Rees polycyanated hydrocarbons and related compounds are extremely powerful Brønsted superacids in the gas-phase and DMSO—a density functional B3LYP study

Abstract

The spatial structure and acidic properties of demethylated Rees hydrocarbons and related fluoradene, as well as their polycyanated derivatives, are considered by the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) method. The molecular frameworks 1c, 2e and 3g involve the π-networks possessing 10, 14 and 18 π-electrons on the molecular rims, held in the rigid tricyclic structures by the central tertiary C(sp³) carbon atom. It is found that 1c is a tricyclic [10]annulene with almost uniformly delocalized π-electrons over the molecular perimeter, whereas other systems with 14 and 18 π-electrons exhibit pronounced bond alternation. It is conclusively shown that extended π-electron networks, like e.g. that in 3g include also smaller π-electron patterns conforming to Hückel’s (4n + 2) rule, thus representing an interesting case of molecular “philogenesis”. Further, all these molecules undergo prototropic tautomerism. The number of prototropic tautomers is considerably increased by polycyanation inter alia by forming keteneimine moiety. The most important result of the present study is that polycyano derivatives of studied molecules are strong organic Brønsted acids both in the gas-phase (GP) and DMSO. More specifically, percyano molecules 1a_CNCN, 2a_CNCN and 3a_CNCN possess gas-phase ΔH_acid values as low as 261.8, 259.0 and 246.3 kcal mol⁻¹, respectively. If the thresholds of superacidity and hyperacidity are accepted as ΔH_acid(HClO₄) = 300.0 kcal mol⁻¹ and the protonation energy of 1,8-bis(dimethylamino)naphthalene DMAN, PA(DMAN) = 245.3 kcal mol⁻¹, respectively, then 1a_CNCN and 2a_CNCN are NH superacids, whereas 3a_CNCN is a C(sp³)H hyperacid. The origin of the dramatic amplification of the acidity in multiply cyanated derivatives is identified as the increase in the anionic resonance of the resulting conjugate bases.