Mixed valence in YBaFe(2)O(5)

YBaFe(2)O(5) has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe(2)O(5) adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO(5) square pyramids separated by Y(3+) ions. At T(N) congruent with 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe(2)O(5) orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions. The magnetic structure is characterized by AFM Fe-O-Fe superexchange coupling within the double layers and a ferromagnetic Fe-Fe direct-exchange coupling between neighboring double layers. Upon cooling below approximately 335 K, a premonitory charge ordering (2Fe(2.5+) --> Fe(2.5+delta) + Fe(2.5)(-delta)) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At approximately 308 K, a complete charge ordering (2Fe(2.5+) --> Fe(2+) + Fe(3+)) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe(2)O(5). The magnetic structure rearranges to a G-type AFM structure, where both the Fe-O-Fe superexchange and the Fe-Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe(2+) and Fe(3+) running along b and an unexpectedly large cooperative Jahn-Teller distortion about the high-spin Fe(2+) ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe(2+) d(xz) orbitals. Comparisons with YBaMn(2)O(5) and YBaCo(2)O(5) are made to highlight the impact of changing the d-electron count.     

Highly enantioselective conjugate addition of AlMe3 to linear aliphatic enones by a designed catalyst

2-Hydroxy-2'-alkylthio-1,1'-binaphthyl compounds are catalytic promoters of the 1,4-addition of AlMe(3) to linear aliphatic enones in THF at -40 to -48 degrees C in the presence of [Cu(MeCN)(4)]BF(4). At ligand loadings of 5-20 mol %, enantioselectivities of 80-93 % are realised for most substrates. To attain these values, the use of highly pure AlMe(3) is mandatory. The presence of methylalumoxane (MAO), derived by hydrolysis, leads to reduced enantioselectivity and a conjugate addition product.     

Lewis acid mediated cyclisations of silylated methylenecyclopropyl hydrazones

Silylated methylenecyclopropyl hydrazones on treatment with BF3 x Et2O cyclise to give heterocyclic products involving a novel sequence of hydride and silyl shifts via a series of increasingly stable cationic intermediates.     

Introduction to the proceedings of the thirteenth symposium on biotechnology for fuels and chemicals

Applied Biochemistry and Biotechnology -     

Reductive Cross-Coupling of a Vinyl Thianthrenium Salt and Secondary Alkyl Iodides

We report the first reductive vinylation of alkyl iodides. The reaction uses a vinyl thianthrenium salt, a palladium catalyst, and an alkyl zinc intermediate formed in situ to trap the L_nPd^II(vinyl) complex formed after oxidative addition before it undergoes undesired homocoupling to form butadiene.     

Hydrolysis of cellobiose by immobilized β-glucosidase entrapped in maintenance-free gel spheres

A crude preparation of Aspergillus niger β-glucosidase (27.5 cello-biase U/mg protein at 40°C, pH 5.0) was immobilized on concanavalin A-Sepharose (CAS). The cellobiase activity of the immobilized enzyme was 1334 U/mg dried CAS or 108 U/mL CAS gel. The β-glucosidase-CAS complex was entrapped within crosslinked propylene glycol alginate/bone-geletin gel spheres that possessed between 0.67 and 2.35 cellobiase U/mL spheres, depending on their size. The effect of cellobiose concentration (10–300 mM) on the activity of native, immobilized, and gel-entrapped enzyme was determined. It was shown that concentrations of cellobiose between 10 and 180 mM were not inhibitory to the entrapped enzyme, although inhibition was found to occur with the native and immobilized enzyme. Exogenous ion addition was not necessary to maintain the structural integrity of the spheres, which were stable for 4 d at 40°C.     

System Maps for Retention of Small Neutral Compounds on a Superficially Porous Ethyl-Bridged,Octadecylsiloxane-Bonded Silica Stationary Phase in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an ethyl-bridged, ocatadecylsiloxane-bonded superficially porous silica stationary phase (Kinetex EVO C18) for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not methanol–water mobile phase compositions. Compared with a superficially porous octadecylsiloxane-bonded silica stationary phase (Kinetex C18) with a similar morphology but different topology statistically significant differences in selectivity at the 95% confidence level are observed for neutral compounds that vary by size and hydrogen-bond basicity with other intermolecular interactions roughly similar. These selectivity differences are dampened with acetonitrile–water mobile phases, but are significant for methanol–water mobile phase compositions containing <30% (v/v) methanol. A comparison of a totally porous ethyl-bridged, octadecylsiloxane-bonded silica stationary phase (XBridge C18) with Kinetex EVO C18 indicated that they are effectively selectivity equivalent.     

Solvent- and metal-free selective oxidation of thiols to disulfides using I2/DMSO catalytic system

We describe herein a simple, fast and inexpensive protocol for the oxidative coupling of thiols employing a stoichiometric amount of DMSO and iodine as the catalyst. Various aromatic disulfides were obtained in good to excellent yields in short reaction times at room temperature, while aliphatic disulfides were achieved in good yields when the reactions were conducted under microwave irradiation.     

Synthese und Kristallstruktur von [(PhCH2)2GaF(tBuNH2)]2 · 2 THF

Synthesis and Crystal Structure of [(PhCH₂)₂GaF(^tBuNH₂)]₂ · 2 THF (PhCH₂)₂GaF reacts with ^tBuNH₂ to the adduct [(PhCH₂)₂GaF(^tBuNH₂)] ( 1 ). 1 was characterized by NMR, IR and MS techniques. 1 can be recrystallized from THF forming crystals of [ 1 ]₂ · 2 THF. According to an X-ray structure analysis [ 1 ]₂ · 2 THF consists of dimers of 1 formed by hydrogen bridges. The THF molecules are coordinated to [ 1 ]₂ by hydrogen bridges, too.     

Uncatalyzed,Regioselective Oxidation of Saturated Hydrocarbons in an Ambient Corona Discharge

Atmospheric pressure chemical ionization (APCI) in air or in nitrogen with just traces of oxygen is shown to yield regioselective oxidation, dehydrogenation, and fragmentation of alkanes. Ozone is produced from ambient oxygen in situ and is responsible for the observed ion chemistry, which includes partial oxidation to ketones and C?C cleavage to give aldehydes. The mechanism of oxidation is explored and relationships between ionic species produced from individual alkanes are established. Unusually, dehydrogenation occurs by water loss. Competitive incorporation into the hydrocarbon chain of nitrogen versus oxygen as a mode of ionization is also demonstrated.