178 results on '"Williams, Travis J."'
Search Results
152. Rhodium-Catalyzed [5+2] Cycloaddition Reactions in Water.
- Author
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Wender, Paul A., Love, Jennifer A., and Williams, Travis J.
- Published
- 2003
- Full Text
- View/download PDF
153. Toward the Ideal Synthesis: New Transition Metal catalyzed Reactions Inspired by Novel Medicinal Leads.
- Author
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Wender, Paul A., Bi, F. Christopher, Gamber, Gabriel G., Gosselin, Francis, Hubbard, Robert D., Scanio, Marc J. C., Sun, Robert, Williams, Travis J., and Zhang, Lei
- Published
- 2002
- Full Text
- View/download PDF
154. Cluster Frustration in the Breathing Pyrochlore Magnet LiGaCr4S8.
- Author
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Pokharel, Ganesh, Arachchige, Hasitha Suriya, Williams, Travis J., May, Andrew F., Fishman, Randy S., Sala, Gabriele, Calder, Stuart, Ehlers, Georg, Parker, David S., Tao Hong, Wildes, Andrew, Mandrus, David, Paddison, Joseph A. M., and Christianson, Andrew D.
- Subjects
- *
PYROCHLORE , *FRUSTRATION , *EXCITATION spectrum , *MAGNETS , *NEUTRON scattering , *SPIN-orbit interactions - Abstract
We present a comprehensive neutron scattering study of the breathing pyrochlore magnet LiGaCr4S8. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional-theory predictions [Ghosh et al., npj Quantum Mater. 4, 63 (2019)]. We explain the origin of geometrical frustration in LiGaCr4S8 in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered-cubic lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids. [ABSTRACT FROM AUTHOR]
- Published
- 2020
- Full Text
- View/download PDF
155. Formation of the Coherent Heavy Fermion Liquid at the Hidden Order Transition in URu2Si2.
- Author
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Chatterjee, Shouvik, Trinckauf, Jan, Hänke, Torben, Shai, Daniel E., Harter, John W., Williams, Travis J., Luke, Graeme M., Shen, Kyle M., and Geck, Jochen
- Subjects
- *
HEAVY fermion superconductors , *HIGH resolution imaging , *ELECTRONIC structure , *CONDUCTION bands , *FERMI level - Abstract
We present high-resolution angle-resolved photoemission spectra of the heavy-fermion superconductor URu2Si2. Detailed measurements as a function of both photon energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low-energy electronic structure across the "hidden order" transition. Above the transition, our measurements reveal the existence of weakly dispersive states that exhibit a large scattering rate and do not appear to shift from above to below the Fermi level, as previously reported. Upon entering the hidden order phase, these states rapidly hybridize with light conduction band states and transform into a coherent heavy fermion liquid, coincident with a dramatic drop in the scattering rate. This evolution is in stark contrast with the gradual crossover expected in Kondo lattice systems, which we attribute to the coupling of the heavy fermion states to the hidden order parameter. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
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156. A new mechanism of metal-ligand cooperative catalysis in transfer hydrogenation of ketones.
- Author
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Demianets, Ivan, Cherepakhin, Valeriy, Maertens, Alexander, Lauridsen, Paul J., Mallikarjun Sharada, Shaama, and Williams, Travis J.
