Synthesis of 1,3,4-oxadiazoles from 1,2-diacylhydrazines using [Et2NSF2]BF4 as a practical cyclodehydration agent

The preparation of 1,3,4-oxadiazoles from 1,2-diacylhydrazines using XtalFluor-E ([Et(2)NSF(2)]BF(4)) as cyclodehydration reagent is described. Various functionalized 1,3,4-oxadiazoles were synthesized and it was found that the use of acetic acid as an additive generally improved the yields.     

Stereoselective nucleophilic substitution of poly(vinyl chloride) with sodium thiophenate in cyclohexanone solution: Influence of the reaction temperature on the mechanism

Nucleophilic substitution of PVC with sodium thiophenate was carried out in cyclohexanone solution at 5, 25, 40, 60, and 70°C. The initial rate obeys an Arrhenius law from 25 to 60°C, with an activation energy of 70 kJ/mol. Conversion limits are observed which strongly depend on the temperature. The stereoselectivity of the reaction with respect to the configurational triads does not depend on the temperature: the distribution of configurations is only dependent on the conversion. Assuming an S_N2 substitution mechanism governed by steric factors, the Monte Carlo simulation procedure described in a prior study is shown to give a good account for all temperatures above 40°C assuming for the mm, mr or rm, and rr triads a reactivity such as R_mm = 2 R_mr and R_rr nil at low temperature and very low at temperatures ≥ 40°C. The low conversion limits observed at 5 and 25°C cannot be explained by a limited accessibility of a part of the polymer. Finally, it is shown that the elimination reaction, which remains limited, does not interfere with the substitution process.     

Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+: chemical and electrochemical access to a new mono-mu-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)Mn(II)(mL)](ClO(4))(2) (1(ClO(4))(2)) has been obtained with the new [N(4)O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0-800 mT at the X band and over 100-1450 mT at the Q band, features that are usually observed for dinuclear Mn(II) complexes. Cyclic voltammetry of 1 exhibits two irreversible oxidation waves at E(1)(p)=0.89 V and E(2)(p)=1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E(1')(p)=0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20 degrees C, 1 is transformed into the mono-mu-oxo species [(mL)Mn(III)-(mu-O)-Mn(III)(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn(III)-(mu-O)(2)-Mn(IV)(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn(III) charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the Mn(III)Mn(III)/Mn(III)Mn(IV) and Mn(III)Mn(IV)/Mn(IV)Mn(IV) oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixed-valent species [(mL)Mn(III)-(mu-O)-Mn(IV)(mL)](3+) (2 ox). The UV/Vis spectrum of 2 ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn(IV) charge-transfer transition. 2 ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2 ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2 ox are proposed.     

An electron impact study of HCN elimination from indolizine by use of 13C labelling

The study of the loss of HCN from the molecular ions of [1-¹³C]-, [2-¹³C]- and [3-¹³C]-indolizine shows that, if the C-3 atom is eliminated predominantly, as may be expected, the C-2 atom, and (a) carbon atom(s) of the hexagonal ring are also involved. The losses of ¹³CCH₃^. and C₂H₃^. from the [M? H¹²CN]^+˙ ions of the three compounds point to the interference of distinct mechanisms of HCN elimination, leading to different structures for the [C₇H₆]^+˙ ions.     

Carboxylate ligands drastically enhance the rates of oxo exchange and hydrogen peroxide disproportionation by oxo manganese compounds of potential biological significance

To mimic the carboxylate-rich active site of the manganese catalases more closely we introduced carboxylate groups into dimanganese complexes in place of nitrogen ligands. The series of dimanganese(III,IV) complexes of tripodal ligands [Mn(2)(L)(2)(O)(2)](3+/+/-/3-) was extended from those of tpa (1) and H(bpg) (2) to those of H(2)(pda) (3) and H(3)(nta) (4) (tpa=tris-picolylamine, H(bpg)=bis-picolylglycylamine, H(2)(pda)=picolyldiglycylamine, H(3)(nta)=nitrilotriacetic acid). While 3 [Mn(2)(pda)(2)(O)(2)][Na(H(2)O)(3)] could be synthesized at -20 degrees C and characterized in the solid state, 4 [Mn(2)(nta)(2)(O)(2)](3-) could be obtained and studied only in solution at -60 degrees C. A new synthetic procedure for the dimanganese(III,III) complexes was devised, using stoichiometric reduction of the dimanganese(III,IV) precursor by the benzil radical with EPR monitoring. This enabled the preparation of the parent dimanganese(III,III) complex 5 [Mn(2)(tpa)(2)(O)(2)](ClO(4))(2), which was structurally characterized. The UV/visible, IR, EPR, magnetic, and electrochemical properties of complexes 1-3 and 5 were analyzed to assess the electronic changes brought about by the carboxylate replacement of pyridine ligands. The kinetics of the oxo ligand exchanges with labeled water was examined in acetonitrile solution. A dramatic effect of the number of carboxylates was evidenced. Interestingly, the influence of the second carboxylate substitution differs from that of the first one probably because this substitution occurs on an out-of-plane coordination while the former occurs in the plane of the [Mn(2)O(2)] core. Indeed, on going from 1 to 3 the exchange rate was increased by a factor of 50. Addition of triethylamine caused a rate increase for 1, but not for 3. The abilities of 1-3 to disproportionate H(2)O(2) were assessed volumetrically. The disproportionation exhibited a sensitivity corresponding to the carboxylate substitution. These observations strongly suggest that the carboxylate ligands in 2 and 3 act as internal bases.     

