Magneto-structural correlation in (mu-alkoxo/hydroxo)(mu-carboxylato)dicopper(II) systems: Synthesis, X-ray structure and magnetic properties of aquo(mu-hydroxo) (mu-arylcarboxylato)bis(N,N,N ',N '-tetramethylethane-1,2-diamine)dicopper(II) diperchlorate

Asymmetrically dibridged dicopper(II) complexes, [Cu-2(OH)(O2CC6H4-p-Me)(tmen)(2)(H2O)](ClO4)(2) (1) and [Cu-2(OH)(O2CC6H4-p-OMe)(tmen)(2)(H2O)](ClO4)(2) (2) (tmen = N,N,N',N'-tetramethylethane-1,2-diamine), were prepared and structurally characterized. Complex 1 crystallizes in the monoclinic space group P2(1)/a with a = 17.718(2), b = 9.869(1), c = 19.677(2) Angstrom, beta = 115.16(1)degrees, V = 3114.3(6) Angstrom(3) and Z = 4. The structure was refined to R(wR(2)) = 0.067(0.178). Complex 2 crystallizes in the monoclinic space group P2(1)/a with a = 17.695(3), b = 9.574(4), c = 20.104(2) Angstrom, beta = 114.18(1)degrees, V = 3107(1) Angstrom(3) and Z = 4. The final residuals are R(wR(2)) = 0.067(0.182). The complexes have a [Cu-2(mu-OH)(mu-OH)(mu-O2CAr)](2+) core with tmen Ligands occupying the terminal sites of the core. In addition, one copper is axially bound to a water molecule. The Cu ... Cu distances and the Cu-OH Cu angles in the core are 3.394(1) Angstrom, 124.4(2)degrees for 1 and 3.374(1) Angstrom, 123.3(3)degrees for 2. The complexes show axial X-band EPR spectral features in methanol glass at 77 K giving g(perpendicular to) = 2.02, g(parallel to) = 2.3 (A(parallel to) = 165 x 10(-4) cm(-1)) and a visible band near similar to 630 nm in methanol. The complexes are weakly antiferromagnetic. A theoretical fit of the magnetic susceptibility data in the temperature range 40-295 K gives -J = 10 cm(-1), g = 2.05 for 1 and -J = 10 cm(-1), g = 2.0 for 2. Plots of -2J versus the Cu-OH-Cu angle (phi) in this class of asymmetrically dibridged dicopper(II) complexes having d(x2-y2)-d(x2-y2) magnetic orbitals show a linear magneto-structural correlation: -2J(cm(-1)) = 11.48 phi(deg) - 1373.

N-6-anisoyladenosine monohydrate

The title compound, C18H19N5O6. H2O, has a syn conformation about the glycosidic bond. Its furanose ring shows a C2'-endo-C3'-exo twist conformation and trans-gauche geometry about the C4'-C5' bond. The angle between the adenine base and the phenyl ring of the anisoyl group is 22.9 degrees. Adenine and anisoyl groups stack along the b axis at a separation of 3.4 Angstrom.

Pronounced hydrogel formation by the self-assembled aggregates of N-alkyl disaccharide amphiphiles

