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rss_1.0 Recursos de colección

Caltech Authors (89,479 recursos)
Repository of works by Caltech published authors.

Type = Conference or Workshop Item

Mostrando recursos 1 - 20 de 407

1. Cavity ringdown spectroscopy of chlorine-​substituted peroxy radicals: Reaction kinetics with nitric oxide - Smarte, Matthew D.; Dodson, Leah G.; Okumura, Mitchio
Peroxy radicals (RO_2) are ubiquitous intermediates in the oxidn. of volatile org. compds. Their competing fates govern subsequent free radical chem. in the troposphere. Reaction of RO_2 with nitric oxide (NO) has two product channels: assocn. to an org. nitrate (RONO_2) or oxidn./redn. to nitrogen dioxide (NO_2) and an alkoxy radical (RO). In this work, we present measurements of the RO_2 + NO rate const. at several pressures for peroxy radicals formed in the chlorine-initiated oxidn. of ethene, propene, 2-butene, and isoprene. We monitor the decay of peroxy radicals through their weak and structured A-X transition in the near-IR using...

2. Robust CNT field emitters: Growth and in situ welding on metal surfaces - Scott, Valerie J.; Manohara, Harish; Toda, Risaku; Murthy, Rakesh; Del Castillo, Linda
Methods to grow carbon nanotubes (CNTs) on titanium (Ti) dies using Chem. Vapor Deposition were developed. Post-growth treatment of the dies at elevated temps. (1050 °cC) under inert atm. allows for the metal surface to deform and subsequently aid in anchoring the surface-grown CNTs to the surface. Addn. of a layer of copper to the grown CNTs prior to heat treatment further enhances the CNT surface adhesion. Mech. tests demonstrated that the welded CNTs were attached to the substrate stronger than the compared to unwelded samples. The field emission properties of these anchored CNTs were tested and compared to analogous...

3. Bimetallic complexes for polymerization catalysis - Sampson, Jessica; Radlauer, Madalyn; Agapie, Theodor
Bimetallic polymn. catalysts based on a rigid terphenyl linker appended with various donors for metal binding have been previously prepd. Access to both meta and para terphenyl linkages allow for variation in metal-metal distance and steric profiles. The chem. of bimetallic complexes supported by phenoxy-imine and bisphenoxy amine ligands has been explored with early and late transition metals. Depending on the nature of the metal and the ligands, these bimetallic catalysts have shown tolerance of amines, polymn. of amino olefins and tacticity control. The mechanism of polymn. of a variety of monomers with these bimetallic catalysts will be discussed.

4. Effects of redox-​inactive metals on heterometallic manganese-​oxido complexes - Tsui, Emily Y.; Agapie, Theodor
Lewis acidic redox-inactive metal ions affect many chem. transformations of metal oxo compds., including electron transfer, hydrogen atom transfer, and oxygen atom transfer processes. Redox-inactive metals also play essential roles in both heterogeneous and biol. metal oxide catalysts such as the oxygen-evolving complex (OEC) of photosystem II, which contains a CaMn_4O_x cluster. A series of structurally related high oxidn. state heterometallic trimanganese dioxido clusters and a related series of trimanganese tetraoxido cubane clusters contg. redox-inactive metals of various charge were prepd. and structurally characterized. Electrochem. studies demonstrated that the redn. potentials of the clusters are linearly correlated to the Lewis...

5. Two electrode platforms for DNA sensing based on DNA charge transport - Furst, Ariel L.; Hill, Michael G.; Barton, Jacqueline K.
Recently, we have developed a new DNA array platform that is formed electrochem. and addressed using a secondary electrode. Electrochem. activation of a copper catalyst, patterned with one electrode, enables precise placement of different DNA sequences onto a second electrode surface. The two-electrode patterning and detection platform allows for both high spatial resoln. of the patterned DNA array and optimization of detection through DNA-mediated charge transport with electrocatalysis. These platforms have been used for the successful detection of single base mismatches, specific hybridization events, and sequence-specific binding of transcription factors. We describe the application of this two-electrode platform in the...

