Dithiophosphonate-Protected Eight-Electron Superatomic Ag21 Nanocluster: Synthesis, Isomerism, Luminescence, and Catalytic Activity

The structure-property relationship considering isomerism-tuned photoluminescence and efficient catalytic activity of silver nanoclusters (NCs) is exclusive. Asymmetrical dithiophosphonate NH4[S2P(OR)(p-C6H4OCH3)] ligated first atomically precise silver NCs [Ag21{S2P(OR)(p-C6H4OCH3)}12]PF6 {where, R = nPr (1), Et (2)} were established by single-crystal X-ray diffraction and characterized by electrospray ionization mass spectrometry, NMR (31P, 1H, 2H), X-ray photoelectron spectroscopy, UV-visible, energy-dispersive X-ray spectroscopy, Fourier transforms infrared, thermogravimetric analysis, etc. NCs 1 and 2 consist of eight silver atoms in a cubic framework and enclose an Ag@Ag12-centered icosahedron to constitute an Ag21 core of Th symmetry, which is concentrically inscribed within the S24 snub-cube, P12 cuboctahedron, and the O12 truncated tetrahedron formed by 12 dithiophosphonate ligands. These NCs facilitate to be an eight-electron superatom (1S21P6), in which eight capping Ag atoms exhibit structural isomerism with documented isoelectronic [Ag21{S2P(OiPr)2}12]PF6, 3. In contrast to 3, the stapling of dithiophosphonates in 1 and 2 triggered bluish emission within the 400 to 500 nm region at room temperature. The density functional theory study rationalized isomerization and optical properties of 1, 2, and 3. Both (1, and 2) clusters catalyzed a decarboxylative acylarylation reaction for rapid oxindole synthesis in 99% yield under ambient conditions and proposed a multistep reaction pathway. Ultimately, this study links nanostructures to their physical and catalytic properties.

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.

Construction of novel Ag(0)-containing silver nanoclusters by regulating auxiliary phosphine ligands

Controlled synthesis of metal clusters through minor changes in surface ligands holds significant interest because the corresponding entities serve as ideal models for investigating the ligand environment's stereochemical and electronic contributions that impact the corresponding structures and properties of metal clusters. In this work, we obtained two Ag(0)-containing nanoclusters (Ag17 and Ag32) with near-infrared emissions by regulating phosphine auxiliary ligands. Ag17 and Ag32 bear similar shells wherein Ag17 features a trigonal bipyramid Ag5 kernel while Ag32 has a bi-icosahedral interpenetrating an Ag20 kernel. Ag17 and Ag32 showed a near-infrared emission (NIR) of around 830 nm. Benefiting from the rigid structure, Ag17 displayed a more intense near-infrared emission than Ag32. This work provides new insight into the construction of novel superatomic silver nanoclusters by regulating phosphine ligands.

Diselenophosphate Ligands as a Surface Engineering Tool in PdH-Doped Silver Superatomic Nanoclusters

The first hydride-doped Pd/Ag superatoms stabilized by selenolates are reported: [PdHAg19(dsep)12] [dsep = Se2P(OiPr)2] 1 and [PdHAg20(dsep)12]+ 2. 1 was derived from the targeted transformation of [PdHAg19(dtp)12] [dtp = S2P(OiPr)2] by ligand exchange, whereas 2 was obtained from the addition of trifluoroacetic acid to 1, resulting in a symmetric redistribution of the capping silver atoms. The transformations are all achieved while retaining an 8-electron superatomic configuration. VT-NMR attests to the good stability of the NCs in solution, and single-crystal X-ray diffraction reveals the crucial role that the interstitial hydride plays in directing the position of the capping silver atoms. The total structures are reported alongside their electronic and optical properties. 1 and 2 are phosphorescent with a lifetime of 73 and 84 μs at 77 K, respectively. The first antibacterial activity data for superatomic bimetallic Pd/Ag nanoclusters are also reported.

