Dissertations / Theses on the topic 'High Oxidation State of Copper'

La galactose oxydase (GOase), une métallo-enzyme contenant du cuivre, est l'un des biocatalyseurs les plus étudiés pour l'oxydation enzymatique des glucides. Le mécanisme couramment accepté implique la forme oxydée clé (GOaseox), dans laquelle une unité glucidique (galactose) se lie au site équatorial (libre) et subit une déprotonation, suivie d'un arrachage d'atome d'hydrogène par un radical et d'un transfert d'électron supplémentaire, conduisant ainsi à la formation du produit final, l'aldéhyde, et à la réduction de la GOase. En raison de leur potentiel pour des oxydations catalytiques hautement sélectives, des modèles moléculaires du site actif de la GOase dérivés de bases de Schiff encombrées ont été développés. Cette approche biomimétique passe par la synthèse de complexes Cu(II)-phénol servant de pré-catalyseurs, qui subissent ensuite une oxydation à un électron pour devenir la forme "active" du catalyseur. Une question cruciale demeure : quels sont les facteurs qui déterminent si l’oxydation est centrée sur le ligand, produisant des radicaux Cu(II)-phénoxyle, ou vers le métal, formant des espèces Cu(III)-phénolate ? Malgré des efforts importants, une réponse définitive à cette question reste elusive.L'objectif de cette thèse est de développer des ligands redox-actifs afin de mieux comprendre les facteurs qui influencent le site d’oxydation du complexe de cuivre correspondant. La stratégie consiste à incorporer des fonctions chimiques stabilisant l'un ou l'autre tautomère de valence (Cu(II)-radical phénoxyle et Cu(III)-phénolate) et à étudier leur impact. À cet effet, plusieurs complexes ont été synthétisés et caractérisés par spectroscopie et électrochimie. Les activités catalytiques ont également été évaluées sur divers substrats contenant des groupes hydroxyle. Enfin, des calculs de chimie quantique (DFT) ont été réalisés pour aider à élucider les mécanismes catalytiques et à mieux comprendre les caractéristiques des différents complexes
Galactose Oxidase (GOase), a copper-containing metallo-enzyme, is one of the most studied biocatalysts for the enzymatic oxidation of carbohydrates. The consensus mechanism involves the key oxidized form (GOaseox), in which an the carbohydrate substrate (galactose unit) binds to the equatorial (free) site and is subsequently deprotonated. It undergoes hydrogen atom abstraction by the radical and further electron transfer to give the final product aldehyde and the reduced form of the GOase. Due to the potential for highly selective catalytic oxidations, the development of small-molecular models of the GOase active site has been carried out. Notably, sterically hindered schiff bases, which stand as one of the most representative mimics, have garnered significant attention. This biomimetic approach has extended to encompass other strategies. Within this framework, a range of Cu(II)-phenol complexes, serving as pre-catalysts, have been synthesized, subsequently undergoing one-electron oxidation to yield the "active" catalyst form. A central question then arises: What factors determine whether the oxidation pathway proceeds toward the ligand, resulting in the formation of Cu(II)-phenoxyl radicals, or toward the metal, giving rise to the Cu(III)-phenolate species? Despite substantial efforts, a definitive answer to this question has yet to be obtained.The aim of this thesis is to develop redox-active ligands aimed at understanding the factors affecting the oxidation state of copper, able to catalyze the oxidation of an alcohol into an aldehyde. The strategy is to include chemical functions that can stabilize either valence tautomer (Cu(II)-phenoxyl radical and Cu(III)-phenolate) and study their effect. For that purpose, several complexes were synthesized and characterized by different ways to understand their properties. The catalytic activities were also tested against different families of substrates comprising hydroxyl functions. Finally, quantum chemistry (DFT) calculations have been carried to help understand the characteristics of different complexes and elucidate of the catalytic mechanisms at work

The reduction of osmium tetroxide by a series of alcohols was studied spectrophotometrically. The reaction was observed to occur in two steps, unlike previously reported studies on this reaction. The identities of both reactants and products were established via a range of techniques. Equilibrium and kinetic data were gathered and reaction models were evaluated using equilibrium and kinetic modelling software. The following complexation reaction model emerged that simulates both the equilibrium and kinetic data. Os(VIII) + RCH2OHOs(VI) + RCHO2 Os(VIII) + Os(VI)k+2k1Complexk-2 Conditional rate constants and equilibrium constants were generated. Rate constants for the alcohol reactions were correlated with the Taft σ* constant. The ρ* value obtained (-1.4) is consistent with a hydride transfer mechanism coupled with synchronous removal of the hydroxyl proton. The identity of the osmium(VIII)-osmium(VI) complex has been suggested. Thermodynamic parameters were also reported. The rate constants for benzyl alcohol and 2-chloroethanol deviated from those predicted by the Taft plot. An explanation of enhanced resonance effects is offered for benzyl alcohol and an alternative reaction mechanism, involving proton abstraction, is offered for 2-chloroethanol. The reaction of the oxidation products of alcohols, namely ketones, with osmium tetroxide produced rate constants that were, perhaps surprisingly, far larger than those of the alcohols. A reaction mechanism for the oxidation of the ketones is suggested, which involves the enolate ion as the reactive starting reagent.

Chapter one is an introduction to the less common coordination and oxidation chemistry of palladium; complexes containing Pd-OR, Pd-NR2 and those in the oxidation states of +IV. An outline of PdII/IV catalysed ligand-directed oxidative functionalisation is also included. Chapter two covers the design and synthesis of a range of tethered N-heterocyclic carbene (NHC) complexes of Pd. In addition, the syntheses of a number of new tethered NHC ligands are described. The use of Density Functional Theory (DFT) to model the complexes in this thesis was explored. Chapter three describes the synthesis and characterisation of PdIV halide complexes. The relevance of these compounds to PdII/IV catalysed ligand-directed oxidative functionalisation is explored. DFT was used to probe the reaction pathway for N-bromosuccinimide and iodobenzene dichloride. Chapter four examines reactions with oxidants used to form C-O and C-C bonds. The reaction pathway for iodobenzene diacetate was investigated using DFT. Chapter five contains experimental details and characterising data for the compounds reported.