- Subjects
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TRANSFER hydrogenation , *KINETIC isotope effects , *IRIDIUM catalysts , *KETONES , *PROPANOLS , *CATALYSIS , *HYDROGENATION , *TRANSITION state theory (Chemistry) - Abstract
We report a new metal-ligand cooperative mechanism for transfer hydrogenation of ketones with alcohols enabled by iridium catalysts 1-Me and 1-F. The reaction proceeds through the nine-membered cyclic transition state, TS , which is stabilized by an extra alcohol molecule that shuttles the proton from the alcohol to the catalyst. We report iridium catalysts IrCl(η5-Cp⁎)(κ2-(2-pyridyl)CH 2 NSO 2 C 6 H 4 X) (1-Me , X = CH 3 and 1-F , X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η5-Cp⁎)(κ2-(C 5 H 4 N)CHNSO 2 Ar) (2), IrH(η5-Cp⁎)(κ2-(2-pyridyl)CH 2 NSO 2 Ar) (3), and Ir(η5-Cp⁎)(κ3-(2-pyridyl)CH 2 NSO 2 Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R 2 CHOH ⇄ 3 + R 2 CO) is suggested by kinetic isotope effects, Eyring parameters (Δ H ≠ = 29.1(8) kcal mol−1 and Δ S ≠ = −17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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157. Asymmetric Catalysis of the [5 + 2] Cycloaddition Reaction of Vinylcyclopropanes and π-Systems.
- Author
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Wender, Paul A., Haustedt, Lars O., Jaehong Lim, Love, Jennifer A., Williams, Travis J., and Joo-Yong Yoon
- Subjects
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ENANTIOSELECTIVE catalysis , *RING formation (Chemistry) , *CHROMOGENIC compounds , *ASYMMETRIC synthesis , *ALKENES - Abstract
The article describes a comparative evaluation of several chiral catalysts system for effecting asymmetric [5 + 2] cycloadditions of alkenes and vinylcyclopropanes (VCP). The researchers conducted a screen of catalyst systems using the VCP as a substrate and obtained high yields and high enantioselectives with several substrates differing in substitution of the reactive functionality and in the tether.
- Published
- 2006
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158. An Immobilized (Carbene)Nickel Catalyst for Water Oxidation.
- Author
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Lu Z, Mitra D, Narayan SR, and Williams TJ
- Abstract
The oxygen evolution reaction (OER) of water splitting is essential to electrochemical energy storage applications. While nickel electrodes are widely available heterogeneous OER catalysts, homogeneous nickel catalysts for OER are underexplored. Here we report two carbene-ligated nickel(II) complexes that are exceptionally robust and efficient homogeneous water oxidation catalysts. Remarkably, these novel nickel complexes can assemble a stable thin film onto a metal electrode through poly-imidazole bridges, making them supported heterogeneous electrochemical catalysts that are resilient to leaching and stripping. Unlike molecular catalysts and nanoparticle catalysts, such electrode-supported metal-complex catalysts for OER are rare and have the potential to inspire new designs. The electrochemical OER with our nickel-carbene catalysts exhibits excellent current densities with high efficiency, low Tafel slope, and useful longevity for a base metal catalyst. Our data show that imidazole carbene ligands stay bonded to the nickel(II) centers throughout the catalysis, which allows the facile oxygen evolution.
- Published
- 2024
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159. Topology stabilized fluctuations in a magnetic nodal semimetal.
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Drucker NC, Nguyen T, Han F, Siriviboon P, Luo X, Andrejevic N, Zhu Z, Bednik G, Nguyen QT, Chen Z, Nguyen LK, Liu T, Williams TJ, Stone MB, Kolesnikov AI, Chi S, Fernandez-Baca J, Nelson CS, Alatas A, Hogan T, Puretzky AA, Huang S, Yu Y, and Li M
- Abstract
The interplay between magnetism and electronic band topology enriches topological phases and has promising applications. However, the role of topology in magnetic fluctuations has been elusive. Here, we report evidence for topology stabilized magnetism above the magnetic transition temperature in magnetic Weyl semimetal candidate CeAlGe. Electrical transport, thermal transport, resonant elastic X-ray scattering, and dilatometry consistently indicate the presence of locally correlated magnetism within a narrow temperature window well above the thermodynamic magnetic transition temperature. The wavevector of this short-range order is consistent with the nesting condition of topological Weyl nodes, suggesting that it arises from the interaction between magnetic fluctuations and the emergent Weyl fermions. Effective field theory shows that this topology stabilized order is wavevector dependent and can be stabilized when the interband Weyl fermion scattering is dominant. Our work highlights the role of electronic band topology in stabilizing magnetic order even in the classically disordered regime., (© 2023. Springer Nature Limited.)