Roles of local He concentration and Si sample orientation on cavity growth in amorphous silicon

(111)- and (100)-oriented Si samples were implanted with Si⁺ ions at 1 MeV to a dose of 1?×?10¹⁶?cm^?2 and with 5?×?10¹⁶ He⁺ cm^?2 at 10?keV or 50?keV and eventually annealed in the 800–1000°C temperature range. Sample characterisation was carried out by cross-section transmission electron microscopy, positron annihilation spectroscopy and nuclear reaction analysis. In addition to the formation of He bubbles at the projected range of He, bubbles were observed after solid-phase epitaxial growth (SPEG) of the embedded amorphous Si layer. The He threshold concentration required to obtain thermally stable bubbles in amorphised Si is between one and four orders of magnitude lower than in c-Si. Since bubble formation and growth take place in the a-Si phase, the interaction with SPEG during annealing was studied by considering (100) and (111) Si. Both the SPEG velocity and the resulting defects play a role on bubble spatial distribution and size, resulting in bigger bubbles in (111) Si with respect to (100) Si.     

Facilitated Transport of Carbohydrates, Catecholamines, and Amino Acids Through Liquid and Plasticized Organic Membranes

A number of methods are described to facilitate the transport of monosaccharides, catecholamines, and amino acids through bulk liquid membranes, supported liquid membranes and plasticized cellulose triacetate membranes. Transport is mediated by carrier compounds, such as boronic acids, quaternary ammonium salts and crown ethers, that are dissolved within the lipophilic membranes. Two types of transport mechanisms are described, carrier diffusion and fixed-site jumping.     

Influence of the protonic state of an imidazole-containing ligand on the electrochemical and photophysical properties of a ruthenium(II)-polypyridine-type complex

The synthesis and characterisation of [Ru(bpy)2(PhenImHPh)]2+ where PhenImHPh represents the 2-(3,5-di-tert-butylphenyl)imidazo[4,5-f][1,10]phenanthroline ligand are described. The compounds issued from the three different protonic states of the imidazole ring [Ru(bpy)2(PhenImPh)]+ (I), [Ru(bpy)2(PhenImHPh)]2+ (II) and [Ru(bpy)2(PhenImH2Ph)]3+ (III) were isolated and spectroscopically characterised. The X-ray structures of [Ru(bpy)2(PhenImPh)](PF6)H2O.6 MeOH, [Ru(bpy)2(PhenImHPh)](NO3)2H2O.3 MeOH and [Ru(bpy)2(PhenImH2Ph)](PF6)3 5 H2O are reported. Electrochemical data obtained on these complexes indicate almost no potential shift for the Ru(III/II) redox couple. Therefore a Coulombic effect between the imidazole ring and the metal centre can be ruled out. The monooxidised forms of I and II have been characterised by EPR spectroscopy and are reminiscent of the presence of a radical species. The emission properties of the parent compound [Ru(bpy)2(PhenImHPh)]2+ were studied as a function of pH and both the lifetimes and intensities decreased upon deprotonation. Photophysical properties, investigated in the absence and presence of an electron acceptor (methylviologen), were distinctly different for the three compounds. Transient absorption features indicate that unique excited states are involved. Theoretical data obtained from DFT calculations in water on the three protonic forms are presented and discussed in the light of the experimental results.     

Polyelectrolyte entry and transport through an asymmetric alpha-hemolysin channel

We study the entry and transport of a polyelectrolyte, dextran sulfate (DS), through an asymmetric alpha-hemolysin protein channel inserted into a planar lipid bilayer. We compare the dynamics of the DS chains as they enter the channel at the opposite stem or vestibule sides. Experiments are performed at the single-molecule level by using an electrical method. The frequency of current blockades varies exponentially as a function of applied voltage. This frequency is smaller for the stem entrance than for the vestibule one, due to a smaller coupling with the electric field and a larger activation energy for entry. The value of the activation energy is quantitatively interpreted as an entropic effect of chain confinement. The translocation time decreases when the applied voltage increases and displays an exponential variation which is independent of the stem or vestibule sides.