The underlying principle is straightforward: a reduction that increases the negative charge on a H-acceptor or an oxidation that increases the positive charge on a H-donor will increase the strength of a H-bond. However, oxidation or reduction could also lead to full proton transfer. If this occurs across the H-bond, the primary H-bond will remain favorable, but the secondary H-bonds will change. This can lead to an increase in unfavorable secondary interactions that oppose the effect of the initial electron transfer. However, with appropriate design, proton transfer could lead to an increase in favorable secondary interactions that enhance the effect of the initial electron transfer. The goal of this project is to demonstrate the latter. For this work, the H-bond DAD array 1ox that contains a N-methyl-4,4'-bipyridinium or “monoquat” redox couple has been synthesized using a Suzuki coupling between 4-pyridylboronic acid and the 4-bromo-2,6-diamidopyridine as the key step. Control experiments with monoquat itself indicate that the ADA array 2 is acidic enough to protonate the two electron reduced form of monoquat. As expected, 1H NMR shows that 1ox forms a three H-bond dimer with 2 with a modest Kb of 650 M−1 in CD2Cl2 due to the unfavorable secondary interactions. Significantly, cyclic voltammetry studies with 1 and 2 together indicate an ~105 increase in binding strength upon two electron reduction of 1ox to 1red. This large effect would not be expected from reduction alone, and, indeed, the resulting Kb of ~108 M−1 is on par with those measured for other DDD-AAA dimers, suggesting proton transfer has occurred as predicted, converting the DAD-ADA dimer with all unfavorable secondary interactions to the DDD-AAA dimer with all favorable secondary interactions. These results demonstrate the utility of incorporating proton transfer to amplify the effect of electron transfer in these systems.

Using electron-transfer-induced proton transfer to control binding strength in a H-bond dimer

2019 Fall National Meeting

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