ACS Organic & Inorganic Au最新文献

Parallel Minisci reactions of nonfluorinated and gem-difluorinated C₄–C₇ cycloalkyl building blocks (trifluoroborates and carboxylic acids) with a series of electron-deficient heterocycles were studied. A comparison of the reaction’s outcome revealed better product yields in the case of carboxylic acids as the radical precursors in most cases, albeit these reagents were used with three-fold excess under optimized conditions. The nature of the heterocyclic core was found to be important for successful incorporation of the cycloalkyl fragment. The impact of the CF₂ moiety on the oxidation potential of fluorinated cycloalkyl trifluoroborates and the reaction outcome, in general, was also evaluated.

In order to prevent the current unsustainable waste handling of the enormous volumes of end-of-use organic polymer material sent to landfilling or incineration, extensive research efforts have been devoted toward the development of appropriate solutions for the recycling of commercial thermoset polymers. The inability of such cross-linked polymers to be remelted once cured implies that mechanical recycling processes used for thermoplastic materials do not translate to the recycling of thermoset polymers. Moreover, the structural diversity within the materials from the use of different monomers as well as the use of such polymers for the fabrication of fiber-reinforced polymer composites make recycling of these materials highly challenging. In this Perspective, depolymerization strategies for thermoset polymers are discussed with an emphasis on recent advancements within our group on recovering polymer building blocks from polyurethane (PU) and epoxy-based materials. While these two represent the largest thermoset polymer groups with respect to the production volumes, the recycling landscapes for these classes of materials are vastly different. For PU, increased collaboration between academia and industry has resulted in major advancements within solvolysis, acidolysis, aminolysis, and split-phase glycolysis for polyol recovery, where several processes are being evaluated for further scaling studies. For epoxy-based materials, the molecular skeleton has no obvious target for chemical scission. Nevertheless, we have recently demonstrated the possibility of the disassembly of the epoxy polymer in fiber-reinforced composites for bisphenol A (BPA) recovery through catalytic C–O bond cleavage. Furthermore, a base promoted cleavage developed by us and others shows tremendous potential for the recovery of BPA from epoxy polymers. Further efforts are still required for evaluating the suitability of such monomer recovery strategies for epoxy materials at an industrial scale. Nonetheless, recent advancements as illustrated with the presented chemistry suggest that the future of thermoset polymer recycling could include processes that emphasize monomer recovery in an energy efficient manner for closed-loop recycling.

We report on three new 9-phenyl-substituted ferroceno[2,3]indenylmethylium dyes 1⁺–3⁺ with electron-donating (OMe, Me) or electron-withdrawing (CF₃) substituents. Complexes 1⁺–3⁺ exist as racemic mixtures of Rp and Sp enantiomers. Pyramidalization at the methyl C atom in the precursor carbinol species 1-OH–3-OH or the corresponding one-electron reduced radicals induces a second stereocenter, as the 9-phenyl substituent may reside in an endo or an exo position. Indeed, alcohol 2-OH crystallizes as a racemate of Rp,S and Sp,R isomers. Cationic complexes 1⁺–3⁺ are of deep green color and show intense electronic absorption in the visible. The oxidation and reduction processes are thoroughly investigated by means of cyclic voltammetry and UV/vis/NIR spectroelectrochemistry, the latter showing their electrochromic behavior. T-dependent EPR spectroscopy, EPR spin counting at 20 °C, as well as the UV/vis/NIR spectra of the reduced samples suggest that the one-electron reduced, neutral radicals dimerize nearly quantitatively (≥99.98%). Chemical reduction of 2⁺ furnished an isomeric mixture of dimeric 2–2. As was shown by cyclic voltammetry and UV/vis/NIR spectroelectrochemistry, the latter dimer redissociates to monomers 2⁺ upon oxidation, thereby closing a reversible cycle of redox-induced C–C bond making and breaking.

