Electrochemical Transformation of Alkanes, Carbon Dioxide and Protons at Iron-Porphyrins and Iron-Sulfur Clusters

Abstract The work contained in this thesis focuses on (i) chemical and electrochemical alkane oxidation using Fe-porphyrin complexes as catalysts (ii) electrochemical and photoelectrochemical CO2 reduction using Fe-porphyrin complexes (iii) electrochemical and photoelectrochemical generation of hydrogen using iron-sulfur cluster. Chapter 1 gives a general overview of the electrochemical techniques which underpin the work presenedt in this thesis. Chapter 2 reports the chemical and electrocatalytic oxidation of hydrocarbons to alcohols and epoxides by using iron (III) porphyrins as catalysts. A series of new basket-handle thiolate Fe (III) porphyrins have been used to mediate anodic oxidation of hydrocarbons, specifically adamantane hydroxylation and cyclooctene epoxidation. The electrocatalytic and chemical catalytic activity oxidation of the thiolate porphyrins are benchmarked against Fe (III) tetraphenyl porphyrin chloride and its tetrapentafluorophenyl analogue. Chapter 3 describes the electrochemical and photoelectrochemical reduction of carbon dioxide to carbon monoxide. This chapter shows that iron (III) porphyrin complexes are capable of carrying out electrocatalytic reduction of carbon dioxide at both vitreous carbon and illuminated p-type silicon surfaces, with reasonable current efficiencies. At illuminated p-type silicon photovoltages of ca 500mV are obtained. 7 Chapter 4 describes the electrochemical and photoelectrochemical reduction of proton to H2 using [Fe4S4 (SPh)4]2- as an electrocatalyst at both vitreous carbon and at illuminated p-type Si electrodes

Biomimetic Peroxo- and Oxo-manganese Complexes: Insights into Structure and Reactivity through Kinetic, Spectroscopic, and Computational Studies

Manganese centers that react with O2 and its reduced derivatives mediate a diverse array of biologically important reactions including the detoxification of superoxide, the conversion of nucleotides to deoxynucleotides, and the generation of O2 from H2O. Peroxo-, oxo-, and hydroxo-manganese motifs are frequently invoked in the catalytic cycles of Mn enzymes. To that end, biomimetic complexes featuring peroxo-, oxo-, and hydroxo-manganese adducts were synthesized and studied using spectroscopic techniques, including variable-temperature electronic absorption, electron paramagnetic resonance (EPR), X-ray absorption (XAS), and magnetic circular dichroism (MCD) spectroscopies along with computational methods, such as density functional theory (DFT) and time-dependent DFT. The structural and spectroscopic properties of these species were investigated in order to better understand how the geometric and electronic structure of these complexes affects reactivity

Dual role for alkali metal cations in enhancing the low-temperature radical polymerization of N,N-dimethylacrylamide

The radical polymerization of N,N-dimethylacrylamide (DMAAm) has been investigated in the presence of several alkali metal salts, including lithium bis(trifluoromethanesulfonyl)imide (LiNTf2). The addition of an alkali metal salt led to a significant increase in the yield and molecular weight of the resulting polymer. NMR analysis of mixtures of DMAAm and LiNTf2 suggested that DMAAm was being activated by the coordination of Li+ to its C=O group. Electron spin resonance analysis of the DMAAm polymerization in the presence of LiNTf2 suggested that the propagating radical was being stabilized by Li+ through a single-electron lithium bond, because a signal for the propagating radical of the acrylamide derivatives was observed for the first time in solution when LiNTf2 was added. Based on these results, we have proposed a mechanism for this polymerization, where the propagation steps occur between a lithium ion-stabilized propagating radical and a lithium ion-activated incoming monomer. Furthermore, polymers with a wide range of stereoregularities, such as isotactic, syndiotactic and heterotactic systems, were successfully prepared using this method by carefully selecting the appropriate combination of solvent and alkali metal salt

Sunlight-Initiated Photochemistry: Excited Vibrational States of Atmospheric Chromophores

Atmospheric chemical reactions are often initiated by ultraviolet (UV) solar radiation since absorption in that wavelength range coincides to typical chemical bond energies. In this review, we present an alternative process by which chemical reactions occur with the excitation of vibrational levels in the ground electronic state by red solar photons. We focus on the O–H vibrational manifold which can be an atmospheric chromophore for driving vibrationally mediated overtone-induced chemical reactions. Experimental and theoretical O–H intensities of several carboxylic acids, alcohols, and peroxides are presented. The importance of combination bands in spectra at chemically relevant energies is examined in the context of atmospheric photochemistry. Candidate systems for overtone-initiated chemistry are provided, and their lowest energy barrier for reaction and the minimum quanta of O–H stretch required for reaction are calculated. We conclude with a discussion of the major pathways available for overtone-induced reactions in the atmosphere

