Jul 24, 2014

Self-control tames the coupling of reactive radicals

  • Guy C. Lloyd-Jones and
  • Liam T. Ball
Science 25 July 2014: 381-382.
Iridium complexes use two points of contact to control carbon-carbon bond formation [Also see Reports by Tellis et al. and Zuo et al.]
Combining two catalysts, one light-activated, facilitates bond formation between saturated and unsaturated carbons. [Also see Perspective by Lloyd-Jones and Ball]
 The routine application of Csp3-hybridized nucleophiles in cross-coupling reactions remains an unsolved challenge in organic chemistry. The sluggish transmetalation rates observed for the preferred organoboron reagents in such transformations are a consequence of the two-electron mechanism underlying the standard catalytic approach. We describe a mechanistically distinct single-electron transfer-based strategy for the activation of organoboron reagents toward transmetalation that exhibits complementary reactivity patterns. Application of an iridium photoredox catalyst in tandem with a nickel catalyst effects the cross-coupling of potassium alkoxyalkyl- and benzyltrifluoroborates with an array of aryl bromides under exceptionally mild conditions (visible light, ambient temperature, no strong base). The transformation has been extended to the asymmetric and stereoconvergent cross-coupling of a secondary benzyltrifluoroborate.

Combining two catalysts, one light-activated, facilitates bond formation between saturated and unsaturated carbons. [Also see Perspective by Lloyd-Jones and Ball]
Over the past 40 years, transition metal catalysis has enabled bond formation between aryl and olefinic (sp2) carbons in a selective and predictable manner with high functional group tolerance. Couplings involving alkyl (sp3) carbons have proven more challenging. Here, we demonstrate that the synergistic combination of photoredox catalysis and nickel catalysis provides an alternative cross-coupling paradigm, in which simple and readily available organic molecules can be systematically used as coupling partners. By using this photoredox-metal catalysis approach, we have achieved a direct decarboxylative sp3–sp2 cross-coupling of amino acids, as well as α-O– or phenyl-substituted carboxylic acids, with aryl halides. Moreover, this mode of catalysis can be applied to direct cross-coupling of Formula–H in dimethylaniline with aryl halides via C–H functionalization.

A core-shell structure provides a cloak for objects within a diffusive medium. [Also see Perspective by Smith]
  • Abstract
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  • Supplementary Materials 
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  • In vacuum, air, and other surroundings that support ballistic light propagation according to Maxwell’s equations, invisibility cloaks that are macroscopic, three-dimensional, broadband, passive, and that work for all directions and polarizations of light are not consistent with the laws of physics. We show that the situation is different for surroundings leading to multiple light scattering, according to Fick’s diffusion equation. We have fabricated cylindrical and spherical invisibility cloaks made of thin shells of polydimethylsiloxane doped with melamine-resin microparticles. The shells surround a diffusively reflecting hollow core, in which arbitrary objects can be hidden. We find good cloaking performance in a water-based diffusive surrounding throughout the entire visible spectrum and for all illumination conditions and incident polarizations of light.

NIH institute considers broad shift to ‘people’ awards

  • Jocelyn Kaiser
Science 25 July 2014: 366-367.
Approach promises freedom from grant writing but could favor established researchers.

Fullerene Van der Waals Oligomers as Electron Traps

Publication Date (Web): July 22, 2014 (Communication)
DOI: 10.1021/ja505949m
 
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Density functional theory calculations indicate that van der Waals fullerene dimers and larger oligomers can form interstitial electron traps in which the electrons are even more strongly bound than in isolated fullerene radical anions. The fullerenes behave like “super atoms”, and the interstitial electron traps represent one-electron intermolecular σ-bonds. Spectroelectrochemical measurements on a bis-fullerene-substituted peptide provide experimental support. The proposed deep electron traps are relevant for all organic electronics applications in which non-covalently linked fullerenes in van der Waals contact with one another serve as n-type semiconductors.
 

