Zinc complexes of anionic NPPN and NP(S)PN ligands and rearrangement to the isomeric NPNP and NP(S)NP ligands in mercury complexes

The lithium (imido)diphosphineimide Li(Et2O)[DippNPhP-P((n)Bu)PhNDipp] (1) (Dipp = 2,6-(i)Pr2C6H3) undergoes simple metathesis reactions with equimolar amounts of zinc halides, ZnCl2 and (t)BuZnBr, to give the respective N,N'-chelated complexes {Zn(micro-Cl)[DippNPhP-P((n)Bu)PhNDipp]}2 (2) and (t)BuZn[NDippPhP-P((n)Bu)PhNDipp] (3). In contrast, the reaction of two equivalents of complex 1 with HgCl2 affords the rearranged bis(imidodiphosphinoamine) complex, Hg[PhP([double bond, length as m-dash]NDipp)(micro-NDipp)P((n)Bu)Ph]2 (4), where the ligand acts as a P-centered anion. The (imido)diphosphineimide backbone of remains intact on oxidation with elemental sulfur to afford the lithium (imido)diphosphineimine sulfide complex, Li(Et2O)[DippNPhP(S)-P((n)Bu)PhNDipp] (6). Reactions of 6 with group 12 metal halides show similar behaviour to those of complex 1. The N,N' chelated metathesis products RZn[DippNPhP(S)-P((n)Bu)PhNDipp] (7, R = Cl; 8, R = (t)Bu) are obtained on reaction with ZnCl2 and (t)BuZnBr, respectively. Isomerization of the ligand backbone occurs on reaction of 6 with HgCl2 to form the homoleptic P,S-chelated mercury complex Hg[Ph(S)P(=NDipp)(micro-NDipp)P((n)Bu)Ph]2 (9). Complexes 2, 3, 4, 6, 8 and 9 have been characterized by X-ray crystallography.     

Chemical probes of UDP-galactopyranose mutase

Many pathogenic prokaryotes and eukaryotes possess the machinery required to assemble galactofuranose (Galf)-containing glycoconjugates; these glycoconjugates can be critical for virulence or viability. Accordingly, compounds that block Galf incorporation may serve as therapeutic leads or as probes of the function of Galf-containing glycoconjugates. The enzyme UDP-galactopyranose mutase (UGM) is the only known generator of UDP-galactofuranose, the precursor to Galf residues. We previously employed a high-throughput fluorescence polarization assay to investigate the Klebsiella pneumoniae UGM. We demonstrate the generality of this assay by extending it to UGM from Mycobacterium tuberculosis. To identify factors influencing binding, we synthesized a directed library containing a 5-arylidene-2-thioxo-4-thiazolidinone core, a structure possessing features common to ligands for both homologs. Our studies offer a blueprint for identifying inhibitors of the growing family of UGM homologs and provide insight into UGM inhibition.     

Sulfur, normally a poison, strongly promotes chemoselective catalytic hydrogenation: stereochemistry and reactivity of crotonaldehyde on clean and S-modified Cu(111)

Sulfur adatoms strongly activate the otherwise inert Cu(111) surface towards chemoselective hydrogenation of crotonaldehyde by electronically perturbing and strongly tilting the reactant.     

Microfluidic production of biopolymer microcapsules with controlled morphology

We report a microfluidic approach to generating capsules of biopolymer hydrogels. Droplets of an aqueous solution of a biopolymer were emulsified in an organic phase comprising a cross-linking agent. Polymer gelation was achieved in situ (on a microfluidic chip) by diffusion-controlled ionic cross-linking of the biopolymer, following the transfer of the cross-linking agent from the continuous phase to the droplets. Gelation was quenched by collecting particles in a large pool of cross-linking agent-free liquid. The structure of microgels (from capsules to gradient microgels to particles with a uniform structure) was controlled by varying the time of residence of droplets on the microfluidic chip and the concentration of the cross-linking agent in the continuous phase. We demonstrated the encapsulation of a controlled number of polystyrene beads in the microgel capsules. The described approach was applied to the preparation of capsules of several polysaccharides such as alginate, kappa-carrageenan, and carboxymethylcellulose.     

Synthesis of C6-to-C6 and C3-to-C3 diamide linked γ-cyclodextrin dimers

The syntheses of three new diamide-linked γ-cyclodextrin dimers joined by substitution at either a glucopyranose C6^A or C3^A carbon are reported. The syntheses involve the reaction of either C6^A or C3^A amino-substituted γ-cyclodextrin with bis(4-nitrophenyl)succinate to form succinamide linked γ-cyclodextrin dimers or reaction of C6^A azide-substituted γ-cyclodextrin with carbon dioxide to form a urea linked γ-cyclodextrin dimer.     

Single-molecule fluorescence imaging of nanocatalytic processes

This tutorial review covers recent developments in using single-molecule fluorescence microscopy to study nanoscale catalysis. The single-molecule approach enables following catalytic and electrocatalytic reactions on nanocatalysts, including metal nanoparticles and carbon nanotubes, at single-reaction temporal resolution and nanometer spatial precision. Real-time, in situ, multiplexed measurements are readily achievable under ambient solution conditions. These studies provide unprecedented insights into catalytic mechanism, reactivity, selectivity, and dynamics in spite of the inhomogeneity and temporal variations of catalyst structures. Prospects, generality, and limitations of the single-molecule fluorescence approach for studying nanocatalysis are also discussed.     

Effect of molecular weight distribution on the rheological behaviour of poly(vinylidene fluoride) (PVDF)

In this study, six samples of commercial poly(vinylidene fluoride) are investigated. Dynamic and steady-state shear flow experiments are carried out in the melt state in order to relate their rheological behaviour to their molecular weight distributions. This rheological investigation is performed through the use of some simple parameters related to the viscosity, elasticity and relaxation times. Plots of flow curves in reduced coordinates are used to confirm the above results.