Chemistry - A European Journal

Stable gold(III)–porphyrin complexes display promising in vitro cyctotoxic and anti-angiogenic activity as described in the Full Paper by C.-M. Che et al. on page 3097 ff. Strongly chelating phorphyrinato ligands were used to stabilize the gold(III) ion and the structure–bioactivity relationship of a range of compounds was investigated. The biological properties and ease of structural modification of the gold(III)–porphyrins suggest that this family of lipophilic cations would be a suitable choice for the development of future therapeutic applications.

Nice necklaces and not just large rings appear to be formed in the reactions described by A. Osuka et al. on page 3009 ff., which enable the efficient synthesis of medium- to large-sized porphyrin nanorings.

In control: An α-amino amide reacted with an aldehyde and a Michael acceptor to form stable imidazolidin-4-ones with high stereoselectivity. A dynamic kinetic aza-Michael addition was discovered and applied to the three-component reaction to enforce high stereoselectivity. A remote group was incorporated to invert the reaction process and direct the reaction towards the desired product (see scheme).

Keep it simple: A new transamination method is reported for the synthesis of diamino-substituted p-quinones (see scheme). Asymmetrically substituted p-quinones are shown to be key intermediates in this reaction. The redox properties of these ligands are discussed and their utility in coordination and redox chemistry is shown with a representative example.

The right to choose: 1,2,4-Diazaphospholide anions support the predictable construction of coordination polymer networks of different topology by switching between bidentate (κN,κN′) and tridentate (κN,κN′,κP) coordination depending on the choice of the metal (see picture); furthermore, the ligand exhibits a remarkable difference to 1,2,4-triazolides because the diffuse lone pair at phosphorus permits μ₂(P)-bridging coordination.

Sugar sugar! The synthesis and evaluation of ribostamycin/kanamycin hybrids incorporating a highly crowded trisubstituted aminocyclitol unit that should provide maximum complementation with the RNA receptor are reported (see picture). Analysis shows that the existing conflicts between the different sugar rings can be significantly alleviated by a simple chemical modification, leading to an improvement in activity.

Fluorescein ver. 2.0: A new, highly fluorescent, pH-sensitive trihydroxytrioxatriangulenium dye (H-TOTA) has been synthesised and characterised. The dye is closely related to fluorescein and may be considered to be a two-dimensional extended version. This new dye can exist in four different protonation states (see graphic) depending on the pH, and its use as a sensitive fluorescent ratiometric pH probe in a physiological buffer is demonstrated.

In a flash! A squaraine-based optical molecular switch functions with striking fluorescence signal changes that are visible to the naked eye (see figure).

Delivery by stealth: Vesicles with porous silica shell layers and poly(ethylene glycol) (PEG) stealth layers have good stability, due to the silica layers, good dispersity in aqueous solution, without aggregation due to the PEG stealth layers, low cytotoxicity, and the capability to enter cells (see figure).

Intramolecular additions of amines to olefinic and acetylenic residues in the presence of B(C₆F₅)₃ affords a “frustrated Lewis Pair” strategy to five- and six-membered-ring heterocyclic ammonium borate derivatives (see picture).

The bridge determines the size of the loop? Palladium-catalyzed cross-coupling of borylporphyrins led to the construction of β-pyridine-bridged porphyrin nanorings in good yields (around 55–60 %; see scheme). The photophysical study revealed that these nanorings exhibit weakly coupled chromophores and longer fluorescent lifetimes, which is an apparent advantage in the process of efficient excited energy transfer.

A surprisingansa! The electronic structure of [1],[1]disilamolybdenocenophane has been analyzed by means of DFT. As predicted from the metal-based nucleophilicity and strained geometry, remarkable reactivity towards nonpolar and polar unsaturated organic substrates has been observed. For instance, treatment with tert-butylisonitrile leads to a highly unusual structural motif, an ansa-carbene (see scheme).

