Angewandte Chemie International Edition

Crystals as you like them: The last decade has witnessed the successful synthesis of metal nanocrystals in a rich variety of shapes. As a zero-order approach, a typical synthesis can be divided into three distinct stages: nucleation, evolution of nuclei into seeds, and growth of seeds into nanocrystals. As illustrated in the cover picture and discussed by Y. Xia et al. in their Review on page 60 ff., the final shape of a nanocrystal is determined primarily by the internal structure of the seed and the binding of the capping agent.

Silicon nanospheres are promising as fluorescent biological probes because of their excellent water dispersibility, high photoluminescence, robust photostability, and favorable biocompatibility. In their Communication on page 128 ff., C.-H. Fan, S.-T. Lee, and co-workers describe how the nanospheres, which are remarkably effective in real-time and long-term imaging, have significant advantages over traditional dyes in biological applications.

A quick fix: The recently reported process of “oxygen-atom metathesis”, which is akin to olefin metathesis, may be involved in the activation of carbon dioxide. Nucleophilic attack by the d⁸ metal center of an in situ generated Fischer carbene complex on carbon dioxide affords an unstable metallacycle. Elimination of an organic component, tert-butyl formate, results in the concurrent formation of an iridium–carbonyl complex.

Going full circle: Pericyclic reactions can be catalyzed by groups that can bind selectively to a pericyclic transition-state structure (blue) either by interacting directly with rearranging electrons (green) or interacting with substituents (red). The recent success in accelerating six-electron electrocyclizations by using Lewis acids (LAs) is discussed in this Highlight. Future prospects for catalyst design are also emphasized.

Classical protocols for carbonyl allylation, propargylation, and vinylation typically rely upon the use of preformed allyl metal, allenyl metal, and vinyl metal reagents, respectively, mandating stoichiometric generation of metallic by-products. Through transfer hydrogenative CC coupling, carbonyl addition may be achieved from the aldehyde or alcohol oxidation level in the absence of stoichiometric organometallic reagents or metallic reductants.

More than an afterthought: Functional polymers possessing precisely controlled molecular weight, composition, and architecture remain difficult to synthesize, despite the advent of modern polymerization techniques. This Review surveys different classes of reactive polymer precursors that bear chemoselective side groups and discusses reactions suitable for their post-polymerization modification into functional polymers.

Function follows form: Controlling the shape of nanocrystals may initially seem like a scientific curiosity, but its goal goes far beyond aesthetic appeal. For metal nanocrystals, shape not only determines their intrinsic physical and chemical properties but also their relevance for electronic, magnetic, optical, catalytic, and sensing applications.

Seeking scaffold diversity: A synthetic approach for the combinatorial variation of the scaffolds of small molecules is described. Using just six basic reaction types, compounds with 84 distinct scaffolds were prepared. The compounds had many natural-product-like structural features including rich stereochemistry, heterocyclic and unsaturated ring systems, and dense functionalization.

Branching out: The use of polypropylene–imine dendrimers as scaffolds leads to the precise control of the pore size of functionalized nanoporous membranes, which can be used for molecular recognition. The nanopores show a finite transition in generation-dependent molecular discrimination capabilities, which arises not only from the size of the final pores, but also from the functional group density of the dendrimers (see picture).

It's hip to be square: A perfectly planar square of phosphorus atoms is prepared by the reduction of [ZrCl₂{PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh}] in the presence of P₄ (see structure). On the basis of the PP single bond lengths and oxidation state of the Zr centers, this unit is formulated as a [P₄]⁴⁻ fragment.

Touching the surface: DNA-labeled gold nanoparticles can be removed from a planar substrate by using a heterogeneous DNA displacement reaction. An advantage of using a planar substrate is that quantitative and kinetic studies of the displacement process could be achieved. This provides a fascinating way to control nanomaterials and to develop novel nanodevices on planar substrates by combining DNA hybridization and displacement reactions.

