Noteworthy Chemistry

May 11, 2014

 

How does steric conformation affect a luminogen’s emission? Tetraphenylethylene (TPE) is a versatile building block for synthesizing luminogenic molecules that exhibit aggregation-induced emission (AIE). TPE derivatives can take E- and Z-conformations, but isolating the individual isomers has been difficult.

T. T. Tasso, T. Furuyama, and N. Kobayashi* at Tohoku University (Sendai, Japan) synthesized and isolated two pairs of TPE derivatives (1 and 2 in the figure) with sterically pure E- and Z-structures. They studied the effects of E- versus Z-conformations on  the behavior of photoluminescence.

Two sets of E- and Z-TPE derivatives

The tetrahydrofuran (THF) solution of (E)-1 emits weak red light. When its molecules aggregate in 99:1 v/v H2O/THF, its emission increases sharply, showing the AIE effect. Its isomer (Z)-1 is similarly AIE-active. The emissions of (E)-1 in solution (λs) and aggregated (λa) peak at wavelengths of 642 nm and 590 nm, respectively. The corresponding emissions of (Z)-1 are red-shifted from those of (E)-1 (648 and 637 nm). The emission efficiency of the Z-conformer is lower than that of its E-isomer.

The authors observed similar conformational effects in the second system. The emissions of (Z)-2s = 564 nm, λa = 572 nm) are red-shifted from those of (E)-2s = 558 nm, λa = 564 nm), and the emission efficiency of the former is lower than that of the latter.

The conformations of the Z-isomers are more twisted than those of their E-counterparts. This elevates the highest occupied molecular orbital (HOMO) levels of the Z-conformers and explains the observed bathochromic shifts in their emissions.

The more twisted Z-conformers have greater molecular motion in solution and are packed less compactly in their aggregates. Hence, they have less efficient emissions. (Chem. Eur. J. DOI: 10.1002/chem.201406128; Ben Zhong Tang)

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How hazardous are lead-containing solar cells? Methylammonium lead(IV) iodide (MeNH3PbI3) films form the basis of affordable, high-voltage hybrid organic–inorganic perovskite (HOIP) solar cells. When MeNH3PbI3 is exposed to water, it decomposes to form lead(II) iodide (PbI2), which is several orders of magnitude more water-soluble than other commonly used solar panel materials. The lead-containing films in solar cells could be encapsulated to limit contact with the environment, but damaged solar cells could release lead.

G. Hodes, D. Cahen, and co-workers at the Weizmann Institute of Science (Rehovot, Israel) exposed 12 mm × 30 mm MeNH3PbI3 films to water at pH values from 4.2 to 8.1 to mimic the effects of rain on exposed films in solar panels. They determined the degree of lead loss with gravimetric measurements and inductively coupled plasma mass spectrometry analysis.

Exposing the films to water vapor caused hydrate formation, which could be reversed by heating the samples. After only a few seconds of contact with liquid water, however, the samples decomposed completely and irreversibly.

The authors calculated that a 1-m2 panel having a 300-nm thick MeNH3PbI3 layer contains ≈0.4 g of lead. Releasing all the lead from a field of solar panels would add ≈70 ppm of lead to the first centimeter of soil beneath the panels. (Lead levels in soil typically range from <10 ppm in pristine areas to 200 ppm or more in urban areas.)

The authors compared the potential for lead contamination with the lead output from coal-fired electric power plants. They concluded that if breakage does not exceed one solar module in 300 over a period of 20 years, the amount of lead released would be less than from the least polluting coal-fired plants over the same time frame. They also noted that the lead from coal-fired plants is released into the atmosphere, whereas the lead from HOIP solar panels can be collected and recycled. (J. Phys. Chem. Lett. DOI: 10.1021/acs.jpclett.5b00504; Nancy McGuire)

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This reaction easily converts nitriles to amides. Amides are important functional groups in organic and medicinal chemistry. They are the core of biological structures such as proteins. One of the most common routes to amides is the hydrolysis of nitriles, but this is tricky because carboxylic acids are formed at faster rates than the corresponding amides.

