Noteworthy Chemistry

September 14, 2009

Nickel etching of graphene to produce graphene nanoribbons
Removal of trace metals with magnetic EDTA
Continuous hydrogenation of an aromatic dinitro compound
Unsubstituted BODIPY’s exceptionally high quantum yields
Particle obstruction influence on patterning of block copolymer films
Gelation-enhanced optical properties of arylethynyl luminogens
Base-directed stereochemistry of β-lactam synthesis

Etch single-layer graphene with nickel nanoparticles to produce graphene nanoribbons. Graphene exists in various nanostructures (e.g., nanoribbons) that have unique properties for advanced applications. According to P. Jarillo-Herrero and coauthors at MIT (Cambridge, MA), the Federal University of Minas Gerais (Belo Horizonte, Brazil), and the University of São Paulo (Brazil), nickel nanoparticles etch single-layer graphene (SLG) by catalytically hydrogenating carbon atoms. The carbon atoms on exposed graphene edges dissociate into the nanoparticles, then react with hydrogen at the nickel surfaces to form methane and leave trenches in the graphene edges (see figure).

The trenches form at angles of 60° and 120° and preserve the chirality of the edges, indicating that almost all the cuts in SLG run along the same crystallographic orientation. In addition, when an etching nanoparticle approaches within ~10 nm of a previously etched trench, it turns away before reaching the trench. Two possible causes of this behavior are: (1) coulomb interactions between nanoparticles (which are charged because of the graphene–Ni work function difference), and (2) the enhanced electronic density of states at the zigzag edge (which may prevent a nanoparticle from intersecting a previously etched trench).

Trenches cut in graphite usually meld or even cross when they approach each other, whereas in graphene the trenches are kept separate. Hence, the process that produces connected nanoribbons and other nanostructures in SLG does not occur in graphite samples. (Nano Lett. 2009, 9, 2600–2604; George Xiu Song Zhao)

Remove trace metals with magnetic EDTA. Chelators such as ethylenediaminetetraacetic acid (EDTA) are widely used to sequester heavy metals in aqueous solutions. The chelators do not remove the metals from solution; they only inactivate them. Removal is only possible via filtration or adsorption techniques, which usually require energy-intensive pumps.

W. J. Stark and co-workers at ETH Zürich (Switzerland) have developed magnetic nanoparticles modified with EDTA to remove heavy metals. Starting with carbon-coated iron nanomagnets, the researchers washed the nanomagnets with HCl, then treated them with polyethyleneimine (PEI) to provide free amine groups (1) on the magnet surfaces. The amine groups reacted with diethylenetriaminepentaacetic acid dianhydride to form an amide-linked EDTA-like ligand (2) after hydrolysis of the unreacted anhydride group. The nanomagnets were characterized by FTIR and elemental analysis.

The authors tested the nanomagnets’ efficacy for extracting various concentrations of Pb²⁺, Cd²⁺, and Cu²⁺. The magnets reduced Pb²⁺ and Cd²⁺ to levels near or below the US Environmental Protection Agency limits for drinking water (15 and 5 μg/L, respectively) in 5 min. Higher concentrations of lighter metals were not extracted efficiently. The efficiency of Cd²⁺ extraction dropped to 85% at pH <2.

Modified nanomagnets without complete carbon coatings or HCl treatment exhibited lower extraction power and contaminated the process’s wastewater with iron oxides. The authors performed a scaled-up version of the experiment using 8 L of water and 1 g of nanomagnets. Separation efficiency was lower because the magnetic field declined with distance. On the whole, however, using nanomagnets combines stability and easy separation with extraction efficiency, but chelating capacity decreases to 65% after the second application. (Chem. Commun. 2009, 4862–4864; José C. Barros)

Convert the batch hydrogenation of an aromatic dinitro compound to a continuous process. J. G. Van Alsten*, M. L. Jorgensen, and D. J. am Ende at Pfizer Global R&D (Groton, CT) describe a proof-of-concept study of the hydrogenation of an aromatic dinitro intermediate in the synthesis of the antismoking drug varenicline (marketed as Chantix in the United States and Champix in the European Union) to form the corresponding diamine.

The authors use a continuous stirred tank reactor system that consists of two reactors in series. Most of the reaction (~80%) takes place in the first reactor, and the reaction is completed in the second reactor. Although this initial study was successful, much work must to be done to develop a commercial continuous process to replace the current batch chemistry. (Org. Process Res. Dev. 2009, 13, 629–633; Will Watson)

Unsubstituted BODIPY displays exceptionally high quantum yields. The original discovery of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (also known as borondipyrromethene, or BODIPY) derivatives led to a significant body of research for potential electronic applications that include fluorescent switches, electroluminescent films, laser dyes, and chemosensors. Oddly, the parent (unsubstituted) system remained elusive until very recently.

