Deciphering the Mechanistic Diversity of Proton-Coupled Electron Transfer Reactions

Proton coupled electron transfer is ubiquitous in biological and artificial reaction systems. Much has been done in order to describe the occurrence of such reactions. However, PCET reactivity is often very complex. For instance, there are multiple (stepwise and concerted) mechanistic pathways through which PCET may occur. The aim of this thesis is to further describe factors and underlying principles governing PCET reactivity. The contents of this thesis can be summarized in three parts: In the first part (chapter 4), the competition between different PCET mechanisms is discussed. Considering all mathematical expressions for the dependence of the rate constants on The Gibbs free energy changes (driving forces) associated with electron and proton transfer, mechanistic Zone-Diagrams are constructed. These show which of the mechanistic pathways is dominant, given a certain electron and proton transfer driving force. It is shown, how these diagrams simplify discussion of PCET reactivity. Strategies for modifying the mechanistic landscape, suppressing or favoring a CEPT mechanism, are demonstrated in the PCET oxidation of 4-methoxyphenol by photogenerated Ru(III) oxidants in the presence of pyridine bases. These are discussed utilizing the zone-diagram methodology. Implications for catalytic applications are discussed. The second part (chapter 5) introduces pressure dependence as a tool for mechanistic characterization of the PCET reactions. The PCET oxidation of tungsten hydrides covalently linked to pyridine bases by photogenerated Ru(III) oxidants was studied, and contributions from multiple mechanistic pathways were uncovered. It is shown, how each pathway has a characteristic pressure dependence. These can be related to changes in electrostriction of the solvent modifying the volume profile of the reaction. Finally, the third part (Chapter 6) deals with the concerted pathway. The possibility of photo-EPT, where electronic excitation directly yields the PCET product state, in Phenol/N-Methyl-4,4’-bipyridine complexes is discussed. It is shown that the optical charge-transfer absorption in these complexes is not coupled to proton transfer, in spite of previous claims. Further, the pressure dependence of the CEPT quenching of excited state fac-Re(CO)3(2,2’-bpy)(4,4’-bpy)+ by substituted Phenols is monitored. It is shown that the observed pressure dependence cannot be rationalized using the electrostriction arguments outlined in chapter 5. Instead, a model relating the observations to pressure induced changes of contributing proton tunneling distances is constructed.