| Coupling electron transfer to proton transfer is key to converting solar energy to chemical fuels, and more generally in a wide range of chemical and biochemical processes. Although electron transfers have been thoroughly studied over many decades, the fundamentals of proton-coupled electron transfer (PCET) are still being worked out. In order to understand how the distance between the electron and proton-accepting sites affects PCET, this dissertation focuses on the preparation and reactivity of Ru complexes with large separations between the redox-active metal and the basic carboxylate site.The Ru(III) complex, RuIII(pydic)(terpy-COO) (Ru IIICOO), has been isolated with a terpyridine-4'-carboxylate ligand, in which there are six bonds and 6.9 A between the redox-active Ru and the basic carboxylate. RuIIICOO oxidizes substrates by removal of e- and H+ to form the protonated Ru(II) complex, RuIICOOH. Thermochemical analyses indicate that these reactions occur by concerted transfer of the proton and electron despite the large distance between the Ru and the H+-accepting oxygen. To further understand how the reactivity has been affected by this distance, the separation between the Ru and basic site has been increased by inserting a phenyl group between the terpyridine and the carboxylate in the analogous Ru(III) complex, RuIIIPhCOO. Even with this separation, Ru IIIPhCOO also oxidizes hydrogen atom donors X-H in a concerted (CPET) process, although the reactions typically proceed more slowly than in the system without the phenyl spacer.The large distance between the metal and carboxylate sites in both systems appears to complicate the patterns of reactivity, so that simple treatments, such as linear free energy relationships and Marcus Theory, do not explain the observed trends. This is the first study to address the distance dependence of the proton and electron accepting sites in PCET reactivity, and has important implications for systems where long distance electron transfer is coupled to proton transfer.A separate project involving PCET with cobalt biimidazoline complexes is also presented. The low-spin Co(III) complex, CoIIIHbim, reacts with hydrogen atom donors to form high-spin Co IIH2bim in a CPET process. This system also deviates from typical Marcus behavior, which is likely due to the change in spin state that occurs during reactivity. |
