Conspectus A comprehensive understanding of chemical bonding and reactions at the surface of nanomaterials is of great importance in the rational design of their functional properties and ...applications. With the rapid development in cluster science, it has become clear that atomically precise metal clusters represent ideal models for resolving various important and/or unsolved issues related to surface science. This Account highlights our recent efforts on the fabrication of ligand-stabilized coinage nanoclusters with atomic precision from the viewpoint of surface coordination chemistry in particular. The successful synthesis of a large variety of metal clusters in our group has greatly benefitted from the development of an effective amine-assisted NaBH4 reduction method. First discussed in this Account is how the introduction of amines in the synthetic protocol enhances the long-term stability and high-yield production of Ag/Cu-based metals in air. Such a method allows the utilization of different organic ligands as surface stabilizing agents to manipulate both the core and surface structures of metal nanoclusters, helping to understand the role of surface ligands in determining the structures of metal nanoclusters. The coordination chemistry of ligands used in the synthesis of metal nanoclusters is crucial in determining their overall shape, metal arrangement, surface ligand binding structure, chirality and also metal exposure. Detailed discussions are given in the following four different systems: (1) The co-use of phosphines and thiolates with rich coordination structures (2 to 4-coordinated) helps to control the formation of a sequence of Ag nanoclusters with a near-perfectly cubic shape; (2) The metal arrangements and surface structures of AuCu clusters highly depend on metal precursors and counter cations used in the synthesis; (3) Metal clusters with intrinsic chirality are readily prepared by introducing chiral ligands or counterions, making it possible to obtain optically active enantiomers and understand the origin of chirality of metal nanoclusters; (4) The variation of metal exposure of the inner metal core of metal nanocluster can be controlled by the surface ligand coordination structure. Such capabilities to manipulate the surface structure of metal nanoclusters allow the creation of model systems for investigating the structure–reactivity relationship of metal nanomaterials. Several important examples are then discussed to highlight the importance of ligand coordination chemistry in tuning the surface reactivity and catalysis of metal nanoclusters. For example, bulky thiolates on Ag are demonstrated to be more labile than small thiolates for making metal nanoclusters with both enhanced ligand exchange capability and catalysis. Alkynyl ligands can be thermally released from metal nanoclusters more easily than thiolates and halides while maintaining the overall structure, thereby serving as ideal systems for understanding the promoting effect of surface stabilizers on catalysis. Finally, we provide a perspective on the principles of surface coordination chemistry of metal nanoclusters and their potential applications with regards to catalysis of protected metal clusters.
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Many metal clusters are intrinsically chiral but are often synthesized as a racemic mixture. By taking chiral Ag14(SPh(CF3)2)12(PPh3)4(DMF)4 (Ag 14 ) clusters with bulky thiolate ligands as an ...example, we demonstrate herein an interesting assembly disassembly (ASDS) strategy to obtain the corresponding, optically pure crystals of both homochiral enantiomers, R-Ag 14m and S-Ag 14m . The ASDS strategy makes use of two bidentate linkers with different chiral configurations, namely, (1R,2R,N 1 E,N 2 E)-N 1,N 2-bis(pyridin-3-ylmethylene)cyclohexane-1,2-diamine (LR) and the corresponding chiral analogue LS. For comparison, we also use the racemic mixture of equimolar of LR and LS (LRS). Three three-dimensional (3D) Ag14-based metal–organic frameworks (MOFs) were characterized by X-ray crystallography to be Ag14(SPh(CF3)2)12(PPh3)4(LR)2 n (Ag 14 -LR), Ag14(SPh(CF3)2)12(PPh3)4(LS)2 n (Ag 14 -LS), and Ag14(SPh(CF3)2)12(PPh3)4(LRS)2 n (Ag 14 -LRS), respectively. As expected, the building blocks in Ag 14 -LR or Ag 14 -LS are homochiral R-Ag 14 or S-Ag 14 , respectively. In contrast, Ag 14 -LRS is achiral and crystallizes with a diamond-like structure containing alternate R-Ag 14 and S-Ag 14 clusters. During the assembly process, the racemic Ag 14 clusters were converted to homochiral building blocks, namely, R-Ag 14 for Ag 14 -LR and S-Ag 14 for Ag 14 -LS. Subsequently, the chiral linkers were removed from the crystals of Ag 14 -LR and Ag 14 -LS via hydrolysis with water, and from the disassembled solid material Ag 14 -DR and Ag 14 -DS, optically pure enantiomers R-Ag 14m and S-Ag 14m were obtained. It is hoped that this simple assembly strategy can be used to construct cluster-based chiral assemblage materials and that the subsequent disassembly protocol can be used to obtain optically pure chiral cluster molecules from as-prepared racemic mixtures.
