Heinz Bandmann

Hydrogen Storage in Magnesium Hydride: The Molecular Approach†

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. Whereas ammonia–borane (NH3BH3) is a nontoxic, nonflammable, H2-releasing solid with a record hydrogen density of 19.8 wt%, it releases hydrogen in an irreversible process. Metal hydrides such as MgH2 are less rich in hydrogen (7.7 wt%) but advantageously display reversible hydrogen release and uptake: MgH2QMg+ H2. [3] Although bulk MgH2 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 and by ball milling or surface modifications. The high hydrogen release temperature (over 300 8C), however, is due to unfavorable thermodynamic parameters (DH= 74.4(3) kJmol ; DS= 135.1(2) Jmol K ), which originate from the enormous lattice energy for [MgH 2]1 (DH= 2718 kJmol ) relative to that of bulk Mg (DH= 147 kJmol ). Although thermodynamic values are intrinsic to the system, recent theoretical calculations demonstrate that for very small (MgH2)n clusters (n< 19), the enthalpy of decomposition sharply reduces with cluster size. Downsizing the particles has a dramatic effect on the stability of saltlike (MgH2)n but much less on that of the metal clusters Mgn. For a Mg9H18 cluster of approximately 0.9 nm diameter a desorption enthalpy of 63 kJmol 1 was calculated, from which a decomposition temperature of about 200 8C can be estimated. At the extreme limit, molecular MgH2 is calculated to be unstable even towards decomposition into its elements (DH= 5.5 kJmol ). The sharp decrease of stability for (MgH2)n clusters with n< 19 can be understood by the rapid increase in surface/volume ratios: surface atoms have a lower coordination number and are loosely bound. It is of interest to note that only clusters with n 19 ( 1.3 nm) have a core with the typical a-MgH2 rutile geometry (six-coordinate Mg and three-coordinate H). Apparently this is the critical size from which clusters start to show bulk behavior. These insights led to increased research activity on the syntheses of MgH2 nanoparticles, either by special ballmilling techniques or by incorporation into confined spaces. Thus nanoparticles in the range of 1–10 nm have been reported. Hydrogen elimination studies indeed show a small reduction of DH and H2 desorption temperatures, but dramatic effects can only be expected for particles smaller than 1 nm. Production of magnesium hydride particles in the subnanometer range would benefit from a molecular “bottomup” approach. We recently reported a simple synthesis protocol for the first soluble calcium hydride complex 1 by the silane route [Eq. (1)], and Jones et al. reported the

Molecular Recognition of Carbohydrates by Artificial Polypyridine and Polypyrimidine Receptors.

Despite their acyclic structure, the simple host molecules 1 and 2 can effectively hydrogen bond to monosaccharides. They show marked binding affinities to glucopyranosides in chloroform, and they are able to participate in three-dimensional recognition of monosaccharides.

Synthesis and Supramolecular Properties of Trimethylene‐Bridged Clips

The novel trimethylene-bridged clips 3 and 4 have been synthesized by using repetitive stereoselective Diels-Alder reactions of the benzo- and naphthobismethylenenorbornenes 8 and 19 as dienes and norbornadiene 9 as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts 10 and 20 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips 3 and 4 serve as receptors for a variety of electron-deficient neutral and cationic aromatic substrates, comparable to the molecular tweezers 1 and 2. The thermodynamic parameters of the complex formation, K(a) and DeltaG, were determined by (1)H NMR titration experiments and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titration microcalorimetry. The finding that clip 4 forms more stable complexes than 3 can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in 4 compared to the corresponding benzene systems in 3. In the complexes with 4 as receptor, the plane of each aromatic substrate molecule is calculated to be oriented almost parallel to the naphthalene sidewalls. However, in the complexes of tweezers 2, the substrate is usually oriented parallel to the central naphthalene spacer unit. Due to the more open topology of 4, most complexes were calculated to consist of two or more equilibrating noncovalent conformers.

Molecular tweezers inhibit islet amyloid polypeptide assembly and toxicity by a new mechanism.

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 disease-modifying therapy for T2D.