- Published
- 2023
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160. An ambient pressure, direct hydrogenation of ketones.
- Author
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Zhang L, Lu Z, Rander AR, and Williams TJ
- Abstract
We report two bifunctional (pyridyl)carbene-iridium(I) complexes that catalyze ketone and aldehyde hydrogenation at ambient pressure. Aryl, heteroaryl, and alkyl groups are demonstrated, and mechanistic studies reveal an unusual polarization effect in which the rate is dependant of proton, rather than hydride, transfer. This method introduces a convenient, waste-free alternative to traditional borohydride and aluminum hydride reagents.
- Published
- 2023
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161. Synthesis of 1,4-Diazacycles by Hydrogen Borrowing.
- Author
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Nalikezhathu A, Tam A, Cherepakhin V, Do VK, and Williams TJ
- Abstract
We report the syntheses of 1,4-diazacycles by diol-diamine coupling, uniquely made possible with a (pyridyl)phosphine-ligated ruthenium(II) catalyst ( 1 ). The reactions can exploit either two sequential N -alkylations or an intermediate tautomerization pathway to yield piperazines and diazepanes; diazepanes are generally inaccessible by catalytic routes. Our conditions tolerate different amines and alcohols that are relevant to key medicinal platforms. We show the syntheses of the drugs cyclizine and homochlorcyclizine in 91% and 67% yields, respectively.
- Published
- 2023
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162. Polystyrene Upcycling into Fungal Natural Products and a Biocontrol Agent.
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Rabot C, Chen Y, Lin SY, Miller B, Chiang YM, Oakley CE, Oakley BR, Wang CCC, and Williams TJ
- Subjects
- Plastics metabolism, Polyethylene metabolism, Aspergillus flavus metabolism, Polystyrenes metabolism, Biological Products metabolism
- Abstract
Polystyrene (PS) is one of the most used yet infrequently recycled plastics. Although manufactured on the scale of 300 million tons per year globally, current approaches toward PS degradation are energy- and carbon-inefficient, slow, and/or limited in the value that they reclaim. We recently reported a scalable process to degrade post-consumer polyethylene-containing waste streams into carboxylic diacids. Engineered fungal strains then upgrade these diacids biosynthetically to synthesize pharmacologically active secondary metabolites. Herein, we apply a similar reaction to rapidly convert PS to benzoic acid in high yield. Engineered strains of the filamentous fungus Aspergillus nidulans then biosynthetically upgrade PS-derived crude benzoic acid to the structurally diverse secondary metabolites ergothioneine, pleuromutilin, and mutilin. Further, we expand the catalog of plastic-derived products to include spores of the industrially relevant biocontrol agent Aspergillus flavus Af36 from crude PS-derived benzoic acid.
- Published
- 2023
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163. SEEMS: A Single Event Effects and Muon Spectroscopy facility at the Spallation Neutron Source.
- Author
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Williams TJ, MacDougall GJ, Riemer BW, Gallmeier FX, Dela Cruz CR, and Louca D
- Abstract
This study outlines a concept that would leverage the existing proton accelerator at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory to enable transformative science via one world-class facility serving two missions: Single Event Effects (SEE) and Muon Spectroscopy (μSR). The μSR portion would deliver the world's highest flux and highest resolution pulsed muon beams for material characterization purposes, with precision and capabilities well beyond comparable facilities. The SEE capabilities deliver neutron, proton, and muon beams for aerospace industries that are facing an impending challenge to certify equipment for safe and reliable behavior under bombardment from atmospheric radiation originating from cosmic and solar rays. With negligible impact on the primary neutron scattering mission of the SNS, the proposed facility will have enormous benefits for both science and industry. We have designated this facility "SEEMS."
- Published
- 2023
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164. Conversion of Polyethylenes into Fungal Secondary Metabolites.
