Reactions of trivacant Wells-Dawson heteropolytungstates. Ionic strength and Jahn-Teller effects on formation in multi-iron complexes

Reaction of alpha-P(2)W(15)O(56)(12-) and Fe(III) in a saturated NaCl solution produces a trisubstituted Wells-Dawson structure with three low-valent metals, alpha-(Fe(III)Cl)(2)(Fe(III)OH(2))P(2)W(15)O(59)(11-) (1). Dissolution of this species into 1 M NaBr (Br(-) is non-coordinating) gives the triaquated species alpha-(Fe(III)OH(2))(3)P(2)W(15)O(59)(9-) (2). Ionic strength values of 1 M or greater are necessary to avoid decomposition of 1 or 2 to the conventional sandwich-type complex, alpha beta beta alpha-(Fe(III)OH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(12-) (3). If the pH is greater than 5, a new triferric sandwich, alpha alpha beta alpha-(NaOH(2))(Fe(III)OH(2))Fe(III)(2)(P(2)W(15)O(56))(2)(14-) (4), forms rather than 3. Like the previously reported Wells-Dawson-derived sandwich-type structures with three metals in the central unit ([TM(II)Fe(III)(2)(P(2)W(15)O(56))(P(2)TM(II)(2)W(13)O(52))],(16-) TM = Cu, Co), this complex has a central alpha-junction and a central beta-junction. Thermal studies suggest that 4 is more stable than 3 over a wide range of temperatures and pH values. The intrinsic Jahn-Teller distortion of d-electron-containing metal ions incorporated into the external sites of the central multi-metal unit impacts the stoichiometry of their incorporation (with a consequent change in the inter-POM-unit connectivity, where POM = polyoxometalate). Reaction of non-distorting Ni(II) with the diferric lacunary sandwich-type POM alpha alpha alpha alpha-(NaOH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(16-) (5) produces alpha beta beta alpha-(Ni(II)OH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(14-) (6), a Wells-Dawson sandwich-type structure with two Ni(II) and two Fe(III) in the central unit. All structures are characterized by (31)P NMR, IR, UV-vis, magnetic susceptibility, and X-ray crystallography. Complexes 4 and 6 are highly selective and effective catalysts for the H(2)O(2)-based epoxidation of alkenes.     

The Suzuki coupling of aryl chlorides in TBAB-water mixtures

Palladium acetate in a mixture of TBAB and water can be used as an effective catalyst for the Suzuki coupling of deactivated aryl chloride substrates.     

A comparative study of some lanthanon chelates of alizarin complexan as reagents for fluoride

A study has been made of the comparative suitability of the cerium^III, lanthanum and praseodymium chelates of alizarin complexan as reagents for the spectrophotometric determination of microgram amounts of fluoride. The cerium^III reagent is most sensitive at pH < 4.5 and the lanthanum reagent at pH 5.0. An enhancement of sensitivity may be obtained for both reagents at pH 4.3 by addition of acetone to 25 % v/v, but the most sensitive means of determination is to use the lanthanum reagent in aqueous solution at pH 5.2 with measurement at 281 mμ. This procedure is 200% more sensitive than the standard method at 620 mμ.     

The infrared spectra (600-150 cm−1 of pyridine N-oxide complexes of metal(II) halides and mixed-ligand (pyridine N-oxide—dimethylsulphoxide) complexes of copper(II) halides

The infrared spectra of the complexes [M(pyO)(H₂O)Cl₂] (M = Mn, Fe, Co, Ni, Cu; pyO = pyridine N-oxide) have been determined. Assignments of ν M-Opy, νM-OH₂ and ν M-Cl are made by observing the effects of deuterating the coordinated pyO and H₂O and replacing chloride by bromide in the Mn(II) complex. Assignments of metal—ligand modes in the mixed ligand complexes [M(pyO)(dmso)X₂] (dmso = dimethylsulphoxide) are made by comparison with the spectra of (ML₂X₂] (L = pyO, dmso) and by observing the effects of deuteration of pyO and dmso. Structural aspects of the spectra are discussed.     

