Chiral metal cluster and nanocluster complexes and their application in asymmetric catalysis

Enantioselective Synthesis of Homochiral Hierarchical Nd8Fe3-Oxo Cluster from Racemic Nd9Fe2-Oxo Cluster

Enantioselective synthesis of homochiral rare earth clusters is still a great challenge. In this work, we developed an efficient "cluster to cluster" approach, that is, a pair of enantiomerical R/S-{Nd8Fe3}-oxo clusters were successfully obtained from the presynthesized racemic {Nd9Fe2}-oxo cluster. R/S-hydrobenzoin ligands trigger the transformation of the pristine clusters by an SN2-like mechanism. Compared to the pristine cluster with an achiral core, the new cluster exhibits hierarchical chirality, from ligand chirality to interface chirality, then to helix chirality, and finally to supramolecular double helix chirality. The spectral experiments monitored the transformation and confirmed distinctly structure-related optical activity. The enantiomeric pure cluster also exhibits a potential asymmetric catalytic activity.

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.

Chiroptical Activity Amplification of Chiral Metal Nanoclusters via Surface/Interface Solidification

It remains a grand challenge to amplify the chiroptical activity of chiral metal nanoclusters (NCs) although it is desirable for fundamental research and practical application. Herein, we report a strategy of surface/interface solidification (SIS) for enhancing the chiroptical activity of gold NCs. Structural analysis of [Au19(2R,4R/2S,4S-BDPP)6Cl2]3+ (BDPP is 2,4-bis(diphenylphosphino)pentane) clusters reveals that one of the interfacial gold atoms is flexible between two sites and large space is present on the surface, thus hampering chirality transfer from surface chiral ligands to metal core and leading to low chiroptical activity. Following SIS by filling the flexible sites and replacing chlorides with thiolate ligands affords another pair of [Au20(2R,4R/2S,4S-BDPP)6(4-F-C6H4S)2]4+, which shows a more compact and organized structure and thus an almost 40-fold enhancement of chiroptical activity. This work not only provides an efficient approach for amplifying the chiroptical activity of metal nanoclusters but also highlights the significance of achiral components in shaping chiral nanostructures.

Emerging trends in chiral inorganic nanomaterials for enantioselective catalysis

Asymmetric transformations and synthesis have garnered considerable interest in recent decades due to the extensive need for chiral organic compounds in biomedical, agrochemical, chemical, and food industries. The field of chiral inorganic catalysts, garnering considerable interest for its contributions to asymmetric organic transformations, has witnessed remarkable advancements and emerged as a highly innovative research area. Here, we review the latest developments in this dynamic and emerging field to comprehensively understand the advances in chiral inorganic nanocatalysts and stimulate further progress in asymmetric catalysis.

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.

Chiral copper-hydride nanoclusters: synthesis, structure, and assembly

An effective strategy is developed to synthesize a novel and stable layered Cu nanocluster using a one-pot reduction method. The cluster, with a molecular formula of [Cu₁₄(^tBuS)₃(PPh₃)₇H₁₀]BF₄ which has been unambiguously characterized by single crystal X-ray diffraction analysis, exhibits different structures from previously reported analogues with core-shell geometries. In the absence of chiral ligands, the cluster displays intrinsic chirality owing to the non-covalent ligand-ligand interactions (e.g., C-H⋯Cu interactions and C-H⋯π interactions) to lock the central copper core. The interlacing of chiral-cluster enantiomers forms a large cavity, which lays the foundation for a series of potential applications such as drug filling and gas adsorption. Moreover, the C-H⋯H-C interactions of phenyl groups between different cluster moieties promote the formation of a dextral helix and realization of the self-assembly of nanostructures.

