Germaborenes: Borylene Transfer Agents for the Synthesis of Iminoboranes

Halide and phenyl substituted germaborenes were shown to react with azides at room temperature and transfer a borylene moiety to give iminoboranes. This iminoborane synthesis based on a borylene transfer route was investigated computationally in the case of the phenyl substituted germaborene.     

Phosphine-Stabilized Pnictinidenes

The reaction of the intramolecular germylene-phosphine Lewis pair (o-PPh₂)C₆H₄GeAr* ( 1 ) with Group 15 element trichlorides ECl₃ (E=P, As, Sb) was investigated. After oxidative addition, the resulting compounds (o-PPh₂)C₆H₄(Ar*)Ge(Cl)ECl₂ ( 2 : E=P, 3 : E=As, 4 : E=Sb) were reduced by using sodium metal or LiHBEt₃. The molecular structures of the phosphine-stabilized phosphinidene (o-PPh₂)C₆H₄(Ar*)Ge(Cl)P ( 5 ), arsinidene (o-PPh₂)C₆H₄(Ar*)Ge(Cl)As ( 6 ) and stibinidene (o-PPh₂)C₆H₄(Ar*)Ge(Cl)Sb ( 7 ) are presented; they feature a two-coordinate low-valent Group 15 element. After chloride abstraction, a cyclic germaphosphene [(o-PPh₂)C₆H₄(Ar*)GeP] [B(C₆H₃(CF₃)₂)₄] ( 8 ) was isolated. The ³¹P NMR data of the germaphosphene were compared with literature examples and analyzed by quantum chemical calculations. The phosphinidene was treated with [iBu₂AlH]₂, and the product of an Al−H addition to the low-valent phosphorus atom (o-PPh₂)C₆H₄(Ar*)Ge(H)P(H)Al(C₄H₉)₂ ( 9 ) was characterized.     

Cation-Induced Stabilization of Protein Complexes in the Gas Phase: Mechanistic Insights From Hemoglobin Dissociation Studies

Collision-induced dissociation (CID) of electrosprayed protein complexes usually involves asymmetric charge partitioning, where a single unfolded chain gets ejected that carries a disproportionately large fraction of charge. Using hemoglobin (Hb) tetramers as model system, we confirm earlier reports that bound metal ions can stabilize protein complexes under CID conditions. We examine the mechanism underlying this effect. Nonvolatile salts cause extensive adduct formation. Significant stabilization was observed for Mg²⁺ and Ca²⁺, whereas K⁺, Rb⁺, and Cs⁺ had no effect. Precursor ion selection was used to examine Hb subpopulations with well-defined metal binding levels. K⁺, Rb⁺, and Cs⁺-adducted tetramers eject monomers that carry roughly one-quarter of the metal ions that were bound to the precursor. This demonstrates that charge migration during CID is exclusively due to proton transfer, not metal ion transfer. Also, replacement of highly mobile charge carriers (protons) with less mobile species (metal ions) does not exert a stabilizing influence under the conditions used here. Interestingly, Hb carrying stabilizing ions (Mg²⁺ and Ca²⁺) generates monomeric CID products that are metal depleted. This effect is attributed to a combination of two factors: (1) Me²⁺ binding stabilizes Hb via formation of chelation bridges (e.g., R-COO^– Me^{2+ –}OOC-R); the more Me²⁺ a subunit contains the more stable it is. (2) More than ~90 % of the tetramers contain at least one subunit with a below-average number of Me²⁺. The prevalence of monomeric CID products with depleted Me²⁺ levels is caused by the tendency of these low metal-containing subunits to undergo preferential unfolding/ejection.

