Spin–Spin Coupling Between Two meta‐Benzyne Moieties In a Quinolinium Tetraradical Cation Increases Their Reactivities

Abstract

The reactivity of a carbon‐centered σ,σ,σ,σ‐type singlet‐ground‐state tetraradical containing two meta‐benzyne moieties was examined in the gas phase. Surprisingly, the tetraradical showed higher reactivity than its individual meta‐benzyne counterparts. The reactivity of meta‐benzynes is controlled by their (calculated) distortion energy ΔE2.3, singlet–triplet spitting ΔES–T, and electron affinity (EA2.3) of the meta‐benzyne moiety at the transition state geometry for hydrogen‐atom abstraction reactions. The addition of a second meta‐benzyne moiety to a meta‐benzyne does not significantly change EA2.3. However, ΔE2.3 is substantially decreased for both meta‐benzyne moieties in the tetraradical, and this explains their higher reactivities. The decrease in ΔE2.3 for each meta‐benzyne moiety in the tetraradical is rationalized by stabilizing spin–spin coupling between one radical site in each meta‐benzyne moiety. Therefore, spin–spin coupling between the meta‐benzyne moieties in this tetraradical increases its reactivity, whereas spin–spin coupling within each meta‐benzyne moiety decreases its reactivity.

Spin‐Spin Coupling Controls the Gas‐Phase Reactivity of Aromatic σ‐Type Triradicals

“…[26,27,[30][31][32] An additional reactivity-controlling factor that only applies to meta-benzyne analogs is the distortion energy (ΔE 2.3 Å) or the energy required for the biradical to reach the dehydrocarbon atom separation (DAS) that they have in their transition states of radical reactions. [26,33,34] This DAS is approximately 2.3 Å for H atom abstraction reactions. [26,33,34] Related tri-and tetraradicals, with three or four electrons in nonbonding molecular orbitals that are similar in energy, contain several low-lying electronic states, which suggests that they might have interesting chemical properties.…”

“…[26,33,34] This DAS is approximately 2.3 Å for H atom abstraction reactions. [26,33,34] Related tri-and tetraradicals, with three or four electrons in nonbonding molecular orbitals that are similar in energy, contain several low-lying electronic states, which suggests that they might have interesting chemical properties. [33,35,36] However, while the reactivity of aromatic σ-type mono-and biradicals has been fairly thoroughly studied, [24,26,27,30,34] this is not true for the related tri-and tetraradicals.…”

“…[26,33,34] Related tri-and tetraradicals, with three or four electrons in nonbonding molecular orbitals that are similar in energy, contain several low-lying electronic states, which suggests that they might have interesting chemical properties. [33,35,36] However, while the reactivity of aromatic σ-type mono-and biradicals has been fairly thoroughly studied, [24,26,27,30,34] this is not true for the related tri-and tetraradicals. [33,[35][36][37][38][39][40][41] The bestunderstood systems are based on the pyridinium skeleton and therefore contain three or four strongly coupled unpaired electrons.…”

“…[33,35,36] However, while the reactivity of aromatic σ-type mono-and biradicals has been fairly thoroughly studied, [24,26,27,30,34] this is not true for the related tri-and tetraradicals. [33,[35][36][37][38][39][40][41] The bestunderstood systems are based on the pyridinium skeleton and therefore contain three or four strongly coupled unpaired electrons. Examination of the reactivity of the 2,4,6-tridehydropyridinium cation revealed that this species is best described as a reactive monoradical with a substantially less reactive, strongly coupled biradical moiety (meta-pyridyne).…”

“…Examination of the reactivity of the 4,5,8tri-and 2,4,6,8-tetradehydroquinolinium cations revealed, surprisingly, that the relatively strong spin-spin coupling between the radical sites at C-4 and C-8 weakens the coupling between the other radical sites (i. e., C-5 and C-8 for the triradical and C-2 and C-4 as well as C-6 and C-8 for the tetraradical), which results in greater than expected radical reactivity for these other radical sites. [33,41] In order to better understand this observation and further explore reactivity trends in quinolinium based, weakly coupled triradical systems, the gas-phase reactions of cyclohexane with six isomeric quinolinium-based triradicals (Figure 1) were examined. The triradicals contain a parabenzyne or a meta-pyridyne moiety and an additional radical site that is more weakly coupled to the benzyne/pyridyne moiety than the benzyne/pyridyne radical sites to each other, with one exception (6).…”