Functionalization of Keggin Fe13-Oxo Clusters

Two Keggin Fe₁₃-oxo clusters, [Pr₁₂Fe₃₃(NO₃)₆(L-van)₄(D-van)₅(TEOA)₁₂(μ₃-OH)₁₂(μ₄-OH)₁₂(μ₄-O)₂₈(H₂O)₄]·(ClO₄)₃·(NO₃)·10H₂O (1) and [Dy₁₂Fe₃₃(NO₃)₂(L-van)₃(D-van)₃(TEOA)₁₂(μ₃-OH)₁₈(μ₄-OH)₆(μ₄-O)₂₈(H₂O)₉]·(ClO₄)₅·(NO₃)₆·15H₂O (2), where L-van = l-valine, D-van = d-valine, and TEOA = triethanolamine, were prepared by using Ln³⁺ as a stabilizer. Cluster 1 crystallizes in a chiral space group of C2, while cluster 2 crystallizes in a centrosymmetric space group of Pnma. Dynamic magnetic measurements of 2 under a zero direct-current field reveal that 2 exhibits single-molecule-magnet characteristics with an energy barrier of 18.79 K. Significantly, the formation of the chiral cluster 1 is closely related to the larger radius of the Pr³⁺ ion.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Structural Control and Chiroptical Response in Intrinsically Tetra- and Pentanuclear Chiral Gold Clusters

Controlling the synthesis of chiral metal clusters in the aspects of nuclearity number, metal-metal interaction, and spatial arrangement of metal atoms is crucial for establishing the correlation of detailed structural factors with chiroptical activity. Herein, a series of enantiopure gold complexes with nuclearity numbers ranging from 2 to 5 were constructed and structurally characterized. On the basis of the annulation reaction between two aurated μ₂-imido nucleophilic units with various aldehydes, we finely adjusted the metal-metal interaction and torsion angles of a characteristic tetranuclear metal cluster by introducing different substituents into the resulting imidazolidine dianionic chiral skeleton. Further structural investigations, contrast experiments, and time-dependent density functional theory calculations confirmed that the chiroptical response of the acquired asymmetric metal clusters was mainly affected by the geometrically twisted arrangement of metal atoms. Finally, the tetranuclear gold cluster compound with the shortest intermetallic interaction and the largest torsion angle of a Au₄ core showed the highest absorption anisotropy factor up to 2.2 × 10^-3. In addition, the correlation of structural factors with the stability of chiral gold clusters was thoroughly evaluated by monitoring the CD, UV-vis, and NMR spectra at elevated temperatures. Insight into the relationship between the structural factors with the chiroptical property and stability of chiral gold clusters in this work will help us to design and achieve more stable chiral metal clusters and stimulate their practical applications in chiroptical functional materials.

Isomer dynamics of the [Au6(NHC-S)4]2+ nanocluster

The use of metal nanoclusters is strongly reliant on their size and configuration; hence, studying the potential isomers of a cluster is extremely beneficial in understanding their performance. In general, the prediction and identification of isomer structures and their properties can be challenging and computationally expensive. Our work describes an investigation to find local isomers for the previously experimentally characterized small gold cluster [Au₆(NHC-S)₄]²⁺ protected by bidentate mixed carbene-thiolate ligands. We employ the molecular dynamics simulation method where the interatomic forces are calculated from density functional theory. We find several isomers that are more stable than the isomer corresponding to the experimental crystal structure, as well as a significant impact of the finite-temperature atom dynamics on the electronic structure and optical properties. Our work highlights the growing need to investigate ligand-stabilized metal clusters to uncover isomerism and temperature effects on their properties.

Synthesis and Characterization of Enantiopure Chiral Bis NHC-Stabilized Edge-Shared Au10 Nanocluster with Unique Prolate Shape

Herein we report the first chiral Au₁₀ nanoclusters stabilized by chiral bis N-heterocyclic carbene (bisNHC) ligands. ESI-MS and single-crystal X-ray crystallography confirmed the molecular formula to be [Au₁₀(bisNHC)₄Br₂](O₂CCF₃)₂. The chiral Au₁₀ nanocluster adopts a linear edge-shared tetrahedral geometry with a prolate shape. DFT calculations provide insight into the electronic structure, optical absorption, and circular dichroism (CD) characteristics of this unique Au₁₀ nanocluster. CD spectra demonstrate chirality transfer from the chiral bisNHC ligand to the inner Au₁₀ nanocluster core. Examination of ESI-MS and UV-vis spectra show that cluster [Au₉(bisNHC)₄Br]Br₂ is formed initially and then transformed into the Au₁₀ nanocluster in solution.

