[Ag 21 {S 2 P(O i Pr) 2 } 12 ] + : An Eight-Electron Superatom

Hydride-doped coinage metal superatoms and their catalytic applications

Superatomic constructs have been identified as a critical component of future technologies. The isolation of coinage metal superatoms relies on partially reducing metallic frameworks to accommodate the mixed valent state required to generate a superatom. Controlling this reduction requires careful consideration in reducing the agent, temperature, and the ligand that directs the self-assembly process. Hydride-based reducing agents dominate the synthetic wet chemical routes to coinage metal clusters. However, within this category, a unique subset of superatoms that retain a hydride/s within the nanocluster post-reduction have emerged. These stable constructs have only recently been characterized in the solid state and have highly unique structural features and properties. The difficulty in identifying the position of hydrides in electron-rich metallic constructs requires the combination and correlation of several analytical methods, including ESI-MS, NMR, SCXRD, and DFT. This text highlights the importance of NMR in detecting hydride environments in these superatomic systems. Added to the complexity of these systems is the dual nature of the hydride, which can act as metallic hydrogen in some cases, resulting in entirely different physical properties. This review includes all hydride-doped superatomic nanoclusters emphasizing synthesis, structure, and catalytic potential.

Synthesis of Two Neutral Silver Alkynyl Nanoclusters by a Single Divalent Tetrahedral Anion Template and a Study of Their Optical Features

The synthesis of nanoclusters from simple structural units is usually a challenging process because of the complexity and unpredictability of the self-assembly process of these types of compounds. Herein, two new neutral 19-nuclearity silver nanoclusters based on alkynyl ligands with the formulas [(CrO₄)@Ag₁₉(C≡C^tBu)₈(Ph₂PO₂)₆(tfa)₃(CH₃OH)₂] (1) and [(SO₄)@Ag₁₉(C≡C^tBu)₈(Ph₂PO₂)₆(tfa)₃(CH₃OH)₂] (2), in which tfa = trifluoroacetate, were synthesized, and their structures were investigated by single-crystal and powder X-ray diffraction, electrospray ionization mass spectrometry, elemental analyses, and Fourier transform infrared spectroscopy. The surface ligands of Ph₂PO₂H and trifluoroacetate were assembled through hydrogen bonding, metal-aromatic interactions, and coordination bonding around 19 silver atoms as the metal skeletons of the nanoclusters. Sulfate and chromate anions, as a template within the metal skeleton of clusters through bonding with silver atoms, stabilized the structure. In addition, the UV-vis absorption spectroscopy, luminescence properties, and thermal stability of the nanoclusters were investigated.

Looking at platinum carbonyl nanoclusters as superatoms

Although the chemistry of carbonyl-protected platinum nanoclusters is well established, their bonding mode remains poorly understood. In most of them, the average Pt oxidation state is zero or slightly negative, leading to the apparent average configuration 5d¹⁰ 6s^ε (ε = 0 or very small) and the apparent conclusion that metal-metal bonding cannot arise from the completely filled 5d shell nor from the empty (or almost empty) 6s orbitals. However, DFT calculations show in fact that in these species the actual average configuration is 5d^10-x 6s^x, which provides to the whole cluster a significant total number of 6s electrons that ensures metal-metal bonding. This ("excited") average configuration is to be related to that of coinage metals in ligated group 11 nanoclusters (nd¹⁰ (n + 1)s^x). Calculations show that metal-metal bonding in most of these platinum nanoclusters can be rationalized within the concepts of superatoms and supermolecules, in a similar way as for group 11 nanoclusters. The "excited" 5d^10-x 6s^x configuration results from a level crossing between 5d combinations and 6s combinations, the former transferring their electrons to the latter. This level crossing, which does not exist in the bare Pt_n clusters, is induced by the ligand shell, the role of which being thus not innocent with respect to metal-metal bonding.

