Angewandte Chemie International Edition

Polyoxoniobates (PONb) differ from nearly all other polyanionic clusters in their unusual solubility behavior. In their Communication on page 12 514 ff., M. Nyman and co-workers show that the Cs salts of PONb are the most soluble and the Li salts the least. The opposite trend holds for other polyoxometalates and anionic clusters, owing to factors such as the strong inner-sphere complexation ability of large alkali-metal ions. The excellent solubility of the Rb and Cs heteropolyniobates offers a unique system for aqueous ion-association studies.

A seed-mediated growth method has been used to synthesize silver nanocrystals enclosed by concave surfaces and thus high-index facets in aqueous solution. Y. Xia and co-workers report in their Communication on page 12 542 ff. that controlling the growth habit of silver cubic seeds leads to concave silver nanocrystals that act as surface-enhanced Raman-scattering substrates with stronger signal enhancements than conventional silver nanocrystals of a similar size but enclosed by flat faces.

Reaching for the stars Although the cyclopentadienyl cation is still synthetically out of reach, the isoelectronic replacement of most of the carbon atoms by elements from the lithosphere, such as silicon and phosphorus, makes a heavier congener accessible. In their Communication on page 12 510 ff., H. W. Roesky, M. Scheer, D. Stalke, and co-workers show how the anti-aromatic five-membered CSi₃P ring is stabilized. Picture of planet Earth courtesy of NASA, 1972 (Image ID: AS17-148-22727) and picture of Globular Cluster Messier 15 courtesy of ESA/Hubble and NASA, 2011.

“When I was eight(een) I wanted to be a boat builder/yacht maker. My favorite drink is whisky (Islay) …” This and more about Rhett Kempe can be found on page 12396.

Columbia University Press, New York, 2011. 272 pp., softcover, $ 19.95.—ISBN 978-0231142717

John Wiley & Sons, Hoboken, 2011. 320 pp., softcover, $ 34.90.—ISBN 978-0470390436

Retooling RNA: RNA aptamers are high-affinity ligands that can be assembled with other structures to yield multivalent molecules. These properties have been addressed in two recent studies: One describes a GFP-like RNA reporter used to study the dynamics of endogenous RNA; the other study reports on an aptamer-templated assembly of multi-enzyme complexes in bacteria for the controlled production of secondary molecules (see picture).

The detection of magnetochiral dichroism (MChD; CD: circular dichroism) in a suspension of J-aggregate particles of an achiral amphiphilic nonmetalated porphyrin renews the interest for porphyrins in supramolecular chemistry and reinforces the concept that novel advanced materials can be obtained through self-assembly and auto-organization processes (see picture).

At warp speed: The speed and cost of sequencing DNA has undergone tremendous improvement based on Solexa sequencing. This technique arose from basic curiosity-driven research and unexpected outcomes, rather than from ideas that were strategically driven. The whole-genome sequencing of individuals now possible may one day transform the way medical disorders are classified, diagnosed, and treated.

The quest for the etiology of a biomolecular structure acquires special significance when the questions asked refer to molecules, the existence of which are fundamental to life, and in particular to the origin of life. In a search for the chemistry of the emergence of life, one needs to pay strict heed to molecular guides—nucleic acids, proteins, cofactors—that all carry a message, which is our job to decipher.

Sucked into the vortex: Hydrogels with embedded Rhodamine B dye showed stir-induced circularly polarized luminescence (CPL; see picture), the sense of which can be controlled by switching the stir direction from clockwise (CW) to counterclockwise (CCW) with slow cooling from the sol to gel states. The chiral alignment of the dye was erased by heating the sample above the gel–sol transition temperature.

You crack me up: A topographically patterned PDMS stamp was coated with thin metal film and swelled under organic vapor to induce the tunable cracking of the brittle film into metallic nanostructures (see SEM images, scale bars 1 μm). UV/Vis spectra, OLED efficiency, and SERS spectra demonstrate the fine controllability of the metallic nanostructures, the well-ordered and highly regulable surface plasmons, and the facile fabrication process.

The one- and two-photon luminescence of stilbene-type solid-state materials can be tuned and controlled from blue to yellow color by a supramolecular cocrystal method.

Two is better than one: Two short locked nucleic acid based probes were used to collectively capture and detect microRNAs by a simple two-temperature hybridization process. Intact microRNAs were directly measured down to attomolar concentrations with a high specificity and nearly four orders of magnitude of dynamic range. Single base mismatches in the microRNAs were potently discriminated from the perfectly matched targets.