The preparation of UF5 has been re-investigated in detail and it has been shown that the most reliable synthetic route is a novel reaction in which UF6 is interacted with hexamethyldisiloxane at room temperature. The pure UF5 produced has been employed in an investigation of the reactions of UF5.nCH3CN and UF5.nTPPO with trimethylsilylchloride which have been claimed to yield compounds of the type UFxC15-x.nCH3CN (0?x?5 and n = 1,2) and UFxC15-x.nTPPO (TPPO = (C6H5)3PO, 0?x?5 and n = 1,2). In the process of these studies a large number of products were obtained as single crystals and the single-crystal X-ray structures of the novel compounds, [UF4.TPPO]2F, [(UF3.3TPPO)2]2+ 2[UF6]-, cis-UO2Cl2.2TPP0 and ?-trans- UO2Cl2.2TPPO have been obtained. Attempts have been made to elucidate the infrared and e.s.r. spectra of these compounds in order to try to understand the nature of the chemical reactions which give rise to them. Efforts to prepare a mixed bromide fluoride of uranium (VI) have yielded only uranium pentafluoride or uranium tetrafluoride. The preparations of UF5.SbF5.L (L = CH3CN and TPPO) and UF5.2SbF5.5CH3CN were repeated and single crystals were successfully grown for X-ray examination. An attempt to obtain the novel UF5.2SbF5.5TPP0, however, gave inconclusive results.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.
Vita.
Includes bibliographical references.
Chapter 1. An olefin metathesis reaction between Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2 (Ar = 2,6diisopropylphenyl) and trans-i 1,4-divinylbenzene or trans, trans- 1,4-di-buta- 1,3-dienylbenzene (5) results in the formation of bimetallic ROMP initiators [(DME)((CF3)2MeCO)2(ArN)MoCH]2-1,4-C6H4 (DME = 1,2-dimethoxyethane; la) and [(DME)((CF3)2MeCO)2(ArN)MoCHCHCH]2-1,4-C6H4 (6), respectively. An X-ray study of [(THF)((CF3)2MeCO)2(ArN)MoCH]2-1,4-C6H4 (THF = tetrahydrofuran; ib), which is closely related to la, showed it to be the expected bimetallic species in which each end is approximately a trigonal bipyramidal monoadduct of a syn alkylidene with the THF coordinated to the NOO face of the metal trans to the Mo=C bond. Treatment of la with hium-t-butoxide yielded [(Bu-t-O)2(ArN)MoCH]2-1,4-C6H4 (2). Addition of four equivalents of Me3CCH2MgCl to la produced the bimetallic species [(Me3CCH2)2(ArN)MoCH]2- 1,4-C6H4 (3), which upon treatment with 2,6diisopropylphenol generated a diastereomeric mixture of [(DME)(ArO)(Me3CCH2)(ArN)MoCH]2-1,4-C6H4 (4). The solid state structure of 3 revealed a "syn/syn" bimetallic species related to lb. In solution two resonances can be observed in the alkylidene region of the 1H NMR spectra for the "syn/anti" isomer of la, Ib, 2, 3, 4 and 5. The total amount of the "syn/anti" isomer varies between 4 and 20% of the total. Bimetallic species la, 2, and 6 initiate at both ends upon addition of less than ten equivalents of 4,5-dicarbomethoxynorbornadiene (DCMNBD),and afford homopolymers of DCMNBD and methyltetracyclododecene (MTD) in a living fashion. MALDI-TOF mass spectra of ferrocene-containing homopolymers have been obtained that are consistent with the polymerization process being living.
(cont.) Triblock copolymers MTDxDCMNBDyMTDx were prepared by adding y equivalents of DCMNBD to the bimetallic ROMP initiators followed (after consumption of DCMNBD) by 2x equivalents of MTD. These triblocks were shown to be of relatively high purity (free of homopolymer and diblock copolymer) and to have a relatively low polydispersity index. Chapter 2 A series of monomers with side chain liquid crystals (SCLCs) were synthesized for ring opening metathesis polymerization (ROMP). The liquid crystals (LCs) used were 4hydroxybenzoic acid 4-methoxyphenyl ester (MPOB-H), which is known to exhibit a nematic liquid crystalline phase, and biphenyl-4-carboxylic acid 4-(1butoxycarbonylethoxy)phenyl ester (BPP4-H), which is known to exhibit a smectic phase. The side chains differed in spacer length, spacer type, and the nature of the LC. Monomers were polymerized using a bimetallic ROMP initiator [(Bu-t-O)2(ArN)MoCH]2-1,4-C6H4 (where Ar = 2,6-diisopropylphenyl); both homopolymers and ABA type triblock copolymers, where the B block is the LC functional monomer, and A is methyltetracyclodocene (MTD) were prepared. The polymers displayed unimodal peak distributions with polydispersities < 1.22. Incorporation of a polyoxyethylene spacer decreased the glass transition temperature (Tg) of the polymer block to -25 from +20 TC, the Tg when an alkyl spacer was used. Although no distinct LC phase was observed with the polyoxyethylene spacer when MPOB was used, use of BPP4 in conjunction with a polyoxyethylene spacer displayed a distinct liquid crystalline transition. Polymers with an alkyl spacer exhibited liquid crystalline behavior and good phase segregation on the basis of differential scanning calorimetry and small-angle X-ray scattering studies.
