Conductance Growth of Single-Cluster Junctions with Increasing Sizes.

Quantum-tunneling-based nanoelectronics has the potential for the miniaturization of electronics toward the sub-5 nm scale. However, the nature of phase-coherent quantum tunneling leads to the rapid decays of the electrical conductance with tunneling transport distance, especially in organic molecule-based nanodevices. In this work, we investigated the conductance of the single-cluster junctions of a series of atomically well-defined silver nanoclusters, with varying sizes from 0.9 to 3.0 nm, using the mechanically controllable break junction (MCBJ) technique combined with quantum transport theory. Our charge transport investigations of these single-cluster junctions revealed that the conductance grows with increasing cluster size. The conductance decay constant was determined to be ∼-0.4 nm-1, which is of opposite sign to that of organic molecules. Comparison between experiment and theory reveals that although charge transport through the silver single-cluster junctions occurs via phase-coherent tunneling, this is compensated by a rapid decrease in the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) with size and the increase in the electrode-cluster coupling, which results in their conductance increase up to lengths of ∼3.0 nm. These results demonstrate that such families of nanoclusters provide unique bottom-up building blocks for the fabrication of nanodevices in the sub-5 nm size range.

Targeting bone microenvironments for treatment and early detection of cancer bone metastatic niches.

Bone tissues are the main metastatic sites of many cancers, and bone metastasis is an important cause of death. When bone metastasis occurs, dynamic interactions between tumor cells and bone tissues promote changes in the tumor-bone microenvironments that are conducive to tumor growth and progression, which also promote several related diseases, including pathological fracture, bone pain, and hypercalcemia. Accordingly, it has obvious clinical benefits for improving the cure rate and reducing the occurrence of related diseases through targeting bone microenvironments for the treatment and early detection of cancer bone metastasis niches. In this review, we briefly analyzed the relationship between bone microstructures and tumor metastasis, as well as microenvironmental changes in osteoblasts, osteoclasts, immune cells, and extracellular and bone matrixes caused when metastatic tumor cells colonize bones. We also discuss novel designs in nanodrugs for inhibiting tumor proliferation and migration through targeting to tumor bone metastases and abnormal bone-microenvironment components. In addition, related researches on the early detection of bone and multi-organ metastases by nanoprobes are also introduced. And we look forward to providing some useful proposals and enlightenments on nanotechnology-based drug delivery and probes for the treatment and early detection of bone metastasis.

Atomically Precise, Thiolated Copper-Hydride Nanoclusters as Single-Site Hydrogenation Catalysts for Ketones in Mild Conditions.

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)18]3- 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)18]3- 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.

Ether-Soluble Cu53 Nanoclusters as an Effective Precursor of High-Quality CuI Films for Optoelectronic Applications.

An effective strategy is developed to synthesize high-nuclearity Cu clusters, [Cu53 (RCOO)10 (C≡CtBu)20 Cl2 H18 ]+ (Cu53 ), which is the largest CuI /Cu0 cluster reported to date. Cu powder and Ph2 SiH2 are employed as the reducing agents in the synthesis. As revealed by single-crystal diffraction, Cu53 is arranged as a four-concentric-shell Cu3 @Cu10 Cl2 @Cu20 @Cu20 structure, possessing an atomic arrangement of concentric M12 icosahedral and M20 dodecahedral shells which popularly occurs in Au/Ag nanoclusters. Surprisingly, Cu53 can be dissolved in diethyl ether and spin coated to form uniform nanoclusters film on organolead halide perovskite. The cluster film can subsequently be converted into high-quality CuI film via in situ iodination at room temperature. The as-fabricated CuI film is an excellent hole-transport layer for fabricating highly stable CuI-based perovskite solar cells (PSCs) with 14.3 % of efficiency.

Surface Chemistry of Atomically Precise Coinage-Metal Nanoclusters: From Structural Control to Surface Reactivity and Catalysis.

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.

Co-crystallization of atomically precise metal nanoparticles driven by magic atomic and electronic shells.

This paper reports co-crystallization of two atomically precise, different-size ligand-stabilized nanoclusters, a spherical (AuAg)267(SR)80 and a smaller trigonal-prismatic (AuAg)45(SR)27(PPh3)6 in 1:1 ratio, characterized fully by X-ray crystallographic analysis (SR = 2,4-SPhMe2). The larger cluster has a four concentric-shell icosahedral structure of Ag@M12@M42@M92@Ag120(SR)80 (M = Au or Ag) with the inner-core M147 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.

From Symmetry Breaking to Unraveling the Origin of the Chirality of Ligated Au13 Cu2 Nanoclusters.

