Pure and doped lanthanum cobaltites obtained by combustion method

We report the synthesis of La_1−xSr_xCoO₃ nanopowders by solution combustion method using metal nitrates and -alanine (alanine method) or urea (urea method) as fuel. The influence of metal nitrates/organic substance molar ratio and the type of fuel was investigated. The isolated complex precursors were characterized by atomic absorption spectroscopy (AAS), FT-IR spectra and DTA–TG analysis. The La_1−xSr_xCoO₃ (x = 0–0.3) powders were characterized by X-ray diffraction (XRD), scanning electron microscopy–energy-dispersive X-ray analysis (SEM–EDX), as well as by specific surface area measurements. XRD patterns indicate the formation of single-phase LaCoO₃ (rhombohedral) when as-synthesized powders were calcined at 873 K, 3 h in the case of the alanine method and at 1073 K, 3 h for urea-based system. Also, strontium doped lanthanum cobaltites obtained by both methods at 1273 K are single phase with rhombohedral perovskite-like structure as XRD data have proved. SEM investigation of pure and doped lanthanum cobaltites reveal that the samples prepared by both methods have fine particles with tendency of agglomerates formation with different shapes, spongy aspect and high porosity. La_1−xSr_xCoO₃ nanopowders obtained by alanine method have larger specific surface area values than those prepared by urea method.     

The C-disaccharide alpha-C(1-->3)-mannopyranoside of N-acetylgalactosamine is an inhibitor of glycohydrolases and of human alpha-1,3-fucosyltransferase VI. Its epimer alpha-(1-->3)-mannopyranoside of N-acetyltalosamine is not

The radical C-glycosidation of (-)-(1S,4R,5R, 6R)-6-endo-chloro-3-methylidene-5-exo-(phenylseleno)-7-ox abi cyclo[2. 2.1]heptan-2-one ((-)-4) with 2,3,4, 6-tetra-O-acetyl-alpha-D-mannopyranosyl bromide gave (+)-(1S,3R,4R, 5R,6R)-6-endo-chloro-5-exo-(phenylseleno)-3-endo-(1',3',4', 5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-7-oxabi cyc lo[ 2.2.1]hept-2-one ((+)-5) that was converted into (+)-(1R,2S,5R, 6R)-5-acetamido-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl)-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-10) and into (+)-(1R,2S,5R, 6S)-5-bromo-3-chloro-2-hydroxy-6-(1',3',4',5'-tetra-O-acetyl-2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)cyclohex -3-en- 1-yl acetate ((+)-19). Ozonolysis of (+)-10 and further transformations provided 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-galac tos e (alpha-C(1-->3)-D-mannopyranoside of N-acetylgalactosamine (alpha-D-Manp-(1-->3)CH(2)-D-GalNAc): 1). Displacement of the bromide (+)-19 with NaN(3) in DMF provided the corresponding azide ((-)-20) following a S(N)2 mechanism. Ozonolysis of (-)-20 and further transformations led to 2-acetamido-2,3-dideoxy-3-C-(2', 6'-anhydro-7'-deoxy-D-glycero-D-manno-heptitol-7'-C-yl)-D-talose (alpha-C(1-->3)-D-mannopyranoside of N-acetyl D-talosamine (alpha-D-Manp-(1-->3)CH(2)-D-TalNAc): 2). The neutral C-disaccharide 1 inhibits several glycosidases (e.g., beta-galactosidase from jack bean with K(i) = 7.5 microM, alpha-L-fucosidase from human placenta with K(i) = 28 microM, beta-glucosidase from Caldocellum saccharolyticum with K(i) = 18 microM) and human alpha-1, 3-fucosyltransferase VI (Fuc-TVI) with K(i) = 120 microM whereas it 2-epimer 2 does not. Double reciprocal analysis showed that the inhibition of Fuc-TVI by 1 displays a mixed pattern with respect to both the donor sugar GDP-fucose and the acceptor LacNAc with K(i) of 123 and 128 microM, respectively.     

Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing

Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N(2) gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at -25 degrees C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235+/-35 microm. Increasing the freezing rate by cooling with liquid N(2) (-196 degrees C) resulted in much smaller aggregates with an average diameter of 20 microm. Adding NaNO(3) electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at -25 degrees C had a porosity of 0.73+/-0.02 ll(-1). SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9+/-0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N(2) gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m(2)g(-1).     

