Reactivity of organic σ,σ,σ,σ,σ-pentaradicals

Abstract

The gas-phase reactivity of the first known σ-type carbon- and nitrogen-centered aromatic pentaradical cations toward cyclohexane was studied using a dual-linear quadrupole ion trap tandem mass spectrometer and compared to that of analogous mono-, bi-, tri- and tetraradicals. The major reaction of all radicals containing an ortho-benzyne moiety, including both pentaradicals, is abstraction of a hydride followed by abstraction of a proton from one cyclohexane molecule, generating a product complex of cyclohexene and a mono-, bi- or triradical (for the tri-, tetra- and pentaradical reactants, respectively) or a closed-shell aromatic molecule (for the biradical reactant). These nonradical reactions are calculated to be highly exothermic (by at least 58 kcal mol−1). After the ortho-benzyne moiety has been quenched via hydride and proton abstraction, radical reactions take place in the product complexes. The most prominent reactions involve [1] abstraction of one, two or three hydrogen atoms, respectively, from cyclohexene and [2] addition to cyclohexene, sometimes followed by the loss of a hydrogen atom or more extensive fragmentation. These radical reactions are fast if one or three radical sites are present (e.g., for the tri- and pentaradical reactants) but slow if two are present (e.g., for the tetraradical reactants) due to reactivity-lowering spin-spin coupling between the radical sites (even when the spin-spin coupling is very weak). Overall, the tetra- and pentaradicals display substantially lower reactivity than the mono- and triradicals, which is likely due to reactivity-lowering spin-spin coupling between all, or most, of the radical sites. Spin-spin coupling overrides the effects of a greater (calculated) vertical electron affinity of the radical sites of the tetra- and pentaradicals, which was expected to make them more reactive than the mono- and biradicals.