Hollow nanoparticles via stepwise complexation and selective decomplexation of poly(ethylene imine)

The preparation of hollow nanoparticles with amino groups on the inner side via the stepwise complexation and selective decomplexation of poly(ethylene imine) is presented.     

Front Cover: A Dibismuthane with Olefin Functional Groups: Towards Tridentate Hybrid Chalcogen/Olefin Ligands (Eur. J. Inorg. Chem. 32/2023)

Bis[dibenzobismepine], a dibismuthane composed of two bismepine units (R₂Bi−BiR₂), was synthesized and fully characterized (R₂=(C₆H₄CH)₂). Reactions of this dibismuthane with diphenyl dichalcogenides, dibenzoylperoxide, and elemental chalcogens have been investigated. All products of these reactions have been isolated and fully characterized, including a series of compounds R₂Bi−E−BiR₂ (E=O−Te). These species contain two olefin units of the bismepine moieties and a chalcogen atom as potential coordination sites. The potential of these species to act as hybrid tridentate chalcogen/olefin ligands with bismuth atoms as structure-determining elements in the backbone has been investigated by theoretical approaches, aiming at the complexation of Co^I, Rh^I, Ir^I and Ni⁰, Pd⁰, Pt⁰. The analytical techniques applied in this work include heteronuclear and 2D NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction analysis, and DFT calculations.     

Synthesis of spherical polymer and titania photonic crystallites

The fabrication of small structured spherical particles that are essentially small photonic crystals is described. The particles are 1-50 microm in diameter and are porous with nearly close-packed monodisperse pores whose size is comparable to the wavelength of light. The solid matrix of the particles is titania, which provides a large refractive index contrast between the particle matrix and pores. The particles are made by encapsulating polymer colloidal particles in emulsion droplets of hexanes in which a titanium alkoxide precursor is dissolved. Subsequent osmotic removal of the hexanes from the droplets and condensation of the alkoxide precursor leads to spherical aggregates of polymer spheres with titania filling the spaces between the polymer spheres. The polymer particles are then burned out leaving behind the desired porous titania particles. The size and structure of the pores and high refractive index of the titania matrix are expected to produce particles that are very efficient scatterers of light, making them useful as pigments.     

Facilitated hydride binding in an Fe-Fe hydrogenase active-site biomimic revealed by X-ray absorption spectroscopy and DFT calculations

Iron-only hydrogenases are high-efficiency biocatalysts for the synthesis and cleavage of molecular hydrogen. Their active site is a diiron center, which carries CO and CN ligands. Remarkably, the two iron atoms likely are connected by a non-protein azadithiolate (adt = S-CH2-NH-CH2-S). To dwell on the role of the adt in H2 catalysis, a specific biomimetic diiron compound, 1 = [Fe2(mu-adt-CH2-Ph)(CO)4(PMe3)2], with unprecedented positive reduction potential, has been synthesized and crystallized previously. It comprises two protonation sites, the N-benzyl-adt nitrogen that can hold a proton (H) and the Fe-Fe bond that will formally carry a hydride (Hy). We investigated changes in the solution structure of 1 in its four different protonation states (1', [1H]+, [1HHy]2+, and [1Hy]+) by X-ray absorption spectroscopy at the iron K-edge. EXAFS reveals that already protonation at the adt nitrogen atom causes a change of the ligand geometry involving a significant lengthening of the Fe-Fe distance and CO and PMe3 repositioning, respectively, thereby facilitating the subsequent binding of a bridging hydride. Hydride binding clearly is discernible in the XANES spectra of [1HHy]2+ and [1Hy]+. DFT calculations are in excellent agreement with the experimentally derived structural parameters and provide complementary insights into the electronic structure of the four protonation states. In the iron-only hydrogenases, protonation of the putative adt ligand may cause the bridging CO to move to a terminal position, thereby preparing the active site for hydride binding en route to H2 formation.     

Multidimensional electrochemical imaging in materials science

In the past 20 years the characterization of electroactive surfaces and electrode reactions by scanning probe techniques has advanced significantly, benefiting from instrumental and methodological developments in the field. Electrochemical and electrical analysis instruments are attractive tools for identifying regions of different electrochemical properties and chemical reactivity and contribute to the advancement of molecular electronics. Besides their function as a surface analytical device, they have proved to be unique tools for local synthesis of polymers, metal depots, clusters, etc. This review will focus primarily on progress made by use of scanning electrochemical microscopy (SECM), conductive AFM (C-AFM), electrochemical scanning tunneling microscopy (EC-STM), and surface potential measurements, for example Kelvin probe force microscopy (KFM), for multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers. Figure Electrochemical and electrical tools like scanning electrochemical microscopy, conductive atomic force microscopy, electrochemical scannig tunneling microscopy and Kelvin probe force microscopy (see figure) are powerful tools for the multidimensional imaging of potential-dependent processes on metals and electrified surfaces modified with polymers and self assembled monolayers.     