J. Dash and co-workers at the Indian Association for the Cultivation of Science (Kolkata) report a way to make amides from nitriles that uses potassium tert-butoxide under anhydrous conditions. Noting that the nitrile group is activated by coordination with potassium, they treated benzonitrile with 3 equiv KO-t-Bu in toluene or tert-butyl alcohol solution at room temperature and obtained benzamide in 96 and 99% yield, respectively (see figure; R = H). 

Improved route to amides from nitriles

Changing the base (e.g., to NaO-t-Bu) gives no product, demonstrating that the potassium is required as the counterion. tert-Butoxide is the oxygen source for the amide.

The authors then tested several aliphatic and aromatic nitriles as substrates. As expected, electron-rich benzonitriles give lower yields. The antitubercular agent pyrazinamide was obtained in 95% yield from the corresponding nitrile.

The authors propose reasonable mechanisms based on the coordination of the nitrile with the potassium cation to form a complex stabilized by cation–π and π–π* stacking interactions. But their experiments could not discriminate between radical (single-electron transfer) and ionic mechanisms.

This widely applicable, scalable reaction does not require metal catalysts or special lab equipment. (J. Org. Chem. DOI: 10.1021/jo502752u, José C. Barros)

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Are alkyl sulfonate salts in APIs really much of a problem? D. Snodin* at Xiphora Biopharma Consulting (Bristol, UK) and A. Teasdale at AstraZeneca (Macclesfield, UK) present an excellent review of the evidence  of alkyl sulfonate salt impurities in active pharmaceutical ingredients (APIs). The review covers toxicology and chemistry. It should be read by anyone who is involved in salt selection and, more importantly, by those involved in regulatory affairs.

The main perceived problem is that alkyl sulfonate impurities can form during sulfonic acid salt formation. These alkyl methanesulfonates (mesylates) are toxic; the concern is that they can contaminate the APIs. The authors show that the chance of forming alkyl mesylates is extremely low, that they are usually not as toxic as the regulations would suggest, and that they are efficiently purged during workup and isolation.

The authors’ primary conclusion is that the regulatory guidelines on alkyl sulfonate impurities are seriously in need of updating. (Org. Process Res. Dev. DOI: 10.1021/op500397h; Will Watson)

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Reduce oxygen to hydrogen peroxide over porous carbon. Hydrogen peroxide (H2O2) is a common fine chemical that, because it is a strong oxidizer, is used in applications that range from disinfectants to rocket propellants. With an annual production of >2 million tonnes, H2O2 is produced mainly in  an anthraquinone process, which is a multistep, energy-intensive method.

A promising alternative is  to reduce oxygen electrochemically via a two-electron pathway, but a practical electrocatalyst has not been identified. X. Quan and co-workers at Dalian University of Technology (China) demonstrate the electroreduction activity of hierarchically porous carbon (HTC) materials for H2O2 production.

The researchers prepared the HTC materials by carbonizing MOF-5, a porous metal–organic framework composed of zinc nodes and terephthalic acid struts. The HTC “inherited” the porous features of MOF-5. With high surface areas (1635–2130 m2/g) and porosities, the HTC materials catalyzed H2O2 production at a rate of as much as 395.7 mmol/(h·g C) with high, stable current efficiency.

The authors attribute the remarkable activity and efficiency of the HTC materials to the high content of sp3-hybridized carbon atoms and defects that can serve as catalytic sites. In addition, the materials’ hierarchical porosity allows reactants ample access to the reaction sites.

The high-performance HTC-based electrocatalysts are a major step toward practical industrial electrochemical H2O2 production. The nonmetallic inorganic material is abundant and inexpensive. Compared with the anthraquinone process, HTC-based electroreduction is much greener and more cost-effective. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201502396; Xin Su)

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