An electrochemical investigation on the reduction path of the arene complexes [CpM(arene)]2+ and [(η-9-SMe2-7,8-C2B9H10)M(arene)]2+ (M=Rh, Ir)

The reduction behavior of the isoelectronic complexes [CpM^III(η⁶-C₆R₆)]²⁺ (M=Rh, Ir; R=H, Me) and [(η-9-SMe₂-7,8-C₂B₉H₁₀)M^III(η⁶-C₆R₆)]²⁺ (M=Rh, Ir; C₆R₆ = C₆H₆, C₆H₅OMe, C₆H₃Me₃) has been studied by cyclic voltammetry and controlled potential coulometry in acetonitrile and propylene carbonate at 253 and 298 K, respectively. The extent of chemical reversibility of the pertinent sequences Rh(III)/Rh(II)/Rh(I) and Ir(III)/Ir(I) is highly dependent on both the nature of the solvent and the intrinsic electronic properties of the arene substituents. The arene η⁶ coordination makes the derivatives in their lower oxidation states notably short lived, even if, in some cases, the use of propylene carbonate improves their stability or causes the increase in their lifetimes before changing the arene coordination from η⁶ to η⁴. Cations [(η-9-SMe₂-7,8-C₂B₉H₁₀)M(η⁶-C₆R₆)]²⁺ were obtained by the bromide abstraction from [(η-9-SMe₂-7,8-C₂B₉H₁₀)MBr₂]₂ with Ag⁺ in the presence of benzene and its derivatives. The structure of [(η-9-SMe₂-7,8-C₂B₉H₁₀)Ir(η⁶-C₆H₅OMe)](BF₄)₂ was determined by X-ray diffraction.     

A fluorescent chemosensor for Zn(II). Exciplex formation in solution and the solid state

The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-1,10-phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol dm(-3) NMe(4)Cl H(2)O-MeCN (1:1, v/v) solution at 298.1 +/- 0.1 K by means of potentiometric (pH metric) titration. The fluorescence emission properties of these complexes were studied in this solvent. For the Zn(II) complex, steady-state and time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular pi-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited state were observed. From the temperature dependence of the photostationary ratio (I(Exc)/I(M)), the activation energy for the exciplex formation (E(a)) and the binding energy of the exciplex (-DeltaH) were determined. The crystal structure of the [ZnLBr](ClO(4)).H(2)O compound was resolved, showing that in the solid state both intra- and inter-molecular pi-stacking interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra, whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no pi-stacking (full overlap of the pi-electron cloud of the two chromophores - anthracene and phenanthroline) is observed. L is a fluorescent chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to the formation of pi-stacking complexes or to exciplex emission in solution.     

Large amplitude out-of-plane vibrations of 1,3-benzodioxole in the S0 and S1 states: an analysis of fluorescence and excitation spectra by ab initio calculations

We report the analytical expressions of the two-dimensional potential energy surfaces (PES) spanned by the puckering and flapping vibrations in the S0 and S1 states of 1,3-benzodioxole (BDO). Both PES are obtained from S0 and S1 energies computed on a grid of 2500 molecular geometries at the CASPT2 level. Both the S0 and S1 PES are anharmonic, and the planar geometry corresponds to a barrier that separates two minima at nonplanar geometries along the puckering/flapping deformations. Eigenvalues and eigenvectors of the mixed puckering/flapping modes are calculated by the Meyer flexible model. Improved vibronic levels, in better agreement with the observed spectra, are obtained by suitably optimized CASPT2 surfaces. To assign the lower-energy (0-500 cm(-1)) portion of emission and absorption spectra, we evaluate the band intensities by estimating the Franck-Condon factors between the puckering/flapping eigenvectors of the S0 and S1 states. From these calculations, we obtain a satisfactory assignment of the ground state IR spectra and of the fluorescence excitation spectrum. Both assignments are supported by the analysis of the vibrational structures of several single vibronic level (SVL) fluorescence spectra. The successful interpretation of these spectra shows that the S0 and S1 PES that we derive for BDO are substantially correct. The barrier heights in the two states are similar: 125.7 and 190.4 cm(-1) in S0 and in S1, respectively. In S0, the barrier is associated essentially with the puckering motion. In S1, it involves to a considerable extent also the flapping coordinate, whose vibrational frequency is much lower in S1 than in S0. This fact introduces a substantial Duschinsky effect in the S0-S1 transitions of BDO.     

