Novel polymer-derived carbide and boride refractory ceramics

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Novel polymer-derived carbide and boride refractory ceramics

Please click Additional Files below to see the full abstrac

Triggering redox activity in a thiophene compound: radical stabilization and coordination chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+

Alkyl Rearrangement Processes in Organozirconium Complexes. Observation of Internal Alkyl Complexes during Hydrozirconation

Isotopically labeled alkyl zirconocene complexes of the form (CpR_n)_2Zr(CH_2CDR‘_2)(X) (CpR_n = alkyl-substituted cyclopentadienyl; R‘ = H, alkyl group; X = H, D, Me) undergo isomerization of the alkyl ligand as well as exchange with free olefin in solution under ambient conditions. Increasing the substitution on the Cp ring results in slower isomerization reactions, but these steric effects are small. In contrast, changing X has a very large effect on the rate of isomerization. Pure σ-bonding ligands such as methyl and hydride promote rapid isomerization, whereas π-donor ligands inhibit β-H elimination and hence alkyl isomerization. For (η^5-C_5H_5)2Zr(R)(Cl), internal alkyl complexes have been observed for the first time. The rate of isomerization depends on the length of the alkyl group:  longer alkyl chains (heptyl, hexyl) isomerize faster than shorter chains (butyl). The transient intermediate species have been identified by a combination of isotopic labeling and ^1H, ^2H, and ^(13)C NMR experiments. The solid-state structure of the zirconocene cyclopentyl chloride complex, Cp_2Zr(cyclo-C_5H_9)(Cl), has been determined by X-ray diffraction

The Formazanate Ligand as an Electron Reservoir: Bis(Formazanate) Zinc Complexes Isolated in Three Redox States

The synthesis of bis(formazanate) zinc complexes is described. These complexes have well-behaved redox-chemistry, with the ligands functioning as a reversible electron reservoir. This allows the synthesis of bis(formazanate) zinc compounds in three redox states in which the formazanate ligands are reduced to "metallaverdazyl" radicals. The stability of these ligand-based radicals is a result of the delocalization of the unpaired electron over four nitrogen atoms in the ligand backbone. The neutral, anionic, and dianionic compounds (L2Zn0/-1/-2) were fully characterized by single-crystal X-ray crystallography, spectroscopic methods, and DFT calculations. In these complexes, the structural features of the formazanate ligands are very similar to well-known β-diketiminates, but the nitrogen-rich (NNCNN) backbone of formazanates opens the door to redox-chemistry that is otherwise not easily accessible. N is better than C: Bis(formazanate) zinc complexes (see picture; Zn yellow, N blue, O red, Na green) show sequential and reversible redox chemistry in which the formazanate ligands are reduced to metallaverdazyl radicals. These ligands are very similar to β- diketiminates, but the nitrogen-rich NNCNN backbone of formazanates opens the door to redox chemistry that is otherwise difficult to access

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

The mechanism of the asymmetric hydrogenation of prochiral enamides by well-defined, neutral bis(phosphine) cobalt(0) and cobalt(II) precatalysts has been explored using(R,R)-iPrDuPhos ((R,R)-iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) as a representative chiral bis(phosphine) ligand. A series of (R,R)-(iPrDuPhos)Co(enamide) (enamide = methyl-2-acetamidoacrylate (MAA), methyl(Z)-α-acetamidocinnamate (MAC), and methyl(Z)-acetamido(4-fluorophenyl)acrylate (4FMAC)) complexes (1-MAA, 1-MAC, and 1-4FMAC), as well as a dinuclear cobalt tetrahydride, [(R,R)-(iPrDuPhos)Co]2(μ2-H)3(H) (2), were independently synthesized, characterized, and evaluated in both stoichiometric and catalytic hydrogenation reactions. Characterization of (R,R)-(iPrDuPhos)Co(enamide) complexes by X-ray diffraction established the formation of the pro-(R) diastereomers in contrast to the (S)-alkane products obtained from the catalytic reaction. In situ monitoring of the cobalt-catalyzed hydrogenation reactions by UV–visible and freeze-quench electron paramagnetic resonance spectroscopies revealed (R,R)-(iPrDuPhos)Co(enamide) complexes as the catalyst resting state for all the three enamides studied. Variable time normalization analysis kinetic studies of the cobalt-catalyzed hydrogenation reactions in methanol established a rate law that is first order in (R,R)-(iPrDuPhos)Co(enamide) and H2 but independent of the enamide concentration. Deuterium-labeling studies, including measurement of an H2/D2 kinetic isotope effect and catalytic hydrogenations with HD, established an irreversible H2 addition step to the bound enamide. Density functional theory calculations support that this step is both rate and selectivity determining. Calculations, as well as HD-labeling studies, provide evidence for two-electron redox cycling involving cobalt(0) and cobalt(II) intermediates during the catalytic cycle. Taken together, these experiments support an unsaturated pathway for the [(R,R)-(iPrDuPhos)Co]-catalyzed hydrogenation of prochiral enamides