6. Mechanistic studies of intramolecular aryl ether carbon-​oxygen bond activation by group 9 and 10 transition metal centers - Edouard, Guy A.; Kelley, Paul; Herbert, David E.; Agapie, Theodor
Aryl ether bond activation is of interest in the context of lignin conversion and org. methodol. To gain mechanistic insight, the intramol. reactivity of group 9 and 10 metals supported by a terphenyl diphosphine bearing an aryl Me ether was investigated. Ni°c and Rh^I were obsd. to activate aryl ether C-O bonds selectivity via oxidative addn. Non-selective activation of aryl and alkyl C-O bonds was obsd. at Ir^I. At Pd°c and Pt°c, alkyl C-O bond activation was obsd. through oxidative addn. With group 10 M^II halides, alkyl C-O bond activation was obsd. through Lewis-acid mediated alkyl halide elimination. Various modes...

7. New ruthenium-​based catalysts for Z-​selective metathesis of hindered olefins - Quigley, Brendan L.; Grubbs, Robert H.
A series of new ruthenium-based metathesis catalysts with varying N-heterocyclic carbene (NHC) substituents and their reactivity toward hindered olefins are reported. Through systematic variation of the sterics and electronics of the NHC substituents, metathesis activity and Z-selectivity are related to structure, resulting in catalysts that perform cross metathesis of a wide variety of allylic-substituted olefins in high yield and excellent Z-selectivity. Further insights into reactivity are obtained from the solid-state structures and soln. dynamics of the catalysts.

8. Catalytic synthesis of ethylene glycol from a C1 source - Swisher, Nicholas A.; Romero, Patricio; Grubbs, Robert H.
Ethylene glycol is an industrially important compd. which is used in a variety of polymeric materials and antifreeze applications. Ethylene glycol is mass produced on a multibillion kilogram scale using ethylene as a C2 source. A potentially sustainable route to ethylene glycol based on synthesis from a C1 source is envisioned. Using formaldehyde as a methanol surrogate, we describe a one-pot two step procedure for ethylene glycol synthesis. In the first step, an N-heterocyclic carbene organocatalyst is used to condense two mols. of formaldehyde to form a C2 compd., glycoaldehdye. In the second step, a late transition metal organometallic catalyst...

9. Upgrading light olefins by trimerization catalysis - Sattler, Aaron; Winkler, Jay R.; Labinger, Jay A.; Gray, Harry B.; Bercaw, John E.
Activation of a (phenoxy-imine) titanium tri-Me complex with one equiv. of B(C_6F_5)_3 effects the catalytic trimerization of ethylene. Stoichiometric activation with B(C_6F_5)_3 allows for mechanistic studies to be conducted, which give insight into catalyst initiation, trimerization, and decompn., and the relative rates of these processes. In addn. to ethylene, α-olefins are oligomerized with high selectivity for trimers (> 95%), of which approx. 85% are one regioisomer.

10. Tungsten photosensitizers - Sattler, Wesley; Winkler, Jay R.; Gray, Harry B.
The development of photosensitizers is of great interest due to their ability to carry out difficult inorg. and org. reactions in their excited states. We have developed a new class of homoleptic arylisocyanide tungsten photosensitizers, W(CNAr)_6, and studied their photoelectrochem. properties. A modular synthesis allows for the prodn. of a variety of oligoaryl isocyanide complexes of tungsten, all of which display intense metal-to-ligand charge transfer (MLCT) absorptions in the visible region. Their excited states, *W(CNAr)_6, are highly emissive with lifetimes ranging from 17 ns to 3.0 μs depending on the arylisocyanide ligand. *W(CNAr)_6, are extremely powerful reductants: [W(CNAr)_6]^+/*W(CNAr)_6 redn. potentials...

11. Redox properties of cobalt μ4-​oxo complexes [MCo_3O]^​n+ incorporating redox inactive metals (M = Li^+, Zn^2+, Co^2+) - Lee, Heui Beom; Herbert, Dave E.; Suseno, Sandy; Agapie, Theodor
The synthesis and reactivity of cobalt oxo cluster complexes will be presented. Using a hexapyridyl-trialkoxide ligand, a trinuclear Co complex was used as a platform for the prepn. of tetrametallic oxido cluster complexes [MCo_3O]^n+ where M = Li^+, Zn^2+, Co^2+. Reactivity of such complexes toward oxygen atom transfer reagents and their redox properties will be discussed.

12. Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surfaces - Blakemore, James D.; Lattimer, Judith R.; Sattler, Wesley; Gul, Sheraz; Yano, Junko; Brunschwig, Bruce S.; Lewis, Nathan S.; Gray, Harry B.
A route to bipyridine-functionalized silicon(111) surfaces has been developed which gave submonolayer surface coverage of immobilized 4-vinyl-2,2'-bipyridyl (vbpy). The remaining atop sites of the silicon surface were passivated with Me groups. The immobilized bipyridyl ligands bound metal ions, thus enabling assembly of metal complexes on the silicon surface. XPS studies demonstrated that [Cp*Rh(vbpy)Cl]Cl, [Cp*Ir(vbpy)Cl]Cl, and Ru(acac)2vbpy were assembled on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepd. with iridium, X-ray absorption spectroscopy at the Ir L_(III) edge showed an edge energy and post-edge features consistent with a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2'-bipyridyl). Cyclic voltammetry...

13. Strategies for synthesizing Mn_4CaOn models relevant to biological water oxidation - Kanady, Jacob S.; Tsui, Emily Y.; Lin, Po-Heng; Agapie, Theodor
The biol. site of water oxidn.-the oxygen-evolving complex (OEC) of photosystem II-is a Mn_4CaO_5 heterometallic cluster, proposed to contain a Mn-_3CaO_4 cubane / open cubane unit with a fourth, dangling Mn bridged to the cubane through two oxido ligands. In past work we synthesized a Mn_3CaO_4 cubane supported by three acetates and a 1, 3,5-triarylbenzene framework contg. six pyridines and three alkoxides. More recently, incorporation of a fifth metal center has been targeted to complete the Mn_4Ca stoichiometry. A desymmetrized Mn_3CaO_4 complex has been made with reactivity complementary to the parent, sym. complex; synthetic strategies from this desymmetrized complex toward...

14. Synthesis and study of [MMn_3O_x] complexes: The effects of redox-​inactive metals on cluster properties and reactivity - Tsui, Emily Y.; Agapie, Theodor
Lewis acidic redox-inactive metal ions are known to affect processes such as electron transfer, hydrogen atom transfer, and oxygen atom transfer in metal oxo compds. A series of structurally related [MMn_3O_2] and [MMn_3O_4] complexes contg. redox-inactive metals of various charge (M = Na^+, Ca^2+, Sr^2+, Zn^2+, Y^3+, and Sc^3+) were prepd. and structurally characterized. These complexes were studied to probe the effects of varying the redox-inactive metals in isostructural complexes on different chem. transformations of the clusters. Cyclic voltammetry of the compds. showed that the redox potentials of the clusters are dependent upon the Lewis acidity of the incorporated redox-inactive...

15. Para-​terphenyl diphosphine molybdenum complexes: Synthesis and reactivity - Buss, Joshua A.; Cheng, Christine; Edouard, Guy A.; Shi , Jade; Agapie, Theodor
We describe the synthesis of a series of molybdenum (Mo) complexes supported by a para-terphenyl diphosphine (P2) ligand, displaying metal-arene interactions. Treatment of P2 with Mo tricarbonyl precursors yields the Mo°c species, P2Mo(CO)3. Two electron oxidn. followed by photolytic decarbonylation gives a Mo^II complex, [P2Mo(MeCN)2]^2+. Sodium azide oxidizes this species to yield a Mo^IV nitride-azide complex. Redn. with magnesium furnishes a Mo°c dinitrogen complex, P2Mo(N2). This dinitrogen species reacts cleanly with CO_2, H_2, or PhSiH_3 providing a μ_2-CO_2 complex, a Mo dihydride, and a Ph silyl hydride species, resp. The structure, characterization, and further reactivity of these P2 Mo complexes...

