1,381 research outputs found
Liv Ullmann standing with Senator Henry M. Jackson and Senator Slade Gorton, Senate Office Building, Washington, D.C., August 17, 1982
No full textHandwritten on verso: 8-17-82"Liv Johanne Ullmann (born December 16, 1938) is an Academy Award-nominated, Golden Globe-winning Norwegian actress, author and film director. She played lead roles in nine films by Ingmar Bergman" (Wikipedia article)
Copper Single Atoms Chelated on Ligand-Modified Carbon for Ullmann-type C−O Coupling
No full textCross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C−O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C−O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries
Mechanistic Picture and Kinetic Analysis of Surface-Confined Ullmann Polymerization
No full textSurface-confined polymerization via Ullmann coupling is a promising route to create one- and two-dimensional covalent π-conjugated structures, including the bottom-up growth of graphene nanoribbons. Understanding the mechanism of the Ullmann reaction is necessary to provide a platform for rationally controlling the formation of these materials. We use fast X-ray photoelectron spectroscopy (XPS) in kinetic measurements of epitaxial surface polymerization of 1,4-dibromobenzene on Cu(110) and devise a kinetic model based on mean field rate equations, involving a transient state. This state is observed in the energy landscapes calculated by nudged elastic band (NEB) within density functional theory (DFT), which assumes as initial and final geometries of the organometallic and polymeric structures those observed by scanning tunneling microscopy (STM). The kinetic model accounts for all the salient features observed in the experimental curves extracted from the fast-XPS measurements and enables an enhanced understanding of the polymerization process, which is found to follow a nucleation-and-growth behavior preceded by the formation of a transient state
Liv Ullmann sitting at a dinner table with Mrs. Slade Gorton and Senator Henry M. Jackson and his family, Washington, D.C., August 17, 1982
No full textHandwritten on verso: 8-17-82"Liv Johanne Ullmann (born December 16, 1938) is an Academy Award-nominated, Golden Globe-winning Norwegian actress, author and film director. She played lead roles in nine films by Ingmar Bergman" (Wikipedia article)
Identification of topotactic surface-confined Ullmann-polymerization
No full textOn-surface Ullmann coupling is an established method for the synthesis of 1D and 2D organic structures. A key limitation to obtaining ordered polymers is the uncertainty in the final structure for coupling via random diffusion of reactants over the substrate, which leads to polymorphism and defects. Here, a topotactic polymerization on Cu(110) in a series of differently-halogenated para-phenylenes is identified, where the self-assembled organometallic (OM) reactants of diiodobenzene couple directly into a single, deterministic product, whereas the other precursors follow a diffusion driven reaction. The topotactic mechanism is the result of the structure of the iodine on Cu(110), which controls the orientation of the OM reactants and intermediates to be the same as the final polymer chains. Temperature-programmed X-ray photoelectron spectroscopy and kinetic modeling reflect the differences in the polymerization regimes, and the effects of the OM chain alignments and halogens are disentangled by Nudged Elastic Band calculations. It is found that the repulsion or attraction between chains and halogens drive the polymerization to be either diffusive or topotactic. These results provide detailed insights into on-surface reaction mechanisms and prove the possibility of harnessing topotactic reactions in surface-confined Ullmann polymerization
Optical probe study of a nonentangling macromolecule solution--bovine serum albumin: Water
No full textThe diffusion coefficient D of polystyrene latex spheres in bovine serum albumin:water was studied as a function of protein concentration c for 0 c D fails by as much as 25 to 50%, D being larger than predicted from the sphere radius R and the solution viscosity. Probe particles with R as large as 0.62 [mu]m were used. D fits well to the form D = Doexp(-[alpha]cv) for [alpha] = 0.004 to 0.008 and v = 0.96 to 0.99. Serum albumin is a globular protein, so chain entanglement cannot cause these non-Stokes--Einsteinian effects, which are presumably due to sphere:albumin interactions. Polystyrene spheres in semidilute polyethylene oxide:water (G. S. Ullmann, K. Ullmann, R. M. Lindner, and G. D. J. Phillies, J. Phys. Chem. 89, 692 (1985)) behave similarly to spheres in serum albumin:water, suggesting that chain entanglement may also not be important in probe diffusion through semidilute polymer solutions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25653/1/0000205.pd