Jul 21, 2014

Publication Date (Web): July 21, 2014 (Communication)
DOI: 10.1021/ja505755k
 
JACS ASAP PDF 
 
Abstract Image
The triplet excited state of acridine orange (3*AO) undergoes a proton-coupled electron transfer (PCET) reaction with tri-tert-butylphenol (ttbPhOH) in acetonitrile. Each of the reaction components possesses a spectroscopic signature, providing a rare opportunity to monitor the individual proton transfer, electron transfer, and H-transfer components in parallel via transient absorption spectroscopy. This enhanced optical tracking, along with excited-state thermochemical analysis, facilitates assignment of the mechanism of excited-state PCET reactivity. 3*AO is quenched via concerted proton–electron transfer (CPET) from ttbPhOH to form acridine radical (AOH) and ttbPhO (kCPET = 3.7 × 108 M–1 s–1, KIE = 1.3). Subsequently, AOH reduces the phenoxyl radical (kET = 5.5 × 109 M–1 s–1), forming AOH+ and ttbPhO, followed by proton transfer (kPT = 1.0 × 109 M–1 s–1) to regenerate the starting reactants.
 
Toward Understanding the Growth Mechanism: Tracing All Stable Intermediate Species from Reduction of Au(I)–Thiolate Complexes to Evolution of Au25 Nanoclusters
Publication Date (Web): July 11, 2014 (Communication)
DOI: 10.1021/ja505429f

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Despite 20 years of progress in synthesizing thiolated gold nanoclusters (Au NCs), the knowledge of their growth mechanism still lags behind. Herein the detailed process from reduction of Au(I)–thiolate complex precursors to the eventual evolution of and focusing to the atomically precise Au25 NCs was revealed for the first time by monitoring the time evolution of Au(I) precursor and Au NC intermediate species with ESI-MS. A two-stage, bottom-up formation and growth process was proposed: a fast stage of reduction-growth mechanism, followed by a slow stage of intercluster conversion and focusing. Balanced reactions of formation for each identified NC were suggested, backed by theoretical calculations of the thermodynamic driving force. This work advances one step further toward understanding the mechanism of formation and growth of thiolated Au NCs.

Jul 18, 2014

Creating antioxidants by oxidation catalysis

  • Boris J. Nachtsheim
Science 18 July 2014: 270-271.
A key component of vitamin E can be synthesized without use of expensive transition metal catalysts. [Also see Report by Uyanik et al.]
An iodide salt with a chiral counterion proves an efficient catalyst for preparation of compounds analogous to vitamin E. [Also see Perspective by Nachtsheim]

Abstract

The diverse biological activities of tocopherols and their analogs have inspired considerable interest in the development of routes for their efficient asymmetric synthesis. Here, we report that chiral ammonium hypoiodite salts catalyze highly chemo- and enantioselective oxidative cyclization of γ-(2-hydroxyphenyl)ketones to 2-acyl chromans bearing a quaternary stereocenter, which serve as productive synthetic intermediates for tocopherols. Raman spectroscopic analysis of a solution of tetrabutylammonium iodide and tert-butyl hydroperoxide revealed the in situ generation of the hypoiodite salt as an unstable catalytic active species and triiodide salt as a stable inert species. A high-performance catalytic oxidation system (turnover number of ~200) has been achieved through reversible equilibration between hypoiodite and triiodide in the presence of potassium carbonate base. We anticipate that these findings will open further prospects for the development of high-turnover redox organocatalysis.


Charge transfer goes the distance

  • Stephen T. Pratt
Science 18 July 2014: 267-268.
Electronic relaxation following x-ray excitation illuminates steps in molecular dissociation. [Also see Report by Erk et al.]
A free-electron laser enables precise tracking of electron movement between segments of a dissociating molecule. [Also see Perspective by Pratt]

Abstract

Studies of charge transfer are often hampered by difficulties in determining the charge localization at a given time. Here, we used ultrashort x-ray free-electron laser pulses to image charge rearrangement dynamics within gas-phase iodomethane molecules during dissociation induced by a synchronized near-infrared (NIR) laser pulse. Inner-shell photoionization creates positive charge, which is initially localized on the iodine atom. We map the electron transfer between the methyl and iodine fragments as a function of their interatomic separation set by the NIR–x-ray delay. We observe signatures of electron transfer for distances up to 20 angstroms and show that a realistic estimate of its effective spatial range can be obtained from a classical over-the-barrier model. The presented technique is applicable for spatiotemporal imaging of charge transfer dynamics in a wide range of molecular systems.