(cont.) Chapter 3. Molybdenum imido alkylidene complexes supported by 2,6-diisopropylphenyl, 2,6dimethylphenyl, 1-adamantyl, or 2-trifluorophenyl that contain relatively electronwithdrawing phenolate (pentafluoro), binaphtholate (3,3'-bis(9-anthracenyl), 3,3'bispentafluorophenyl, or 3,3'-bis(3,5-bis(trifluoromethyl)phenyl)) or biphenolate (3,3'-ditert-butyl-5,5'-bistrifluoromethyl-6,6'-dimethyl-1,1'-biphenyl) ligands have been prepared to be tested in olefin metathesis reactions. A series of new monomeric pyrrolide complexes, Mo(NR)(CHCMe2R')(2,5-dimethylpyrrolide)2 (where R' = Me or Ph) and Mo(NAd)(CHCMe2Ph)(2,4-dimethylpyrrolide)2, were also synthesized and treated with alcohols, biphenols or binaphthols in order to generate Mo(NR)(CHCMe2Ph)(diolate) species. In several cases the new alkylidene complexes could be prepared only through reaction of two equivalents of pentafluorophenols or an equivalent of binaphthol (3,3'bis(pentafluorophenyl)binaphthol or 3,3'-bis(3,5-bis(trifluoromethyl)phenyl binaphthol) with a bis(2,5-dimethylpyrrolide) complex. The pyrrolide approach can be employed either to isolate catalysts on a preparative scale or to generate catalysts in situ. Several simple preliminary ring-closing metathesis reactions show that the new complexes are catalytically competent. Chapter 4. Living ring opening metathesis polymerization of cyclopropenes using Mo(NAr)(CHCMe2R)(O-t-Bu)2 (where R = Me; la or R = Ph; ib) and Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)2 (2) has been achieved. The polydispersity indices of the polymers generated are recorded to be < 1.10 for la and lb based on gel permeation chromatography (GPC). Living polymerization of 3-methyl-3phenylcyclopropene (MPC) by t-butoxide derived molybdenum imido alkylidene initiators is utilized in the synthesis of block copolymers.
(cont.) Diblock copolymers MPCPxDCMNBDy (where DCMNBD = 2,3- dicarbomethoxynorbornadiene and x and y are the number of equivalents of MPC and DCMNBD, respectively) were prepared in quantitative yield with unimodal peak distribution via sequential addition of monomers to the initiator 1b. Triblock copolymers MPC100MEMC100MPCoo0 and MTDiooMEMCIooMTDjoo were prepared using [(O-t-Bu)2(ArN)MoCH]2-1,4-C6H4 (IC) as an initiator. The block copolymers revealed good phase segregation based on the differential scanning calorimetry (DSC). ROMP of MPC by molybdenum imido alkylidene initiators containing electron-withdrawing diolates resulted in highly tactic polymers as determined by 'H and 13C NMR spectroscopy. IR spectroscopy of all MPCl00 revealed absorptions at 963 and/or 982 cm-1 of approximately equal intensity that are most consistent with a trans structure. Chapter 5. Addition of one equivalent of ROH to Mo(NAr)(CHCMe2Ph)(2,5-dimethylpyrrolide)2 (Ar = 2,6-diisopropylphenyl) in diethyl ether or THF yielded Mo(NAr)(CHCMe2Ph)(OR)(2,5-dimethylpyrrolide) species where R = (CH3)3C (1), (CH3)2CH (2), Ar (3), (CF3)2CH (4), (CF3)2MeC (5), (Bu-t-O)3SiO (6) or C6F50 (7). Treatment of an equivalent of PMe3 to 5 resulted in the formation of (Me3P)Mo(NAr)(CHCMe2Ph)(OCMe(CF3)2)(PyrMe) (Me3P-5), which showed trans binding of PMe3 with respect to 2,5-dimethylpyrrolide ligand as determined by the 'H NOESY spectrum. The solid state structure of 3 depicts a psuedo-tetrahedral geometry around the metal center with the 2,5-dimethylpyrrolide ligand bound rq' to the metal cener.
(cont.) Complexes 1-6 show rapid reaction when treated with one atmosphere of ethylene and catalyze ring-closing metathesis reactions. Hetereogenous analogs of 6, Mo(NAr)(CHCMe2Ph)(PyrMe)(OSisurf) (10a), Mo(NAd)(CHCMe2Ph)(PyrMe)(OSisurf) (10b) and Mo(NArF)(CHCMe2Ph)(PyrMe)(OSisurf) (10c) (Ad = 1-adamantyl and ArF = 2trifluoromethylphenyl) showed great enhancement in the catalytic activity when employed in self-metathesis of propene and ethyloleate. Ring-closing enyne metathesis catalyzed by 1 - 6 leads to cyclic products that arise through initial addition of the triple bond to a methylene species (initially neophylidene) to yield an cC- or a 3-substituted metallacyclobutene intermediate.
by Rojendra Singh.
Ph.D.

