Hydride-Containing Eight-Electron Pt/Ag Superatoms: Structure, Bonding, and Multi-NMR Studies

Relationship between Structural Defects and Free Electrons in Icosahedral Nanoclusters

According to the classic superatom model, metal nanoclusters with a "magic number" of free valence electrons display high stability, manifesting as the closed-shell-dependent electronic robustness. The icosahedral nanobuilding blocks containing eight free electrons were the most common in constructing metal nanoclusters; however, the structure defect-dependent variations of the free electron count in icosahedral configurations are still far from thorough research. Here, we reported a hydride-containing [Pt2Ag15(SAdm)4(DPPOE)4H]2+ nanocluster with two largely defective Pt1Ag8 icosahedral cores. Together with previously reported complete or slightly defective icosahedra in metal nanoclusters, the largely defective Pt1Ag8 core provided important clues to reveal the evolutionary mode of structural defects and free electrons in icosahedral nanoclusters; the free electron count of icosahedron was reduced two-by-two (i.e., from 8e to 6e and then to 4e) accompanied by the structure defection. Overall, the work presented a novel Pt2Ag15 nanocluster with a largely defective core structure that enables an atomic-level understanding of the relationship between structural defects and free electrons in icosahedral nanoclusters.

Controlled aggregation of Pt/PtH/Rh/RhH doped silver superatomic nanoclusters into 16-electron supermolecules

The assembly of discrete superatomic nanoclusters into larger constructs is a significant stride towards developing a new set of artificial/pseudo-elements. Herein, we describe a novel series of 16-electron supermolecules derived from the combination of discrete 8-electron superatomic synthons containing interstitial hydrides as vertex-sharing building blocks. The symmetric (RhH)2Ag33[S2P(OPr)2]17 (1) and asymmetric PtHPtAg32[S2P(OPr)2]17 (2) are characterized by ESI-MS, SCXRD, NMR, UV-vis absorption spectra, electrochemical and computational methods. Cluster 1 represents the first group 9-doped 16-electron supermolecule, composed of two icosahedral (RhH)@Ag12 8-electron superatoms sharing a silver vertex. Cluster 2 results from the assembly of two distinct icosahedral units, Pt@Ag12, and (PtH)@Ag12. In both cases, the presence of the interstitial hydrides is unprecedented. The stability of the supermolecules is investigated, and 2 spontaneously transforms into Pt2Ag33[S2P(OPr)2]17 (3) with thermal treatment. The lability of the hydride within the icosahedral framework in solution at low-temperature was confirmed by the VT-NMR.

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.

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.

Atom-Precise Ligated Copper and Copper-Rich Nanoclusters with Mixed-Valent Cu(I)/Cu(0) Character: Structure-Electron Count Relationships

Copper homometallic and copper-rich heterometallic nanoclusters with some Cu(0) character are reviewed. Their structure and stability are discussed in terms of their number of "free" electrons. In many aspects, this structural chemistry differs from that of their silver or copper homologs. Whereas the two-electron species are by far the most numerous, only one eight-electron species is known, but more electron-rich nanoclusters have also been reported. Owing to the relatively recent development of this chemistry, it is likely that more electron-rich species will be reported in the future.

Catalytic Hydrogenation of Nitroarenes over Ag33 Nanoclusters: The Ligand Effect

Using ligand-protected metallic nanoclusters with atomic precision as catalysts and elucidating its ligand effect in the catalysis are the prerequisites to deepen the structure-catalysis relationship of nanoclusters at the molecular level. Herein, a series of Ag33 nanoclusters protected with different thiolate ligands (2-phenylethanethiol, 4-chlorobenzyl mercaptan, and 4-methoxybenzyl mercaptan as precursors) were synthesized and used as heterogeneous catalysts for the conversion of nitroarenes to arylamine with NaBH4 as reductant. The obtained nanoclusters exhibited ligand-dependent catalytic activity, with benzyl thiolate ligands distinctly superior to the phenethyl thiolate ligands. DFT calculations revealed that the ligand regulated catalytic activity of the nanoclusters was ascribed to the H-π and π-π interactions between the ligands and the substrates, owing to the presence of phenyl rings in these structures. This work highlighted the importance of the ligands on the metallic nanoclusters in catalysis and provides a strategy to regulate the catalytic activity by utilizing weak interactions between the catalysts and the substrates.