Six disaccharide amphiphiles were synthesized and their hydrogel-forming behavior was extensively studied. These amphiphiles were based on maltose and lactose. Since the gels formed from some of these systems showed the ability to "trap" water molecules upon gelation, these gels were described as "hydrogels". When these gels were heated to similar to 70 degrees C, the samples turned into clear, isotropic fluids, and upon gradual cooling, the hydrogels could be reproduced. Thus these systems were also "thermoreversible". The low molecular mass (MW 565) of the gelators compared to that of a typical polymeric gelator forming substance implies pronounced aggregation of the disaccharide amphiphiles into larger microstructures during gelation. To discern the aggregate textures and morphologies, the specimen hydrogel samples were examined by high-resolution scanning electron microscopy (SEM). A possible reason for the exceptionally high water gelating capacities (>6000 molecules of water per gelator molecule) exhibited by these N-alkyl disaccharide amphiphiles is the presence of large interlamellar spaces into which the water molecules get entrapped due to surface tension. In contrast to their single-chain counterparts, the double-chain lactosyl and maltosylamine amphiphiles upon solubilization in EtOH-H2O afforded hydrogels with reduced mechanical strengths. Interestingly, the corresponding microstructures were found to be quite different from the corresponding hydrogels of their single-chain counterparts. Rheological studies provided further insights into the behavior of these hydrogels. Varying the chain length of the alcohol cosolvent could modulate the gelation capacities, melting temperatures, and the mechanical properties of these hydrogels. To explain the possible reasons of gelation, the results of molecular modeling and energy minimization studies were also included.

Thermal analysis studies on hydroxobridged homo and hetero trinuclear cobalt(III) complexes

The synthesis and thermal analysis studies of several hydroxobridged homo and hetero trinuclear cobalt(III) complexes are reported. The complexes are of the type [M(H2O)(x) {(OH)(2)Co(en)(2)}(2)](SO4)(2). nH(2)O and [M(H2O)(x){(OH)(2)Co(NH3)(4)}(2)] (SO4)(2). nH(2)O where en denotes ethylenediamine and M =Co(II), Ni(II), Cu(II) and Zn(II) with x=0 for Cu(II), and 2 for other metal ions, and n =3, 4 or 5. The TG and DTA studies of these compounds show that one or more intermediate compounds are formed in each case before the metal oxides are produced.

Studies on Cu/CeO2: A new NO reduction catalyst

Fine particle and large surface area Cu/CeO2 catalysts of crystallite sizes in the range of 100-200 Angstrom synthesized by the solution combustion method have been investigated for NO reduction. Five percent Cu/CeO2 catalyst shows nearly 100% conversion of NO by NH3 below 300 degrees C, whereas pure ceria and Zr, Y, and Ca doped ceria show 85-95% NO conversion above 600 degrees C. Similarly NO reduction by CO has been observed over 5% Cu/CeO2 with nearly 100% conversion below 300 degrees C. Hydrocarbon (n-butane) oxidation by NO to CO2, N-2, and H2O has also been demonstrated over this catalyst below 350 degrees C making Cu/CeO2 a new NO reduction catalyst in the low temperature window of 150-350 degrees C. Kinetics of NO reduction over 5% Cu/CeO2 have also been investigated. The rate constants are in the range of 1.4 x 10(4) to 2.3 x 10(4) cm(3) g(-1) s(-1) between 170 and 300 degrees C. Cu/CeO2 catalysts are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy where Cu2+ ions are shown to be dispersed on the CeO2 surface. (C) 1999 Academic Press.

An open-framework zincoborate formed by Zn6B12O24 clusters

A novel zincoborate, Zn(H2O)B2O4.xH(2)O (xapproximate to0.12), I, with open architecture has been synthesized hydrothermally. The 3-dimensional structure is built up of Zn6B12O24 clusters formed by the capping of the polycyclic borate anion, B12O2412-, by Zn3O3 clusters. The open-framework structure of I has one-dimensional 8-membered channels wherein the water molecules reside. Formation of trimeric Zn3O3 clusters as well as the presence of boron in dual coordination, both triangular and tetrahedral, are important structural features of this new zincoborate.