16. Synthesis, characterization, and multi-​proton​/multi-​electron chemistry of molybdenum-​quinoid complexes supported by a non-​innocent ligand - Henthorn, Justin T.; Agapie, Theodor
Incorporation of redox non-innocent ligands as well as pendant acid/base moieties has proven effective as a method for overcoming the propensity for earth-abundant transition metals to preferentially access single-electron chem. and facilitate efficient multi-proton, multi-electron activation of small mols. such as dioxygen. A series of molybdenum-quionoid complexes supported by a non-innocent ligand have been synthesized and characterized demonstrating both hydrogen atom and proton and electron transfer chem. spanning a total of four electrons and two protons. Five different protonation/oxidn. state combinations were isolated varying from Mo(0)-catechol to Mo(II)-quinone. Substrate-based multi-proton, multi-electron reactivity was demonstrated through redn. of dioxygen to water,...

17. Transition metal mediated carbon-​oxygen bond activation of aryl ethers bearing pendant phosphines - Edouard, Guy A.; Kelley, Paul; Herbert, David H.; Agapie, Theodor
Cleavage of aryl C-O bonds of lignin affords phenols and aroms. that might serve as useful precursors in chem. synthesis. C-O bond activation of aryl ethers allows for their use as cross coupling partners and removable protecting groups. An aryl Me ether bearing pendant phosphines was synthesized to investigate ether bond cleavage mediated by different transition metals. Three modes of bond activation were obsd. at group 9 and 10 transition metals: oxidative addn. across the aryl C-O bond, oxidative addn. across the alkyl C-O bond, and Lewis-acid mediated alkyl halide elimination. Metal oxidn. state as well as π-backbonding interactions between...

18. Modulation of redox potential of Fe-​3 oxo​/hydroxo clusters by Lewis acids - Lionetti, Davide; Herbert, David E.; Rittle, Jonathan; Agapie, Theodor
Lewis acidic redox-inactive metal centers have long been known to affect the redox reactivity of transition metal complexes. To further our understanding of the relationship between a cation's Lewis acidity and its effects on redox processes, a systematic study of structurally analogous heteromultimetallic complexes of redox inactive metals (M) displaying a broad range of Lewis acidities is necessary. Inspired by recent reports on [Mn_3MO_n] complexes indicating a link between Lewis acidity and the tuning of redox potentials, a series of triiron complexes was prepd. incorporating a range of redox-inactive metals. Crystallog. characterization revealed a tetrametallic Fe_3M (M = Ca, Sr,...

19. Synthesis of mixed-​metal transition metal oxido cubanes (MMn_3O_4) and their potential as prescursors for water oxidation electrocatalysts - Suseno, Sandy; Kaiser, Jens; Tran, Rosalie; McCrory, Charles C.; Rittle, Jonathan; Yano, Junko; Agapie, Theodor
The study of mixed transition metal oxido cubane complexes is crucial to the development of materials for oxygen evolution reaction, oxygen redn. reaction, and energy storage. A discreet series of heterometallic trimanganese-tetraoxido cubanes (MMn_3O_4 ; M = Fe, Co, Ni, Cu) supported by a trisalkoxy-hexapyridyl ligand with a 1, 3,5-triarylbenzene spacer were synthesized and characterized by single crystal X-ray diffraction, dispersive difference diffraction anal., cyclic voltammetry, Mossbauer spectroscopy, and X-ray absorption spectroscopy. To det. their potential as precursors for heterogeneous water oxidn. electrocatalysts, these well-defined complexes were drop-casted on electrodes, and electrolysis of the material was performed in basic aq....

20. m-​Terphenyl diphosphines bearing a central phenolate ring as coordinatively flexible frameworks for supporting transition metals - Low, Choon Heng; Edouard, Guy A.; Agapie, Theodor
Based on earlier studies of m-terphenyl-supported complexes, a m-terphenyl diphosphine framework bearing a central phenolate ring has been studied. This ligand framework imparts coordinative lability to stabilize metal centers of a variety of oxidn. states by shifting between arene/phenolate coordination. Metalation has been explored with iron, cobalt and nickel and their redox properties and reactivities will be discussed. Variations in the substituent para to the phenolate oxygen have been synthesized to investigate their electronic effects on the metal complex.

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