High-oxidation state main-group metal complexes are potential alternatives to transition metals for electrophilic C-H functionalization reactions. However, there is little known about how selection of the p-block, main-group metal and ligand impact C-H activation and functionalization thermodynamics and reactivity. Chapter 2 reports density functional theory (DFT) calculations used to determine qualitative and quantitative features of C-H activation and metal-methyl functionalization energy landscapes for reaction between high-oxidation state d10s0 InIII, TlIII, SnIV, and PbIV carboxylate complexes with methane. While the main-group metal influences the C-H activation barrier height in a periodic manner, the carboxylate ligand has a much larger quantitative impact on C-H activation with stabilized carboxylate anions inducing the lowest barriers. For metal-methyl reductive functionalization reactions, the barrier heights, are correlated to bond heterolysis energies as model two-electron reduction energies.In Chapter 3, DFT calculations reveal that arene C-H functionalization by the p-block main-group metal complex TlIII(TFA)3 (TFA = trifluoroacetate) occurs by a C-H activation mechanism akin to transition metal-mediated C-H activation. For benzene, toluene, and xylenes a one-step C-H activation is preferred over electron transfer or proton-coupled electron transfer. The proposed C-H activation mechanism is consistent with calculation and comparison to experiment, of arene thallation rates, regioselectivity, and H/D kinetic isotope effects. For trimethyl and tetramethyl substituted arenes, electron transfer becomes the preferred pathway and thermodynamic and kinetic calculations correctly predict the experimentally reported electron transfer crossover region.In Chapter 4, DFT calculations are used to understand the C-H oxidation reactions of methane and isobutane with SbVF5. SbVF5 is generally assumed to oxidize methane through a methanium-methyl cation mechanism. DFT calculations were used to examine methane oxidation by SbVF5 in the presence of CO leading to the acylium cation, [CH3CO]+. While there is a low barrier for methane protonation by [SbVF6]-[H]+ to give the [SbVF5]-[CH5]+ ion pair, H2 dissociation is a relatively higher energy process, even with CO assistance, and so this protonation pathway is reversible. The C-H activation/[]-bond metathesis mechanism with formation of an SbV-Me intermediate is the lowest energy pathway examined. This pathway leads to [CH3CO]+ by functionalization of the SbV-Me intermediate by CO, and is consistent with no observation of H2. In contrast to methane, due to the much lower carbocation hydride affinity, isobutane significantly favors hydride transfer to give tert-butyl carbocation with concomitant SbV to SbIII reduction. In this mechanism, the resulting highly acidic SbV-H intermediate provides a route to H2 through protonation of isobutane.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2014.
Cataloged from PDF version of thesis. Vita.
Includes bibliographical references.
Chapter 1 describes work toward solid-supported W olefin metathesis catalysts. Attempts to tether derivatives of the known Z-selective catalyst W(NAr)(C₃H₆)(pyr)(OHIPT) (Ar = 2,6- diisopropylphenyl, pyr = pyrrolide; HIPT = 2,6-bis-(2,4,6-triisopropylphenyl)phenyl) to a modified silica surface by covalent linkages are unsuccessful due to destructive interactions between W precursors and silica. W(NAr)(C₃H₆)(pyr)(OHIPT) and W(NAr)(CHCMe₂Ph)(pyr)(OHIPT-NMe₂) (HIPT-NMe 2 = 2,6-bis-(2,4,6-triisopropylphenyl)-4- dimethylaminophenyl) are adsorbed onto calcined alumina. W(NAr)(C 3H6 )(pyr)(OHIPT) is destroyed upon binding to alumina, while W(NAr)(CHCMe 2Ph)(pyr)(OHIPT-NMe 2) appears to bind through a non-destructive interaction between the dimethylamino group and an acidic surface site. The heterogeneous catalysts perform non-stereoselective metathesis of terminal olefins, and W(NAr)(CHCMe₂Ph)(pyr)(OHIPT-NMe₂) can be washed off the surface with polar solvent and perform solution-phase Z-selective metathesis. Chapter 2 details selective metathesis homocoupling of 1,3-dienes with Mo and W monoalkoxide pyrrolide (MAP) catalysts. A catalytically relevant vinylalkylidene complex, Mo(NAr)(CHCHCH(CH₃)₂)(Me₂pyr)(OHMT) (HMT = 2,6-bis(2,4,6-trimethylphenyl)phenyl; Me₂pyr = 2,5-dimethylpyrrolide), is isolated. A series of Mo and W MAP catalysts is synthesized and tested for activity, stereoselectivity, and chemoselectivity in 1,3-diene metathesis homocoupling. Catalysts containing the OHIPT ligand display excellent selectivity in general, and W catalysts are less active but more selective than their Mo counterparts. Chapter 3 recounts the synthesis and characterization of several heteroatom-substituted alkylidene complexes with the formula Mo(NAr)(CHER)(Me₂pyr)(OTPP) (TPP = 2,3,5,6- tetraphenylphenyl; ER = OPr, N-pyrrolidinonyl, N-carbazolyl, pinacolborato, trimethylsilyl, SPh, or PPh2). Synthesis proceeds via alkylidene exchange between Mo(NAr)(CHR)(Me₂pyr)(OTPP) (R = H, CMe₂Ph) and a CH₂CHER precursor. Each complex behaves similarly to known MAP complexes in olefin metathesis processes; the electronic identity of ER has little effect on catalytic properties. Distinctive features of alkylidene isomerism and catalyst resting state are examined. Chapter 4 contains synthetic and catalytic studies of thiolate-containing Mo and W imido alkylidene complexes. The species M(NAr)(CHCMe 2Ph)(pyr)(SHMT) (M = Mo or W), Mo(NAr)(CHCMe₂Ph)(Me₂pyr)(STPP), and Mo(NAr)(CHCMe₂Ph)(STPP)₂ are synthesized by substitution of the appropriate thiol or thiolate ligands for pyrrolide or triflate ligands in metal precursors. These complexes show similar structural and spectral characteristics to alkoxidecontaining species. The thiolate complexes and their alkoxide analogues are compared for activity and selectivity in metathesis homocoupling and ring-opening metathesis polymerization processes. In general, thiolate catalysts are slower and less selective than alkoxide catalysts.
by Erik Matthew Townsend.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.
Vita. Cataloged from PDF version of thesis.
Includes bibliographical references.
