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End date: 2022 × Start date: 2022 × Subject: Chemical bonds ×

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Now showing 1 - 4 of 4
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    Entropy driven chain effects on ligation chemistry
    (Cambridge : RSC, 2014) Pahnke, Kai; Brandt, Josef; Gryn'ova, Ganna; Lindner, Peter; Schweins, Ralf; Schmidt, Friedrich Georg; Lederer, Albena; Coote, Michelle L.; Barner-Kowollik, Christopher
    We report the investigation of fundamental entropic chain effects that enable the tuning of modular ligation chemistry – for example dynamic Diels–Alder (DA) reactions in materials applications – not only classically via the chemistry of the applied reaction sites, but also via the physical and steric properties of the molecules that are being joined. Having a substantial impact on the reaction equilibrium of the reversible ligation chemistry, these effects are important when transferring reactions from small molecule studies to larger or other entropically very dissimilar systems. The effects on the DA equilibrium and thus the temperature dependent degree of debonding (%debond) of different cyclopentadienyl (di-)functional poly(meth-)acrylate backbones (poly(methyl methacrylate), poly(iso-butyl methacrylate), poly(tert-butyl methacrylate), poly(iso-butyl acrylate), poly(n-butyl acrylate), poly(tert-butyl acrylate), poly(methyl acrylate) and poly(isobornyl acrylate)), linked via a difunctional cyanodithioester (CDTE) were examined via high temperature (HT) NMR spectroscopy as well as temperature dependent (TD) SEC measurements. A significant impact of not only chain mass and length with a difference in the degree of debonding of up to 30% for different lengths of macromonomers of the same polymer type but – remarkably – as well the chain stiffness with a difference in bonding degrees of nearly 20% for isomeric poly(butyl acrylates) is found. The results were predicted, reproduced and interpreted via quantum chemical calculations, leading to a better understanding of the underlying entropic principles.
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    Indirect reduction of CO2 and recycling of polymers by manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes
    (Cambridge : RSC, 2021) Liu, Xin; Werner, Thomas
    The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO2as a C1 building block the reported reaction represents an approach to the indirect reduction of CO2. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed. © The Royal Society of Chemistry 2021.
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    Two-photon, visible light water splitting at a molecular ruthenium complex
    (Cambridge : RSC Publ., 2021) Schneidewind, Jacob; Argüello Cordero, Miguel A.; Junge, Henrik; Lochbrunner, Stefan; Beller, Matthias
    Water splitting to give molecular oxygen and hydrogen or the corresponding protons and electrons is a fundamental four-electron redox process, which forms the basis of photosynthesis and is a promising approach to convert solar into chemical energy. Artificial water splitting systems have struggled with orchestrating the kinetically complex absorption of four photons as well as the difficult utilization of visible light. Based on a detailed kinetic, spectroscopic and computational study of Milstein's ruthenium complex, we report a new mechanistic paradigm for water splitting, which requires only two photons and offers a new method to extend the range of usable wavelengths far into the visible region. We show that two-photon water splitting is enabled by absorption of the first, shorter wavelength photon, which produces an intermediate capable of absorbing the second, longer wavelength photon (up to 630 nm). The second absorption then causes O–O bond formation and liberation of O2. Theoretical modelling shows that two-photon water splitting can be used to achieve a maximum solar-to-hydrogen efficiency of 18.8%, which could be increased further to 28.6% through photochemical instead of thermal H2 release. It is therefore possible to exceed the maximum efficiency of dual absorber systems while only requiring a single catalyst. Due to the lower kinetic complexity, intrinsic utilization of a wide wavelength range and high-performance potential, we believe that this mechanism will inspire the development of a new class of water splitting systems that go beyond the reaction blueprint of photosynthesis.
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    Mg3(Bi,Sb)2 single crystals towards high thermoelectric performance
    (Cambridge : RSC Publ., 2020) Pan, Yu; Yao, Mengyu; Hong, Xiaochen; Zhu, Yifan; Fan, Fengren; Imasato, Kazuki; He, Yangkun; Hess, Christian; Fink, Jörg; Yang, Jiong; Büchner, Bernd; Fu, Chenguang; Snyder, G. Jeffrey; Felser, Claudia
    The rapid growth of the thermoelectric cooler market makes the development of novel room temperature thermoelectric materials of great importance. Ternary n-type Mg3(Bi,Sb)2 alloys are promising alternatives to the state-of-the-art Bi2(Te,Se)3 alloys but grain boundary resistance is the most important limitation. n-type Mg3(Bi,Sb)2 single crystals with negligible grain boundaries are expected to have particularly high zT but have rarely been realized due to the demanding Mg-rich growth conditions required. Here, we report, for the first time, the thermoelectric properties of n-type Mg3(Bi,Sb)2 alloyed single crystals grown by a one-step Mg-flux method using sealed tantalum tubes. High weighted mobility ∼140 cm2 V−1 s−1 and a high zT of 0.82 at 315 K are achieved in Y-doped Mg3Bi1.25Sb0.75 single crystals. Through both experimental angle-resolved photoemission spectroscopy and theoretical calculations, we denote the origin of the high thermoelectric performance from a point of view of band widening effect and electronegativity, as well as the necessity to form high Bi/Sb ratio ternary Mg3(Bi,Sb)2 alloys. The present work paves the way for further development of Mg3(Bi,Sb)2 for near room temperature thermoelectric applications.