Non-Innocent Base Properties of 3- and 4‑Pyridyl-dithia- and Diselenadiazolyl Radicals: The Effect of N‑Methylation

Condensation of persilylated nicotinimideamide and isonicotinimideamide with sulfur monochloride affords double salts of the 3-, 4-pyridyl-substituted 1,2,3,5-dithiadiazolylium DTDA cations of the general formula [3-, 4-pyDTDA]­[Cl]­[HCl] in which the pyridyl nitrogen serves as a noninnocent base. Reduction of these salts with triphenylantimony followed by deprotonation of the intermediate-protonated radical affords the free base radicals [3-, 4-pyDTDA], the crystal structures of which, along with those of their diselenadiazolyl analogues [3-, 4-pyDSDA], have been characterized by powder or single-crystal X-ray diffraction. The crystal structures consist of “pancake” π-dimers linked head-to-tail into ribbonlike arrays by η²-S₂---N­(py) intermolecular secondary bonding interactions. Methylation of the persilylated (iso)­nicotinimide-amides prior to condensation with sulfur monochloride leads to N-methylated double chloride salts Me­[3-, 4-pyDTDA]­[Cl]₂, which can be converted by metathesis into the corresponding triflates Me­[3-, 4-pyDTDA]­[OTf]₂ and then reduced to the N-methylated radical triflates Me­[3-, 4-pyDTDA]­[OTf]. The crystal structures of both the N-methylated double triflate and radical triflate salts have been determined by single-crystal X-ray diffraction. The latter consist of trans-cofacial π-dimers strongly ion-paired with triflate anions. Variable temperature magnetic susceptibility measurements on both the neutral and radical ion dimers indicate that they are diamagnetic over the temperature range 2–300 K.