Effect of molecular association and reactivity on substitution in chromium(III)-salprn complexes

A new chromium(III)-Schiff base complex, [Cr(5-chlorosalprn)(H2O)(2)]ClO4, where salprn=N,N'-propylenebis(salicylideneimine) has been prepared and characterized by electrospray ionization mass spectrometric (ESIMS) analysis and other spectroscopic techniques. Single crystal X-ray data reveal that the complex assumes a trans-diaquo structure, [Cr(C17H18Cl2N2O4)]ClO4.H2O. The effect of phenyl ring substituents on the rate of formation of [O=Cr-V Schiff base](+) has been investigated. The bimolecular rate constant for the formation of O=Cr-V species by the [Cr(Schiff base)(H2O)(2)]ClO4, where the Schiff base=salprn, (1) and 5-chlorosalprn, (2) with PhOI was compared. In the case of (2) the rate was found to be faster by an order of magnitude at pH=4 compared to (1). The introduction of a chloro-substituent on the phenyl ring not only influences the rate of redox reactivity but also the pKa values of aquo ligands of the complexes, indicating the difference in the electronic environment around the metal ion in both (1) and (2).

Half-sandwich (eta(6)-arene)ruthenium(II) chiral Schiff base complexes: Analysis of the diastereomeric mixtures in solution by 2D-NMR spectroscopy

2D NMR spectroscopy has been used to determine the metal configuration in solution of three complexes, viz. [(eta(6)-p-cymene)Ru(L*)Cl] (1) and [(eta(6)-p-cymene)Ru(L*)(L')] (ClO4) (L' = H2O, 2; PPh3, 3), where L* is the anion of (S)-(1-phenylethyl)salicylaldimine. The complexes exist in two diastereomeric forms in solution. Both the (R-Ru,S-C)- and (S-Ru,S-C)-diastereomers display the presence of attractive, CH/pi interaction involving the phenyl group attached to the chiral carbon and the cymene ring hydrogens. This interaction restricts the rotation of the C*-N single bond and, as a result, two structural types with either the hydrogen atom attached to the chiral carbon (C*) or the methyl group attached to C* in close proximity of the cymene ring protons get stabilized. Using 2D NMR spectroscopy as a tool, the spatial interaction involving these protons are studied in order to obtain the metal configuration(s) of the diastereomeric complexes in solution. This technique has enabled us to determine the metal configuration as (R-Ru,S-C) for the major isomers of 1-3 in solution.

Organically templated linear and layered cadmium sulfates

By reacting cadmium salts with H2SO4 in the presence of organic amines or directly with amine sulfates under hydrothermal conditions, it has been possible to prepare three linear cadmium sulfates of linarite topology, with the compositions [H3N(CH2)(2)NH3](2)[CdCl2(SO4)][SO4].H2O, I, [HN(CH2)(6)NH][CdBr2(SO4)], II, [HN(CH2)(6)NH][CdCl2-(SO4)], III. A layered cadmium sulfate of composition [H3N(CH2)(3)NH3][Cd-2(H2O)(2)(SO4)(3)], IV, has also been obtained. These sulfates are the first examples of a family of organically templated metal sulfates with interesting structural features. In the linarite chains, the CdX4O2 (X = Cl, Br) octahedron shares two trans-edges to form an [Mphi(4)] (phi = anionic ligand) chain decorated by the SO4 tetrahedron that adopts a staggered arrangement on either side of the chain. IV is constructed by the fusion of four-membered ring ladders involving edge sharing between the sulfate tetrahedron and metal octahedron. IV appears to be the first member of a family of organically templated metal sulfates containing an octahedral-tetrahedral 2D net wherein the sulfate tetrahedron is connected at all four corners.

Oxidation and decomposition of NH3 over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts

The catalytic oxidation and decomposition of NH3 have been carried out over combustion synthesized Al2O3 and CeO2 supported Pt, Pd and Ag catalysts using temperature programmed reaction (TPR) technique in a packed bed tubular reactor. Metals are ionically dispersed over CeO2 and fine metal particles are found on Al2O3. NH3 oxidation occurs over 1% Pt/Al2O3, 1% Pd/Al2O3 and 1% Ag/Al2O3 at 175, 270 and 350 C respectively producing N-2, NO, N2O and H2O, whereas 1% Pt/CeO2, 1% Pd/CeO2 and 1% Ag/CeO2 give N-2 along with NO, N2O and H2O at 200, 225 and 250degreesC respectively. N-2 predominates over other nitrogen-containing products during the reaction on all catalysts. At less O-2 concentration, N-2 and H2O are the only products obtained during NH3 Oxidation. NH3 decomposition over all the catalysts occurs above 450degreesC.

