A pyrazine bis‐adduct of a binuclear rhodium(II) carboxylate containing 3,4,5‐triethoxybenzoate as the ­equatorial ligand

The title compound, tetrakis(μ‐3,4,5‐triethoxy­benzoato‐κ²O:O′)­bis­[(pyrazine‐κN)­rhodium(II)](Rh—Rh), [Rh₂(C₁₃H₁₇O₅)₄(C₄H₄N₂)₂], crystallizes on an inversion centre in the triclinic space group . The equatorial carboxyl­ate ligands bridge the two Rh^II atoms, giving a binuclear lantern‐like structure. The pyrazine mol­ecules occupy the two axial coordination sites. The phenyl rings are tilted by ca 10° with respect to the attached carboxyl­ate groups. The pyrazine planes have a torsion angle of ca 19° around the Rh—N bond with respect to the plane of the nearer carboxyl­ate group and are not coplanar with the Rh—Rh bond.     

Optical Nanocomposite Planar Waveguides Doped with Rare-Earth and Noble Metal Elements

The present paper is focused on multilayer Er³⁺-doped silica-titania planar waveguides, co-doped with silver, which were prepared by spin-coating on silica glass, or buffered single crystal silicon substrates. The single layer thickness (0.4 m) and refractive index (1.60–1.63) were measured by spectroscopic ellipsometry at 715 nm. The thickness of the waveguides (measured by mechanical profilometry) was 1 m and their optical propagation losses were measured at different laser wavelengths (488 nm, 514 nm and 633 nm), exhibiting an approximately Rayleigh-like behavior. The thermal precipitation of silver nanocrystallites was achieved, both in air and under a controlled atmosphere (dry nitrogen) and these were characterized by visible absorption spectroscopy, which clearly showed the development of a plasmon absorption band near 415 nm, by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The Er³⁺ metastable level lifetimes for the emission at ca. 1.54 m were found to be ca. 4–6 ms, for Er³⁺ concentrations varying between 0.2–2.0 mol% (or (0.4–4.4) × 10²⁰ ions/cm³), but no significant variation was observed with the Ag concentration added (up to 2.5 mol%).     

Contrasting behavior in azide pyrolyses: an investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.     

Copper-assisted oxidation of catechols into quinone derivatives

Catechols are ubiquitous substances often acting as antioxidants, thus of importance in a variety of biological processes. The Fenton and Haber–Weiss processes are thought to transform these molecules into aggressive reactive oxygen species (ROS), a source of oxidative stress and possibly inducing degenerative diseases. Here, using model conditions (ultrahigh vacuum and single crystals), we unveil another process capable of converting catechols into ROSs, namely an intramolecular redox reaction catalysed by a Cu surface. We focus on a tri-catechol, the hexahydroxytriphenylene molecule, and show that this antioxidant is thereby transformed into a semiquinone, as an intermediate product, and then into an even stronger oxidant, a quinone, as final product. We argue that the transformations occur via two intramolecular redox reactions: since the Cu surface cannot oxidise the molecules, the starting catechol and the semiquinone forms each are, at the same time, self-oxidised and self-reduced. Thanks to these reactions, the quinone and semiquinone are able to interact with the substrate by readily accepting electrons donated by the substrate. Our combined experimental surface science and ab initio analysis highlights the key role played by metal nanoparticles in the development of degenerative diseases.

An antioxidant catechol transforms following intramolecular redox reactions into highly reactive oxygen species, a semiquinone and a quinone, on copper.     

Anaerobic reduction of a sulfonated azo dye, Congo Red, by sulfate-reducing bacteria

The capacity for anaerobic decolorization of a sulfonated azo dye, Congo Red, by a strain of a sulfate-reducing bacterium was evaluated. After optimizing the growth rate of the bacteria on a simple carbon source and terminal electron acceptor pair, lactate and sulfate, respectively, the effect of the dye concentration on their growth rate was analyzed. The decolorization rate was affected by the dye concentration in the growth medium. The azo-bond cleavage mechanism of reductive decolorization with the formation of benzidine was consistent with the results, as this metabolite was identified by high-performance liquid chromatography. Several fractions of the culture medium, including lysed cell extracts, were examined for the capacity to reduce the azo dye. This reduction capacity was found in the culture medium in which the cells had previously grown. The results showed that the mechanism of reductive decolorization of this sulfonated azo dye was extracellular and nonenzymatic, consistent with the production of sulfide anion by the microorganisms while growing on lactate and sulfate. The sulfide anions were the cause of the reduction leading to the disappearance of color in the medium. To increase the rate of decolorization, the presence of ferrous ion was also necessary together with the lactate and sulfate substrates.     

