Probing the charge delocalization mode in methyl-, dimethyl- and methylene-bridged phenanthrenium ions. NMR studies of persistent mono- and di-cations and AM1 calculations

Abstract

Superacid protonation studies are reported on 2-methyl- (1), 3-methyl- (2), 3,6-dimethyl-phenanthrene (3) and 4H-cyclopenta[def]phenanthrene (4). In FSO3H–SO2ClF persistent monocations 1H+–4H+ are generated. 3 gives a symmetrical dication by protonation with FSO3H·SbF5 (4∶1)–SO2ClF or with ‘magic acid’–SO2ClF. 1 and 2 are partially diprotonated in ‘magic acid’–SO2ClF. The observed mono- and di-cations are in most cases those predicted by AM1 to be the most stable. Correlation between the magnitude of Δδ13C and AM1 calculated changes in carbon changes [Δq = qc (ion) – qc (neutral)] is also examined. The charge alternation paths in the mono- and di-cations are compared and discussed. A parallel is drawn between stable ion and AM1 studies of methylphenanthrenes and the solvolytic studies of K-region and non-K-region phenanthrene oxides.