Dimers of delocalised radicals: pentamethylcyclopentadienyl and pentadienyl

Abstract

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(Me₅C₅˙)= 10.7 kcal mol ^–1, DH°(Me₅C₅–H)= 70.6 kcal mol^–1 and E_g^MeH(Me₅C₅˙)= 34.2 kcal mol^–1(where E_g^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.