Step by step: The cluster [3,3′-Co(1,2-C₂B₉H₁₁)₂]⁻ is an excellent platform for making a stepwise tunable redox potential system by dehydroiodination. With the addition of up to eight iodine substituents (purple; see picture), there is a fall in the E_1/2(Co^III/Co^II) value from −1.80 V to −0.68 V (vs. Fc⁺/Fc; Fc=ferrocene). A practical application of this tunability has been observed in the growth of polypyrrole.

Fine-tuning: Three types of optically active trans-2,3-disubstituted-2,3-dihydrobenzofurans having three contiguous stereogenic centers can be efficiently accessed by one-pot reaction cascades (see scheme; TMS=trimethylsilyl). High substitution diversity of the final products can be achieved from the same common precursors by fine-tuning of their reactivity through simple structural modifications.

Double take: β-Borylallylsilanes have been synthesized by the regioselective silaboration of terminal alkynes followed by palladium-catalyzed double-bond migration of the resulting β-borylalkenylsilanes (see scheme). The stereoselectivity of the double-bond migration can be controlled by additives, thus leading to the stereocomplementary synthesis of (E)- and (Z)-β-borylallylsilanes.

Bloodless surgical knife: Nano-biotechnology has been introduced into imaging-guided high-intensity focused ultrasound (HIFU) cancer surgery by adopting engineered multifunctional manganese-based mesoporous composite nanocapsules as the contrast agents for T₁-weighted magnetic resonance imaging (MRI) and simultaneously as synergistic agents for MRI-guided HIFU cancer surgery.

Don′t count on counterions: The cyclic five-membered CSi₃P cation 1 is synthesized in the reaction of benzamidinato-stabilized chlorosilylene and methyl phosphaalkyne. The presence of four π electrons in 1 means it can be considered as a formal, heavier analogue of the cyclopentadienyl cation. Surprisingly the small counteranion (Cl⁻) does not contribute to the ring stability.

Surprising solubility: While it is already well known that [Nb₆O₁₉]⁸⁻ salts exhibit an unusual solubility trend, that is, Cs>Rb>K>Na>Li, the heteropolyniobates of Cs and Rb had not yet been crystallized. These very soluble entities have now been obtained from solution by a simple and universal process. New polyoxoniobate geometries are thus unveiled, and the [SiNb₁₂O₄₀]¹⁶⁻ Keggin ion is characterized in solution for the first time.

Tuning MOFs: When a metal–organic framework (MOF) with an ordered three-dimensional macroporous structure is integrated into a film, the resulting materials have an additional optical element, which can be used as a general and effective signal transducer. This, in combination with the hierarchical pore structure, makes these films interesting dynamic photonic materials with potential applications in sensors.

What a core-ker! By the appropriate combination of promesogenic bent-core structures and the C₆₀ unit, lamellar polar liquid-crystal phases were induced (see scheme). The supramolecular organization of the functional fullerene-based assemblies, the temperature range of the soft phase, the stabilization of the mesophase-like order at room temperature, and the molecular switching under an electric field can be tuned, depending on the molecular structure.

Let it grow: The conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was synthesized with heparin as the counterion to form a cell culture substrate. The surface of PEDOT:heparin in the neutral state associated biologically active growth factors (see picture). Electrochemical in situ oxidation of PEDOT during live cell culture decreased the bioavailability of the growth factor and created an exact onset of neural stem cell differentiation.

Sweet screens: A high-throughput screening platform for identification of inhibitors of sialyl- and fucosyltransferases based on fluorescence polarization (FP) has been developed. An analogue of the natural donor substrate carrying a fluorescent label (green star) is transferred to a glycoprotein acceptor, which results in robust FP (see picture). The screening of 16 000 compounds against different glycosyltransferases has identified various interesting inhibitors.

A proton-conductive material based on a crystalline assembly of trimesic acid and melamine (TMA⋅M, see picture) is reported. Because of the ordered structure of the assembly, the water-saturated proton conductivity for the TMA⋅M assembly is 5.5 S cm⁻¹, which is the highest proton conductivity measured to date. This exceptionally high conductivity and low-cost fabrication of the material make applications feasible for fuel-cell devices.

Shaped and dimpled: Silver nanocrystals enclosed by concave surfaces and thus high-index facets have been prepared by simply controlling the growth habit of Ag cubic seeds. Four types of concave nanocrystals, including octahedron, cube, octapod, and trisoctahedron, were obtained (see picture).

