Synthesis and study of hexanuclear molybdenum clusters containing thiolate ligands

Four hexanuclear molybdenum chloride cluster complexes containing terminal thiolate ligands have been synthesized and fully characterized. (Bu 4N) 2[Mo 6Cl 8(SEt) 6] was prepared by reacting Na 2[Mo 6Cl 8(OMe) 6] with an excess of ethanethiol in refluxing tetrahydrofuran. (PPN) 2[Mo 6Cl 8(SBu) 6], (Bu 4N) 2[Mo 6Cl 8(SBn) 6], and (Bu 4N) 2[Mo 6Cl 8(SNC 8H 6) 6] (C 8H 6NS (-) = 3-indolylthiolate) were subsequently prepared in the reaction of [Mo 6Cl 8(SEt) 6] (2-) with an excess of HSR (R = Bu, Bn or 3-indolyl). Single crystal X-ray diffraction analyses were performed on two of these complexes: (PPN) 2[Mo 6Cl 8(SEt) 6].Et 2O, crystallizes in the triclinic space group P1 with a = 12.3894(11), b = 13.7651(12), c = 15.0974(13), alpha = 103.975(2), beta = 99.690(2), gamma = 98.062(2), and Z = 1; (PPh 3Me) 2[Mo 6Cl 8(SBn) 6].2NO 2CH 3, also crystallizes in the P1 space group with a = 12.1574(16), b = 13.4441(17), c = 14.2132(18), alpha = 89.654(2), beta = 88.365(2), gamma = 71.179(2), and Z = 1. Our studies demonstrate that [Mo 6Cl 8(SEt) 6] (2-) displays luminescent properties and that the same complex undergoes substitution reactions with different thiols, as well as reaction with electrophilic reagents such as MeI.     

Rationalization and in vitro modeling of the chemical mechanisms of the enzymatic oxidation of phenolic compounds in planta: from flavonols and stilbenoids to lignins

Enzymatic oxidation of phenolic compounds is a widespread phenomenon in plants. It is responsible for the formation of many oligomers and polymers, which are generally described as the result of a combinatorial coupling of the different radicals formed through oxidation of the phenol group and delocalization of the radical. We focused our interest on several phenolic compounds that are present in plants and known to form, under enzymatic oxidation, oligomers with different type of linkages between monomers. To explain this diversity of inter-monomer linkages and their variation according to the experimental procedure used for the enzymatic oxidation, we report an alternative mechanistic pathway involving dismutation of the radicals, leading to the formation of carbocations which, thereafter, react with nucleophilic species present in the medium. This alternative pathway allows the understanding of peculiar linkages between monomeric units in the oligomer and offers new insights for understanding the formation of phenolic biopolymers in plants.     

Thermal degradation of α-pinene using a Py–GC/MS

Mediterranean forest fires may be accelerated, partly due to biogenic volatile organic compounds produced by vegetation, mainly monoterpenes largely represented by α-pinene. To model the propagation of biomass combustion, it is necessary to study the flammability of the produced gas mixture, and thus, necessary to identify the emitted volatile compounds. However, thermal degradation of monoterpenes is rarely experimented above 300 °C, whereas forest fires reach higher temperatures. Thus, in this work, we experimented a 2-min pyrolysis of α-pinene at temperatures from 300 to 800 °C using a Py–GC/MS device. Less than 1% of pyrolysis products were detected at 300 and 400 °C. The pyrolysis products increased then from 14 compounds at 500 °C to 31 compounds at 800 °C. Degradation of α-pinene started with its isomerization at 500 °C. At 800 °C, alkenes detected increased as well as aromatics produced through the Diels–Alder mechanism. These results are consistent with the literature on thermal degradation of α- and β-pinene presented in our article.

     

Preparation and Characterization of Poly(Lactic Acid)-g-Maleic Anhydride + Starch Blends

Poly(lactic acid) (PLA) and starch copolymers are obtained by reactive blending - varying the starch compositions from 0 to 60%. PLA is functionalized with maleic anhydride (MA), obtaining PLA-g-MA copolymers using dicumyl peroxide as an initiator of grafting in order to improve the compatibility and interfacial adhesion between the constituents. PLA + starch blends without a compatibilizer do not have sufficient interfacial adhesion. Decomposition temperature of PLA is not affected by grafting. Glass transition temperatures and dynamic mechanical properties are affected since MA has a plasticizing effect. Along with an increasing starch content friction decreases while wear loss volume in pin-on-disk tribometry has a minimum at nominal 15% wt. starch but increases at higher starch concentrations. The residual depth in scratching and sliding wear testing has a maximum at 15% starch; there is a minimum of storage modulus E′ determined in dynamic mechanical testing at the same concentration. Microhardness results also reflect the plasticization by MA.     

The crystal and molecular structure of bicyclo[2.2.1]-hept-2-en-5,6-dinitrosocobalt-η5-cyclopentadienide. Evidence for a nitroxyl diradical bidentate ligand

An accurate X-ray crystallographic structural study of a metal complex having a nitroxyl diradical as a bidentate ligand has been carried out. Bicyclo[2.2.1]-hept-2-en-5,6-dinitrosocobalt-η⁵-cyclopentadienide crystallizes in space group P$\text{1}$ with Z 2, a 8.245 ± 0.007, b 10.285 ± 0.016, c 7.888 ± 0.008 Å and α 59.276 ± 0.073, β 101.099 ± 0.053, γ 108.802 ± 0.067°. Intensities of 1831 independent reflections were used in the refinement of the structure to an R(F) of 4.6%. The bonding of the Co atom to the cyclopentadienyl ligand appears to be a normal pentahapto linkage. The lone double bond and the CC single bonds adjacent to the endo hydrogens of the norbornene fragment of the molecule are shorter than have previously been reported for this moiety. This study represents the first structural report of NO groups bridging to Co. The CoNO angles (129.4(3) and 128.9(3)°) and the NO stretching frequency (1357 cm^?1) of the title compound resemble values found in nitroxyl free radical compounds more closely than values found in bridging NO group compounds.