Aug 31, 2015

Multimetallic catalysed cross-coupling of aryl bromides with aryl triflates

NATURE | LETTER
Link to Full Article

Multimetallic catalysed cross-coupling of aryl bromides with aryl triflates

  • Laura K. G. Ackerman,
  • Matthew M. Lovell
  • Daniel J. Weix
    • Nature
       
      524,
       
      454–457
       
       
      doi:10.1038/nature14676
      • The advent of transition-metal catalysed strategies for forming new carbon-carbon bonds has revolutionized the field of organic chemistry, enabling the efficient synthesis of ligands, materials, and biologically active molecules123. In cases where a single metal fails to promote a selective or efficient transformation, the synergistic cooperation4 of two distinct catalysts—multimetallic catalysis—can be used instead. Many important reactions rely on multimetallic catalysis5678910, such as the Wacker oxidation of olefins678and the Sonogashira coupling of alkynes with aryl halides910, but this approach has largely been limited to the use of metals with distinct reactivities, with only one metal catalyst undergoing oxidative addition1112. Here, we demonstrate that cooperativity between two group 10 metal catalysts—(bipyridine)nickel and (1,3-bis(diphenylphosphino)propane)palladium—enables a general cross-Ullmann reaction (the cross-coupling of two different aryl electrophiles)131415. Our method couples aryl bromides with aryl triflates directly, eliminating the use of arylmetal reagents and avoiding the challenge of differentiating between multiple carbon–hydrogen bonds that is required for direct arylation methods1617. Selectivity can be achieved without an excess of either substrate and originates from the orthogonal reactivity of the two catalysts and the relative stability of the two arylmetal intermediates. While (1,3-bis(diphenylphosphino)propane)palladium reacts preferentially with aryl triflates to afford a persistent intermediate, (bipyridine)nickel reacts preferentially with aryl bromides to form a transient, reactive intermediate. Although each catalyst forms less than 5 per cent cross-coupled product in isolation, together they are able to achieve a yield of up to 94 per cent. Our results reveal a new method for the synthesis of biaryls, heteroaryls, and dienes, as well as a general mechanism for the selective transfer of ligands between two metal catalysts. We anticipate that this reaction will simplify the synthesis of pharmaceuticals, many of which are currently made with pre-formed organometallic reagents123, and lead to the discovery of new multimetallic reactions.
        • A general cross-Ullmann reaction catalysed by a combination of nickel and palladium.

Aug 28, 2015

Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation

Abstract Image

Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2(rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites, and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 10–12 Pa to 105 Pa results in (i) increase in the band gap from 2.42 to 2.91 eV (direct transitions) or 2.88 to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process.
J. Phys. Chem. A, Article ASAP
DOI: 10.1021/acs.jpca.5b05031

Aug 27, 2015

Saved: Size-Dependent Appearance of Intrinsic Oxq “Activated Oxygen” Molecules on Ceria Nanoparticles


Size-Dependent Appearance of Intrinsic Oxq “Activated Oxygen” Molecules on Ceria Nanoparticles by Xing Huang and Matthew J. Beck via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1MQ8zWI

Aug 25, 2015

Saved: High Stability of Immobilized Polyoxometalates on TiO2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation


High Stability of Immobilized Polyoxometalates on TiO2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation by Sarah M. Lauinger, Jordan M. Sumliner, Qiushi Yin, Zihao Xu, Guijie Liang, Elliot N. Glass, Tianquan Lian and Craig L. Hill via Chemistry of Materials: Latest Articles (ACS Publications) http://ift.tt/1JtFHy4
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Switching on elusive organometallic mechanisms with photoredox catalysis

  • Jack A. Terrett,
  • James D. Cuthbertson,
  • Valerie W. Shurtleff
  • David W. C. MacMillan
    • Link to Full Article
      • Nature
         
