Local electrostatic effects on the selectivity of catalytic reactions [electronic resource]
Keywords: Chemistry.

Controlling the selectivity of catalytic reactions remains a major challenge for improving the efficiency, cost and utility of many chemical transformations. The development of a selective catalyst for a chemical reaction requires control over the relative energies of competing activation barriers. A complementary strategy to changing catalyst or substrate structure is to modify the local chemical environment in which the reaction takes place. Previously, there has been very little effort to control the local electrostatic environment in the vicinity of an active catalyst. A strong local electric field could in principle change the outcome of a reaction through interactions with the catalyst, substrates, or solvent molecules surrounding them. This work examines two different methods for controlling the electrostatic environment of a catalyst: localizing a reaction to an electrode--electrolyte or liquid--liquid interface. A unique electrostatic environment is present in the double layer region at the interface between a polarized electrode and an electrolyte solution, arising from the combination of surface charge density on the polarized electrode and accumulation of oppositely charged ions from the electrolyte solution. To study reactions in this electrostatic environment, Si electrodes coated with thin films of insulating dielectric layers were used as the opposing walls of a reaction vessel. The charge density was varied at the interface by changing the voltage across the two electrodes. The reaction vessel was first used to study interfacial electrostatic effects on a stilbene oxide rearrangement directly catalyzed by an insulating oxide. The product ratio was changed by up to a factor of ~160 as the charge density at the metal oxide--electrolyte interface was increased by applying a voltage. Catalysts localized to the interface also demonstrated charge-density dependent selectivity. Rh-catalyzed carbene reactions changed up to ~200-fold in response to increasing charge density in a direction that depended on the surface chemistry of the dielectric layer. With TiO2 as the oxide, increasing the interfacial charge density induced a TiO2--Rh interaction that changed the selectivity of the catalyst to favor cyclopropanation. With an Al2O3 surface, increasing the interfacial charge density by applying a voltage resulted in a change in selectivity to favor the insertion product, the more polar transition state, with no specific Al2O3--Rh interaction. The interface between two immiscible electrolyte solutions also provides local solvation and electrostatic environments that are unattainable in a homogeneous phase. To assess whether these environments can affect the selectivity of catalytic reactions, metalloporphyrin--catalyzed reactions at the interface between immiscible solutions were compared to reactions in homogeneous solutions. Our strategy for performing reactions at an interface is to confine the substrate to one phase and the catalyst to the other, in an ideal case forcing the reaction to occur at the interface. Up to ~100 fold changes to selectivity were observed if an interfacial environment was used, with the magnitude dependent on the composition of the electrolyte solution. Additional changes in selectivity were observed if the interface was further polarized by the application of a potential. Overall, these results indicate localizing a catalytic reaction to a unique electrostatic environment influences selectivity.