Ligand Engineering toward the Trade-Off between Stability and Activity in Cluster Catalysis

We report the structures, stability and catalysis properties of two Ag₂₁ nanoclusters, namely [Ag₂₁ (H₂ BTCA)₃ (O₂ PPh₂ )₆ ]SbF₆ (1) and [Ag₂₁ (C≡CC₆ H₃ -3,5-R₂ )₆ (O₂ PPh₂ )₁₀ ]SbF₆ (2) (H₄ BTCA=p-tert-butylthiacalix[4]arene, R=OMe). Both Ag₂₁ structures possess an identical icosahedral kernel that is surrounded by eight peripheral Ag atoms. Single-crystal structural analysis and ESI-MS revealed that 1 is an 8-electron cluster and 2 has four free electrons. Theoretical results show that the P-symmetry orbitals are found as HOMO-1 and HOMO states in 1, and the frontier unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) show D-character, indicating 1 is a superatomic cluster with an electronically closed shell 1S² 1P⁶ , while 2 has an incomplete shell configuration 1S² 1P² . These two Ag₂₁ clusters show superior stability under ambient conditions, and 1 is robust even at 90 °C in toluene and under oxidative conditions (30 % H₂ O₂ ). Significantly, 2 exhibits much higher activity than 1 as catalyst in the reduction of 4-nitrophenol. This work demonstrates that ligands can influence the electronic structures of silver clusters, and further affect their stability and catalytic performance.

Interstitial hydrides in nanoclusters can reduce M(I) (M = Cu, Ag, Au) to M(0) and form stable superatoms

High-nuclearity clusters resemble the closest model between the determination of atomically precise chemical species and the bulk metallic version thereof, and both impacts on a variety of applications, including catalysis, optics, sensors, and new energy sources.  Our interest lies with the nanoclusters of the Group 11 (Cu, Ag, Au) metals stabilized by dichalcogenido and hydrido ligands.  Herein, we describe superatoms formed by the clusters and their relationship with precursor hydrido clusters.  Specifically, our concept seeks to demonstrate a possible correlation that exist between hydrido clusters (and nanoalloys) and the formation of superatoms, with the loss of hydrides and typically with release of H 2 gas.  These reactions appear to be internal self-redox reactions and require no additional reducing agent, but does seem to require a similar core structure.  Knowledge of such processes could provide insight into how clusters grow and an understanding in bridging the atomically precise cluster - metal nanoparticle mechanism.

A Molecular Hybrid of an Atomically Precise Silver Nanocluster and Polyoxometalates for H2 Cleavage into Protons and Electrons

Atomically precise silver (Ag) nanoclusters are promising materials as catalysts, photocatalysts, and sensors because of their unique structures and mixed-valence states (Ag⁺ /Ag⁰ ). However, their low stability hinders the in-depth study of their intrinsic reactivity and catalytic property accompanying their redox processes. Herein, we demonstrate that a molecular hybrid of an atomically precise {Ag₂₇ }¹⁷⁺ nanocluster and polyoxometalates (POMs) can efficiently cleave H₂ into protons and electrons. The Ag nanocluster accommodates electrons through the redox reaction from {Ag₂₇ }¹⁷⁺ to {Ag₂₇ }¹³⁺ , and the POM ligands play the following important roles: (i) a significant stabilization of the typically unstable Ag nanocluster to preserve its structure during the redox reaction with H₂ , (ii) formation of a unique interface between the Ag nanocluster and metal oxides for efficient H₂ cleavage, and (iii) storage of the generated protons on the negatively charged basic surface.