Unexpected ferromagnetic interaction in a new tetranuclear copper(II) complex: synthesis, crystal structure, magnetic properties, and theoretical studies

The new tetranuclear carbonate complex [Cu2L)2(CO3)] x 8H2O (1 x 8H2O) (H3L = (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) has been obtained by two different synthetic routes and fully characterized. Recrystallization of 1 x 8H2O in methanol yields single crystals of {[(Cu2L)2(CO3)]}2 x 12H2O (1 x 6H2O), suitable for X-ray diffraction studies. The crystal structure of 1 x 6H2O shows two crystallographically different tetranuclear molecules in the asymmetric unit, 1a and 1b. Both molecules can be understood as self-assembled from two dinuclear [Cu2L]+ cations, joined by a mu4-eta(2):eta(1):eta(1) carbonate ligand. The copper atoms of each crystallographically different [(Cu2L)2(CO3)] molecule present miscellaneous coordination polyhedra: in both 1a and 1b, two metal centers are in square pyramidal environments, one displays a square planar chromophore and the other one has a geometry that can be considered as an intermediate between square pyramid and trigonal bipyramid. Magnetic studies reveal net intramolecular ferromagnetic coupling between the metal atoms. Density functional calculations allow the assignment of the different magnetic coupling constants and explain the unexpected ferromagnetic behavior, because of the presence of an unusual NCN bridging moiety and countercomplementarity of the phenoxo (or carbonate) and NCN bridges.     

Early-late heterobimetallic alkoxides as model systems for late-transition-metal catalysts supported on titania

Titanium complexes with chelating alkoxide ligands [TiCp*(O(2)Bz)(OBzOH)] (1) and [TiCp*(Me)((OCH(2))(2)Py)] (2) were synthesised by reaction of [TiCp*Me(3)] (Cp*=eta(5)-C(5)Me(5)) with 2-hydroxybenzyl alcohol ((HO)(2)Bz) and 2,6-pyridinedimethanol ((HOCH(2))(2)Py), respectively. Complex 1 reacts with [(M(mu-OH)(cod))(2)] (M=Rh, Ir) to yield the early-late heterobimetallic complexes [TiCp*(O(2)Bz)(2)M(cod)] [M=Rh (3), Ir (4)]. Carbon monoxide readily replaces the COD ligand in 3 to give the rhodium dicarbonyl derivative [TiCp*(O(2)Bz)(2)Rh(CO)(2)] (5). Compound 2 reacts with [(M(mu-OH)(cod))(2)] (M=Rh, Ir) with protonolysis of a Tibond;Me bond to give [TiCp*((OCH(2))(2)Py)(mu-O)M(cod)] [M=Rh (6), Ir (7)]. The molecular structures of complexes 3, 5 and 7 were established by single-crystal X-ray diffraction studies.     

Convergent stereocontrolled synthesis of substituted exo-glycals by Stille cross-coupling of halo-exo-glycals and stannanes

(Z)-exo-Glycals can be conveniently prepared in a convergent manner by Stille cross-coupling of (Z)-halo(Br,I)-exo-glycals and aryl or alkenyl stannanes, the latter are readily obtained by addition of tributylstannyl radicals to terminal alkynes.     

Three oxo complexes with a tetra­nuclear [Cu4(μ2‐Cl)6(μ4‐O)] unit

The three title compounds, namely 4‐phenyl‐1H‐imidazolium hexa‐μ₂‐chloro‐chloro‐μ₄‐oxo‐tris­(4‐phenyl‐1H‐imidazole‐κN¹)­tetra­copper(II) monohydrate, (C₉H₉N₂)[Cu₄Cl₇O(C₉H₈N₂)₃]·H₂O, hexa‐μ₂‐chloro‐μ₄‐oxo‐tetra­kis­(pyridine N‐oxide‐κO)tetra­copper(II), [Cu₄Cl₆O(C₅H₅NO)₄], and hexa‐μ₂‐chloro‐tetra­kis(2‐methyl‐1H‐imidazole‐κN¹)‐μ₄‐oxo‐tetra­copper(II) methanol trisolvate, [Cu₄Cl₆O(C₄H₆N₂)₄]·3CH₄O, exhibit the same Cu₄OCl₆ framework, where the O atom at the centre of an almost regular tetra­hedron bridges four copper cations at the corners. This group is in turn surrounded by a Cl₆ octa­hedron, leading to a rather globular species. This special arrangement of the Cu^II cations results in a diversity of magnetic behaviours.     

Flow system to gas capture from a nebulized solution

The use of aerosol produced in a nebulization chamber is proposed as an alternative to gas sample capture in flow systems. This paper describes the coupling of a sampling interface with a flow system, for in situ gas monitoring. Aspects related with the behavior of aerosol formation and gas solubilization in liquid drops are discussed. The method is applied to the determination of residual lime in acidic soils. Aliquots of 5.0 ml of 1.0 mol l⁻¹ HCl were mixed with soil samples (1 g). The CO₂ released from these samples was captured by a nebulized aerosol and determined conductivity. The analytical curve from 1.0×10⁻² to 5.0×10⁻² mol kg⁻¹ CaCO₃ was ploted applying the matrix matching approach. This proposition, allowed an increase in the sensibility with detection limit of 6.0×10⁻³ mol kg⁻¹. The precision was good (R.S.D. <3%) for an analytical frequency of 22 determinations per hour. A fair agreement, at 95% confidence level, was found between the results from the proposed method and certified values of the investigated samples.