Isomerism in Supramolecular Adducts of Atomically Precise Nanoparticles

Carborane-Cluster-Wrapped Copper Cluster with Cyclodextrin-like Cavities for Chiral Recognition

Chiral atomically precise metal clusters, known for their remarkable chiroptical properties, hold great potential for applications in chirality recognition. However, advancements in this field have been constrained by the limited exploration of host-guest chemistry, involving metal clusters. This study reports the synthesis of a chiral Cu16(C2B10H10S2)8 (denoted as Cu16@CB8, where C2B10H12S2H2 = 9,12-(HS)2-1,2-closo-carborane) cluster by an achiral carboranylthiolate ligand. The chiral R-/S-Cu16@CB8 cluster features chiral cavities reminiscent of cyclodextrins, which are surrounded by carborane clusters, yet they crystallize in a racemate. These cyclodextrin-like cavities demonstrated the specific recognition of amino acids, as indicated by the responsive output of circular dichroism and circularly polarized luminescence signals of Cu16 moieties of the Cu16@CB8 cluster. Notably, a quantitative chiroptical analysis of amino acids in a short time and a concomitant deracemization of Cu16@CB8 were achieved. Density functional tight-binding molecular dynamics simulation and noncovalent interaction analysis further unraveled the great importance of the cavities and binding sites for chiral recognition. Dipeptide, tripeptide, and polypeptide containing the corresponding amino acids (Cys, Arg, or His residues) display the same chiral recognition, showing the generality of this approach. The functional synergy of dual clusters, comprising carborane and metal clusters, is for the first time demonstrated in the Cu16@CB8 cluster, resulting in the valuable quantification of the enantiomeric excess (ee) value of amino acids. This work opens a new avenue for chirality sensors based on chiral metal clusters with unique chiroptical properties and inspires the development of carborane clusters in host-guest chemistry.

Structural studies of supramolecular complexes and assemblies by ion mobility mass spectrometry

Recent advances in instrumentation and development of computational strategies for ion mobility mass spectrometry (IM-MS) studies have contributed to an extensive growth in the application of this analytical technique to comprehensive structural description of supramolecular systems. Apart from the benefits of IM-MS for interrogation of intrinsic properties of noncovalent aggregates in the experimental gas-phase environment, its merits for the description of native structural aspects, under the premises of having maintained the noncovalent interactions innate upon the ionization process, have attracted even more attention and gained increasing interest in the scientific community. Thus, various types of supramolecular complexes and assemblies relevant for biological, medical, material, and environmental sciences have been characterized so far by IM-MS supported by computational chemistry. This review covers the state-of-the-art in this field and discusses experimental methods and accompanying computational approaches for assessing the reliable three-dimensional structural elucidation of supramolecular complexes and assemblies by IM-MS.

A concise guide to chemical reactions of atomically precise noble metal nanoclusters

Nanoparticles (NPs) with atomic precision, known as nanoclusters (NCs), are an emerging field in materials science in view of their fascinating structure-property relationships. Ultrasmall noble metal NPs have molecule-like properties that make them fundamentally unique compared with their plasmonic counterparts and bulk materials. In this review, we present a comprehensive account of the chemistry of monolayer-protected atomically precise noble metal nanoclusters with a focus on the chemical reactions, their diversity, associated kinetics, and implications. To begin with, we briefly review the history of the evolution of such precision materials. Then the review explores the diverse chemistry of noble metal nanoclusters, including ligand exchange reactions, ligand-induced structural transformations, and reactions with metal ions, metal thiolates, and halocarbons. Just as molecules do, these precision materials also undergo intercluster reactions in solution. Supramolecular forces between these systems facilitate the creation of well-defined hierarchical assemblies, composites, and hybrid materials. We conclude the review with a future perspective and scope of such chemistry.