Bifunctional catalysis: Excellent agreement between experimental observations and theoretical calculations was found in the understanding of the mechanism of the chiral phosphoric acid catalyzed asymmetric Baeyer–Villiger reaction of cyclobutanones with H₂O₂ as the oxidant. The transition state for the reaction is depicted.

DNA sensors: A systematic study of guanidiniocarbonylpyrrole–aryl derivatives designed to interact with DNA or RNA both through intercalation and through groove binding is presented. Distinctively different interactions with either DNA or RNA are seen by UV/Vis, fluorescence and CD spectroscopy. Compound 1 (see graphic), for example, shows base-pair-selective recognition of ds-DNA.

Melting temperature versus pK_a: Quantum chemical calculations on the Watson–Crick-type base pairing and stacking properties of triazine- and aminopyrimidine-based primordial informational oligomers (see graphic) show that neither the intrinsic base pairing nor the stacking interactions are responsible for the experimentally observed relationship between the pK_a values of the participating heterocycles and the melting points of the duplexes.

Positionally isomeric gelators: Members of a series of positionally isomeric gelators based on long alkyl chains, a hydrogen-bonding phenylglycine unit, and a terminal carboxylic acid or sodium carboxylate moiety (see figure) have been found to display significantly different gelation properties. Dramatic differences in gelator effectiveness (G_eff) values of up to 70 times could be observed between racemate/enantiomer gelator pairs, an effect that was observable even in the gelation of very similar solvents such as isomeric xylenes.

Salvation for the guilty: Protonation of a π-delocalizing, non-innocent ligand complex of iron developed as a pentacoordinate, dicarbonyl analogue of the [Fe]-H₂ase or Hmd (H₂-forming methylene-tetrahydromethanopterin dehydrogenase) enzyme active site, decreases the electron density at iron and engenders CO uptake as expected for d⁶ Fe^II and as seen in the [Fe]-H₂ase enzyme active site. Deprotonation releases the extrinsic CO and reclaims the pentacoordinate iron in a state of electrophilic deficiency (see picture).

Chiral bis(oxazolinylphenyl)amines (Bopa-R) proved to be efficient auxiliary ligands for iron and cobalt catalysts that exhibit high activity for asymmetric hydrosilylation of ketones and asymmetric conjugate hydrosilylation of enones to give products such as those shown in the picture (4-morph=morpholino).

Fantastic gold! A family of stable lipophilic cations of the type [Au^III(Por)]⁺ (Por=porphyrinato ligand) display promising in vitro cytotoxic and anti-angiogenic activities, and a synergistic property in cytotoxicity upon co-incubation with the fluorescent dye 4′,6-diamidino-2-phenylindole (DAPI, see graphic).

Fueling the fire: N-Alkyl-N,N-dimethylhydrazinium dicyanamide and nitrocyanamide ionic liquids are hypergolic with white-fuming nitric acid (see graphic). With extremely low vapor pressure, and high thermal and chemical stability, they should be less toxic than hydrazine and its derivatives, and have potential as liquid hypergolic fuels.

See the light! Photochemical reactions of a series of tricyclo[5.2.2.0^2,6]undeca-4,10-dien-8-ones were studied (see scheme). The oxa-di-π-methane rearranged products were further converted to linear triquinanes. This methodology was successfully applied to the total synthesis of (±)-Δ⁹⁽¹²⁾-capnellene (1).

Bimetallic Au–Ni synergy: Bimetallic gold–nickel nanoparticles (NPs) with diameters of 3–4 nm embedded in silica nanospheres of around 15 nm have been prepared by a reversed-micelle technique and by in situ reduction in an aqueous NaBH₄/NH₃BH₃ solution. The combination of gold and nickel leads to a remarkable synergy not only in terms of catalytic activity (see figure), but also in durability in the hydrolysis of ammonia borane.

Shown the ropes! The rotated π stacking of dendronized oligo(phenylene ethynylene)s (see figure) results in the formation of nanofibrillar structures several micrometers long that can interact by the hydrophilic triethyleneglycol chains to form thicker strands. These supramolecular structures are able to intercalate and release dye molecules.