Magnetic attraction: The cubic perovskite BaFeO₃ (see picture, Ba blue, Fe brown, O white), which is obtained by a low-temperature reaction using ozone as an oxidant, exhibits ferromagnetism with a fairly large moment of 3.5 μ_B per Fe ion above a small critical field of approximately 0.3 T. This specific ferromagnetism is attributed to the enhancement of O→Fe charge transfer that arises from deepening of the Fe⁴⁺ d levels.

AUROlite, consisting of gold supported on titania (picture shows extrudates in a steel net cage), is a robust catalyst for the production of catalyst-free HCOOH/NEt₃ adducts from H₂, CO₂, and neat NEt₃. Pure HCOOH is freed from the adducts by amine exchange.

Channel zapping: PKU-1 and newly synthesized PKU-2 (Al₂B₅O₉(OH)₃⋅n H₂O; see picture) possess microporous structures with 18-ring and 24-ring channels, respectively. They show high reactivity and size selectivity in the cyanosilylation of aldehydes as heterogeneous Lewis acid catalysts. The different channel sizes determine the substrate selectivity. These examples demonstrate the potential of octahedron-based aluminoborate channels in catalysis.

Light can play: Irradiation causes dramatic changes in the shape of rigid-rod polymers incorporating azobenzene photochromes in the main chain. The embedded photoswitches act as hinges, which upon light-induced isomerization lead to reversible shrinking and stretching of the polymer backbone (see scheme), resembling light-orchestrated macromolecular accordions.

Lined up water molecules: Artificial transmembrane channels from pillar[5]arene monomeric and dimeric derivatives have been prepared. Single-channel conductance measurements and isotopeeffect experiments under acidic conditions showed selective proton transport through the channels, which were mediated by water wires formed in the pillar[5]arene backbones (see picture).

The conformational equilibrium of a pH-dependent switch based on an intramolecularly H-bonded diphenylacetylene can be predictably biased by using electron-donating or -withdrawing groups (see scheme). Furthermore, protonation of the electron-donating dimethylamino group converts it into an electron-withdrawing dimethylammonium cation with a concomitant switch in conformation.

Single-crystal calcite nanowires are formed by crystallization of morphologically equivalent amorphous calcium carbonate (ACC) particles within the pores of track etch membranes. The polyaspartic acid stabilized ACC is drawn into the membrane pores by capillary action, and the single-crystal nature of the nanowires is attributed to the limited contact of the intramembrane ACC particle with the bulk solution. The reaction environment then supports transformation to a single-crystal product.

Not one but two: The title reaction proceeds through the dual CH bond cleavage of both aniline and acetonitrile (see scheme). The reaction affords a variety of cyano-bearing indolinones in excellent yield. Mechanistic studies demonstrate that this reaction involves a fast arylation of the olefin and a rate-determining CH activation of the acetonitrile.

Let's twist! The Scholl reaction with quinquephenyl derivatives has been shown to have an unexpectedly strong preference for forming twisted, helicene aromatic polycycles, instead of their flat counterparts. This tendency is so strong that it will overcome even severe steric hindrance, and the procedure can be used in the efficient synthesis of hexa-tert-butylhexabenzotriphenylene from a simple biaryl starting material (see scheme).

Syn-gled out: The syn diastereo- and enantioselective addition of azlactones to 3-vinylindoles was accomplished by using a chiral, binapthol-derived, Brønsted acid catalyst (see scheme). This method enables facile access to tryptophan derivatives with adjacent quaternary and tertiary stereogenic centers, which are potentially useful for the synthesis of peptidomimetics.

Structural signatures: The analysis of Si–Si and Si–Li connectivities by solid-state NMR spectroscopy allows the different types of silicon clusters to be discriminated in the model lithium silicide compound Li₁₂Si₇ (see picture, Si clusters red and blue, Li ions gray). The results provide new NMR spectroscopic strategies with which to differentiate and study the structures formed in silicon-based electrode materials.

Something functional: The title reaction proceeds in the presence of azide and water to deliver amides in high yields, and it can be used in a ring-expansion strategy to generate lactams. A mechanism is proposed based on experimental results. This reaction offers a new approach to functionalizing simple and readily available hydrocarbons. DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Basic difference: The importance of a pendent base in promoting proton-coupled electron-transfer reactions with low activation barriers has been discussed for H⁺ reduction or H₂ oxidation in acetonitrile. Investigation of the interaction between a base positioned in the second coordination sphere of a complex and a water ligand in water oxidation reactions using geometric isomers of [Ru(tpy)(pynap)(OH₂)]²⁺ (see picture) gave intriguing results.