Chapter 1: Reaction of W(CCMe3)Cl 3(dme) with one equivalent of (3,5-Me 2C6H3NCH2CH 2)3N)Li3 affords yellow, crystalline W(CCMe3)(N3N) in good yield. The reactivity of this new alkylidyne complex towards terminal alkynes was investigated. Two other new tungsten alkylidynes, W(CCMe3)(pyr) 3 (pyr = 2,5-dimethylpyrrolide) and W(CCMe3)(Ph 2N)3 were prepared by the addition of three equivalents of lithium dimethylpyrrolide or lithium diphenylamide, respectively, to W(CCMe3)Cl 3(dme). The reactivity of these new alkylidynes with various alcohols is reported. The reactivity of several tungsten alkylidyne compounds towards ligand displacement by surface silanols is reported, resulting in the synthesis of several new silicasupported tungsten alkylidynes. The alkyne metathesis activity of all new homogeneous and heterogeneous alkylidyne complexes is reported. Chapter 2: Addition of one equivalent of 2,4,6,2',4',6'-hexaisopropylterphenol to Mo(NAd)(CHCMe 2Ph)(pyr)2 results in the formation of Mo(NAd)(CHCMe 2Ph)(pyr)(HIPTO) (HIPTO = hexaisopropylterphenoxide). This new alkylidene compound was found to catalyze the metathesis of 1-hexene in 20% yield to 95% cis 5-decene, which represents the first report of highly Z-selective metathesis homocoupling of a terminal olefin. The decomposition of the catalyst in the presence of ethylene is explored. The syntheses of several new bulky achiral phenoxide ligands are presented, along with the syntheses of the corresponding MAP (monoalkoxide monopyrrolide) molybdenum imido alkylidene compounds. The reactivity of new MAP compounds containing bulky phenoxide ligands towards the Z-selective metathesis of terminal and internal olefins is presented. The cis-selectivity of this system is proposed to arise from the combination of a relatively small imido ligand in conjunction with a very bulky alkoxide forcing the substituents of the substrate to point in this same direction with each insertion. Photolysis of MAP compounds with 366 nm radiation was found to produce significant amounts of anti alkylidenes, and the kinetics of decay of unstable anti alkylidenes are investigated. Chapter 3: The reaction of 2,3-dicarbomethoxynorbomadiene (DCMNBD) with Mo(NAd)(CHCMe 2Ph)(pyr)(HIPTO) (Ad = 1-adamantyl, HIPTO = hexaisopropylterphenoxide) affords >98% cis, >98% tactic polyDCMNBD. The tacticity of this polymer is proved to be syndiotactic through polymerization of DCMenthNBD (2,3-dicarbomenthoxynorbomadiene) and IH- H COSY. A variety of related MAP alkylidene compounds are also investigated towards the ROMP of DCMNBD and found to produce polyDCMNBD in a range of tacticities and cis contents. Highly cis polyNBDF6 (poly-bis(CF 3)-norbomadiene) was also prepared using molybdenum MAP compounds, and the resulting polymer was found to be essentially insoluble in common organic solvents. Solid state CPMAS 13C NMR spectroscopy revealed insoluble polyNBDF6 to be highly tactic, and the tacticity is proposed to be syndiotactic. Cis, tactic polymer was prepared through the addition of 3,3-methylphenylcyclopropene (MPCP) to molybdenum MAP compounds. Attempts towards determination of the tacticity of cispolyMPCP are presented, including the synthesis of three 3,3-disubstituted cyclopropene monomers containing chiral tags. The cis-selective ROMP of cyclooctene and 1,5- cyclooctadiene are reported. The syndioselectivity of the catalysts is proposed to be controlled by the configuration of the 4-coordinate metal center, which alternates with each insertion of monomer. Chapter 4: Racemic 2,3-dicarbomethoxynorbornene (rac-DCMNBE) is polymerized by Mo(NAd)(CHCMe 2Ph)(pyr)(HMTO) (Ad = 1-adamantyl, pyr = pyrrolide, HMTO = hexamethylterphenoxide) to afford an all-cis polymer that is syndiotactic and composed of alternating enantiomers. The cis, syndiotactic ROMP of several other racemic chiral monomers are reported, also affording structures containing a high degree of enantiomer alteration. Attempts towards the alternating copolymerization of two different monomers are reported. The ROMP of enantiomerically pure (+)-dicarbomethoxynorbornene with Mo(NAd)(CHCMe 2Ph)(pyr)(HIPTO) leads to the production of 92% trans-isotactic polyDCMNBE. The structure of trans-isotactic polyDCMNBD is proved through hydrogenation and comparison of its 3C NMR spectrum with that of known cis-isotactic polyDCMNBE. Both cis/syndiotactic/alternating poly-rac-DCMNBE and trans/isotactic poly-(+)-DCMNBE are polymer structures that have not been previously reported. The thermal properties of all new polymers and their hydrogenated counterparts are reported and are found to correlate closely with polymer structure.
by Margaret McGuigan Flook.
Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Chapter 1 details the synthesis of tungsten imidoalkylidene compounds bearing strongly electron-withdrawing imido ligands. An alternative synthesis involving the treatment of WCl6 with 4 equivalents of N-trimethylsilyl-substituted anilines and subsequent workup with 1,2-dimethoxyethane (DME) has been employed to form complexes of the type W(NAr)2C12(dme); syntheses employing WO2C 2(dme) as the tungsten precursor were unsuccessful. Alkylation with neopentylmagnesium chloride (ClMgNp) and subsequent treatment with trifluoromethanesulfonic acid (HOTf) affords imidoalkylidene species W(NAr)(CHCMe 3)(OTf)2(dme) (OTf = trifluoromethanesulfonate); analogous neophylidene ([W]CHCMe 2Ph) species could not be made under these conditions. Treatment of these compounds with two equivalents of LiO(2,6-(CHCPh 2)C6H3)-Et2O affords the bisaryloxide complexes of the type W(NAr)(CHCMe3)(OR)2. Ring-Opening Metathesis Polymerization (ROMP) studies using a series of these bisaryloxides show that rates of ROMP increase as the electron-withdrawing power of the substituents on the imido ligand increase if steric bulk about the metal center is held constant. A similar trend between two bisaryloxides is observed for anti-to-syn alkylidene rotation rates at 50*C in toluene-d8 . Difficulties synthesizing bis-pyrrolide complexes of the type W(NAr)(CHCMe3)(pyr)2 precluded their use as catalyst precursors; some MAP species containing the more sterically encumbering 2,5-dimethylpyrrolide ligand are presented and the metathesis activity of MAP species bearing the 2,5-dimethylpyrrolide ligand is discussed. Chapter 2 introduces Mo and W complexes bearing the current extreme in sterically bulky imido ligands, the NHIPT (HIPT = 2,6-(2,4,6-iPr 3CH2)CH3) ligand, in an effort to generate all anti alkylidene species. A non-traditional synthetic route is employed in order to install this ligand first as an anilide, and after subsequent proton transfer, as an imido ligand to form a mixed imido species of the type M(NHIPT)(N'Bu)(NH'Bu)Cl. Addition of one equivalent of 2,6-lutidinium chloride, followed by alkylation affords dialkyl species M(NHIPT)(N'Bu)Np 2, and treatment with three equivalents of pyridinium chloride yields all anti imidoalkylidene dichloride species as mono-pyridine adducts, M(NHIPT)(CHCMe 3)C 2(py) (M = Mo, W). General reactivity, including strategies for removal of the pyridine adduct as well as substitution and metathesis chemistry, are discussed. ROMP of MPCP (MPCP = 3-methyl-3-phenylcyclopropene) by a Mo-based MAP species bearing the NHIPT ligand yields predominantly cis,syndiotactic poly(MPCP) and in the homo-metathesis of 1 -octene yields ~81% cis-7-tetradecene. The possible source of trans olefinic product is addressed. Chapter 3 presents the synthesis of the first (1-adamantyl)imido species of tungsten. The functional equivalent of common bisimido precursors for other Mo/W alkylidene species, [W(NAd) 2C 2(AdNH2)1 2, is shown to be a dimer stabilized by hydrogen-bonding interactions between adamantylamine protons and adjacent chlorides bound to the second metal of the dimer. Subsequent alkylation with ClMgNp affords the expected dialkyl species, and treatment with three equivalents of 3,5-lutidinium chloride affords imidoalkylidene complex W(NAd)(CHCMe 3)(C) 2(lut)2 (lut = 3,5-dimethylpyridine). The most desirable synthetic route toward monoalkoxide pyrrolide (MAP) species proceeds through a monoaryloxide monochloride intermediate W(NAd)(CHCMe 3)(Cl)(OAr)(lut) (Ar = 2,6-(2,4,6-Me 3)C6H3, 2,6-(2,4,6-'Pr 3)C6H3). Removal of lutidine with B(C6 F5 )3 and subsequent treatment with lithium pyrrolide affords W(NAd)(CHCMe3)(pyr)(OAr) (pyr = pyrrolide); 2,5-dimethylpyrrolide analogues (W(NAd)(CHCMe3)(Me2pyr)(OAr) can be accessed via protonolysis by HOAr from W(NAd)(CHCMe3)(Me2pyr)2(lut).
by Jonathan Clayton Axtell.
Ph. D.

Oxidative processes are linked to a number of major disease states as well as the breakdown of many materials. Of particular importance are reactive oxygen species (ROS), as they are known to be endogenously produced in biological systems as well as exogenously produced through a variety of different means. In hopes of better understanding what controls the behavior of ROS, researchers have studied radical chemistry on a fundamental level. Fundamental knowledge of what contributes to oxidative processes can be extrapolated to more complex biological or macromolecular systems. Fundamental concepts and applied data (i.e. interaction of ROS with polymers, biomolecules, etc.) are critical to understanding the reactivity of ROS. A detailed review of the literature, focusing primarily on the hydroxyl radical (HO•) and hydrogen atom (H•) abstraction reactions, is presented in Chapter 1. Also reviewed herein is the literature concerning high density polyethylene (HDPE) degradation. Exposure to treated water systems is known to greatly reduce the lifetime of HDPE pipe. While there is no consensus on what leads to HDPE breakdown, evidence suggests oxidative processes are at play. The research which follows in Chapter 2 focuses on the reactivity of the hydroxyl radical and how it is controlled by its environment. The HO• has been thought to react instantaneously, approaching the diffusion controlled rate and showing little to no selectivity. Both experimental and calculational evidence suggest that some of the previous assumptions regarding hydroxyl radical reactivity are wrong and that it is decidedly less reactive in an aprotic polar solvent than in aqueous solution. These findings are explained on the basis of a polarized transition state that can be stabilized via the hydrogen bonding afforded by water. Experimental and calculational evidence also suggest that the degree of polarization in the transition state will determine the magnitude of this solvent effect. Chapter 3 discusses the results of HDPE degradation studies. While HDPE is an extremely stable polymer, exposure to chlorinated aqueous conditions severely reduces the lifetime of HDPE pipes. While much research exists detailing the mechanical breakdown and failure of these pipes under said conditions, a gap still exists in defining the species responsible or mechanism for this degradation. Experimental evidence put forth in this dissertation suggests that this is due to an auto-oxidative process initiated by free radicals in the chlorinated aqueous solution and propagated through singlet oxygen from the environment. A mechanism for HDPE degradation is proposed and discussed. Additionally two small molecules, 2,3-dichloro-2-methylbutane and 3-chloro-1,1-di-methylpropanol, have been suggested as HDPE byproducts. While the mechanism of formation for these products is still elusive, evidence concerning their identification and production in HDPE and PE oligomers is discussed. Finally, Chapter 4 deals with concluding remarks of the aforementioned work. Future work needed to enhance and further the results published herein is also addressed.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2004.