Interplay of three catalytic cycles explains the main features of the Soai reaction (see picture). These are the fully enantioselective catalytic cycle of the homochiral dimers (the dominant one), the catalytic cycle of the heterochiral dimer (in fact ineffective, as required for the Soai reaction to manifest chiral amplification), and the non-enantioselective catalytic cycle of the homochiral dimers (necessary to explain the observed enantioselectivity).

Effective cooperation of the ligands 2,3-bis(2-pyridyl)benzoquinoxaline (dpb) and 4,5,9,16-tetraaza-dibenzo[a,c]naphthacene (dppn) gives the tris-heteroleptic complex [Ru(bpy)(dpb)(dppn)]²⁺ (bpy=2,2′-bipyridine), which has a metal-to-ligand charge-transfer maximum at 548 nm, a singlet-oxygen quantum yield of 0.43, strong binding affinity towards double-stranded DNA, and thus promising photodynamic activity.

Cage fighter: In contrast to the typically extreme stability and inertness of metal ions trapped within macrobicyclic cage ligands, the six-coordinate Cu^II complex of a mixed-donor N₃S₃ cage ligand rapidly rearranges to a bromide complex (see figure) in which the ligand adopts a novel tetradentate coordination mode.

From flexible fibrils to hierarchically self-assembled microcrystals: The structural transition from mesostable dipeptide gels to thermodynamically stable microcrystals is achieved merely by addition of the co-solvent; this provides insights into the role of solvents in controlling molecular arrangements (see scheme).

Dipolar organic dyes containing diarylamine as a donor and fluorene and a heteroaromatic ring as the conjugating bridge (see scheme) have been developed for highly efficient nanocrystalline TiO₂ dye-sensitized solar cells (DSSCs). The overall efficiencies of DSSCs range from 4.92–6.88 %, which reach 68–96 % of a standard device of N-719 that was fabricated and measured under the same conditions.

Off the coast: A marine-derived actinomycete, Nocardiopsis sp., from a sediment sample collected at the coast of Brisbane, Australia, yielded two new macrolide polyketides, nocardiopsins A (see figure) and B. The nocardiopsins exhibit low-micromolar binding to FKBP12, consistent with their structural and biosynthetic relationship to the immunosuppressive agents FK506 and rapamycin.

The wizard of Os: A novel class of highly modular diphosphane/diamine–osmium complexes (see structures) are found to be extremely active catalysts in the hydrogenation of ketones and aldehydes (substrate/catalyst ratios and turnover frequencies of up to 2×10⁵ and 3×10⁵ h⁻¹, respectively). High enantioselectivity is achieved with the osmium complexes obtained from the new precursor [Os₂Cl₄{P(m-tolyl)₃}₅] and the correctly matched chiral ligands.

Fullerene antioxidants: The mechanism for degradation of the biologically relevant superoxide radical (O₂^.−) involving the tris-malonyl fullerene compound known as C₃ was investigated in detail by means of DFT calculations (see figure). The capability of C₃ and related fullerenes to remove highly reactive oxygen species is of immediate interest for future applications in the fields of biology and medicine.

Regioselective H/D exchange occurs between isoalkanes and acidic zeolites at room temperature and it is shown that the main factor controlling the reaction is not the accessibility to the acid sites (see scheme). The deuterium-labeling in various small alkanes is best rationalized by mechanistic schemes typical for classic organic chemistry involving carbocationic intermediates related to the intrinsic reactivity of the alkanes.

Measurement and calculation: A comprehensive variable B₀⁹³Nb broadline solid-state NMR study of a large suite of niobate-based materials is reported. Correlations involving the elucidated ⁹³Nb NMR parameters suggest that the Nb^V site is directly influenced by both the immediate oxo environment and the more distant heavy elements comprising the material structure (see figure).

Soft and controlled: Photocontrolled living anionic ring-opening polymerization (ROP) of phosphorus-bridged [1]ferrocenophanes (see scheme) initiated by the mild nucleophile NaCp allows access to well-defined polyferrocenylphosphines with molecular weight control and narrow polydispersities.