Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered pi- and sigma-bonds

摘要

Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or a-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2(center dot)) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1(center dot)) leads to only the sigma-dimer. Further encumbrances of V with a pair of alpha, beta- or beta, beta-tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3(center dot) - 6 center dot) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-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 alpha-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.