Noteworthy Chemistry

May 16, 2011

New phosphorogens emit efficiently in the crystalline state. Conventional phosphorescent molecules that emit efficiently in solution often cannot maintain their high emission efficiencies in the crystalline state. For example, a solution of tris(phenylpyridine)iridium has a phosphorescence quantum yield (ΦP) as high as 97%, whereas its solid film has a ΦP value as low as 3% because of the self-quenching effect caused by π-π stacking interactions.

T. Naota and co-workers at Osaka University (Japan) synthesized a series of trans-bis(salicylaldiminato)platinum(II) complexes (e.g., 1 and 2) from which they identified phosphorogens that are highly emissive in the crystalline state.

The complexes do not emit in the solution state. Their crystals’ emission efficiencies vary widely, depending on their conformational and packing structures. For example, crystals of 1 are nonphosphorescent (left photo). Its bent conformation hampers close packing, and the molecular motions in the loose crystals quench any potential emission.

In contrast, crystals of 2 emit strongly (right photo). Its neighboring molecules form dimeric structures that are firmly held by the coordination plane, the Pt–Pt interactions, and hydrogen bonding between the oligo(ethylene glycol) units. This collective structural rigidification makes 2 resistant to thermal perturbations and emissive at temperatures between 77 to 298 K. (J. Am. Chem. Soc. 2011, 21, 6493–6496; Ben Zhong Tang)

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Remove palladium and magnesium from Kumada products. M. J. Zacuto*, C. S. Shultz, and M. Journet at Merck (Rahway, NJ) carried out a Kumada coupling reaction between allylmagnesium chloride and 4-bromoisoindoline in toluene–THF. The product was an amine that has high water solubility at low pH, which would be the normal condition for an aqueous workup. Quenching the reaction mixture onto NH4Cl followed by adding NH4OH gave two clear phases that could be separated to isolate the product in the organic layer.

The organic layer, however, was dark colored; and the isolated product contained ≥500 ppm palladium. The authors improved the workup by quenching onto aqueous citric acid and separating and discarding the organic layer. They added fresh toluene and then NH4OH to adjust the pH of the aqueous layer to 10 to form a colorless organic layer. The product, isolated as its HCl salt, contained ≤25 ppm palladium. The citric acid also chelated the magnesium ion byproduct to solubilize it in the aqueous layer. (Org. Process Res. Dev. 2011, 15, 158–161; Will Watson)

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Benzotrithiophene copolymers may be useful semiconductors. Highly aromatic benzotrithiophene derivative 1 is a promising donor constituent that may be useful in donor–acceptor copolymers for organic photovoltaic materials. It also has the potential for forming a hole-transporting semiconductor for organic field-effect transistors.

C. B. Nielsen and co-workers at Imperial College London note that only the C3h-symmetric isomer 2 has been evaluated for optoelectronic applications. However, asymmetric structure 1 permits a direct conjugation path between the two free α-positions in the unsubstituted thiophene rings, a useful feature for highly conjugated polymers that contain this structure. The authors believe that the extended aromatic core will be useful for intermolecular π-stacking and charge transport, and they developed a method for synthesizing 1 and incorporating it into soluble benzothiadiazole copolymers.

The authors synthesized ketone-substituted 1 in three steps from 2,3-dibromothiophene with ≈23% overall yield. They converted the ketone to dibrominated comonomer 3 with N-bromosuccinimide (NBS). They also made corresponding alkyl-substituted dibromide 4 for possible use in the subsequent polymer-forming reaction.

Initial attempts to copolymerize 3 and 4 with 2,1,3-benzothiadiazolebis(boronic ester) 6 were marginal because of poor polymer solubility, even at modest molecular weights. The authors then modified 3 by converting the ketone to the more soluble neopentyl ketal 5, which was copolymerized with 6 under Suzuki–Miyaura coupling conditions. (p-TsOH is p-toluenesulfonic acid; dba is dibenzylideneacetone; o-Tol is o-tolyl.) Resulting soluble polymer 7 had a molecular weight (Mn) of 16.5 kDa. Treating 7 with acid formed target structure 8, but it had limited solubility.

The optical properties of copolymer 7 include an absorption maximum in solution of ≈550 nm and a 10–20-nm red shift from solution to the solid state. The planar, electron-rich aromatic core in 1 has potential for promoting the formation of donor–acceptor copolymers with tunable optoelectronic properties. (Org. Lett. 2011, 13, 2414–2417; W. Jerry Patterson)

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Be aware of toxic substances in toys and other children’s products. Children’s exposure to toxic substances is one of the greatest environmental health threats to families. The U.S. Consumer Product Safety Commission has restricted 14 chemicals, including 8 metals and metalloids (Sb, As, Ba, Cd, Cr, Pb, Hg, and Se) and 6 phthalates [bis(2-ethylhexyl), dibutyl, benzyl butyl, diisononyl, diisodecyl, and di-n-octyl], from toys and other children’s products.

M. Becker at Monica Becker & Associates Sustainability Consultants (Rochester, NY) and coauthors at the University of Massachusetts (Lowell) recommended that additional toxic chemicals be added to this list (Environ. Sci. Technol. 2010, 44, 7986–7991). Brominated flame retardants and bisphenol A are banned in some states and Canada. Carcinogenic azo dyes are not regulated in North America, and as many as 22 aromatic amines are limited in Europe. E. Gray, Jr. and coauthors at the U.S. Environmental Protection Agency and the National Institutes of Health (both in Research Triangle Park, NC) identified dipentyl phthalate as a toxicant (Toxicol. Sci. 2011, 120, 184–193). It is more toxic than the banned phthalates.