Peroxo-bridged divanadate as selective bromide oxidant in bromoperoxidation

Diperoxovanadate is effective only in presence of free vanadate in vanadium-dependent bromoperoxidation at physiological pH. Peroxide in the form of bridged divanadate complex (VOOV-type), but not the bidentate form as in diperoxovanadate, is proposed to be the oxidant of bromide. In order to obtain direct evidence, peroxo-divanadate complexes with glycyl-glycine, glycyl-alanine and glycyl-asparagine as heteroligands were synthesized. By elemental analysis and spectral studies they were characterized to be triperoxo-divanadates, [V2O2(O-2)(3)(peptide)(3)]. H2O, with the two vanadium atoms bridged by a peroxide and a heteroligand. The dipeptide seems to stabilize the peroxo-bridge by inter-ligand interaction, possibly hydrogen bonding. This is indicated by rapid degradation of these compounds on dissolving in water with partial loss of peroxide accompanied by release of bubbles of oxygen. The V-51-NMR spectra of such solutions showed diperoxovanadate and decavanadate (oligomerized from vanadate) as the products. Additional oxygen was released on treating these solutions with catalase as expected of residual diperoxovanadate. The solid compounds when added to the reaction mixtures showed transient, rapid bromoperoxidation reaction, but not oxidation of NADH or inactivation of glucose oxidase, the other two activities shown by a mixture of diperoxovanadate and vanadyl. This demonstration of peroxide-bridged divanadate as powerful, selective oxidant of bromide, active at physiological pH, should make it a possible candidate of mimic in the action of vanadium in bromoperoxidase proteins.

Rayleigh-Benard convection during solidification of an eutectic solution cooled from the top

The interaction between laminar Rayleigh-Benard convection and directional solidification is studied for the case of an eutectic solution kept in a rectangular cavity cooled from the top. Experiments and numerical simulations are carried out using an NH4Cl-H2O solution as the model fluid. The flow is visualized using a sheet of laser light scattered by neutrally buoyant, hollow-glass spheres seeded in the fluid. The numerical modeling is performed using a pressure-based finite-volume method according to the SIMPLER algorithm. The present configuration enables us to visualize flow vortices in the presence of a continuously evolving solid/liquid interface. Clear visualization of the Rayleigh-Benard convective cells and their interaction with the solidification front are obtained. It is observed that the convective cells are characterized by zones of up-flow and down-flow, resulting in the development of a nonplanar interface. Because of the continuous advancement of the solid/liquid interface, the effective liquid height of the cavity keeps decreasing. Once the height of the fluid layer falls below a critical value, the convective cells become weaker and eventually die out, leading to the growth of a planar solidification front. Results of flow visualization and temperature measurement are compared with those from the numerical simulation, and a good agreement is found.

Synthesis of acicular hydrogoethite (alpha-FeOOH center dot xH(2)O; 0.1 < x < 0.22) particles using morphology controlling cationic additives and magnetic properties of maghemite derived from hydrogoethite