Giant Emission Enhancement of Solid-State Gold Nanoclusters by Surface Engineering

Ligand-induced surface restructuring with heteroatomic doping is used to precisely modify the surface of a prototypical [Au₂₅ (SR¹ )₁₈ ]^- cluster (1) while maintaining its icosahedral Au₁₃ core for the synthesis of a new bimetallic [Au₁₉ Cd₃ (SR² )₁₈ ]^- cluster (2). Single-crystal X-ray diffraction studies reveal that six bidentate Au₂ (SR¹ )₃ motifs (L2) attached to the Au₁₃ core of 1 were replaced by three quadridentate Au₂ Cd(SR² )₆ motifs (L4) to create a bimetallic cluster 2. Experimental and theoretical results demonstrate a stronger electronic interaction between the surface motifs (Au₂ Cd(SR² )₆ ) and the Au₁₃ core, attributed to a more compact cluster structure and a larger energy gap of 2 compared to that of 1. These factors dramatically enhance the photoluminescence quantum efficiency and lifetime of crystal of the cluster 2. This work provides a new route for the design of a wide range of bimetallic/alloy metal nanoclusters with superior optoelectronic properties and functionality.

Evidence for the Superatom-Superatom Bonding from Bond Energies

Metal clusters with specific number of valence electrons are described as superatoms. Super valence bond (SVB) model points out that superatoms could form the superatomic molecules through SVBs by sharing nucleus and electrons. The existence of superatom-superatom bonding was verified by the shape of their orbitals in former studies. In this paper, another important evidence-bond energy is studied as the criterion for the SVBs using the density functional theory method. In order to get the reliable values of bond energies, the series of Zn-Cu and Mg-Li superatomic molecules composed of two tetrahedral superatoms which do not share their nucleus are designed. Considering the number of the valence electrons in one tetrahedral superatomic unit, (Zn₄)₂/(Mg₄)₂, (Zn₃Cu)₂/(Mg₃Li)₂, (Zn₂Cu₂)₂/(Mg₂Li₂)₂, and (ZnCu₃)₂/(MgLi₃)₂ clusters are 8e-8e, 7e-7e, 6e-6e, and 5e-5e binary superatomic molecules with super nonbond, single bond, double bond, and triple bond, respectively, which are verified by chemical bonding analysis depending on the SVB model. Further calculations reveal that the bond energies increase and the bond lengths decrease along with the bond orders in Zn-Cu and Mg-Li systems which is in accordance with the classical nonbond, single bond, double bond, and triple bond in C-H systems. Thus, these values of bond energies confirm the existence of the SVBs. Moreover, electron localization function analysis is also carried on to describe the similarity between the superatomic bonds and atomic bonds in simple molecules directly. This study reveals the new evidence for the existence of the superatom-superatom bonding depending on the bond energies, which gives the new insight for the further investigation of the superatomic clusters.

Kernel Homology in Gold Nanoclusters

Homology is well known in organic chemistry; however, it has not yet been reported in nanochemistry. Herein, we introduce the concept of kernel homology to describe the phenomenon of metal nanoclusters sharing the same "functional group" in kernels with some similar properties. To illustrate this point, we synthesized two novel gold nanoclusters, Au₄₄ (TBBT)₂₆ and Au₄₈ (TBBT)₂₈ (TBBTH=4-tert-butylbenzenethiol), and solved their total structures by X-ray crystallography, which reveals that they have the same Au₂₃ bi-icosahedron capped with a similar bottom cap (Au₆ and Au₈ , respectively) in the kernels. The two novel gold nanoclusters, together with the existing Au₃₈ (PET)₂₄ nanocluster (PETH=phenylethanethiol), have the same "functional group"-Au₂₃ -in their kernels and have some similar properties (e.g., electrochemical properties); therefore, they are comparable to the homologues in organic chemistry.

The Origin of the Photoluminescence Enhancement of Gold-Doped Silver Nanoclusters: The Importance of Relativistic Effects and Heteronuclear Gold-Silver Bonds

The weak photoluminescence of silver nanoclusters prevents their broad application as luminescent nanomaterials. Recent experiments, however, have shown that gold doping can significantly enhance the photoluminescence intensity of Ag₂₉ nanoclusters but the molecular and physical origins of this effect remain unknown. Therefore, we have computationally explored the geometric and electronic structures of Ag₂₉ and gold-doped Ag_29-x Au_x (x=1-5) nanoclusters in the S₀ and S₁ states. We found that 1) relativistic effects that are mainly due to the Au atoms play an important role in enhancing the fluorescence intensity, especially for highly doped Ag₂₆ Au₃ , Ag₂₅ Au₄ , and Ag₂₄ Au₅ , and that 2) heteronuclear Au-Ag bonds can increase the stability and regulate the fluorescence intensity of isomers of these gold-doped nanoclusters. These novel findings could help design doped silver nanoclusters with excellent luminescence properties.

