N-Heterocyclic Carbene-Stabilized Atomically Precise Metal Nanoclusters.

This perspective highlights advances in the preparation and understanding of metal nanoclusters stabilized by organic ligands with a focus on N-heterocyclic carbenes (NHCs). We demonstrate the need for a clear understanding of the relationship between NHC properties and their resulting metal nanocluster structure and properties. We emphasize the importance of balancing nanocluster stability with the introduction of reactive sites for catalytic applications and the importance of a better understanding of how these clusters interact with their environments for effective use in biological applications. The impact of atom-scale simulations, development of atomic interaction potentials suitable for large-scale molecular dynamics simulations, and a deeper understanding of the mechanisms behind synthetic methods and physical properties (e.g., the bright fluorescence displayed by many clusters) are emphasized.

N-Heterocyclic Carbene-Stabilized Hydrido Au24 Nanoclusters: Synthesis, Structure, and Electrocatalytic Reduction of CO2.

Atomically precise hydrido gold nanoclusters are extremely rare but interesting due to their potential applications in catalysis. By optimization of molecular precursors, we have prepared an unprecedented N-heterocyclic carbene-stabilized hydrido gold nanocluster, [Au24(NHC)14Cl2H3]3+. This cluster comprises a dimer of two Au12 kernels, each adopting an icosahedral shape with one missing vertex. The two kernels are joined through triangular faces, which are capped with a total of three hydrides. The hydrides are detected by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy, with density functional theory calculations supporting their position bridging the six uncoordinated gold sites. The reactivity of this Au24H3 cluster in the electrocatalytic reduction of CO2 is demonstrated and benchmarked against related catalysts.

Robust, Highly Luminescent Au13 Superatoms Protected by N-Heterocyclic Carbenes.

Gold superatom nanoclusters stabilized entirely by N-heterocyclic carbenes (NHCs) and halides are reported. The reduction of well-defined NHC-Au-Cl complexes produces clusters comprised of an icosahedral Au13 core surrounded by a symmetrical arrangement of nine NHCs and three chlorides. X-ray crystallography shows that the clusters are characterized by multiple CH-π and π-π interactions, which rigidify the ligand and likely contribute to the exceptionally high photoluminescent quantum yields observed, up to 16.0%, which is significantly greater than that of the most luminescent ligand-protected Au13 superatom cluster. Density functional theory analysis suggests that clusters are 8-electron superatoms with a wide HOMO-LUMO energy gap of 2 eV. Consistent with this, the clusters have high stability relative to phosphine stabilized clusters.

Crystalline Superlattices of Nanoscopic CdS Molecular Clusters: An X-ray Crystallography and 111Cd SSNMR Spectroscopy Study.

Systematic 111Cd solid-state (SS) NMR experiments were performed to correlate X-ray crystallographic data with SSNMR parameters for a set of CdS-based materials, varying from molecular crystals of small complexes [Cd(SPh)4]2- and [Cd4(SPh)10]2- to superlattices of large monodisperse clusters [Cd54S32(SPh)48(dmf)4]4- and 1.9 nm CdS. Methodical data analysis allowed for assigning individual resonances or resonance groups to particular types of cadmium sites residing in different chemical and/or crystallographic environments. For large CdS frameworks, 111Cd resonances were found to form three groups. This result is noteworthy, since for related systems with size polydispersity and variations in composition, such as CdS or CdSe nanoparticles protected with an organic ligand shell, typically only two groups of resonances were observed. The generalized information obtained in this work can be used for the interpretation of 111/113Cd SSNMR data for large CdS clusters and nanoparticles, for which crystal structure analysis remains inaccessible. Comparison of the powder X-ray diffraction patterns for freshly prepared and dried superlattices of large CdS clusters revealed an interesting superstructure rearrangement that was not observed for the smaller frameworks.

Luminescent CdSe Superstructures: A Nanocluster Superlattice and a Nanoporous Crystal.