A general method, using mixed ligands (here diphosphines and thiolates) is devised to turn an achiral metal cluster, Au13 Cu2 , into an enantiomeric pair by breaking (lowering) the overall molecular symmetry with the ligands. Using an achiral diphosphine, a racemic [Au13 Cu2 (DPPP)3 (SPy)6 ]+ was prepared which crystallizes in centrosymmetric space groups. Using chiral diphosphines, enantioselective synthesis of an optically pure, enantiomeric pair of [Au13 Cu2 ((2r,4r)/(2s,4s)-BDPP)3 (SPy)6 ]+ was achieved in one pot. Their circular dichroism (CD) spectra give perfect mirror images in the range of 250-500 nm with maximum anisotropy factors of 1.2×10-3 . DFT calculations provided good correlations with the observed CD spectra of the enantiomers and, more importantly, revealed the origin of the chirality. Racemization studies show high stability (no racemization at 70 °C) of these chiral nanoclusters, which hold great promise in applications such as asymmetry catalysis.

Divergent synthesis of N-heterocycles via controllable cyclization of azido-diynes catalyzed by copper and gold.

Gold-catalyzed intermolecular alkyne oxidation by an N-O bond oxidant has proven to be a powerful method in organic synthesis during the past decade, because this approach would enable readily available alkynes as precursors in generating α-oxo gold carbenes. Among those, gold-catalyzed oxidative cyclization of dialkynes has received particular attention as this chemistry offers great potential to build structurally complex cyclic molecules. However, these alkyne oxidations have been mostly limited to noble metal catalysts, and, to our knowledge, non-noble metal-catalyzed reactions such as diyne oxidations have not been reported. Herein, we disclose a copper-catalyzed oxidative diyne cyclization, allowing the facile synthesis of a wide range of valuable pyrrolo[3,4-c]quinolin-1-ones. Interestingly, by employing the same starting materials, the gold-catalyzed cascade cyclization leads to the divergent formation of synthetically useful pyrrolo[2,3-b]indoles. Furthermore, the proposed mechanistic rationale for these cascade reactions is strongly supported by both control experiments and theoretical calculations.

Microporous Cyclic Titanium-Oxo Clusters with Labile Surface Ligands.

By using ethylene glycol and monocarboxylic acid as surface ligands, a series of cyclic Ti-oxo clusters (CTOC) with permanent microporosity are successfully synthesized. With a cyclic {Ti32 O16 } backbone made of eight connected Ti4 tetrahedral cages that are arranged in a zigzag fashion, the clusters have a "donut" shape with an inner diameter of 8.3 Å, outer diameter of 26.9 Šand height of 10.4 Å. While both inner and outer walls of the "donut" clusters are modified by double-deprotonated ethylene glycolates, their upper and lower surfaces are bound by carboxylates and mono-deprotonated ethylene glycolates. The clusters are readily packed into one-dimensional tubes which are further arranged in two different modes into crystalline microporous solids with surface areas over 660 m2 g-1 , depending on the surface carboxylates. The solid with olefin-bearing carboxylates exhibits a superior CO2 adsorption capacity of 40 cm3 g-1 at 273 K under 1 atm. Moreover, the mono-deprotonated ethylene glycolates on the clusters are demonstrated to be highly exchangeable by other alcohols, providing a nice platform for creating microporous solids or films with a wide variety of surface functionalities.

From Racemic Metal Nanoparticles to Optically Pure Enantiomers in One Pot.

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.

Supercubes, Supersquares, and Superrods of Face-Centered Cubes (FCC): Atomic and Electronic Requirements of [Mm(SR)l(PR'3)8]q Nanoclusters (M = Coinage Metals) and Their Implications with Respect to Nucleation and Growth of FCC Metals.

Understanding the nucleation and growth pathways of nanocrystallites allows precise control of the size and shape of functional crystalline nanomaterials of importance in nanoscience and nanotechnology. This paper provides a detailed analysis of the stereochemical and electronic requirements of three series of nanoclusters based on face-centered cubes (fcc) as the basic building blocks, namely, 1-, 2-, and 3-D assemblages of fcc to form superrods (n), supersquares (n2), and supercubes (n3). The generating functions for calculating the numbers (and arrangements) of surface and interior metal atoms, as well as the number and dispositions of the ligands, for these particular sequences of fcc metal clusters of the general formula [Mm(SR)l(PR'3)8]q (where M = coinage metals; SR = thiolates (or group XI ligands), and PR'3 = phosphines) are presented. An electron-counting scheme based on the jelliumatic shell nodel, a variant of the jellium model, predicts the electron requirements and hence the chemical compositions that are critical in the design and synthesis of the next generation of giant nanoclusters in the nanorealm. The ligand binding specificities, which are keys to effective surface ligand control of the size and shape of these nanoclusters, are defined. Finally, a connection is made with regard to the growth of fcc metals, n3, from fcc supercubes (n < 10) to fcc nanocrystallites/particles (10 < n < 102) and to fcc bulk phase (n > 102).