[Ag15(N-triphos)4(Cl4)](NO3)3: a stable Ag-P superatom with eight electrons (N-triphos = tris((diphenylphosphino)methyl)amine)

A first and stable Ag-P superatom nanocluster [Ag₁₅(N-triphos)₄(Cl₄)](NO₃)₃ (1) has been successfully synthesized and characterized. X-ray analysis shows that this Ag₁₅ cluster has a hexacapped body-centered cubic (bcc) framework which is consolidated by four tripodal N-triphos ligands. The identity of 1 is confirmed by high resolution ESI-MS. Cluster 1 has an electronic and geometric shell closure structure with 8 free electrons, matching the stability idea of superatom theory for a nanocluster. DFT calculation of this Ag₁₅ cluster reveals the superatom feature with a 1S²1P⁶ configuration. The chelation of multidentate phosphines enhances the stability of this Ag₁₅ cluster. The AgAg distances between the centered and the vertical Ag atoms of this bcc (Ag@Ag₈) are in the range of 2.57-2.71 Å, and the distances between the face-capped and the vertical silver atoms are in the range of 2.84-2.92 Å, showing strong AgAg interactions within this cluster core. This superatom complex exhibits a relatively high thermal and photolytic stability.

Johnson Solids: Anion-Templated Silver Thiolate Clusters Capped by Sulfonate

Sulfonates were incorporated into six novel high-nuclearity silver(I) thiolate clusters under the guidance of anion templates varied from S^2- , SO₄^2- , α-[Mo₅ O₁₈ ]^6- , β-[Mo₅ O₁₈ ]^6- , [Mo₂ O₈ ]^4- , to [Mo₄ O₁₄ (SO₄ )]^6- . Single crystal X-ray analysis revealed that SD/Ag1, SD/Ag3, SD/Ag5, and SD/Ag6 are discrete [S@Ag₆₀ ], [α-Mo₅ O₁₈ @Ag₃₆ ], [Mo₂ O₈ @Ag₃₀ ]₂ , and [Mo₄ O₁₄ (SO₄ )@Ag₇₃ ] clusters, respectively, whereas SD/Ag2 and SD/Ag4 are one-dimensional (1D) chains based on the [SO₄ @Ag₂₀ ] and [β-Mo₅ O₁₈ @Ag₃₆ ] cluster subunits, respectively. Their silver skeletons are protected exteriorly by thiolates and sulfonates and interiorly supported by diverse anions as templates. Structurally, cluster SD/Ag1 is a typical core-shell structure comprised of an inner Ag₁₂ cuboctahedron and an outer Ag₄₈ shell. The sulfate-templated drum-like Ag₂₀ cluster subunits are bridged by PhSO₃^- to give a 1D chain of SD/Ag2. Complex SD/Ag3 and SD/Ag4 are spindle-like Ag₃₆ clusters with isomeric [Mo₅ O₁₈ ]^6- inside, and the latter is further extended to a 1D chain through PhSO₃^- bridges. A pair of [Mo₂ O₈ ]^4- templated gourd-like Ag₃₀ clusters are dimerized in a head-to-head fashion to form SD/Ag5. Complex SD/Ag6 is the largest cluster in this family and doubly templated by unprecedented [Mo₄ O₁₄ (SO₄ )]^6- anions. Geometrically, the silver shells of SD/Ag1-SD/Ag5 show the polyhedral features of Johnson solids, instead of the usual Platonic or Archimedean solids. Solution behaviors and luminescent properties were also investigated in detail.