Effects of bromine-containing counterion salts in directing the structures of medium-sized silver nanoclusters

The preparation and structural determination of silver nanoclusters (especially the medium-sized Ag clusters) remain more challenging relative to those of their gold counterparts because of the comparative instability of the former. In this work, three medium-sized Ag clusters were controllably synthesized and structurally determined, namely, [Ag52(S-Adm)30Br4H20]2- (Ag52 for short), Ag54(S-Adm)30Br4H20 (Ag54 for short), and [Ag58(S-Adm)30Br4(NO3)2H22]2+ (Ag58 for short) nanoclusters. Specifically, the introduction of PPh4Br gave rise to the generation of Ag52 and Ag54 nanoclusters with homologous compositions and configurations, while the TOABr salt selected Ag58 as the sole cluster product, whose geometric structure was completely different from those of Ag52 and Ag54 nanoclusters. In addition, the optical absorptions and emissions of the three medium-sized silver nanoclusters were compared. The findings in this work not only provide three uniquely medium-sized nanoclusters to enrich the silver cluster family but also point out a new approach (i.e., changing the counterion salt) for the preparation of new nanoclusters with novel structures.

Thiacalix[4]arene-Protected Silver Nanoclusters Encapsulating Different Two-Electron Superatom Oligomers

The subvalent silver kernel represents the nascent state of silver cluster formation, yet the growth mechanism has long been elusive. Herein, two silver nanoclusters (Ag30 and Ag34) coprotected by TC4A4- (H4TC4A = p-tert-butylthiacalix[4]arene) and TBPMT- (TBPMTH = 4-tert-butylbenzenemethanethiol) containing 6e and 4e silver kernels are synthesized and characterized. The trimer of the 2e superatom Ag14 kernel in Ag30 is built from a central Ag6 octahedron sandwiched by two orthogonally oriented Ag5 trigonal bipyramids through sharing vertexes, whereas a double-octahedral Ag10 kernel in Ag34 is a dimer of 2e superatoms. They manifest disparate polyhedron fusion growth patterns at the beginning of the silver cluster formation. Their excellent solution stabilities are contributed by the multisite and multidentate coordination fashion of TC4A4- and the special valence electron structures. This work demonstrates the precise control of silver kernel growth by the solvent strategy and lays a foundation for silver nanocluster application in photothermal conversion.

Constructing perfect cubic Ag-Cu alloyed nanoclusters through selective elimination of phosphine ligands

The aspiration of chemists has always been to design and achieve control over nanoparticle morphology at the atomic level. Here, we report a synthesis strategy and crystal structure of a perfect cubic Ag-Cu alloyed nanocluster, [Ag55Cu8I12(S-C6H32,4(CH3)2)24][(PPh4)] (Ag55Cu8I12 for short). The structure of this cluster was determined by single-crystal X-ray diffraction (SCXRD) and further validated by X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), Energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), and 1H and 31P nuclear magnetic resonance (NMR). The surface deviation of the cube was measured to be 0.291 Å, making it the flattest known cube to date.

Unveiling the Remarkable Stability and Catalytic Activity of a 6-Electron Superatomic Ag30 Nanocluster for CO2 Electroreduction

Nanocluster catalysts face a significant challenge in striking the right balance between stability and catalytic activity. Here, we present a thiacalix[4]arene-protected 6-electron [Ag30(TC4A)4(iPrS)8] nanocluster that demonstrates both high stability and catalytic activity. The Ag30 nanocluster features a metallic core, Ag104+, consisting of two Ag3 triangles and one Ag4 square, shielded by four {Ag5@(TC4A)4} staple motifs. Based on DFT calculations, the Ag104+ metallic kernel can be viewed as a trimer comprising 2-electron superatomic units, exhibiting a valence electron structure similar to that of the Be3 molecule. Notably, this is the first crystallographic evidence of the trimerization of 2-electron superatomic units. Ag30 can reduce CO2 into CO with a Faraday efficiency of 93.4% at -0.9 V versus RHE along with excellent long-term stability. Its catalytic activity is far superior to that of the chain-like AgI polymer ∞1{[H2Ag5(TC4A)(iPrS)3]} (∞1Agn), with the composition similar to Ag30. DFT calculations elucidated the catalytic mechanism to clarify the contrasting catalytic performances of the Ag30 and ∞1Agn polymers and disclosed that the intrinsically higher activity of Ag30 may be due to the greater stability of the dual adsorption mode of the *COOH intermediate on the metallic core.

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.