A method for the preparation of acicular hydrogoethite (alpha -FeOOH.xH(2)O, 0.1 < x < 0.22) particles of 0.3-1 mm length has been optimized by air oxidation of Fe( II) hydroxide gel precipitated from aqueous (NH4)(2)Fe(SO4)(2) solutions containing 0.005-0.02 atom% of cationic Pt, Pd or Rh additives as morphology controlling agents. Hydrogoethite particles are evolved from the amorphous ferrous hydroxide gel by heterogeneous nucleation and growth. Preferential adsorption of additives on certain crystallographic planes thereby retarding the growth in the perpendicular direction, allows the particles to acquire acicular shapes with high aspect ratios of 8-15. Synthetic hydrogoethite showed a mass loss of about 14% at similar to 280 degreesC, revealing the presence of strongly coordinated water of hydration in the interior of the goethite crystallites. As evident from IR spectra, excess H2O molecules (0.1- 0.22 per formula unit) are located in the strands of channels formed in between the double ribbons of FeO6 octahedra running parallel to the c- axis. Hydrogoethite particles constituted of multicrystallites are formed with Pt as additive, whereas single crystallite particles are obtained with Pd (or Rh). For both dehydroxylation as well as H-2 reduction, a lower reaction temperature (similar to 220 degreesC) was observed for the former (Pt treated) compared to the latter (Pd or Rh) (similar to 260 degreesC). Acicular magnetite (Fe3O4) was prepared either by reducing hydrogoethite (magnetite route) or dehydroxylating hydrogoethite to hematite and then reducing it to magnetite (hematite- magnetite route). According to TEM studies, preferential dehydroxylation of hydrogoethite along < 010 > leads to microporous hematite. Maghemite (gamma -Fe2O3 (-) (delta), 0 < < 0.25) was obtained by reoxidation of magnetite. The micropores are retained during the topotactic transformation to magnetite and finally to maghemite, whereas cylindrical mesopores are formed due to rearrangement of the oxygen sublattice from hexagonal to cubic close packing during the conversion of hydrogoethite to magnetite and then to maghemite. Accordingly, three different types of maghemite particles are realized: strongly oriented multicrystalline particles, single crystalline acicular particles with micropores or crystallites having mesopores. Higher values of saturation magnetization ((s) = 74 emu g(-1)) and coercivity (H-c = 320 Oe) are obtained for single crystalline mesoporous particles. In the other cases, the smaller size of particles and larger distribution of micropores decreases sigma (s) considerably ( < 60 emu g(-1)) due to relaxation effects of spins on the surface atoms as revealed by Mossbauer spectroscopy.

A layered chlorophosphate, Na-3[Cd4Cl3(HPO4)(2)(H2PO4)(4)], containing Na+ ions in the interlamellar space

Reaction between CdCl2.H2O and NaH2PO4.H2O Under hydrothermal conditions gives rise to a new cadmium chlorophosphate of the formula Na-3[Cd4Cl3(HPO4)(2)(H2PO4)(4)] I. This material crystallizes in the orthorhombic system with space group Fmm2(no. 42). I has macroanionic layers of [Cd4Cl3(HPO4)(2)(H2PO4)(4)](3-) with Na+ ions in the interlamellar space. The discovery of such compounds suggests that metathetic reactions carried out under hydrothermal conditions may provide a novel route for the synthesis of new open-framework structures.

Internal displacement reactions in multicomponent oxides: Part I. Line compounds with narrow homogeneity range

Structure and phase transition of LaO1−xF1+2x, prepared by solid-state reaction of La2O3 and LaF3, was investigated by X-ray powder diffraction and differential scanning calorimetry for both positive and negative values of the nonstoichiometric parameter x. The electrical conductivity was investigated as a function of temperature and oxygen partial pressure using AC impedance spectroscopy. Fluoride ion was identified as the migrating species in LaOF by coulometric titration and transport number determined by Tubandt technique and EMF measurements. Activation energy for conduction in LaOF was 58.5 (±0.8) kJ/mol. Conductivity increased with increasing fluorine concentration in the oxyfluoride phase, suggesting that interstitial fluoride ions are more mobile than vacancies. Although the values of ionic conductivity of cubic LaOF are lower, the oxygen partial pressure range for predominantly ionic conduction is larger than that for the commonly used stabilized-zirconia electrolytes. Thermodynamic analysis shows that the oxyfluoride is stable in atmospheres containing diatomic oxygen. However, the oxyfluoride phase can degrade with time at high temperatures in atmospheres containing water vapor, because of the higher stability of HF compared with H2O.