Precision nanoengineering for functional self-assemblies across length scales

As nanotechnology continues to push the boundaries across disciplines, there is an increasing need for engineering nanomaterials with atomic-level precision for self-assembly across length scales, i.e., from the nanoscale to the macroscale. Although molecular self-assembly allows atomic precision, extending it beyond certain length scales presents a challenge. Therefore, the attention has turned to size and shape-controlled metal nanoparticles as building blocks for multifunctional colloidal self-assemblies. However, traditionally, metal nanoparticles suffer from polydispersity, uncontrolled aggregation, and inhomogeneous ligand distribution, resulting in heterogeneous end products. In this feature article, I will discuss how virus capsids provide clues for designing subunit-based, precise, efficient, and error-free self-assembly of colloidal molecules. The atomically precise nanoscale proteinic subunits of capsids display rigidity (conformational and structural) and patchy distribution of interacting sites. Recent experimental evidence suggests that atomically precise noble metal nanoclusters display an anisotropic distribution of ligands and patchy ligand bundles. This enables symmetry breaking, consequently offering a facile route for two-dimensional colloidal crystals, bilayers, and elastic monolayer membranes. Furthermore, inter-nanocluster interactions mediated via the ligand functional groups are versatile, offering routes for discrete supracolloidal capsids, composite cages, toroids, and macroscopic hierarchically porous frameworks. Therefore, engineered nanoparticles with atomically precise structures have the potential to overcome the limitations of molecular self-assembly and large colloidal particles. Self-assembly allows the emergence of new optical properties, mechanical strength, photothermal stability, catalytic efficiency, quantum yield, and biological properties. The self-assembled structures allow reproducible optoelectronic properties, mechanical performance, and accurate sensing. More importantly, the intrinsic properties of individual nanoclusters are retained across length scales. The atomically precise nanoparticles offer enormous potential for next-generation functional materials, optoelectronics, precision sensors, and photonic devices.

Nanohybrids of atomically precise metal nanoclusters

Atomically precise metal nanoclusters (NCs) with molecule-like structures are emerging nanomaterials with fascinating chemical and physical properties. Photoluminescence (PL), catalysis, sensing, etc., are some of the most intriguing and promising properties of NCs, making the metal NCs potentially beneficial in different applications. However, long-term instability under ambient conditions is often considered the primary barrier to translational research in the relevant application fields. Creating nanohybrids between such atomically precise NCs and other stable nanomaterials (0, 1, 2, or 3D) can help expand their applicability. Many such recently reported nanohybrids have gained promising attention as a new class of materials in the application field, exhibiting better stability and exciting properties of interest. This perspective highlights such nanohybrids and briefly explains their exciting properties. These hybrids are categorized based on the interactions between the NCs and other materials, such as metal-ligand covalent interactions, hydrogen-bonding, host-guest, hydrophobic, and electrostatic interactions during the formation of nanohybrids. This perspective will also capture some of the new possibilities with such nanohybrids.

Ligand effects on the photoluminescence of atomically precise silver nanoclusters

Photoluminescence (PL) is one of the most exciting properties of atomically precise metal nanoclusters (NCs), making them a prime choice for various applications, from sensing to bio-imaging. While there are several advantages of metal NCs for PL-based applications, their PLQY is significantly low compared to other PL-active nanomaterials or organic dyes. It is essential to understand the PL mechanism in detail to tune the PLQY of NCs. There are numerous reports on gold NCs with a known structure where the origin of PL has been explored, and it was found that ligands play a vital role in their PL properties along with the kernel (core). Reports on understanding the ligand effects on PL properties are also evolving for the case of atomically precise silver NCs. This mini-review will summarize the ligands' role in PL of 29 atom Ag NCs, the most reported NCs with diversity in the silver family. The ligands were classified as primary and secondary, and their effects on tuning the PL properties were explained. The review will also address some of the answers to open questions for AgNCs, such as the origin of PL, dynamics, and the tunability of PLQY using ligand modifications.

Tunable reactivity of silver nanoclusters: a facile route to synthesize a range of bimetallic nanostructures

Quantized energy levels and unique optoelectronic properties of atomically precise noble metal nanoclusters (NCs) have made them important in materials science, catalysis, sensors, and biomedicine. Recent studies on the profound chemical interactions of such NCs within themselves and with ultrasmall plasmonic nanoparticles (NPs) indicate that depending on the size, shape, and composition of the second reactant, NCs can either take part in colloidal assembly without any chemical modifications or lead to products with atoms exchanged. Anisotropic NPs are a unique class of plasmonic nanomaterials as their sharp edges and protrusions show higher chemical reactivity compared to flat surfaces, often leading to site-specific growth of foreign metals and metal oxide shells. Here, using chemical interactions between gold nanotriangles (AuNTs) and Ag NCs of different compositions, we show for the first time that metal atom etching, alloying/atom exchange, and colloidal assembly can all happen at a particular length scale. Specifically, Ag₂₅(DMBT)₁₈ NCs (denoted as 1), upon reacting with AuNTs of ∼57 nm edge length, etch gold atoms from their sharp tips and edges. Simultaneously, the two nanosystems exchange metal atoms, resulting in Ag-doped AuNTs and Au_xAg_24-x(DMBT)₁₈ (x = 1, 2). However, another Ag NC with the same metallic core, but a different ligand shell, namely, Ag₂₅H₂₂(DPPE)₈ (denoted as 2), creates dendritic shells made of Ag, surrounding these AuNTs under the same reaction conditions. Furthermore, we show that in the case of a more reactive thiol-protected Ag NC, namely, Ag₄₄(pMBA)₃₀ (denoted as 3), gold etching is faster from the edges and tips, which drastically alters the identities of both the reactants. Interestingly, when the AuNTs are protected by pMBA, 3 systematically assembles on AuNTs through H-bonding, resulting in an AuNT core-Ag NC shell nanocomposite. Thus, while shedding light on various factors affecting the reactivity of Ag NCs towards AuNTs, the present study proposes a single strategy to obtain a number of bimetallic nanosystems of targeted morphology and functionality.