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Metal nanoclusters whose surface ligands are removable while keeping their metal framework structures intact are an ideal system for investigating the influence of surface ligands on catalysis of ...metal nanoparticles. We report in this work an intermetallic nanocluster containing 62 metal atoms, Au34Ag28(PhCC)34, and its use as a model catalyst to explore the importance of surface ligands in promoting catalysis. As revealed by single-crystal diffraction, the 62 metal atoms in the cluster are arranged as a four-concentric-shell Ag@Au17@Ag27@Au17 structure. All phenylalkynyl (PA) ligands are linearly coordinated to the surface Au atoms with staple “PhCC–Au–CCPh” motif. Compared with reported thiolated metal nanoclusters, the surface PA ligands on Au34Ag28(PhCC)34 are readily removed at relatively low temperatures, while the metal core remains intact. The clusters before and after removal of surface ligands are used as catalysts for the hydrolytic oxidation of organosilanes to silanols. It is, for the first time, demonstrated that the organic-capped metal nanoclusters work as active catalysts much better than those with surface ligands partially or completely removed.
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Copper–hydrides are known catalysts for several technologically important reactions such as hydrogenation of CO, hydroamination of alkenes and alkynes, and chemoselective hydrogenation of unsaturated ...ketones to unsaturated alcohols. Stabilizing copper-based particles by ligand chemistry to nanometer scale is an appealing route to make active catalysts with optimized material economy; however, it has been long believed that the ligand–metal interface, particularly if sulfur-containing thiols are used as stabilizing agent, may poison the catalyst. We report here a discovery of an ambient-stable thiolate-protected copper–hydride nanocluster Cu25H10(SPhCl2)183– that readily catalyzes hydrogenation of ketones to alcohols in mild conditions. A full experimental and theoretical characterization of its atomic and electronic structure shows that the 10 hydrides are instrumental for the stability of the nanocluster and are in an active role being continuously consumed and replenished in the hydrogenation reaction. Density functional theory computations suggest, backed up by the experimental evidence, that the hydrogenation takes place only around a single site of the 10 hydride locations, rendering the Cu25H10(SPhCl2)183– one of the first nanocatalysts whose structure and catalytic functions are characterized fully to atomic precision. Understanding of a working catalyst at the atomistic level helps to optimize its properties and provides fundamental insights into the controversial issue of how a stable, ligand-passivated, metal-containing nanocluster can be at the same time an active catalyst.
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Surface organic ligands play a critical role in stabilizing atomically precise metal nanoclusters in solutions. However, it is still challenging to prepare highly robust ligated metal nanoclusters ...that are surface‐active for liquid‐phase catalysis without any pre‐treatment. Now, an N‐heterocyclic carbene‐stabilized Au25 nanocluster with high thermal and air stabilities is presented as a homogenous catalyst for cycloisomerization of alkynyl amines to indoles. The nanocluster, characterized as Au25(iPr2‐bimy)10Br72+ (iPr2‐bimy=1,3‐diisopropylbenzimidazolin‐2‐ylidene) (1), was synthesized by direct reduction of AuSMe2Cl and iPr2‐bimyAuBr with NaBH4 in one pot. X‐ray crystallization analysis revealed that the cluster comprises two centered Au13 icosahedra sharing a vertex. Cluster 1 is highly stable and can survive in solution at 80 °C for 12 h, which is superior to Au25 nanoclusters passivated with phosphines or thiols. DFT computations reveal the origins of both electronic and thermal stability of 1 and point to the probable catalytic sites. This work provides new insights into the bonding capability of N‐heterocyclic carbene to Au in a cluster, and offers an opportunity to probe the catalytic mechanism at the atomic level.
An atomically precise N‐heterocyclic carbene‐stabilized Au25 nanocluster is successfully synthesized in a one‐pot reaction. It exhibits much higher thermal stability in the solution form compared to Au25 protected by thiol or phosphine ligands. The cluster displays excellent catalytic activity in the cycloisomerization of alkynyl amine as a homogeneous catalyst.
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A general strategy, using mixed ligands, is utilized to synthesize atomically precise, intrinsically chiral nanocluster Ag78(DPPP)6(SR)42 (Ag78) where DPPP is the achiral ...1,3-bis(diphenyphosphino)propane and SR = SPhCF3. Ag78 crystallizes as racemates in a centric space group. Using chiral diphosphines BDPP = 2,4-bis(diphenylphosphino)pentane, the enantiomeric pair Ag78(R/S-BDPP)6(SR)42 can be prepared with 100% optical purity. The chiral diphosphines gives rise to, separately, two asymmetric surface coordination motifs composed of tetrahedral R3PAg(SR)3 moieties. The flexible nature of C–C–C angles between the two phosphorus atoms restricts the relative orientation of the tetrahedral R3PAg(SR)3 moieties, thereby resulting in the enantiomeric selection of the intrinsic chiral metal core. This proof-of-concept strategy raises the prospect of enantioselectively synthesizing optically pure, atomically precise chiral noble metal nanoclusters for specific applications.