Nonlinear desingularization in certain free-boundary problems

We consider a nonlinear, elliptic, free-boundary problem involving an initially unknown setA that represents, for example, the cross-section of a steady vortex ring or of a confined plasma in equilibrium. The solutions are characterized by a variational principle which allows us to describe their behaviour under a limiting process such that the diameter ofA tends to zero, while the solutions degenerate to the solution of a related linear problem. This limiting solution is the sum of the Green function of the linear operator and of a smooth function satisfying the boundary conditions. Mathematically speaking, this limiting process, that we call nonlinear desingularization, is a novel kind of bifurcation phenomenon since the nonlinear effect here involves smoothing the singularity of the associated linear problem.Research partially supported by A FOSR and NSF grants     

Two New Spermidine Alkaloids from Chisocheton weinlandii

The investigation of the MeOH extract of the leaves of Chisocheton weinlandii Harms (Meliaceae) revealed two new open‐chain spermidine alkaloids, chisitine 1 ( 1 ) and chisitine 2 ( 2 ). Their structures were elucidated by NMR spectroscopy, tandem‐mass spectrometry, and independant syntheses (Scheme 3). Detailed MS/MS fragmentation pathways are discussed for both compounds based on H/D exchange and ¹⁸O‐labeling experiments (Schemes 1 and 2).     

Radical amination with sulfonyl azides: a powerful method for the formation of C-N bonds

A novel reaction for the introduction of an azide moiety by means of a mild radical process is currently under development. Sulfonyl azides are suitable azidating agents for nucleophilic radicals, such as secondary and tertiary alkyl radicals. More electrophilic radicals, such as enolate radicals, do not react with sulfonyl azides. This feature allowed the development of efficient intra- and intermolecular carboazidations of olefins. Due to the versatility of the azido group, this reaction has an important synthetic potential, as already demonstrated by the preparation of the core of several alkaloids, particularly those containing an amino-substituted quaternary carbon center, such as FR901483.     

Mixed-metal sandwich complexes [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14- (M(II) = Co, Mn): synthesis and stability. The molecular structure of [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14-

Two new mixed-metal sandwich complexes [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14- (abbreviated [M2Fe2P4W30], M(II) = Co(II), Mn(II)) were obtained at pH 3 by addition of M2+ to [Na2(H2O)2Fe(III)2(P2W15O56)2]16- (abbreviated [Na2Fe2P4W30]) without substitution in the alpha-[P2W15O56]12- (abbreviated [P2W15]) units. Their X-ray structures are reported. At lower pH, back conversion to [Na2Fe2P4W30] was followed by 31P NMR, electrochemistry and UV-visible spectroscopy. The preparation and the characterization in solution of the lacunary intermediate [NaCo(II)(H2O)2Fe(III)2(P2W15O56)2]15- (abbreviated [NaCoFe2P4W30]) is also described.     

Organometallic complex compounds,part 6(1). Property-specific ligand control of stability and reactivity of (1,3-dimethyl-η3-allyl)methylnickel-ligand complexes. Separation of electronic and steric effects

Summary The property-specific ligand control of 28 ligands on the decomposition temperatures in solution, measured by d.t.a. of a four-coordinate nickel(II)-complex is reported. A quantitative separation of electronic and steric effects by a multilinear regression analysis (75% electronic and 25% steric influence for the chosen ligands) is presented. The controlling effect of the selectivity on the decomposition (fraction of the C-C-linked product) (25 P-ligands) leads to an electronic: steric ratio of the property-specific ligand control of 5545 for the chosen ligands. An increase in the relative acceptor character of the P-ligands relatively destabilizes the complexes and thereby favours formation of a C-C-bond. An increase in steric hindrance also favours C-C-bond formation. A method for revising the steric parameter of P-ligands is presented and is used to correct the -value of (PhCH₂)₃P is corrected to 135°. SCCC-MO-calculations for testing the chemical reasoning of the separated electronic and steric ligand property control are shown.     

Human immunoglobulin adsorption investigated by means of quartz crystal microbalance dissipation, atomic force microscopy, surface acoustic wave, and surface plasmon resonance techniques

Time-resolved adsorption behavior of a human immunoglobin G (hIgG) protein on a hydrophobized gold surface is investigated using multitechniques: quartz crystal microbalance/dissipation (QCM-D) technique; combined surface plasmon resonance (SPR) and Love mode surface acoustic wave (SAW) technique; combined QCM-D and atomic force microscopy (AFM) technique. The adsorbed hIgG forms interfacial structures varying in organization from a submonolayer to a multilayer. An "end-on" IgG orientation in the monolayer film, associated with the surface coverage results, does not corroborate with the effective protein thickness determined from SPR/SAW measurements. This inconsistence is interpreted by a deformation effect induced by conformation change. This conformation change is confirmed by QCM-D measurement. Combined SPR/SAW measurements suggest that the adsorbed protein barely contains water after extended contact with the hydrophobic surface. This limited interfacial hydration also contributed to a continuous conformation change in the adsorbed protein layer. The viscoelastic variation associated with interfacial conformation changes induces about 1.5 times overestimation of the mass uptake in the QCM-D measurements. The merit of combined multitechnique measurements is demonstrated.     

Single metal four-electron reduction by U(ii) and masked “U(ii)” compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a “clear-cut” single metal four-electron transfer in f-element chemistry.

Both a masked and the actual complex [U(ii){N(SiMe₃)₂}₃]⁺ effect the reduction of azobenzene to yield a U(vi) bis-imido species providing the first example of a “clear-cut” metal centred four-electron reduction in f-element chemistry.