Science Concentrates MATERIALS
▸ Unstable allotropes stored safely in carbon Although the elemental allotropes white phosphorus (P4) and yellow arsenic (As4) have the potential to be useful reagents, these compounds aren’t commonly used by chemists because of their notorious instability. White phosphorus will burst into flame when exposed to air and is subject to strict shipping regulations. Light-sensitive yellow arsenic turns to gray arsenic so quickly that solutions of the element must be used in complete darkness. Chemists
C R E D I T: A DA PT E D FRO M ACS N A NO ( 3 -D MO D E L) ; CO U RT ESY O F MA N F R ED S CH E E R (P L AT ES )
White phosphorus bursts into flame when exposed to air but is stable when encapsulated in activated carbon (front left). Yellow arsenic is also stable when stored in activated carbon (front right). have been working to create materials in which these allotropes can be stored stably, but their success has been limited. Now, University of Regensburg’s Manfred Scheer and colleagues have found that the pores within activated carbon work well at storing both white phosphorus and yellow arsenic. What’s more, the elements can be released from the activated carbon into solution, where they can subsequently be used as reagents (Nat. Commun. 2018, DOI: 10.1038/s41467-017-02735-2). Scheer’s group prepares the material by adsorbing a solution of P4 or As4 in tetrahydrofuran onto activated carbon with a defined pore size and distribution. After centrifugation, decanting, and drying, the resulting black powder can be stored on a benchtop and exposed to light and air with only minimal decomposition (Scheer’s group recommends, instead, storing the arsenic-filled carbon carefully in a closed container because of its unknown toxicity). The researchers note that this opens new avenues for reactions with white phosphorus and yellow arsenic, as well as for activated carbon as a storage material for unstable chemicals.—BETHANY HALFORD
High conductivity and the presence of –O, –OH, and –F surface groups make this titanium carbide MXene useful for sensing gases (ethanol and ammonia shown). Ti = yellow, C = black, H = white, O = red, F = blue, N = green.
2-D MATERIALS
MXenes sense gas maximally Gas sensors have long served critical roles in industrial applications, including monitoring air quality and engine emissions. Analyzing volatile organic compounds in breath to screen for disease markers is a developing application that is being advanced by highly sensitive “homemade” gas sensors, often based on semiconducting oxides, nanomaterials, and two-dimensional materials. That field may advance even more quickly now thanks to a study showing that a titanium carbide MXene compound can be fashioned into a gas sensor that provides part-per-billion-level sensitivity and record-setting signal-to-noise ratios (ACS Nano 2018, DOI: 10.1021/acsnano.7b07460). Various 2-D materials, including black phosphorus and molybdenum disulfide, rank among the top-performing gas-sensing media. That motivated a team led by Hee-Tae Jung of Korea Advanced Institute of Science & Technology and Yury Gogotsi of Drexel University to see how MXenes, a family of 2-D metal carbides and nitrides, stack up in that application. Head-to-head comparisons in tests with acetone, ethanol, ammonia, propanal, and other gases relevant to breath analysis show that the MXene compound evaluated, Ti3C2, provides signal-to-noise ratios roughly 100 times as high as those of the other 2-D materials. The team attributes the excellent performance to the MXene’s porosity, metal-like conductivity, and abundant surface groups that adsorb analytes.—MITCH JACOBY
SYNTHESIS
▸ Dual activation fuses polyaromatics
This type of π-conjugated architecture holds precious properties for modern applications such as organic light-emitting diodes and photovoltaics. Itami notes that the partially fused arenes can be successfully “zipped up” to the fully fused polyaromatic structure using the iron-catalyzed Scholl reaction. Previous attempts to use this route starting from nonfused arenes have been plagued with undesirable side reactions. This additional step provides access to graphene nanoribbon materials, which are coveted for their unique optoelectronic properties.—TIEN NGUYEN
More than a century ago, chemists discovered the homocoupling of aryl halides, in which a copper catalyst convinces twin halogen atoms to give up their positions to bring together two aryl rings. This mighty dimerization reaction has long dominated biarene synthesis. Now, scientists are offering an upgrade. A research team at Nagoya University, led by Kei Murakami and Kenichiro Itami, has developed Simple annulative dimerization provides access an annulative dimerization reaction to polyaromatics for optoelectronic applications. of chlorophenylenes that connects the rings with two new bonds instead of one (Science 2018, DOI: Cl 10.1126/science.aap9801). In the Pd catalyst + Cl reaction, a palladium catalyst with bulky adamantane ligands promotes dual carbon-hydrogen bond activation events to produce the partially Chlorophenylenes Triphenylene-based fused aromatics fused triphenylene core structure. FEBRUARY 5, 2018 | CEN.ACS.ORG | C&EN
11
