Interfacial polarization effects on molecular catalysis [electronic resource]
Keywords: Chemistry.

The local environment at highly polarized solid--liquid interfaces provides a unique medium for chemical reactions that could be exploited to control selectivity of non-Faradaic reactions. One such example is the charged electrode--electrolyte interface that arises from the application of an external voltage to a pair of electrodes immersed in an electrolyte solution. Catalysts covalently attached to the electrode surface can then be exposed to a chemical environment that is very different from the bulk solution. Using a reaction vessel that consists of consists of two planar electrodes separated by a thin inert gasket and functionalized on one side with catalyst molecules, it was found that the polarized interface between an electrode and an organic solvent could change the selectivity of an NHC--Au-catalyzed aryl alkynyl sulfoxide rearrangement. Reactions taking place in more highly polarized environments enjoyed the largest changes in catalyst selectivity. Even greater changes in selectivity were accessible by using a different NHC--Au catalyst that placed the catalytic Au center closer to the electrode surface; conversely, the change in selectivity was greatly attenuated if the Au center was located further away from the surface. Doubly functionalizing an electrode with NHC--Au catalyst molecules followed by an aliphatic alkylphosphonate or perfluoroalkylsilane also led to increased catalyst response to interfacial polarization, which likely stemmed from a reduction in charge screening by oppositely charged surface adsorbates. While polarized interfaces are commonly prepared by applying a voltage to an electrode immersed in an electrolyte solution, it is nevertheless challenging to achieve high surface charge densities while suppressing undesired Faradaic processes. Ferroelectric materials have permanent high surface charge densities that arise from the dipole moments of ferroelectric domains and can be used to create polarized solid--liquid interfaces without application of a voltage. The effects of ferroelectric BaTiO3 materials on the selectivity of catalytic reactions were compared to the effects of the voltage-polarized electrode--electrolyte interface described earlier. The reactions investigated were a Rh porphyrin--catalyzed intramolecular carbene rearrangement in addition to the NHC--Au-catalyzed rearrangement. In both cases, the addition of ferroelectric BaTiO3 nanoparticles to the reaction solutions changed the product ratios in the same direction and by a similar magnitude as performing the reactions at a polarized electrode--electrolyte interface. Similar behavior was observed with nanoparticles of PbTiO3 and LiNbO3, two other ferroelectric materials. In contrast, non-ferroelectric TiO2, CaTiO3, or SrTiO3 nanoparticles had little or no effect on the product ratios. The results demonstrate that colloidal suspensions of BaTiO3 nanoparticles effectively act as a dispersible polarized interface that can influence the selectivity of catalytic reactions in a similar manner to an electrically polarized electrode--electrolyte interface. The insights presented in this thesis will enable chemists to efficiently exploit interfacial polarization as a complementary method of controlling catalytic reactivity.