A Cages-on-Cluster Structure Constructed by Post-Clustering Covalent Modifications and Guest-Enabled Stimuli-Responsive Luminescence.

A novel gold(I)-cluster-based twin-cage is constructed by post-clustering covalent modification of a hexa-aldehyde cluster precursor with triaminotriethylamines. The cages-on-cluster structure have double cavities and four binding sites, showing site discriminative binding for silver(I) and copper(I) guests. The guests in the tripodal hats affect the luminescence of the cluster: tetra-silver(I) host-guest complex is weak red emissive, while bis-copper(I)-bis-silver(I) one is non-emissive but a stimuli-responsive supramolecule. The copper(I) ion inside the tri-imine cavity is oxidation sensitive, which enable the release of the bright emissive precursor cluster triggered by H2O2 solution. The hybridization of a cluster with cavities to construct a cluster-based cage presents an innovative concept for functional cluster design, and the post-clustering covalent modification opens up new avenues of finely tuning the properties of clusters.

Near-unity NIR phosphorescent quantum yield from a room-temperature solvated metal nanocluster.

Metal nanoclusters have emerged as promising near-infrared (NIR)-emissive materials, but their room-temperature photoluminescence quantum yield (PLQY), especially in solution, is often low (<10%). We studied the photophysics of Au22(tBuPhC≡C)18 (Au22) and its alloy counterpart Au16Cu6(tBuPhC≡C)18 (Au16Cu6) (where tBu is tert-butyl and Ph is phenyl) and found that copper (Cu) doping suppressed the nonradiative decay (~60-fold less) and promoted intersystem crossing rate (~300-fold higher). The Au16Cu6 nanocluster exhibited >99% PLQY in deaerated solution at room temperature with an emission maximum at 720 nanometers tailing to 950 nanometers and 61% PLQY in the oxygen-saturated solution. The approach to achieve near-unity PLQY could enable the development of highly emissive metal cluster materials.

Heterometallic AuI 6 AgI 6 Macrocyclic Cluster Templated by a Supramolecular Melamine Dimer.

The application of supramolecular templates in aligning atomically precise heterometal arrays is important for pursuing functional materials. Herein, we report that a bilayered supramolecular tri-deprotonated melamine dimer functions as an effective template in the construction of a heterometallic gold(I)-silver(I) macrocyclic cluster [µ6 -(C3 N6 H3 )3- ]2 -AuI 6 AgI 6 . X-ray single crystal structural analysis showed that a crown-like AuI 6 AgI 6 macrocycle is aligned around two parallelly stacked µ6 -(C3 N6 H3 )3- moieties hold together with π-π interactions. Theoretical calculations revealed that the [µ6 -(C3 N6 H3 )3- ]2 motif dominantly contributes to the near-occupied orbitals in the electronic structure, which is closely related to its luminescence properties. This work demonstrates that the supramolecular templates containing multiple symmetric binding sites may present a facile approach in the construction of functional metal clusters.

Alkynyl-Protected Bimetallic Nanoclusters with a Hybrid Mackay Icosahedral Ag42 Cu12 Cl Kernel and an Octahedral Ag22 Cu12 Kernel.

A facile strategy that directly reduces alkynyl-silver precursors and copper salts for the synthesis of bimetallic nanoclusters using the weak reducing agent Ph2 SiH2 is demonstrated. Two alkynyl-protected concentric-shell nanoclusters, (Ph4 P)2 [Ag22 Cu12 (C≡CR)28 ] and (Ph4 P)3 [Ag42 Cu12 Cl(C≡CR)36 ] (Ag22 Cu12 and Ag42 Cu12 Cl, R=bis(trifluoromethyl)phenyl), were successfully obtained and characterized by single-crystal X-ray diffraction and electro-spray ionization mass spectrometry. For the first time, a hybrid 55-atom two-shell Mackay icosahedron was found in Ag42 Cu12 Cl, which is icosahedral M54 Cl instead of M55 . The incorporation of a chloride in the metal icosahedron contributes to the stability of the cluster from both electronic and geometric aspects. Alkynyl ligands show various binding-modes including linear "RC≡C-Cu-C≡CR" staple motifs.

Integration of Metal Catalysis and Organocatalysis in a Metal Nanocluster with Anchored Proline.

