Chemistry - A European Journal

A mosaic of Orion's Great Nebula (M42), a nearby starbirth region, is shown on the cover (Hubble space telescope; C. R. O'Dell (Vanderbilt University) and NASA). Dust grains covered by ice mantles are expected to be UV photoprocessed in the nebula, and in circumstellar regions during early phases of star formation. Experimental simulations revealed that precursors of biological cofactors, such as hexahydrotriazines, are constantly synthesized under these conditions. More details are given by G. M. Muñoz Caro et al. on page 4895 ff. Stéphane Le Saint at the University of Nice designed the picture.

What's a couple of CF₂groups among friends? The old adage “never mix pure organic compounds” is trumped by the adage “never say never”. The enantiopure synthesis of quasienantiomers (almost enantiomers, see figure) and subsequent mixing to make quasiracemates is providing a powerful new suite of tools for working with chiral molecules.

Size is important! To elucidate the crucial role of amphiphile head-group size on lipase activity in w/o microemulsions, three different series of surfactants with different head-group size and hydrophilicity were synthesized. For the first time, the activity profile of lipase clearly indicates the dominance of the head-group-size factor (see scheme) over the hydrophilicity in regulating the enzyme activity.

Water growth! The growth mechanism of water clusters in carbon nanopores (illustrated schematically here) is clearly elucidated by in situ small-angle X-ray scattering (SAXS) studies and grand canonical Monte Carlo (GCMC) simulations at 293–313 K.

Chemical evolution: Hexahydrotriazine structures (see scheme) were formed from the photoprocessing of circumstellar/interstellar ice analogues. These structures are proposed to be precursors of uroporphyrinogenoctanitrile, from which various biological cofactors develop.

Oxidation state distributions for various accessible redox states of [(bpy)₂Ru^II(μ-H₂L²⁻)Ru^II(bpy)₂](ClO₄)₂, 1-(ClO₄)₂, and [(acac)₂Ru^III(μ-H₂L²⁻)Ru^III(acac)₂], 2, (where H₂L²⁻=2,5-dioxido-1,4-benzoquinonediimine) have been investigated by using a combination of UV-visible-NIR and EPR spectroelectrochemistry. Though in 1ⁿ⁺ electron transfer occurs largely at the bridging ligand μ-H₂Lⁿ⁻, higher metal oxidation states (Ru^III, Ru^IV) seem to be favored in 2^m, leading to more complex and ambivalent oxidation state combinations.

High-coordination frameworks that produce band gaps are realised in intermetallic zinc antimonides such as that depicted here (Zn •, Sb •). These frameworks are electron precise due to confined four-centre four-electron bonded structural entities representing rhomboid rings Zn₂Sb₂. Zn/Sb materials are promising thermoelectrics but display at the same time immense structural complexity and involved temperature polymorphism, which impedes thorough structure–property relationship studies.

Ligand-induced charge concentrations (LICCs) are shown to cause the non-VSEPR structures, determined by experiment and calculation, for the heteroleptic d⁰ transition-metal alkyls Me₃NbCl₂ and Me₂NbCl₃ (see figure). The natural bond order (NBO) method was employed to partition the total charge density in a physically meaningful way, and we demonstrate for the first time that all information about the origin of LICCs in d⁰ transition-metal compounds is directly accessible through the wavefunction. LICCs arise naturally as part of an MX bond in which metal d-functions are involved; also a sound chemical and physical basis is outlined for their occurrence at a transition-metal center.

Macrolides with a broad biological activity based on recorcinol-based compounds are described. An expedient and modular synthesis of radicicol (Monorden) and pochonin C is reported. A combination of solid-phase chemistry and polymer-bound reagents was used to assemble both natural products in seven and eight steps, respectively, from the intermediates depicted.

Chains of supramolecular rhomboids pack to form highly porous structures during cocrystallization of bis-pyridinyl homoquadricyclanes with AgNO₃ in 3:2 ratio (see figure). Large cavities in this structure are filled with solvent molecules. Rigidity of bis-aryl homoquadricyclanes can be fine-tuned with substitution at the aliphatic moiety that induces restricted rotation of the aromatic rings.

Frustration helps the nonlinear polarizations in a constrained geometry of calixarenes. Quantum-chemical calculations show that the all-parallel arrangement, as depicted above, exhibits the maximum magnitude for the nonlinear optical coefficients at small interdipolar angles. In the frustrated orientations, however, the NLO response reaches a maximum at large interdipolar angles corresponding to the flattened calixarene rings.

A glucose/O₂biofuel cell (BFC) with pH-dependent power output was constructed by using porous carbon (PC) as the matrix to load glucose oxidase or fungi laccase as catalysts (shown here). The power of the BFC was enhanced significantly by using PC as the matrix, rather than a glassy carbon electrode. Furthermore, the BFC can operate at pH 6.0 and 7.0, which overcomes the requirement for cathode solutions of pH<5.0 when using fungi laccase as catalyst.

Metal ions captured in tunnels: Highly dispersed, stable metal (Cu and Pd) and bimetal (Pd/Pt) nanoparticles were encapsulated in a size- and site-controlled manner within rigid nanoscopic containers (see scheme), by the chemical reduction of metal-ion-exchanged polyaminoamine (PAMAM)-dendron-propagated mesoporous silica.

Remarkable differences in the susceptibility of MFI zeolites to desilication and related mesoporosity development are reported. Aluminium in framework positions of the zeolites (see figure) directs Si extraction towards controlled intracrystalline mesopore formation upon treatment in NaOH solution.

Adding up to rings: A simple change in the nature of the metal in the alkenylcarbene complex, the R substituent of the methyl ketone lithium enolate, or the reaction conditions allows the diastereoselective synthesis of several functionalized five- or seven-membered carbocycles through different formal carbocyclization patterns (see scheme).

