Noteworthy Chemistry

May 21, 2012

BODIPY-like structures stabilize primary phosphanes. Primary phosphanes (RPH2) are notorious for spontaneously burning in air. L. J. Higham and co-workers at Newcastle University (Newcastle upon Tyne, UK), however, found that incorporating the PH2 group into highly conjugated systems can stabilize it without steric protection. They confirmed this observation by modeling the complexes with density functional theory calculations.

The researchers first treated a 4-bromophenyldipyrromethene boron difluoride (BODIPY) derivative (1) with 2 equiv PhLi to give B-diphenylated compound 2a. Using palladium catalysis, they coupled 2a with diethyl phosphonate to give phosphonate 3a. [The ligand dppb is 1,4-bis(diphenylphosphino)butane.] Treating 3a with LiAlH4 and Me3SiCl quantitatively reduced it to primary phosphane 4a. Dimethyl derivative 4b was synthesized similarly.

The authors showed that phosphanes 4a and 4b are air-stable in the solid state and in solution for >7 days. They attribute this air stability to the highly π-conjugated BODIPY backbone, which increases the energy levels of the neutral molecules and their radical cations. The photophysical properties of the BODIPY units change very little after the phosphano group is introduced. Replacing fluorine with phenyl drastically lowers the quantum yield of BODIPY, but methyl substitution does not decrease the quantum yield.

The authors also prepared chiral rhenium complex 5, which shows a much higher quantum yield (0.28) than common transition-metal fluorescence probes. The authors believe that replacing rhenium in 5 with 99mTc would produce a useful combination fluorescence–nuclear imaging reagent and have begun work toward this goal. (Angew. Chem., Int. Ed. 2012, 51, 4921–4924; Xin Su)

Manage quality parameters . . . and other factors. S. B. Bruggemeier and colleagues at Bristol-Myers Squibb (New Brunswick, NJ) describe a modeling-based approach to Quality by Design for a telescoped two-step process. An aspartic acid salt is neutralized in aq NaOH and CH2Cl2; then the resulting free amidine is treated with 4-chlorobenzenesulfonyl chloride under Schotten–Baumann conditions to make the amidine sulfonamide. The product is solvent-switched to EtOH for crystallization.

One of the critical quality attributes is the enantiomeric purity of the final product, which can degrade during the distillative crystallization. Distillation temperature and pressure and the amount of residual free amidine were among the critical process parameters that affected the enantiopurity of the final product.

When the authors determined the design space of these parameters, they had to consider other factors, such as CH2Cl2 emission limits during the distillation, the precision of the available pressure-control equipment, and the effect of residual amidine limits on the conditions for the sulfonylation step. (Org. Process Res. Dev. 2012, 16, 567–576; Will Watson)

This efficient field-effect transistor emits intense blue light. Creating highly fluorescent molecules with high charge mobilities in the solid state would lead to a variety of high-tech applications. For example, the fluorophore could be used to make organic light-emitting transistors that combine the functions of organic light-emitting diodes and field-effect transistors. These devices can potentially simplify the structures of active matrix displays. High charge mobilities usually require crystalline materials, but fluorescence is often suppressed in the crystalline state.

D. F. Perepichka and coauthors at McGill University (Montreal), the National Institute of Scientific Research (Varennes, QU), Tohoku University (Sendai, Japan), and Waseda University (Tokyo) report that crystals of 2-(4-hexylphenylvinyl)anthracene (1) exhibit a unique combination of charge mobility as high as 2.6 cm2(V·s) and strong blue-light emission (fluorescence quantum yields as high as 70%). This result disproves the common belief that the crystallinity required for high charge mobility necessarily leads to fluorescence quenching in the solid state. (Angew. Chem., Int. Ed. 2012, 51, 3837–3841; Ben Zhong Tang)

1-D metal–organic nanotubes conduct protons. Much research has been reported on metal–organic frameworks (MOFs), but there are only a handful of MOFs that form metal–organic nanotubes (MONTs). Of these, most are interconnected by metal ions; few self-assemble through hydrogen bonding.

Reports of MONTs are scarce, and accounts of their fundamental applications are even scarcer. So one might ask: What fundamental properties might 1-D MONTs possess that would make them useful for real-world applications?

Research reported by T. Panda, T. Kundu, and R. Banerjee* at the National Chemical Laboratory (Pune, India) begins to answer that question. They synthesized MONTs made from a 5-triazoleisophthalic acid (5TIA) building block with an In(III) or Cd(II) metal center. The resulting In-5TIA and Cd-5TIA MONTs are held together by hydrogen bonds to form 1-D nanotubes with inner dimensions of 7.85 and 8.23 Å, respectively.

Characterization studies demonstrated that the MONTs form crystals that are stable up to 150 °C. At ambient temperature and 98% relative humidity, the MONTs have proton conductivities of 5.35 x 10–5 and 3.61 x 10–5 S/cm, respectively, making them two of the first (if not the first) examples of self-assembled MONTs capable of conducting protons.

The authors believe that the observed proton conduction occurs by a carrier-mediated pathway in the pores of the materials that follows a Grotthus proton-hopping mechanism. The presence of water plays an important role in the conductivity, which decreases with decreasing humidity. This study may lead to new applications for these emerging materials. (Chem. Commun. 2011, 48, 4998–5000; Gary A. Baker)

Use this metal–organic material as a carbon dioxide trap. One way to curb CO2 emissions is to selectively capture CO2 from coal-fired power plant effluents. Aqueous amine solutions are currently the most viable adsorbents, but their use may increase electricity prices and reduce power plant efficiency. Solid CO2 adsorbents may be a better alternative, and they would have other applications such as removing CO2 from air prior to cryogenic distillation for nitrogen and oxygen production; controlling CO2 levels in confined spaces (e.g., in space stations and submarines); and even trapping CO2 directly from the atmosphere.

C. S. Hong at Korea University (Seoul), J. R. Long at the University of California, Berkeley, and colleagues synthesized amine-functionalized metal–organic adsorbents. They treated 4,4′-dihydroxybiphenyl (1) with 2 equiv MgBr2·6H2O to produce a metal–organic framework (MOF) symbolized by 2, and then functionalized the MOF with 1.6 equiv N,N’-dimethylethylenediamine (mmen).

Adding the diamine functionality to 2 causes it to adsorb 15 times more CO2 than 2 alone. Moreover, 2–mmen has high affinity for CO2 at very low pressures and adsorbs more than twice the amount of CO2 as zeolite 5A, which is currently used in the International Space Station. Complex 2–mmen is also highly selective for CO2, and it is regenerated easily with no apparent loss of capacity.

More testing is needed, but 2–mmen may be the precursor to a new generation of CO2 trapping agents. The authors plan to test it under humid conditions and to attempt to improve separation performance by introducing other polyamines in place of mmen. (J. Am. Chem. Soc. 2012, 134, 7056–7065, Chaya Pooput)

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