Triggering Redox Activity in a Thiophene Compound: Radical Stabilization and Coordination Chemistry

The synthesis, metalation, and redox properties of an acyclic bis(iminothienyl)methene L− are presented. This π-conjugated anion displays pronounced redox activity, undergoing facile one-electron oxidation to the acyclic, metal-free, neutral radical L* on reaction with FeBr2. In contrast, reaction of L− with CuI forms the unique, neutral Cu2I2(L*) complex of a ligand-centered radical, whereas reaction with the stronger oxidant AgBF4 forms the metal-free radical dication L*2+

After-effects of geomagnetic storms: statistical analysis and theoretical explanation

Our previous studies have shown the presence of daytime positive electron density disturb-ances during several days after the start of the recovery phase. The aim of this paper is to study after-effects of geomagnetic storms (after-storm effects), i.e. ionospher-ic effects observed on the 3–5th day after the beginning of the storm recovery phase. From numerical calcula-tions with the GSM TIP model, we have found the main mechanisms for the formation of the after-storm effects. Using Irkutsk (52° N, 104° E) and Kaliningrad (54° N, 20° E) ionosonde data, we have carried out a statistical analysis of daytime ionospheric responses to geomagnetic storms. As a result of the analysis, we obtained averaged ionospheric responses at the beginning of the storm recovery phase and for five consecutive days. The statistical analysis results received near the beginning of the recovery phase are in good agreement with the well-known ionospheric effects of geomagnetic storms obtained in previous studies. For the first time, the obtained statistics of iono-spheric responses observed on the 3–5th day after the beginning of the recovery phase allowed us to reveal the dependence of after-storm ionospheric effects on season, storm intensity, and ionosonde geomagnetic latitude. In addition, we for the first time present the interpretation of after-storm ionospheric effects from numerical simulation results

Alkyl Rearrangement Processes in Organozirconium Complexes. Observation of Internal Alkyl Complexes during Hydrozirconation

Isotopically labeled alkyl zirconocene complexes of the form (CpR_n)_2Zr(CH_2CDR‘_2)(X) (CpR_n = alkyl-substituted cyclopentadienyl; R‘ = H, alkyl group; X = H, D, Me) undergo isomerization of the alkyl ligand as well as exchange with free olefin in solution under ambient conditions. Increasing the substitution on the Cp ring results in slower isomerization reactions, but these steric effects are small. In contrast, changing X has a very large effect on the rate of isomerization. Pure σ-bonding ligands such as methyl and hydride promote rapid isomerization, whereas π-donor ligands inhibit β-H elimination and hence alkyl isomerization. For (η^5-C_5H_5)2Zr(R)(Cl), internal alkyl complexes have been observed for the first time. The rate of isomerization depends on the length of the alkyl group:  longer alkyl chains (heptyl, hexyl) isomerize faster than shorter chains (butyl). The transient intermediate species have been identified by a combination of isotopic labeling and ^1H, ^2H, and ^(13)C NMR experiments. The solid-state structure of the zirconocene cyclopentyl chloride complex, Cp_2Zr(cyclo-C_5H_9)(Cl), has been determined by X-ray diffraction