Pyridine-4-thiol as halogen-bond (HaB) acceptor: influence of the noncovalent interaction in its reactivity

The study of pyridine-4-thiol as a halogen-bond (HaB) acceptor has allowed the isolation of its cocrystal with the HaB donor IC 6 F 4 I, namely, 1,2,4,5-tetrafluoro-3,6-diiodobenzene bis(pyridin-1-ium-4-ylsulfanide), C 6 F 4 I 2 ·2C 5 H 5 NS ( 1 ), where the S atom is the HaB acceptor, while the pyridine position is blocked by the proton. Furthermore, the S atom acts a dual acceptor and also establishes an interaction with the pyridinium proton from an adjacent molecule. The presence of these interactions in 1 contributes to the stabilization of the zwitterionic form. This pre-organization seems to have an influence on the reactivity of the compound since when left in dichloromethane solution, an unusual activation of the C—Cl bond is observed that leads to the formation of the bis[(pyridin-1-ium-4-yl)sulfanyl]methane dication, while the Cl atoms are still present as chloride counter-ions, i.e. 4,4′-[methanediyldi(sulfanediyl)]dipyridinium dichloride, C 11 H 12 N 2 S 2 2+ ·2Cl − ( 2 ). In the crystal structure of 2 it is observed that the S atom is now acting as the donor part of a chalcogenide bond with the chloride anions.