Regio- and stereoselective double alkylation of β-enamino esters with organolithium reagents followed by one-pot reduction: convenient method for the synthesis of tertiary γ-amino alcohols

An easy, high yielding and stereoselective procedure for the preparation of tertiary γ-amino alcohols starting from β-enamino esters is presented. In this procedure, the double alkylation of β-enamino esters with organolithium reagents is followed by one-pot reduction with sodium borohydride in methanol/acetic acid. A hypothesis of mechanism is given, explaining the observed diastereoselectivity through molecular modeling. The configuration of the products was determined by ¹H NMR spectroscopy coupled with conformational analysis.     

Exciton states of dynamic DNA double helices: alternating dCdG sequences

The present communication deals with the excited states of the alternating DNA oligomer (dCdG)5.(dCdG)5 which correspond to the UV absorption band around 260 nm. Their properties are studied in the frame of the exciton theory, combining molecular dynamics simulations and quantum chemistry data. It is shown that the dipolar coupling undergoes important variations with the site and the helix geometry. In contrast, the energy of the monomer transitions within the double helix is not sensitive to the local environment. It is thus considered to be distributed over Gaussian curves whose maximum and width are derived from the experimental absorption spectra of nucleosides in aqueous solution. The influence of the spectral width on the excited state delocalization and the absorption spectra is much stronger than that of the oligomer plasticity. About half of the excited states are delocalized over at least two bases. Many of them result from the mixing of different monomer states and extend on both strands. The trends found in the simulated spectra, when going from non-interacting monomers to the duplex, are in agreement with experimental observations. Conformational changes enhance the diversity of the states which can be populated upon excitation at a given energy. The states with larger spatial extent are located close to the maximum of the absorption spectrum.     

Guaiazulene-based phenolic radical scavengers: synthesis, properties, and EPR studies of their reaction with oxygen-centred radicals

A variety of phenolic derivatives 4, carrying the guaiazulene moiety, were prepared starting from guaiazulene. Compounds 4 react with oxygen-centred radicals exhibiting chromotropic behaviour. The radical scavenging power of these compounds was evaluated by different methods. Compounds 4 are less efficient than some of the most common radical scavengers but show quite selective behaviour towards different oxygen-centred radicals. A correlation is found between the antioxidant activity of the compounds 4 and the corresponding phenolic O-H bond dissociation energy. Some aspects of the reaction of the compounds 4 with oxygen-centred radicals were elucidated by EPR and DFT studies.     

Complexation of anionic polyelectrolytes with cationic liposomes: evidence of reentrant condensation and lipoplex formation

We have studied the complexation process taking place in cationic liposomes in the presence of anionic polyelectrolytes, in the polyion concentration range from the dilute to the concentrated regime, by combining dynamic light scattering and transmission electron microscopy techniques. We employed as the cationic lipid a two-chained amphiphile (Dioleoyltrimethylammoniumpropane) and sodium polyacrylate salt as the flexible anionic polyelectrolyte. The results evidence a variety of different structures, mainly depending on the liposome-polyion charge ratio, whose peculiar dynamical and structural features are briefly described. In particular, three different polyion concentration regions are found, within which a monomodal or bimodal distribution of aggregates, with a well-defined time evolution, is present. At low polyion content, close to the isoelectric point, large aggregates are formed, deriving from the collapse of the liposomal bilayers into extended charged surfaces, where adsorbed polyions form a two-dimensional strongly correlated array and organize into a two-dimensional Wigner liquid. At high polyion content, above a critical concentration, the size distributions of the complexes are clearly bimodal and a large-component aggregate, continuously increasing with time, coexists with a population of smaller-size aggregates. At an intermediate polyion concentration, spherical, small-size vesicular structures are reformed, connected in a network by polymer chains. A brief discussion tries to summarize our results into a consistent picture.     

Preparation of bicyclo[3.2.0]heptane-2-endo,7-endo-diols: 1,3-diols with a chiral rigid backbone

The easily available bicyclo[3.2.0]hept-3-en-6-ones (1a-f) have been converted into the corresponding bicyclo[3.2.0]heptane-2-endo,7-endo-diols (4a-f) in an efficient and stereoselective fashion. This preparation opens a route to a family of 1,3-diols with a chiral rigid backbone, potentially suitable as nonracemic precursors for bidentate ligands in asymmetric synthesis.     

GC-MS studies of the chemical composition of two inedible mushrooms of the genus Agaricus

Background  

Mushrooms in the genus Agaricus have worldwide distribution and include the economically important species A. bisporus. Some Agaricus species are inedible, including A. placomyces and A. pseudopratensis, which are similar in appearance to certain edible species, yet are known to possess unpleasant odours and induce gastrointestinal problems if consumed. We have studied the chemical composition of these mushrooms using GC-MS.