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.

Thiacalix[4]arene Etching of an Anisotropic Cu70H22 Intermediate for Accessing Robust Modularly Assembled Copper Nanoclusters.

Atom-precise metal nanoclusters (NCs) with large bulk (nuclearity >60) are important species for insight into the embryonic phase of metal nanoparticles and their top-down etching synthesis. Herein, we report a metastable rod-shaped 70-nuclei copper-hydride NC, [Cl@Cu70H22(PhC≡C)29(CF3COO)16]2+ (Cu70), with Cl- as the template, in which the Cl@Cu59 kernel adopts a distinctive metal packing mode along the bipolar direction, and the protective ligand shell exhibits corresponding site differentiation. In terms of metal nuclearity, Cu70 is the largest alkynyl-stabilized Cu-hydride cluster to date. As a typical highly active intermediate, Cu70 could undergo a transformation into a series of robust modularly assembled Cu clusters (B-type Cu8, A-A-type Cu22, A-B-type Cu23, and A-B-A-type Cu38) upon etching by p-tert-butylthiacalix[4]arene (H4TC4A), which could not be achieved by "one-pot" synthetic methods. Notably, the patterns of A and B blocks in the Cu NCs could be effectively modulated by employing appropriate counterions and blockers, and the modular assembly mechanism was illustrated through comprehensive solution chemistry analysis using HR-ESI-MS. Furthermore, catalytic investigations reveal that Cu38 could serve as a highly efficient catalyst for the cycloaddition of propargylic amines with CO2 under mild conditions. This work not only enriched the family of high-nuclear copper-hydride NCs but also provided new insights into the growth mechanism of metal NCs.

Chiral Hydride Cu18 Clusters Transform to Superatomic Cu15Ag4 Clusters: Circularly Polarized Luminescence Lighting.

The manipulation of metal cluster enantiomers and their reconstruction remain challenging. Here, for the first time, we report an enantiomeric pair of hydride copper clusters [Cu18H(R/S-PEA)12](BF4)5 (R/S-Cu18H) made using designed chiral ligands. By manipulation of R/S-Cu18H with Ag+ ions, H- ions are released, leading to the reconstruction of 15 Cu atoms. Moreover, 4 Ag atoms replaced Cu atoms at the specific sites, resulting in the formation of homochiral [Cu15Ag4(R/S-PEA)12](BF4)5 (R/S-Cu15Ag4) with an isomorphic metal skeleton. This process was accompanied by a reduction reaction generating two free valence elections in the chiral alloying counterparts, which displayed orange emission. The solid-state R/S-Cu15Ag4 exhibited a photoluminescence quantum yield of 7.02% and excellent circularly polarized luminescence. The chiral transformations were resolved by single-crystal X-ray diffraction. The development of chiral copper hydride precursor-based metal clusters with chiroptical activities holds tremendous promise for advancing the field of optoelectronics and enabling new applications in lighting, displays, and beyond.

Photoluminescence Anisotropy in Eutectic Crystals of Polynuclear Lanthanide Complexes and Silver Clusters.

Anisotropy is an intrinsic property of crystalline materials. However, the photoluminescence anisotropy in eutectic crystals of organometallic complexes has remained unexplored. Herein, the eutectic of polynuclear lanthanide complexes and Ag clusters was prepared, and the crystal shows significant photoluminescence anisotropy. The polarization anisotropy of emission δ and degree of excitation polarization P are 2.62 and 0.53, respectively. The rare excitation polarization properties have been proved to be related to the regular arrangement of electric transition dipole moments of luminescent molecules in the crystal. Our design provides a reference for developing new photoluminescence anisotropy materials and expanding their applications.

Predesigned Cluster-Based Spacers for Versatile Luminescent Metallacages.