Total Structure, Electronic Structure and Catalytic Hydrogenation Activity of Metal-Deficient Chiral Polyhydride Cu57 Nanoclusters

Accurate identifying and in-depth understanding of the defect sites in a working nanomaterial could hinge on establishing specific defect-activity relationships. Yet, atomically precise coinage-metal nanoclusters (NCs) possessing surface vacancy defects are scarce primarily owing to challenges in the synthesis and isolation of such defective NCs. Herein we report a mixed-ligand strategy to synthesizing an intrinsically chiral and metal-deficient copper hydride-rich NC [Cu57 H20 (PET)36 (TPP)4 ]+ (Cu57 H20 ). Its total structure (including hydrides) and electronic structure are well established by combined experimental and computational results. Crystal structure reveals Cu57 H20 features a cube-like Cu8 kernel embedded in a corner-missing metal-ligand shell of Cu49 (PET)36 (TPP)4 . Single Cu vacancy defect site occurs at one corner of the shell, evocative of mono-lacunary polyoxometalates. Theoretical calculations demonstrate that the above-mentioned point vacancy causes one surface hydride exposed as an interfacial capping μ3 -H- , which is accessible in chemical reaction, as proved by deuterated experiment. Moreover, Cu57 H20 shows catalytic activity in the hydrogenation of nitroarene. The success of this work opens the way for the research on well-defined chiral metal-deficient Cu and other metal NCs, including exploring their application in asymmetrical catalysis.

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.

Locating Interstitial Hydrides in MH2@Cu14 (M = Cu, Ag) Clusters by Single-Crystal X-ray Diffraction

Two structures, [Cu15H2(S2CNnBu2)6(C≡CPh)6][CuCl2] (1) and [AgH2Cu14{S2P(OiPr)2}6(C≡CPh)6][PF6] (2), are characterized by X-ray crystallography with high-quality single crystals. The position of interstitial hydrides can be accurately located. In addition, the refinement of the hydrides with anisotropic displacement parameters (ADPs) was successful. The distances between the central atom and copper atoms, as well as the distances within the metal cages surrounding the hydrides, are analyzed and compared with similar MH2@Cu14 (M = Cu, Ag, Pd) compounds. This work provides a thoughtful and accurate assessment of the considerations and challenges associated with anisotropic refinement for H atoms, particularly in X-ray data collection.

Restriction of intramolecular rotation for functionalizing metal nanoclusters

The restriction of intramolecular rotation has been extensively exploited to trigger the property enhancement of nanocluster-based materials. However, such a restriction is induced mainly by intermolecular aggregation. The direct restriction of intramolecular rotation of metal nanoclusters, which could boost their properties at the single molecular level, remains rarely explored. Here, ligand engineering was applied to activate intramolecular interactions at the interface between peripheral ligands and metallic kernels of metal nanoclusters. For the newly reported Au4Ag13(SPhCl2)9(DPPM)3 nanocluster, the molecule-level interactions between the Cl terminals on thiol ligands and the Ag atoms on the cluster kernel remarkably restricted the intramolecular rotation, endowing this robust nanocluster with superior thermal stability, emission intensity, and non-linear optical properties over its cluster analogue. This work presents a novel case of the restriction of intramolecular rotation (i.e., intramolecular interaction-induced property enhancement) for functionalizing metal clusters at the single molecular level.

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.

DppfCuBH4: new reducing agents for the synthesis of ferrocene-functionalized metal nanoclusters

A facile synthesis of atomically precise metal nanoclusters, especially those decorated with functional groups, is the prerequisite for finding applications in special fields and studying structure-and-property relationships. The exploration of simple and efficient synthetic prototypes for introducing functional ligands (such as ferrocene) into cluster moieties is thus of high interest. In this work, a type of reducing agent of dppfCuBH₄ (dppf is 1,1'-bis(diphenyphosphino)ferrocene) is introduced for the first time to prepare ferrocene-functionalized metal nanoclusters. Two new clusters of [Ag₂₅Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆]³⁺ (1) and [Ag₄(dppf)₅Cl₂]²⁺ (2) have been obtained from the simple synthetic method. The two compounds have been fully characterized by advanced techniques of electrospray ionization mass spectroscopy (ESI-MS), nuclear magnetic resonance (NMR), and ultraviolet-visible spectroscopy (UV-Vis). The total structure of the clusters, as determined by X-ray single-crystal diffraction, describes the Ag₁₃@Ag₁₂Cu₄(dppf)₆(3-F-PhCC)₁₂Cl₆ core-shell structure of 1 and [Ag₂Cl(dppf)₂]⁺-dppf-[Ag₂Cl(dppf)₂]⁺ polymeric structure of 2. This work opens the door to employing dppfCuBH₄ as a functional reducing agent to discover many underlying metal nanoclusters and even other nanomaterials which feature ferrocene-groups.

