Steric Modulations in the Reversible Dimerizations of Phenalenyl Radicals via Unusually Weak Carbon-Centered π- and σ-Bonds

Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either π- or
σ-dimers, depending on alkyl-substitution patterns at the α- and β-positions. Thus, the sterically encumbered
all-β-substituted tri-tert-butylphenalenyl radical (2•) affords only the long-bonded π-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1•) leads to only the σ-dimer.
Further encumbrances of 1• with a pair of α, β- or β, β- tert-butyl substituents and additional methyl and
ethyl groups (as in sterically hindered phenalenyl radicals 3• − 6•) do not inhibit σ-dimerization. ESR
spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers;
and UV−vis spectroscopy specifically identifies the π-dimer by its intense near-IR band. The different
temperature-dependent spectral (ESR and UV−vis) behaviors of these phenalenyl radicals allow the
quantitative evaluation of the bond enthalpy of 12 ± 2 kcal mol-1 for σ-dimers, in which the unusually
low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) π-resonance
energy attendant upon such bond formation.