Chemistry - A European Journal

The cover picture shows the pictogram of the complex tetranuclear iron(III) cluster [Fe₄(L¹)₄]. With the growing size of supramolecular systems, the fundamental question, how to visualize science, becomes more and more challenging. In the case of [Fe₄(L¹)₄], this difficult task was solved by placing four Fe^III centers (silver balls) in the vertices of a tetrahedron. The four tris-bidentate ligands (L¹)³⁻ (golden tridents) are located above the triangular faces and linked to three Fe^III cations, each. The [Fe₄(L¹)₄] cage has idealized T-molecular symmetry. Thus, the four iron(III) centers have the same chirality (all Δ or Λ). Hence, [Fe₄(L¹)₄] generates homoconfigurational racemic crystals. Further details of the preparation and structure of tetrahedral [Fe₄(L¹)₄], together with antiprismatic [Fe₆(L²)₆], are reported by R. W. Saalfrank, K. N. Raymond et al. on p. 493 ff.

The synthetic chemistry of colloidal nanocrystals (schematically depicted) could be environmentally benign and user friendly. This constraint encourages us to better understand crystallization phenomena in general.

With inorganic, oxygen-centered radicals X−O^. reactions could be performed, which are difficult or impossible with their organic counterparts because of fast side reactions (see scheme): Upon addition of X−O^. to C−C triple bonds in alkynes, a stereoselective, oxidative radical cyclization cascade is induced, which is terminated by release of an unreactive radical X^. and formation of a carbonyl group.

The structure of the lithium salt of the new oxothiomolybdate ion [Mo^V₈S₈O₈(OH)₈{HW^VIO₅(H₂O)}]³⁻ has been studied by X-ray and neutron diffraction. The ring-shaped polyoxothioanions are anchored to partially filled lithium columns, which consist of alternating LiO₂(H₂O)₄ octahedra and Li(H₂O)₄ tetrahedra (see diagram). The ionic conductivity at room temperature (10⁻⁵ S cm⁻¹) is typical of a good ionic conductor.

The Hagihara–Sonogashira cross-coupling reaction provides access to shape-persistent macrocycles of the kind shown. These compounds are of interest for their behavior in bulk and at interfaces, as well as for their properties in host–guest chemistry, catalysis etc. Their structures were determined by single-crystal X-ray crystallography.

Polymerization inside the catalyst beads only and continuous release for the newly formed macromolecules into the solvent phase was observed for the reaction depicted. For this purpose the [Rh(cod)Cl]₂ complex was anchored on polybenzimidazole porous beads to yield the supported catalyst; the suspension of the later induces polymerization of substituted phenylacetylenes to cis–transoid polyphenylvinylenes.

Reactions of [{MCl₂(Cp*)}₂] (M=Ir, Rh) with bidentate ligands (L¹=diisocyanide, L²=pyrazine, L³=4,4′-dipyridyl) gave [{MCl₂(Cp*)(Lⁿ)}₂], which were converted into tetranuclear complexes [{M(μ-Cl)(Cp*)}₄(Lⁿ)₂](OTf)₄ or [{{M(Cp*)}₂(Lⁿ)(Lⁿ′)}₂](OTf)₄ containing different ligands on treatment with Ag(OTf). An example of a tetranuclear complex is shown here.

Under anaerobic conditions, reversible thermal S−N bond scission occurs in S-nitrosothiols. Under aerobic conditions, a fast decomposition occurs by an autocatalytic mechanism induced by nitric oxide and dioxygen (see scheme).

The 16 e half-sandwich complexes [M(Cp*){E₂C₂(B₁₀H₁₀)}], (E = S, Se, M = Rh, Ir), and [M(η⁶-p-cymene){S₂C₂(B₁₀H₁₀)}] (M = Ru, Os) react with acetylene, propyne, and 3-methoxypropyne to give 18 e complexes, each containing a metal–boron bond (see scheme). Insertion of the alkyne into one of the metal–chalcogen bonds is followed by B−H activation, transfer of one hydrogen atom from the carborane through the metal to the terminal carbon of the alkyne and concomitant ortho-metalation of the carborane.

With the reducible V^IVcenter as well as the functionalized channeling apertures (5.38×7.55 Å) enclosed by oxovanadyl dimers, squarates, and pairs of π-π interactions of phenathroline groups, the presence of vacant channels in 1 affords redox potential sites for lithium-ion intercalation (Li-1).

An efficient route to tetrasubstituted alkenes with the four substituents as part of two ring systems was achieved by a domino-Heck double cyclisation of arylbromides containing an allylsilane and an alkyne moiety (see scheme). UV irradiation leads to a reversible isomerisation of the central double bond. Such compounds are of interest for optical data storage.