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Rabot C, Chen Y, Bijlani S, Chiang YM, Oakley CE, Oakley BR, Williams TJ, and Wang CCC
- Subjects
- Plastics chemistry, Catalysis, Polyethylenes chemistry, Aspergillus nidulans metabolism
- Abstract
Waste plastics represent major environmental and economic burdens due to their ubiquity, slow breakdown rates, and inadequacy of current recycling routes. Polyethylenes are particularly problematic, because they lack robust recycling approaches despite being the most abundant plastics in use today. We report a novel chemical and biological approach for the rapid conversion of polyethylenes into structurally complex and pharmacologically active compounds. We present conditions for aerobic, catalytic digestion of polyethylenes collected from post-consumer and oceanic waste streams, creating carboxylic diacids that can then be used as a carbon source by the fungus Aspergillus nidulans. As a proof of principle, we have engineered strains of A. nidulans to synthesize the fungal secondary metabolites asperbenzaldehyde, citreoviridin, and mutilin when grown on these digestion products. This hybrid approach considerably expands the range of products to which polyethylenes can be upcycled., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)
- Published
- 2023
- Full Text
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165. Pressurized Formic Acid Dehydrogenation: An Entropic Spring Replaces Hydrogen Compression Cost.
- Author
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Do VK, Vargas NA, Chavez AJ, Zhang L, Cherepakhin V, Lu Z, Currier RP, Dub PA, Gordon JC, and Williams TJ
- Abstract
Formic acid is unique among liquid organic hydrogen carriers (LOHCs), because its dehydrogenation is highly entropically driven. This enables the evolution of high-pressure hydrogen at mild temperatures that is difficult to achieve with other LOHCs, conceptually by releasing the "spring" of energy stored entropically in the liquid carrier. Applications calling for hydrogen-on-demand, such as vehicle filling, require pressurized H
2 . Hydrogen compression dominates the cost for such applications, yet there are very few reports of selective, catalytic dehydrogenation of formic acid at elevated pressure. Herein, we show that homogenous catalysts with various ligand frameworks, including Noyori-type tridentate (PNP, SNS, SNP, SNPO), bidentate chelates (pyridyl)NHC, (pyridyl)phosphine, (pyridyl)sulfonamide, and their metallic precursors, are suitable catalysts for the dehydrogenation of neat formic acid under self-pressurizing conditions. Quite surprisingly, we discovered that their structural differences can be related to performance differences in their respective structural families, with some tolerant or intolerant of pressure and others that are significantly advantaged by pressurized conditions. We further find important roles for H2 and CO in catalyst activation and speciation. In fact, for certain systems, CO behaves as a healing reagent when trapped in a pressurizing reactor system, enabling extended life from systems that would be otherwise deactivated., Competing Interests: Conflicts of interest T.J.W. and Z.L. are founders of startup company, Catapower Inc., that is commercializing 10 for lactate synthesis. LANL (P.A.D.) commercialized complexes 4, 7, and 8, which are available from STREM Chemicals. N. A. V. is a founder of a company, Solargonic LLC, that aims to commercialize liquid hydrogen carriers.- Published
- 2022
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166. Out of Thin Air? Catalytic Oxidation of Trace Aqueous Aldehydes with Ambient Dissolved Oxygen.