Publication Date (Web): July 16, 2014 (Article)
DOI: 10.1021/ja503603k
JACS ASAP PDF 
 
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Interfacial hole transfer between n-SrTiO3 and OH was investigated by surface sensitive transient optical spectroscopy of an in situ photoelectrochemical cell during water oxidation. The kinetics reveal a single rate constant with an exponential dependence on the surface hole potential, spanning time scales from 3 ns to 8 ps over a ≈1 V increase. A voltage- and laser illumination-induced process moves the valence band edge at the n-type semiconductor/water interface to continuously change the surface hole potential. This single step of the water oxidation reaction is assigned to the first hole transfer h+ + OH → OH. The kinetics quantify how much a change in the free energy difference driving this first hole transfer reduces the activation barrier. They are also used to extrapolate the kinetic rate due to the activation barrier when that free energy difference is zero, or the Nernstian potential. This is the first time transient spectroscopy has enabled the separation of the first hole transfer from the full four hole transfer cycle and a direct determination of these two quantities. The Nernstian potential for OH/OH is also suggested, in rough agreement with gas-phase studies. The observation of a distinct, much longer time scale upon picosecond hole transfer to OH suggests that a dominant, more stable intermediate of the water oxidation reaction, possibly a surface bound oxo, may result.
 

Jul 15, 2014

A method for estimating the uncertainty of calculated properties in density functional theory is introduced.
We introduce a general method for estimating the uncertainty in calculated materials properties based on density functional theory calculations. We illustrate the approach for a calculation of the catalytic rate of ammonia synthesis over a range of transition-metal catalysts. The correlation between errors in density functional theory calculations is shown to play an important role in reducing the predicted error on calculated rates. Uncertainties depend strongly on reaction conditions and catalyst material, and the relative rates between different catalysts are considerably better described than the absolute rates. We introduce an approach for incorporating uncertainty when searching for improved catalysts by evaluating the probability that a given catalyst is better than a known standard.

Fishing for peroxidase protons

  • John T. Groves and
  • Nicholas C. Boaz
Science 11 July 2014: 142-143.
Where are the protons in heme protein catalysis? [Also see Report by Casadei et al.]
The sensitivity of neutron scattering to proton locations clarifies the acid/base chemistry of a biochemical oxidation. [Also see Perspective by Groves and Boaz]
Heme enzymes activate oxygen through formation of transient iron-oxo (ferryl) intermediates of the heme iron. A long-standing question has been the nature of the iron-oxygen bond and, in particular, the protonation state. We present neutron structures of the ferric derivative of cytochrome c peroxidase and its ferryl intermediate; these allow direct visualization of protonation states. We demonstrate that the ferryl heme is an Fe(IV)=O species and is not protonated. Comparison of the structures shows that the distal histidine becomes protonated on formation of the ferryl intermediate, which has implications for the understanding of O–O bond cleavage in heme enzymes. The structures highlight the advantages of neutron cryo-crystallography in probing reaction mechanisms and visualizing protonation states in enzyme intermediates.

Publication Date (Web): June 30, 2014 (Article)
DOI: 10.1021/ja504460w
 
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We present a systematic electrochemical and spectroelectrochemical study of the catalytic activity for water oxidation of an iridium-N-dimethylimidazolin-2-ylidene (Ir–NHC–Me2) complex adsorbed on a polycrystalline gold electrode. The work aims to understand the effect of the electrolyte properties (anions and acidity) on the activity of the molecular catalyst and check its stability toward decomposition. Our results show that the iridium complex displays a very strong dependence on the electrolyte properties such that large enhancements in catalytic activity may be obtained by adequately choosing pH and anions in the electrolyte. The stability of the adsorbed compound was investigated in situ by Surface Enhanced Raman Spectroscopy and Online Electrochemical Mass Spectrometry showing that the catalyst exhibits good stability under anodic conditions, with no observable evidence for the decomposition to iridium oxide.
 