Fourier-transformed infrared absorption spectroscopy and kinetics studies of Gas phase small molecules

Ph.DDOCTOR OF PHILOSOPH

Thermochemistry and kinetics: hydrogen atom addition reactions with alkenes, oxidation of cyclopentadienone, trifluoroethene and transport properties

Thermochemical and transport properties and reaction kinetic parameters are important to understand and to model atmospheric chemistry, combustion and other thermal systems. These processes are all important to the environment. Thermochemical properties kinetic parameters and models for several atmospheric and combustion related chemical systems are determined using computational chemistry coupled with fundamentals of thermodynamics and statistical mechanics. Transport properties of hydrocarbon and oxygenated hydrocarbons which are important to the calculation of fluid dynamics of gas phase flow reactions and mixing needed for evaluation in the combustion and thermal (flow) fluid dynamic modeling. Transport properties of radicals cannot be measured so computational chemistry is method of choice. The dissertation determine dipole moment, polarizability and molecular diameters of hydrocarbon and oxygenated hydrocarbons needed for calculation of multicomponent viscosities, thermal conductivities, and thermal diffusion coefficients. Cyclopentadienone with cyclic five-member ring aromatic structure is an important intermediate in combustion systems. Thermochemical and kinetic parameters for the initial reactions of cyclopentadienone radicals with O2 and the thermochemical properties for cyclopentadienone - hydroperoxides, alcohols, alkenyl , alkoxy and peroxy radicals are determined by use of computational chemistry via Density Functional Theory (DFT) and the composite, Complete Basis Set (CBS) methods. Enthalpies of formation (ΔfH°298) with the isodesmic reaction schemes with several work reactions for each species are used for standard enthalpies. Entropy and heat capacity, S° (T) and CP° (T) (50 K ≤ T ≤ 5000 K) are also determined. Chemical activation kinetics using quantum RRK analysis for k(E) and master equation for fall-off are used for kinetic parameters and to show significance of chain branching as a function of temperature and pressure. The cyclopentadienone vinylic carbon radicals of with molecular oxygen appear to undergo chain branching via reaction with O2, to a higher extent to that of vinyl and phenyl radicals. Reaction kinetics of hydrogen atom addition to primary (P), secondary (S), tertiary (T) vinylic (olefin) carbons to form an alkyl radical is investigated using Density Functional Theory (DFT) and ab initio composite level methods. Results from calculations at different levels are compared with the experimental literature data for hydrogen atom addition to Ethylene, Propene, 1-Butene, E-2-Butene, Z-2-Butene, and Isobutene. Activation energy and rate constants for forward and reverse paths are investigated and compared with available experimental data. One goal of the study is to determine accurate calculation method for use on large molecules. Chlorofluorocarbons are widely present in the environment. Thermochemical and kinetic properties work will aid in the understanding the chlorofluorocarbons reactions in combustion and atmospheric environments. Trifluoroethene (CF2=CHF) reaction in atmospheric and combustion environment initiated via OH radical system is investigated. The HF generated channel is currently not reported in any kinetic study. It is important as the toxic gas that can cause severe respiratory damage in humans

Reactivity Selectivity Relationships in Reactions of Carbocations with Nucleophiles

Benzhydrylium ions, which have previously been characterized as reference electrophiles in the Mayr group, have been employed for the construction of general nucleophilicity and electrophilicity scales. The kinetics of the reactions of benzhydrylium ions with 15 n-nucleophiles in water and DMSO were measured to yield the N- and s-parameters. Most of the nucleophiles have closely similar slope parameters indicating that the reactions of most n-nucleophiles approximately follow Ritchie's constant selectivity relationship. The different slope parameter for water is recognized as the main reason for deviations from the Ritchie relationship reported in 1986. The rates of the reactions of benzhydrylium ions with solvent mixtures of variable composition have been determined. From plots of the first-order rate constants (log k) versus E of benzydrylium ions, the solvent nucleophilicity parameters are derived. This allow the systematic design of Friedel-Crafts reactions with solvolytically generated carbocations. Rate constants for the reactions of laser flash photolytically generated benzhydrylium ions with chloride and bromide anions have been determined in various solvents and compared with literature data. The rate constants for the ionization of benzhydryl halides and for the reactions of benzhydrylium ions with halide anions and with solvent are combined to give complete free energy profiles for solvolysis reactions. The first SN1 reaction, where ionization and trapping of the carbocation by the solvent could directly be observed, is reported

