Superacid protonation of dihydrocyclobuta[e]pyrene and its C60–o-quinodimethane adduct. An NMR, ab initio/GIAO and AM1/PM3 study

Abstract

Dihydrocyclobuta[e]pyrene 1 is protonated in FSO₃H–SO₂ClF to give a ca. 2∶1 mixture of two arenium ions of α-protonation 1H⁺ and 1aH⁺. Ab initio/GIAO theory at the B3LYP/6-31G(d,p) level was utilized to calculate the NPA charges, relative energies and to predict the NMR chemical shifts for 1H⁺ and 1aH⁺ as a means to enhance the solution NMR-based assignments. The resulting regioisomeric arenium ions are highly delocalized and exhibit a uniform charge alternation path within a pyrenium moiety. Reasonable predictions of their relative energies and the overall charge delocalization mode can also be obtained at the AM1/PM3 levels. In an attempt to determine the influence of C₆₀ on tropicity in an anchored-pyrenium ion, the C₆₀–o-quinodimethane adduct 2 was protonated in FSO₃H·SbF₅–SO₂ClF; formation of regioisomeric bound-arenium ions (2H⁺, 2aH⁺) which are very poorly soluble in superacid media was inferred from ¹H NMR and AM1/PM3 calculations were used to model these cations. Further model studies using a fragment of C₆₀ bound to regioisomeric pyrenium ions were realized at the LSDA/DN level of theory with electrostatic potential derived charges at the B3LYP/6-31G(d,p) level.