Angewandte Chemie International Edition

Hunter and hunted is not only a principle of nature but is also found in host–guest systems. J. de Mendoza and co-workers describe in their Communication on page 198 ff. how a host molecule (a small calix[4]arene or cavitand) can itself become the “prey” for a larger host (a metallocavitand made from a calix[4]arene with [3,8]phenanthroline subunits linked through Re atoms). The cover picture shows the complexation of a cavitand (by P. Ballester) superimposed on an underwater picture at Cap Andritxol, Mallorca (by E. Botana).

A molecular Solomon knot was isolated from a dynamic library of molecular knots by kinetically controlled crystallization. By incorporating mixed-metal templates into the usual protocol employed in the synthesis of Borromean Rings, J. F. Stoddart and co-workers obtained a molecular Solomon link as the only crystalline product under the reaction conditions described and were able to determine its X-ray crystal structure, as they report in their Communication on page 218 ff.

Far-reaching influence: How did the cultural atmosphere in the Anglo-Saxon world shape the sciences and in particular chemistry at the beginning of the 19th century? Who were the most important figures, and what role did they play in academia as well as in the public? Impressions regarding these themes are given in this Essay.

Energy for everybody? The ever increasing world energy demand cannot be satisfied much longer with fossil fuels; alternatives are required to limit the chance of a climate collapse and the spreading of wars for natural resources. The 21st century will be largely defined by the way we face and resolve the energy crisis. This is an intricate and fascinating scientific challenge, in which chemistry will play a fundamental role, and also an unprecedented opportunity to shape a more peaceful world.

Exercising demons: Although the notion of synthetic molecular-level motors and machines has been around for 150 years (for example, Maxwell's demon, see picture), it is only recently that research in this area has flourished. The current state of the art in regard to how the components of molecular-level structures can be switched, rotated, speeded up, slowed down, and directionally driven in response to stimuli is described in the Review.

Twist your tubes: The self-assembly of small organic molecules is an entirely new approach to the synthesis of nanotubular structures possessing helical chirality. Amino acid derivatives of naphthalenediimide form such assemblies both in solution and in the solid state. The chirality of the nanotubes is determined by the constituent amino acid but is independent of the nature of the side chains.

A hunter hunted! A highly preorganized, deep metallocavitand of nanoscale dimensions containing rhenium atoms at the four corners can be readily synthesized from a simple formyl-substituted calix[4]arene and 3,8-phenanthroquinone. Unsubstituted calix[4]arenes and cavitands without substituents on the lower rim are the ideal guests for such an expanded calixarene (see picture), and are included with binding constants of 10³–10⁵ M⁻¹.

All wrapped up: An easy, highly selective separation of C₇₀ and the higher fullerenes has been achieved by solid–liquid extraction with a 2-ureido-4-[1H]-pyrimidinone-based scaffold which wraps around the guest as a dimeric capsule (see picture), thereby solubilizing it in THF. Acidification disrupts the hydrogen-bonding network and allows easy recovery of the guest and recycling of the receptor without chromatography or tedious procedures.

Stick with me: Electrostatic charges can be induced in functionalized polystyrene beads. Oppositely charged beads then aggregate to form superstructures. A coat of small beads can self-assemble around a large bead (see optical microscopy image). After annealing, another layer of beads can be added. The technique, based on contact electrification, avoids the use of expensive equipment and enables the use of large quantities of material.

Without modification of its periphery, cucurbit[7]uril (a macrocyclic cavitand comprising seven glycoluril units) forms a hydrogel. The gelation is thermoreversible and sensitive to pH, but is inhibited by the presence of alkali-metal ions and shows guest-induced stimuli-responsive behavior: upon addition of a small amount of a guest, it undergoes a reversible gel–sol transition with alternating UV irradiation and heat treatment (see picture).

Big is beautiful: A folded synthetic molecule with a conformation that compares in size with the tertiary folds of a small protein and yet only consist of non-natural units is described. By not controlling the helical handedness allows the effect of tertiary interactions between helical modules through helix–helix side-by-side induction of handedness to be observed.

Knots galore: With a judicious choice of ions and solvents, it is possible to amplify a molecular Solomon link by kinetically controlled crystallization from a dynamic combinatorial library of molecular knots (see scheme).

Recognize this face? Quadruplex DNA has been used as a noncovalent scaffold to project binding groups for protein recognition and denaturation, with the best quadruplex decreasing the melting temperature of cytochrome c by about 45 K.

Around the bend: A right-angled bend in the RNA of the hepatitis C virus is stabilized by a core of divalent metal ions (shown as green and violet spheres). Crystal-structure analysis and fluorescence labeling have been used to investigate the structure and metal-ion-dependent stabilization of the RNA domain that plays a key role in viral protein synthesis.

Let's do the twist: Tripodal squaraines self-assemble from acetonitrile to form hollow spheres, the complexation of which with Ca²⁺ or Mg²⁺ results in extended networks. An analogous chiral dye exhibits a bisignate CD couplet and a helical morphology upon Ca²⁺ binding (see figure). Thus, the molecular chirality of a functional dye is expressed through specific cation binding and manifested in the form of supramolecular helicity.

