Redox-Responsive Dimerization in a Ferrocene-Containing Ureidopyrimidone Supramolecular Assembly

Abstract

An important goal in supramolecular chemistry is the development of stimuli-responsive systems, where the strength of the intermolecular interactions can be altered by external signals such as changes in light, temperature, pH or voltage of an electrode. Applications include self-healing supramolecular polymers and gels, controlled release of entrapped molecules, and smart materials. In this paper, the results of our recent electrochemical studies with a ferrocene-containing ureidopyrimidone (UPy) will be described. The UPy system, introduced by Meijer, can form two self-complementary 4 H-bond arrays (AADD or ADAD) which dimerize in relatively non-polar solvents such as methylene chloride. Due to their ease of synthesis and relatively strong H-bonding they have been heavily investigated in a variety of functional supramolecular assemblies and polymers. By introducing an electroactive ferrocene it may be possible to control the strength of interaction to create a redox-responsive UPy. There are two different places an electroactive group can be attached to the UPy: on the urea side (R1 position ) and on the pyrimidinone side (R2 position). Following earlier studies by Alexander et al. (Chem Comm 2007, 22, 2246) we have prepared a UPy with a ferrocene in the R2 position. Concentration-dependent CV studies indicate a square scheme type mechanism. In the starting ferrocene state, the AADD dimer is preferred. Oxidation to the ferrocenium results in conversion to another form with a less positive E1/2. Preference for this new form increases at lower concentrations suggesting that it is a monomeric species, which would indicate that oxidation breaks apart the dimer as desired. Alternatively it is possible that the observed concentration dependence results from rate-limiting dissociation of the dimer, but the monomer then tautomerizes to the ADAD form which re-dimerizes. More detailed investigations are currently underway to try and distinguish between these possibilities.