Luminescent metal-organic cages are of great interest in contemporary research; however, their designed synthesis remains challenging. Here, we created metal-cluster-derived spacers, where emissive C3-symmetric Cu4 clusters have three arms modified by benzene alkynyl ligands, which are terminally functionalized by extensile -COOH and 15-crown-5-ether groups with directional coordination ability. Through vertex orientation, -COOH-functionalized cluster-based spacers coassembled with paddle-wheel Cu(I)xZn(II)2-x(COO)3 nodes in 3+3 mode, generating an emissive cubic cage, which subsequently gave another distorted cubic cage by synthetic modification on the nodes. Through face orientation, 15-crown-5-ether-containing cluster-based spacers capturing K+ ions in 3+2 mode produced an octahedral cage whose empty phase showed dual emission peaks, leading to diverse stimuli-responsive photoluminescence. This work provides new design and synthesis strategies for the integration of nodes and spacers based on metal clusters for cage materials as well as prototypes of luminescent metal-cluster cages for important sensing applications.

Modular Cocrystallization of Customized Carboranylthiolate-Protected Copper Nanoclusters via Host-Guest Interactions.

Cocrystals containing distinct atom-precise metal nanoclusters (NCs) provide an opportunity to elucidate the crystallization process, architectural complexity, and newly emerging properties of condensed-state metal NC-assembled materials. However, the controllable preparation of such cocrystals is still challenging. Herein, we present a modular strategy to cocrystallize two customized carboranylthiolate-protected copper NCs, Cu14(C2B10H10S2)6(CH3CN)6 (Cu14) and Cu16(C2B10H10S2)8 (Cu16), which adopt matched surface patterns by host-guest chemistry. The Cu14·Cu16 cocrystals show integrated UV-vis adsorption and dual emission stemming from the Cu14 and Cu16 NCs. Moreover, the component NCs are selectively doped by gold atoms, which is a promising way to incorporate diverse properties of metal cluster-based cocrystals. This work not only provides a copper NC-based cocrystal for a profound study on a condensed-state copper nanomaterial but also develops a modular strategy for the cocrystallization of metal NCs.

Achiral-Core-Metal Change in Isomorphic Enantiomeric Ag12Ag32 and Au12Ag32 Clusters Triggers Circularly Polarized Phosphorescence.

Understanding how the chiral or achiral section in chiral nanostructures contributes to circularly polarized light emission (CPLE) at the atomic level is of fundamental importance. Here, we report two pairs of atomically precise enantiomers of homosilver (R/S-Ag12Ag32) and heterometal (R/S-Au12Ag32) clusters. The geometrical chirality of R/S-Ag12Ag32 arises from the chiral ligand and interface consisting of positive moieties of Ag32(R/S-PS)24. The circular dichroism of R/S-Ag12Ag32 is active, but CPLE-silent. A complete metal change from Ag12 to Au12 in the achiral core section of S2-@M12@S8 engenders isomorphous heterometal R/S-Au12Ag32, which activates CPLE. We further quantify the contributions of achiral and chiral sections and for the first time unveil that heterometal bonding (Au12-Ag32) at the linkage varies the delocalization of orbitals and proportion of achiral and chiral section in electron transition-involved orbitals, thus activating CPLE. Based on these unique atomically precise homochiral metal clusters, our work provides a new insight into the contributions of achiral and chiral sections to the origin of chiroptical response of chiral metal clusters, paving the way to advance the development of CPLE nanoparticles.

Aggregation-induced barrier to oxygen-a new AIE mechanism for metal clusters with phosphorescence.

Metal clusters are useful phosphors, but highly luminescent examples are quite rare. Usually, the phosphorescence of metal clusters is hindered by ambient O2 molecules. Transforming this disadvantage into an advantage for meaningful applications of metal clusters presents a formidable challenge. In this work, we used ligand engineering to judiciously prepare colour-tuneable and brightly emitting Cu(I) clusters that are ultrasensitive to O2 upon dispersion in a fluid solution or in a solid matrix. When the O2 scavenger dimethyl sulfoxide (DMSO) was used as the solvent, joint photo- and oxygen-controlled multicolour switches were achieved for the first time for metal cluster-based photopatterning and photo-anticounterfeiting. More importantly, an aggregation-induced barrier to oxygen, a new aggregation-induced emission mechanism for metal clusters, was proposed, providing a new pathway to realizing the intense emission of metal clusters in the aggregated state. These results are expected to promote the application of metal clusters and enrich the luminescence theory of metal cluster aggregates.

