| This thesis presents electrochemical studies of p-phenylenediamine-based ureas. The results of the first study show that what appears to be a one electron oxidation of the compounds with a urea N-H as part of the p-phenylenediamine redox couple actually corresponds to a two-electron oxidation to the quinoidal form with transfer of a proton to another urea, which deactivates the second urea. Thus the overall process corresponds to one electron per urea. Interestingly, there are strong structural and solvent effects on the voltammetry which require that H-bonding steps be considered along with the electron transfer and proton transfer steps in the mechanism. Because acetonitrile is more polar than methylene chloride, it can better solvate the quinoidal urea and the protonated, unoxidized urea to prevent them from hydrogen bonding to each other. This results in chemically irreversible electrochemistry. In contrast, stronger H-bonding between the quinoidal urea and the protonated, unoxidized urea is expected in the less polar solvent methylene chloride and it is believed that this is what facilitates the observed reversible electrochemistry in this solvent. In the second study, the addition of a basic guest, 1,4-dimethylpiperizine-2,3-dione, PZD, to p-phenylenediamine-based ureas was re-investigated in a light of the better understanding of the electrochemistry of ureas resulting from the first study. Previous work showed that addition of PZD caused a negative shift in the potential of oxidation of one of the ureas in methylene chloride, consistent with strong H-bonding of PZD to the oxidized urea. According to the new NMR titration data, PZD is H-bonded to urea even without oxidation. The binding constants measured in different electrolyte solutions between PZD and urea show that there is an electrolyte effect. Re-examination of the cyclic voltammetry of the ureas suggests that the same overall reaction, two-electron oxidation of half the urea with proton transfer to the other half is occurring with and without PZD present. This makes it difficult to explain the reversible electrochemistry observed in methylene chloride. A three-way H-bond complex between PZD, the oxidized urea and the protonated reduced urea is proposed as a possible explanation. |
