Chiral π-Conjugated Double Helical Aminyl Diradical with the Triplet Ground State.

We report a neutral high-spin diradical of chiral C 2 -symmetric bis[5]diazahelicene with Δ E ST ≈ 0.4 kcal mol -1 , as determined by EPR spectroscopy/SQUID magnetometry. The diradical is the most persistent among all high-spin aminyl radicals reported to date by a factor of 20, with a half-life of up to 6 days in 2-MeTHF at room temperature. Its triplet ground state and excellent persistence may be associated with the unique spin density distribution within the dihydrophenazine moiety, which characterizes two effective 3-electron C-N bonds analogous to the N-O bond of a nitroxide radical. The enantiomerically enriched (ee ≥ 94%) ( MM )- and ( PP )-enantiomers of the precursors to the diradicals are obtained by either preparative chiral supercritical fluid chromatography or resolution via functionalization with the chiral auxiliary of the C 2 -symmetric racemic tetraamine. The barrier for the racemization of the solid tetraamine is Δ G ‡ = 43 ± 0.01 kcal mol -1 in the 483-523 K range. The experimentally estimated lower limit of the barrier for the racemization of a diradical, Δ G ‡ ≥ 26 kcal mol -1 in 2-MeTHF at 293 K, is comparable to the DFT-determined barrier of Δ G ‡ = 31 kcal mol -1 in the gas phase at 298 K. While the enantiomerically pure tetraamine displays strong chiroptical properties, with anisotropy factor | g | = |Δε|/ε = 0.036 at 376 nm, | g | ≈ 0.005 at 548 nm of the high-spin diradical is comparable to that recently reported triplet ground-state diradical dication. Notably, the radical anion intermediate in the generation of diradical exhibits a large SOMO-HOMO inversion, SHI = 35 kcal mol -1 .