Design, synthesis and isolation of a new 1,2,5-selenadiazolidyl and structural and magnetic characterization of its alkali-metal salts

5,6-Dicyano[1,2,5]selenadiazolo[3,4-b]pyrazine 1 was synthesized and electrochemically and chemically reduced into its radical anion [1]˙−, which was characterized by EPR spectroscopy and DFT calculations, and isolated in the form of homospin salts [K(18-crown-6)]+[1]˙− (2) and [Na(18-crown-6)]+[1]˙− (3). The crystal structure of 2 revealed π-dimers of [1]˙− featuring a shortened interplanar separation of 3.18 A, and that of 3 showed chains of alternating cations and anions. DFT and CASSCF calculations on the π-dimers of [1]˙− suggest a closed-shell singlet ground state with a noticeable contribution of diradical character. According to EPR and SQUID magnetometry, salt 2 is diamagnetic in the range of 2–300 K, whereas salt 3 is paramagnetic and reveals weak antiferromagnetic exchange interactions.