Yttrium-Catalyzed Intramolecular Hydroalkoxylation/Claisen Rearrangement Sequence: Efficient Synthesis of Medium-Sized Lactams.

An efficient yttrium-catalyzed intramolecular hydroalkoxylation/Claisen rearrangement sequence has been achieved, thus enabling facile access to a diverse array of valuable medium-sized lactams. Furthermore, a mechanistic rationale for this novel cascade reaction is well supported by a variety of control experiments.

Peculiar holes on checkerboard facets of a trigonal prismatic Au9Ag36(SPhCl2)27(PPh3)6 cluster caused by steric hindrance and magic electron count.

We report herein the synthesis and structure of a 45-atom trigonal-prismatic Au-Ag bimetallic nanocluster, formulated as Au9Ag36(SPhCl2)27(PPh3)6, based on single-crystal X-ray crystallographic determination. The structure can be described as a core-shell structure with a tricapped trigonal prismatic (ttp1) Au9 core encaged in a larger (frequency-two) tricapped trigonal prismatic (ttp2) Ag30 shell. The cluster is terminated by six Ag(PPh3) moieties which, along with ttp2 and 27 thiolates, constitute the outer trigonal-prismatic (TP) shell. Each of the three nearly coplanar yet severely distorted "square" faces of TP contains 13 Ag atoms which are arranged in a way reminiscent of the (100) face of a face-centered cubic (fcc) structure. Of the 30 edges formed by these quasi-(100) faces of the TP, only 27 are bridged by the thiolate ligands; three are vacant, one on each "square" face. It is believed that these peculiar vacant ligand sites are caused by steric hindrance of the thiolate ligands in combination with the superatomic electronic shell closing of 1S21P61D10 rendering 9(ttp1) + 30(ttp2) + 6(TP) - 27(SR) = 18 jellium electrons.

Highly Site Selective Formal [5+2] and [4+2] Annulations of Isoxazoles with Heterosubstituted Alkynes by Platinum Catalysis: Rapid Access to Functionalized 1,3-Oxazepines and 2,5-Dihydropyridines.

Platinum-catalyzed formal [5+2] and [4+2] annulations of isoxazoles with heterosubstituted alkynes enabled the atom-economical synthesis of valuable 1,3-oxazepines and 2,5-dihydropyridines, respectively. Importantly, this Pt catalysis not only led to unique reactivity dramatically divergent from that observed under Au catalysis, but also proceeded via unprecedented α-imino platinum carbene intermediates.

Embryonic Growth of Face-Center-Cubic Silver Nanoclusters Shaped in Nearly Perfect Half-Cubes and Cubes.

Demonstrated herein are the preparation and crystallographic characterization of the family of fcc silver nanoclusters from Nichol's cube to Rubik's cube and beyond via ligand-control (thiolates and phosphines in this case). The basic building block is our previously reported fcc cluster [Ag14(SPhF2)12(PPh3)8] (1). The metal frameworks of [Ag38(SPhF2)26(PR'3)8] (22) and [Ag63(SPhF2)36(PR'3)8]+ (23), where HSPhF2 = 3,4-difluorothiophenol and R' = alkyl/aryl, are composed of 2 × 2 = 4 and 2 × 2 × 2 = 8 metal cubes of 1, respectively. All serial clusters share similar surface structural features. The thiolate ligands cap the six faces and the 12 edges of the cube (or half cube) while the phosphine ligands are terminally bonded to its eight corners. On the basis of the analysis of the crystal structures of 1, 22, and 23, we predict the next "cube of cubes" to be Ag172(SR)72(PR'3)8] (33), in the evolution of growth of this cluster sequence.

Asymmetric Synthesis of Chiral Bimetallic [Ag28Cu12(SR)24]4- Nanoclusters via Ion Pairing.

In this work, a facile ion-pairing strategy for asymmetric synthesis of optically active negatively charged chiral metal nanoparticles using chiral ammonium cations is demonstrated. A new thiolated chiral three-concentric-shell cluster, [Ag28Cu12(SR)24]4-, was first synthesized as a racemic mixture and characterized by single-crystal X-ray structure determination. Mass spectrometric measurements revealed relatively strong ion-pairing interactions between the anionic nanocluster and ammonium cations. Inspired by this observation, the as-prepared racemic mixture was separated into enantiomers by employing chiral quaternary ammonium salts as chiral resolution agents. Subsequently, direct asymmetric synthesis of optically active enantiomers of [Ag28Cu12(SR)24]4- was achieved by using appropriate chiral ammonium cations (such as N-benzylcinchoninium vs N-benzylcinchonidinium) in the cluster synthesis. These simple strategies, ion-pairing enantioseparation and direct asymmetric synthesis using chiral counterions, may be of general use in preparing chiral metal nanoparticles.