Publication Date (Web): July 10, 2014 (Communication)
DOI: 10.1021/ja505387c
 
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Relaxation of photoexcited chromophores is a key factor determining diverse molecular properties, from luminescence to photostability. Radiationless relaxation usually occurs through state intersections caused by distortions in the nuclear geometry of the chromophore. Using excited-state nonadiabatic dynamics simulations based on algebraic diagrammatic construction, it is shown that this is the case of 9H-adenine in water cluster, but not of 7H-adenine in water cluster. 7H-adenine in water cluster relaxes via a state intersection induced by electron transfer from water to the chromophore. This result reveals an unknown reaction pathway, with implications for the assignment of relaxation mechanisms of exciton relaxation in organic electronics. The observation of photorelaxation of 7H-adenine induced by water–chromophore electron transfer is a proof of principle calling for further computational and experimental investigations to determine how common this effect is.
 

Shape-Dependent Hydrogen-Storage Properties in Pd Nanocrystals: Which Does Hydrogen Prefer, Octahedron (111) or Cube (100)?

Publication Date (Web): July 14, 2014 (Communication)
DOI: 10.1021/ja504699u


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Pd octahedrons and cubes enclosed by {111} and {100} facets, respectively, have been synthesized for investigation of the shape effect on hydrogen-absorption properties. Hydrogen-storage properties were investigated using in situ powder X-ray diffraction, in situ solid-state 2H NMR and hydrogen pressure–composition isotherm measurements. With these measurements, it was found that the exposed facets do not affect hydrogen-storage capacity; however, they significantly affect the absorption speed, with octahedral nanocrystals showing the faster response. The heat of adsorption of hydrogen and the hydrogen diffusion pathway were suggested to be dominant factors for hydrogen-absorption speed. Furthermore, in situ solid-state 2H NMR detected for the first time the state of 2H in a solid-solution (Pd + H) phase of Pd nanocrystals at rt.

Uniform Doping of Metal Oxide Nanowires Using Solid State Diffusion

Publication Date (Web): July 15, 2014 (Article)
DOI: 10.1021/ja505734s
 
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The synthesis of one-dimensional nanostructures with specific properties is often hindered by difficulty in tuning the material composition without sacrificing morphology and material quality. Here, we present a simple solid state diffusion method utilizing atomic layer deposition to controllably alter the composition of metal oxide nanowires. This compositional control allows for modification of the optical, electronic, and electrochemical properties of the semiconductor nanowires. Using this method and a novel process for manganese oxide atomic layer deposition, we produced manganese-doped rutile TiO2 nanowires and investigated their structural and photoelectrochemical properties. A homogeneous incorporation of the Mn dopant into the rutile lattice was observed, and the local chemical environment of the Mn was determined using X-ray absorption spectroscopy. The doping process resulted in a tunable enhancement in the electrocatalytic activity for water oxidation, demonstrating that this simple and general method can be used to control the properties of one-dimensional nanostructures for use in a variety of applications including solar-to-fuel generation.
 

Reducing CO2 to Methanol Using Frustrated Lewis Pairs: On the Mechanism of Phosphine–Borane-Mediated Hydroboration of CO2

Publication Date (Web): June 20, 2014 (Article)
DOI: 10.1021/ja5047846
 
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The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2–C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.
 