Chiral metal nanoclusters have recently been attracting great attention. It is challenging to realize asymmetric catalysis via atomically precise metal nanoclusters. Herein, we report the synthesis and total structure determination of chiral clusters [Au7Ag8(dppf)3(l-/d-proline)6](BF4)2 (l-/d-Au7Ag8). Superatomic clusters l-/d-Au7Ag8 display intense and mirror-image Cotton effects in their CD spectra. Density functional theory (DFT) calculations were carried out to understand the correlation between electronic structures and the optical activity of the enantiomeric pair. Surprisingly, the incorporation of proline in a metal nanocluster can significantly promote the catalytic efficiency in asymmetric Aldol reactions. The increase of catalytic activity of Au7Ag8 in comparison with organocatalysis by proline is attributed to the cooperative effect of the metal core and prolines, showing the advantages of the integration of metal catalysis and organocatalysis in a metal nanocluster.

Insufficiency of collagenases in establishment of primary chondrocyte culture from cartilage of elderly patients receiving total joint replacement.

Background Collagenases are frequently used in chondrocyte isolation from articular cartilage. However, the sufficiency of this enzyme in establishing primary human chondrocyte culture remains unknown. Methods Cartilage slices shaved from femoral head or tibial plateau of patients receiving total joint replacement surgery (16 hips, 8 knees) were subjected to 0.02% collagenase IA digestion for 16 h with (N = 19) or without (N = 5) the pre-treatment of 0.4% pronase E for 1.5 h. Chondrocyte yield and viability were compared between two groups. Chondrocyte phenotype was determined by the expression ratio of collagen type II to I. The morphology of cultured chondrocytes was monitored with a light microscope.Results Cartilage with pronase E pre-treatment yielded significantly higher chondrocytes than that without the pre-treatment (3,399 ± 1,637 cells/mg wet cartilage vs. 1,895 ± 688 cells/mg wet cartilage; P = 0.0067). Cell viability in the former group was also significantly higher than that in the latter (94% ± 2% vs. 86% ± 6%; P = 0.03). When cultured in monolayers, cells from cartilage with pronase E pre-treatment grew in a single plane showing rounded shape while cells from the other group grew in multi-planes and exhibited irregular shape. The mRNA expression ratio of collagen type II to I was 13.2 ± 7.5 in cells isolated from cartilage pre-treated with pronase E, indicating a typical chondrocyte phenotype. Conclusions Collagenase IA was not sufficient in establishing primary human chondrocyte culture. Cartilage must be treated with pronase E prior to collagenase IA application.

Disc-Like Silver Nanocluster Ag93 Built with Bicapped Hexagonal Prismatic Ag15 and Ino Decahedral Ag13 Units.

The reduction of alkynyl-silver and phosphine-silver precursors with a weak reducing reagent Ph2 SiH2 led to the formation of a novel silver nanocluster [Ag93 (PPh3 )6 (C≡CR)50 ]3+ (R=4-CH3 OC6 H4 ), which is the largest structurally characterized cluster of clusters. This disc-shaped cluster has a Ag69 kernel consisting of a bicapped hexagonal prismatic Ag15 unit wrapped by six Ino decahedra through edge-sharing. This is the first time that Ino decahedra are used as a building block to assemble a cluster of clusters. Moreover, the central silver atom has a coordination number of 14, which is the highest in metal nanoclusters. This work provides a diverse metal packing pattern in metal nanoclusters, which is helpful for understanding metal cluster assembling mechanisms.

Face-Centered Cubic Silver Nanoclusters Consolidated with Tetradentate Formamidinate Ligands.

Growing attention has been paid to nanoclusters with face-centered cubic (fcc) metal kernels, due to its structural similarity to bulk metals. We demonstrate that the use of tetradentate formamidinate ligands facilitate the construction of two fcc silver nanoclusters: [Ag52(5-F-dpf)16Cl4](SbF6)2 (Ag52, 5-F-Hdpf = N,N'-di(5-fluoro-2-pyridinyl)formamidine) and [Ag53(5-Me-dpf)18](NO3)5 (Ag53, 5-Me-Hdpf = N,N'-di(5-methyl-2-pyridinyl)formamidine). Single-crystal X-ray structural analysis revealed that the silver atoms in both clusters are in a layer-by-layer arrangement, which can be viewed as a portion of the fcc packing of silver. The nitrogen donors of amidinate ligands selectively passivate the {111} facets. All silver atoms are involved in the fcc packing, that is, no staple motifs are observed due to the linear arrangement of the four N donors of the dpf ligands. The characteristic optical absorption bands of Ag52 and Ag53 have been studied with a time-dependent density functional theory. This work provides a facile access to assembling atomically precise fcc-type nanoclusters and shows the prospect of amidinates as protecting ligands in synthesizing metal nanoclusters.

