An important goal in supramolecular chemistry is the development of stimuli-responsive systems in which supramolecular assembly is controlled by external signals. Our group focusses on one way to do this - use electron transfer to perturb the strength of H-bonds. The principle is straightforward: reduction that increases negative charge on a H-acceptor (A) or oxidation that increases positive charge on a H-donor (D) will increase the strength of a H-bond. However, oxidation or reduction can also lead to full proton transfer, resulting in an increase in unfavorable secondary interactions that counteracts the effect of the initial electron transfer. But, with proper design, proton transfer could lead to an increase in favorable secondary interactions, enhancing the effect of the initial electron transfer. The success of the latter strategy is demonstrated with the 3 H-bond DAD array, 1, that contains a N-methyl-4,4'-bipyridinium redox couple. In CH2Cl2, 1 forms a three H-bond dimer with ADA array, 2, with a modest Kb of 650 M−1 due to the unfavorable secondary interactions. Cyclic voltammetry studies show that two electron reduction of the bipyridinium increases the binding strength by 105 giving a very strong Kb of ~108 M−1. This value is consistent with expectations for a DDD-AAA H-bond complex, indicating that electron transfer not only created an ionic H-bond but also led to proton transfer and conversion of the secondary interactions from unfavorable to favorable.

Conversion of a weak DAD-ADA H-bond dimer to a much stronger DDD-AAA dimer via proton-coupled electron transfer

2019 Fall National Meeting

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