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

Abstract

We report a neutral high-spin diradical of chiral C 2 -symmetric bis [5]diazahelicene with ΔE 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 ΔG ‡ = 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, ΔG ‡ ≥ 26 kcal mol −1 in 2-MeTHF at 293 K, is comparable to the DFTdetermined barrier of ΔG ‡ = 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 .

A Stable Aluminum Tris(dithiolene) Triradical

“…Significant advances in the chemistry of stable organic diradicals and polyradicals, usually involving carbon, nitrogen, and oxygen elements, have been reported. − Organic di- or polyradicals with high-spin ground states and large energy gaps (Δ E ) between the high-spin ground state and low-spin excited state are intriguing due to their remarkable application potentials in magnetic materials. , Notably, main group element-based polyradicals beyond carbon, nitrogen, and oxygen are scarce. − Since the radical character of the dithiolene ligand in transition-metal complexes was first proposed by Gray et al in the early 1960s, , transition-metal dithiolene complexes with radical dithiolene ligands have received significant attention . Typically, tris­(dithiolene) metal complexes involve three dianionic dithiolate ligands (L 2– ).…”

peri‐Benzo‐Diindenotetracenyl: Helically π‐Extended Allyl Radical with Robust Stability

“…[11] It should also be noted that stable helical and doubly helical diradicals have been reported recently. [13,14] These radicals, however, often suffer from instability under ambient conditions, and kinetic protection on the periphery has been indispensable for stabilization. Therefore, general and reliable molecular design strategies for constructing stable helical π-radicals are demanded.…”