Instrumental barriers in biological Fourier transform infrared spectroscopy

Biological applications of infrared spectroscopy have pressed for ever greater instrumental capabilities in terms of spectral sensitivity and quantitative exactness. Improved instrumentation has provided measurement of many vibrational modes in biological samples that previously were lost in noise. With highly optimized sampling conditions, useful measurements have been made with a peak-to-peak noise level less than 5 microabsorbance (5×10^–6 absorbance), at 0.5 cm^–1 resolution. However, optical and instrumental instabilities often result in sine waves that are not totally removed by the ratio of sample to reference. These often limit effective spectral sensitivity to 50 or 100 microabsorbance, peak-to-peak, and constitute a non-random noise. Non-atmospheric absorptions, especially one at 1959 cm^–1 with 0.8 cm^–1 band width (FWHM) are reported. The latter is due to a trace impurity in the KBr beam splitter substrate and compensator plate. Improvements in instrumentation and sampling conditions are expected to yield measurements of absorption bands as small as 50 microabsorbance with excellent signal/noise.     

Surface modification of nanoporous alumina surfaces with poly(ethylene glycol)

Nanoporous alumina surfaces have a variety of applications in biosensors, biofiltration, and targeted drug delivery. However, the fabrication route to create these nanopores in alumina results in surface defects in the crystal lattice. This results in inherent charge on the porous surface causing biofouling, that is, nonspecific adsorption of biomolecules. Poly(ethylene glycol) (PEG) is known to form biocompatible nonfouling films on silicon surfaces. However, its application to alumina surfaces is very limited and has not been well investigated. In this study, we have covalently attached PEG to nanoporous alumina surfaces to improve their nonfouling properties. A PEG-silane coupling technique was used to modify the surface. Different concentrations of PEG for different immobilization times were used to form PEG films of various grafting densities. X-ray photoelectron spectroscopy (XPS) was used to verify the presence of PEG moieties on the alumina surface. High-resolution C1s spectra show that with an increase in concentration and immobilization time, the grafting density of PEG also increases. Further, a standard overlayer model was used to calculate the thickness of PEG films formed using the XPS intensities of the Al2p peaks. The films formed by this technique are less than 2.5 nm thick, suggesting that such films will not clog the pores which are in the range of 70-80 nm.     

Synthesis and DNA binding properties of iminodiacetic acid-linked polyamides: characterization of cooperative extended 2:1 side-by-side parallel binding

A series of iminodiacetic acid (IDA)-linked polyamides (DpPyPyPy-IDA-PyPyPyDp) were prepared and constitute polyamides joined head-to-head by a functionalizable five-atom linker. It was found that the IDA linker exerts a unique influence over the DNA binding conformation differing from both the beta-alanine (extended) or gamma-aminobutyric acid (hairpin) linkers, resulting in cooperative parallel side-by-side 2:1 binding in an extended conformation most likely with a staggered versus stacked alignment. A generalized variant of a fluorescent intercalator displacement (FID) assay conducted on a series of hairpin deoxyoligonucleotides containing a systematically varied A/T-rich binding-site size was used to distinguish between the binding modes of the IDA-linked polyamides.     

Imaging the quantum-state specific differential cross sections of HCl formed from reactions of chlorine atoms with methanol and dimethyl ether

Center-of-mass frame scattering angle distributions obtained directly from crossed molecular beam velocity map images are reported for HCl formed in different rotational levels of its vibrational ground state by reaction of Cl atoms with CH3OH and CH3OCH3. Products are observed to scatter over all angles, with peaks in the distribution in the forward and backward directions (theta = 0 and 180 degrees with respect to the relative velocity vectors of the Cl atoms). Products of both reactions exhibit differential cross sections that vary with the rotational quantum number of the HCl, with a greater propensity for forward scatter for J = 2, shifting to more pronounced backward scatter for J = 5. This trend is, however, more evident for reaction of dimethyl ether than for methanol. The mean fractions of the available energy channeled into product kinetic energy vary with scattering angle, but the angle-averaged fractions are, respectively, 0.37 and 0.42 for the methanol and dimethyl ether reactions. On average, 46% or more of the available energy of the reactions becomes internal energy of the radical co-product. Results are interpreted with the aid of computed energies of transition states and molecular complexes along the reaction pathways, and comparisons are drawn with recent measurements of the scattering distributions and energy release for reactions of Cl atoms with small alkanes.