Experimental and theoretical studies of fused-ring persistent [1,2,4]thiadiazinyl radicals

Five persistent radicals 1a-1e were generated by the oxidation of 4H-[1,2,4]thiadiazines 2a-2e and studied with ESR and UV-vis spectroscopy. Three of the radicals, 1a, 1d, and 1e, were generated in high yields (>90%) using either SO₂Cl₂/amine in toluene or AgO/K₂CO ₃ in a toluene/ MeCN mixture. Halogenated radicals 1d and 1e were sufficiently stable for chromatographic isolation and vacuum sublimation. The solution stability of the fluorinated 1d was measured at t_1/2 ≈ 4 months in the absence of oxygen, and 1e at t_1/2 ≈ 40 min in the presence of air. The crystal and molecular structures of 1d were determined by X-ray crystallography showing columns of parallel almost evenly spaced planar heterocycles connected by infinite ⋯N⋯S⋯ chains. Cyclic voltammetry of Id and 1e shows reversible reduction waves at about 0 V and irreversible oxidations at about 1.2 V. Spectral and electrochemical properties of 1 were well reproduced by DFT methods.