Full-Color Tunable Circularly Polarized Luminescence Induced by the Crystal Defect from the Co-assembly of Chiral Silver(I) Clusters and Dyes

Four pairs of defective crystals exhibiting full-color emission and circularly polarized luminescence (CPL) with high luminescence dissymmetry factor (g_lum) values (∼3 × 10^-3) were successfully obtained by doping dye molecules into the chiral crystalline metal cluster-based matrixes. The dye molecules function as defect inducers and confer fluorescence on the crystals. Studies reveal that electrostatic interactions provide the main impetus in generating defective crystals, and the restricted effect of chiral space and the weak interactions in defect crystal enable the efficient chiral transfer from the intrinsically chiral host silver(I) clusters to achiral luminescent dopants and finally induce them to emit bright CPL. This defect engineering strategy opens a new way to versatile functions for crystalline cluster-based materials.

Synthesis and enantioseparation of chiral Au13 nanoclusters protected by bis-N-heterocyclic carbene ligands

A series of chiral Au13 nanoclusters were synthesized via the direct reduction of achiral dinuclear Au(i) halide complexes ligated by ortho-xylyl-linked bis-N-heterocyclic carbene (NHC) ligands. A broad range of functional groups are tolerated as wingtip substituents, allowing for the synthesis of a variety of functionalized chiral Au13 nanoclusters. Single crystal X-ray crystallography confirmed the molecular formula to be [Au13(bisNHC)5Cl2]Cl3, with a chiral helical arrangement of the five bidentate NHC ligands around the icosahedral Au13 core. This Au13 nanocluster is highly luminescent, with a quantum yield of 23%. The two enantiomers of the Au13 clusters can be separated by chiral HPLC, and the isolated enantiomers were characterized by circular dichroism spectroscopy. The clusters show remarkable stability, including configurational stability, opening the door to further investigation of the effect of chirality on these clusters.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Stepwise Achievement of Circularly Polarized Luminescence on Atomically Precise Silver Clusters

The weakly coordinated anionic nitrate ligands in a centrosymmetric Ag20 cluster are replaced in a stepwise manner by chiral amino acids and two achiral luminescent sulfonic-group-containing ligands while nearly maintaining the original silver(I) cage structure. This surface engineering enables the atomically precise Ag20 clusters to exhibit the high-efficiency synergetic effects of chirality and fluorescence, producing rare circularly polarized luminescence among the metal clusters with a large dissymmetry factor of (|glum|) ≈ 5 × 10-3. This rational approach using joint functional ligands further opens a new avenue to diverse multifunctional metal clusters for promising applications.

Light-Induced Size-Growth of Atomically Precise Nanoclusters

A photo-induced transformation from [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ to Au₂₈(S-c-C₆)₂₀ nanocluster was first reported in this work. The [Au₂₃(S-c-C₆)₁₆]^-(TOA)⁺ nanocluster is first excited to [Au₂₃(S-c-C₆)₁₆]^•-(TOA)⁺ by photons with energy higher than its E_g (E_g = HOMO - LUMO energy gap), and then, the negatively charged [Au₂₃(S-c-C₆)₁₆]^•- nanocluster was oxidized to the neutral state by transfering one electron to O₂. The unstable neutral cluster [Au₂₃(S-c-C₆)₁₆]⁰ obtained was decomposed into smaller nanocluster and finally reassembled into the Au₂₈(S-c-C₆)₂₀ nanocluster. Time-dependent UV-vis, matrix-assisted laser desorption/ionization time of flight mass spectrometry, electron paramagnetic resonance, and electrospray ionization mass spectrometry characterizations were performed to monitor the nanocluster size transformation.