Build a bridge: The first catalytic intramolecular [3+2] cycloaddition of monodonor–monoacceptor cyclopropanes (see scheme) provides a general and efficient strategy for construction of structurally diverse acetal[n.2.1] and 1,4-dioxygen-substituted cyclic skeletons, which are widely distributed in biologically important natural products.

Sugar coat: The nitrogen-fixing soil bacterium Bradyrhizobium sp. BTAi1 is coated with a unique lipopolysaccharide that does not induce innate immune responses in its host plant Aeschynomene indica or in different plant families. The chemical nature of the monosaccharide forming the polymer (see picture) is unprecedented in nature, which helps to avoid “harmful” recognition by its symbiotic host.

The diboranes(4) bis(catecholato)diborane (B₂Cat₂) and bis(pinacolato)diborane (B₂Pin₂) are important precursors for organoboronic esters, which are versatile reagents for the formation of carbon–carbon bonds. A new catalytic synthesis for these compounds starts from catecholborane or pinacolborane and gives the dehydrocoupling products B₂Cat₂ and B₂Pin₂ with turnover numbers of up to 11 600 (see scheme).

Swing bridge: The triplet species ethenedithione has been generated within the coordination sphere of cobalt, leading to a dinuclear μ-η²-η²-C₂S₂ complex (see picture: C gray, Co blue, P purple, S yellow). Depending on the solvent, the C₂S₂ moiety displays a transoid or a cisoid geometry. This isomerization step changes the sign of the magnetic coupling between the cobalt centers.

When three become one: Within one nickel-based model system, the three reactants CO, MeI, and PhSH have been assembled to yield an acetyl thioester. The reactivity is of relevance for the functioning of the acetyl coenzyme A synthase active site and provides insights into possible binding sequences.

Faster, higher, stronger! The N-heterocyclic carbene (NHC) catalyzed diastereo- and enantioselective hydroacylation of cyclopropenes affords structurally valuable acylcyclopropanes. A new family of electron-rich, 2,6-dimethoxyphenyl-substituted NHCs induces excellent reactivity and enantioselectivity. Preliminary kinetic studies unambiguously demonstrated the superiority of this family of catalysts over known NHCs in this challenging transformation.

Your wish is my command: Deracemization is a powerful strategy wherein a racemate is converted into a 100 % yield of a single enantiopure product. A new concept in catalytic deracemization is presented, in which a racemate with n stereogenic elements can be selectively converted into each one of 2^m (m=number of chiral centers of the product) different enantiopure products, by simple tuning of the reaction conditions.

Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions.

12 not so angry men: Hexaphenylethane is unstable, a phenomenon traditionally attributed to steric repulsion between the six phenyl rings. However, adding 12 bulky tert-butyl groups, one to each of the 12 meta positions, gives a stabile ethane derivative (see space-filling model and potential energy curve for the dissociation of the central CC bond). This unexpected stabilization is shown to result from attractive dispersion interactions between the substituents.

There you go: How to get kinetic information from trajectories of single biomolecules? A method based on correlating signal ranges (here for a three-state DNA hairpin with F=folded, I= intermediate, and U=unfolded state) is reported and shows how to get the kinetic scheme and the corresponding rates, even for states with low occupancy or very short lifetime, states which overlap because of noise, and systems with very similar or very different rates.

Subtle differences in the local sequence and conformation of amino acids can result in diversity and specificity in electron transfer (ET) in proteins, despite structural conservation of the redox partners. For individual ET steps, distance is not necessarily the decisive parameter; orientation and solvent accessibility of the ET partners, and thus the stabilization of the charge-separated states, contribute substantially.

One at a time or all at once? Electronic fluxes during a pericyclic reaction in the electronic ground state—exemplified for the degenerate Cope rearrangement of semibullvalene—may proceed either synchronously or asynchronously. Quantum simulations show that the mechanism is determined by the preparation of the reactants, for example, synchronous at cryogenic temperatures (tunneling) but asynchronous when induced by selective laser pulses (with energy over the barrier).

As you like it: xCrAsH, a dimeric derivative of the arsenical compound FlAsH, enables the highly specific, covalent cross-linking of two proteins containing a 12 amino acid peptide tag. This inducible and (by addition of dithiols) reversible system can be used to detect and manipulate protein–protein interactions both in vitro and in living cells (see picture).