- Author
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Kim E, Cardosa GB, Stanley KE, Williams TJ, and McCurry DL
- Subjects
- Aldehydes, Carbon, Oxidation-Reduction, Water, Oxygen, Wastewater
- Abstract
Water reuse is expanding due to increased water scarcity. Water reuse facilities treat wastewater effluent to a very high purity level, typically resulting in a product water that is essentially deionized water, often containing less than 100 μg/L organic carbon. However, recent research has found that low-molecular-weight aldehydes, which are toxic electrophiles, comprise a significant fraction of the final organic carbon pool in recycled wastewater in certain treatment configurations. In this manuscript, we demonstrate oxidation of trace aqueous aldehydes to their corresponding acids using a heterogeneous catalyst (5% Pt on C), with ambient dissolved oxygen serving as the terminal electron acceptor. Mass balances are essentially quantitative across a range of aldehydes, and pseudo-first-order reaction kinetics are observed in batch reactors, with k
obs varying from 0.6 h-1 for acetaldehyde to 4.6 h-1 for hexanal, while they are low for unsaturated aldehydes. Through kinetic and isotopic labeling experiments, we demonstrate that while oxygen is essential for the reaction to proceed, it is not involved in the rate-limiting step, and the reaction appears to proceed primarily through a base-promoted β-hydride elimination mechanism from the hydrated gem -diol form of the corresponding aldehyde. This is the first report we are aware of that demonstrates useful abiotic oxidation of a trace organic contaminant using dissolved oxygen.- Published
- 2022
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167. Kinetics and mechanistic details of bulk ZnO dissolution using a thiol-imidazole system.
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Koskela KM, Quiton SJ, Sharada SM, Williams TJ, and Brutchey RL
- Abstract
Oxide dissolution is important for metal extraction from ores and has become an attractive route for the preparation of inks for thin film solution deposition; however, oxide dissolution is often kinetically challenging. While binary "alkahest" systems comprised of thiols and N -donor species, such as amines, are known to dissolve a wide range of oxides, the mechanism of dissolution and identity of the resulting solute(s) remain unstudied. Here, we demonstrate facile dissolution of both bulk synthetic and natural mineral ZnO samples using an "alkahest" that operates via reaction with thiophenol and 1-methylimidazole (MeIm) to give a single, pseudotetrahedral Zn(SPh)
2 (MeIm)2 molecular solute identified by X-ray crystallography. The kinetics of ZnO dissolution were measured using solution1 H NMR, and the reaction was found to be zero-order in the presence of excess ligands, with more electron withdrawing para -substituted thiophenols resulting in faster dissolution. A negative entropy of activation was measured by Eyring analysis, indicating associative ligand binding in, or prior to, the rate determining step. Combined experimental and computational surface binding studies on ZnO reveal stronger, irreversible thiophenol binding compared to MeIm, leading to a proposed dissolution mechanism initiated by thiol binding to the ZnO surface with the liberation of water, followed by alternating MeIm and thiolate ligand additions, and ultimately cleavage of the ligated zinc complex from the ZnO surface. Design rules garnered from the mechanistic insight provided by this study should inform the dissolution of other bulk oxides into inks for solution processed thin films., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)- Published
- 2022
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168. Decay and renormalization of a longitudinal mode in a quasi-two-dimensional antiferromagnet.
- Author
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Do SH, Zhang H, Williams TJ, Hong T, Garlea VO, Rodriguez-Rivera JA, Jang TH, Cheong SW, Park JH, Batista CD, and Christianson AD
- Abstract
An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different modes types are important. Here we approach this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an easy-plane quantum magnet near a continuous quantum phase transition. Our inelastic neutron scattering measurements of Ba
2 FeSi2 O7 reveal the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center. To account for the many-body effects of the interacting low-energy modes in anisotropic magnets, we generalize the standard spin-wave theory. The measured mode decay and renormalization is reproduced by including all one-loop corrections. The theoretical framework developed here is broadly applicable to quantum magnets with more than one type of low energy mode., (© 2021. The Author(s).)- Published
- 2021
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169. Heterobimetallic complexes of IrM (M = Fe II , Co II , and Ni II ) core and bridging 2-(diphenylphosphino)pyridine: electronic structure and electrochemical behavior.
- Author
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Cherepakhin V, Hellman A, Lan Z, Mallikarjun Sharada S, and Williams TJ
- Abstract
Three complexes based on an Ir-M (M = Fe
II , CoII , and NiII ) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph2 PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph2 PPy)2 ] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph2 PPy)2 FeCl2 ]·2CH2 Cl2 (2), [IrCl(CO)(μ-Ph2 PPy)2 CoCl2 ]·2CH2 Cl2 (3), and [Ir(CO)(μ-Cl)(μ-Ph2 PPy)2 NiCl2 ]·2CH2 Cl2 (4). Time-dependent DFT computations suggest a donor-acceptor interaction between a filled 5dz orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high-spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through an ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals.2 - Published
- 2020
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170. Surface coordination chemistry of germanium nanocrystals synthesized by microwave-assisted reduction in oleylamine.
