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Noteworthy Chemistry

July 15, 2013

A metal–organic framework separates hexane isomers. Hexane isomers, one of the major components in commercial gasoline, have research octane numbers (RONs) that range from 30 to 105. To increase the RON of a gasoline, it is desirable to separate the two dibranched hexanes (2,2- and 2,3-dimethylbutane) with higher RONs (94 and 105, respectively) from the others.

Traditional separation materials such as zeolites, however, are inefficient for enriching dibranched hexanes because hexanes are chemically inert and have similar polarizabilities. R. Krishna, J. R. Long, and colleagues at the University of California, Berkeley, the University of Amsterdam, the National Institute of Standards and Technology (Gaithersburg, MD), the University of Silesia (Katowice, Poland), the University of Delaware (Newark), and the University of Insubria (Como, Italy) developed a method to separate hexanes efficiently that uses the metal–organic framework (MOF) Fe2(BDP)3. (BDP is 1,4-benzenedipyrazolate).

Fe2(BDP)3 is produced in the reaction between Fe(acac)3 and H2BDP (1) in DMF (acac is acetylacetonato). Octahedral Fe(III) centers linked by μ2-pyrazolate units form 1-D chains that stack into triangular channels. Fe2(BDP)3 has great chemical and thermal stability and a Brunauer–Emmett–Teller (BET) surface area of 1230 m2/g.

The ligand H2BDP combines with Fe(III) to form an MOF

In pure-component equilibrium adsorption isotherm measurements at 200 °C and 100 mbar pressure, Fe2(BDP)3 adsorbed 60 vol% (100 wt%) more n-hexane than zeolite 5A. n-Hexane is disordered within the MOF. In contrast, the two dibranched hexane isomers undergo an ordered stepwise adsorption process at 160 °C and 100 mbar.

The strength of the interactions between hexane isomers and the Fe2(BDP)3 framework decreases as the degree of branching increases. The authors believe that this is a function of the enthalpies and entropies of adsorption.

In the breakthrough experiment, equimolar eutectic hexane isomers were passed over a Fe2(BDP)3 bed at 160 °C under nitrogen. The dibranched isomers eluted first, then monobranched isomers, and then n-hexane. The RON of the hexanes that eluted initially was >90, much greater than typical industrially refined hexanes (83). (Science 2013, 340, 960–964; Xin Su)

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Use triaxial electrospinning to tailor biodegradable nanofibers. J. F. Rabolt and colleagues at the University of Delaware (Newark) prepared the first triaxial electrospun fibers made from biodegradable polymers. The multilayered fibers consist of a poly(ε-caprolactone) (PCL) middle layer with a gelatin core and sheath.

The authors incorporated fluorescent markers in the spinning solutions to confirm the trilayered structure and to allow visual observation of the core and sheath thicknesses. Investigation of the fiber cross-sections with focused ion beam and field-emission scanning electron microscopy showed that the layers had the following thicknesses:

Data table. Layer Gel sheath : 130 nm. Layer PCL middle layer : 240 nm. Layer Gel core L 230 nm.

This work demonstrates the possibility of incorporating a variety of bioactive molecules into biodegradable nanofibers and tailoring release profiles by altering the layered morphology. (ACS Macro Lett. 2013, 2, 466–468; LaShanda Korley)

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Model zeolite framework flexibility effects on molecular diffusion rapidly and accurately. For calculating molecular diffusion through nanoporous materials, stipulating a rigid host framework saves considerable computation time and compensates for a lack of force field information. This technique, however, can limit the accuracy of the results. R. V. Awati, P. I. Ravikovitch, and D. S. Sholl* at Georgia Tech (Atlanta) and ExxonMobil Research and Engineering (Annandale, NJ) introduce two new methods for approximating framework flexibility that are based on a set of discrete rigid snapshots obtained from simulating the dynamics of an empty small-pore eight-member–ring silica zeolite framework.

In the first method, the authors introduced random snapshots into a molecular dynamics calculation at a frequency corresponding to the breathing motion of the nanopore windows. The positions of the methane adsorbate molecules are kept constant. The left-hand plot in the figure shows the accuracy of predicting self-diffusivity (Ds) of methane in the flexible structures of three zeolites using this method (black), time averaging (red), and energy minimization (green), compared with theoretical calculations that use a flexible structure.

Comparison of modeling methods to theoretical calculations

In the second method, the researchers performed transition-state theory calculations of cage-to-cage hopping rates in each snapshot and then averaged them over a distribution of snapshots. The right-hand plot shows a comparison for this method.