Structural Engineering toward Gold Nanocluster Catalysis.

Atomically precise gold nanoclusters provide great opportunities to explore the relationship between the structure and properties of nanogold catalysts. A nanocluster consists of a metal core and a surface ligand shell, and both the core and shell have significant effects on the catalytic properties. Thanks to their precise structures, the active metal site of the clusters can be readily identified and the effects of ligands on catalysis can be disclosed. In this Minireview, we summarize recent advances in catalytic research of gold nanoclusters, emphasizing four strategies for constructing open metal sites, including by post-treatment, the bulky ligands strategy, the surface geometric mismatch method, and heteroatom doping procedures. We also discuss the effects of ligands on the catalytic activity, selectivity, and stability of gold cluster catalysts. Finally, we present future challenges relating to gold cluster catalysis.

Site-specific doping of silver atoms into a Au25 nanocluster as directed by ligand binding preferences.

For the first time site-specific doping of silver into a spherical Au25 nanocluster has been achieved in [Au19Ag6(MeOPhS)17(PPh3)6] (BF4)2 (Au19Ag6) through a dual-ligand coordination strategy. Single crystal X-ray structural analysis shows that the cluster has a distorted centered icosahedral Au@Au6Ag6 core of D 3 symmetry, in contrast to the I h Au@Au12 kernel in the well-known [Au25(SR)18]- (R = CH2CH2Ph). An interesting feature is the coexistence of [Au2(SPhOMe)3] dimeric staples and [P-Au-SPhOMe] semi-staples in the title cluster, due to the incorporation of PPh3. The observation of only one double-charged peak in ESI-TOF-MS confirms the ordered doping of silver atoms. Au19Ag6 is a 6e system showing a distinct absorption spectrum from [Au25(SR)18]-, that is, the HOMO-LUMO transition of Au19Ag6 is optically forbidden due to the P character of the superatomic frontier orbitals.

Identification of the Active Species in Bimetallic Cluster Catalyzed Hydrogenation.

Identification of the authentic active species of cluster catalysis is rather challenging, and direct structural evidence is quite valuable and difficult to obtain. Two "isostructural" clusters, Ag25Cu4Cl6(dppb)6(PhC≡C)12(SO3CF3)3 (1) and Ag25Cu4Cl6H8(dppb)6(PhC≡C)12(SO3CF3)3 (2H) (dppb is 1,4-bis(diphenylphosphine)butane), have been successfully isolated and structurally characterized. Both these clusters have a centered icosahedron Ag13 core with the same peripheral composition and structure. The only difference is that 2H has eight hydrides but 1 has none, that is, the kernels are Ag135+ and Ag13H85+ in 1 and 2H, respectively. The catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction is assessed with the two clusters. Cluster 2H is very active with 100% yield within 2 h, whereas 1 shows a very low conversion (∼8%) under the same conditions. Interestingly, high catalytic activity was observed when 1 was converted to 2H with the oxidation of H2O2 under catalytic conditions. The unprecedented transformation of a reduced nanocluster to an Ag(I)Cu(I) bimetallic cluster compound provides an excellent platform to determine the real active cluster in terms of metal cluster catalysis. The present work presents clear structural evidence that the catalytic performance of metal nanoclusters can be modulated by properly regulating the oxidation state of their constituted metal atoms.

Anion-Directed Regulation of Structures and Luminescence of Heterometallic Clusters.

Anions have been used to regulate the structures and luminescence of heterometallic clusters. Introducing ClO4 - into orange-emissive, butterfly-like [(C)(Au-PPhpy2 )6 Ag4 ](BF4 )6 (1, PPhpy2 =bis(2-pyridyl)phenylphosphine) leads to the formation of red-emissive [(C)(Au-PPhpy2 )6 Ag5 (ClO4 )3 ](ClO4 )4 (2) with a novel trigonal bipyramidal structure; employing PhCO2 - gives yellow-emissive, hexagram-like [(C)(Au-PPhpy2 )6 Ag6 (PhCO2 )3 ](BF4 )5 (3). Notably, 1 exhibits weak luminescence in CH2 Cl2 /CH3 OH=1 : 1 (v : v) with a quantum yield (QY) of 0.05, whereas it was dramatically increased to 0.49 and 0.83 for 2 and 3, respectively. Theoretical calculation confirms that the involvement of anions in the electronic structures is responsible for the shifts of emission. The high QYs of 2 and 3 are attributed to the protection provided by ligands and anions. This work demonstrates that anions may serve as an extra designable factor beyond just counterions for functional metal clusters.

