Proton Transfer Controlled Reactions at Liquid-Liquid Interfaces

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Pekka Peljo Name of the doctoral dissertation Proton Transfer Controlled Reactions at Liquid-Liquid Interfaces Publisher School of Chemical Technology Unit Department of Chemistry Series Aalto University publication series DOCTORAL DISSERTATIONS 39/2013 Field of research Physical Chemistry Manuscript submitted 10 December 2012 Date of the defence 5 April 2013 Permission to publish granted (date) 12 February 2013 Language English Monograph Article dissertation (summary + original articles) Abstract Electrochemistry at liquid-liquid interfaces has been a versatile area of contemporary electrochemistry for almost 40 years, with research mainly focusing on the aqueous-organic solution interface. The Galvani potential difference across such an interface, controlling the distribution of ions between the phases, can be generated either chemically or with an external voltage source. An aqueous-organic solvent interface shares similarities with the cell membrane, where several important biological reactions such as photosynthesis or cell respiration take place. Hence electrochemistry at liquid-liquid interfaces is an excellent means to study biomimetic oxygen reduction. In this reaction, proton transfer across the interface controlled by the Galvani potential difference is coupled with electron transfer. Therefore oxygen reduction in the oil phase mediated by an electron donor occurs only at potentials where proton transfer becomes feasible. In this thesis oxygen and hydrogen peroxide reduction by 1,2-diferrocenylethane was investigated. The results show that the reduction of hydrogen peroxide is faster than oxygen reduction, but both reactions were slow due to the low thermodynamic driving force. The reaction rate can be increased by the addition of molecular catalysts, and electrochemistry at liquid-liquid interfaces provides an excellent means to compare the activity and selectivity of the catalysts. Cofacial biscobalt porphyrins, biomimetic analogues of the active centre of cytochrome c oxidase responsible of oxygen reduction in nature, were investigated. Surprisingly, significant amounts of hydrogen peroxide were produced, contradicting previous results. The reaction was shown to proceed to hydrogen peroxide when oxygen was bound to the exo side (dock-on) of the catalyst, while four-electron reduction took place with oxygen bound to the endo side (dock-in) of the molecule. A proof of concept of a novel type of a fuel cell utilising the liquid-liquid interface is presented. In this fuel cell hydrogen is oxidized on the anode as in a conventional fuel cell, but oxygen reduction takes place at the liquid-liquid interface. The redox mediator in the oil phase is regenerated at the cathode, completing the electric circuit. Proton transfer across the interface was also utilised for performing acid catalysed SN1 substitutions on ferrocene methanol. This is a novel method to perform synthesis of organic chemicals, as the presence of protons is controlled by the applied Galvani potential difference. In situ biphasic electrospray ionization mass spectroscopy was demonstrated to be a very efficient way to follow this kind of reactions.Electrochemistry at liquid-liquid interfaces has been a versatile area of contemporary electrochemistry for almost 40 years, with research mainly focusing on the aqueous-organic solution interface. The Galvani potential difference across such an interface, controlling the distribution of ions between the phases, can be generated either chemically or with an external voltage source. An aqueous-organic solvent interface shares similarities with the cell membrane, where several important biological reactions such as photosynthesis or cell respiration take place. Hence electrochemistry at liquid-liquid interfaces is an excellent means to study biomimetic oxygen reduction. In this reaction, proton transfer across the interface controlled by the Galvani potential difference is coupled with electron transfer. Therefore oxygen reduction in the oil phase mediated by an electron donor occurs only at potentials where proton transfer becomes feasible. In this thesis oxygen and hydrogen peroxide reduction by 1,2-diferrocenylethane was investigated. The results show that the reduction of hydrogen peroxide is faster than oxygen reduction, but both reactions were slow due to the low thermodynamic driving force. The reaction rate can be increased by the addition of molecular catalysts, and electrochemistry at liquid-liquid interfaces provides an excellent means to compare the activity and selectivity of the catalysts. Cofacial biscobalt porphyrins, biomimetic analogues of the active centre of cytochrome c oxidase responsible of oxygen reduction in nature, were investigated. Surprisingly, significant amounts of hydrogen peroxide were produced, contradicting previous results. The reaction was shown to proceed to hydrogen peroxide when oxygen was bound to the exo side (dock-on) of the catalyst, while four-electron reduction took place with oxygen bound to the endo side (dock-in) of the molecule. A proof of concept of a novel type of a fuel cell utilising the liquid-liquid interface is presented. In this fuel cell hydrogen is oxidized on the anode as in a conventional fuel cell, but oxygen reduction takes place at the liquid-liquid interface. The redox mediator in the oil phase is regenerated at the cathode, completing the electric circuit. Proton transfer across the interface was also utilised for performing acid catalysed SN1 substitutions on ferrocene methanol. This is a novel method to perform synthesis of organic chemicals, as the presence of protons is controlled by the applied Galvani potential difference. In situ biphasic electrospray ionization mass spectroscopy was demonstrated to be a very efficient way to follow this kind of reactions.