        524,
         
        330–334
         
         
        doi:10.1038/nature14875

        Transition-metal-catalysed cross-coupling reactions have become one of the most used carboncarbon and carbonheteroatom bond-forming reactions in chemical synthesis. Recently, nickel catalysis has been shown to participate in a wide variety of C−C bond-forming reactions, most notably Negishi, SuzukiMiyaura, Stille, Kumada and Hiyama couplings12. Despite the tremendous advances in C−C fragment couplings, the ability to forge C−O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C–O bond-forming step (formally known as the reductive elimination step) to occur via a Ni(III) alkoxide intermediate. Here we demonstrate that visible-light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbonoxygen coupling reaction using abundant alcohols and aryl bromides. More notably, we have developed a general strategy to ‘switch on’ important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron-transfer catalysts.
        Modulating oxidation states of nickel enables challenging carbon-heteroatom coupling.

Aug 24, 2015

Saved: Toward Facet Engineering of CdS Nanocrystals and Their Shape-Dependent Photocatalytic Activities


Toward Facet Engineering of CdS Nanocrystals and Their Shape-Dependent Photocatalytic Activities by Xixi Wang, Maochang Liu, Zhaohui Zhou and Liejin Guo via The Journal of Physical Chemistry C: Latest Articles (ACS Publications) http://ift.tt/1h90mkv

Saved: Three-terminal energy harvester with coupled quantum dots


Three-terminal energy harvester with coupled quantum dots by Holger Thierschmann via Nature Nanotechnology - AOP - nature.com science feeds http://ift.tt/1h90m44

Aug 20, 2015

Time-Resolved Detection and Analysis of Single Nanoparticle Electrocatalytic Impacts

Department of Chemistry and MOAC Doctoral Training Centre, University of Warwick, Coventry, CV4 7AL, U.K.
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b05856
Publication Date (Web): August 12, 2015
Copyright © 2015 American Chemical Society

Abstract

Abstract Image
There is considerable interest in understanding the interaction and activity of single entities, such as (electro)catalytic nanoparticles (NPs), with (electrode) surfaces. Through the use of a high bandwidth, high signal/noise measurement system, NP impacts on an electrode surface are detected and analyzed in unprecedented detail, revealing considerable new mechanistic information on the process. Taking the electrocatalytic oxidation of H2O2 at ruthenium oxide (RuOx) NPs as an example, the rise time of current–time transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distance-dependent NP diffusion-coefficient. NP release from the electrode appears to be aided by propulsion from the electrocatalytic reaction at the NP. High-frequency NP impacts, orders of magnitude larger than can be accounted for by a single pass diffusive flux of NPs, are observed that indicate the repetitive trapping and release of an individual NP that has not been previously recognized. The experiments and models described could readily be applied to other systems and serve as a powerful platform for detailed analysis of NP impacts.

Y-doped Li8ZrO6: A Li-Ion Battery Cathode Material with High Capacity

Department of Chemistry, Chemical Theory Center, and Supercomputing Institute and Department of Chemistry, University of Minnesota, 207 Pleasant Street S.E., Minneapolis, Minnesota 55455-0431, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b04690
Publication Date (Web): August 11, 2015
Copyright © 2015 American Chemical Society
ACS AuthorChoice - This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Abstract

Abstract Image
We study—experimentally and theoretically—the energetics, structural changes, and charge flows during the charging and discharging processes for a new high-capacity cathode material, Li8ZrO6 (LZO), which we study both pure and yttrium-doped. We quantum mechanically calculated the stable delithiated configurations, the delithiation energy, the charge flow during delithiation, and the stability of the delithiated materials. We find that Li atoms are easier to extract from tetrahedral sites than octahedral ones. We calculate a large average voltage of 4.04 eV vs Li/Li+ for delithiation of the first Li atom in a primitive cell, which is confirmed by galvanostatic charge/discharge cycling data. Energy calculations indicate that topotactic delithiation is kinetically favored over decomposition into Li, ZrO2, and O2 during the charging process, although the thermodynamic energy of the topotactic reaction is less favorable. When one or two lithium atoms are extracted from a primitive cell of LZO, its volume and structure change little, whereas extraction of the third lithium greatly distorts the layered structure. The Li6ZrO6 and Li5ZrO6 delithiation products can be thermodynamically metastable to release of O2. Experimentally, materials with sufficiently small particle size for efficient delithiation and relithiation were achieved within an yttrium-doped LZO/carbon composite cathode that exhibited an initial discharge capacity of at least 200 mAh/g over the first 10 cycles, with 142 mAh/g maintained after 60 cycles. Computations predict that during the charging process, the oxygen ion near the Li vacancy is oxidized for both pure LZO and yttrium-doped LZO, which leads to a small-polaron hole.