Regulating the assembly and expansion of the silver cluster from the Ag37 to Ag46 nanowheel driven by heteroanions

Precise control over the shape and size of metal nanoclusters through anion template-driven self-assembly is one of the key scientific goals in the nanocluster community, however, it is still not understood comprehensively. In this work, we report the controllable synthesis and atomically precise structures of silver nanowheels Ag37 and Ag46, using homo (Cl^- ions) and heteroanion (Cl^- and CrO₄ ^2- ions) template strategies, along with macrocyclic p-phenyl-thiacalix[4]arene and small ⁱPrS^- ligands. Structural analyses revealed that in Ag37, Cl^- ions serve as both local and global templates, whereas CrO₄ ^2- ions function as local and Cl^- ions as global templates in Ag46, resulting in a pentagonal nanowheel (Ag37) and a hexagonal (Ag46) nanowheel. The larger ionic size and more negative charges of CrO₄ ^2- ions than Cl^- ions offer more coordination sites for the silver atoms and are believed to be the key factors that drive the nanowheel core to expand significantly. Also, by taking advantage of the deep cavity of thiacalix[4]arene with an extended phenyl group, Ag46 has been used as a host material for dye adsorption depending on the charge and size of organic dyes. The successful use of heteroanions to control the expansion of well-defined silver nanowheels fills the knowledge gap in understanding the directing role of heteroanions in dictating the shape and size of nanoclusters at the atomic level.

Supramolecular hybrids of chiral Waugh polyoxometalate with cyclodextrins

The development of novel systems for chiral polyoxometalates (POMs) is an attractive research field because of their fascinating topological structures and well-defined functions. Herein, we have developed a new reaction route for the synthesis of two unprecedented chiral Waugh POM-based supramolecular architectures. Single-crystal X-ray diffraction reveals that the architectures exhibit a wavy three-dimensional framework and bamboo-rod-connected framework upon regulating the size of the cyclodextrin and the stacking pattern of the D₃ symmetric Waugh {MnMo₉}. Solution studies using NMR, circular dichroism and isothermal titration calorimetry corroborate nicely the very weak interactions between the components. The intricate chiral microenvironment originating from the hybrid frameworks may be responsible for the selective recognition of the Λ-{MnMo₉} enantiomer. This study highlights the importance of the asymmetric configuration of the POM for designing CD/POM assemblies and understanding their chirality.

[Cu18H3(S-Adm)12(PPh3)4Cl2]: fusion of Platonic and Johnson solids through a Cu(0) center and its photophysical properties

Structural elucidation of atom-precise thiolate-protected copper nanoclusters (Cu NCs) containing Cu(0) is quite challenging. Here, we report a new adamantane-thiol-protected NC, [Cu₁₈H₃(S-Adm)₁₂(PPh₃)₄Cl₂] (Cu₁₈), which represents the first observation of a rare mononuclear Cu(0)-containing Cu₁₀H₃Cl₂ core that is constructed via kernel fusion through vertex sharing of the Platonic-solid- and Johnson-solid-geometry-like kernels and hydride-bridging. The unique core is surrounded by a Cu₈S₁₂P₄ metal–ligand motif shell and adopts a butterfly-like structure. In comparison to its closest structural analogue, the predominant effect of the principal Cu atom vacancy-induced structural rearrangement is evidenced. The occupied orbitals of this NC have a major d-orbital contribution to the distorted Cu₆ octahedral kernel, whereas unoccupied orbitals owe a contribution to the distorted Cu₅ square-pyramidal kernel. Thus, the charge transfer phenomenon is uniquely instigated between the two fused kernels through Cu(d) → Cu(d) transition via the Cu(0) center. This NC exhibits violet emission due to kernel-dominated relaxation at room temperature, which is further enhanced by confining the surface protecting ligands through recognition-site-specific host–guest supramolecular adduct formation by β-cyclodextrin. The unique electronic structure of this NC further facilitates its application toward photocurrent generation. Thus, this study offers a unique strategy for the controllable synthesis of a Cu(0)-containing Cu NC, which enables atomic-level insights into their optoelectronic properties.

