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.

Temperature-Controlled Selective Formation of Silver Nanoclusters and Their Transformation to the Same Product

Herein, two atomically precise silver nanoclusters, Ag54 and Ag33, directed by inner anion templates (CrO4 2- and/or Cl-), are initially isolated as a mixed phase from identical reactants across a wide temperature range (20-80 °C). Interestingly, fine-tuning the reaction temperature can realize pure phase synthesis of the two nanoclusters; that is, a metastable Ag54 is kinetically formed at a low temperature (20 °C), whereas such a system is steered towards a thermodynamically stable Ag33 at a relatively high temperature (80 °C). Electrospray ionization mass spectrometry illustrates that the stability of Ag33 is superior to that of Ag54, which is further supported by density functional theory calculations. Importantly, the difference in structural stability can influence the pathway of 1,4-bis(pyrid-4-yl)benzene induced transformation reaction starting from Ag54 and Ag33. The former undergoes a dramatic breakage-reorganization process to form an Ag31 dimer (Ag31), while the same product can be also achieved from the latter following a noninvasive ligand exchange process. Both the Ag54 and Ag33 have the potential for further remote laser ignition applications. This work not only demonstrates how temperature controls the isolation of a specific phase, but also sheds light on the structural transformation pathway of nanoclusters with different stability.

Eight-Electron Copper Nanoclusters for Photothermal Conversion

Owing to distinct physicochemical properties in comparison to gold and silver counterparts, atomically precise copper nanoclusters are attracting embryonic interest in material science. The introduction of copper cluster nanomaterials in more interesting fields is currently urgent and desired. Reported in this work are novel copper nanoclusters of [XCu54Cl12(tBuS)20(NO3)12] (X=S or none, tBuSH=2-methyl-2-propanethiol), which exhibit high performance in photothermal conversion. The clusters have been prepared in one pot and characterized by combinatorial techniques including ultraviolet-visible spectroscopy (UV-vis), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS). The molecular structure of the clusters, as revealed by single crystal X-ray diffraction analysis (SCXRD), shows the concentric three-shell Russian doll arrangement of X@Cu14@Cl12@Cu40. Interestingly, the [SCu54Cl12(tBuS)20(NO3)12] cluster contains 8 free valence electrons in its structure, making it the first eight-electron copper nanocluster stabilized by thiolates. More impressively, the clusters possess an effective photothermal conversion (temperature increases by 71 °C within ~50 s, λex=445 nm, 0.5 W cm-2) in a wide wavelength range (either blue or near-infrared). The photothermal conversion can be even driven under irradiation of simulated sunlight (3 sun), endowing the clusters with great potency in solar energy utilization.

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.

Understanding the decomposition process of the Pt1Ag24(SPhCl2)18 nanocluster at the atomic level

We report the decomposition of the Pt1Ag24(SPhCl2)18 nanocluster into a crown-like Pt1Ag4(SR)8 (SR = 2,4-SPhCl2 and 4-SPhBr) complex. UV-vis spectra and single crystal X-ray diffraction were used to track the structure-conversion process. Based on the total structure, the differences in ligand exchange rates at different sites and the effects on the stability were mapped out. This work can not only help us understand the ligand exchange behavior of the clusters, but also provide experimental support for the design of stable metal clusters.

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.

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.

A bimetallic Ag15Cu12(S-c-C6H11)18(CH3COO)3 nanocluster featuring an irregular Ag12 kernel

Here, we report the synthesis and atomic structure of a Ag₁₅Cu₁₂(SR)₁₈(CH₃COO)₃·(C₆H₁₄) nanocluster (Ag₁₅Cu₁₂ for short, SR denotes cyclohexanethiol), confirmed by single-crystal X-ray diffraction (SC-XRD), electrospray ionization mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). X-ray crystallographic analysis revealed that Ag₁₅Cu₁₂ consisted of an irregular Ag₁₂ core, stabilized by the Ag₃Cu₁₂(SR)₁₈(CH₃COO)₃ shell. The shell consisted of two nearly planar Cu₃(SR)₆ moieties, three monomeric [-SR-Ag-SR-] units and three Cu₂(CH₃COO) staples. Furthermore, time-dependent density functional theory (TD-DFT) simulation was performed to interpret the optical absorption features of Ag₁₅Cu₁₂. Overall, this work will broaden and deepen the understanding of Ag-Cu alloy nanoclusters.

Co-reactant-Free Transformation in Atomically Precise Metal Nanoclusters

Transformation chemistry has advanced significantly in recent years as an excellent methodology for synthesizing new nanoclusters and functionalizing the existing ones. However, rational synthesis and fundamental understanding of the structural evolution among clusters have not yet been achieved in nanocluster science. A deeper understanding of the fundamental aspects of structure-property correlation is necessary for the employment of befitting nanoclusters for specific applications. Very recently, the transformation of nanoclusters without the use of conventional co-reactants has been brought to light. These co-reactant-less transformations are triggered by various conditions, such as pH, solvent, light, temperature, etc. In this perspective, we discuss how this unique method of transformation without any co-reactant benefits the basic understanding of growth patterns and the corresponding property evolution in nanoclusters.