Plasmonic twinned silver nanoparticles with molecular precision.

Determining the structures of nanoparticles at atomic resolution is vital to understand their structure-property correlations. Large metal nanoparticles with core diameter beyond 2 nm have, to date, eluded characterization by single-crystal X-ray analysis. Here we report the chemical syntheses and structures of two giant thiolated Ag nanoparticles containing 136 and 374 Ag atoms (that is, up to 3 nm core diameter). As the largest thiolated metal nanoparticles crystallographically determined so far, these Ag nanoparticles enter the truly metallic regime with the emergence of surface plasmon resonance. As miniatures of fivefold twinned nanostructures, these structures demonstrate a subtle distortion within fivefold twinned nanostructures of face-centred cubic metals. The Ag nanoparticles reported in this work serve as excellent models to understand the detailed structure distortion within twinned metal nanostructures and also how silver nanoparticles can span from the molecular to the metallic regime.

Identifying the Molecular Structures of Intermediates for Optimizing the Fabrication of High-Quality Perovskite Films.

During the past two years, the introduction of DMSO has revolutionized the fabrication of high-quality pervoskite MAPbI3 (MA = CH3NH3) films for solar cell applications. In the developed DMSO process, the formation of (MA)2Pb3I8·2DMSO (shorted as Pb3I8) has well recognized as a critical factor to prepare high-quality pervoskite films and thus accomplish excellent performances in perovskite solar cells. However, Pb3I8 is an I-deficient intermediate and must further react with methylammonium iodide (MAI) to be fully converted into MAPbI3. By capturing and solving the molecular structures of several intermediates involved in the fabrication of perovskite films, we report in this work that the importance of DMSO is NOT due to the formation of Pb3I8. The use of different PbI2-DMSO ratios leads to two different structures of PbI2-DMSO precursors (PbI2·DMSO and PbI2·2DMSO), thus dramatically influencing the quality of fabricated perovskite films. However, such an influence can be minimized when the PbI2-DMSO precursor films are thermally treated to create mesoporous PbI2 films before reacting with MAI. Such a development makes the fabrication of high-quality pervoskite films highly reproducible without the need to precisely control the PbI2:DMSO ratio. Moreover, the formation of ionic compound (MA)4PbI6 is observed when excess MAI is used in the preparation of perovskite film. This I-rich phase heavily induces the hysteresis in PSCs, but is readily removed by isopropanol treatment. On the basis of all these findings, we develop a new effective protocol to fabricate high-performance PSCs. In the new protocol, high-quality perovskite films are prepared by simply treating the mesoporous PbI2 films (made from PbI2-DMSO precursors) with an isopropanol solution of MAI, followed by isopropanol washing. The best efficiency of fabricated MAPbI3 PSCs is up to 19.0%. As compared to the previously reported DMSO method, the devices fabricated by the method reported in this work display narrow efficiency distributions in both forward and reverse scans. And the efficiency difference between forward and reverse scans is much smaller.

Total Structure and Electronic Structure Analysis of Doped Thiolated Silver [MAg24(SR)18](2-) (M = Pd, Pt) Clusters.

With the incorporation of Pd or Pt atoms, thiolated Ag-rich 25-metal-atom nanoclusters were successfully prepared and structurally characterized for the first time. With a composition of [PdAg24(SR)18](2-) or [PtAg24(SR)18](2-), the obtained 25-metal-atom nanoclusters have a metal framework structure similar to that of widely investigated Au25(SR)18. In both clusters, a M@Ag12 (M = Pd, Pt) core is capped by six distorted dimeric -RS-Ag-SR-Ag-SR- units. However, the silver-thiolate overlayer gives rise to a geometric chirality at variance to Au25(SR)18. The effect of doping on the electronic structure was studied through measured optical absorption spectra and ab initio analysis. This work demonstrates that modulating electronic structures by transition-metal doping is expected to provide effective means to manipulate electronic, optical, chemical, and catalytic properties of thiolated noble metal nanoclusters.

High-yield synthesis and crystal structure of a green Au30 cluster co-capped by thiolate and sulfide.

A green gold-cluster, Au30S(StBu)18, was successfully prepared in high yield and crystallographically characterized. Each cluster consists of an Au22 core capped by a mixed layer of staple Au-thiolate units, bridging thiolates and a µ3-S(2-).