Very fast: UV excitation of the right-handed B form or the left-handed Z form of [poly(dG-dC)]₂ results in transient IR absorption bands that remain long after the ¹ππ* state is predicted to have decayed (see picture). A biexponential decay is observed for B-DNA, which is assigned to the vibrationally excited ground state and the ¹n_Nπ* state. The decay of Z-DNA is dominated by single-exponential decay, which is assigned to an exciplex state.

In silico to in vivo: The design strategy for silicon-based fluorescent nanospheres with controllable sizes was based on the results of theoretical calculations. These nanospheres are fully water dispersible, highly photoluminescent, extremely photostable, and suitably biocompatible. Cell imaging results further demonstrate the nanospheres are remarkably efficacious for real-time and long-term cell monitoring (see picture).

Tracking the changes: The switchable translocation of a nucleic acid strand with a methylene blue (MB⁺) redox group on a DNA track associated with a surface can be triggered by adenosine monophosphate (AMP, green) and adenosine deaminase (purple, see picture); the latter converts AMP into inosine monophosphate (orange). The motion of the nucleic acid can be followed by the electrochemical, photo-electrochemical, and fluorescence signals.

Caught in a trap: Colloids of gold nanoparticles coated with a thermally responsive poly-(N-isopropylacrylamide) (pNIPAM) microgel can trap molecules in different ways as a function of temperature (see scheme). The porous pNIPAM shells prevent electromagnetic coupling between metal particles, thus providing highly reproducible surface-enhanced Raman scattering (SERS) signals and intensity.

Nitty gritty: A chemically amplified poly(methylsilsesquioxane) resist based on the acid-catalyzed condensation of silanol end groups was developed for direct fabrication of hybrid 3D microstructures (see picture) by conformal and maskless proximity-field nanopatterning. Fibers, colloidal particles, helical arrays, and photonic crystals were fabricated by varying phase-mask design and exposure conditions.

Magic pentagons: Exploitation of versatile pentagonal units/ligands has previously led to giant molybdenum oxide based curved species, including spherical Keplerates. Similar methodology is now also applicable to the related tungstate scenario (see corresponding basic central pentagonal unit in green).

Nickel box: The first catalytic asymmetric method for cross-coupling arylzinc reagents with α-bromoketones has been developed (see scheme). This stereoconvergent carbon–carbon bond-forming process occurs under unusually mild conditions and without activators, thereby allowing the generation of potentially labile tertiary stereocenters.

The medicinal properties of garlic, thought to derive at least in part from the antioxidant activity of its sulfur-containing secondary metabolites, have been recognized for hundreds of years. The ability of garlic to scavenge peroxyl radicals can be accounted for in terms of the action of transient sulfenic acids, which are predicted to react by diffusion-controlled five-center proton-coupled electron transfer (see scheme and transition state).

Self-decontaminating textiles were prepared using layer-by-layer deposition of highly active WO₃/titanate nanotubes for photocatalytic removal of sulfide and organophosphonate simulants and authentic chemical warfare agents by using solar light.

Reduction deduction: Precipitation of an insoluble copper triazole complex onto a carbon black support leads to the formation of an efficient catalyst for the four-electron reduction of O₂ to H₂O. The oxygen-reduction activity is reported over a wide pH range from 1 to 13 and the onset of the reaction occurs at potentials as high as 0.86 V. Ex situ magnetic susceptibility measurements demonstrate the presence of multicopper sites.

It comes out in the wash: In the esterification of citric acid with n-butanol, heteropolyanion-based ionic liquid (IL) catalysts show high catalytic activity, self-separation, and easy reuse. The good solubility in reactants, nonmiscibility with ester product, and high melting point of the IL catalysts enable the reaction-induced switching from homogeneous (b in the picture) to heterogeneous (c) with subsequent precipitation of the catalyst (d).

An efficient and practical protocol for the highly selective preparation of substituted allyl sulfones has been developed. Arenesulfonyl cyanides, Baylis–Hillman adducts, and simple allylic alcohols give an unforeseen outcome (see scheme).