Hydration of Gaseous m-Aminobenzoic Acid: Ionic vs Neutral Hydrogen Bonding and Water Bridges

Publication Date (Web): June 27, 2014 (Article)
DOI: 10.1021/ja5045874
 
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Hydration of a protonated amine and a neutral carboxylic acid were investigated for protonated m-aminobenzoic acid (MABAH+) with up to 15 water molecules attached using infrared photodissociation spectroscopy, laser-induced dissociation kinetics, and computational chemistry. A free COO–H stretch in the spectra of MABAH+·(H2O)1–5 indicates that water does not bind to the carboxylic acid H atom. This band is absent in the spectrum of MABAH+ with six or more water molecules attached, and there is a hydrogen-bonded (HB) COO–H stretch indicating that water hydrogen bonds to the carboxylic acid H atom for these larger clusters. Photodissociation kinetic data for MABAH+·(H2O)6 indicate that greater than 74 ± 13% of the ion population consists of the HB COO–H isomer, consistent with this isomer being ≥0.5 kJ mol–1 lower in energy than isomers where the carboxylic acid H atom does not donate a hydrogen bond. Calculations at the B3LYP/6-31+G** and MP2/6-31+G**//B3LYP/6-31+G** levels of theory indicate that this energy difference is 3–5 kJ mol–1, in agreement with the experimental results. Lower effective ion heating rates, either by attenuation of the laser power or irradiation of the ions at a lower frequency, result in more time for interconversion between the free and HB COO–H isomers. These data suggest that the barrier to dissociation for the free COO–H isomer is less than that for the HB COO–H isomer but greater than the barrier for interconversion between the two isomers. These results show the competition between hydration of a primary protonated amine vs that of a neutral carboxylic acid and the effect of water bridging between the two functional groups, which provide valuable insight into the hydration of protonated amino acids and establish rigorous benchmarks for theoretical modeling of water–biomolecule interactions.
 

Jul 9, 2014

Connecting the dots

  • Michael A. Boles and
  • Dmitri V. Talapin
Science 20 June 2014: 1340-1341.
Understanding nanocrystal surfaces helps to direct their assembly into novel material architectures. [Also see Reports by Boneschanscher et al. and Zherebetskyy et al.]
Metal-chalcogenide nanocrystals undergo necking and large-scale atomic rearrangements when forming a surface lattice. [Also see Perspective by Boles and Talapin]
The surfaces of lead sulfide nanocrystals capped with an organic acid can also bear hydroxyl groups. [Also see Perspective by Boles and Talapin]

Sulfur-mediated electron shuttling during bacterial iron reduction

Bacterial respiration of ferric iron involves sulfur intermediates in alkaline conditions [Also see Perspective by Friedrich and Finster]

 

Abstract

Microbial reduction of ferric iron [Fe(III)] is an important biogeochemical process in anoxic aquifers. Depending on groundwater pH, dissimilatory metal-reducing bacteria can also respire alternative electron acceptors to survive, including elemental sulfur (S0). To understand the interplay of Fe/S cycling under alkaline conditions, we combined thermodynamic geochemical modeling with bioreactor experiments using Shewanella oneidensis MR-1. Under these conditions, S. oneidensis can enzymatically reduce S0 but not goethite (α-FeOOH). The HS produced subsequently reduces goethite abiotically. Because of the prevalence of alkaline conditions in many aquifers, Fe(III) reduction may thus proceed via S0-mediated electron-shuttling pathways.

X-ray birefringence imaging

An x-ray birefringence imaging technique, analogous to polarized optical microscopy, can detect bond orientation in crystals [Also see Perspective by Lidin]

 

Abstract

The polarizing optical microscope has been used since the 19th century to study the structural anisotropy of materials, based on the phenomenon of optical birefringence. In contrast, the phenomenon of x-ray birefringence has been demonstrated only recently and has been shown to be a sensitive probe of the orientational properties of individual molecules and/or bonds in anisotropic solids. Here, we report a technique—x-ray birefringence imaging (XBI)—that enables spatially resolved mapping of x-ray birefringence of materials, representing the x-ray analog of the polarizing optical microscope. Our results demonstrate the utility and potential of XBI as a sensitive technique for imaging the local orientational properties of anisotropic materials, including characterization of changes in molecular orientational ordering associated with solid-state phase transitions and identification of the size, spatial distribution, and temperature dependence of domain structures.

Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation

  • Shu Hu,
  • Matthew R. Shaner,
  • Joseph A. Beardslee,
  • Michael Lichterman,
  • Bruce S. Brunschwig,
  • and Nathan S. Lewis
Science 30 May 2014: 1005-1009.
An amorphous titanium oxide coating protects semiconductors by conducting photogenerated charge carriers to nickel catalysts.

 

Abstract

Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel production, these materials are unstable in aqueous media. We show that TiO2 coatings (4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidation of 1.0 molar aqueous KOH to O2 for more than 100 hours at photocurrent densities of >30 milliamperes per square centimeter and ~100% Faradaic efficiency. TiO2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V and light-limiting photocurrent densities of 14.3 and 3.4 milliamperes per square centimeter, respectively, for water oxidation.

Real-space imaging of molecular structure and chemical bonding by single-molecule inelastic tunneling probe

  • Chi-lun Chiang,
  • Chen Xu,
  • Zhumin Han,
  • and W. Ho
Science 23 May 2014: 885-888.
Vibrational excitation of a carbon monoxide molecule on a scanning tunneling microscope tip reveals bonding within adsorbed molecules

 

Abstract

The arrangement of atoms and bonds in a molecule influences its physical and chemical properties. The scanning tunneling microscope can provide electronic and vibrational signatures of single molecules. However, these signatures do not relate simply to the molecular structure and bonding. We constructed an inelastic tunneling probe based on the scanning tunneling microscope to sense the local potential energy landscape of an adsorbed molecule with a carbon monoxide (CO)–terminated tip. The skeletal structure and bonding of the molecule are revealed from imaging the spatial variations of a CO vibration as the CO-terminated tip probes the core of the interactions between adjacent atoms. An application of the inelastic tunneling probe reveals the sharing of hydrogen atoms among multiple centers in intramolecular and extramolecular bonding.

Recyclable, Strong Thermosets and Organogels via Paraformaldehyde Condensation with Diamines

  • Jeannette M. García,
  • Gavin O. Jones,
  • Kumar Virwani,
  • Bryan D. McCloskey,
  • Dylan J. Boday,
  • Gijs M. ter Huurne,
  • Hans W. Horn,
  • Daniel J. Coady,
  • Abdulmalik M. Bintaleb,
  • Abdullah M. S. Alabdulrahman,
  • Fares Alsewailem,
  • Hamid A. A. Almegren,
  • and James L. Hedrick
Science 16 May 2014: 732-735.
A strong polymer formed by heating can be digested with strong acid to recover and recycle its bisaniline monomers. [Also see Perspective by Long]

 

Abstract

Nitrogen-based thermoset polymers have many industrial applications (for example, in composites), but are difficult to recycle or rework. We report a simple one-pot, low-temperature polycondensation between paraformaldehyde and 4,4ʹ-oxydianiline (ODA) that forms hemiaminal dynamic covalent networks (HDCNs), which can further cyclize at high temperatures, producing poly(hexahydrotriazine)s (PHTs). Both materials are strong thermosetting polymers, and the PHTs exhibited very high Young’s moduli (up to ~14.0 gigapascals and up to 20 gigapascals when reinforced with surface-treated carbon nanotubes), excellent solvent resistance, and resistance to environmental stress cracking. However, both HDCNs and PHTs could be digested at low pH (<2) to recover the bisaniline monomers. By simply using different diamine monomers, the HDCN- and PHT-forming reactions afford extremely versatile materials platforms. For example, when poly(ethylene glycol) (PEG) diamine monomers were used to form HDCNs, elastic organogels formed that exhibited self-healing properties.
pp 8459–8466
Publication Date (Web): May 8, 2014 (Article)
DOI: 10.1021/ja503494b
 
JACS 2014, 136 (23), pp 8459–8466 PDF
 
 
 