Cations Controlling the Chiral Assembly of Luminescent Atomically Precise Copper(I) Clusters

Chiral assembly and asymmetric synthesis are critically important for the generation of chiral metal clusters with chiroptical activities. Here, a racemic mixture of [K(CH₃ OH)₂ (18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1⋅CH₃ OH) in the chiral space group was prepared, in which the chiral red-emissive anionic [Cu₅ (S^t Bu)₆ ]^- cluster was arranged along a twofold screw axis. Interestingly, the release of the coordinated CH₃ OH of the cationic units turned the chiral 1⋅CH₃ OH crystal into a mesomeric crystal [K(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (1), which has a centrosymmetric space group, by adding symmetry elements of glide and mirror planes through both disordered [Cu₅ (S^t Bu)₆ ]^- units. The switchable chiral/achiral rearrangement of [Cu₅ (S^t Bu)₆ ]^- clusters along with the capture/release of CH₃ OH were concomitant with an intense increase/decrease in luminescence. We also used cationic chiral amino alcohols to induce the chiral assembly of a pair of enantiomers, [d/l-valinol(18-crown-6)]⁺ [Cu₅ (S^t Bu)₆ ]^- (d/l-Cu_5V ), which display impressive circularly polarized luminescence (CPL) in contrast to the CPL-silent racemic mixture of 1⋅CH₃ OH and mesomeric 1.

Atomically Precise Gold-Levonorgestrel Nanocluster as a Radiosensitizer for Enhanced Cancer Therapy

Gold nanoclusters have become promising radiosensitizers due to their ultrasmall size and robust ability to adsorb, scatter, and re-emit radiation. However, most of the previously reported gold nanocluster radiosensitizers do not have a precise atomic structure, causing difficulties in understanding the structure-activity relationship. In this study, a structurally defined gold-levonorgestrel nanocluster consisting of Au₈(C₂₁H₂₇O₂)₈ (Au₈NC) with bright luminescence (58.7% quantum yield) and satisfactory biocompatibility was demonstrated as a nanoradiosensitizer. When the Au₈NCs were irradiated with X-rays, they produced reactive oxygen species (ROS), resulting in irreversible cell apoptosis. As indicated by in vivo tumor formation experiments, tumorigenicity was significantly suppressed after one radiotherapy treatment with the Au₈NCs. In addition, compared with tumors treated with X-rays (4 Gy) alone, tumors treated with the nanosensitizer exhibited an inhibition rate of 74.2%. This study contributes to the development of atomically precise gold nanoclusters as efficient radiosensitizers.

Spontaneous Resolution by Crystallization of an Octanuclear Iron(III) Complex Using Only Racemic Reagents

The P and M enantiomers of the octanuclear [Fe₈ (μ₄ -O)₄ (μ-4-Cl-pz)₁₂ Cl₄ ] complex, having T symmetry, were resolved by temporary substitution of chloride ligands by racemic 4-^s Bu-phenolates and subsequent crystallization, where the (S)- and (R)-phenolates coordinate selectively to the M and P complexes, respectively. The complexes were characterized by circular dichroism analysis and X-ray structure determination. This work constitutes a rare example of enantiomeric recognition resulting in spontaneous resolution upon crystallization.

Decorated single-enantiomer phosphoramide-based silica/magnetic nanocomposites for direct enantioseparation

The nano-composites Fe₃O₄@SiO₂@PTA(+) and Fe₃O₄@SiO₂@PTA(−) (PTA: phosphoric triamide) were prepared and used for the chiral separation of five racemic mixtures.

Temperature dependent chiroptical response of sigmoidal gold clusters: probing the stability of chiral metal clusters

The stability of chiral metal clusters is of great importance for their practical applications. Herein we select three structurally well-defined gold cluster compounds to probe how structural factors influence the stability of chiral metal clusters upon heating. Through monitoring the variation of CD, UV-vis and NMR spectra at elevated temperatures, the biased chiroptical response of three sigmoidal Au6 clusters is finally ascribed to the synergistic effect of the distinct structural tunability of central diamino ligands, inter-cluster aurophilic interactions and steric hindrance. The rigid skeleton of chiral ligands and the strong metal-metal interaction effectively enhance the stability of asymmetric structural motifs in chiral metal clusters. In addition, some central diamino ligands lead to a destructive decomposition of corresponding chiral clusters in the heating process due to the reduction of Au(i) to Au(0). The relationship between structural characteristics and the stability of chiral clusters addressed in this study will facilitate our understanding on how to achieve stable chiral metal clusters and potentiate their practical applications.