Acidic properties of N,N-naphthoquinonylamines

The new system in which two naphthoquinonic moieties are bridged via a NH group shows acidic properties due to its vinylogous-imide nature. In the dichloro derivative (BClNQA) strong mesomeric and inductive effects result in unusually high acidity. Its pKa value in methanol is equal to 7.51±0.02, which is several orders of magnitude lower than that of normal aliphatic or aromatic imides or even aliphatic carboxylic acids. The new system can form stable salts with various organic bases (e.g. triethylamine). The chromophoric nature of BClNQA and its anion is expressed by a dramatic bathochromic shift of its λ3 in polar solvents from the orange (441–473 nm) to the blue region (608–624 nm). This unusual red shift is due to substantial polarization of the quinonic system and delocalization of its anionic form.