- Author
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Smock SR, Tabatabaei K, Williams TJ, Kauzlarich SM, and Brutchey RL
- Abstract
As surface ligands play a critical role in the colloidal stability and optoelectronic properties of semiconductor nanocrystals, we used solution NMR experiments to investigate the surface coordination chemistry of Ge nanocrystals synthesized by a microwave-assisted reduction of GeI
2 in oleylamine. The as-synthesized Ge nanocrystals are coordinated to a fraction of strongly bound oleylamide ligands (with covalent X-type Ge-NHR bonds) and a fraction of more weakly bound (or physisorbed) oleylamine, which readily exchanges with free oleylamine in solution. The fraction of strongly bound oleylamide ligands increases with increasing synthesis temperature, which also correlates with better colloidal stability. Thiol and carboxylic acid ligands bind to the Ge nanocrystal surface only upon heating, suggesting a high kinetic barrier to surface binding. These incoming ligands do not displace native oleylamide ligands but instead appear to coordinate to open surface sites, confirming that the as-prepared nanocrystals are not fully passivated. These findings will allow for a better understanding of the surface chemistry of main group nanocrystals and the conditions necessary for ligand exchange to ultimately maximize their functionality.- Published
- 2020
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171. Quantifying the Thermodynamics of Ligand Binding to CsPbBr 3 Quantum Dots.
- Author
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Smock SR, Williams TJ, and Brutchey RL
- Abstract
Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chemistry. To this point, the thermodynamics of ligand binding remain unstudied. Herein,
1 H NMR methods were used to quantify the thermodynamics of ligand exchange on CsPbBr3 QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr3 QD surface, having individual surface densities of 1.2-1.7 nm-2 . 10-Undecenoic acid undergoes an exergonic exchange equilibrium with bound oleate (Keq =1.97) at 25 °C while 10-undecenylphosphonic acid undergoes irreversible ligand exchange. Undec-10-en-1-amine exergonically exchanges with oleylamine (Keq =2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr3 QDs without etching of the nanocrystal surface; increases in the steady-state PL intensities correlate with more strongly bound conjugate base ligands., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)- Published
- 2018
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172. Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy Composites.
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Navarro CA, Kedzie EA, Ma Y, Michael KH, Nutt SR, and Williams TJ
- Abstract
Carbon fiber-reinforced polymer (CFRP) materials are widely used in aerospace and recreational equipment, but there is no efficient procedure for their end-of-life recycling. Ongoing work in the chemistry and engineering communities emphasizes recovering carbon fibers from such waste streams by dissolving or destroying the polymer binding. By contrast, our goal is to depolymerize amine-cured epoxy CFRP composites catalytically, thus enabling not only isolation of high-value carbon fibers, but simultaneously opening an approach to recovery of small molecule monomers that can be used to regenerate precursors to new composite resin. To do so will require understanding of the molecular mechanism(s) of such degradation sequences. Prior work has shown the utility of hydrogen peroxide as a reagent to affect epoxy matrix decomposition [1]. Herein we describe the chemical transformations involved in that sequence: the reaction proceeds by oxygen atom transfer to the polymer's linking aniline group, forming an N -oxide intermediate. The polymer is then cleaved by an elimination and hydrolysis sequence. We find that elimination is the slower step. Scandium trichloride is an efficient catalyst for this step, reducing reaction time in homogeneous model systems and neat cured matrix blocks. The conditions can be applied to composed composite materials, from which pristine carbon fibers can be recovered.
- Published
- 2018
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173. Ruthenium-Catalyzed Ammonia Borane Dehydrogenation: Mechanism and Utility.