Complexes NCN de Ni(II) et Ni(III) : synthèse, caractérisation et rôle dans le mécanisme de couplage C-O, C-N et C-halogènes

La présente thèse décrit la synthèse, caractérisation et réactivité des complexes pinceurs de type NCN du Ni(II) et Ni(III). Elle comporte trois parties. La première traite de la synthèse d’une nouvelle famille de complexes de NiII basé sur le ligand 1,3-bis(pyrazole),5-R-C6H3 (R= H, OMe). La synthèse du ligand est effectuée par couplage d’Ullman entre le pyrazole et le 1,3-diiodobenzène ou le 1,3-dibromo,5-méthoxybenzène. La réaction à reflux des ligands avec le précurseur de nickel {[NiBr2(iPrCN)]}n en présence de triéthylamine mène à la formation des complexes pinces (NCNpz)Ni(II)Br et (MeO-NCNpz)Ni(II)Br par activation du lien C-H (pz = pyrazole). Le complexe de NiII de type alcoolate (NCNpz)Ni(BHT) (BHT = 2,6-t-Bu2-4-Me-OC6H2) est isolé à partir de la réaction du complexe (NCNpz)Ni(II)Br avec NaBHT. Les tentatives d’oxydation des complexes isolés n’ont pu mener à la caractérisation d’authentiques complexes de Ni(III). La réaction des complexes bromo avec H2O2 mène à la formation de Br-NC(OH)Npz et NC(OH)Npz à partir du complexe (NCNpz)Ni(II)Br et MeO-NC(OH)Npz à partir du complexe (MeO-NCNpz)Ni(II)Br. La réaction aérobique du complexe (NCNpz)Ni(II)Br avec différents alcools et amines mène aux produits de fonctionnalisation du ligand NC(OR)Npz et NC(NR2)Npz. Le deuxième thème montre la fonctionnalisation du lien Cispo-Ni(III) du complexe (NCN)Ni(III)Br2. La réaction de (NCN)Ni(III)Br2 avec H2O, divers alcools, amines et acides forts mène à la formation de lien C-O, C-N et C-halogènes. Une étude cinétique montre que la réaction est d’ordre 1 en NiIII excluant la possibilité d’une réaction de disproportionation entre 2 Ni(III) menant à un complexe de Ni(IV). La mesure d’un effet cinétique isotopique inverse de 0.47 de la réaction avec MeOH/CD3OD indique un transfert de proton provenant d’un pré-équilibre ayant lieu avant l’étape déterminante. L’observation de rendements inférieurs à 50% pour la fonctionnalisation s’explique par une réaction de comproportionation entre le Ni(I) issu de l’élimination réductrice avec le Ni(III) de départ. Ces observations permettent la proposition d’un mécanisme pour la formation de lien C-O, C-N et C-halogènes à partir du complexe (NCN)Ni(III)Br2. La troisième partie traite de la synthèse de nouveaux complexes de Ni(III) cationiques et dicationique à partir du complexe (NCN)Ni(III)Br2. En présence d’AgSbF6 et d’acétonitrile, le complexe (NCN)Ni(III)Br2 réagit pour former le complexe dicationique [(NCN)NiIII(MeCN)3]2+ avec un rendement de 83%. La réaction entre (NCN)Ni(III)Br2 et [(NCN)Ni(III)(MeCN)3]2+ mène à la formation du complexe cationique [(NCN)Ni(III)(Br)(MeCN)]+ avec un rendement de 84%. Ces complexes sont caractérisés par diffraction des rayons-X et résonance paramagnétique de l’électron. La réaction de ces complexes avec MeOH mènent à la formation du produit fonctionnalisé NC(OMe)N et d’un complexe de nickel divalent cationique suggérant une comproportionation entre [(NCN)Ni(III)(Br)(MeCN)]+ ou [(NCN)Ni(III)(MeCN)3]2+ avec le Ni(I) généré par la formation du lien C-O. La réaction avec MeNH2 mène à la formation du produit fonctionnalisé NC(NHMe)N avec des rendements typiques de 30%. Ces rendements s’expliquent par l’oxydation de la méthylamine par les Ni(III) cationiques et dicationiques. Une étude DFT exhaustive met en lumière le mécanisme de réaction pour la formation de lien C-O et C-N. La première étape est la coordination du substrat au centre métallique trivalent. Ensuite la déprotonnation du MeOH est assurée par un des bras azotés de la pince alors que celle de MeNH2 provient de la sphère externe. L’élimination réductrice est l’étape limitante pour la formation de