The sensitive type: A lower critical solution temperature (LCST) is observed for dendrimers with N-isopropylamide (NIPAM) groups at all chain terminals and for poly(N-isopropylacrylamide) (PNIPAAm; see picture). A much smaller endothermic peak occurs around the LCST for NIPAM-terminated dendrimers. The globular structure of the dendrimers may cause inefficient hydration and dehydration around NIPAM groups below and above the LCST, respectively.

Doped and wired: The organization of the π functional units in amide-functionalized tetrathiafulvalene in a gel by hydrogen-bonding interactions has given rise to nanofibers. Doping with iodine generates a conducting material, which upon annealing gives rise to nanowires with metal-like conductivity (see current-sensing AFM image).

Different, yet compatible: A hybrid capsule is formed through hydrogen bonding between two different subunits, each of which is capable of self-assembly into homodimeric capsules. The hybrid forms in response to a guest that fills its space properly. Formation of the molecular capsule (see picture) is supported by ¹H NMR spectroscopy experiments.

Nanopatterning of multifunctional architectures, that is, 1D or 2D arrays on surfaces, can be achieved through bottom-up fabrication of 1D coordination networks by self-assembly processes. A coordinating tecton bearing both monodentate and tridentate coordination poles is combined with suitable metal centers (see scanning tunneling microscopy image).

A helical turn full of metal ions! The metal–salen base-pair concept allows the incorporation of up to 10 transition-metal ions into double strands of DNA (see picture; Mn gray spheres, C gray, N blue, P orange, O red). The crosslinking provided by the metal–salen complexes conveys a high stability to the self-assembled systems.

Tying up loose ends: Oligoamide thread molecules can spontaneously transform into open knots, whose terminal functional groups can subsequently be chemically linked to form closed, cyclic knots (see scheme; left: threads; middle: open knots; right: molecular knots; X=N, C).

A molecular basket (see picture) and a molecular tube can be reversibly switched between closed and open forms. In their closed forms, these novel container molecules encapsulate cycloalkanes such as cyclohexane. Their binding capabilities can be completely turned off by the addition of acid. Neutralization with base leads to restoration of the original complexes.

Threads of cesium ions can be generated through supramolecular coordination chemistry in a one-pot synthesis. The cesium ions are threaded together, like in a pearl necklace, with surprisingly short interatomic distances. The figure shows the space-filling model of the dicationic repeating unit of the 1D coordination polymer (left) and the cesium threads alone in the unit cell (right). C black, Cs green, Cu yellow, O red, N blue.

The counterion, not the binding pocket, shifts the absorption of the protonated Schiff base of 11-cis-retinal into the visible region. This conclusion was drawn from high-level CASPT2 calculations on the visual pigment rhodopsin (the optimized structure of the binding pocket is shown).

Resilient hosts: The porous coordination solids CUK-1 (see structure; Co purple, C gray, N blue, O red/pink) and CUK-2 are synthesized from unsymmetrical ligands and Co^II ions. Even after the removal of co-crystallized guests, the host frameworks are exceptionally stable. CUK-1 shows a high capacity for H₂ adsorption and is one of very few materials capable of separating N₂ from O₂.

Aluminum sites in zeolites are difficult to locate experimentally, and hence an indirect theoretical approach is proposed that identifies the most likely positions of the aluminum atoms by matching simulation results with available experimental data. The picture shows a model unit cell of FER-type zeolite with the four distinct T-sites for aluminum in green.

Au-dacious ring synthesis: An efficient synthesis of the A–D rings of azaspiracid has been developed by using a Co-catalyzed oxaetherification to form the fused 2,5-trans-configured trisubstituted tetrahydrofuran D ring, and an unprecedented Au-catalyzed bis-spiroketal formation wherein a bridging alkyne served as a surrogate for the C10 ketal (see scheme; PMB=para-methoxybenzyl, TBDPS=tert-butyldiphenylsilyl).

Turned by gold: The gold(I)-catalyzed enantioselective hydroalkoxylation of allenes proceeded rapidly to give useful oxygen heterocycles in high yields and with high stereoselectivity. The procedure was also effective for the cyclization of γ-hydroxyallenes that possess an axially chiral allenyl moiety and for the cyclization of δ-hydroxyallenes.

Thinking it through: Details of the transition-state structures of the chorismate to prephenate rearrangement in the active site of the 1F7 (N33S) mutant were obtained by computer-aided rational design. This method may be used to determine whether antibody scaffolds are evolutionary dead ends or if they can be further improved.

Shining light on local order: The relations between local crystalline order and peak intensities in the infrared spectra of calcite are explained in terms of different sensitivities to CaO and O⋅⋅⋅O distances (see picture) by comparing ab initio phonon spectra for ideal and distorted calcite unit cells with experimental spectra of various biogenic and geological calcites.

“Green” batteries: The cycling of an aqueous rechargeable lithium battery (ARLB) with an output voltage of 1.05 V is markedly improved by choosing the appropriate intercalation compounds as electrode materials (see diagram). This system is, in principle, very stable and provides a means to explore new energy-storage and conversion systems, especially for use in future electric vehicles.

This most ethereal of cascades: Water-resistant single isolated Lewis acids within the framework of molecular sieves act as excellent general catalysts for the synthesis of ethers (see scheme). On this basis, an environmentally friendly process has been developed for the preparation of fine chemicals that involves a one-pot Meerwein–Ponndorf–Verley reduction/etherification cascade.