Hydride Doping Effects on the Structure and Properties of Eight-Electron Rh/Ag Superatoms: The [RhHx@Ag21-x{S2P(OnPr)2}12] (x = 0-2) Series

Three hitherto unknown eight-electron rhodium/silver alloy nanoclusters, [RhAg₂₁{S₂P(OⁿPr)₂}₁₂] (1), [RhHAg₂₀{S₂P(OⁿPr)₂}₁₂] (2), and [RhH₂Ag₁₉{S₂P(OⁿPr)₂}₁₂] (3), have been isolated and fully characterized. Cluster 1 contains a regular Rh@Ag₁₂ icosahedral core, whereas 2 and 3 exhibit distorted RhH@Ag₁₂ and RhH₂@Ag₁₂ icosahedral cores. The single-crystal neutron structure of 2 located the encapsulated hydride at the center of an enlarged RhAg₃ tetrahedron. A similar position was found by neutron diffraction for one of the hydrides in 3, whereas the other hydride is trigonally coordinated to Rh and an elongated Ag-Ag edge. The solid-state structures of 1-3 possess C₁ symmetry due to the asymmetric arrangement of the surrounding capping Ag atoms. Our investigation shows that the insertion of one hydride dopant provokes the elimination of one capping silver atom on the cluster surface, resulting in the general formula [RhH_x@Ag_21-x{S₂P(OⁿPr)₂}₁₂] (x = 0-2), which maintains the same number of cluster electrons as well as neutral charge. Clusters 1-3 exhibit an intense emission band in the NIR region. Contrarily to their PdAg₂₁ and PdHAg₂₀ relatives, the 4d orbitals of the encapsulated heterometal are somewhat involved in the optical processes.

Surface-exposed silver nanoclusters inside molecular metal oxide cavities

The surfaces of metal nanoclusters, including their interface with metal oxides, exhibit a high reactivity that is attractive for practical purposes. This high reactivity, however, has also hindered the synthesis of structurally well-defined hybrids of metal nanoclusters and metal oxides with exposed surfaces and/or interfaces. Here we report the sequential synthesis of structurally well-defined {Ag₃₀} nanoclusters in the cavity of ring-shaped molecular metal oxides known as polyoxometalates. The {Ag₃₀} nanoclusters possess exposed silver surfaces yet are stabilized both in solution and the solid state by the surrounding ring-shaped polyoxometalate species. The clusters underwent a redox-induced structural transformation without undesirable agglomeration or decomposition. Furthermore, {Ag₃₀} nanoclusters showed high catalytic activity for the selective reduction of several organic functional groups using H₂ under mild reaction conditions. We believe that these findings will serve for the discrete synthesis of surface-exposed metal nanoclusters stabilized by molecular metal oxides, which may in turn find applications in, for example, the fields of catalysis and energy conversion.

Anion-templated silver nanoclusters: precise synthesis and geometric structure

Metal nanoclusters (NCs) are gaining much attention in nanoscale materials research because they exhibit size-specific physicochemical properties that are not observed in the corresponding bulk metals. Among them, silver (Ag) NCs can be precisely synthesized not only as pure Ag NCs but also as anion-templated Ag NCs. For anion-templated Ag NCs, we can expect the following capabilities: 1) size and shape control by regulating the central anion (anion template); 2) stabilization by adjusting the charge interaction between the central anion and surrounding Ag atoms; and 3) functionalization by selecting the type of central anion. In this review, we summarize the synthesis methods and influences of the central anion on the geometric structure of anion-templated Ag NCs, which include halide ions, chalcogenide ions, oxoanions, polyoxometalate, or hydride/deuteride as the central anion. This summary provides a reference for the current state of anion-templated Ag NCs, which may promote the development of anion-templated Ag NCs with novel geometric structures and physicochemical properties.

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.

