Comparison of Redox-Dependent H-Bonding in Simple, Electroactive Ureas Containing Either Ferrocene or Phenylenediamine Redox Couples – Similar Results with Different Mechanisms

Abstract

An important goal in supramolecular chemistry is the design of stimuli-responsive supramolecular systems in which binding between components can be controlled by external signals. Supramolecular chemistry research in our group focuses in particular on using electron transfer to control the strength of H-Bonding interactions. In this presentation the redox-dependent H-bonding behavior of two simple phenylureas containing either a phenylenediamine redox couple, Figure 1a, or a ferrocene redox couple, FcUHH, Figure 1b, 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. Experimentally, this should result in a negative shift in the E1/2 of the cyclic voltammetric wave upon addition of PZD, with little change in wave shape or height. This behavior is indeed observed for both ureas in 0.1 M NBu4PF6/CH2Cl2. Plots of the ΔE1/2 of FcUHH vs [PZD] are fit well using a 1:1 binding isotherm, with initial results giving estimates of the binding constants to the oxidized and reduced forms as Kred = 80 M−1 and Kox = 500 M−1. The same type of experiments have been run with the phenylenediamine urea, UHH. The data also fits well to a simple 1:1 binding isotherm and gives similar values for the binding constants, Kred = 30 M−1 and Kox = 700 M−1. This is interesting because from our previous work1 we know that the electrochemistry is quite a bit more complex for UHH than FcUHH. In the former case 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, Figure 1a. Given this, it is somewhat surprising that any potential shift is observed upon addition of PZD, since the oxidation product can only form a single H-bond with PZD. However, the key realization was that the other product, the protonated urea, can still form a 2 H-bond complex, and that should be stronger than the one formed with the starting urea due to the increased acidity of one of the urea NH’s. From this perspective, it is not surprising that we see such similar behavior between FcUHH and UHH. In the former, oxidation creates a +1 charge on one side of the urea (ferrocenium), Figure 1b, and, in the latter, proton transfer creates +1 charge, Figure 1a. This conclusion suggests an entirely 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! 1 L. A. Clare, A. T. Pham, F. Magdaleno, J. Acosta, J. E. Woods and D. K. Smith, Journal of the American Chemical Society 2013, 135, 18930-18941. Figure 1