Redox-Responsive Dimerization in a Ferrocene-Ureidopyrimidinone Supramolecular Assembly

Abstract

At one extreme, supramolecular polymers can have a high degree of randomness, existing as entangled coils with the mechanical properties of plastics and elastomers. At the other extreme, supramolecular polymers can be formed through directional intermolecular interactions among designed molecular subunits to achieve dynamic behavior as well as high degrees of internal order. The strength of these intermolecular interactions can be altered via external signals such as the absorption of light, temperature, pH, or the voltage of an electrode. Such is one of the important goals in supramolecular chemistry: the development of stimuli-responsive systems where the strength of intermolecular interactions can be perturbed through the use of external signals. Applications of supramolecular chemistry include self-healing polymers and gels, controlled release of entrapped molecules, and smart materials. In this presentation the results of electrochemical studies with a ferrocene-containing ureidopyrimidone (UPy) will be described. The UPy system – introduced by Meijer – can form two self-complementary 4 hydrogen bond arrays (AADD or ADAD) which dimerize in relatively non-polar organic solvents. By attaching an electroactive substituent such as ferrocene, the strength of electrostatic interactions can be perturbed to create a redox-responsive ureidopyrimidinone. Following earlier studies by Graham and co-workers, a ferrocene-functionalized ureidopyrimidinone UPy(Fc) was prepared. Concentration and scan-rate-dependent cyclic voltammetric studies indicate a square scheme type mechanism in dichloromethane. In the starting UPy(Fc) state, the AADD dimer is preferred. Oxidation to UPy(Fc+) results in conversion to another form with a less positive E1/2. Preference for this new form increases at lower concentrations which suggests that UPy(Fc+) exists as a monomeric species. This would indicate that oxidation breaks the dimer apart 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 enol form and re-dimerizes. To distinguish between these possibilities the relative diffusion coefficients (D’s) of UPy(Fc+) to UPy(Fc) were measured by steady-state voltammetry at a microdisk electrode. They were then compared to the relative D’s of ferrocenium to ferrocene. If UPy(Fc) is a dimer in both oxidation states, the DUPy(Fc+)/DUPy(Fc) ratio should be similar to that observed for ferrocene (DFc+/DFc), which will be a monomer in both oxidation states. However, if UPy(Fc+) is a monomer, the DUPy(Fc+)/DUPy(Fc) ratio will be larger than DFc+/DFc, since the D of the UPy(Fc+) monomer will be relatively smaller than the D of the UPy(Fc) dimer due to its smaller size. The latter is observed, supporting the conclusion that oxidation breaks apart the UPy(Fc) dimer at mM concentrations in dichloromethane. The results of these studies support the conclusion that UPy(Fc+) is monomeric at milli-molar concentrations in dichloromethane, meaning that oxidation of the UPy(Fc) breaks apart the dimer as hypothesized. Furthermore the process appears to be completely reversible. CV simulations were performed to determine if the square mechanism can quantitatively explain the observed voltammetry. The resulting simulated CV data supported the use of a square scheme mechanism to explain the cyclic voltammetry. However, the mechanism only accounts for the predominant keto AADD tautomer and excludes the enol ADAD tautomer. HNMR studies mimicking experimental conditions in cyclic voltammetry have been performed, revealing the presence of the enol ADAD tautomer in dichloromethane. Further HNMR studies in dichloromethane are underway, along with a comparative study in chloroform which does not appear to contain the enol ADAD tautomer. Cyclic voltammetry of the Upy(Fc) system will also be performed in chloroform. The results of these experiments will determine if the enol ADAD contributes to the redox mechanism of UPy(Fc). In addition, the synthesis of covalently linked UPy(Fc) oligomers is being initiated with the goal of creating a redox-responsive supramolecular polymer.