Phosphate anion-induced silver-chalcogenide cluster-based metal organic frameworks as dual-functional catalysts for detoxifying chemical warfare agent simulants.

Two porphyrinic silver-chalcogenide cluster-based MOFs were achieved using a phosphate anionic template strategy, and the highly photoactive organic building modules combined with Lewis acidic silver clusters allow both SCC-MOFs to be used as versatile catalysts for the simultaneous degradation of sulfur mustard and nerve agent simulants.

Ligand-Shell Engineering of a Au28 Nanocluster Boosts Electrocatalytic CO2 Reduction.

Subtle tailoring of gold nanoclusters (NCs) could significantly change their physicochemical properties. However, direct comparison of the catalytic performance of gold NCs with identical metal cores but distinct ligand shells is rarely elucidated. In this work, a novel gold NC, Au28 (C2 B10 H11 S)12 (tht)4 Cl4 (Au28 -S), was isolated by a facile self-reducing synthesis. Au28 -S adopts an identical Au28 metal framework to that of the reported alkynyl-protected Au28 -C. The different protective layers lead to distinctions in their electronic structure and optical properties. Furthermore, Au28 -S shows better catalytic activity for the electrochemical reduction of CO2 to CO. Theoretical calculations identified the active sites and shed light on the catalytic mechanism to elucidate the different catalytic performances. This work provides an ideal platform to study the protective layer-activity relationship of gold NCs, and may also provide guidance in the design of metal NC-based catalysts.

Mediating CO2 Electroreduction Activity and Selectivity over Atomically Precise Copper Clusters.

Atomically precise copper clusters are highly desirable catalysts for electrocatalytic CO2 reduction reaction (CO2 RR) and provide an ideal platform for elaborating structure-activity relationships. However, systematic comparative studies of Cu cluster isomers for electrocatalytic CO2 RR are lacking because they are challenging to synthesize. A group of structurally precise Cu8 cluster isomers with different core structures (cube- and ditetrahedron-shaped) were developed and investigated for highly active and selective CO2 reduction. Electrocatalytic measurements showed that the ditetrahedron-shaped Cu8 cluster exhibited a higher FEHCOOH (≈92 %) at -1.0 V and higher selectivity than the cube-shaped cluster. Theoretical investigations revealed different levels of competitiveness with the hydrogen evolution reaction on the respective core-shaped Cu8 clusters and decreased free energies for the adsorbed HCOO* intermediates on the ditetrahedron-shaped Cu8 clusters.

Directional Doping and Cocrystallizing an Open-Shell Ag39 Superatom via Precursor Engineering.

Metal precursors employed in the bottom-up synthesis of metal nanoclusters (NCs) are of great importance in directing their composition and geometrical structure. In this work, a silver nanocluster co-protected by phosphine and thiolate, namely, [Ag39(PFBT)24(TPP)8]2- (Ag39, PFBT = pentafluorobenzenethiol, TPP = triphenylphosphine), was isolated and structurally characterized. It adopts a three-layered Ag13@Ag18@Ag8S24P8 core-shell structure. The Ag13@Ag18 kernel is unusual in multilayer noble metal NCs. By introducing a copper precursor in the synthesis, a bimetallic nanocluster [Ag37Cu2(PFBT)24(TPP)8]2- (Ag37Cu2) with an identical structure to Ag39 apart from two outer Ag atoms being substituted by Cu atoms was obtained. Astoundingly, the Cu precursor used in the synthesis was found to be critical in determining the final structure. The alteration of the Cu precursor led to the cocrystallization of the above alloy nanocluster with a Ag14 nanocluster, namely, [Ag37Cu2(PFBT)24(TPP)8]2-·[Ag14(PFBT)6(TPP)8] (Ag37Cu2·Ag14). The electronic structure analyzed by theoretical calculation reveals that Ag39 is a 17-electron open-shell superatom. The optical absorption of Ag39, Ag37Cu2, and Ag37Cu2·Ag14 was compared and studied in detail. This work not only enriches the family of alloy metallic nanoclusters but also provides a metal NC-based cocrystal platform for in-depth study of its crystal growth and photophysical property.

