The Effect of Electron Transfer on a Strongly H-Bonded Ureidopyrimidinone Dimer

There is much current interest on the role that H-bonding intermediates play in the mechanism of proton-coupled electron transfer. Recently we have been examining this phenomena in detail using an electroactive phenylenediamine based urea, U(H)H (Clare et al., J. Am. Chem. Soc. 2013, 135, 18930-41).  This compound appears to undergo a reversible one electron oxidation to the radical cation in methylene chloride.  We have shown that in fact the oxidation corresponds to two electron oxidation of half the ureas to the quinoidal form, accompanied by proton transfer to the dimethylamino group and subsequent deactivation of the other half of the ureas, 2U(H)H = U(H)⁺ + H(U)H⁺ + 2 e-. The reaction gives chemically irreversible voltammetry in acetonitrile as would be expected given that the product quinoidal cation, U(H)⁺, is harder to reduce than the initially formed radical cation.  However, it gives reversible voltammetry in methylene chloride because the reverse reaction can proceed through the H-bonded complex formed between U(H)⁺ and H(U)H⁺, which is easier to reduce than the radical cation alone.  
    We have also studied the voltammetry of U(H)H in the presence of guest molecules that can H-bond to the two urea NH’s.  In these cases, we observed similar overall electrochemistry, with proton transfer accompanying the oxidation of U(H)H.  In contrast, different behavior is observed when the U(H)H redox couple is incorporated into a ureidopyrimidinone framework, 1.  This structure is well-known to form strong H-bonded homo-dimers in less polar solvents such as methylene chloride, and this behavior is maintained with the U(H)H derivative.  As with U(H)H, cyclic voltammetry of 1 in methylene chloride shows a reversible, apparent one electron oxidation. However, there is also an irreversible, apparent half electron oxidation at slightly more positive potentials.  While we were expecting that the reversible one electron oxidation with 1 corresponded to a similar reaction as seen with U(H)H by itself and with the two H-bond guests, UV/Vis spectroelectrochemical evidence along with titration studies with a non-electroactive ureidopyrimidinone strongly suggest that in this case the apparent one electron oxidation is really that, with both ureas in the dimer being oxidized to the radical cation without proton transfer.  Despite the electrostatic repulsion, the dimer holds together at mM concentrations as evidenced by the reversibility of the CV wave.  However, it appears with the removal of one more electron, the dimer breaks apart at these concentrations.  This behavior is notable because the ureidopyrimidinones are currently under intensive investigation as components of supramolecular polymers that possess unique self-healing properties.  Existing systems use heat to break apart the dimers and allow the polymer to self heal.  This work shows that electron transfer may be able to provide a more directed approach to control the fluidity of these materials.