Electrochemically controlled hydrogen bonding. Redox-dependent formation of a 2:1 diarylurea/dinitrobenzene2- complex.

[reaction: see text] The electrochemistry of 1,2-dinitrobenzene (1,2-DNB), 1,3-dinitrobenzene (1,3-DNB), and 1,4-dinitrobenzene (1,4-DNB) is strongly affected by the presence of 1,3-diphenylurea. In DMF, the second reduction potential of all three DNBs shifts substantially positive in the presence of the urea, indicating very strong hydrogen bonding to the dianions. With 1,2- and 1,3-DNB, the hydrogen bonding leads to irreversible chemistry, likely due to proton transfer from the urea to the dianions. No such irreversible behavior is observed with 1,4-DNB. Instead, the second reduction shifts into the first reduction, producing a single, reversible, two-electron cyclic voltammetric wave at high urea concentrations. Computer simulations show that the changes in wave shape accompanying this process are well accounted for by the stepwise formation of a 1:1 and 2:1 1,3-diphenylurea/DNB2- complex, with sequential binding constants of approximately 5.5 x 10(4) M(-1) and approximately 4.0 x 10(3) M(-1) in DMF.