Redox-Dependent Binding to an Electroactive Urea: Comparison of One and Two Electron Redox Couples

Tuesday, 30 May 2017
Grand Ballroom (Hilton New Orleans Riverside)
K. Logan, L. A. Clare, and D. K. Smith (San Diego State University)
In previous work in our lab, the redox-dependent binding behavior of a phenylene-diamine urea, UHH, was investigated in the presence of different guests in CH2Cl2. The urea functionality contains two good H-donors in the two urea NH bonds (a DD motif) that are capable of H-bonding with two appropriately spaced H-acceptors (AA motif). An example is 1,8-naphthyridine, naph, which contains two pyridine-type N’s appropriately spaced and orientated to H-bond to the two NH’s in UHH. 1H NMR studies indicated a modest Kbinding of 30 M-1 in CH2Cl2 between UHH and naph. The expectation was that oxidation of the phenylenediamine couple in UHH would increase the acidity of one of the NH’s leading to stronger H-bonding and a negative shift in the observed redox potential upon addition of naph. However, instead what happened was that the current for the oxidation increased with little change in peak potential. This can be explained by proton transfer from the UHH radical cation to the naphthyridine. The resulting neutral radical will be immediately oxidized by a second electron leading to the increase in current. The proton transfer will change the H-bonding motif from AA-DD to AD-DA, which is inherently weaker because of unfavorable secondary interactions. In addition because of the two electron oxidation, both binding partners in the resulting complex, (naphH+)(UH+), will be positively charged. Therefore it is not surprising that the H-bonding does not strengthen upon oxidation. In this study the phenylenediamine couple is replaced by a ferrocene, FcUHH, which is only capable of a one electron oxidation. Cyclic voltammetry experiments will be run with FcUHH and naphthyridine to see if eliminating the possibility of the second electron transfer prevents proton transfer and results in the expected stronger H-bonding to the oxidized form.