Mechanistic Study of the Hydride Migration-Induced Reversible Isomerization in Au22(SR)15H Isomers

Unraveling the evolution mechanism of metal nanoclusters is of great importance in understanding the formation and evolution of metallic condensed matters. In this work, the specific evolution process between a pair of gold nanocluster (Au NC) isomers is completely revealed by introducing hydride ligands to simplify the research system. A hydride-containing Au NC, Au₂₂(SR)₁₅H, was synthesized by kinetic control, and the positions of the hydrides were then confirmed by combining X-ray diffraction, neutron diffraction, and DFT calculations. Importantly, a reversible structural isomerization was found to occur on this Au₂₂(SR)₁₅H. By combining the crystal structures and theoretical calculations, the focus was placed on the hydride-binding site, and a [Au-H] migration mechanism of this isomerization process is clearly shown. Furthermore, this [Au-H] migration mechanism is confirmed by the subsequent capture and structural determination of theoretically predicted intermediates. This work provides insight into the dynamic behavior of hydride ligands in nanoclusters and a strategy to study the evolution mechanism of nanoclusters by taking the hydride ligand as the breakthrough point.

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.

Simple Approach toward N-Heterocyclic Carbene-Protected Gold Nanoclusters

Little advance has been made toward developing alternative bottom-up synthetic strategies for N-heterocyclic carbene (NHC)-stabilized gold nanoclusters, although this unique class of nanomaterials has exhibited exciting properties. We report in this work a simple and straightforward approach toward NHC-ligated gold nanoclusters by using imidazolium salts rather than free carbenes or NHC-coordinated gold complexes (NHC-Au-X, X is counterions) as precursors. Illustrated here is a one-pot and one-step preparation of an NHC-stabilized Au₁₃Br₄ cluster that features a distinct molecular formula, surface motifs, and assembling modes via chemical reduction of dpaAu, NaOMe, and ^FNHC^Bn·HBr by NaBH₄ (Hdpa is dipyridylamine; ^FNHC^Bn·HBr is 1,3-dibenzyl-5,6-difluoro-1H-benzo[d]imidazole-3-ium bromide). In situ UV-vis and NMR studies have elucidated the base-assisted formation of NHCs from imidazolium salts for the protection of the metal core. This work not only reports a new NHC-ligated superatom that completes the Au₁₃ library, thus facilitating structure-property studies, but also opens the door to explore underlying analogues in a facile and reasonable way.

From 8- to 18-Cluster Electrons Superatoms: Evaluation via DFT Calculations of the Ligand-Protected W@Au12(dppm)6 Cluster Displaying Distinctive Electronic and Optical Properties

The iconic W@Au₁₂ icosahedral bare cluster reaches the favorable closed-shell superatomic electron configuration 1S² 1P⁶ 1D¹⁰, making it an 18-cluster electron (18-ce) superatom. Here, we pursue the evaluation of a ligand-protected counterpart based on the construction of a fully phosphine-protected [W@Au₁₂(dppm)₆] cluster strongly related to the characterized [Au₁₃(dppm)₆]⁵⁺ homometallic counterpart. The later cluster has the same total number of valence electrons as the former but is considered an 8-ce superatom with 1S² 1P⁶ configuration. The fundamental differences between 8- and 18-ce species are investigated. The character of the frontier orbitals varies from 1P/1D in the 8-ce case to a 1D/ligand for 18-ce species, enabling an efficient charge transfer toward the ligands upon irradiation, being interesting for electron injection in optoelectronic devices and black absorbers applications. Excited-state properties are also revisited, showing different geometrical and electronic structure variations between 8- and 18-ce species. Moreover, the continuum between the 8- and 18-ce limits has been explored by varying the nature of the encapsulated dopant between group 6 and group 11. The transition between the 8- and 18-ce counts can be formally situated between Pt (8-ce) and Ir (18-ce). Thus, 18-ce derivatives obtained as doped counterparts of homometallic gold clusters can introduce useful alternatives to achieve different properties in related structural motifs, which can be further explored owing to their extension of the well-established versatility of current gold nanoclusters.