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Zhang X, Kam L, Trerise R, and Williams TJ
- Abstract
One of the greatest challenges in using H
2 as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density (19.6 wt %) and ease of handling and transport. Hydrogen release from ammonia borane is mediated by either hydrolysis, thus giving borate products that are difficult to rereduce, or direct dehydrogenation. Catalytic AB dehydrogenation has thus been a popular topic in recent years, motivated both by applications in hydrogen storage and main group synthetic chemistry. This Account is a complete description of work from our laboratory in ruthenium-catalyzed ammonia borane dehydrogenation over the last 6 years, beginning with the Shvo catalyst and resulting ultimately in the development of optimized, leading catalysts for efficient hydrogen release. We have studied AB dehydrogenation with Shvo's catalyst extensively and generated a detailed understanding of the role that borazine, a dehydrogenation product, plays in the reaction: it is a poison for both Shvo's catalyst and PEM fuel cells. Through independent syntheses of Shvo derivatives, we found a protective mechanism wherein catalyst deactivation by borazine is prevented by coordination of a ligand that might otherwise be a catalytic poison. These studies showed how a bidentate N-N ligand can transform the Shvo into a more reactive species for AB dehydrogenation that minimizes accumulation of borazine. Simultaneously, we designed novel ruthenium catalysts that contain a Lewis acidic boron to replace the Shvo -OH proton, thus offering more flexibility to optimize hydrogen release and take on more general problems in hydride abstraction. Our scorpionate-ligated ruthenium species (12) is a best-of-class catalyst for homogeneous dehydrogenation of ammonia borane in terms of its extent of hydrogen release (4.6 wt %), air tolerance, and reusability. Moreover, a synthetically simplified ruthenium complex supported by the inexpensive bis(pyrazolyl)borate ligand is a comparably good catalyst for AB dehydrogenation, among other reactions. In this Account, we present a detailed, concise description of how our work with the Shvo system progressed to the development of our very reactive and flexible dual-site boron-ruthenium catalysts.- Published
- 2017
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174. Synthesis, Structure, and Conformational Dynamics of Rhodium and Iridium Complexes of Dimethylbis(2-pyridyl)borate.
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Pennington-Boggio MK, Conley BL, Richmond MG, and Williams TJ
- Abstract
Rhodium(I) and Iridium(I) borate complexes of the structure [Me
2 B(2-py)2 ]ML2 (L2 = (tBuNC)2 , (CO)2 , (C2 H4 )2 , cod, dppe) were prepared and structurally characterized (cod = 1,5-cyclooctadiene; dppe = 1,2-diphenylphosphinoethane). Each contains a boat-configured chelate ring that participates in a boat-to-boat ring flip. Computational evidence shows that the ring flip proceeds through a transition state that is near planarity about the chelate ring. We observe an empirical, quantitative correlation between the barrier of this ring flip and the π acceptor ability of the ancillary ligand groups on the metal. The ring flip barrier correlates weakly to the Tolman and Lever ligand parameterization schemes, apparently because these combine both σ and π effects while we propose that the ring flip barrier is dominated by π bonding. This observation is consistent with metal-ligand π interactions becoming temporarily available only in the near-planar transition state of the chelate ring flip and not the boat-configured ground state. Thus, this is a first-of-class observation of metal-ligand π bonding governing conformational dynamics.- Published
- 2014
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175. A noncovalent, fluoroalkyl coating monomer for phosphonate-covered nanoparticles.