Insights into the Mechanism of Protein Electrospray Ionization From Salt Adduction Measurements

The mechanisms whereby protein ions are liberated from charged droplets during electrospray ionization (ESI) remain under investigation. Compact conformers electrosprayed from aqueous solution in positive ion mode likely follow the charged residue model (CRM), which envisions analyte release after solvent evaporation to dryness. The concentration of nonvolatile salts such as NaCl increases sharply within vanishing CRM droplets, promoting nonspecific pairing of Cl^- and Na⁺ with charged groups on the protein surface. For unfolded proteins, it has been proposed that ion formation occurs via the chain ejection model (CEM). During the CEM proteins are expelled from the droplet long before complete solvent evaporation has taken place. Here we examine whether salt adduction levels support the view that folded and unfolded proteins follow different ESI mechanisms. Solvent evaporation during the CEM is expected to be less extensive and, hence, the salt concentration at the point of protein release should be substantially lower than for the CRM. CEM ions should therefore exhibit lower adduction levels than CRM species. We explore the adduction behavior of several proteins that were chosen to allow comparative studies on folded and unfolded structures in the same solution. In-source activation eliminates chloride adducts via HCl release, generating protein ions that are heterogeneously charged because of sodiation and protonation. Sodiation levels measured under such conditions provide estimates of the salt adduction behavior experienced by the “nascent” analyte ions. Sodiation levels are significantly reduced for unfolded proteins, supporting the view that these species are indeed formed via the CEM.

Thermogravimetry study of the effectiveness of different reducing agents during sintering of Cr-prealloyed PM steels

The La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF48) cathode material was used as a protective-conducting coating on an interconnect made of Crofer 22 APU ferritic steel intended for application in intermediate-temperature solid oxide fuel cell (IT-SOFC) stacks. The LSCF48 coating was deposited on the surface of the steel via screen-printing followed by appropriate thermal treatment. The oxidation kinetics of the Crofer 22 APU steel—uncoated and coated with LSCF48—approximately obeys the parabolic rate law in air at 1,073 K under isothermal and cyclic oxidation conditions. The oxidation rate for uncoated steel is higher than that for coated steel. SEM–EDS and XRD investigations showed that the LSCF48 coating interacts with the steel during long-term oxidation in the afore-mentioned thermal conditions, and an intermediate multilayer interfacial zone is formed. This intermediate layer leads to lower area specific resistance in air at 1,073 K in comparison to the Crofer 22 APU steel without surface modification.     

Influence of the steel powder type and processing parameters on the debinding of PM compacts with gelatin binder

Binders present an important part of the powder metallurgy technology as they are vital to provide efficient powder agglomeration and/or lubrication during shaping. At the same time, they have to be easily removed from the compacts during initial stages of sintering without any harmful effect for the base material, as well as for the environment. Therefore, behavior of gelatin as a binder for stainless- and tool-steel gas-atomised powder compacts was studied by thermal analysis and electron microscopy. Thermal analysis showed that peak mass-loss occurred in the range between 340 and 370 °C, depending on the base powder and heating rate. Risk for base powder oxidation at temperatures below 425 °C was detected. Based on the obtained results, it is recommended to perform debinding at ~425 °C after applying a heating rate of around 7.5 °C min^?1. Only in this way efficient enough binder removal can be obtained concurrently to avoiding base powder oxidation.     

Regiospecific Formation and Unusual Optical Properties of 2,5‐Bis(arylethynyl)rhodacyclopentadienes: A New Class of Luminescent Organometallics

A series of 2,5‐bis(arylethynyl)rhodacyclopentadienes has been prepared by a rare example of regiospecific reductive coupling of 1,4‐(p‐R‐phenyl)‐1,3‐butadiynes (R?H, Me, OMe, SMe, NMe₂, CF₃, CO₂Me, CN, NO₂, ?C?C‐(p‐C₆H₄?NHex₂), ?C?C?(p‐C₆H₄?CO₂Oct)) at [RhX(PMe₃)₄] ( 1 ) (X=?C?C?SiMe₃ ( a ), ?C?C‐(p‐C₆H₄?NMe₂) ( b ), ?C?C?C?C?(p‐C₆H₄?NPh₂) ( c ) or ?C?C?{p‐C₆H₄‐C?C?(p‐C₆H₄‐N(C₆H₁₃)₂)} ( d ) or Me ( e )), giving the 2,5‐bis(arylethynyl) isomer exclusively. The rhodacyclopentadienes bearing a methyl ligand in the equatorial plane (compound 1 e ) have been converted into their chloro analogues by reaction with HCl etherate. The rhodacycles thus obtained are stable to air and moisture in the solid state and the acceptor‐substituted compounds are even stable to air and moisture in solution. The photophysical properties of the rhodacyclopentadienes are highly unusual in that they exhibit, exclusively, fluorescence between 500–800 nm from the S₁ state, with quantum yields of Φ=0.01–0.18 and short lifetimes (τ=0.45–8.20 ns). The triplet state formation (Φ_ISC=0.57 for 2 a ) is exceptionally slow, occurring on the nanosecond timescale. This is unexpected, because the Rh atom should normally facilitate intersystem crossing within femto‐ to picoseconds, leading to phosphorescence from the T₁ state. This work therefore highlights that in some transition‐metal complexes, the heavy atom can play a more subtle role in controlling the photophysical behavior than is commonly appreciated.     

