Electrochemically-Controlled Dimerization in Ferrocene-Containing Ureidopyrimidones: Effect of Ferrocene Position

Abstract

Ureidopyrimidones (UPy’s) are well-known to dimerize in weakly polar solvents via the formation of four strong, linear H-bonds. Application of heat or mechanical stress can be used to break these bonds, which has led to their use as a linker in self-healing supramolecular polymers. On-going research in our lab is focused on further increasing the versatility of the UPy system by creating electroactive UPy’s in which dimerization strength can also be controlled through oxidation and reduction. Previously, we have shown that dimers based on the ferrocene-UPy derivative, 1, break apart upon oxidation of the ferrocene to the ferrocenium form at mM concentrations in CH2Cl2. This could be due both to the creation of electrostatic repulsion and a decrease in H-bond strength due to the reduced H-donor ability of the O and N on the pyrimidone side. In this project, another Fc-UPy has been prepared, 2, in which the ferrocene is attached to the urea side of the molecule. In contrast to 1, oxidation of 2 shows a single reversible ferrocene CV wave in CH2Cl2 at mM concentrations. Under these conditions, 1H NMR indicates that 2 is fully dimerized, thus, it appears that, unlike with 1, oxidation has no significant effect on dimerization of 2. In both cases, oxidation would increase electrostatic repulsion. However, unlike in 1, oxidation of 2 should actually increase H-bond strength by making the urea NH a stronger H-donor. Therefore, it appears that the effect of oxidation on the H-donating or accepting ability is a more crucial factor for dimerization control than electrostatics in these systems. Figure 1