B. Z. Tang, E. Peña-Cabrera, and coauthors at the University of Guanajuato (Mexico) and the Hong Kong University of Science and Technology observed that 8-methylthio-BODIPY derivatives (1) had been prepared by a mild general method. They also noted that the Fukumaya thioester reduction allows the alkylthio group in the ester to serve as a proton surrogate in aldehyde synthesis. Based on this, it seemed feasible to replace the methylthio group on 1 with a proton under Fukumaya conditions to give the elusive BODIPY parent compound (2). This approach proved successful and led to a highly efficient synthesis of 2 under reductive conditions in the presence of Et₃SiH. The ligand dba is dibenzylideneacetone; TFP is tri-2-furylphosphine; and CuTC is Cu(I) thienyl-2-carboxylate.

The authors’ method produced 2 in near-quantitative (98%) yield under mild conditions. The electronic properties of 2 were also exciting: BODIPY generates fluorescence quantum yields up to 93% in polar solvents, including a value of 90.3% in water at 10^–5 M.

Compound 2 is a crystalline red solid with excellent shelf life. It can be handled in air with no decomposition, although the authors caution that in powder form it should be handled in an efficient fume hood because it is a rather strong respiratory irritant. Compound 2 is quite emissive; it produces high quantum yields in nonpolar and polar solvents with absorption at 498 nm and fluorescence at 516 nm. The researchers suggest that structure 2 is a useful starting point for synthesizing similar dyes for specific electronic applications. They also propose exploiting the use of the alkylthio group as a proton surrogate in other systems because it gives high yields, is easy to remove, and uses mild reaction conditions. (J. Org. Chem. 2009, 74, 5719–5722; W. Jerry Patterson)

Particle obstruction influences patterning of block copolymer thin films. X. Zhang, J. F. Douglas, and colleagues at the National Institute of Standards and Technology (Gaithersburg, MD) have tuned block copolymer (BCP) morphology in thin films with stationary, adsorbed, nanoparticles. They coated polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) films onto 150-nm silica particles (comparable in size to the film thickness), then annealed the coated particles at ~164 °C for 22 h. In contrast to identically annealed PS-b-PMMA films that exhibit a cylindrical (parallel orientation) morphology, films of filled PS-b-PMMA display a periodic cylindrical pattern, alternating between parallel and perpendicular cylinders in a concentric ring.

The authors observed a primarily close-packed, perpendicular orientation at the nanoparticle–BCP boundary. They note that nanoparticle aggregates functioned as a single large “particle” in terms of influencing BCP pattern oscillation; more periodic variations (five versus two) were generated with the larger “particle”, suggesting substantial long-range interactions.

The authors listed several key parameters that influence the observed target-like oscillating BCP pattern: film thickness perturbations, residual stresses from entrapped solvent and thermal expansion upon annealing, and the presence of adsorbed nanoparticle constraints. For example, a two-stage annealing protocol, in which thermal treatment first occurs near the glass-transition temperature, relieved the residual stresses in the thin film and prevented the oscillating morphology. This research effort offers a new perspective on the influence of thermal processing conditions, film quality, and topological constraints in polymer manufacturing. (ACS Nano 2009, 3, 2115–2120; LaShanda Korley)

Manipulate the optical properties of arylethynyl luminogens by a gelation process. Self-aggregation of gelator molecules brings about entangled supramolecular fibrillar networks, or organogels, that have potential applications in sensing, catalysis, and molecular electronics. S.-S. Sun and coauthors at Academia Sinica (Taipei, Taiwan), National Taiwan Normal University (Taipei), and Tamkang University (Tamsui, Taiwan) have developed a series of functional organogels and found that their luminescence properties can be tuned to a great extent by the gelation process.

The researchers synthesized organogelators 1–4 with π-conjugated arylethynyl frameworks. The molecules form stable organogels in a variety of organic solvents with minimum gelation concentrations as low as 0.1 wt%. Gelation occurs by a combination of intermolecular hydrogen bonding, π–π stacking, and van der Waals interactions. The light-emitting behaviors of the gelators can be dramatically manipulated by the gelation process: Whereas the sols of the gelators are practically nonluminescent with negligible fluorescence quantum yields (Φ_F for 1 is 0.052%), their gels are highly efficient emitters (Φ_F for 3 is 17%). (Langmuir 2009, 25, 8714–8722; Ben Zhong Tang)

Direct cis- or trans-β-lactam synthesis by using the appropriate base. The extreme importance of the β-lactam scaffold is well established on the basis of its use in antibiotics. M. Shimizu and co-workers at Mie University (Tsu City, Japan) report an innovative method for synthesizing these structures in which an alkynyl imine undergoes conjugate addition with a ketene silyl acetal to form an iminocyclobutenone intermediate (1).

Structure 1 is reduced to the corresponding amino derivative (2), which undergoes thermal rearrangement in the presence of 1,4-dimethylpiperazine (DMP) to give β-lactams (cis-3) with cis-selectivities as high as cis/trans = 98:2. The same reaction in the presence of a stronger base such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) gives β-lactams (trans-3) with similarly high trans-selectivities (trans/cis as high as 97:3). The authors suggest that this method provides a practical alternative synthesis when compared with current preparations of chiral β-lactams because 2 can be easily prepared from readily available starting materials. (Org. Lett. 2009, 11, 3266–3268; W. Jerry Patterson)