Superstructures, combining nanoscopic constituents into micrometer-size assemblies, have a great potential for utilization of the size-dependent quantum-confinement properties in multifunctional electronic and optoelectronic devices. Two diverse superstructures of nanoscopic CdSe were prepared using solvothermal conversion of the same cadmium selenophenolate precursor (Me4N)2[Cd(SePh)4]: the first is a superlattice of monodisperse [Cd54Se32(SePh)48(dmf)4]4- nanoclusters; the second is a unique porous CdSe crystal. Nanoclusters were crystallized as cubic crystals (≤0.5 mm in size) after solvothermal treatment at 200 °C in DMF. UV-vis absorption and PLE spectra of the reported nanoclusters are consistent with previously established trends for the known families of tetrahedral CdSe frameworks. In contrast to these, results of PL spectra are rather unexpected, as distinct room temperature emission is observed both in solution and in the solid state. The porous CdSe crystals were isolated as red hexagonal prisms (≤70 µm in size) via solvothermal treatment under similar conditions but with the addition of an alkylammonium salt. The presence of a three-dimensional CdSe network having a coherent crystalline structure inside hexagonal prisms was concluded based on powder X-ray diffraction, selected area electron diffraction and electron microscopy imaging. Self-assembly via oriented attachment of crystalline nanoparticles is discussed as the most probable mechanism of formation.

Stable -ESiMe3 Complexes of Cu(I) and Ag(I) (E=S, Se) with NHCs: Synthons in Ternary Nanocluster Assembly.

As a part of efforts to prepare new "metallachalcogenolate" precursors and develop their chemistry for the formation of ternary mixed-metal chalcogenide nanoclusters, two sets of thermally stable, N-heterocyclic carbene metal-chalcogenolate complexes of the general formula [(IPr)Ag-ESiMe3] (IPr=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; E=S, 1; Se, 2) and [(iPr2-bimy)Cu-ESiMe3]2 (iPr2-bimy=1,3-diisopropylbenzimidazolin-2-ylidene; E=S, 4; Se, 5) are reported. These are prepared from the reaction between the corresponding carbene metal acetate, [(IPr)AgOAc] and [(iPr-bimy)CuOAc] respectively, and E(SiMe3 )2 at low temperature. The reaction of [(IPr)Ag-ESiMe3] 1 with mercury(II) acetate affords the heterometallic complex [{(IPr)AgS}2Hg] 3 containing two (IPr)Ag-S(-) fragments bonded to a central Hg(II), representing a mixed mercury-silver sulfide complex. The reaction of [(iPr2-bimy)Cu-SSiMe3]2, which contains a smaller N-heterocyclic-carbene, with mercuric(II) acetate affords the high nuclearity cluster, [(iPr2-bimy)6Cu10S8Hg3]6. The new N-heterocyclic carbene metal-chalcogenolate complexes 1, 2, 4, 5 and the ternary mixed-metal chalcogenolate complex 3 and cluster 6 have been characterized by multinuclear NMR spectroscopy ((1)H and (13)C), elemental analysis and single-crystal X-ray diffraction.

CXCL14 is an autocrine growth factor for fibroblasts and acts as a multi-modal stimulator of prostate tumor growth.

This study explored the role of secreted fibroblast-derived factors in prostate cancer growth. Analyses of matched normal and tumor tissue revealed up-regulation of CXCL14 in cancer-associated fibroblasts of a majority of prostate cancer. Fibroblasts over-expressing CXCL14 promoted the growth of prostate cancer xenografts, and increased tumor angiogenesis and macrophage infiltration. Mechanistic studies demonstrated that autocrine CXCL14-stimulation of fibroblasts stimulate migration and ERK-dependent proliferation of fibroblasts. CXCL14-stimulation of monocyte migration was also demonstrated. Furthermore, CXCL14-producing fibroblasts, but not recombinant CXCL14, enhanced in vitro proliferation and migration of prostate cancer cells and in vivo angiogenesis. These studies thus identify CXCL14 as a novel autocrine stimulator of fibroblast growth and migration, with multi-modal tumor-stimulatory activities. In more general terms, our findings suggest autocrine stimulation of fibroblasts as a previously unrecognized mechanism for chemokine-mediated stimulation of tumor growth, and suggest a novel mechanism whereby cancer-associated fibroblasts achieve their pro-tumorigenic phenotype.

NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters.

Herein, we describe the synthesis of a toroidal Au10 cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC-Au-X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au25 was observed even upon the reduction of NHC-Au-I. The isolated bromide derivative of the Au25 cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH-π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au10 cluster. 13C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

From molecule to materials: crystalline superlattices of nanoscopic CdS clusters.

Make way for a superlattice! A crystalline 3D superlattice of 2.3 nm molecular CdS nanoclusters was prepared from a convenient mononuclear cadmium thiophenolate precursor. HRTEM and STEM tomography show highly crystalline repetition of monodisperse frameworks. This combined with elemental and thermogravimetric analyses suggests an approximate formula [Cd(130)S(103)(SPh)(54)].

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.

Therapeutic antibodies targeting angiomotin inhibit angiogenesis in vivo.

We have previously shown that angiomotin (Amot) mediates angiostatin inhibition of endothelial migration and tube formation in vitro. A crucial role of angiomotin in regulating endothelial cell motility is indicated by the findings that knockdown of Amot in zebrafish reduces the number of filopodia of endothelial tip cells and severely impairs the migration of intersegmental vessels. In addition, targeting angiomotin using DNA vaccination inhibits angiogenesis and tumor growth in vivo. In this report, we have generated antibodies that, similar to angiostatin, bind to angiomotin on the endothelial cell surface. These antibodies inhibited FGF-2 and vascular endothelial growth factor (VEGF) -induced endothelial migration in the Boyden chamber assay. Furthermore, the anti-Amot B06 antibody significantly reduced the number of endothelial filopodia and inhibited vessel migration during retinal angiogenesis in vivo. We also show that systemic or local treatment with this antibody inhibits pathological blood vessel formation associated with tumor growth or laser-induced choroid neovascularization of the eye. These findings provide a rationale for using angiomotin antibodies for specifically targeting endothelial migration in angiogenesis-dependent diseases.

Angiomotin expression promotes hemangioendothelioma invasion.

Angiomotin was identified by its ability to bind angiostatin and has been shown to mediate its activity in vitro. The family of angiomotin-like protein consists of three members that have coiled-coil domains and conserved c-terminal PDZ-binding motifs. We show here that expression of angiomotin in mouse aortic endothelial (MAE) cells results in stabilization of tubes in the Matrigel assay. Control tubes start to regress after 72 h, whereas MAE-angiomotin (MAE Amot) tubes were stable for over 30 days. In contrast, cells expressing a functional mutant lacking the PDZ protein interaction motif did not migrate and form tubes. Cells from the established tubes invaded into the solidified matrigel. We therefore tested whether angiomotin promotes endothelial invasion. In microcarrier-based invasion in vitro assay, angiomotin-expressing cells invaded collagen matrix and formed tube-like branches. This was confirmed in vivo as injection of MAE-Amot cells promoted tumor growth and invasion into surrounding muscle tissue. Injection of cells transfected with the functional mutant resulted in establishment of noninvasive tumors surrounded by a capsule of fibrous tissue. These tumors remained in constant size or dormant over 3 weeks. Zymogel analysis of the transfected cells did not reveal any differences in proteolytic activity. However, time-lapse photography showed a significant increase in random motility in MAE-Amot cells. We conclude that angiomotin may promote angiogenesis by both stimulating invasion as well as stabilizing established tubes.

Polydentate chalcogen reagents for the facile preparation of Pd2 and Pd4 complexes.