Topochemical acetylation of cellulose nanopaper structures for biocomposites: mechanisms for reduced water vapour sorption

Moisture sorption decreases dimensional stability and mechanical properties of polymer matrix biocomposites based on plant fibers. Cellulose nanofiber reinforcement may offer advantages in this respect. Here, wood-based nanofibrillated cellulose (NFC) and bacterial cellulose (BC) nanopaper structures, with different specific surface area (SSA), ranging from 0.03 to 173.3 m²/g, were topochemically acetylated and characterized by ATR-FTIR, XRD, solid-state CP/MAS ¹³C-NMR and moisture sorption studies. Polymer matrix nanocomposites based on NFC were also prepared as demonstrators. The surface degree of substitution (surface-DS) of the acetylated cellulose nanofibers is a key parameter, which increased with increasing SSA. Successful topochemical acetylation was confirmed and significantly reduced the moisture sorption in nanopaper structures, especially at RH = 53 %. BC nanopaper sorbed less moisture than the NFC counterpart, and mechanisms are discussed. Topochemical NFC nanopaper acetylation can be used to prepare moisture-stable nanocellulose biocomposites.     

Novel aspects of nanocellulose

Novel nanoscaled cellulose particles were prepared using high-pressure homogenization of aqueous media contenting treated cellulose samples in a Microfluidizer^® processor (MF). Here, we present the generation of spherical cellulose nanoparticles as an extension of previously published reports of nano fibrillated cellulose. Although MF treatment of unmodified cellulose yields nanofibrils which are reported in several publications, in the current work different kinds of pretreatments were proven to be necessary to obtain spherical structured cellulose nanoparticles. One such treatment may be the decrystallization of cellulose regenerating it from N-methylmorpholine-N-oxid-monohydrate (NMMNO*H₂O). Nanocellulose was then obtained by a subsequent high-pressure mechanical treatment of the precipitate in aqueous dispersion. Decrystallization was also realized by grinding cellulose in a planetary ball mill. The resulting amorphous intermediates were characterized by Raman spectroscopy. Another approach tested was hydrolysis and subsequent mechanical treatment using an Ultra-Turrax^® and MF. Another alternative was given by the mechanical treatment of aqueous dispersions of low substituted cellulose derivatives such as carboxymethyl cellulose and oxidized cellulose without any further hydrolysis.     

New insights into the mechanisms behind the strengthening of lignocellulosic fibrous networks with polyamines

Polyelectrolytes have been used extensively in the papermaking industry for various purposes. Although recent studies have shown that polyamines can be efficient dry-strength additives, the mechanism governing the strength enhancement of paper materials following the adsorption of polyamines onto pulp fibres is still not well understood. In this study, the effect of the adsorption of polyallylamine hydrochloride (PAH) onto the surface of unbleached kraft pulp fibres was investigated on both the fibre and the network scale. Isolated fibre crosses were mechanically tested to evaluate the impact of the chemical additive on the interfibre joint strength on the microscopic scale and the effect was compared with that previously observed on the paper sheet scale. X-ray microtomography was used to understand structural changes in the fibrous network following the adsorption of a polyamine such as PAH. Using image analysis methods, it was possible to determine the number of interfibre contacts (or joints) per unit length of fibre as well as the average interfibre joint contact area. The results showed that the median interfibre joint strength increased by 18 % upon adsorption of PAH. This can be achieved both by a larger molecular contact area in the contact zones and by a stronger molecular adhesion. The addition of the polymer also increased the number of efficient interfibre contacts per sheet volume. This combination of effects is the reason why polyamines such as PAH can increase the dry tensile strength of paper materials.