Kristin Kirschbaum

Ultrastable silver nanoparticles

Noble-metal nanoparticles have had a substantial impact across a diverse range of fields, including catalysis, sensing, photochemistry, optoelectronics, energy conversion and medicine. Although silver has very desirable physical properties, good relative abundance and low cost, gold nanoparticles have been widely favoured owing to their proved stability and ease of use. Unlike gold, silver is notorious for its susceptibility to oxidation (tarnishing), which has limited the development of important silver-based nanomaterials. Despite two decades of synthetic efforts, silver nanoparticles that are inert or have long-term stability remain unrealized. Here we report a simple synthetic protocol for producing ultrastable silver nanoparticles, yielding a single-sized molecular product in very large quantities with quantitative yield and without the need for size sorting. The stability, purity and yield are substantially better than those for other metal nanoparticles, including gold, owing to an effective stabilization mechanism. The particular size and stoichiometry of the product were found to be insensitive to variations in synthesis parameters. The chemical stability and structural, electronic and optical properties can be understood using first-principles electronic structure theory based on an experimental single-crystal X-ray structure. Although several structures have been determined for protected gold nanoclusters, none has been reported so far for silver nanoparticles. The total structure of a thiolate-protected silver nanocluster reported here uncovers the unique structure of the silver thiolate protecting layer, consisting of Ag2S5 capping structures. The outstanding stability of the nanoparticle is attributed to a closed-shell 18-electron configuration with a large energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, an ultrastable 32-silver-atom excavated-dodecahedral core consisting of a hollow 12-silver-atom icosahedron encapsulated by a 20-silver-atom dodecahedron, and the choice of protective coordinating ligands. The straightforward synthesis of large quantities of pure molecular product promises to make this class of materials widely available for further research and technology development.

Crystal Structure of Barrel-Shaped Chiral Au130(p-MBT)50 Nanocluster.

We report the structure determination of a large gold nanocluster formulated as Au130(p-MBT)50, where p-MBT is 4-methylbenzenethiolate. The nanocluster is constructed in a four-shell manner, with 55 gold atoms assembled into a two-shell Ino decahedron. The surface is protected exclusively by -S-Au-S- staple motifs, which self-organize into five ripple-like stripes on the surface of the barrel-shaped Au105 kernel. The Au130(p-MBT)50 can be viewed as an elongated version of the Au102(SR)44. Comparison of the Au130(p-MBT)50 structure with the recently discovered icosahedral Au133(p-TBBT)52 nanocluster (where p-TBBT = 4-tert-butylbenzenethiolate) reveals an interesting phenomenon that a subtle ligand effect in the para-position of benzenethiolate can significantly affect the gold atom packing structure, i.e. from the 5-fold twinned Au55 decahedron to 20-fold twinned Au55 icosahedron.

Emergence of hierarchical structural complexities in nanoparticles and their assembly

Probing packing rules The crystals of a well-defined ligand-covered gold nanoparticle can reveal how packing into a lattice happens. Zeng et al. synthesized nanoparticles with a 246-atom gold core surrounded by 80 4-methylbenzenethiol ligands. These nearly spherical nanoparticles did not pack into a cubic arrangement but instead formed a lower-symmetry monoclinic structure. A hierarchy of interparticle ligand interactions controlled the packing, including sets of chiral packing arrangements that reversed between layers. Science, this issue p. 1580 Structural studies of a well-defined ligand-covered gold nanoparticle reveal rules for hierarchical packing. We demonstrate that nanoparticle self-assembly can reach the same level of hierarchy, complexity, and accuracy as biomolecules. The precise assembly structures of gold nanoparticles (246 gold core atoms with 80 p-methylbenzenethiolate surface ligands) at the atomic, molecular, and nanoscale levels were determined from x-ray diffraction studies. We identified the driving forces and rules that guide the multiscale assembly behavior. The protecting ligands self-organize into rotational and parallel patterns on the nanoparticle surface via C-H⋅⋅⋅π interaction, and the symmetry and density of surface patterns dictate directional packing of nanoparticles into crystals with orientational, rotational, and translational orders. Through hierarchical interactions and symmetry matching, the simple building blocks evolve into complex structures, representing an emergent phenomenon in the nanoparticle system.

Gold-Catalyzed Transannular [4+3] Cycloaddition Reactions

Macrocyclic propargyl acetates containing a furan ring were prepared by using a CrCl(2)-promoted reaction. In the presence of either a Au(I) or Au(III) catalyst, a tandem 3,3-rearrangement/transannular [4+3] cycloaddition reaction occurred to give propargyl acetates that are regio- and diastereospecific. The regiochemistry of the product is controlled by the position of the acetoxy group in the starting material and the stereochemistry of the reaction depends on the ring size.

An open-flow helium cryostat for single-crystal X-ray diffraction experiments

An open-flow helium cryostat for single-crystal X-ray diffraction experiments capable of reaching 14 K has been developed using off-the-shelf components. Solid/liquid air build-up is prevented using the transfer-line helium back-flow and a heated nozzle. The system has run for over 30 h with no frost build-up.In a recent experiment the system has run for over 60 h with no frost build-up. The effectiveness of the system has been demonstrated using test data for oxalic acid, terbium vanadate and dysprosium vanadate.

