Strong acidity of some polycyclic aromatic compounds annulated to a cyclopentadiene moiety and their cyano derivatives - A density functional B3LYP study

The acidity of the parent H-1-benz[f]indene (1a), 1H-benz[e]indene (2a), 1H-benz[fg]acenaphthylene (3a) and 1H-cyclopenta[I]phenanthrene (4a) and of their polycyano derivatives is examined in the gas phase and in DMSO by the DFT-B3LYP methods. It is shown that the parent hydrocarbons exhibit modest acidities, whereas a dramatic increase in acidity is observed upon multiple cyanation leading to hyperstrong neutral organic superacids, It is found that polycyano derivatives undergo prototropic tautomerism with a consequence that the most stable tautomer contains a ketene imine C=C=NH functionality, The origin of their highly pronounced acidity is identified as a very strong anionic resonance effect efficiently assisted by a large number of CN groups in a concerted manner, It is stressed that the polycyanated anions are very stable and have a highly dispersed negative charge, which should lead to a low nucleophilicity and weak coordinating properties, thus making them potentially very useful in academic research and technological applications. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).