Iodide···π Interactions of Perhalogenated Quinoid Rings in Co-crystals with Organic Bases

First anion···π contacts with quinoid rings have been described in novel co-crystals of tetrabromo- and tetrachloroquinone with iodide salts of substituted N-methylpyridinium cations. In seven crystal structures of these co-crystals, a centrosymmetric unit I–···quinone···I– is observed involving close contacts between iodide anions and electron-depleted carbon skeletons of the quinoid rings. However, the salt with N-methyl-4-methylcarboxypyridinium base crystallizes in two polymorphs characterized by OC···quinone···CO interaction instead of I–···quinone···I– one. A possible charge transfer, suggested by the black color of the crystals, is probed by solid-state NMR and IR spectroscopies and analyzed by DFT calculations.