[1,2,5]Selenadiazolo[3,4-c][1,2,5]thiadiazole and [1,2,5]Selenadiazolo[3,4-c][1,2,5jthiadiazolidyl -: A synthetic, structural, and theoretical study

A new heterocyclic system, namely [1,2,5]selenadiazolo[3,4c] [1,2,5]thiadiazole (1), has been prepared in 60 % yield by the 1:1 condensation of 3,4-diamino-1,2,5-thiadiazole (4) with SeCl4 in the presence of pyridine. The structures of 1 and its 1:1 complex with pyridinium chloride (5) have been elucidated by X-ray diffraction (XRD). The reversible electrochemical reduction of 1, as well as its chemical reduction with PhS- anion, provide the long-lived [1,2,5]selenadiazolo[3,4-c][1,2,5]thiadiazolidyl radical anion (2), which has been isolated in the form of the thermally stable salt [K(18crown-6)1[2] (3) in 75% yield. The experimental structural and spectral characteristics of I and 2 agree fairly well with those obtained from density functional (DFT) calculations. The salt 3 has been characterized by ESR spectroscopy (in the solid state and in solution) and XRD. The radical anion 2 acts as a bridging ligand in crystalline 3. Magnetic measurements on salt 3 reveal weak antiferromagnetic interactions (J = -1.65 cm(-1)). The magnetic structures of 3 and its disulfur congener 11 have been analyzed in terms of dimeric exchange integrals calculated by spin-unrestricted broken-symmetry DFT and post-HF methods. (0 (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)