Structural transformation and catalytic hydrogenation activity of amidinate-protected copper hydride clusters.

Copper hydrides are important hydrogenation catalysts, but their poor stability hinders the practical applications. Ligand engineering is an effective strategy to tackle this issue. An amidinate ligand, N,N'-Di(5-trifluoromethyl-2-pyridyl)formamidinate (Tf-dpf) with four N-donors has been applied as a protecting agent in the synthesis of stable copper hydride clusters: Cu11H3(Tf-dpf)6(OAc)2 (Cu11) with three interfacial µ5-H and [Cu12H3(Tf-dpf)6(OAc)2]·OAc (Cu12) with three interstitial µ6-H. A solvent-triggered reversible interconversion between Cu11 and Cu12 has been observed thanks to the flexibility of Tf-dpf. Cu11 shows high activity in the reduction of 4-nitrophenol to 4-aminophenol, while Cu12 displays very low activity. Deuteration experiments prove that the type of hydride is the key in dictating the catalytic activity, for the interfacial µ5-H species in Cu11 are involved in the catalytic cycle whereas the interstitial µ6-H species in Cu12 are not. This work highlights the role of hydrides with regard to catalytic hydrogenation activity.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters.

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Ligand Engineering toward the Trade-Off between Stability and Activity in Cluster Catalysis.

We report the structures, stability and catalysis properties of two Ag21 nanoclusters, namely [Ag21 (H2 BTCA)3 (O2 PPh2 )6 ]SbF6 (1) and [Ag21 (C≡CC6 H3 -3,5-R2 )6 (O2 PPh2 )10 ]SbF6 (2) (H4 BTCA=p-tert-butylthiacalix[4]arene, R=OMe). Both Ag21 structures possess an identical icosahedral kernel that is surrounded by eight peripheral Ag atoms. Single-crystal structural analysis and ESI-MS revealed that 1 is an 8-electron cluster and 2 has four free electrons. Theoretical results show that the P-symmetry orbitals are found as HOMO-1 and HOMO states in 1, and the frontier unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) show D-character, indicating 1 is a superatomic cluster with an electronically closed shell 1S2 1P6 , while 2 has an incomplete shell configuration 1S2 1P2 . These two Ag21 clusters show superior stability under ambient conditions, and 1 is robust even at 90 °C in toluene and under oxidative conditions (30 % H2 O2 ). Significantly, 2 exhibits much higher activity than 1 as catalyst in the reduction of 4-nitrophenol. This work demonstrates that ligands can influence the electronic structures of silver clusters, and further affect their stability and catalytic performance.

Superatomic Orbital Splitting in Coinage Metal Nanoclusters.

The superatomic orbital splitting (SOS) method is developed to understand the electronic structures of coinage metal nanoclusters, in which delocalized electron counts are not magic numbers. Because the symmetry of a metal core can significantly affect the electronic structure of a nanocluster, this method takes the shape of the core into account in determining the order of group orbital levels. By taking nanoclusters as superatoms, a highly positively charged core is established by removing the ligands and staples. The superatomic orbitals split into group orbitals at different energy levels because of the nonspherical shape of the cluster core. Therefore, the electron configuration of the nonmagic-number nanocluster can be qualitatively analyzed without quantum chemical calculations, which is very important for understanding the stability of the cluster.

A 59-Electron Non-Magic-Number Gold Nanocluster Au99(C≡CR)40 Showing Unexpectedly High Stability.

An atomically resolved gold nanocluster Au99(C≡CC6H3-2,4-F2)40 (Au99) with an unusual 59 valence electrons has been synthesized. Single-crystal X-ray diffraction reveals that its Au79 kernel is a Au49 Marks decahedron capped by two Au15 units. The surface structure of Au99 consists of 20 linear Au(C≡CR)2 staples. Intercluster interactions are observed between these D5 symmetric clusters. The existence of an unpaired electron is verified by magnetic measurement. Interestingly, this open-shell gold cluster Au99 stays intact in toluene solution at 80 °C for more than a week, and it has good charging-discharging capability under electrochemical conditions. The compact ligand shell protection around the symmetric core accounts for the high stability. This work suggests that geometric factors may play a crucial role in determining the stability of a metal nanocluster, even though the cluster has an open-shell electronic structure.