The role of halogens in on-surface Ullmann polymerization
No full textUllmann coupling is the most common approach to form surface-confined one- and two-dimensional conjugated structures from haloaryl derivatives. The dimensions of the formed nanostructures can be controlled by the number and location of halogens within the molecular precursors. Our study illustrates that the type of halogen plays an essential role in the design, orientation, and extent of the surface-confined organometallic and polymeric nanostructures. We performed a comparative analysis of five 1,4-dihalobenzene molecules containing chlorine, bromine, and iodine on Cu(110) using scanning tunneling microscopy, fast-X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies. Our experimental data identify different molecular structures, reaction temperatures and kinetics depending on the halogen type. Climbing image nudged elastic band simulations further clarify these observations by providing distinct diffusion paths for each halogen species. We show that in addition to the structure of the building blocks, the halogen type has a direct influence on the morphology of surface-confined polymeric structures based on Ullmann coupling
An unexpected organometallic intermediate in surface-confined Ullmann coupling
No full textUllmann coupling or, more generally, dehalogenative aryl-aryl coupling, is one of the most widely exploited chemical reactions to obtain one- and two-dimensional polymers on metal surfaces. It is generally described as a two-step reaction: (i) dehalogenation, resulting in the formation of a stable intermediate organometallic phase and subsequent (ii) C-C coupling. The topology of the resulting polymer depends on the number and positions of the halogen atoms in the haloaromatic precursor, although its orientation and order are determined by the structure of the intermediate phase. Hitherto, only one intermediate structure, identified as an organometallic (OM) phase, has been reported for such a reaction. Here we demonstrate the formation of two distinct OM phases during the temperature-induced growth of poly(para-phenylene) from 1,4-dibromobenzene precursors on Cu(110). Beyond the already known linear-OM chains, we show that a phase reorganization to a chessboard-like 2D-OM can be activated in a well-defined temperature range. This new intermediate phase, revealed only when the reaction is carried out at low molecular coverages, was characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, and modeled by density functional theory calculations. Our data show that the 2D-OM remains stable after cooling down the sample and is stabilized by four-Cu clusters at each node. The observation of such unexpected intermediate phase shows the complexity of the mechanisms underlying on-surface synthesis and broadens the understanding of Ullmann coupling, which continues to be astonishing despite its extensive use
A Pd[0]-catalyzed Ullmann cross-coupling/reductive cyclization approach to C-3 mono-alkylated oxindoles and related compounds
No full textThe Pd[0]-catalyzed Ullmann cross-coupling of o-nitrohaloarenes 1a-e with the brominated heterocycles 2a-f delivers the expected products 3a-j in good to excellent yields. The reductive cyclization of such products, as well as N-acyl derivatives 3k, l, and m, has been investigated and provided the C-3 mono-substituted oxindoles 5a-d, f, g, k, and m, the direct reduction products 4i and j or indole 5l
Silk−Fibroin‐Supported Palladium Catalyst for Suzuki‐Miyaura and Ullmann Coupling Reactions of Aryl Chlorides
No full textRecently, we have reported the preparation of a silk fibroin-supported Palladium catalyst (Pd/SF) and its use in the Suzuki-Miyaura cross-coupling of aryl iodides. Since its synthetic applicability and structural features are still far from being fully explored, a deeper investigation is reported here. Pd/SF is tested as a catalyst for the Suzuki-Miyaura and Ullmann coupling reactions of aryl chlorides, in H2O/EtOH solvent mixture under atmospheric conditions. Starting from the analysis of the products of the Pd/SF promoted Ullmann reactions of poly-haloarenes, we can hypothesize the existence of catalytic pockets where monoatomic palladium species can form stable complexes with SF. To shed light on this hypothesis, Pd/SF is characterized by Wide-Angle X-ray Scattering (WAXS) analysis, which supports the supposition of catalytic pockets. Finally, using a computational model developed by Molecular Mechanic Energy Minimization we have estimated the dimension of the catalytic pocket of Pd/SF (about 15 angstrom), in good agreement with the experimental results
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