Diamagnetic π-dimers of the [1,2,5]thiadiazolo[3,4-c][1,2,5] thiadiazolidyl radical anion in the crystalline state: Preparation and X-ray crystal structure of a [(Me2N)2CC(NMe2) 2]2+[(C2N4S2) 2]2- salt

Abstract

Chemical reduction of [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (2) with 1,1,2,2-tetrakis(dimethylamino)ethene (3) leads to the [3]2+[1 2]2- salt (4) whose X-ray crystal structure reveals diamagnetic π-dimers of the [1,2,5]thiadiazolo[3,4-c][1,2,5]-thiadiazolidyl radical anion (1). The interplanar separation of 324.5 pm in the [1 2]2- dimers is significantly shorter than the sum of the isotropic van der Waals radii of two S atoms of 348-358 pm. CASSCF calculations suggest that the largest contributor to the singlet ground state of these π-dimers is the closed-shell electronic configuration, although a noticeable contribution from a configuration with a singlet biradical character was also found. © Wiley-VCH Verlag GmbH & Co. KGaA, 2008.