Small symmetry-breaking triggering large chiroptical responses of Ag70 nanoclusters.

The origins of the chiroptical activities of inorganic nanostructures have perplexed scientists, and deracemization of high-nuclearity metal nanoclusters (NCs) remains challenging. Here, we report a single-crystal structure of Rac-Ag70 that contains enantiomeric pairs of 70-nuclearity silver clusters with 20 free valence electrons (Ag70), and each of these clusters is a doubly truncated tetrahedron with pseudo-T symmetry. A deracemization method using a chiral metal precursor not only stabilizes Ag70 in solution but also enables monitoring of the gradual enlargement of the electronic circular dichroism (CD) responses and anisotropy factor gabs. The chiral crystals of R/S-Ag70 in space group P21 containing a pseudo-T-symmetric enantiomeric NC show significant kernel-based and shell-based CD responses. The small symmetry breaking of Td symmetry arising from local distortion of Ag-S motifs and rotation of the apical Ag3 trigons results in large chiroptical responses. This work opens an avenue to construct chiral medium/large-sized NCs and nanoparticles, which are promising for asymmetric catalysis, nonlinear optics, chiral sensing, and biomedicine.

Photo/Electrochromic Dual Responsive Behavior of a Cage-like Zr(IV)-Viologen Metal-Organic Polyhedron (MOP).

Stable stimulus-responsive materials are highly desirable due to their widespread potential applications and growing demand in recent decades. Despite the fact that viologen derivatives have long been known as excellent photochromic and electrochromic materials, the development of stable viologen-based multifunctional smart materials with short coloration times remains an exciting topic. To obtain photochromic and electrochromic dual responsive materials, embedding the viologen ligand into a robust metal oxide cluster to increase its stability and sensitivity is an effective strategy. Herein, a viologen-based metal-organic polyhedron (MOP) {[Zr6L3(µ3-O)2(µ2-OH)6Cp6]·8Cl·CH3OH·DMF} [Zr-MOP-1; H2L·2Cl = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium dichloride, and Cp = η5-C5H5] was successfully prepared and characterized. It consists of trinuclear Zr-oxygen secondary building units and exhibits reversible photochromic and electrochromic dual responsive behaviors. As expected, the designed robust viologen-based nanocage with a V2E3 (V = vertex, and E = edge) topology can maintain its stability and rapid photo/electrochromic behaviors with an obvious reversible change in color from purple (brown) to green, mainly due to the enclosed cluster structure and the abundant free viologen radicals that originate from the effective Cl → N and O → N electron transfers. Spectroelectrochemistry and theoretical calculations of this Zr-MOP were also performed to verify the chromic mechanism.

Electropolymerization of Metal Clusters Establishing a Versatile Platform for Enhanced Catalysis Performance.

Atomically precise metal clusters are attractive as highly efficient catalysts, but suffer from continuous efficiency deactivation in the catalytic process. Here, we report the development of an efficient strategy that enhances catalytic performance by electropolymerization (EP) of metal clusters into hybrid materials. Based on carbazole ligand protection, three polymerized metal-cluster hybrid materials, namely Poly-Cu14 cba, Poly-Cu6 Au6 cbz and Poly-Cu6 Ag4 cbz, were prepared. Compared with isolated metal clusters, metal clusters immobilizing on a biscarbazole network after EP significantly improved their electron-transfer ability and long-term recyclability, resulting in higher catalytic performance. As a proof-of-concept, Poly-Cu14 cba was evaluated as an electrocatalyst for reducing nitrate (NO3 - ) to ammonia (NH3 ), which exhibited ≈4-fold NH3 yield rate and ≈2-fold Faraday efficiency enhancement compared to that of Cu14 cba with good durability. Similarly, Poly-Cu6 Au6 cbz showed 10 times higher photocatalytic efficiency towards chemical warfare simulants degradation than the cluster counterpart.