Better performance than their blended counterparts in light-emittting diodes are displayed by the copolymers that were synthesized by Suzuki polycondensation of A-A- and B-B-type monomers (see scheme). The tBu- substituted β-diketonate ligand makes the Ir complex monomer more soluble in organic solvents, which affords better compatibility with other comonomers and avoids gelation during copolymerization.

Photoreversible system: Simple molybdenum methyl, carbene, and carbyne complexes ([CH₃MoF], [CH₂MoHF], and [CHMoH₂F]: see figure) were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through α-hydrogen migration.

A source of the complex fragment [Ni(iPr₂Im)₂] for stoichiometric and catalytic transformations has been obtained with complex 1, for example, in the reaction of 1 with CO, ethylene, and alkynes, and also in the activation of CF and CC bonds. The dinuclear complex [Ni₂(iPr₂Im)₄(cod)] (1) has been obtained from the reaction of 1,3-diisopropylimidazole-2-ylidene (iPr₂Im) with [Ni(cod)₂] in good yield.

From trinuclear to three-dimensional networks (see picture), the structures of the heterometallic oxovanadium–lanthanide complexes obtained from VOSO₄, Ln₂O₃, and triethylenetetraaminehexaacetic acid change as the lanthanide coordination number increases from eight (Yb, Ho,Gd) to nine (Eu, Pr).

A CO ligand switch based on a monolayer of metal clusters: Electrochemically controlled reversible CO-ligand exchange in a self-assembled monolayer (SAM) of a CO-bound triruthenium cluster has been realized on the surface of a gold electrode. Multistep electron transfer (see figure), oxidation states, and CO exchange kinetics in the SAM have been extensively studied by electrochemistry as well as in situ IR and sum frequency generation (SFG) vibrational spectroscopy.

Handy pockets! An immobilized Ru–porphyrin catalyst with a substrate pocket (see graphic) was synthesized by molecular imprinting. The catalytic performance of the polymer was found to be by far superior to the corresponding homogeneous catalyst.

The intrinsic stability of Watson–Crick d(A⋅T) and r(A⋅U) hydrogen bonds (depicted left and right, respectively) was analyzed by using quantum-mechanical techniques, such as energy calculations, determination of reactivity indexes, and analysis of electron density topology. The data support the traditional view that the greater stability of RNA relative to DNA is due to a variety of effects (e.g., stacking, sugar puckering, solvation) rather than to a significant difference in the hydrogen bonding of the DNA and RNA base pairs.

Small [n.n.n]propellanes and bicyclo[n.n.n]alkanes (1≤n≤3) with different Group 14 elements at the bridgehead positions and in the bridges have so far not been prepared. Quantum-chemical calculations were performed to determine their structure, strain, and aptitude for oligomerizations (shown schematically in the picture). Several of the propellanes will only exist under cryogenic conditions, whereas most of the bicycloalkanes should be stable at ambient temperatures.

To dimerize or not? That is the question! Nonaromatic unsymmetrically N,N′-substituted diaminocarbenes 3 (n=1, 2) with a spirocyclic structure can be obtained in a two-step reaction starting from secondary amines and ketimines 1 via the imidazolidin-2-thiones 2. The proton-catalyzed dimerization of carbenes 3 is reversible under special circumstances.

Solid-phase solutions! While the Grubbs 2nd generation pre-catalyst affords a greater degree of conversion to product than the Hoveyda–Grubbs pre-catalyst in a solution-phase enyne-metathesis reaction (a), this trend is reversed in the solid-phase variant (b). Factors effecting this crossover have been explored and a mechanistic rationale is proposed which suggests an expanded role for phosphine-free pre-catalysts in solid-phase metathesis reactions.

Phosphate-dependent catalysis of nitrophenyl glycoside cleavage was performed by cyclodextrin diacids with k_cat/k_uncat ratios of up to 10³ (see picture).

Bioactive conformations of antibiotic and nucleotide: The molecular recognition of streptomycin by Bacillus subtilis aminoglycoside-6-adenyl transferase, a protein involved in bacterial defence against aminoglycoside antibiotics by selective adenylation of position 6 of the streptidine ring (shown in red), has been analysed by a combination of NMR techniques and molecular dynamics simulations. This provides valuable information about the bioactive conformations of both the antibiotic and the nucleotide and shows that the molecular recognition process for streptomycin implies a conformational selection phenomenon.

An old postulate is disproved! Experimental structural methods in the solid and gas phases, and theoretical methods, have shown that an α-effect in the form of a dative bond between geminal silicon and nitrogen atoms in aminomethylsilanes (see picture) does not exist. However, new and unusual electronic properties in SiCN units have been found.

Specific, strong, and spatially well-defined host–guest interactions occur between the periphery of adamantylurea-functionalized dendrimers (host) and ureido acetic acid derivatives (guest). The binding constants could be tuned by changing the design of the acidic part of the guest. The supramolecular interactions were studied by using fluorescence spectroscopy on a model system and collision-induced dissociation mass spectrometry on adamantylurea-modified poly(propylene imine) dendrimers (see picture).

Enantiopure hydroxysulfinyl- and sulfonyldienes undergo Diels–Alder cycloadditions with high π-facial selectivity controlled in some cases by the oxidation state at sulfur (see scheme).

The electronic structure of a family of organometallic Cu^IIIcomplexes containing monoanionic triazamacrocyclic ligands (Lⁱ) has been investigated by X-ray diffraction analysis, ¹H NMR and UV/Vis spectroscopy, cyclic voltammetry (see picture), and DFT calculations.