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Mechanistic details pertaining to the SmI2–H2O-mediated reduction and reductive coupling of 6-membered lactones, the first class of simple unactivated carboxylic acid derivatives that had long been thought to lie outside the reducing range of SmI2, have been elucidated. Our results provide new experimental evidence that water enables the productive electron transfer from Sm(II) by stabilization of the radical anion intermediate rather than by solely promoting the first electron transfer as originally proposed. Notably, these studies suggest that all reactions involving the generation of ketyl-type radicals with SmI2 occur under a unified mechanism based on the thermodynamic control of the second electron transfer step, thus providing a blueprint for the development of a broad range of novel chemoselective transformations via open-shell electron pathways.
 
pp 8907–8910
Publication Date (Web): June 12, 2014 (Communication)
DOI: 10.1021/ja504484a
 
JACS 2014, 136 (25), pp 8907–8910 PDF
 
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Electron transfer in azobenzene derivatives bearing two carboxylic acid groups is coupled with intramolecular proton transfer in a stepwise manner in the title 2e + 2H+ redox couple. The presence of the pendant acid–base functions pushes the redox chemistry of the azo/hydrazo couple toward positive potentials by as much as 0.75 V. This is essentially the result of H-bonding of one of the nitrogen atoms by the neighboring carboxylic group and H-bonding of one carboxylate by the neighboring protonated nitrogen atom. The two electron-transfer reactions, particularly the second one, are accompanied by strong structural changes, which results in the occurrence of a square scheme mechanism in which electron transfer and structural change are not concerted. These are typical phenomena that are likely to be encountered when attempting to boost proton-coupled electron-transfer stoichiometric or catalytic processes by installing pendant acid–base functionalities in the close vicinity of the reacting center.
 

Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric–Superoxo Complex

Publication Date (Web): June 22, 2014 (Article)
DOI: 10.1021/ja503105b
 
JACS ASAP PDF
 
Abstract Image
To obtain mechanistic insights into the inherent reactivity patterns for copper(I)–O2 adducts, a new cupric–superoxo complex [(DMM-tmpa)CuII(O2•–)]+ (2) [DMM-tmpa = tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine] has been synthesized and studied in phenol oxidation–oxygenation reactions. Compound 2 is characterized by UV–vis, resonance Raman, and EPR spectroscopies. Its reactions with a series of para-substituted 2,6-di-tert-butylphenols (p-X-DTBPs) afford 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) in up to 50% yields. Significant deuterium kinetic isotope effects and a positive correlation of second-order rate constants (k2) compared to rate constants for p-X-DTBPs plus cumylperoxyl radical reactions indicate a mechanism that involves rate-limiting hydrogen atom transfer (HAT). A weak correlation of (kBT/e) ln k2 versus Eox of p-X-DTBP indicates that the HAT reactions proceed via a partial transfer of charge rather than a complete transfer of charge in the electron transfer/proton transfer pathway. Product analyses, 18O-labeling experiments, and separate reactivity employing the 2,4,6-tri-tert-butylphenoxyl radical provide further mechanistic insights. After initial HAT, a second molar equiv of 2 couples to the phenoxyl radical initially formed, giving a CuII–OO–(ArO′) intermediate, which proceeds in the case of p-OR-DTBP substrates via a two-electron oxidation reaction involving hydrolysis steps which liberate H2O2 and the corresponding alcohol. By contrast, four-electron oxygenation (O–O cleavage) mainly occurs for p-R-DTBP which gives 18O-labeled DTBQ and elimination of the R group.
 

A Crystalline Phosphaalkene Radical Anion

Publication Date (Web): June 30, 2014 (Communication)
DOI: 10.1021/ja504001x
 
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Abstract Image
Salts containing phosphaalkene radical anions have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV–vis absorption spectroscopy, and single-crystal X-ray diffraction. The radical anions feature elongated P–C bonds and an aromatization of fulvene compared to the neutral phosphaalkene. Their EPR spectra and theoretical calculations indicate the spin density of the radicals mainly resides on phosphorus atoms. This work provides the first example of a crystalline phosphaalkene radical anion.
 

