Mono- and diprotonation of dihydropyrene, 2,7-di-tert-butyl-dihydropyrene,and their conversion to pyrenium ions; Influence of the radical cation and its potential utility in NMR assignments of the arenium ions of readily oxidizable PAHs

Parent dihydropyrene 1 and 2,7-di-tert-butyldihydropyrene 3 are monoprotonated with FSO₃H/SO₂CIF to give their persistent monoarenium ions 1H ⁺ and 3H ⁺ by the attack of proton at C-3 (peri to the ethano-bridge not at C-1 as previously suggested). Dihydropyrene 3 is diprotonated in FSO₃H.SbF₅ 1∶1 “Magic Acid”^R/SO₂CIF to give the symmetrical dication 3H ₂ ⁺², similar diprotonation of 1 with “Magic Acid”/SO₂CIF or with FSO₃H.SbF₅ (4∶1)/SO₂CIF gave the diprotonated species 1H ₂ ⁺² in a mixture. NMR characteristics of the mono- and dications are discussed. On raising temperature or on prolonged cold storage, 1H ⁺ and 3H ⁺ are converted to their corresponding pyrenium cations (2H ⁺ and 4H ⁺). Formation of 2H ⁺ from 1 is more rapid than conversion of 3 to 4H ⁺. Parent pyrenium cation was independently generated by protonation with FSO₃H/SO₂CIF. When a mixture of 2 and 1 is reacted with FSO₃H/SO₂CIF (dry ice/acetone temperature) only 2H ⁺ is seen in the NMR (concomitant presence of the radical cation 1 ⁺⁺ is inferred from EPR). Similar protonation of a mixture of 6-chlorochrysene 5 and 1 with FSO₃H/SO₂CIF leads to NMR observation of 1H ⁺ (with concomitant presence of 5 ⁺⁺); on raising temperature 1H ⁺ is converted to 2H ⁺. The nature of the paramagnetic radical cation (RC) present in the arenium ion samples influences the position, and resolution of the NMR spectra. This approach may prove useful in NMR studies of large polycyclic aromatic hydrocarbons PAHs where concomitant RC formation greatly diminishes the quality of the NMR spectra.