Dimers of delocalised radicals: Pentamethylcyclopentadienyl and pentadienyl

The thermal dissociation of bis(pentamethylcyclopentadienyl) to pentamethylcyclopentadienyl radicals has been studied by EPR spectroscopy in the temperature range 365-393 K. The enthalpy of dissociation of the dimer is found to be 18.8 kcal mol^-1 and combination of this result with empirical and semi-empirical estimates of the enthalpies of formation of the dimer and of pentamethylcyclopentadiene, leads to ΔH°_f (Me ₅C₅ ^.) = 10.7 kcal mol^-1, DH°(Me₅C₅-H) = 70.6 kcal mol^-1 and E _s ^MeK(Me₅C₅ ^.) = 34.2 kcal mol^-1 (where E_s ^MeH is the methane-based stabilisation energy). Steric strain is shown to be a major factor leading to weakening of the inter-ring bond of the dimer. The pentaethyl and pentamethoxycarbonyl analogues have been investigated, but neither is suitable for quantitative measurements. Thermal decomposition of trans,trans-deca-1,3,7, 9-tetraene and trans,trans-3,8-dimethyldeca-1,3,7,9-tetraene gives rise to pentadienyl and 3-methylpentadienyl radicals respectively, each of which isomerises to form a mixture of trans- and cis-radicals which then couple in both the 'end to end' and 'end to centre' modes to give mixtures of tetraenes. Hydrogen abstraction from 1,2-diphenylhydrazine is not competitive with pentadienyl radical combination in the temperature range 450-490 K. The EPR spectra of pentadienyl and 3-methylpentadienyl radicals have been observed in this temperature range.