Eight-Electron Superatomic Cu31 Nanocluster with Chiral Kernel and NIR-II Emission

Owing to the inherent instability caused by the low Cu(I)/Cu(0) half-cell reduction potential, Cu(0)-containing copper nanoclusters are quite uncommon in comparison to their Ag and Au congeners. Here, a novel eight-electron superatomic copper nanocluster [Cu₃₁(4-MeO-PhC≡C)₂₁(dppe)₃](ClO₄)₂ (Cu₃₁, dppe = 1,2-bis(diphenylphosphino)ethane) is presented with total structural characterization. The structural determination reveals that Cu₃₁ features an inherent chiral metal core arising from the helical arrangement of two sets of three Cu₂ units encircling the icosahedral Cu₁₃ core, which is further shielded by 4-MeO-PhC≡C^- and dppe ligands. Cu₃₁ is the first copper nanocluster carrying eight free electrons, which is further corroborated by electrospray ionization mass spectrometry, X-ray photoelectron spectroscopy and density functional theory calculations. Interestingly, Cu₃₁ demonstrates the first near-infrared (750-950 nm, NIR-I) window absorption and the second near-infrared (1000-1700 nm, NIR-II) window emission, which is exceptional in the copper nanocluster family and endows it with great potential in biological applications. Of note, the 4-methoxy groups providing close contacts with neighboring clusters are crucial for the cluster formation and crystallization, while 2-methoxyphenylacetylene leads only to copper hydride clusters, Cu₆H or Cu₃₂H₁₄. This research not only showcases a new member of copper superatoms but also exemplifies that copper nanoclusters, which are nonluminous in the visible range may emit luminescence in the deep NIR region.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

Surface modifications of eight-electron palladium silver superatomic alloys

Atomically precise thiolate-protected coinage metal nanoclusters and their alloys are far more numerous than their selenium congeners, the synthesis of which remains extremely challenging. Herein, we report the synthesis of a series of atomically defined dithiophosph(in)ate protected eight-electron superatomic palladium silver nanoalloys [PdAg₂₀{S₂PR₂}₁₂], 2a-c (where R = OⁱPr, a; OⁱBu, b; Ph, c) via ligand exchange and/or co-reduction methods. The ligand exchange reaction on [PdAg₂₀{S₂P(OⁿPr)₂}₁₂], 1, with [NH₄{Se₂PR₂}₁₂] (where R = OⁱPr, or OⁿPr) leads to the formation of [PdAg₂₀{Se₂P(OⁱPr)₂}₁₂] (3) and [PdAg₂₀{Se₂P(OⁿPr)₂}₁₂] (4), respectively. Solid state structures of 2a, 2b, 3 and 4 unravel different PdAg₂₀ metal frameworks from their parent cluster, originating from the different distributions of the eight-capping silver(I) atoms around a Pd@Ag₁₂ centered icosahedron with C_2, D_3, T_h and T_h symmetries, respectively. Surprisingly ambient temperature crystallization of the reaction product 3 obtained by the ligand exchange reaction on 1 has resulted in the co-crystallization of two isomers in the unit cell with overall T (3a) and C₃ (3b) symmetries, respectively. To our knowledge, this is the first ever characterized isomeric pair among the selenolate-protected NCs. Density functional theory (DFT) studies further rationalize the preferred geometrical isomerism of the PdAg₂₀ core.

Carboranethiol-Protected Propeller-Shaped Photoresponsive Silver Nanomolecule

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag₂₁ nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag₁₃ central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag-S-Ag bridging motifs. While 12 MCT ligands protect the core through metal-thiolate bonds in a 3-6-3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV-vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag₂₁ nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120-160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 μs for the carriers, spread over a broad wavelength region of 520-650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.

Variable control of the electronic states of a silver nanocluster via protonation/deprotonation of polyoxometalate ligands

The properties of metal nanoclusters depend on both their structures and electronic states. However, in contrast to the significant advances achieved in the synthesis of structurally well-defined metal nanoclusters, systematic control of their electronic states is still challenging. In particular, stimuli-responsive and reversible control of the electronic states of metal nanoclusters is attractive from the viewpoint of their practical applications. Recently, we developed a synthesis method for atomically precise Ag nanoclusters using polyoxometalates (POMs) as inorganic ligands. Herein, we exploited the acid/base nature of POMs to reversibly change the electronic states of an atomically precise {Ag27} nanocluster via protonation/deprotonation of the surrounding POM ligands. We succeeded in systematically controlling the electronic states of the {Ag27} nanocluster by adding an acid or a base (0-6 equivalents), which was accompanied by drastic changes in the ultraviolet-visible absorption spectra of the nanocluster solutions. These results demonstrate the great potential of Ag nanoclusters for unprecedented applications in various fields such as sensing, biolabeling, electronics, and catalysis.