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Surface ligands play critical roles in determining the surface properties of metal clusters. However, modulating the properties and controlling the surface structure of clusters through ...surface‐capping‐agent displacement is challenging. Herein, Ag14(SPh(CF3)2)12(PPh3)4(DMF)4 (Ag14‐DMF; DMF=N,N‐dimethylformamide), with weakly coordinated DMF ligands on surface silver sites, was synthesized by a mixed‐ligands strategy. Owing to the high surface reactivity of Ag14‐DMF, the surface ligands are labile, easily dissociated or exchanged by other ligands. Based on the enhanced surface reactivity, easy modulation of the optical properties of Ag14 by reversible “on‐off” DMF ligation was realized. When chiral amines were introduced to as‐prepared products, all eight surface ligands were replaced by amines and the racemic Ag14 clusters were converted to optically pure homochiral Ag14 clusters as evidenced by circular dichroism (CD) activity and single‐crystal X‐ray diffraction (SCXRD). This work provides a new insight into modulation of the optical properties of metal clusters and atomically precise homochiral clusters for specific applications are obtained.
Bulky surface ligands greatly enhance the surface ligands reactivity of metal clusters, allowing easy modulation of their optical properties by reversible surface ligation. When chiral amines are introduced into the solution of racemic clusters, they are readily converted in full to optically pure homochiral clusters.
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Bottom-up design of functional device components based on nanometer-sized building blocks relies on accurate control of their self-assembly behavior. Atom-precise metal nanoclusters are ...well-characterizable building blocks for designing tunable nanomaterials, but it has been challenging to achieve directed assembly to macroscopic functional cluster-based materials with highly anisotropic properties. Here, we discover a solvent-mediated assembly of 34-atom intermetallic gold-silver clusters protected by 20 1-ethynyladamantanes into 1D polymers with Ag-Au-Ag bonds between neighboring clusters as shown directly by the atomic structure from single-crystal X-ray diffraction analysis. Density functional theory calculations predict that the single crystals of cluster polymers have a band gap of about 1.3 eV. Field-effect transistors fabricated with single crystals of cluster polymers feature highly anisotropic p-type semiconductor properties with ≈1800-fold conductivity in the direction of the polymer as compared to cross directions, hole mobility of ≈0.02 cm
V
s
, and an ON/OFF ratio up to ≈4000. This performance holds promise for further design of functional cluster-based materials with highly anisotropic semiconducting properties.
To report the first clinical results and value assessment of prompt gamma imaging for in vivo proton range verification in pencil beam scanning mode.
A stand-alone, trolley-mounted, prototype prompt ...gamma camera utilizing a knife-edge slit collimator design was used to record the prompt gamma signal emitted along the proton tracks during delivery of proton therapy for a brain cancer patient. The recorded prompt gamma depth detection profiles of individual pencil beam spots were compared with the expected profiles simulated from the treatment plan.
In 6 treatment fractions recorded over 3 weeks, the mean (± standard deviation) range shifts aggregated over all spots in 9 energy layers were -0.8 ± 1.3 mm for the lateral field, 1.7 ± 0.7 mm for the right-superior-oblique field, and -0.4 ± 0.9 mm for the vertex field.
This study demonstrates the feasibility and illustrates the distinctive benefits of prompt gamma imaging in pencil beam scanning treatment mode. Accuracy in range verification was found in this first clinical case to be better than the range uncertainty margin applied in the treatment plan. These first results lay the foundation for additional work toward tighter integration of the system for in vivo proton range verification and quantification of range uncertainties.
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This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)
(SR)
and a smaller trigonal-prismatic (AuAg)
(SR)
(PPh
)
in 1:1 ...ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe
). The larger cluster has a four concentric-shell icosahedral structure of Ag@M
@M
@M
@Ag
(SR)
(M = Au or Ag) with the inner-core M
icosahedron observed here for metal nanoparticles. The cluster has an open electron shell of 187 delocalized electrons, fully metallic, plasmonic behavior, and a zero HOMO-LUMO energy gap. The smaller cluster has an 18-electron shell closing, a notable HOMO-LUMO energy gap and a molecule-like optical spectrum. This is the first direct demonstration of the simultaneous presence of competing effects (closing of atom vs. electron shells) in nanocluster synthesis and growth, working together to form a co-crystal of different-sized clusters. This observation suggests a strategy that may be helpful in the design of other nanocluster systems via co-crystallization.
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