In situ K-edge X-ray absorption spectroscopy of the ligand environment of single-site Au/C catalysts during acetylene hydrochlorination

The replacement of HgCl₂/C with Au/C as a catalyst for acetylene hydrochlorination represents a significant reduction in the environmental impact of this industrial process. Under reaction conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state under operating conditions has been investigated in catalysts prepared from aqua regia and thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of three distinct Cl species in the materials; inorganic Cl⁻, Au–Cl, and C–Cl and how these species evolve during reaction. Direct evidence of Au–S interactions is confirmed in catalysts prepared using thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This stability was clear during gas switching experiments, where exposure to C₂H₂ alone did not dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate catalyst.

In situ chlorine and sulphur XAS shows a dynamic ligand environment around cationic Au single-sites during acetylene hydrochlorination.     

Aromaticity as a Quantitative Concept. 6. Aromaticity Variation with Molecular Environment

The classical aromaticity of most heterocycles, and of some carbocycles such as azulene, increases with the polarity of the medium as shown by experimental and calculated bond lengths, aromaticity indices, and dipole moments.     

How are tris(triazolyl)borate ligands electronically different from tris(pyrazolyl)borate ligands? A study of (Ttz(tBu,Me))CuCO [Ttz(tBu,Me) = tris(3-t-butyl-5-methyl-1,2,4-triazolyl)borate

(Ttz(tBu,Me))CuCO [Ttz(tBu,Me) = tris(3-t-butyl-5-methyl-1,2,4-triazolyl)borate] was prepared and fully characterized to test whether the Ttz(tBu,Me) ligand, which is sterically similar to Tp(tBu,Me) is electronically different; Ttz(tBu,Me) is a weaker electron donor and (Ttz(tBu,Me))CuCO is water stable and has a propensity to form hydrogen bonds.     

Poly(arylene sulfide)s via nucleophilic aromatic substitution reactions of halogenated pyromellitic diimides

Nucleophilic aromatic substitution (SNAr) reactions are exploited to prepare poly(arylene sulfide)s (PAS's) via the reaction of bis-thiolates and dibrominated pyromellitic diimide (PMDI) derivatives. Small-molecule model studies reveal the reaction is well-defined and proceeds in quantitative yield in practical times at room temperature. Variation in comonomer feed ratios allowed some control over target polymer molecular weights in the step polymerization, but control was likely limited by the relatively poor polymer solubility in the dipolar aprotic solvents typically employed to promote SNAr reactions. One substitution pattern produces a steric “pocket” around the PMDI units, inducing a peculiar solubility trend in halogenated solvents; that is, greatly reduced solubility in CHCl₃ relative to CH₂Cl₂ and C₂H₂Cl₄. One example small-molecule readily dissolves in CHCl₃ at room temperature, then rapidly grows poorly soluble crystals revealed by single-crystal XRD to contain CHCl₃ molecules in the steric pockets. Finally, the recently demonstrated depolymerization of phthalonitrile-based PAS's via ipso substitution with monothiolates as chain scission agents yields quantitative molecular weight reduction to monomeric species from the polymers reported here.     

Establishing the feasibility of coupled solid-phase extraction-solid-phase derivatization for acidic herbicides

The significance of this research is that it improves analytical methodology used for organic chemicals in aqueous solutions by establishing the feasibility of heterogeneous chemical derivatization at the liquid-solid interface (i.e., solid-phase reaction or solid-phase derivatization). A solid-phase derivatization method for determining chlorinated herbicide acids was developed. Solid-phase extraction was used to concentrate and retain analytes on sorbents for subsequent solid-phase derivatization. Background interferences were removed from the chromatograms by electronically subtracting the responses of blank, nonfortified analyses from spiked samples. Two extraction sorbents (octadecyl bonded silica and polystyrene-divinylbenzene) and two derivatizing reagents (BF3-MeOH and trimethylsilyldiazomethane) were investigated. Recovery of 13 chlorinated herbicide acids--including pentachlorophenol, dinoseb, and bentazon (having a derivatizable functional group, -OH or -NH, bonded directly to a phenyl group); dicamba, picloram, acifluorfen, 3,5-dichlorobenzoic acid, and dacthal (having a derivatizable functional group, -COOH, bonded directly to a phenyl group); and 2,4-D, 2,4,5-T, dichlorprop, 2,4,5-TP, and 2,4-DB (having a derivatizable functional group, -COOH, bonded directly to a sp3 carbon atom)--was tested. The analytical method developed was proven successful for determining acidic herbicides, except for the dacthal diacid metabolite, in aqueous samples.     

Enantioselective oxidation of O-methyl-N-hydroxylamines using monoamine oxidase N as catalyst

Enantioselective oxidation of racemic O-methyl-N-hydroxycyclohexylethylamine, using a variant of monoamine oxidase N (MAO-N) from Aspergillus niger, yields unreacted (R)-enantiomer (e.e. = 99%) together with the oxime exclusively in the (E)-configuration.     

A potential pitfall in 18O-based N-linked glycosylation site mapping

A common procedure for identifying N-linked glycosylation sites involves tryptic digestion of the glycoprotein, followed by the conversion of glycosylated asparagine residues into (18)O-labeled aspartic acids by PNGase F digestion in (18)O water. The 3 Da mass tag created by this process is readily observable by liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis, and is often used to identify the sites of N-linked glycosylation. While using this procedure, we noticed that 60% of the asparagines identified as being glycosylated were not part of the consensus sequence required for N-linked glycosylation, and thus were not biologically possible. Investigation into the source of this unacceptably high false positive rate demonstrated that even after reversed-phase cleanup and heat denaturation, the trypsin used for proteolysis was still active and led to the incorporation of (18)O into the C-termini of the peptides during the deglycosylation step. The resulting mass shift accounted for most of the false positive sites, as the database search algorithm confused it with an (18)O-labeled Asp residue near the C-terminus of a peptide. This problem can be overcome by eliminating trypsin from the solution prior to performing the deglycosylation process, by resuspending the peptides in natural abundance water following deglycosylation, or by allowing (18)O incorporation into the C-terminus as a variable modification during the database search. These methods have been demonstrated on a model protein, and are applicable to the analyses of glycoproteins that are digested with trypsin or another serine protease prior to enzymatic release of the carbohydrate side chains. This study should alert investigators in the field to this potential and unexpected pitfall and provide strategies to overcome this phenomenon.