Both methods produce excellent agreement with direct molecular dynamics simulations that use fully flexible structures. These methods can predict infinite dilution and finite loading diffusivities of simple molecules through nanoporous frameworks even in the absence of accurate flexible force field information. (J. Phys. Chem. C 2013, 117, 13462–13473; Nancy McGuire)

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Capture a “naked” chloride ion in an imidotitanium cage. C. Lorber* and L. Vendier at the University of Toulouse (France) prepared an imidotitanium-based hexameric host cage and trapped an isolated chloride ion inside it. This is the newest member of the small collection of metal-based halide-encapsulating polyoxoanions.

The authors treated the dimeric imido-bridged complex [Ti(μ-NAr)(NMe2)2]2 (1, Ar = 2,6-i-Pr2C6H3) with 12 equiv Me3SiCl to obtain {[Ti(=NAr)Cl2]6Cl}Q+ (2) as green crystals in 82% yield. (Cation Q+ is a ≈4:1 mixture of [(Me3Si)2NMe2]+ and [(Me3Si)NHMe2]+.) The crystal structure of 2 contains a homometallic Ti6Cl12 spherical skeleton self-assembled in near–D3-symmetry (see figure). The average Ti–N bond length is 2.450 Å, and the internal void of the cage is 5.6 Å in diam.

Formation of an imidotitanium cage that contains a “naked” chloride ion

The “naked” chloride ion is trapped in the center of the neutral host cage with weak Ti–Cl interactions (2.824 Å Ti–Cl distance). The Ti–Cl attractions counterbalance the repulsions from the 12 surrounding chlorine atoms.

The authors estimate the volume of the inner space of the cage to be 3.1 Å3. This eliminates the possibility of exchange of the encapsulated chloride with larger halides (bromide and iodide).

When the authors used Me3SiBr instead of Me3SiCl to form the cage, the product was bromide-encapsulated {[Ti(=NAr)Brl2]6Br}Q+, the bromide equivalent of 2. The authors believe that the templating effect of chloride or bromide favors the formation of the hexameric cage. (Inorg. Chem. 2013, 52, 4756–4758; Xin Su)

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Enhance the selectivity of a chiral methylation. E. O’Brien and coauthors at Millennium: The Takeda Oncology Company (Cambridge, MA), Takeda Pharmaceutical (Osaka, Japan), AMRI (Albany, NY), Takeda California (San Diego), and Asymchem Life Science (Tianjin, China) report their progress toward a scalable synthesis of a drug that inhibits signaling processes of a rapamycin kinase.

One step called for the preparation of (S)-methyl 3-methylmorpholine-3-carboxylate. Classical resolution of the racemic mixture and cyclization of chiral N-benzyl-2-methylserine and CH2ClCOCl were ineffective, so the authors investigated the methylation of N-protected (S)-methyl morpholine-3-carboxylate at the 3-position.

The reaction is highly enantioselective (99% ee) when the N-butoxycarbonyl (Boc)–protected morpholine derivative is alkylated with MeI at –78 °C using sodium hexamethyldisilazide (NaHMDS) as the base. If the reaction is run between –50 and –40 °C, the enantioselectivity drops to 74%. KHMDS gives comparable selectivity to NaHMDS, but using LiHMDS as the base gives only 77% ee, possibly because chelation with the Boc group disrupts the morpholine conformation required for high selectivity.

The importance of the Boc group for the success of the chiral alkylation is underscored by the observation that when the N-benzyl–protected morpholine is used as substrate, the alkylation product is racemic with NaHMDS or LiHMDS at –78 °C. (Org. Process Res. Dev. 2013, 17, 829–837; Will Watson)

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“See” chemical bonds and molecular structures directly. Researchers have long wanted to watch how chemical reactions proceed., but molecules have always been too small to see. M. F. Crommie, F. R. Fischer, and many colleagues in Berkeley, CA (the University of California and Lawrence Berkeley National Laboratory) and San Sebastián, Spain (the Materials Physics Center, Donostia International Physics Center, and the University of the Basque Country) succeeded in directly monitoring single-molecule reactions and the structural changes involved.

Thermal cyclization reaction observed by nc-AFM

The authors accomplished the direct visualization with the aid of a technique called noncontact atomic force microscopy (nc-AFM). Whereas scanning tunneling microscopy gave blurred images, the powerful nc-AFM method, with subnanometer resolution, enabled the researchers to unambiguously see molecular structures of reactant 1 and its thermal cyclization products 24. The bond-resolved single-molecule images allowed the authors to obtain mechanistic insight into the chemistry of the complex reaction. (Science 2013, 340, 1434–1437; Ben Zhong Tang)

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