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 IC₆F₄I, namely, 1,2,4,5-tetrafluoro-3,6-diiodobenzene bis(pyridin-1-ium-4-ylsulfanide), C₆F₄I₂·2C₅H₅NS (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, C₁₁H₁₂N₂S₂²⁺·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.