Vita.
Includes bibliographical references.
Enantiomerically pure molybdenum imido alkylidene complexes of the type Mo(NAr)(CHCMe₂Ph)[Ar'](THF), which contained 3,3'-diaryl (Ar' = Mes (21), Ph (22)) substituted binaphtholate ligands, were prepared and structurally characterized. Complex 21 was a trigonal bipyramidal anti alkylidene complex; both 21 and 22 were anti-THF adducts. Temperature and concentration dependent enantioselective desymmetrization of prochiral trienes by catalyst 21 afforded five- and six-membered heterocycles in >90% conversion and >90% ee. Consumption of the six-membered substrate followed pseudo first-order kinetics. The new catalyst was compatible with a variety of common functional groups. The longevity of the catalysts depended on the concentration of ethylene in solution. The resting state of the catalytic cycle, unsubstituted molybdacyclobutane 70, slowly decomposed to yield propylene. This observation suggests that β-hydride elimination can compete with bimolecular coupling of methylene complexes under some conditions. Chapter 2: Racemic and enantiomerically pure tungsten imido alkylidene complexes, W(NAr)-(CHCMe₂Ph)(Biphen) (23a, Ar = 2,6-i-Pr₂C₆H₃) and W(NAr')(CHCMe₂Ph)(Biphen) (23b, Ar' = 2,6-Me₂C₆H₃) were prepared and shown to be viable catalysts for representative asymmetric ring- closing metathesis reactions. Both catalysts were compatible with a variety of common functional groups. Intermediate tungstacyclobutane complexes were observed when a stoichiometric amount of desymmetrization substrate 62 was added to 23a. The final stable complex 78, formed between the ring-closed product and a tungsten methylene complex, was structurally characterized. Isotopic labeling experiments with ¹³C₂H₄ allowed the observation of
(cont.) unsubstituted tungstacyclobutane complexes (82), ethylene complexes (84), tungstacyclopentane complexes (86), and a heterochiral methylene dimer (85a). The tungstacyclopentane complexes catalyzed slow dimerization of ethylene to 1-butene. The observation of the methylene dimer provides the first direct evidence of a bimolecular decomposition pathway for methylene complexes. Chapter 3 Racemic and enantiomerically pure molybdenum alkylimido alkylidene complexes, Mo(NAd)(CHCMe₂Ph)(Biphen) (19d, Ad = 1-adamantyl) and Mo(NAd)(CHCMe₂Ph)[Trip]-(THF) (20d) were prepared and structurally characterized. Complex 19d was observed almost exclusively as a syn alkylidene isomer, in contrast with 20d which was observed almost exclusively as an anti-THF adduct. Complexes 19d and 20d are the only reported chiral alkylimido alkylidene complexes for enantioselective olefin metathesis reactions. Complex 19d is the first crystallographically characterized four-coordinate adamantylimido alkylidene complex in its base-free form. It offers unique reactivity and selectivity profiles in tandem AROM/RCM and AROM/CM reactions. Complex 19d is compatible with a variety of common functional groups, including boron-containing reagents. Van't Hoff analyses suggest that the bias toward syn-19d isomer is entropy-driven. Chapter 4: Solvent- and base-free molybdenum methylene complexes, Mo(NAr)(Biphen)(CH₂) (114a, Ar = 2,6-i-Pr₂C₆H₃) and Mo(NAd)(Biphen)(CH₂) (114d, Ad = 1-adamantyl) ...
by W.C. Peter Tsang.
Ph.D.

The stoichiometry of the Nd_1+xBa_2-xCu₃O_7-δ solid solution has been investigated using XRD and EPMA. At 980°C in air, an essentially continuous solid solution forms with limiting compositions x_min = 0.03(1) and x_max = 0.92(2). The solid solution limits are independent of temperature over the range 300 to 1050°C, i.e. stoichiometric Nd123 does not form. Preliminary studies show that annealing in an Ar atmosphere does not affect x_min. Three structurally distinct polymorphs of Nd123ss exist - tetragonal Nd123ss, orthorhombic Nd123ss and orthorhombic Nd123ss. The stability range of each in air has been determined. Quenched samples with 0.03 ≤ x ≤ 0.6 have the tetragonal Nd123ss structure. On oxygenation, samples with 0.03 ≤ x <˜0.2 are orthorhombic. The orthorhombic-tetragonal phase transition is second order, both with increasing temperature and increasing x. Samples with 0.7-0.9 have the orthorhombic Nd213ss structure at all oxygen contents. Tetragonal Nd123ss is isostructural with tetragonal Y123 and orthorhombic Nd123ss is isostructural with orthorhombic Y123. Orthorhombic Nd213ss has the ideal stoichiometry Nd₂BaCu₃O_7-δ and is based on a 2a x b x 2c superstructure of the Nd123ss structure. The supercell is due to ordering of the Nd and Ba atoms, which leads to ordering of the oxygen atoms. Melting temperatures decrease with x. Two distinct regions of melting behaviour are observed; the first for 0.03 ≤ x ≤ 0.6 with a thermal minimum at x˜0.4, and the second for 0.7 ≤ x ≤ 0.9. Oxygen contents increase with x. Samples with larger x values have a smaller range of oxygen contents. High pressure oxygen annealing results in a constant Cu valence state of ˜2.35 for all values of x. Average copper valence states <2 are only obtained readily for x ≤ 0.3. T_c decreases with x and samples become non-superconducting at x˜0.5. For samples annealed in 1 bar O₂, 'double plateau' behaviour is observed.

