Strength and directionality are the hallmark binding characteristics of molecules with multiple hydrogen bonds. This is important in the area of molecular recognition and supramolecular construction. When coupled to a redox center, oxidation or reduction can affect the electrostatic property of hydrogen bonds to either strengthen or weaken hydrogen bonding and thus allow for more control over this type of binding. A good example of strengthening hydrogen bonds can be seen in the use of a bi-substituted urea. Previous investigations from our lab showed that as a substrate, with its two side-by-side NH hydrogen donor binding sites (DD), N,N'-diphenylurea forms two, linear, primary hydrogen bonds with both dicarbonyl oxygens on a hydrogen bond acceptor (AA), 9,10-phenanthraquinone as shown below. In addition to the primary hydrogen bonds the DD-AA motif provides secondary hydrogen bond forces that enhanced the overall binding strength. Upon reduction of the 9,10-phenanthraquinone in the presence of the diphenylurea, both primary and secondary binding forces from the DD-AA motif stabilize the radical anion causing the half-wave potential to shift to more positive values with more urea added.

With the addition of a dimethylamino moiety onto one of the phenyl substituents to form a phenylenediamine redox center, the urea substrate was converted to a redox anion receptor, 1-phenyl-3-(4-dimethylamino)phenyl urea, (UHH). This meant that the phenylenediamine-urea N-H site could be perturbed upon electron transfer. The first oxidation forms a radical cation where delocalization of the positive charge in the redox center could also strengthen hydrogen bonding to the N-H, but upon a second electron transfer, the delocalization of two positive charges forms a very acidic NH site that could easily undergo proton transfer. Initial CV studies with UHH conducted in a limited potential window and in the presence of 1,4-dimethylpiperizine-2,3-dione (PZD), a diamide that has two carbonyl oxygens structurally compatible to the two urea N-H’s, showed a single, reversible oxidation wave shifting in the negative direction with increasing amounts of PZD, indicating an increase in binding strength. Upon expansion of the potential window a second oxidation wave is seen that also shifts in the negative direction with increasing amounts of PZD but its current height is comparatively smaller and the wave becomes irreversible which can not be explained by hydrogen bonding only. CV investigations of UHH alone revealed that what was thought to be a simple one electron transfer process, is a much more complicated two electron, one proton transfer. It turns out that the dimethylamino moiety on a fully reduced UHH is basic enough to remove a proton from another singly oxidized UHH allowing for an immediate second electron transfer at the same potential. Ultimately, a fully oxidized phenylenediamine redox center will not enhance binding strength due to proton transfer.

Our new strategy and the subject of this poster presentation, is the replacement of the phenylenediamine redox center with a ferrocene redox center (FcUHH). This change limits the oxidation to a single electron transfer process without a basic site for proton transfer. Initial CV experiments of FcUHH in methylene chloride show a reversible, one electron oxidation as expected. With the addition of PZD, small negative shifts in the halfwave potential are seen, consistent with stronger H-bonding to the oxidized form.