Quinoxaline-1,2,3-dithiazolyls — Synthesis, EPR characterization, and redox chemistry

Oxidation of quinoxalineaminothiol with SCl₂ or S₂Cl₂/Cl₂ affords a series of compounds based on the quinoxaline-1,2,3-dithiazole framework QDTA. Under highly oxidizing conditions, the 1,2,3-dithiazolyl ring is opened to afford the acyclic dichlorosulfimino-sulfenyl chlorides Cl_x-QDTA-Cl₃ (x = 0, 1, 2). Reduction of these "trichloro" compounds leads to ring closure. For x = 2, reduction using S₂Cl₂ affords the dithiazolylium chloride [Cl₂-QDTA][Cl]. For all values of x, reduction with iodide ion (3 mol equiv) affords the corresponding dithiazolyl radical [Cl_x-QDTA]. The radicals can be isolated in good yield in crude form, but attempts to purify them by vacuum sublimation lead to thermal degradation. The radicals have nonetheless been fully characterized by EPR spectroscopy, and the assignments of the observed hyperfine coupling constants cross-matched with those obtained by computation at the B3LYP/6-31G** level. The structures of the trichloro compounds Cl_x-QDTA-Cl₃ (x = 1, 2) have been confirmed by X-ray crystallography. Crystal data: Cl-QDTA-Cl₃, monoclinic, space group C2/c, a = 30.561(5) Å, b = 4.9764(9) Å, c = 22.247(4) Å, β = 131.822(14)°, V = 2521.4(8) ų, Z = 8, R(F) = 0.043, and R_w(F) [I [Formula: see text] σ (I)] = 0.049; Cl₂-QDTA-Cl₃, orthorhombic, space group Pnma, a = 18.627(12) Å, b = 6.848(4) Å, c = 10.926(7) Å, V = 1393.7(15) ų, Z = 4, R(F) = 0.047, and R_w(F) [I [Formula: see text] 3σ(I)] = 0.060.Key words: thiazyl radicals, molecular conductors, EPR spectroscopy, quinoxaline, DFT calculations.