Homoleptic Silver-Rich Trimetallic M20 Nanocluster

Two silver-rich M₂₀ alloy nanoclusters (NCs), [Cu_3.5Ag_16.5{S₂P(OⁿPr)₂}₁₂] (1) and [Cu_2.5AuAg_16.5{S₂P(OⁿPr)₂}₁₂] (2), were synthesized and fully characterized by electrospray ionization mass spectrometry, NMR spectroscopy, and X-ray crystallography. Cluster 2, the first structurally characterized trimetallic M₂₀ NC, was produced by doping one Au atom into a bimetallic M₂₀ NC. Structural analyses showed the preferred positions of Group 11 metals in the yielded M₂₀ NCs. Their antioxidation ability has been investigated, and the time-dependent UV-vis spectrum shows that the presence of Cu^I atoms in structures 1 and 2 can improve the antioxidant ability.

Nuclearity enlargement from [PW9O34@Ag51] to [(PW9O34)2@Ag72] and 2D and 3D network formation driven by bipyridines

The structural transformations of metal nanoclusters are typically quite complex processes involving the formation and breakage of several bonds, and thus are challenging to study. Herein, we report a case where two lacunary Keggin polyoxometallate templated silver single-pods [PW9O34@Ag51] (SD/Ag51b) fuse to a double-pod [(PW9O34)2@Ag72] by reacting with 4,4’-bipyridine (bipy) or 1,4-bis(4-pyridinylmethyl)piperazine (pi-bipy). Their crystal structures reveal the formation of a 2D 44-sql layer (SD/Ag72a) with bipy and a 3D pcu framework (SD/Ag72c) with pi-bipy. The PW9O349− retains its structure during the cluster fusion and cluster-based network formation. Although the two processes, stripping of an Ag-ligands interface followed by fusion, and polymerization, are difficult to envisage, electrospray ionization mass spectrometry provides enough evidences for such a proposal to be made. Through this example, we expect the structural transformation to become a powerful method for synthesizing silver nanoclusters and their infinite networks, and to evolve from trial-and-error to rational.

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.

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.

Surface environment complication makes Ag29 nanoclusters more robust and leads to their unique packing in the supracrystal lattice

Silver nanoclusters have received unprecedented attention in cluster science owing to their promising functionalities and intriguing physical/chemical properties. However, essential instability significantly impedes their extensive applications. We herein propose a strategy termed “surface environment complication” to endow Ag₂₉ nanoclusters with high robustness. The Ag₂₉(S-Adm)₁₈(PPh₃)₄ nanocluster with monodentate PPh₃ ligands was extremely unstable and uncrystallizable. By substituting PPh₃ with bidentate PPh₂py with dual coordination sites (i.e., P and N), the Ag₂₉ cluster framework was twisted because of the generation of N–Ag interactions, and three NO₃ ligands were further anchored onto the nanocluster surface, yielding a new Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ nanocluster with high stability. The metal-control or ligand-control effects on stabilizing the Ag₂₉ nanocluster were further evaluated. Besides, Ag₂₉(S-Adm)₁₅(NO₃)₃(PPh₂py)₄ followed a unique packing mode in the supracrystal lattice with several intercluster channels, which has yet been observed in other M₂₉ cluster crystals. Overall, this work presents a new approach (i.e., surface environment complication) for tailoring the surface environment and improving the stability of metal nanoclusters.

A strategy of “surface environment complication” has been exploited to endow Ag₂₉ nanoclusters with high robustness and a unique packing mode in the supracrystal lattice.