The term “polytopal rearrangement” describes any shape changing process operating on a coordination “polyhedron”─the solid figure defined by the positions of the ligand atoms directly attached to the central atom of a coordination entity. Developed in the latter third of the last century, the polytopal rearrangement model of stereoisomerization is a general mathematical approach for analyzing and accommodating the complexity of such processes for any coordination number. The motivation for the model was principally to deal with the complexity, such as Berry pseudorotation in pentavalent phosphorus species, arising from rearrangements in inorganic coordination complexes of higher coordination numbers. The model is also applicable to lower coordination centers, for example, thermal “inversion” at nitrogen in NH₃ and amines. We present the history of the model focusing on its essential features, and review some of the more subtle aspects addressed in recent literature. We then introduce a more detailed and rigorous modern approach for describing such processes using an assembly of existing concepts, with the addition of formally described terminology and representations. In our outlook, we contend that the rigorous and exhaustive application of the principles of the polytopal rearrangement model, when combined with torsional isomerism, will provide a basis for a mathematically complete, general, and systematic classification for all stereoisomerism and stereoisomerization. This is essential for comprehensively mapping chemical structure and reaction spaces.

Escalating levels of carbon dioxide (CO₂) in the atmosphere have motivated interest in CO₂ capture and concentration from dilute streams. A guanidino-functionalized aromatic 1,4-bis(tetramethylguanidino)benzene (1,4-btmgb) was evaluated both as a redox-active sorbent and as a pH swing mediator for electrochemical CO₂ capture and concentration. Spectroscopic and crystallographic studies demonstrate that 1,4-btmgb reacts with CO₂ in water to form 1,4-btmgbH₂(HCO₃^–)₂. The product suggests that 1,4-btmgb could be used in an aqueous redox pH swing cycle for the capture and concentration of CO₂. The synthesis and characterization of the mono- and diprotonated forms (1,4-btmgbH⁺ and 1,4-btmgbH₂²⁺) and their pK_a values were measured to be 13.5 and 11.0 in water, respectively. Electrochemical pH swing experiments indicate the formation of an intermediate radical species and other degradation pathways, which ultimately inhibited fully reversible redox-induced pH cycling.

The synthesis of selenofunctionalized oxazolines and isoxazolines from N-allyl benzamides and unsaturated oximes with diselenides was studied by utilizing a continuous flow electrochemical approach. At mild reaction conditions and short reaction times of 10 min product yields of up to 90% were achieved including a scale-up reaction. A broad substrate scope was studied and the reaction was shown to have a wide functional group tolerance.

Near-infrared (NIR)-emitting phosphate glasses containing Nd³⁺ ions are attractive for applications in laser materials and solar spectral converters. The composition–structure–property relation in this type of glass system is thus of interest from fundamental and applied perspectives. In this work, Nd³⁺-containing glasses were made by melting with 50P₂O₅-(50 – x)BaO-xNd₂O₃ (x = 0, 0.5, 1.0, 2.0, 3.0, 4.0 mol %) nominal compositions and studied comprehensively by density and related physical properties, X-ray diffraction (XRD), Raman spectroscopy, O 1s X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), dilatometry, ultraviolet–visible (UV–vis)–NIR optical absorption, and photoluminescence (PL) spectroscopy with decay dynamics assessment. The densities and molar volumes of the Nd³⁺-containing glasses generally increased with Nd₂O₃ concentration also resulting in shorter Nd³⁺–Nd³⁺ distances. XRD supported the amorphous nature of the glasses, whereas the Raman spectra evolution was indicative of glass depolymerization being induced by Nd³⁺ ions. Oxygen (1s) and phosphorus (2p) analysis by XPS for the glass with 4.0 mol % Nd₂O₃ agreed with the increase in nonbridging oxygens relative to the undoped host. DSC results showed that the glass transition temperatures increased with Nd³⁺ concentration, with the glasses also displaying a decreased tendency toward crystallization. Dilatometry showed trends of increasing softening temperatures and decreasing thermal expansion coefficients with increasing Nd₂O₃ content. A glass strengthening/tightening effect was then indicated to be induced by Nd³⁺ with higher field strength compared to Ba²⁺ ions. The UV–vis–NIR absorption by Nd³⁺ ions increased consistently with Nd³⁺ concentration. The UV–vis absorption edges of the Nd-containing glasses were also analyzed via Tauc and Urbach plots for comparison with the undoped host. Concerning the PL behavior, the Nd³⁺ NIR emission intensity was highest for 1.0 mol % Nd₂O₃ and decreased thereafter. The decay kinetics of the ⁴F_3/2 emitting state in Nd³⁺ ions analyzed revealed decreasing lifetimes where the decay rate analysis pointed to the prevalence of ion–ion excitation migration leading to PL quenching at high Nd³⁺ concentrations.