Design and Synthesis of Bipyridine Platinum(II) Bisalkynyl Fullerene Donor–Chromophore–Acceptor Triads with Ultrafast Charge Separation

Publication Date (Web): July 2, 2014 (Article)
DOI: 10.1021/ja5040073


JACS ASAP PDF

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Donor–chromophore–acceptor triads, (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet–triplet energy transfer from the 3MLCT/LLCT excited state within 4 ps to give the 3C60* state, while in (PTZ)2-Pt(bpy)-C60 and (tBuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the 3MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of 3C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.

A Mechanistic Change Results in 100 Times Faster CH Functionalization for Ethane versus Methane by a Homogeneous Pt Catalyst

Publication Date (Web): June 13, 2014 (Article)
DOI: 10.1021/ja504368r
 
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Abstract Image
The selective, oxidative functionalization of ethane, a significant component of shale gas, to products such as ethylene or ethanol at low temperatures and pressures remains a significant challenge. Herein we report that ethane is efficiently and selectively functionalized to the ethanol ester of H2SO4, ethyl bisulfate (EtOSO3H) as the initial product, with the PtII “Periana-Catalytica” catalyst in 98% sulfuric acid. A subsequent organic reaction selectively generates isethionic acid bisulfate ester (HO3S-CH2-CH2-OSO3H, ITA). In contrast to the modest 3–5 times faster rate typically observed in electrophilic CH activation of higher alkanes, ethane CH functionalization was found to be 100 times faster than that of methane. Experiment and quantum-mechanical calculations reveal that this unexpectedly large increase in rate is the result of a fundamentally different catalytic cycle in which ethane CH activation (and not platinum oxidation as for methane) is now turnover limiting. Facile PtII-Et functionalization was determined to occur via a low energy β-hydride elimination pathway (which is not available for methane) to generate ethylene and a PtII-hydride, which is then rapidly oxidized by H2SO4 to regenerate PtII-X2. A rapid, non-Pt-catalyzed reaction of formed ethylene with the hot, concentrated H2SO4 solvent cleanly generate EtOSO3H as the initial product, which further reacts with the H2SO4 solvent to generate ITA.
 

Jul 8, 2014

Photochemical Pump and NMR Probe: Chemically Created NMR Coherence on a Microsecond Time Scale

Photochemical Pump and NMR Probe: Chemically Created NMR Coherence on a Microsecond Time Scale

Publication Date (Web): June 19, 2014 (Article)
DOI: 10.1021/ja504732u
 
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We report pump–probe experiments employing laser-synchronized reactions of para-hydrogen (para-H2) with transition metal dihydride complexes in conjunction with nuclear magnetic resonance (NMR) detection. The pump–probe experiment consists of a single nanosecond laser pump pulse followed, after a precisely defined delay, by a single radio frequency (rf) probe pulse. Laser irradiation eliminates H2 from either Ru(PPh3)3(CO)(H)2 1 or cis-Ru(dppe)2(H)2 2 in C6D6 solution. Reaction with para-H2 then regenerates 1 and 2 in a well-defined nuclear spin state. The rf probe pulse produces a high-resolution, single-scan 1H NMR spectrum that can be recorded after a pump–probe delay of just 10 μs. The evolution of the spectra can be followed as the pump–probe delay is increased by micro- or millisecond increments. Due to the sensitivity of this para-H2 experiment, the resulting NMR spectra can have hydride signal-to-noise ratios exceeding 750:1. The spectra of 1 oscillate in amplitude with frequency 1101 ± 3 Hz, the chemical shift difference between the chemically inequivalent hydrides. The corresponding hydride signals of 2 oscillate with frequency 83 ± 5 Hz, which matches the difference between couplings of the hydrides to the equatorial 31P nuclei. We use the product operator formalism to show that this oscillatory behavior arises from a magnetic coherence in the plane orthogonal to the magnetic field that is generated by use of the laser pulse without rf initialization. In addition, we demonstrate how chemical shift imaging can differentiate the region of laser irradiation thereby distinguishing between thermal and photochemical reactivity within the NMR tube.