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A partial charge transfer between iodide ions and quinoid rings

A series of cocrystals of tetrachloro- and tetrabromoquinone with organic iodide salts has been prepared and structurally characterised. Derivatives N-methylpyridinium were chosen as cations, due to their planarity and similar size to the quinones, while their electronic properties are radically different. While the iodide usually acts as an electron donor, reducing the quinone into the semiquinone radical, sometimes the co-crystals of the neutral quinone and the iodide appear. In the studied crystals, the common motive, or supramolecular synthon, has been identified: a sandwich-like I-∙∙∙quinone∙∙∙I- moiety with close contacts between the iodide anion and carbon skeleton of the quinoid ring. Distances between the iodide and the ring centroid range between 3.6 and 3.85 Å, which is slightly shorter than sum of van der Waals radii for C and I. Interactions between π system aromatic ring and an anion are rarely observed, since aromatics are usually electron-rich ; in the case of electron-depleted quinoid rings, anion∙∙∙π contacts are more likely to form. In the studied cases, however, there is also a partial electron transfer between the iodide and the quinone which is noted by a change of colour: the neutral quinone is yellow, while the co-crystals and the semiquinone crystals are black.

cocrystals ; iodide salts ; quinoid ring

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