M. Guney and G. J. Zagury* at école Polytechnique Montréal report additional information about toxic chemicals in toys and children’s products. They warn the public of toxic chemicals that are not covered by the regulations. They have found more toxics in children’s products, such as potentially carcinogenic nitrosamines in rubber toys, and they appeal for additional regulations on toxic chemicals under new policies based on risk assessment—similar to those implemented in the European Union. (Environ. Sci. Technol. 2011, 45, 3819; Sally Peng Li)

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[We welcome returning contributor Gary A. Baker, who wrote for Heart Cut in the early 2000s.—Ed.]

Actinium-225–doped particles sequester daughter radionuclides. S. Mirzadeh and coauthors at Oak Ridge National Laboratory (TN) and the University of Tennessee (Knoxville) demonstrate the near-quantitative retention of the in vivo α-particle generator 225Ac (half-life [t½] = 10 days) within LaPO4 nanoparticles. The associated daughter radionuclides (221Fr, t½ = 4.9 min and 213Bi, t½ = 46.8 min) are also partially (≈50%) sequestered.

The La(225Ac)PO4 nanoparticles (1), which were subsequently coupled to the monoclonal antibody mAb 201B (2) that targets murine endothelium within the vasculature, accumulated rapidly in mouse lungs after intravenous injection, whereas nanoparticles coupled to control antibodies were redistributed primarily to the liver and spleen. Simultaneous analyses of the biodistribution of 225Ac and 213Bi showed that ≈80% of the daughters were retained at the target site at 24 h after injection.

This work is a significant step toward realizing the potential of 225Ac in targeted tumor therapy. The decay of 225Ac, followed by the rapid decay of its daughters, releases four energetic α-particles that range in energy from 5.7 to 8.4 MeV and are highly cytotoxic over a short range (≈100 μm) in tissue. If they are targeted to a tumor site, they can cause highly efficient, localized cell death within a three- to five-cell radius. 225Ac and its daughters, however, have high systemic toxicity and must be fully contained at the target site.

The challenge is to contain the daughter nuclides. Traditional organic chelators can hold 225Ac tightly but cannot maintain their grip on the energetically recoiling 221Fr nucleus after the initial decay. The rigid inorganic lattice introduced in this work, transparent to α-particles, can contain about half the daughters; and the authors suggest a clear path toward even better containment with the use of core–shell nanoparticles. Furthermore, using inorganic nanoparticles as carriers may allow a single delivery vehicle to contain multiple radionuclides for more effective therapy or therapy coupled with imaging.

This work is a milestone in the development of nanotechnology for medical applications. (Bioconjugate Chem. 2011, 22, 766–776; Gary A. Baker)

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Here’s a desalination method that doesn’t use membranes. Based on projected population growth rates, 90% of the Earth’s available fresh water is expected to be consumed by 2025. Consequently, efficient methods are needed to desalinate ocean water. Current desalination methods are membrane-based reverse osmosis and multistage flash distillation, both of which require high amounts of energy.

G. Chen and co-workers at MIT (Cambridge, MA) developed a desalination protocol that uses directional solvents that dissolve water but reject water-soluble salts. They selected decanoic acid as an effective directional solvent because it dissolves ≈3.8 wt% water at 34 °C and 5.9% at 80 °C, and its solubility in water is negligible.

The desalination method consists of a four-step cycle that in its unoptimized form can be completed in 74 h:

  1. Form a saline water-in-decanoic acid emulsion.
  2. Heat the emulsion to a top brine temperature between 40 and 80 °C to dissolve water in the oil.
  3. Settle and remove the brine.
  4. Cool the oil to 34 °C to separate the desalinated water.

The saline content of recovered water is between 0.07 and 0.11%, which falls within regulatory requirements of the World Health Organization and the U.S. Environmental Protection Agency. The energy consumed in this process is similar to or less than existing technologies if the top brine temperature is <50 °C. Heat recovery on the industrial scale would reduce the cost of the method. (Energy Environ. Sci. 2011, 4, 1672–1675; José C. Barros)

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Run a mild decarboxylative activation of malonic acid derivatives. D. Lafrance and co-workers at Pfizer (Groton, CT) evaluated enhanced synthetic methods for cyclopentanecarboxylic acid and found that treating cyclopentane-1,1-dicarboxylic acid with 1,1’-carbonyldiimidazole at ambient temperature forms the monocarbonylimidazole derivative cleanly, quickly, and quantitatively. This result suggests that the decarboxylation step proceeds rapidly under very mild conditions. The authors used this method to form carboxylic acid derivatives in a mild one-pot process.

In a typical example, treating malonic acid derivative 1 with 1,1’-carbonyldiimidazole reagent 2 produces N-acylimidazole intermediate 3 in high yield. A subsequent reaction with a nucleophile gives a high yield of amide derivative 4, the result of direct activation of the carbonyl position.

The reaction proceeds readily with a variety of alkyl-, cycloalkyl-, and arylmalonic acid derivatives. This one-pot process can also be used to form carboxylic acids, esters, Weinreb amides, sulfonamides, and β-keto esters. (Org. Lett. 2011, 13, 2322–2325 ol200575c; W. Jerry Patterson)

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