Aug 12, 2015

Organometallics

Synthesis and Structural Characterization of an Unusual Platinum π-Arene Complex: (η6-C6H3Me3)Pt[(C2F5)2PMe]Me+

Department of Chemistry, University of Wyoming, Dept. 3838, 1000 E. University Avenue, Laramie, Wyoming 82071, United States
Organometallics, Article ASAP
DOI: 10.1021/acs.organomet.5b00410


Abstract Image
Treatment of cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) with the mesitylenium acid (C6Me3H4)+B(C6F5)4 in 1,2-difluorobenzene cleanly produces an unusually stable arene complex, [(η6-C6Me3H3)Pt(dfmp)(CH3)]+(B(C6F5)4) (1). Facile arene exchange and competitive binding equilibria have been quantified for mesitylene relative to toluene (K = 0.0030(3)) and durene (K = 20(2)). Reaction of 1 with H2 at 80 °C results in hydrogenolysis to form the arene hydride (η6-C6Me3H3)Pt(dfmp)(H)+ (2), while treatment of 1 with CO givestrans-(dfmp)2Pt(CO)Me+ as the major phosphine product. Addition of excess Me3P to 1results in both arene and dfmp displacement to form (Me3P)3PtMe+. (η6-C6Me3H3)Pt(dfmp)(CH3)+ is a moderately active ethylene dimerization catalyst to form 2-butenes (∼7 TO h–1, 20 °C).
Organometallics

Iridium Pincer-Catalyzed Dehydrogenation of Ethers Featuring Ethylene as the Hydrogen Acceptor

Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States


Abstract Image
We describe efficient methods to dehydrogenate ethers by using iridium pincer complexes (iPr4Anthraphos)-Ir(H)(Cl), 4iPr4PC(sp3)P–Ir(H)(Cl), 5, and (iPr4PCP)-Ir(H)(Cl), 6. At 120 °C, cyclic ethers were dehydrogenated with tert-butylethylene as the hydrogen acceptor with high turnover numbers (over 400 in many cases). Acyclic ethers such as diethyl ether can also be dehydrogenated catalytically with TONs up to 90. The efficient dehydrogenation of cyclic and acyclic ethers using ethylene as a more practical hydrogen acceptor has been demonstrated for the first time.
NATURE | LETTER

Dosage delivery of sensitive reagents enables glove-box-free synthesis

 Link to Article

& Stephen L. Buchwald

Contemporary organic chemists employ a broad range of catalytic and stoichiometric methods to construct molecules for applications in the material sciences1, and as pharmaceuticals2345, agrochemicals, and sensors6. The utility of a synthetic method may be greatly reduced if it relies on a glove box to enable the use of air- and moisture-sensitive reagents or catalysts. Furthermore, many synthetic chemistry laboratories have numerous containers of partially used reagents that have been spoiled by exposure to the ambient atmosphere. This is exceptionally wasteful from both an environmental and a cost perspective. Here we report an encapsulation method for stabilizing and storing air- and moisture-sensitive compounds. We demonstrate this approach in three contexts, by describing single-use capsules that contain all of the reagents (catalysts, ligands, and bases) necessary for the glove-box-free palladium-catalysed carbon–fluorine789, carbon–nitrogen1011, and carbon–carbon12 bond-forming reactions. This strategy should reduce the number of error-prone, tedious and time-consuming weighing procedures required for such syntheses and should be applicable to a wide range of reagents, catalysts, and substrate combinations.

Encapsulated reagents for dispensing air-sensitive compounds.