An important goal in supramolecular chemistry is the design of stimuli-responsive systems in which binding between components can be controlled by external signals. In this presentation the redox-dependent H-bonding behavior of two simple phenylureas containing either a phenylenediamine redox couple, UHH, or a ferrocene redox couple, FcUHH, will be compared. The urea functional group provides two good H-donors (a DD array) which can be paired with two good H-acceptors (an AA array) in the PZD guest. Oxidation of either the ferrocene or phenylenediamine would be expected to increase the acidity of the attached urea NH, increasing the H-Bonding strength to PZD. This is indeed observed for both ureas in CH2Cl2. Plots of the ΔE1/2 of FcUHH vs [PZD] are fit well using a 1:1 binding isotherm giving Kred = 80 M−1 and Kox = 500 M−1. Similar results are observed for UHH with Kred = 30 M−1 and Kox = 700 M−1, despite the fact that the electrochemistry is much more complex for UHH than FcUHH. With UHH, the reversible, apparent 1 e− cyclic voltammetric (CV) wave observed in CH2Cl2 actually corresponds to 2 e− oxidation of half of the UHH’s to a quinoidal cation accompanied by proton transfer and electro-deactivation of the other half, Fig. 1a. In contrast, FcUHH can only undergo a simple 1 e− oxidation, Fig. 1b. The explanation for the similar binding despite the different reactions is that in both cases a +1 urea is formed upon oxidation. With FcUHH this is due to removal of one electron, with UHH it is due to addition of one proton. This conclusion suggests a new strategy to perturb H-bonding through electron transfer. Rather than doing this directly through oxidation/reduction introduced charges, it can also be done indirectly through oxidation-reduction induced proton transfer.

Comparison of redox dependent H-bonding in simple electroactive ureas containing either ferrocene or phenylenediamine redox couples: Similar results with different mechanisms

2019 Fall National Meeting

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