Activating a Low Overpotential CO2 Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu₃tpy)(bpy)(NCCH₃)]²⁺ (tBu₃tpy=4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine; bpy=2,2′‐bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO₂ to CO. The methyl group enables CO₂ binding already at the one‐electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO₂ reduction catalysts.     

Determination of the elastic moduli of polymer films by a new ultrasonic reflection method

The development and first applications of a new ultrasonic reflection method for determination of the viscoelastic properties of polymer films are reported. The complex shear modulus G* and the complex longitudinal modulus L*=K*+ 4/3 G* of the samples are derived from the measured complex reflection coefficients of an ultrasonic shear and longitudinal wave, respectively. From G and L the Young's modulus E, the compression modulus K and the Poisson ratio ν can be calculated for isotropic materials. A LiNbO₃-transducer (10° rotated Y-cut) is used for the simultaneous excitation of longitudinal and transversal ultrasonic waves, which allows to determine different elastic constants by one measurement. A measuring cell with normal incidence of the ultrasonic waves is used. The equipment has been applied to study the time dependence of the moduli during film formation from an aqueous polymer dispersion and the isothermal curing of an epoxy resin. Furthermore, the temperature dependence of the elastic constants of a carbon-black filled rubber and during non-isothermal crystallization of a semi-crystalline polymer has been studied.     

Targeted Bottom–Up Mass Spectrometry Approach for the Relative Quantification of Post-Translational Modification of Bovine κ-Casein during Milk Fermentation

κ-casein (κ-CN) is one of the key components in bovine milk, playing a unique role in the structuration of casein micelles. It contains in its chemical structure up to sixteen amino acid residues (mainly serine and threonine) susceptible to modifications, including glycosylation and phosphorylation, which may further be formed during milk processing. In this study, changes in post-translational modification (PTM) of κ-CN during bovine milk fermentation were investigated. One-to-five-day fermented milk samples were produced. A traditional bottom–up proteomics approach was used to establish a multiple-reaction monitoring (MRM) method for relative quantification of κ-CN PTM. Endoproteinase Glu-C was found to efficiently digest the κ-CN molecule. The developed LC-MS method was validated by performing assessments of linearity, precision, repeatability, reproducibility, limit of detection (LOD), and limit of quantification (LOQ). Among the yielded peptides, four of them containing serine and threonine residues were identified and the unmodified as well as the modified variants of each of them were relatively quantified. These peptides were (1) IPTINTIASGEPTSTTE _{[140, 158]}, (2) STVATLE _{[162, 168]}, (3) DSPE _{[169, 172]}, and (4) INTVQVTSTAV _{[180, 190]}. Distribution analysis between unmodified and modified peptides revealed that over 50% of κ-CN was found in one of its modified forms in milk. The fermentation process further significantly altered the composition between unmodified/modified κ-CN, with glycoslaytion being predominant compared to phosphorylation (p < 0.01). Further method development towards α and β-CN fractions and their PTM behavior would be an asset to better understand the changes undergone by milk proteins and the micellar structure during fermentation.     

[2 + 2] Cycloaddition of phosphaalkenes as a key step for the reductive coupling of diaryl ketones to tetraaryl olefins

Procedures for the reductive coupling of carbonyl compounds to alkenes in the literature rely either on a radical coupling strategy, as in the McMurry coupling, or ionic pathways, sometimes catalysed by transition metals, as in more contemporary contributions. Herein, we present the first example of a third strategy that is based on the [2 + 2] cycloaddition of ketone-derived phosphaalkenes. Removal of P-trimethylsilyl groups at the intermediary 1,2-diphosphetane dimer results in its collapse and concomitant release of the tetraaryl-substituted alkene. In fact, the presented strategy is the only alternative to the McMurry coupling in the literature that allows tetraaryl alkene formation from diaryl ketones, with yields as high as 85%. The power of the methodology is illustrated in the reaction of tethered bis-benzophenones which engage in intramolecular reductive carbonyl couplings to form unusual macrocycles without the need for high dilution conditions or templating.

Mechanistically distinct from all other carbonyl-to-alkene conversion methodologies, the [2 + 2] cycloaddition of phosphaalkene intermediates is established as the enabling elementary step in this transformation.