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 Δδ_¹³C and AM1 calculated changes in carbon changes [Δq = q_c (ion) – q_c (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.