Size-controlled clusters: Clusters of preformed 15 nm oleic acid coated superparamagnetic iron oxide nanoparticles were prepared through partially destabilizing suspensions of the dispersed nanoparticles with CN-modified silica. This effect induced gradual formation of monodisperse, superparamagnetic clusters by competition between the solid phases for the surfactant. The cluster size can be controlled within the range 15–200 nm as the growth process can be stopped and restarted.

Thanks to CH activation: 3-Aryl-3,4-dihydroisoquinolines (2) are synthesized from bromobenzenes (1) by a sequence comprising a C(sp³)H activation, a Curtius rearrangement, and a tandem electrocyclic ring-opening/6π electrocyclization. This method is applied to the synthesis of various isoquinoline-containing molecules, including the tetrahydroprotoberberine alkaloid coralydine.

Simple epitaxial growth of a cyano-bridged coordination network on different core particles can lead to core–multishell nanoparticles. Shell growth is controlled on the nanometer scale and can be repeated with different metal ions. Particles can be prepared with a distinct composition in each layer (see images), and synergy is observed between their magnetic properties.

Substrate-dependent reactivity: The nickel(II) alkylperoxo complex 1 (see structure), obtained by the dehydrative condensation of the nickel(II) hydroxo complex with tert-butylhydroperoxide has a unique structure; the coordination mode of the OO moiety is an intermediate between η¹ and η². Compound 1 exhibits substrate-dependent reactivity toward aliphatic CH, phosphines, carbon monoxide, and aldehydes.

Enantioselectivity switch: A catalytic enantioselective [4+2] cycloaddition reaction of alkylarylketenes with N-aryl-N′-benzoyldiazenes or N,N′-dibenzoyldiazenes to give 1,3,4-oxadiazin-6-ones 1 was developed by employing N-heterocyclic carbene (NHC) catalysts. The enantioselectivities could be switched for most reactions by changing the substituents on the NHC catalyst. TBS=tert-butyldimethylsilyl, Mes=2,4,6-trimethylphenyl.

BaF₂fled no more: Monodisperse cubic- and orthorhombic-phase BaF₂ nanocrystals were formed readily by the liquid–solid–solution approach. Nucleus growth of BaF₂ occurs under kinetic control, whereby their size and morphology depend greatly on the ripening time and concentration of surfactants (see scheme). The phase of the BaF₂ crystals is under thermodynamic control.

A toss up: A highly active palladium complex enabled the first direct arylation of heteroarenes through CH bond functionalization using tosylates or mesylates as electrophiles with ample scope.

Palladium, radically different: A wide range of polyfunctional aryl- and heteroarylmagnesium reagents undergo fast Kumada cross-couplings (see scheme) with functionalized aryl bromides in the presence of a palladium catalyst and an alkyl iodide as additive. These reactions proceed by a radical pathway.

Totally tubular electrodes: A new design is proposed for nanostructured electrodes for high-performance lithium batteries (see picture) based on the use of carbon tube-in-tube (CTIT) as both a nanoreactor and an efficient ion- and electron-conducting network. A V₂O₅/CTIT nanocomposite electrode is prepared to demonstrate this concept. The resulting materials exhibit a significant improved lithium-storage performance.

No pressure: The most-oxidized iridium oxides known to date are prepared in a hydroxide flux under normal pressure. They contain iridium centers exclusively in the +VI oxidation state and are characterized crystallographically. The picture shows the structure of the Ln₂K₂IrO₇ (Ln=Nd, Sm) and its structural components: IrO₆ octahedra (black), KO₁₀ polyhedra (beige), LnO₁₀ polyhedra (blue).

Silver-plated DNA: The deposit of a thin metal layer on biomolecules, such as DNA, requires the formation of small, “magic-sized” metal nuclei. Through the careful design of a reducing chemical functionality in the form of a dialdehyde, the nucleation process and thus the metallization step can be controlled (see scheme).

Beyond the age of silicon: A combination of radical chemistry under standard conditions and clean-room lithography can alter the electronic structure of graphene layers permanently through covalent chemical functionalization. The potential change follows the Hammett correlation. This simple method is a promising approach for graphene-based electronics.