Analysis of a Four H-Bond Array Using Cyclic Voltammetry: Introducing a New Redox Center to Strengthen Dimerization

Hydrogen bonding arrays are useful building blocks for supramolecular chemistry due to their strong hydrogen bond interactions that can form dimers. The strength of the H-bond interactions can be perturbed by applying an external stimuli, in this case electron transfer. Recently, our group has modified Meijer’s four H-bond ureidopyrimidinone (UPy) array¹ to make it electroactive by adding a dimethylaminophenyl group. The modified structure forms strong H-bond interactions in CH₂Cl₂, creating a dimer that can undergo a reversible 2e^-oxidation, followed by an irreversible oxidation at more positive potentials. The latter results in the break-up of the dimer due to electrostatic repulsion.

For this study, we aim to modify another four H-bond Upy array by introducing a methyl pyridinium redox center as shown below. We hypothesize that this array will prefer the tautomer that presents an ADAD H-bond motif in the starting oxidation state. This motif will form a dimer with relatively weak H-bond interactions due to electrostatic repulsions and also unfavorable secondary H-bond interactions. Upon 2e^- reduction, where 1e^- is gained for each methyl pyridinium redox center, the H-bond strength should increase due to the loss of repulsive charges and also allows the nitrogen to become a stronger hydrogen acceptor. Since the resulting nitrogen is now a better H-acceptor, there is a possibility of intermolecular proton transfer which will induce the tautomer to have an AADD motif that will further strengthen the H-bonds by favorable secondary H-bond interactions.

1. Beijer, F.H.; Sijbesma, R.P.; Kooijman, H.; Spek, A.L.; Meijer, E.W. J. Am. Chem. Soc., 1998, 120 (27), pp 6761–6769.