak transitions in the range. The ligand-core interaction is enhanced for gold counterparts and decreased for lighter counterparts resulting in the Au > Cu > Ag trend for the interaction stabilization. Hence, the Ag and Cu counterparts of the Au₁₃(dppm)₆ cluster appear as useful alternatives, which can be further explored towards different cluster alternatives for building blocks for nanostructured materials.

Rod-Shaped Silver Supercluster Unveiling Strong Electron Coupling between Substituent Icosahedral Units

The first linear silver supercluster based on icosahedral Ag₁₃ units has been constructed via bridging of dpa ligands: Ag₆₁(dpa)₂₇(SbF₆)₄ (Hdpa = dipyridylamine) (Ag₆₁). Single-crystal X-ray diffraction reveals that this rod-shaped cluster consists of four vertex-sharing Ag₁₃ icosahedra in a linear arrangement. This Ag₆₁ cluster represents the longest one-dimensional metal nanocluster with a resolved structure. Unprecedented electron coupling develops between their constituent Ag₁₃ units. Theoretical studies disclose that the stabilities of the two superclusters are dictated by a strong interaction between the Ag₁₃ units as well as the ligand effect of the dpa-Ag motifs. The quantum size effect accounts for the significant enhancement of the metal-related absorptions and the red shift at the near-infrared region as the length of the cluster increases. This work sheds light on the evolution of one-dimensional materials and an understanding of the electronic communication between the constituent clusters.

Total structural determination of alloyed Au15.37Cu16.63(S-Adm)20 nanoclusters with double superatomic chains

Thiolated-alloy nanocluster Au15.37Cu16.63(S-Adm)20 ((AuCu)32 for short) has been synthesized. Single crystal X-ray crystallography (SC-XRD) proved that this cluster contains a Au14Cu6 core, which is composed of double superatom chains, two M4(SR)5 (M = Au/Cu) motifs, four Cu(SR)2 monomer staples and two SR molecules at the waist. The composition of this nanocluster is confirmed by SC-XRD and further verified by XPS, EDX and 2H-NMR. Surprisingly, double superatom chains connect to each other only via two Au-Au bonds, which makes the kernel resemble a tunnel. Combined with DFT calculation and electronic structure analysis, it is further proved that the (AuCu)32 nanocluster contains six 2e alloy superatomic Au3Cu2+ units. This work is the first report showing that superatom units (CuAu32+) self-assembling to a hollow kernel maintain the superatom characteristic of metal nanoclusters, which enriches the fundamental knowledge of metal superatom clusters. The stable electronic structure of the nanocluster provides an experimental basis for theoretical analysis in the future. In addition, the hollow structure may be promising in catalytic applications.

Chiroptical activity of Au13 clusters: experimental and theoretical understanding of the origin of helical charge movements

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.

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.

Aggregation-induced phosphorescence sensitization in two heptanuclear and decanuclear gold-silver sandwich clusters

The strategy of aggregation-induced emission enhancement (AIEE) has been proven to be efficient in wide areas and has recently been adopted in the field of metal nanoclusters. However, the relationship between atomically precise clusters and AIEE is still unclear. Herein, we have successfully obtained two few-atom heterometallic gold-silver hepta-/decanuclear clusters, denoted Au6Ag and Au9Ag, and determined their structures by X-ray diffraction and mass spectrometry. The nature of the Au^I⋯Ag^I interactions thereof is demonstrated through energy decomposition analysis to be far-beyond typical closed-shell metal-metal interaction dominated by dispersion interaction. Furthermore, a positive correlation has been established between the particle size of the nanoaggregates and the photoluminescence quantum yield for Au6Ag, manifesting AIEE control upon varying the stoichiometric ratio of Au : Ag in atomically-precise clusters.

Electron counting and bonding patterns in assemblies of three and more silver-rich superatoms

DFT calculations were carried out on a series of cluster cores, the framework of which was made of the condensation of several Pt@Ag12-centered icosahedra. Icosahedral condensations through vertex-sharing, face-sharing, and interpenetration were considered and their favored electron counts were determined from their stable closed-shell configurations. A large number of the computed assemblies of n icosahedral superatomic units can be considered as isolobal analogs of stable, closed-shell n-atom molecules, most of them obeying the octet rule. The larger the degree of fusion between icosahedra, the stronger the interaction between them. For example, it was possible to design 3-icosahedral supermolecular cores analogous to CO2, SF2, or [I3]-, but also to the not-yet-isolated cyclic O3. Supermolecules equivalent to non-stable molecules can also be designed. Indeed, differences exist between atoms and superatoms, and original icosahedra assemblies with no "molecular" analogs are also likely to exist, especially with compact structures and/or systems made of a large number of fused superatoms.

Halogen effects on the electronic and optical properties of Au13 nanoclusters

We report an experimental and theoretical investigation of the electronic and optical properties of a series of icosahedral Au₁₃ nanoclusters, protected using different halogen ligands (Cl, Br, and I), as well as 1,2-bis(diphenylphosphino)ethane (dppe) ligands. All three clusters are comprised of the same Au₁₃ kernel with two halogens coordinated to the poles of the icosahedral cluster along with five dppe ligands. UV-vis absorption spectra indicate a systematic red shift from Cl to Br to I, as well as a sudden enhancement of the second excitonic peak for the I-coordinated cluster. Density functional theory (DFT) calculations suggest that all clusters possess a wide HOMO-LUMO energy gap of ∼1.79 eV and are used to assign the first two excitonic bands. Frontier orbital analyses reveal several HOMO → LUMO transitions involving halogen-to-metal charge transfers. For the I-coordinated cluster, more complicated I-to-metal charge transfers give rise to different excitation features observed experimentally. The current findings show that halogen ligands play important roles in the electronic structures of gold clusters and can be utilized to tune the optical properties of the clusters.

Polymorphism of Au11(PR3)7Cl3 clusters: understanding C-H⋯π interaction and C-H⋯Cl-C van der Waals interaction on cluster assembly by surface modification

The C-H⋯π interaction and the C-H⋯Cl-C van der Waals interaction play a crucial role in the crystallization of nanoclusters. In this paper, we present an example of a crystal system transformation of Au11(PR3)7Cl3 from monoclinic (M) to trigonal (T) by surface modification. Atomically-resolved gold nanoclusters containing tris(4-chlorophenyl)phosphine and chloride ligands were synthesized and determined to be Au11(p-ClPPh3)7Cl3 (p-ClPPh3 = tris(4-chlorophenyl)phosphine) by X-ray crystallography. Crystal data demonstrated that the C-H⋯Cl-C interaction is dominant in a trigonal crystal system of Au11(p-ClPPh3)7Cl3 with a R3̄ space group. However, the C-H⋯π interaction is the major driving force to form a monoclinic crystal system of Au11(PPh3)7Cl3 (PPh3 = triphenylphosphine) with a P2(1)/n space group. Moreover, UV-vis absorption spectra and X-ray photoelectron spectra reveal that the electronic structure of the Au11(p-ClPPh3)7Cl3 nanocluster is greatly influenced by p-ClPPh3. This work provides critical implications for the crystallization of metal nanoclusters, as well as a better understanding of the non-covalent interaction on the nanocluster assembly and the crystal engineering by surface modification.

Heterogeneous metal alloy engineering: embryonic growth of M13icosahedra in Ag-based alloy superatomic nanoclusters

Alloying is an effective tool to comprehend the packing mechanism and adjust the properties of nanomaterials.