Pressure-Induced Cycloadditions of Dicyanoacetylene to Strained Arenes: The Formation of Cyclooctatetraene, 9,10-Dihydronaphthalene, and Azulene Derivatives; A Degenerate [1,5] Sigmatropic Shift—Comparison between Theory and Experiment

The reaction of dicyanoacetylene (DCA) with the partially hydrogenated dimethano-bridged anthracene and pentacene derivs. I and II lead to the (1:1) Diels - Alder adducts III and IV, resp., and to the 9,10-dihydronaphthalene derivs. V and VI, resp. These compds. are intensely colored, most likely due to a charge-transfer absorption. Photolysis of the (1:1) adducts III and IV produces the corresponding cyclooctatetraene derivs. , which are not planar despite the torsional constraints caused by the fusion of the eight-membered ring to norbornane and norbornene units. The mechanisms of formation of the dihydronaphthalene derivs. V and VI were elucidated by the use of high pressure; this allowed the (2:1) adducts VII and VIII to be detected as intermediates in the reaction of DCA with III or IV to give V and VI, resp. A degenerate rearrangement consisting formally of a [1,5] vinyl shift in V or VI could be detected by their temp.-dependent 1H NMR spectra. The mechanisms of the [1,5]-sigmatropic shift reactions in V and IX have been examd. at the Becke3LYP/6-31G* level of theory. Stepwise mechanisms are predicted, and the calcd. activation barrier is in good agreement with the measured DH.thermod.. The finding that the irreversible rearrangement of V accompanied by the elimination of HCN to give the azulene deriv. proceeds only on heating of V to 80 Deg in a polar solvent is good evidence for a polar mechanism in this case.

Thermal Rearrangements of Di- and Triphenyl-Substituted Benzocyclobutenes and Corresponding o-Quinodimethanes

7,8-Dimethoxy-7,8-diphenyl- (1c), 7,8-dimethyl-7,8-diphenyl- (1d), 7-methoxy-7,8,8-triphenyl- (1e), 7-methyl-7,8,8-triphenyl- (1f), 7-isocyano-7,8,8-triphenyl- (1g), and 7,7,8-triphenylbenzocyclobutene (1h) are amenable to a variety of thermal rearrangements following initial electrocyclic ring-opening to the corresponding 7,8-diphenyl- (2c,d) and 7,8,8-triphenyl-o-quinodimethanes (2e–h). meso-1c was found to undergo a facile meso/rac isomerization at room temperature, indicating that other processes such as a symmetry-forbidden disrotatory ring-opening or a stepwise reaction compete with the symmetry-allowed conrotatory process. An estimate of the energy profile of the 1c/2c reaction system was made by kinetic simulation in combination with oxygen trapping of the intermediate o-quinodimethanes (2c) and semiempirical PM3 calculations, and revealed that the barrier for the symmetry-forbidden pathway is merely about 4 kJ·mol–1 higher than that for the symmetry-allowed one. o-Quinodimethanes 2c, 2g, 2e, and 2h underwent further electrocyclic hexatriene-cyclohexadiene ring-closure to give 4a,10-dihydroanthracene derivatives at temperatures between 20 and 80 °C. The 4a,10-dihydroanthracenes were further transformed to 9,10-disubstituted anthracenes by elimination of methanol or HCN, as well as to 9,10-substituted 9,10-dihydroanthracene derivatives. ESR and ENDOR spectroscopic detection of related 9-anthryl radicals lends support to the view that 9,10-dihydroanthracene products are formed by a homolytic hydrogen-transfer reaction (retrodisproportionation). By way of contrast, the aforementioned transformations play only a minor role in the case of methyl-substituted benzocyclobutenes 1d, 1f as here they are overruled by faster 1,5-H shift reactions of the corresponding o-quinodimethanes 2d, 2f, leading to styrene derivatives.

Synthesis, Structure, Tautomerism, and Reactivity of Methanetrisamidines

N,N’-chelating monoanionic amidinate ligands have been studied in detail over the last decades. The easy tunability of their steric and electronic properties allows the synthesis of tailormade metal complexes for technical applications in catalysis and materials sciences. Surprisingly, multifunctional ligands containing two or more amidine moieties, in the following referred to as polyamidines, have been only scarcely investigated. They are of potential interest for the synthesis of (hetero)multimetallic complexes, which may show improved catalytic properties. Moreover, neutral aromatic tetraamidines have been investigated in cancer research owing to their antiproteinase activity. Unfortunately, only very few polyamidines, almost all containing a central phenyl spacer, have been synthesized and multidentate polyamidines, in which the amidine moieties are bound to a single atom, are limited to two Me2Siand CH2-bridged derivatives. [4,5] In contrast, isoelectronic tetranitromethane C(NO2)4 and tetramethylmethanetetracarboxylate C(COOMe)4 are well known. [6]