Thesis (Ph. D.)--University of Maryland, College Park, 2006.
Thesis research directed by: Chemistry. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Les alliages réfractaires à base de nickel et de chrome sont parmi les plus utilisés à haute température en atmosphère oxydante sur le domaine de température de 700 à 1100°C. Ils sont généralement capables de développer des couches à base de chromine dont la vitesse de croissance est suffisamment lente pour limiter cinétiquement la dégradation des pièces. L’addition d’éléments d’alliage peut modifier considérablement leur comportement en oxydation et/ou leurs propriétés mécaniques. Si des relations entre la composition, la microstructure et la cinétique d’oxydation sont aujourd’hui connues, l’évaluation du comportement en oxydation des nouvelles nuances industrielles demeure systématique car le rôle des différentes additions et leur synergie éventuelle sont mal compris. Aussi, cette thèse vise à affiner la compréhension des relations existant entre la composition chimique et la réactivité chimique des matériaux métalliques en milieu oxydant dans le but de contribuer au design d’alliages aux propriétés en oxydation optimisées. Pour cela, ce travail repose sur l’étude du comportement d’alliages modèles, simplifiés par rapport à ceux proposés aujourd’hui par l’industrie. Ils sont traités en atmosphères oxydantes, puis caractérisés à l’aide des techniques actuellement disponibles (EBSD, SEM-FIB, MET-HR) de façon à évaluer les mécanismes d’oxydation et le rôle des éléments manganèse et silicium présents dans ces alliages en quantités mineures (~ 1% massique). Les premiers stades de l’oxydation des alliages modèles au manganèse ont été suivi in-situ en MEB environnemental (Collaboration ICSN de Marcoule) et ont permis d’observer que le spinelle Mn1+xCr2-xO4 est l’oxyde se formant le plus tôt à 1050°C quelle que soit la teneur en Mn dans l’alliage. Ces ajouts à un alliage de référence Ni-25Cr (% massique) conduisent d’une part à une augmentation de la vitesse d’oxydation et d’autre part à une diminution de la vitesse de volatilisation de l’oxyde de chrome suite à la formation d’une couche continue de spinelle à l’interface oxyde/gaz. A l’opposé, l’ajout de silicium diminue d’un facteur cinq la constante parabolique d’oxydation, suite à la formation de silice aux joints de grains de la couche de chromine. L’ajout à la fois de manganèse et de silicium conduit à des vitesses d’oxydation, en condition isotherme, similaires à celle de l’alliage de référence. Par contre, la durée de vie de tels alliages est considérablement augmentée en condition cyclique d’utilisation. La connaissance du sens de croissance des couches d’oxyde et de leur type de semi-conductivité, par le biais de mesures photo-électrochimiques réalisées en collaboration avec le laboratoire SIMaP de Grenoble, a permis d’identifier les défauts ponctuels majoritaires au sein des couches d’oxyde étudiées. Les mécanismes diffusionnels à l’œuvre lors de la croissance des oxydes ont ainsi pu être caractérisés
Nickel-based refractory alloys are mainly used in oxidizing atmosphere at high temperature. These alloys are generally capable to form chromia oxide scales at temperature between 700 and 1100°C. A few relationships between composition, microstructure and kinetic are known, but the systematic study of the oxidation behaviour of new industrial alloys is required because the impact of minor elements and their possible synergy are not fully understood. Thus, this PhD thesis aims to refine the understanding of the relationship between the chemical composition and the chemical reactivity of metallic materials in an oxidizing atmosphere in order to contribute to the design of alloys with optimized oxidation properties. To reach this purpose, the oxidation behaviour of model alloys, simplified compared to those proposed today by industry, was studied. The techniques available nowadays (EBSD, SEM-FIB, MET-HR) were used in order to determine the oxidation mechanism and the effect of manganese and silicon additions on these mechanisms. The characterization of the first oxidation step was performed in-situ in an environmental SEM (Collaboration with the ICSN in Marcoule) and allowed the observation of the former oxide, the Mn1+xCr2-xO4 spinel regardless the alloy Mn content. Manganese addition led to an increase of the oxidation rate in comparison to the one of a Ni25Cr (% wt.) and to a decrease of the chromia oxidation rate thanks to the presence of the spinel Mn1+xCr2-xO4 at the oxide/gaz interface. In opposite, the silicon addition decreased the parabolic constant by a factor 5. This dramatic effect was attributed to the presence of silica at chromia grain boundaries which decreased the chromium diffusion rate across the oxide scale. The addition of both manganese and silicon led to the same oxidation rate than the reference alloy in isothermal condition whereas, the lifetime in cyclic oxidation was considerably increased. The knowledge of the growth direction coupled to that of the semiconductivity type, determined by photoelectrochemical measurements at SiMaP laboratory in Grenoble, allowed the identification of the major point defects in formed oxide scales. Based on the collected results, the involved solid state diffusion mechanisms responsible of the oxide layer growth were proposed

Le compose pb#2sr#2ln#1##xca#xcu#3o#8#+# est un cuprate supraconduceur a haute tc. L'oxyde pb#2sr#2ycu#3o#8 devient supraconducteur lorsqu'on substitue une partie de l'y par du ca. Nous avons mis au point de nouvelles methodes de synthese de poudres et de monocristaux. Le mecanisme de l'oxydation a ete etudie par thermogravimetrie et analyse structurale des phases oxydees. L'oxygene est intercale sur deux sites situes dans la couche cu, et provoque l'oxydation du cu#1#+ en cu#2#+ et d'une partie du pb#2#+ en pb#4#+. La formation de pb#4#+ desequilibre les charges dans la structure, et l'oxygene apical 01 s'eloigne des plans cuo#2. Plusieurs monocristaux (0