Effect of Added Bases on the Redox-Responsive Dimerization of a 4 H-Bond Array Containing a Phenylenediamine Redox Couple
Authors: Vuong Katrina , Clare Laurie A. , Smith Diane K. 
Published: February 2020

Ureidopyrimidinone (UPy) derivatives, introduced by Meijer and co-workers, are capable of forming dimers linked by four strong H-bonds. They have been widely used as polymer crosslinkers in self-healing supramolecular polymers and gels. While their donor-donor-acceptor-acceptor (DDAA) H-bond motif contributes to polymer stretchiness, their relatively high association constant of 106 also contributes to their ability to autonomously self-repair. This presentation shows the result of substituting a UPy with a N,N- dimethyl-p-phenylenediamine and therefore adding a redox reactive dimension to self-repair. In particular, this work involves the electrochemical investigation of UPyH (shown below) with a series of different bases, specifically pyridine and substituted pyridines. The focus is on finding the appropriate electroinactive base that facilitates both the dimer break up upon oxidation and proton transfer and dimer reformation by reduction and back proton transfer.
Cyclic voltammetry (CV) scans of UPyH only, conducted in CH2Cl2 at slower scan rates, show two reversible redox waves. The first wave is of one electron height, corresponding to a two-electron transfer per dimer. The second oxidation wave at more positive potential is one half the height of the first and less reversible than the first. At a faster scan rate, the second wave becomes irreversible and a new reduction wave appears at a more negative potential than the reduction wave for the first electron transfer process. This type of CV return wave behavior for UPyH at faster scan rates was previously seen in another dimethylphenylurea redox active compound with only two H-bonding sites, UHH. In the UHH investigation, CV and spectroelectrochemical results pointed to the dimethylamino moiety being basic enough to abstract an N-H proton in the radical cation oxidation state and therefore allowing for a two electron transfer wave to give the quinoidal cation. A similar reduction wave negative of the first redox wave was seen in acetonitrile at all scan rates, while in CH2Cl2 this wave was seen only at faster scan rates. At slower scans the first oxidation remained reversible. Based upon the UHH study, interpretation of the UPyH second oxidation wave is that it correspond with a second e- transfer per redox center. Each urea-phenylenediamine N-H site in the dimer then becomes acidic enough to transfer its proton to the carbonyl oxygen on the UPyH isocytosine. This would then change the favorable binding motif of DDAA to a weaker binding motif of ADAD. In addition, a 2+ charge forms on each monomer in the dimer contributing to strong repulsive forces. This results in dimer break-up, where upon the protonated isocytosine carbonyl on each monomer is exposed to the more basic dimethylamino site on a fully reduced UPyH dimer. Proton transfer from the isocytosine to the dimethyl amino renders the fully reduced dimer electroinactive, explaining the half wave height. Thus, the overall reaction, shown in the accompanying scheme, corresponds to break up of the half the starting dimers, with two electron oxidation and deprotonation of each monomer to give two UPy+. However, the other half of the UPy’s remain dimerized in the protonated form as (HUPyH+)2. Further investigation focuses on the addition of a series of substituted pyridines as a base and their effect on dimer break up and reformation as the added base competes with the dimethylamino moiety on the phenylenediamine. The goal is to find an external base that will allow full oxidation and break up of all the UPyH dimers, while retaining the chemical reversibility of the process.

Figure 1