Synthesis of amino ester-embedded benzimidazoles: a one-pot sequential protocol under metal-free neutral conditions

A one-pot three-component protocol has been developed for the synthesis of amino ester-embedded benzimidazoles under metal-free neutral conditions. Sequentially, the methodology involves coupling of an amino ester with 1-fluoro-2-nitrobenzene, reduction of the coupled nitroarene by sodium dithionite, and cyclization of the corresponding diamine with an aldehyde.     

N,N-bis-(2-benzimidazolylmethyl)L-methionine: An efficient Ag(I) extractant

A new ligand N,N-bis-(2-benzimidazolylmethyl)L-methionine has been synthesized and well characterized by different analytical techniques. The ligand has a very strong affinity for silver(I) and is used for the selective extraction of silver ions from aqueous solution into 1-butanol. The effect of concentration of Ag(I), pH, diverse ions, and temperature on the extraction is reported. The developed method has success-fully been applied to the determination of silver in different samples. Radiotracer technique, using ^110mAg(I) as a tracer, has been used for monitoring the concentration of silver ion.     

Improving Physical Properties via C?H Oxidation: Chemical and Enzymatic Approaches

Physicochemical properties constitute a key factor for the success of a drug candidate. Whereas many strategies to improve the physicochemical properties of small heterocycle‐type leads exist, complex hydrocarbon skeletons are more challenging to derivatize because of the absence of functional groups. A variety of C H oxidation methods have been explored on the betulin skeleton to improve the solubility of this very bioactive, yet poorly water‐soluble, natural product. Capitalizing on the innate reactivity of the molecule, as well as the few molecular handles present on the core, allowed oxidations at different positions across the pentacyclic structure. Enzymatic oxidations afforded several orthogonal oxidations to chemical methods. Solubility measurements showed an enhancement for many of the synthesized compounds.     

A new family of 1D exchange biased heterometal single-molecule magnets: observation of pronounced quantum tunneling steps in the hysteresis loops of quasi-linear {Mn2Ni3} clusters

First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.     

Heat load of a GaAs photocathode in an SRF electron gun

A great deal of effort has been made over the last decades to develop a better polarized electron source for high energy physics. Several laboratories operate DC guns with a gallium arsenide photocathode, which yield a highly polarized electron beam. However, the beam's emittance might well be improved by using a superconducting radio frequency (SRF) electron gun, which delivers beams of a higher brightness than that from DC guns because the field gradient at the cathode is higher. SRF guns with metal and CsTe cathodes have been tested successfully. To produce polarized electrons, a Gallium-Arsenide photo-cathode must be used: an experiment to do so in a superconducting RF gun is under way at BNL. Since a bulk gallium arsenide (GaAs) photocathode is normal conducting, a problem arises from the heat load stemming from the cathode. We present our measurements of the electrical resistance of GaAs at cryogenic temperatures, a prediction of the heat load and verification by measuring the quality factor of the gun with and without the cathode at 2 K. We simulate heat generation and flow from the GaAs cathode using the ANSYS program. By following the findings with the heat load model, we designed and fabricated a new cathode holder (plug) to decrease the heat load from GaAs.     

Magnetodielectric effect in Co3O4 nanoparticles grown within a silica glass

Co₃O₄ nanocrystals of sizes in the range 17.5-33.1 nm were grown within a sol-gel-derived silica glass matrix. Rietveld analysis of the X-ray diffraction data showed that the crystals had tetragonal symmetry. This was explained as arising due to a tensile strain induced at the glass-crystal interface as a result of thermal expansion mismatch between the phases. The crystals showed ferromagnetic behaviour with superparamagnetism setting-in at temperatures above 15 K. They also exhibited ferroelectric characteristics with large remanent polarization. Change in dielectric constant as a function of magnetic field was observed in these nanocomposites. This was discussed on the basis of magnetostriction effect.     

Efficient Access to Substituted Silafluorenes by Nickel‐Catalyzed Reactions of Biphenylenes with Et2SiH2

The reaction of biphenylene ( 1 ) with Et₂SiH₂ in the presence of [Ni(PPhMe₂)₄] results in the formation of a mixture of 2‐diethylhydrosilylbiphenyl [ 2 (Et₂HSi)] and 9,9,‐diethyl‐9‐silafluorene ( 3 ). Silafluorene 3 was isolated in 37.5 % and 2 (Et₂HSi) in 36.9 % yield. The underlying reaction mechanism was elucidated by DFT calculations. 4‐Methyl‐9,9‐diethyl‐9‐silafluorene ( 7 ) was obtained selectively from the [Ni(PPhMe₂)₄]‐catalyzed reaction of Et₂SiH₂ and 1‐methylbiphenylene. By contrast, no selectivity could be found in the Ni‐catalyzed reaction between Et₂SiH₂ and the biphenylene derivative that bears tBu substituents in the 2‐ and 7‐positions. Therefore, two pairs of isomers of tBu‐substituted silafluorenes and of the related diethylhydrosilylbiphenyls were formed in this reaction. However, a subsequent dehydrogenation of the diethylhydrosilylbiphenyls with Wilkinson’s catalyst yielded a mixture of 2,7‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 8 ) and 3,6‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 9 ). Silafluorenes 8 and 9 were separated by column chromatography.     

A triad of rhenium-mediated transformations

The title transformations are oxygen atom transfer, twin isomerization and regiospecific imine oxidation.Bispyridyldiazole ligands have furnished new oxygen atom transfer reagents of coordination type Re^VOCl₃(NN) which undergo a slower transfer to PPh₃ than the corresponding azole reagents. The rate of twin isomerization (linkage and geometrical) of meridional azole complexes of coordination type Re^III(OPnP)Cl₃(NN) to facial Re^III(PnPO)Cl₃(NN) decreases rapidly asn increases in the interval 1–4 (PnP is Ph₂P(CH₂)_nPPh₂). An α-diimine chelate of type Re^V(NPh)Cl₃(NN) is shown to undergo facile oxidation to the corresponding iminoamide complex Re_VI(NPh)Cl₃(NN) upon treating with dilute nitric acid. The reaction proceeds via regiospecific nucleophilic addition of waterto the more polarized imine function. Dedicated to Professor C N R Rao on his 70th birthday