Molecular Gold Nanocluster Au156 Showing Metallic Electron Dynamics.

The boundary between molecular and metallic gold nanoclusters is of special interest. The difficulty in obtaining atomically precise nanoclusters larger than 2 nm limits the determination of such a boundary. The synthesis and total structural determination of the largest all-alkynyl-protected gold nanocluster (Ph4P)6[Au156(C≡CR)60] (R = 4-CF3C6H4-) (Au156) are reported. It presents an ideal platform for studying the relationship between the structure and the metallic nature. Au156 has a rod shape with the length and width of the kernel being 2.38 and 2.04 nm, respectively. The cluster contains a concentric Au126 core structure (Au46@Au50@Au30) protected by 30 linear RC≡C-Au-C≡CR staple motifs. It is interesting that Au156 displays multiple excitonic peaks in the steady-state absorption spectrum (molecular) and pump-power-dependent excited-state dynamics as revealed in the transient absorption spectrum (metallic), which indicates that Au156 is a critical crossover cluster for the transition from molecular to metallic state. Au156 is the smallest-sized gold nanocluster showing metal-like electron dynamics, and it is recognized that the cluster shape is one of the important factors determining the molecular or metallic nature of a gold nanocluster.

Migrating Progenitor Cells Derived From Injured Cartilage Surface Respond to Damage-Associated Molecular Patterns.

OBJECTIVE: To delineate the response of migrating chondrogenic progenitor cells (CPCs) that arose from the surface of mechanically injured articular cartilage to proinflammatory damage-associated-molecular-patterns (DAMPs). DESIGN: Bovine CPCs and non-CPC chondrocytes isolated from either impacted or scratched articular cartilage were studied. Those 2 types of cells were treated with mitochondrial DAMPs (MTDs; 10 nM fMLF and 10 µg/mL CpG DNA), or 10 nM HMGB1, or 10 ng/mL IL-1b for 24 hours. At the end of experiments, conditioned media and cell lysates were collected for analysis of expression levels of matrix metalloproteinases (MMPs), chemokines, and cytokines that are associated with cartilage degeneration with Western blotting and quantitative polymerase chain reaction. The difference of expression levels was compared by Welch's t-test. RESULTS: Our data indicated that HMGB1 and MTDs remarkably upregulated pro-MMP-13 expression in CPCs. Compared with non-CPCs, CPCs expressed significantly more baseline mRNAs of MMP-13, CXCL12, and IL-6. MTDs greatly increased the expression of MMP-13 and IL-6 in CPCs by over 100-fold (P < 0.001). MTDs also significantly increased IL-8 expression in CPCs to a similar extent (P < 0.001). However, when IL-1b was present, CPCs expressed less MMP-3 and active MMP-13 proteins as well as less CCL2 and IL-6 than did non-CPCs. CONCLUSIONS: We concluded that CPCs were more sensitive than non-CPCs in response to DAMPs, especially MTDs. The proinflammatory nature of CPCs implied their critical role in the early phase of posttraumatic osteoarthritis development.

Rod-Shaped Silver Supercluster Unveiling Strong Electron Coupling between Substituent Icosahedral Units.

The first linear silver supercluster based on icosahedral Ag13 units has been constructed via bridging of dpa ligands: Ag61(dpa)27(SbF6)4 (Hdpa = dipyridylamine) (Ag61). Single-crystal X-ray diffraction reveals that this rod-shaped cluster consists of four vertex-sharing Ag13 icosahedra in a linear arrangement. This Ag61 cluster represents the longest one-dimensional metal nanocluster with a resolved structure. Unprecedented electron coupling develops between their constituent Ag13 units. Theoretical studies disclose that the stabilities of the two superclusters are dictated by a strong interaction between the Ag13 units as well as the ligand effect of the dpa-Ag motifs. The quantum size effect accounts for the significant enhancement of the metal-related absorptions and the red shift at the near-infrared region as the length of the cluster increases. This work sheds light on the evolution of one-dimensional materials and an understanding of the electronic communication between the constituent clusters.