Cooperative Jahn-Teller induced phase transition of TbVO4: single crystal structure analyses of the tetragonal high temperature phase and the twinned orthorhombic phase below 33 K

The rare earth vanadate TbVO4, undergoes a crystallographic phase transition below 33 K induced by a cooperative Jahn-Teller effect. Twinning of the crystal occurs upon this transition from the tetragonal high temperature phase to the orthorhombic low temperature phase resulting in a domain structure. Single crystal x-ray analyses of the tetragonal and the twinned orthorhombic phase verify a reduction in the site symmetry of the Tb ion from D2d (m2-dodecahedral) for the high temperature phase to D2 (222-distorted dodecahedral) for the low temperature phase. Concomitantly, the space group symmetry is lowered from D194h (I41/amd) to D242h (Fddd). The twinned orthorhombic phase is described as domains related to each other by a 180? rotation about the orthorhombic [110] axis.

Di- and triindolylmethanes: molecular structures and spectroscopic characterization of potentially bidentate and tridentate ligands

Abstract4-Bromophenyldi(3-methylindol-2-yl)methane (2) and 2-methoxyphenyldi(3-methylindol-2-yl)methane (3) were prepared by sulfuric-acid-catalyzed reactions of 3-methylindole with 4-bromobenzaldehyde and o-anisaldehyde, respectively. Di(3-methylindol-2-yl)phenylmethane (1) and tri(3-methylindol-2-yl)methane (4) were similarly prepared as described previously. Spectroscopic data (1H, 13C NMR) and the X-ray crystal structures for 1⋅C2H5OH and 2–4 are reported. The molecular structure of 1⋅C2H5OH shows hydrogen bonding of both indolyl NH protons to the oxygen of an ethanol molecule. Crystal data for 1⋅C2H5OH: Orthorhombic, Pca21, a = 23.9782(17) Å, b = 8.4437(7) Å, c = 11.3029(9) Å, V = 2288.4(3) Å3, R1 = 0.0597. Crystal data for 2: Orthorhombic, P212121, a = 8.911(3) Å, b = 9.584(4) Å, c = 24.040(11) Å, V = 2053.0(14) Å3, R1 = 0.0454. Crystal data for 3: Monoclinic, P21/c, a = 9.737(2) Å, b = 25.035(6) Å, c = 9.359(2) Å, β = 114.853(4)○, V = 2070.2(8) Å3, R1 = 0.0511. Crystal data for 4: Trigonal, R3, a = 14.2214(10) Å, c = 9.6190(10) Å, V = 1684.8(2) Å3, R1 = 0.0425.

Synthesis, crystal structure and electrochemistry of bis(N,N-dimethylammonium) tris(1,2-benzenedithiolato)titanate(IV)

Abstract Synthesis, electrochemical and structural data for [NH2(CH3)2]2[Ti(S2C6H4)3] (1 are described. Treating Ti{N(CH3)2}4 with 1,2-(HS)2C6H4 in a 1 : 3 ratio affords 1 in 94% isolated yield. Dark red single crystals of 1, suitable for X-ray crystallography, were obtained from CH3CNEt2O mixtures. The molecular structure of the anion of 1, [Ti(S2C6H4)3]2−, provides a rare example of a homoleptic thiolate complex of titanium, but more importantly, the titanium centre contains the unusual TiS6 coordination of a skew-trapezoidal bipyramid (S-TzBP) with angular coordinates φA = 60°, φB = 140E = 75°. The S6 polyhedron in 1 can be alternatively described as a pentagonal bipyramid with an equatorial vertex missing. A spectropotentiostatic experiment for 1 verified that a reversible one-electron reduction occurs at −1.51 V (vs Ag/AgCl, DMF). The electrochemical product was characterized by UV-vis (347, 414 and 434 nm) and ESR (77 K, rhombic: g1 = 2.18 g2 = 2.04 and g3 = 2.00) spectroscopies.

Energetic materials: The preparation and structural characterization of melaminium dinitramide and melaminium nitrate

Two energetic salts of the melaminium cation have been prepared and structurally characterized from room temperature X-ray single crystal diffraction data. Melaminium dinitramide (I), triclinic, P1¯, a = 6.6861(11), b = 6.9638(16), c = 10.447(2) Å , α = 99.07(3), β = 98.30(3), γ = 108.50(3)°, V = 445.6(2) Å3, and Z = 2. Melaminium nitrate (II), monoclinic, P21/c, a = 3.5789(7), b = 20.466(4), c = 10.060(2) Å, β = 94.01(2)°, V = 735.0(3) Å3, and Z = 4. The crystal structures of both salts show distinct monoprotonated melaminium cations and dinitramide- or nitrate anions, respectively. Efficient packing in the solid state is achieved by extensive hydrogen bonding between two-dimensional zigzag ribbons of the melaminium cations and the respective anions resulting in high densities of the solid state structures of 1.74 (I) and 1.71 g/cm3 (II).

Quantification of CH···π interactions: implications on how substituent effects influence aromatic interactions.

Attractive interactions between a substituted benzene ring and an α-substituted acetate group were determined experimentally by using the triptycene model system. The attractive interaction correlates well with the Hammett constants σ(m) (R(2)=0.90), but correlates much better with the acidity of the α-protons (R(2)=0.98).