Conversion of a Weak Three H-Bond Dimer to a Strong Three H-Bond Dimer Using Electron Transfer Induced Proton Transfer
Authors: Choi Hyejeong , Smith Diane K. 
Published: February 2020

An important goal in supramolecular chemistry is the development of stimuli-responsive systems in which the strength of the intermolecular interactions can be altered by external signals such as changes in light, temperature, pH or voltage of an electrode. Our group is exploring the latter possibility, primarily by trying to develop systems in which electron transfer perturbs the strength of H-bonding interactions between molecules. The underlying principle is straightforward: an oxidation that decreases the negative charge on a H-acceptor (A) or a reduction that decreases the positive charge on a H-donor (D) will weaken a H-bond. Alternatively, reduction that increases the negative charge on a H-acceptor or oxidation that increases the positive charge on a H-donor will increase the strength of a H-bond. However, in the latter case, it is possible that oxidation or reduction could also lead to full proton transfer. If this occurs across the H-bond, the primary H-bonds will remain, but the secondary H-bonds will change. This can lead to an increase in unfavorable secondary interactions, which would counteract the effect of the initial proton transfer. However, with proper design, proton transfer could lead to an increase in favorable secondary interactions, which would enhance the effect of initial transfer. The goal of this project is to do the latter. In particular, 3-H-bond DAD-ADA arrays will be studied in which the DAD component contains a N-alkyl-4,4'-bipyridinium or “monoquat” redox couple (see figure). DAD-ADA arrays have relatively weak association constants (102 in non-competitive solvents such as CHCl3) due to the three favorable primary H-bonds (shown as solid double-headed arrows in the figure) being counterbalanced by four unfavorable secondary interactions (cross interactions shown as dashed double-headed arrows). As shown in the figure, reduction of the monoquat by one electron should lead to proton transfer, creating not only a strong ionic primary H-bond, but converting all the secondary H-bonds to favorable ones. Examples of similar ionic DDD-AAA complexes have been reported in the literature with association constants greater than 109, suggesting it may be possible see a binding enhancement on the order of 107 upon reduction in this system. Initial voltammetric studies with just monoquat itself (x = nothing) and the diimide ADA guest show a significant positive shift in the E1/2 of the second reduction of monoquat, but little to no shift in the first reduction in both acetonitrile and methylene chloride. The shift is larger in acetonitrile, consistent with it being due to proton transfer between the guest and the fully reduced monoquat. Currently, synthesis of the compete DAD array is underway, which will have X = NC(=O)R to make the H-donor a strongly donating amide NH. In contrast to the monoquat results, CV studies with this compound and the diimide ADA guest are expected to show a E1/2 shift in the first reduction that is significantly larger in methylene chloride than acetonitrile, indicating very strong H-bonding to the radical form.

Figure 1