Science Concentrates CHEMICAL BONDING
▸ Dioxygen’s stability lies in resonance
Resonance of O2’s system stabilizes the molecule. North Texas, Roald Hoffmann of Cornell University, and their colleagues (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b04232). Confirming a 1931 proposal by Linus Pauling, the researchers determined that O2’s π bond can be thought of as a pair of two-center, three-electron bonds, with resonance contributing a net stabilization energy of 418 kJ/mol. S2 is stabilized by only about 213 kJ/mol. The consequence is that trimerization of O2 is endothermic, while S2 is exothermic. Meanwhile, O2’s σ bond is relatively weak, so oxidation reactions are ultimately exothermic when they do occur.—JYLLIAN KEMSLEY
MEDICINAL CHEMISTRY
▸ Making ring compounds for DNAencoded libraries A simple tweak to a tool for making macrocyclic compounds could help increase
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C&EN | CEN.ACS.ORG | JULY 17, 2017
Wrapping sulfur spheres in thin MoS2 flakes protects the material and enables repeated lithiation reactions in batteries.
ENERGY STORAGE
MoS2 wrap protects lithium-sulfur batteries A simple procedure for wrapping sulfur particles in thin sheets of molybdenum disulfide may offer a way to capitalize on the promise of high-performance lithium-sulfur batteries, according to a study (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b05371). Lithium batteries made with sulfur, a low-cost and abundant element, have the potential to store five times as much energy by weight as conventional Li-ion batteries. But long-lasting Li-S batteries have remained elusive because of several materials and chemistry problems. For example, electrochemical reactions in these batteries form troublesome lithium polysulfides, which dissolve in the electrolyte solution, reducing the availability of energy-rich lithium. In addition, lithiation reactions lead to substantial swelling of the cathode, which can trigger cracking and failure of that electrode. In an attempt to bypass those problems, Wei Tang and Kian Ping Loh of National University of Singapore and coworkers reacted a polyvinylpyrrolidone suspension of hollow sulfur particles with ultrathin MoS2 flakes, causing the flakes to tightly encapsulate the sulfur particles. The team made batteries fitted with cathodes prepared from the MoS2-S hybrid material and found that they retained much of their high initial charge capacity even after 1,000 charging cycles. The researchers attribute the good performance to strong van der Waals forces between the MoS2 layers, which help prevent escape of lithium polysulfides. They also found that wrinkles in the MoS2 wrap provide ample space to accommodate swelling of the sulfur particles upon lithiation.—MITCH JACOBY
H N
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Ru catalyst, Mg2+
O By adding magnesium ions, researchers can perform ring-closing metathesis reactions using a ruthenium catalyst on DNA-tagged molecules. the diversity of DNA-encoded libraries used by drug developers to rapidly screen and identify promising drug candidates (Bioconjugate Chem. 2017, DOI: 10.1021/ acs.bioconjchem.7b00292). Building such libraries involves facilitating reactions between small molecules tagged with short, unique DNA sequences to create myriad products, which are then tagged with additional unique DNA sequences. The DNA serves as a sort of bar code to identify the compounds in a library that successfully
bind to a particular drug target. H However, these N N libraries genO erally have not been able to include ring compounds in the drug screening because transition-metal catalysts, essential for ring-closing reactions, can bind to charged DNA backbones and cause the strands to fall apart. Now, a team led by Xiaojie Lu and Lijun Fan of GlaxoSmithKline has found that by protecting the DNA tags with magnesium ions, they can produce a variety of DNA-encoded heterocycles and macrocycles using ruthenium-catalyzed ring-closing metathesis. The team hypothesizes that because the magnesium ions occupy all the DNA’s binding sites, the ruthenium catalyst is forced to react with the substrates instead of the DNA.—XIAOZHI LIM, special to C&EN
C R E D I T: J . A M. CH EM . S OC. ( S UL FUR S P HE R ES , D I OXYGE N )
Oxygen as O2 is stable enough to be abundant in the environment and is required for many forms of life. But from the standpoint of theory, dioxygen’s stability is curious: Its highest occupied molecular orbitals contain two unpaired electrons, making it a diradical. Instead of wafting around as O2, the molecule should be busy abstracting hydrogen atoms or forming oligomers; isoelectronic sulfur, for example, is most stable as S8. The key to dioxygen’s reactivity lies in resonance. That finding comes from experimental and computational analysis by Weston T. Borden of the University of