- Author
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Li V, Chang AY, and Williams TJ
- Abstract
Gadolinium-containing phosphonate-coated gold nanoparticles were prepared and then non-covalently coated with an amphiphilic fluorous monomer. The monomer spontaneously self-assembles into a non-covalent monolayer shell around the particle. The binding of the shell utilizes a guanidinium-phosphonate interaction analogous to the one exploited by the Wender molecular transporter system. Particle-shell binding was characterized by a 27% decrease in
19 F T1 of the fluorous shell upon exposure to the paramagnetic gadolinium in the particle and a corresponding increase in hydrodynamic diameter from 3 nm to 4 nm. Interestingly, a much smaller modulation of19 F T1 is observed when the shell monomer is treated with a phosphonate-free particle. By contrast, the phosphonate-free particle is a much more relaxive1 H T1 agent for water. Together, these observations show that the fluoroalkylguanidinium shell binds selectively to the phosphonate-covered particle. The system's relaxivity and selectivity give it potential for use in19 F based nanotheranostic agents.- Published
- 2013
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176. Optical spectroscopy shows that the normal state of URu2Si2 is an anomalous Fermi liquid.
- Author
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Nagel U, Uleksin T, Rõõm T, Lobo RP, Lejay P, Homes CC, Hall JS, Kinross AW, Purdy SK, Munsie T, Williams TJ, Luke GM, and Timusk T
- Subjects
- Electric Conductivity, Spectrum Analysis, Temperature, Optical Phenomena, Ruthenium chemistry, Silicon Compounds chemistry, Uranium chemistry
- Abstract
Fermi showed that, as a result of their quantum nature, electrons form a gas of particles whose temperature and density follow the so-called Fermi distribution. As shown by Landau, in a metal the electrons continue to act like free quantum mechanical particles with enhanced masses, despite their strong Coulomb interaction with each other and the positive background ions. This state of matter, the Landau-Fermi liquid, is recognized experimentally by an electrical resistivity that is proportional to the square of the absolute temperature plus a term proportional to the square of the frequency of the applied field. Calculations show that, if electron-electron scattering dominates the resistivity in a Landau-Fermi liquid, the ratio of the two terms, b, has the universal value of b = 4. We find that in the normal state of the heavy Fermion metal URu(2)Si(2), instead of the Fermi liquid value of 4, the coefficient b = 1 ± 0.1. This unexpected result implies that the electrons in this material are experiencing a unique scattering process. This scattering is intrinsic and we suggest that the uranium f electrons do not hybridize to form a coherent Fermi liquid but instead act like a dense array of elastic impurities, interacting incoherently with the charge carriers. This behavior is not restricted to URu(2)Si(2). Fermi liquid-like states with b ≠ 4 have been observed in a number of disparate systems, but the significance of this result has not been recognized.
- Published
- 2012
- Full Text
- View/download PDF
177. A Ruthenium-Catalyzed Coupling of Alkynes with 1,3-Diketones.
- Author
-
Pennington-Boggio MK, Conley BL, and Williams TJ
- Abstract
Ruthenium(III) chloride hydrate is a convenient catalyst for the addition of active methylene compounds to aryl alkynes. These reactions are rapid, operationally simple, and high yielding in cases. Most significantly, no precautions are required to exclude air or water from the reactions. All reagents are commercially available at reasonable prices, and the reactions can be conducted in disposable glassware with minimal solvent.
- Published
- 2012
- Full Text
- View/download PDF
178. Reactions of Indene and Indoles with Platinum Methyl Cations: Indene C-H Activation, Indole π vs. Nitrogen Lone-Pair Coordination.
- Author
-
Williams TJ, Labinger JA, and Bercaw JE
- Abstract
Reactions of indene and various substituted indoles with [(diimine)Pt
II (Me)(TFE)]+ cations have been studied (diimine = ArN = C(Me) - C(Me) = NAr; TFE = 2,2,2-trifluoroethanol). Indene displaces the TFE ligand from platinum to form a stable π coordination complex that, upon heating, undergoes C-H activation with first order kinetics, ΔH‡ = 29 kcal/mol, ΔS‡ = 10 eu, and a kinetic isotope effect of 1.1 at 60 °C. Indoles also initially form coordination complexes through the C2=C3 olefin, but these undergo rearrangement to the corresponding N -bound complexes. The relative rates of initial coordination and rearrangement are affected by excess acid or methyl substitution on indole.- Published
- 2007
- Full Text
- View/download PDF
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