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

Recent reports on hydride-doped noble metal nanoclusters strongly suggest that the encapsulated hydride is a part of the superatom core, but no accurate location of the hydride could be experimentally proved, so far. We report herein a hydride-doped eight-electron platinum/silver alloy nanocluster in which the position of four-coordinated hydride was determined by neutron diffraction for the first time. X-ray structures of [PtHAg₁₉(dtp/desp)₁₂] (dtp = S₂P(OⁿPr)₂, 1; dsep = Se₂P(OⁱPr)₂, 2) describe a central platinum hydride (PtH) unit encapsulated within a distorted Ag₁₂ icosahedron, the resulting (PtH)@Ag₁₂ core being stabilized by an outer sphere made up of 7 capping silver atoms and 12 dichalcogenolates. Solid-state structures of 1 and 2 differ somewhat in the spatial configuration of their outer spheres, resulting in overall different symmetries, C₁ and C₃, respectively. Whereas the multi-NMR spectra of 2 in solution at 173 K reveal that the structure of C₃ symmetry is the predominant one, ¹H and ¹⁹⁵Pt NMR spectra of 1 at the same temperature disclose the presence of isomers of both C₁ and C₃ symmetry. DFT calculations found both isomers to be very close in energy, supporting the fact that they co-exist in solution. They also show that the [PtH@Ag₁₂]⁵⁺ kernel can be viewed as a closed-shell superatomic core, the μ₄-hydride electron contributing to its eight-electron count. On the other hand, the 1s(H) orbital contributes only moderately to the superatomic orbitals, being mainly involved in the building of a Pt-H bonding electron pair with the 5d_z²(Pt) orbital.

Two Heterometallic Nanoclusters [DyIII4NiII8] and [DyIII10MnIII4MnII2]: Structure, Assembly Mechanism, and Magnetic Properties

A full understanding of the assembly mechanisms of coordination complexes is of great importance for a directional synthesis under control. We thus explored here the formation mechanisms of the two new heterometallic nanoclusters [Dy^III₄Ni^II₈(μ₃-OH)₈(L)₈(OAc)₄(H₂O)₄]·3.25EtOH·4CH₃CN (1) and [Dy^III₁₀Mn^III₄Mn^II₂O₄(OH)₁₂(OAc)₁₆(L)₄(HL)₂(EtOH)₂]·2EtOH·2CH₃CN·2H₂O (2) with different cubane-based squarelike ring structures, which were obtained from the reactions of 4-bromo-2-[(2-hydroxypropylimino)methyl]phenol (H₂L) with Dy(NO)₃·6H₂O and the transition metal salt Ni(OAc)₂·4H₂O or Mn(OAc)₂·4H₂O. The high-resolution electrospray ionization mass spectrometry (HRESI-MS) tests showed that the skeletons of clusters 1 and 2 have a high stability under the measurement conditions for HRESI-MS. The intermediates formed in the reaction courses of clusters 1 and 2 were tracked using time-dependent HRESI-MS, which helped to determine the proposed hierarchical assembly mechanisms for 1 (H₂L → NiL → Ni₂L₂ → Ni₃L₄ → Ni₄L₄ → DyNi₄L₅ → Dy₂Ni₆L₆ → Dy₃Ni₆L₆ → Dy₃Ni₇L₇ → Dy₄Ni₈L₈) and 2 (H₂L → MnL → DyMnL → DyMn₂L → Dy₂Mn₂L_x → Dy₈Mn₂L₂ → Dy₁₀Mn₂L₂ → Dy₁₀Mn₆L_x and H₂L → DyL → Dy₄L₂ → Dy₆L₂ → Dy₈Mn₂L₂ → Dy₁₀Mn₂L₂ → Dy₁₀Mn₆L_x). This is one of the rare examples of investigating the assembly mechanisms of 3d-4f heterometallic clusters. Magnetic studies indicated that the title complexes both show slow magnetic relaxation behaviors and cluster 1 is a field-induced single-molecule magnet.

[Pt1Ag37(SAdm)21(Dppp)3Cl6]2+: intercluster transformation and photochemical properties

A novel [Pt1Ag37(SAdm)21(Dppp)3Cl6]2+ nanocluster is reported, and the reaction with PPh3 triggers an intercluster transformation into [Pt1Ag28(SAdm)18(PPh3)4]2+. Using chiral Bdpp, the enantiomeric Pt1Ag37(SAdm)21(R/S-Bdpp)3Cl6 can be prepared.