An example of a butterfly-like [Cu₁₈H₃(S-Adm)₁₂(PPh₃)₄Cl₂] cluster with vertex-sharing kernels through a Cu(0) center. Combined experimental and theoretical results correlate its photophysical properties with its unique structural architecture.

Solvent-Induced Isomeric Cu13 Nanoclusters: Chlorine to Copper Charge Transfer Boosting Molecular Oxygen Activation in Sulfide Selective Oxidation

Isomers with minimal structural dissimilarities are promising research objects to obtain a comprehensive understanding of structure-property relationships; however, comparability of isomeric structures is a prerequisite. Herein, two quasi-structurally isomeric 13-nuclei copper nanoclusters (Cu NCs) (Cu13a and Cu13b) containing highly similar Cu₁₃ kernels and different arrangements of peripheral ligands were obtained using a solvent-induced strategy. The exotic chloride ion is shown to play a prominent role in inducing the selective formation of two quasi-isomers, where the comparative study to establish a structure-property relationship was realized. Due to the charge transition from chlorine to the copper core (X_(Cl)M_(Cu)CT), the molecular oxygen activation of Cu13a showed higher singlet oxygen (¹O₂) and lower superoxide radical (O₂^•-) yields compared to those of Cu13b, which gives it better catalytic selectivity for the ¹O₂ involved selective oxidation of sulfides. The present work not only offers a controllable strategy for the rational design and synthesis of quasi-structurally isomeric Cu NCs but also provides a pathway to boost catalytic selectivity by a halogen to metal core charge transition.

Assembling Atomically Precise Noble Metal Nanoclusters Using Supramolecular Interactions

Supramolecular chemistry (SC) of noble metal nanoclusters (NMNCs) is one of the fascinating areas of contemporary materials science. It is principally concerned with the noncovalent interactions between NMNCs, as well as between NMNCs and molecules or nanoparticles. This review focuses on recent advances in the supramolecular assembly of NMNCs and applications of the resulting structures. We have divided the topics into four distinct subgroups: (i) SC of NMNCs in gaseous and solution phases, (ii) supramolecular interactions of NMNCs in crystal lattices, (iii) supramolecular assemblies of NMNCs with nanoparticles and NMNCs, and (iv) SC of NMNCs with other molecules. The last explores their interactions with fullerenes, cyclodextrins, cucurbiturils, crown ethers, and more. After discussing these topics concisely, various emerging properties of the assembled systems in terms of their mechanical, optical, magnetic, charge-transfer, etc. properties and applications are presented. SC is seen to provide a crucial role to induce new physical and chemical properties in such hybrid nanomaterials. Finally, we highlight the scope for expansion and future research in the area. This review would be useful to those working on functional nanostructures in general and NMNCs in particular.

Stepwise Assembly of Ag42 Nanocalices Based on a MoVI-Anchored Thiacalix[4]arene Metalloligand

Metalloligand strategy has been well recognized in the syntheses of heterometallic coordination polymers; however, such a strategy used in the assembly of silver nanoclusters is not broadly available. Herein, we report the stepwise syntheses of a family of halogen-templated Ag₄₂ nanoclusters (Ag42c-Ag42f) based on Mo^VI-anchored p-tert-butylthiacalix[4]arene (H₄TC4A) as a metalloligand (hereafter named MoO₃-TC4A). X-ray crystallography demonstrates that they are similar C₃-symmetric silver-organic nanocalices capped by six MoO₃-TC4A metalloligands, which are evenly distributed up and down the base of 42 silver atoms. These nanoclusters can be disassembled to six bowl-shaped [Ag₁₁(MoO₃-TC4A)(RS)₃] secondary building units (SBUs, R = Et or ⁿPr), which are fused together in a face-sharing fashion surrounding Cl^- or Br^- as a central anion template. The electrospray mass spectrometry (ESI-MS) indicates their high stabilities in solution and verifies the formation of the MoO₃-TC4A metalloligand, thereby rationalizing the overall stepwise assembly process for them. Moreover, Ag42c shows lower cytotoxicity and better activity against the HepG-2 cell line than MCF-7 and BGC-823. These results not only exemplify the effectiveness of a thiacalix[4]arene-based metalloligand in the assembly of silver nanoclusters but also give us profound insight about the step-by-step assembly process in silver nanoclusters.