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 (glum) 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.

Symmetry Breaking of Atomically Precise Fullerene-like Metal Nanoclusters.

Here we report a neutral fullerene-like core-shell homosilver Ag13@Ag20 nanocluster that is fully protected by an achiral bidentate thiolate ligand (9,12-dimercapto-1,2-closo-carborane, C2B10H10S2H2), which crystallizes in centrosymmetric space group R3̅. Continuous Cu doping in the dodecahedral shell first induced symmetry breaking to generate chiral Ag13@Ag20-nCun (6 ≥ n ≥ 2) containing two acetonitrile ligands in space group P212121, and then produced symmetric all-thiolated Ag13@Ag20-nCun (20 ≥ n ≥ 13) in the higher space group Im3̅. The selectively copper-doped Ag13@Ag20-nCun (6 ≥ n ≥ 2) cluster has its structure reorganized to a lower symmetry that shows chiroptical activity. Moreover, structural distortion of Ag13@Ag20-nCun (6 ≥ n ≥ 2) further expanded in chiral R-/S-propylene oxide, which induced a more prominent core-based CD response. This work revealed a novel mechanism of chirality generation at the atomic level through asymmetric shell-doping of metal nanoclusters, which provides new insight into the origin of chirality in inorganic nanostructures.

Construction of Core-Shell MOF@COF Hybrids with Controllable Morphology Adjustment of COF Shell as a Novel Platform for Photocatalytic Cascade Reactions.

Recently, novel core-shell MOF@COF hybrids display excellent performance in various fields because of their inherited advantages from their parent MOFs and/or COFs. However, it is still a grand challenge to adjust the morphology of MOFs and/or COFs for consequent performance improvement. Herein, a Ti-MOF@TpTt hybrid coated with ultra-thin COF nanobelt, which is different from the fibrillar-like parent COF, is successfully synthesized through a sequential growth strategy. The as-obtained Pd decorated Ti-MOF@TpTt catalyst exhibits much higher photocatalytic performance than those of Ti-MOF, TpTt-COF, and Ti-MOF@TpTt hybrids with fibrillar-like COF shell for the photocatalytic cascade reactions of ammonia borane (AB) hydrolysis and nitroarenes hydrogenation. These can be attributed to its high BET surface area, core-shell structure, and type II heterojunction, which offers more accessible active sites and improves the separation efficiency of photo-generated carriers. Finally, the possible mechanisms of the cascade reaction are also proposed to well explain the improved performance of this photocatalytic system. This work presents a constructive route for designing core-shell MOF@COF hybrids with controllable morphology adjustment of COF shell, leading to the improved photocatalytic ability to broaden the applications of MOF/COF hybrid materials.

Pyrolysis-Free Synthesized Catalyst towards Acidic Oxygen Reduction by Deprotonation.

Acidic oxygen reduction is vital for renewable energy devices such as fuel cells. However, many aspects of the catalytic process are still uncertain-especially the large difference in activity in acidic and alkaline media. Thus, the design and synthesis of model catalysts to determine the active centers and the inactivation mechanism are urgently needed. We report a pyrolysis-free synthesis route to fabricate a catalyst (CPF-Fe@NG) for oxygen reduction in acidic conditions. By introducing a deprotonation process, we extended the oxygen reduction reaction (ORR) activity from alkaline to acidic conditions. CPF-Fe@NG demonstrated outstanding performance with a half-wave potential of 853 mV (vs. RHE) and good stability after 10000 cycles in 1 M HClO4 . The pyrolysis-free route could also be used to assemble fuel cells, with a maximum power density of 126 mW cm-2 . Our findings offer new insights into the ORR process to optimize catalysts for both mechanistic studies and practical applications.