Safe and convenient storage of hydrogen is one of the nearfuture challenges. For mobile applications there are strict volume and weight limitations, and these limitations have steered investigations in the direction of compact, solid, lightweight main-group hydrides.[1] Whereas ammonia-borane (NH 3 BH 3 ) is a nontoxic, nonflammable, H 2 -releasing solid with a record hydrogen density of 19.8 wt %, it releases hydrogen in an irreversible process.[2] Metal hydrides such as MgH 2 are less rich in hydrogen (7.7 wt %) but advantageously display reversible hydrogen release and uptake: MgH 2 QMg + H 2 .[3]Although bulk MgH 2 seems an ideal candidate for reversible hydrogen storage, it is plagued by high thermodynamic stability, which translates into relatively high hydrogen desorption temperatures and slow release and uptake kinetics. The kinetics can be improved drastically by doping the magnesium hydride with transition metals [4][5][6] and by ball milling [7,8] or surface modifications.[9] The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH = 74.4(3) kJ mol À1 ; DS = 135.1(2) J mol À1 K À1 ), [10] which originate from the enormous lattice energy for [MgÀ1 ) relative to that of bulk Mg (DH = 147 kJ mol À1 ). [11] Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH 2 ) n clusters (n < 19), the enthalpy of decomposition sharply reduces with cluster size.[12] Downsizing the particles has a dramatic effect on the stability of saltlike (MgH 2 ) n but much less on that of the metal clusters Mg n . For a Mg 9 H 18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJ mol À1 was calculated, [12] from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH 2 is calculated to be unstable even towards decomposition into its elements (DH = À5.5 kJ mol À1

A model solution to hydrogen storage? The recently introduced hydrogen storage material [{Ca(NH2BH3)2}n] has been investigated on a molecular level. A hydrocarbon‐soluble calcium amidoborane complex eliminates H2 spontaneously at the very low temperature of 20–40 °C (see scheme). The decomposition product shows a dimeric species with a bridging [HNBHNHBH3]2− ion that is isolobal to the allylic dianion [HCCHCHCH3]2−.

In type-2 diabetes (T2D), islet amyloid polypeptide (IAPP) self-associates into toxic assemblies causing islet β-cell death. Therefore, preventing IAPP toxicity is a promising therapeutic strategy for T2D. The molecular tweezer CLR01 is a supramolecular tool for selective complexation of K residues in (poly)peptides. Surprisingly, it inhibits IAPP aggregation at substoichiometric concentrations even though IAPP has only one K residue at position 1, whereas efficient inhibition of IAPP toxicity requires excess CLR01. The basis for this peculiar behavior is not clear. Here, a combination of biochemical, biophysical, spectroscopic, and computational methods reveals a detailed mechanistic picture of the unique dual inhibition mechanism for CLR01. At low concentrations, CLR01 binds to K1, presumably nucleating nonamyloidogenic, yet toxic, structures, whereas excess CLR01 binds also to R11, leading to nontoxic structures. Encouragingly, the CLR01 concentrations needed for inhibition of IAPP toxicity are safe in vivo, supporting its development toward diseasemodifying therapy for T2D.

Linear copolymers have been developed which carry binding sites tailored for sulfated sugars. All binding monomers are based on the methacrylamide skeleton and ensure statistical radical copolymerization. They are decorated with o-aminomethylphenylboronates for covalent ester formation and/or alkylammonium ions for noncovalent Coulomb attraction. Alcohol sidechains maintain a high water solubility; a dansyl monomer was constructed as a fluorescence label. Statistical copolymerization of comonomer mixtures with optimized ratios was started by AIBN (AIBN=2,2'-azoisobutyronitrile) and furnished water-soluble comonomers with an exceptionally high affinity for glucosaminoglucans. Heparin can be quantitatively detected with an unprecedented 30 nM sensitivity, and a neutral polymer without any ammonium cation is still able to bind the target with almost micromolar affinity. From this unexpected result, we propose a new binding scheme between the boronate and a sulfated ethylene glycol or aminoethanol unit. Although the mechanism of heparin binding involves covalent boronate ester formation, it can be completely reversed by protamine addition, similar to heparin's complex formation with antithrombin III.

The reactions of (E)-1-methoxy-1,3-butadiene (1) and 1,1-dimethoxy-1,3-butadiene (2) with a series of dienophiles of increasing electrophilicity are described. Stereochemical studies reveal that the cycloadditions of 1 are concerted processes, even for the most electron-deficient olefins dimethyl dicyanofumarate and dimethyl dicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under our conditions (dilute solutions and temperatures ≤60 °C) only with those dienophiles which can give zwitterions out of the antiperiplanar conformation of the diene. Zwitterionic intermediates can be trapped by methanol. In the case of tetracyanoethene the kinetics of decay of an intermediate, interpreted as the zwitterion, can be followed by stopped flow techniques:  E a = 14.8 ± 0.2 kcal mol-1, log A = 11.9 ± 0.1, ΔH ⧧ = 10.8 ± 0.1 kcal mol-1, ΔS ⧧ = −6.2 ± 0.1 cal mol-1 K-1, and ΔG ⧧ = 11.40 ± 0.03 kcal mol-1.

Safe and convenient storage of hydrogen is one of the nearfuture challenges. For mobile applications there are strict volume and weight limitations, and these limitations have steered investigations in the direction of compact, solid, lightweight main-group hydrides.[1] Whereas ammonia-borane (NH 3 BH 3 ) is a nontoxic, nonflammable, H 2 -releasing solid with a record hydrogen density of 19.8 wt %, it releases hydrogen in an irreversible process.[2] Metal hydrides such as MgH 2 are less rich in hydrogen (7.7 wt %) but advantageously display reversible hydrogen release and uptake: MgH 2 QMg + H 2 .[3]Although bulk MgH 2 seems an ideal candidate for reversible hydrogen storage, it is plagued by high thermodynamic stability, which translates into relatively high hydrogen desorption temperatures and slow release and uptake kinetics. The kinetics can be improved drastically by doping the magnesium hydride with transition metals [4][5][6] and by ball milling [7,8] or surface modifications.[9] The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH = 74.4(3) kJ mol À1 ; DS = 135.1(2) J mol À1 K À1 ), [10] which originate from the enormous lattice energy for [MgÀ1 ) relative to that of bulk Mg (DH = 147 kJ mol À1 ). [11] Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH 2 ) n clusters (n < 19), the enthalpy of decomposition sharply reduces with cluster size.[12] Downsizing the particles has a dramatic effect on the stability of saltlike (MgH 2 ) n but much less on that of the metal clusters Mg n . For a Mg 9 H 18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJ mol À1 was calculated, [12] from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH 2 is calculated to be unstable even towards decomposition into its elements (DH = À5.5 kJ mol À1

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.

Cryptophanes, composed of two bowl-shaped cyclotriveratrylene subunits linked by three aliphatic linker groups, are prototypal organic host molecules which bind reversibly neutral small guest compounds via London forces. The binding constants for these complexes are usually measured in tetrachloroethane and are in the range of 10 2 -10 3 M À 1 . Here we show that tetrachloroethane is-in contrast to the scientific consensus-enclosed by the cryptophane-E cavity. By means of NMR spectroscopy we show that the binding constant for CHCl 3 @cryptophane-E is in larger solvents two orders of magnitudes higher than the one measured before. Ab initio calculations reveal that attractive dispersion energy is responsible for high binding constants and for the formation of imploded cryptophanes which seem to be more stable than cryptophanes with empty cavities.