lien C-O et C-N. La valeur de l’effet cinétique isotopique de 0.62 est aussi rationalisée par des calculs vibrationnels issus de la DFT.This thesis describes the synthesis, characterization and reactivity of NCN type Ni(II) and Ni(III) pincer complexes. The thesis is divided in three parts. The first discusses the synthesis of a new family of Ni(II) complexes based on the 1,3-bis(pyrazole),5-R-C6H3 ligand (R = H, OMe). Ligand synthesis is executed by Ullman coupling between pyrazole and 1,3-diiodobenzene or 1,3-dibromo,5-methoxy-benzene. Refluxing ligands with nickel precursor {[NiBr2(iPrCN)]}n in the presence of triethylamine leads to the formation of pincer complexes (NCNpz)Ni(II)Br and (MeO-NCNpz)Ni(II)Br by C-H activation (pz = pyrazole). The alkoxide-type Ni(II) complex (NCNpz)Ni(BHT) (BHT = 2,6-t-Bu2-4-Me-OC6H2) is isolated from the reaction between (NCNpz)Ni(II)Br and NaBHT. Oxidation attempts did not lead to the characterization of an authentic Ni(III) species. Reacting bromo complexes with H2O2 leads to the formation of Br-NC(OH)Npz and NC(OH)Npz when starting from (NCNpz)Ni(II)Br and MeO-NC(OH)Npz when starting from (MeO-NCNpz)Ni(II)Br. The aerobic reaction of (NCNpz)Ni(II)Br with different alcohols and amines leads to the functionalization products of the ligand NC(OR)Npz and NC(NR2)Npz. The second theme is the functionalization of the Cispo-Ni(III) bond in (NCN)Ni(III)Br2. The reaction of (NCN)Ni(III)Br2 with H2O, various alcohols, amines and strong acids leads to the formation of C-O, C-N and C-halogen bonds. A kinetic study shows that Ni(III) displays first order behavior excluding the possibility of a disproportionation reaction between 2 Ni(III) which would lead to a Ni(IV) species. Measurement of an inverse kinetic isotopic effect of 0.47 for the reaction with MeOH/CD3OD indicates a proton transfer arising from one or several pre-equilibria occuring before the rate determining step. The observation of yields of less than 50% for the functionalization reaction indicates a comproportionation reaction between the Ni(I) resulting from the reductive elimination and the starting Ni(III). These observations allow proposition of a mechanism for C-O, C-N and C-halogen bond formation from the (NCN)Ni(III)Br2 complex. The third part reports the synthesis of new cationic and dicationic NiI(III)complexes starting from (NCN)Ni(III)Br2. In the presence of AgSbF6 and acetonitrile, (NCN)Ni(III)Br2 reacts to give dicationic complex [(NCN)Ni(III)(MeCN)3]2+ in 83% yield. The reaction between (NCN)Ni(III)Br2 and [(NCN)Ni(III)(MeCN)3]2+ leads to the formation of the cationic [(NCN)Ni(III)(Br)(MeCN)]+ with a yield of 84%. These complexes are characterized by X-ray diffraction and electron paramagnetic resonance. Reacting these complexes with MeOH leads to the formation of the functionalized product NC(OMe)N and a cationic divalent nickel complex suggesting a comproportionation between [(NCN)Ni(III)(Br)(MeCN)]+ or [(NCN )Ni(III)(MeCN)3]2+ with the NiI generated by the formation of the C-O bond. Reacting [(NCN)Ni(III)(Br)(MeCN)]+ or [(NCN)Ni(III)(MeCN)3]2+ with MeNH2 leads to the formation of the functionalized product NC(NHMe)N with typical yields of 30%. These low yields are explained by the ability of the ionized Ni(III) complexes to oxidize MeNH2. A comprehensive DFT study highlights the reaction mechanism steps for C-O and C-N bond formation. The first step is the coordination of the substrate with the trivalent metal center. Then deprotonation of MeOH is provided by the pincer’s amine moiety while deprotonation of MeNH2 arises from an outer sphere process. Reductive elimination was found to be the limiting step for C-O and C-N bond formation. The value of the isotopic kinetic effect of 0.62 is also rationalized by a DFT vibrational analysis