Noteworthy Chemistry

February 16, 2015


Bioinspired gels convert nitrogen to ammonia. Soybeans do it, peanuts do it, even educated cowpeas do it—they convert atmospheric nitrogen to ammonia in the soil, in the dark. The chemical industry does it faster, but the Haber–Bosch process requires an iron catalyst, high (≈400 °C) temperature, and high (≈250 bar) pressure. Making ≈200 million t of ammonia annually consumes >1% of the world’s energy supply, according to M. G. Kanatzidis and co-workers at Northwestern University (Evanston, IL).

Nitrogen-fixing plants use nitrogenase enzymes that bind elemental nitrogen at iron molybdenum sulfide (FeMoS) core clusters. Nearby Fe–S clusters provide electrons that reduce the nitrogen to ammonia. The Northwestern group developed an accelerated version of the natural process by using FeMoS clusters and white light (or sunlight). They synthesized “chalcogels”: black, spongy, porous gels composed of inorganic Mo2Fe6S8 clusters linked by Sn2S6 ligands to form random, amorphous networks (see figure).

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Mo2Fe6S8–Sn2S6 chalcogel formation and structure (left); black chalcogel (right)

FeMoS catalysts previously were used to reduce hydrazine or hydrogen cyanide to ammonia, but they were ineffective at reducing elemental nitrogen except under strongly reducing conditions with electrochemical methods. Photochemical ammonia production without a catalyst requires ultraviolet light, semiconductor thin films, inert atmospheres, and temperatures as low as –78 °C.

In this study, the authors combined catalysis with photochemistry to produce ammonia from nitrogen with visible light, at room temperature, and under ambient pressure. They bubbled nitrogen through an aqueous solution of pyridinium hydrochloride (a proton source) and sodium ascorbate (an electron source) in which pieces of the chalcogel were immersed. The solutions were photolyzed with a 150-W xenon lamp. Isotope labeling tests confirmed that nitrogen gas was the source of the product ammonia.

Ammonia production began almost immediately and increased steadily over the 32-h test period. In a separate 72-h test, the researchers generated ≈8 equiv ammonia for each equivalent of catalyst, with no catalyst degradation or loss of activity. The chalcogel and light, proton, and electron sources are all necessary to make this method work. (J. Am. Chem. Soc. DOI: 10.1021/ja512491v; Nancy McGuire)

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This work helps us understand an N-demethylation reaction. Z. Časar* and T. Mesar at Lek Pharmaceuticals (Mengeš) and the University of Ljubljana (both in Slovenia) investigated the N-demethylation of a dihydrolysergic acid derivative with 2,2,2-trichloroethyl chloroformate. The study was conducted during the development of industrial-scale syntheses of the Parkinson’s disease drugs pergolide and cabergoline.

Standard conditions that use potassium hydrogen carbonate as the base gave low conversions (37–51%) and even lower isolated yields (30–35%). Analysis of the impurities formed in the reaction led to a proposed set of reaction pathways that show that the effect of trace amounts of water in the system is far more serious than was initially expected.

On the basis of the authors’ findings, water would be expected to react with trichloroethyl chloroformate to give HCl, CO2, and 2,2,2-trichloroethanol. The HCl would react with the base to generate another molecule of water, along with potassium chloride and CO2, to start the cycle again. HCl also can stop the desired N-demethylation reaction by protonating the N-methyl nitrogen atom.

The use of 4-(N,N-dimethylamino)pyridine as the base dramatically shortens the reaction time, conversion, and isolated yield—94% on a 51-kg scale. (Org. Process Res. Dev. DOI: 10.1021/op500394f, Will Watson)

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“Click” a functional group onto graphene oxide. Graphene, the 2010 Nobel Prize–winning substance, has many extraordinary physical and chemical properties. To take full advantage of this material, it is necessary to chemically functionalize it. Most graphene derivatives are made from graphene oxide (GO), but this method is limited by the low density and reactivity of oxygen-containing groups in GO.

J. Seppälä and co-workers at Aalto University School of Chemical Technology Espoo (Aalto, Finland) used thiol–ene click chemistry to develop an easy, efficient one-step process for functionalizing GO. Instead of targeting oxygen-containing groups, their procedure adds substituents directly to the C=C bonds in the graphene network.

The authors chose cysteamine hydrochloride [HS(CH2)2NH2·HCl, 2 in the figure] as the thiol source. In N,N-dimethylformamide (DMF) solvent, it reacts with GO (1) in the presence of the thermal radical catalyst 2,2′-azobisisobutyronitrile (AIBN). After heating at 70 ºC for 12 h and subsequent purification, nitrogen- and sulfur-containing GO (NS-GO, 3) was obtained as a black powder.

Click reaction of graphene oxide with cysteamine hydrochloride

Solubility tests showed that NS-GO disperses readily in water, ethanol, and ethylene glycol as single sheets. NS-GO forms nanocomposites with platinum nanoparticles, which can be made from chloroplatinic acid (H2PtCl6) in situ.

