Redox-Responsive Dimerization in a Ferrocene-Ureidopyrimidone Supramolecular Assembly

Ureidopyrimidones (UPy's) are well-known to dimerize via 4 strong H-bonds in noncompetitive solvents. Although UPy dimers have been widely studied, there are few examples of UPy's containing redox-active groups, and even fewer in which oxidation/reduction has been shown to affect dimerization. In this study, a thorough electrochemical investigation of a UPy with a ferrocene attached to the 6-position of the pyrimidone ring, UPy(Fc), demonstrates that strong and reversible redox-control of dimerization is possible in the UPy system. ¹H NMR shows that the reduced UPy(Fc) is fully dimerized under electrochemical conditions in CH₂Cl₂. Cyclic voltammetry (CV) shows that oxidation of the ferrocene to the ferrocenium converts the dimer to another species with a less positive E_1/2, and that reduction of this species reforms the dimer in a chemically reversible fashion. Analysis of the scan rate and concentration dependence of the CV's along with the relative diffusion coefficient measurements strongly suggest that the oxidized form is the monomer. Simulation of the CV data gives a K_dis of 2.1 × 10^-6 M in the reduced state and 12 M in the oxidized state. This large redox dependence is likely due to oxidation creating both electrostatic repulsion between monomers and greatly decreasing the H-accepting ability of the pyrimidone.