Thermally Hypsochromic or Bathochromic Emissions? The Silver Nuclei Does Matter

Structural modulation of core-shell silver nanoclusters from the inside is a huge challenge but of great importance in their syntheses. Herein, two silver nanoclusters [Ag₃ S₉ @Ag₄₂ ] (SD/Ag45b) and [Ag₉ S₉ @Ag₄₂ ] (SD/Ag51a) are isolated in the presence of different kinds of sulfonic acids. Uniquely, SD/Ag45b and SD/Ag51a show typical core-shell structures with the similar Ag₄₂ shell but different cores. The outer shell of 42 silver atoms comprises two Ag₃ trigons at two poles encircled by three equatorial distorted square cupolas (J₄ , Ag₁₂ ). The core in SD/Ag45b is a silver trigon ligated by nine S^2- ions (Ag₃ S₉ ), while a tricapped triangular prismatic Ag₉ also ligated by the same amount of S^2- ions (Ag₉ S₉ ) is observed in the inner core of SD/Ag51a. The electrospray ionization mass spectrometry (ESI-MS) indicates that the introduction of p-toluenesulfonic acid can realize the transformation from SD/Ag45b to Ag₅₁ . SD/Ag45b and SD/Ag51a show inverse luminescence thermochromic behaviors in the near-infrared (NIR) region, mainly dictated by the inner silver cores. This work not only realizes the synthesis of new silver nanoclusters by core modulation but also provides a prototype to get molecular-level insight into the correlation between structure and luminescence thermochromism.

Alloying dichalcogenolate-protected Ag21 eight-electron nanoclusters: a DFT investigation

The isoelectronic doping of dichalcogenolato nanoclusters of the type [Ag₂₁{E₂P(OR)₂}₁₂]⁺ (E = S, Se) by any heteroatom belonging to groups 9-12 was systematically investigated using DFT calculations. Although they can differ in their global structure, all of these species have the same M@M₁₂-centered icosahedral core. In any case, the different structure types are all very close in energy. In all of them, three different alloying sites can be identified (central, icosahedral, peripheral) and calculations allowed the trends in heteroatom site occupation preference across the group 9-12 family to be revealed. These trends are supported by complementary experimental results. They were rationalized on the basis of electronegativity, potential involvement in the bonding of valence d-orbitals and atom size. TD-DFT calculations showed that the effect of doping on optical properties is sizable and this should stimulate research on the modulation of luminescence properties in the dithiolato and diseleno families of complexes.

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 double helical 4H assembly pattern with secondary hierarchical complexity in an Ag70 nanocluster crystal

The hierarchical assemblies of well-defined structural nanoclusters can help to better understand those of biologically important molecules such as DNA and proteins. Herein, we disclose the synthesis and characterization of a new silver nanocluster, that is Ag₇₀(SR)₄₂(PPh₃)₅ (Ag70-TPP). Directed by the ligands, Ag70-TPP nanoclusters undergo self-hierarchical assembly into a highly space-efficient complex secondary structure of a double helical 4H (DH4H) close packing pattern. The chirality of Ag70-TPP, and the van der Waals forces interactions between the ligands are believed to drive its DH4H arrangement, and the observed interlocking of the phosphine ligands of adjacent Ag70-TPP nanoclusters also contributed. Overall, this work has yielded important and unprecedented insights into the internal structure and crystallographic arrangement of nanoclusters.

Keplerate Ag192 Cluster with 6 Silver and 14 Chalcogenide Octahedral and Tetrahedral Shells

Silver clusters with more than 2 concentric silver shells are scarce. Here, we enable self-assembly and crystallize SD/Ag192a, a highly symmetric silver chalcogenide cluster (SCC) with 192 silver cations in 6 shells and 136 anionic groups in 14 shells. All but 1 of these 20 concentric shells are Platonic or Archimedean solids. All have octahedral or tetrahedral symmetry and align the maximum number of their 2-, 3-, and 4-fold axes of rotational symmetry, thus identifying the cluster as a Keplerate. A rhombic dodecahedron supershell, formed from the first 3 anionic shells, is the keystone for the entire structure. But, nearly all of the edges in these polyhedral shells are too long to represent bonds. What mechanism of coordination chemistry holds the shells together? Like Na⁺ ions held electrostatically inside adjacent cube-shaped anionic compartments in a crystal of NaCl, individual Ag⁺ ions sit inside adjacent octahedron-shaped anionic compartments that fill space. Similarly, like Cl^- ions in NaCl, individual anionic groups sit inside adjacent cationic (Ag⁺) compartments, mostly uniform polyhedra, that also fill space.

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.

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.