The C=C bonds in GO are more available to reactants than heteroatom-containing groups, which makes it easier to control the degree of functionalization. More importantly, NS-GO materials have excellent processibility and can be further chemically modified for various applications. (Chem. Eur. J. DOI: 10.1002/chem.201405734; Xin Su)

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Use an ionic liquid to recycle a rare-earth phosphor. Rare-earth (lanthanide) elements are used in several high-tech applications, but when devices that contain them reach the end of their useful lives, the lanthanides are rarely recycled. D. Dupont and K. Binnermans* at the Catholic University of Leuven (Belgium) prepared a betaine-based ionic liquid (IL) to recover these elements efficiently.

A typical lamp phosphor consists of 40–50% of the halophosphate phosphor HALO [(Sr,Ca)10(PO4)6(Cl,F)2:Sb3+,Mn2+], 20% of the red phosphor YOX (Y2O3:Eu3+), 5% of the blue phosphor BAM (BaMgAl10O17:Eu2+), and 6–7% of the green phosphor LAP (LaPO4:Ce3+,Tb3+) (see figure). For the solvent, the authors chose the Brønsted-acidic IL betainium bis(trifluoromethanesulfonyl)imide ([Hbet][Tf2N]) because it dissolves rare-earth and transition-metal oxides.

Recovery of the red phosphor YOX from a lamp phosphor

The researchers tested the IL with a mixture of the phosphors and observed that it selectively dissolves YOX as [(Y,Eu)bet][Tf2N]3 at 90 ºC in the presence of 5% water. The dissolved phosphor can be stripped from the IL phase by using aqueous hydrochloric acid or solid oxalic acid (H2C2O4). The better of the two methods is oxalic acid because it leaves the ionic liquid intact for recycling through the process.

After the rare-earth oxalate is precipitated, it is calcined at 950 ºC to recover YOX in the correct stoichiometry for reuse in lamps. This process is a green, efficient alternative to traditional recovery of phosphors by leaching. (Green Chem. DOI: 10.1039/C4GC02107J; José C. Barros)

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Extracellular DNA acts as an antimicrobial. Neutrophils are white blood cells that defend the body’s immune system by killing invading microbes. One way they do this is by producing neutrophil extracellular traps (NETs), weblike structures made of DNA scaffolding woven with antimicrobial proteins. NETs trap and kill microbes by an unknown mechanism.

S. Lewenza and colleagues at the University of Calgary (Alberta) hypothesized that the NET DNA backbone is itself antimicrobial. They previously showed that extracellular DNA chelates divalent metal cations; at higher concentrations, this can disrupt bacterial envelope integrity and lead to lysis and rapid cell death. The authors suggest that the DNA phosphodiester backbone contributes to NETs’ antibacterial activity by sequestering cations on the bacterial surface, destabilizing the membrane.

Pseudomonas aeruginosa and other bacteria typically die in the presence of extracellular DNA, but the authors found that bacterial survival is restored if the DNA is pretreated with the enzymes deoxyribonuclease (DNase), which degrades DNA, or alkaline phosphatase (PTase), which cleaves 5’-phosphates. Excess magnesium ion (Mg2+) also restores DNA by saturating its cation-chelating ability. All of these treatments neutralize DNA’s antibacterial activity.

The researchers confirmed that extracellular DNA kills bacteria by disrupting cell membranes via cation chelation. When the cation-chelation potential of the DNA backbone is neutralized with DNase, PTase, or Mg2+ pretreatments, extracellular DNA cannot destabilize the membrane and NETs’ bactericidal activity is blocked.

The authors also found that extracellular DNA antibacterial activity requires direct contact. Their observations suggest that the NET scaffold is not a passive defense mechanism, but that the NET DNA phosphodiester backbone actively lyses bacteria by membrane destabilization and is an antibacterial component of the structure. (PLoS Pathogens DOI: 10.1371/journal.ppat.1004593; Abigail Druck Shudofsky)

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These tiny supramolecular nanocapsules fluoresce efficiently. Organic fluorophores with planar polyaromatic structures often form weakly fluorescent aggregates spontaneously in aqueous media. Developing fluorogens that exhibit the opposite effect of aggregation-induced emission (AIE) is of great interest. Highly emissive AIE aggregates with sizes >30 nm have been made.

AIE-active supramolecular capsules, however, have not been prepared. M. Yoshizawa and colleagues at the Tokyo Institute of Technology (Yokohama) report the preparation of AIE nanocapsules with sizes as small as ≈2 nm.

The researchers designed and synthesized V-shaped polyaromatic amphiphiles that contain phenanthrene (1) or naphthalene (2) rings (see figure). The amphiphile molecules spontaneously and quantitatively form micelle-like spherical capsules with ≈2-nm diameters in water at room temperature. In contrast to “normal” polyaromatic aggregates with weak emission, the nanocapsules of 1 and 2 emit photoluminescence more efficiently than their monomeric counterparts.

The authors believe that AIE effects are caused by steric repulsion between the phenanthryl and phenyl rings in 1 and the naphthyl and phenyl rings in 2. (J. Am. Chem. Soc. DOI: 10.1021/ja511463k; Ben Zhong Tang)

Amphiphiles that form emissive nanocapsules