Stable long chain oligoynes were synthesized and completely characterized. The synthesis and isolation of three series of differently 3,5-disubstituted α,ω-diphenylpolyynes was achieved under optimized Cadiot–Chodkiewicz conditions. By the use of dendritic end-caps R stable molecules with 12, 16, and up to 20 sp carbon atoms (see figure) were obtained.

Whereas conventional reference systems fail to describe the aromaticity of oxocarbon acids and their anions (see figure), only the “nucleus-independent chemical shift” (NICS) criterion gives satisfactory results. Wiberg bond indexes and ¹⁷O NMR chemical shifts are also useful to study the aromatic characteristics of this interesting class of compounds.

Amiclenomycin: cis or trans ? It has long been assumed that the natural product Amiclenomycin had the trans structure 1. Both this and the cis isomer 2 have been obtained by a Diels–Alder strategy, followed by transformation through a Strecker reaction then enzymatic hydrolysis. Comparison of the NMR characteristics of the natural product with those of 1 and 2 led to the conclusion that amiclenomycin is actually the cis isomer.

A sharp increase in the constant-volume heat capacity (C_v) of a CO₂–n-pentane binary mixture is observed as the pressure approaches the critical point (CP) or bubble point (BP), while the C_v is nearly independent of pressure and composition at the pressures well above the CP or BP pressure. The figure shows the dependence of the C_v of pure CO₂ and CO₂–n-pentane mixtures on pressure.

For the entire range of bases, used in this study, we found that density functional calculation of the free energy change on formation of the 1:1 and 2:1 complexes of hydrogen-bond bases with hydrogen fluoride reproduce the experimental pK_HB scale. A 2:1 complex of HF with methyl formamide is shown here.

A novel amphiphilic architecture, depicted here, has been built by polycondensation of α-dicarboxy-functionalized polystyrene and α-dihydroxy-functionalized polyethyleneoxide. The α-dicarboxy-functionalized PS were synthesized by using the atom-transfer radical polymerization technique. The polymacromonomers obtained were characterized, and the formation of lyotropic liquid crystals was detected by polarized light microscopy. The tendency of these macromolecules to form stable micelles was demonstrated by light scattering.

Quadrupolar [1_n]heterophanes have emerged as a new class of macrocyclic synthetic receptors. The first example of a convergent synthetic strategy for constructing the title macrocycles from both highly π-excessive and highly π-deficient heteroaromatic betaine moieties linked in a 1,3-alternating fashion, as shown, is reported here.

Rh^I-catalyzed ortho-alkylation of aromatic ketimines (see scheme, R=CH₂Ph, R′=alkyl, SiMe₃) or ketones with various olefins was investigated and was found to be highly efficient and general.

In a convenient one-pot reaction and simply by replacing a tBu with a pTol group in the starting material dialkyl malonate, the tetra- and hexanuclear iron(III) clusters [Fe₄(L¹)₄] 1 and [Fe₆(L²)₆] 2 are accessible. The tetrahedral and antiprismatic iron(III) cores of 1 and 2 are linked by the chelate anions (L^1/2)³⁻, formally generated by coupling of bis-tBu or bis-pTol malonate and benzene-1,3,5-tricarboxylic acid trichloride in the presence of methyllithium.

A general strategy to 1D and 2D polyrotaxanes with high structural regularity by utilizing the principles of self-assembly and coordination chemistry is described. This approach involves threading a cucurbituril “bead” with a short “string” to form a stable pseudorotaxane, followed by linking the pseudorotaxanes with metal ions as “linkers” for the organization into a 1D or 2D polyrotaxane in the solid state (see diagram).

Thermal retro[2+2]/[4+2]cycloaddition strategies were pursued to generate highly pyramidalized dodecahedral C₂₀H₁₄/C₂₀H₁₂ cage olefins (see diagram) and the derived C₂₀H₁₄/C₂₀H₁₂ cations and anions for which DFT calculations predicted an intriguing “inner life”.

A quantitative analysis of the thermal variations of the magnetic properties of lanthanide complexes comprising a Ln^III ion with first-order orbital momentum in exchange interactions with aminoxyl radicals has been carried out. The ligand-field effect was considered within the Simple Overlap Model, and the exchange interaction was treated exactly within the appropriate Racah formalism (see scheme). Values of the intramolecular exchange parameter were rigorously deduced, and ferromagnetic interactions were found for both the Dy and Ho derivatives.

The kinetics and regiochemistry of the proton-transfer reactions of the radical cations of benzyl alcohol (see scheme) and of some of its derivatives have been examined by FT-ICR mass spectrometry.