Doping effect on the structure and properties of eight-electron silver nanoclusters

The bimetallic M₂₀ and M₂₁ compounds, {[Cu₃Ag₁₇{S₂P(OⁱPr)₂}₁₂]_0.5 [Cu₄Ag₁₆{S₂P(OⁱPr)₂}₁₂]_0.5} ({[1a]_0.5[1b]_0.5}) and [Cu₄Ag₁₇{S₂P(OⁱPr)₂}₁₂](PF₆) (2), have been structurally characterized, in which the Cu(I) ions are randomly distributed on the eight outer positions capping the eight-electron [Ag₁₃]⁵⁺ core. DFT calculations show that the statistical disorder results from the nearly neutral preference of copper to occupy any of the eight outer positions. Surprisingly, the UV-Vis absorption spectra of the M₂₀ and M₂₁ bimetallic nanoclusters display an almost identical absorption profile as that of their homometallic [Ag₂₀{S₂P(OⁱPr)₂}₁₂] and [Ag₂₁{S₂P(OⁱPr)₂}₁₂]⁺ relatives. This is rationalized by TD-DFT calculations, which show that the frontier orbitals of such eight-electron alloys are largely independent from the nature of the capping metal ions. A blue-shifted absorption is observed upon replacing by Au the central Ag atom in 2, forming the trimetallic compound [Cu₄AuAg₁₆{S₂P(OⁱPr)₂}₁₂](PF₆) (3).

Synthesis and Luminescence Properties of Two-Electron Bimetallic Cu-Ag and Cu-Au Nanoclusters via Copper Hydride Precursors

The synthesis, structural characteristics, and photophysical properties of luminescent Cu-rich bimetallic superatomic clusters [Au@Cu₁₂(S₂CNⁿPr₂)₆(C≡CPh)₄]⁺ (1a⁺), [Au@Cu₁₂{S₂P(OR)₂}₆(C≡CPh)₄]⁺ (2⁺), (2a⁺ = ⁱPr; 2b⁺ = ⁿPr), [Au@Cu₁₂{S₂P(C₂H₄Ph)₂}₆(C≡CPh)₄]⁺ (2c⁺), and [Ag@Cu₁₂{S₂P(OⁿPr)₂}₆(C≡CPh)₄]⁺ (3⁺) were studied. Compositionally uniform clusters 1⁺-3⁺ were isolated from the reaction of dithiolato-stabilized, polyhydrido copper clusters with phenylacetylene in the presence of heterometal salts. By using X-ray diffraction, the structures of 1a⁺, 2a⁺, 2b⁺, and 3⁺ were able to be determined. ESI-mass spectrometry and elemental analysis confirmed their compositions and purity. The structural characteristics of these clusters are similar with respect to displaying gold (or silver)-centered Cu₁₂ cuboctahedra surrounded by six dithiocarbamate/dithiophosph(in)ate and four alkynyl ligands. The doping of Au and Ag atoms into the polyhydrido copper nanoclusters significantly enhances their PL quantum yields from Ag@Cu₁₂ (0.58%) to Au@Cu₁₂ (55%) at ambient temperature in solution. In addition, the electrochemical properties of the new alloys were investigated by cyclic voltammetry.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

The ligand effect on the interface structures and electrocatalytic applications of atomically precise metal nanoclusters

Metal nanoclusters, also known as ultra-small metal nanoparticles, occupy the gap between discrete atoms and plasmonic nanomaterials, and are an emerging class of atomically precise nanomaterials. Metal nanoclusters protected by different types of ligands, such as thiolates, alkynyls, hydrides, and N-heterocyclic carbenes, have been synthesized in recent years. Moreover, recent experiment and theoretical studies also indicated that the metal nanoclusters show great promise in many electrocatalytic reactions, such as hydrogen evolution, oxygen reduction, and CO₂reduction. The atomically precise nature of their structures enables the elucidation of structure-property relationships and the reaction mechanisms, which is essential if nanoclusters with enhanced performances are to be rationally designed. Particularly, the ligands play an important role in affecting the interface bonding, stability and electrocatalytic activity/selectivity. In this review, we mainly focus on the ligand effect on the interface structure of metal nanoclusters and then discuss the recent advances in electrocatalytic applications. Furthermore, we point out our perspectives on future efforts in this field.

