4,4' Functionalized 2,2'-Bipyridines

This thesis presents two applications of 2,2'-bipyridine (bipy) derivatives, the first involves their use as stabilizers for nanoparticle (NP) catalysts and the second is concerned with the use of bipy ligands for ruthenium dye molecules applied in dye sensitized solar cells (DSCs). Chapters 2 and 3 describe the work concerning the ability of bipy derivatives to stabilize catalytically active Rh NPs for ionic liquid based, biphasic arene hydrogenation, with cationic bipy derivatives being first evaluated with a series of monocyclic aromatic hydrocarbons. The recyclability of the cationic bipy stabilized Rh NP systems were evaluated in catalysis and analyzed further by X-ray spectroscopy. The stereoelectronics were examined with respect to catalytic activity using various neutral bipy ligands. The differences in catalytic activity were rationalized with steric and electronic arguments throughout. Chapter 4 concerns the application of a cationic bipy derivative as a modifier/stabilizer in the Pd NP catalyzed partial hydrogenation of hexyne to 1-hexene. Factors for the high selectivity have been studied, including the presence of a ligand and/or an IL. The Pd NPs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Leaching was investigated by atomic absorption spectroscopy and the kinetcis of the reaction analyzed using a Langmuir-Hinshelwood model. Chapter 5 describes three heteroleptic ruthenium complexes incorporating new ancillary bipy based ligands synthesized by sequential connection of different alkyl functionalities using a triazole linker, prepared via click chemistry. These complexes were effective dyes when applied in DSCs with both a volatile and non-volatile IL electrolyte. The complexes were characterized and compared using their photovoltaic conversion efficiencies as well as their photophysical and electrochemical properties. A stability study was conducted at 60 °C for 1000 h to determine the most stable substituent.

Novel Ionic Liquids For Electrochemical Applications

Pui Shan Genevieve Lau

This thesis is organized into three main sections, and is concerned with the design and synthesis of novel ionic liquids (ILs), and their associated electrochemical applications. In the first section, a general introduction to the field of ionic liquids is presented, followed by the synthesis and characterization details of all novel compounds used in the study. The second section summarizes work done on the development of more stable dye-sensitized solar cells (DSCs). An introduction to the field of photovoltaics and DSCs is first delivered in Chapter 3, which is followed by two chapters of original work on ionic liqiud-based electrolytes for DSC applications. In Chapter 4, it is shown that novel triazolium-based ionic liquids can be used to fabricate dye-sensitized solar cells, giving power conversion efficiencies of up to 6.00 %. The ionic liquid-based electrolytes are found to be compatible with a range of organic and inorganic photosensitizers, although the ruthenium-based dye (coded C106) exhibits the best performance. A device employing one of these newly-developed electrolytes is also shown to be very stable, retaining ca. 90% of its initial performance even after 1000 hours of continuous full sun illumination at 60 âŠC. In Chapter 5, we focus on the development of stable porphyrin-sensitized solar cells, using IL-based electrolytes. Our studies reveal that the long-term stability of porphyrin-based DSCs are highly dependent on additives in the electrolyte. In particular, it is found that the additive, guanidinium thiocyanate, is essential for the preparation of long-lived devices. It is also shown that the beneficial effect of this additive originates from the thiocyanate anion, rather than the guanidinium cation. The third section of this thesis deals with the ionic liquid-catalyzed electroï¿Œchemical reduction of carbon dioxide. Following a general introduction on carbon dioxide mitigation using ionic liquids (Chapter 6), the results of our work in this area are presented in Chapter 7. Our studies reveal that not all ionic liquids are stable at the potentials required for CO2 reduction. However, the imidazolium-based salts generally perform quite well. The catalytic effect of the IL is found to mainly stem from the cation, with the anion playing a somewhat secondary role. Our results also demonstrate that substitutions on the imidazolium ring can have a huge impact on catalysis. Product analysis shows that the best-performing catalyst produces carbon monoxide as the dominant product, with Faradaic efficiencies of between 70 to 80 %.

EPFL2015

Catalytic hydrogenation of alkenes under biphasic conditions

Corinne Daguenet

Ruthenium(II)-arene diphosphine complexes are shown to be effective precursors for the catalytic hydrogenation of the C=C bond in an aqueous biphase using molecular hydrogen as the hydrogen source. A mechanistic study performed in water, utilizing high gas pressure NMR spectroscopy and electrospray ionization mass spectrometry, and experiments involving the variation of pH, enables indirect identification of the catalytically active species resulting from the substitution of the chloride with the dihydrogen ligand. Similar catalytic experiments are carried out in ionic liquids (ILs), such as [bmim][BF4] and [bmim][TfO] using an analogous precursor, but no conversion is observed and the ruthenium(II) species does not undergo any chemical change during the reaction. The dissociation of chloride, leading to the catalyst, is inhibited in these ILs due to a lower solvation of chloride. An estimate of the bmim-chloride interaction, which is lower than the water-chloride interaction, is determined by 1H NMR spectroscopy. To ensure the formation of the active species in such ILs, the coordinated chloride is abstracted beforehand and replaced by acetonitrile, which restores the catalytic activity. An alternative approach to overcome the chloride dissociation problem consists of improving the solvation of the chloride in the IL medium. To accomplish this, water is added as a co-solvent in the IL phase and, under these conditions, the activity is restored. In order to assess the reactivity of chloride ions in ILs, a relative scale for the solvation of chloride is given for a series of [cation][Tf2N] ILs. This scale, based on the reaction enthalpy for the dissociation of a chloride-templated metallacage, yields the following order of chloride solvation: [C5e3am][Tf2N] < [bmpy][Tf2N] < [bmim][Tf2N] ≤ [bdmim][Tf2N] < [bpy] [Tf2N] ≪ [mimeOH][Tf2N]. In Tf2N-based ILs, the catalytic activity of hydrogenation reactions using the same Ru(II) precursor in the presence of chloride is totally modified. In a series of [cation][Tf2N] ILs, the rate of formation of the catalyst, which is promoted by chlorides, is shown to depend on the solvation of chloride.

EPFL2006

Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation

Gabor Laurenczy

Reaction pathways during CO2 hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)2H2] (dmpe=Me2PCH2CH2PMe2) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO2 and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)2(OCHO)2] and trans-[Ru(dmpe)2H(OCHO)], were formed at low CO2 pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuH⋅⋅⋅ HOCHO dihydrogen-bonded complex of the trans form was identified at H2 partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H2 insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H2 insertion is attributed to the elongated Ru−O bond. The H2 insertion and the subsequent formation of formic acid proceed via Ru(η2-H2)-like complexes, in which apparently formate ion and Ru+ or Ru(η2-H2)+ interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuH⋅⋅⋅HOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.