The silylated organochalcogen reagents 1,2-(Me3SiSCH2)2C6H4, and 1,2-(Me3SiSeCH2)2C6H4, were prepared from the corresponding organobromides and lithium trimethylsilanechalcogenolate Li[ESiMe3] (E = S, Se). They have been characterized by multinuclear NMR spectroscopy ((1)H, (13)C, (77)Se) and electrospray ionization mass spectrometry. and react under mild conditions with (1,3-bis(diphenylphosphino)propane)palladium(ii) chloride, [PdCl2(dppp)], to provide the dinuclear organochalcogenolate-bridged complexes [(dppp)2Pd2-µ-κ(2)S-{1,2-(SCH2)2C6H4}]X2, []X2 and [(dppp)2Pd2-µ-κ(2)Se-{1,2-(SeCH2)2C6H4}]X2, []X2 (X = Cl, Br) in good yield, respectively. Furthermore, the tetranuclear palladium complex [(dppp)4Pd4-µ-κ(4)S-{1,2,4,5-(SCH2)4C6H2}]X4, []X4 (X = Cl, Br) can be synthesized from the reaction of the tetrathiotetrasilane 1,2,4,5-(Me3SiSCH2)4C6H2, and [PdCl2(dppp)]. The structures of []X2, []X2 and []X4 were determined by single crystal X-ray diffraction methods. A variety of NMR experiments including two-dimensional homonuclear and heteronuclear correlated spectra were used to probe the solution behaviour of the dinuclear complexes in more detail. These complexes were further characterized by electrospray ionization (ESI) mass spectrometry, and for []X2 and []X2, UV-Vis absorption spectroscopy.

A DNA vaccine targeting angiomotin inhibits angiogenesis and suppresses tumor growth.

Endogenous angiogenesis inhibitors have shown promise in preclinical trials, but clinical use has been hindered by low half-life in circulation and high production costs. Here, we describe a strategy that targets the angiostatin receptor angiomotin (Amot) by DNA vaccination. The vaccination procedure generated antibodies that detected Amot on the endothelial cell surface. Purified Ig bound to the endothelial cell membrane and inhibited endothelial cell migration. In vivo, DNA vaccination blocked angiogenesis in the matrigel plug assay and prevented growth of transplanted tumors for up to 150 days. We further demonstrate that a combination of DNA vaccines encoding Amot and the extracellular and transmembrane domains of the human EGF receptor 2 (Her-2)/neu oncogene inhibited breast cancer progression and impaired tumor vascularization in Her-2/neu transgenic mice. No toxicity or impairment of normal blood vessels could be detected. This work shows that DNA vaccination targeting Amot may be used to mimic the effect of angiostatin.

Loss of responsiveness to chemotactic factors by deletion of the C-terminal protein interaction site of angiomotin.

We have recently identified a novel protein, named angiomotin, by its ability to bind the angiogenesis inhibitor angiostatin in the yeast two-hybrid system. Angiomotin belongs to a family with two other members, AmotL-1 and -2 characterized by coiled-coil and C-terminal PDZ binding domains. Here we show that the putative PDZ binding motif of angiomotin serves as a protein recognition site and that deletion of three amino acids in this site results in inhibition of chemotaxis. Furthermore, endothelial cells expressing mutant angiomotin failed to migrate and form tubes in an in vitro tube formation assay. To study the effect of angiomotin on embryonic angiogenesis, we generated transgenic mice expressing wild-type angiomotin and the C-terminal deletion mutant driven by the endothelial cell-specific receptor tyrosine kinase (TIE) promoter. Expression of mutant angiomotin in endothelial cells inhibited migration into the neuroectoderm and intersomitic regions resulting in death at embryonic day 9.5. In contrast, mice expressing wild-type angiomotin developed normally and were fertile. These results suggest that the putative PDZ binding motif of angiomotin plays a critical role in regulating the responsiveness of endothelial cells to chemotactic cues.