Nonradiative relaxation dynamics in the [Au25-nAgn(SH)18]-1 (n = 1, 12, 25) thiolate-protected nanoclusters

Evaluation of the electron-nuclear dynamics and relaxation mechanisms of gold and silver nanoclusters and their alloys is important for future photocatalytic, light harvesting, and photoluminescence applications of these systems. In this work, the effect of silver doping on the nonradiative excited state relaxation dynamics of the atomically precise thiolate-protected gold nanocluster [Au_25-nAg_n(SH)₁₈]^-1 (n = 1, 12, 25) is studied theoretically. Time-dependent density functional theory is used to study excited states lying in the energy range 0.0-2.5 eV. The fewest switches surface hopping method with decoherence correction was used to investigate the dynamics of these states. The HOMO-LUMO gap increases significantly upon doping of 12 silver atoms but decreases for the pure silver nanocluster. Doped clusters show a different response for ground state population increase lifetimes and excited state population decay times in comparison to the undoped system. The ground state recovery times of the S₁-S₆ states in the first excited peak were found to be longer for [Au₁₃Ag₁₂(SH)₁₈]^-1 than the corresponding recovery times of other studied nanoclusters, suggesting that this partially doped nanocluster is best for preserving electrons in an excited state. The decay time constants were in the range of 2.0-20 ps for the six lowest energy excited states. Among the higher excited states, S₇ has the slowest decay time constant although it occurs more quickly than S₁ decay. Overall, these clusters follow common decay time constant trends and relaxation mechanisms due to the similarities in their electronic structures.

Tailoring silver nanoclusters via doping: advances and opportunities

Atomically precise noble metal nanoclusters (especially Au and Ag) have been pursued due to their fascinating molecule-like properties. In spite of the significant progress on Au nanoclusters (NCs), the structure and property evolution of Ag NCs is still in high demand. Doping is a useful strategy for improving the physicochemical performances of Ag NCs. Herein we summarize the recent advances in tailoring silver NC structures and properties via doping. First, we reviewed the recent studies on the synthesis of hetero metal atom doped silver bimetallic nanoclusters, which are classified by the dopants, including Au, Pt, Pd, Cu, Ni and Cd. Second, the doping effects on their properties were reviewed, including the locations of hetero metal atoms, the influence on their stability, and the charge state evolution. Moreover, we highlighted the doping-dependent improvement of the photo-luminescence (PL) performance and catalytic activity of Ag NCs.

[Ag9(1,2-BDT)6]3-: How Square-Pyramidal Building Blocks Self-Assemble into the Smallest Silver Nanocluster

The emerging promise of few-atom metal catalysts has driven the need for developing metal nanoclusters (NCs) with ultrasmall core size. However, the preparation of metal NCs with single-digit metallic atoms and atomic precision is a major challenge for materials chemists, particularly for Ag, where the structure of such NCs remains unknown. In this study, we developed a shape-controlled synthesis strategy based on an isomeric dithiol ligand to yield the smallest crystallized Ag NC to date: [Ag9(1,2-BDT)6]3- (1,2-BDT = 1,2-benzenedithiolate). The NC's crystal structure reveals the self-assembly of two Ag square pyramids through preferential pyramidal vertex sharing of a single metallic Ag atom, while all other Ag atoms are incorporated in a motif with thiolate ligands, resulting in an elongated body-centered Ag9 skeleton. Steric hindrance and arrangement of the dithiolated ligands on the surface favor the formation of an anisotropic shape. Time-dependent density functional theory based calculations reproduce the experimental optical absorption features and identify the molecular orbitals responsible for the electronic transitions. Our findings will open new avenues for the design of novel single-digit metal NCs with directional self-assembled building blocks.

Mass spectrometry guided surface modification of a tellurate ion templated 36-nucleus silver alkynyl nanocluster

Under the guidance of the ESI-MS result for [TeO6@Ag36(CCtBu)18(tfa)12] (1, tfa = trifluoroacetate), a new 36-nucleus silver-alkynyl cluster substituted by four pentafluorobenzoates, named as [TeO6@Ag36(CCtBu)18(tfa)8(F5PhCO2)4] (2), has been fabricated.

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.