Effect of H-Bonding on the Electrochemistry of a p-Phenylenediamine-Based Urea. Competition Between Another Urea and a Cyclic Diamide Guest

In previous work done by our group, we reported the observation of strong oxidation-based redox dependent hydrogen bonding between an electroactive urea 1-phenyl-3-(p-N,N-dimethylaminophenyl)urea U(H)H and a cyclic diamide 1,4-dimethylpiperizine-2,3-dione (PZD) in methylene chloride.¹Large negative shifts in the apparent, reversible one electron oxidation of U(H)H are observed upon addition of PZD, consistent with strong H-bonding of PZD to the radical cation of U(H)H.

However, with further study of U(H)H and similar molecules, it is clear that the electrochemistry and the mechanism is not as straightforward as once believed. Instead of undergoing a simple one electron oxidation, a variety of data now suggests that U(H)H actually undergoes a two electron oxidation to the quinoidal form with transfer of a proton to another U(H)H, which deactivates the second U(H)H. Thus the overall process corresponds to one electron per urea. 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. In contrast, stronger H-bonding between the quinoidal urea and the protonated, unoxidized urea is expected in the less polar solvent, methylene chloride and we believe this is what facilitates the observed reversible electrochemistry.

In the current study we are re-investigating the addition of PZD to U(H)H in light of our better understanding of the electrochemistry of U(H)H by itself. According to our new NMR titration data, PZD is H-bonded to U(H)H even without oxidation, which is consistent with our previous study. The binding constants, K, measured in different electrolyte solutions between PZD and U(H)H show that there is an electrolyte effect. In the NMR titration experiment without electrolyte, due to the solvent effect, PZD is more strongly H-bonded to U(H)H in the less polar solvent CH₂Cl₂ than in CH₃CN (K in CH₂Cl₂ = 60.4 M⁻¹and K in CH₃CN = 17.8 M⁻¹). With a large anion electrolyte, NBu₄B(C₆F₅)₄ in CH₂Cl₂ , the binding constant is greater (K = 95.9 M⁻¹) than with a small anion electrolyte, NBu₄PF₆ in CH₂Cl₂ (K = 48.6 M⁻¹). This is consistent with the larger potential shift observed with the addition of PZD in NBu₄B(C₆F₅)₄ in CH₂Cl₂ solution relative to NBu₄PF₆ in CH₂Cl₂. This suggests that the smaller PF6⁻ion may be competing with PZD for the binding site.

Re-examination of the cyclic voltammetry of U(H)H suggests the same overall reaction, 2e⁻ oxidation of half the urea with proton transfer to the other half, is occurring with or without PZD present. However, the peak potentials are affected by the presence of PZD indicating the overall reaction is facilitated by PZD.

1. Jessica E. Woods, Yu Ge, and Diane K. Smith, Journal of the American Chemical Society 2008 130 (31), 10070-10071.