Revealing the chirality origin and homochirality crystallization of Ag14 nanocluster at the molecular level

Although chirality is an ever-present characteristic in biology and some artificial molecules, controlling the chirality and demystifying the chirality origin of complex assemblies remain challenging. Herein, we report two homochiral Ag₁₄ nanoclusters with inherent chirality originated from identical rotation of six square faces on a Ag₈ cube driven by intra-cluster π···π stacking interaction between pntp⁻ (Hpntp = p-nitrothiophenol) ligands. The spontaneous resolution of the racemic (SD/rac-Ag14a) to homochiral nanoclusters (SD/L-Ag14 and SD/R-Ag14) can be realized by re-crystallizing SD/rac-Ag14a in acetonitrile, which promotes the homochiral crystallization in solid state by forming C–H···O/N hydrogen bonds with nitro oxygen atoms in pntp⁻ or aromatic hydrogen atoms in dpph (dpph = 1,6-bis(diphenylphosphino)hexane) on Ag₁₄ nanocluster. This work not only provides strategic guidance for the syntheses of chiral silver nanoclusters in an all-achiral environment, but also deciphers the origin of chirality at molecular level by identifying the special effects of intra- and inter-cluster supramolecular interactions.

The preparation of chiral monolayer-protected metal clusters is interesting for their potential applications in a variety of fields, including catalysis. Here, the authors synthesize chiral Ag₁₄ nanoclusters in an all-achiral environment, and decipher the origin of chirality at the molecular level; the solvent choice is key to achieve homochiral crystallization.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

Cocrystals of Atomically Precise Noble Metal Nanoclusters

Cocrystallization is a phenomenon involving the assembly of two or more different chemical entities in a lattice, occurring typically through supramolecular interactions. In this concept, recent advancements in the cocrystallization of atomically precise noble metal clusters and their potential future directions are presented. Different strategies to create coassemblies of thiolate-protected noble metal nanoclusters are presented first. An approach is the simultaneous synthesis, and cocrystallization of two clusters having similar structures. A unique pair of clusters found recently, namely Ag₄₀ and Ag₄₆ with same core but different shell are taken to illustrate this. In another category, the case of the same core is presented, namely Ag₁₁₆ with different shells, as in a mixture of Ag₂₁₀ and Ag₂₁₁ . Next, an intercluster reaction is presented to create cocrystals through selective crystallization of the reaction products. The coexistence of competing effects, magic sizes, and magic electron shells in a coassembly of alloy nanoclusters is discussed next. Finally, an assembly strategy for nanoclusters using electrostatic interactions is described. This concept is concluded with a future perspective on the emerging possibilities of such solids. Advancements in this field will certainly help the development of novel materials with exciting properties.

Shell engineering to achieve modification and assembly of atomically-precise silver clusters

This tutorial review focuses on the modification and assembly of atomically-precise silver clusters by changing shell layers for more stability and functionalities, especially for brighter luminescence.

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.

Hierarchical structural complexity in atomically precise nanocluster frameworks

The supramolecular chemistry of nanoclusters is a flourishing area of nano-research; however, the controllable assembly of cluster nano-building blocks in different arrays remains challenging. In this work, we report the hierarchical structural complexity of atomically precise nanoclusters in micrometric linear chains (1D array), grid networks (2D array) and superstructures (3D array). In the crystal lattice, the Ag29(SSR)12(PPh3)4 nanoclusters can be viewed as unassembled cluster dots (Ag29–0D). In the presence of Cs+ cations, the Ag29(SSR)12 nano-building blocks are selectively assembled into distinct arrays with different oxygen-carrying solvent molecules―Cs@Ag29(SSR)12(DMF)x as 1D linear chains (Ag29–1D), Cs@Ag29(SSR)12(NMP)x as 2D grid networks (Ag29–2D), and Cs@Ag29(SSR)12(TMS)x as 3D superstructures (Ag29–3D). Such self-assemblies of these Ag29(SSR)12 units have not only been observed in their crystalline state, but also in their amorphous state. Due to the diverse surface structures and crystalline packing modes, these Ag29-based assemblies manifest distinguishable optical absorptions and emissions in both solutions and crystallized films. Furthermore, the surface areas of the nanocluster crystals are evaluated, the maximum value of which occurs when the cluster nano-building blocks are assembled into 2D arrays (i.e. Ag29–2D). Overall, this work presents an exciting example of the hierarchical assembly of atomically precise nanoclusters by simply controlling the adsorbed molecules on the cluster surface.

A covalently linked dimer of [Ag25(DMBT)18]−

We report the first example of a covalently bound dimer of monolayer protected atomically precise silver nanocluster [Ag₂₅(DMBT)₁₈]⁻ (DMBT stands for 2,4-dimethylbenzenethiol).