An atomically precise Ag18Cu8 nanocluster with rich alkynyl-metal coordination structures and unique SbF6- assembling modes

Elucidating the coordination structures and assembling modes of atomically precise metal nanoclusters (NCs) remains a hot topic as it gives answers to the underlying mechanism of nanomaterials and bulk materials in terms of structure-property relationships. Here we report a novel silver-copper alloy NC featuring rich alkynyl-metal coordination modes and unique SbF₆^- assembling structures. The NC, with the composition of [Ag₁₈Cu₈(dppp)₄(^tBu-C₆H₄CC)₂₂](SbF₆)₄ (dppp = 1,3-bis(diphenylphosphino)-propane), was prepared by a stepwise synthetic approach. Single-crystal X-ray diffraction analysis revealed that such a NC featured a staircase-like Ag₁₈Cu₈ kernel, which was protected by hybrid alkynyl and dppp ligands in diverse coordination structures and multiple environments. The structural analysis also revealed the unique function of SbF₆^- in inducing the assembly of cluster moieties, highlighting the importance of counterions in assembling nanomolecules. The diverse coordination structures of the protective ligands with metal ions and the indispensable roles of counterions in assembling the cluster moieties have also been supported by nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS) studies, making it a model system to showcase the uniqueness of atomically precise metal NCs in illustrating the coordination chemistry of nanomaterials and bulk materials at the molecular level.

Alloying and dealloying of Au18Cu32 nanoclusters at precise locations via controlling the electronegativity of substituent groups on thiol ligands

The doping site of metals in an alloy nanocluster plays a key role in determining the cluster properties. Herein, we found that alloying engineering was achieved by replacing Cu at specific positions in the second layer Cu₂₀ shell of the [Au18Cu32(SR-O)36]2- or [Au18Cu32(SR-F)36]3- (SR-O = -S-PhOMe; SR-F = -SC₆H₃^3,4F₂) nanocluster with Au to generate a core-shell [Au20.31Cu29.69(SR-O)36]2- protected by mercaptan ligands with electron-donating substituents, which could be stable obtained compared with the alloyed nanocluster with electron-withdrawing substituent ligands. Moreover, dealloying engineering was accomplished by an electron-withdrawing substituent ligand exchange strategy (i.e., [Au18Cu32(SR-F)36]2-). The abovementioned reaction was analyzed using single-crystal X-ray crystallography, electrospray ionization mass spectrometry, and X-ray photoelectron spectroscopy and monitored via time-dependent ultraviolet-visible absorption spectroscopy. This reversible and precise location of alloying and dealloying provides the possibility for studying the relationship between the structure and properties of nanoclusters at the atomic level.

Bimetallic Nanomaterials: A Promising Nanoplatform for Multimodal Cancer Therapy

Bimetallic nanomaterials (BMNs) composed of two different metal elements have certain mixing patterns and geometric structures, and they often have superior properties than monometallic nanomaterials. Bimetallic-based nanomaterials have been widely investigated and extensively used in many biomedical fields especially cancer therapy because of their unique morphology and structure, special physicochemical properties, excellent biocompatibility, and synergistic effect. However, most reviews focused on the application of BMNs in cancer diagnoses (sensing, and imaging) and rarely mentioned the application of the treatment of cancer. The purpose of this review is to provide a comprehensive perspective on the recent progress of BNMs as therapeutic agents. We first introduce and discuss the synthesis methods, intrinsic properties (size, morphology, and structure), and optical and catalytic properties relevant to cancer therapy. Then, we highlight the application of BMNs in cancer therapy (e.g., drug/gene delivery, radiotherapy, photothermal therapy, photodynamic therapy, enzyme-mediated tumor therapy, and multifunctional synergistic therapy). Finally, we put forward insights for the forthcoming in order to make more comprehensive use of BMNs and improve the medical system of cancer treatment.

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.