The nature of π-hole interaction between iodide anion and quinoid ring in the crystalline state

Abstract The investigated co-crystal of 3-chloro-N-methylpyridinium iodide with tetrabromoquinone (3−Cl−N−MePy∙I∙Br4Q) reveals a π-hole interaction between an iodide anion and a quinoid ring involving a n→π* charge transfer. The quinoid ring has a partial negative charge (estimated to be in the range of 0.08 to 0.11 e) and a partial radical character, which is related to black colour of the crystals (the crystals of neutral tetrabromoquinone are yellow). A detailed X-ray charge density study revealed two symmetry-independent bonding critical points between the iodide and carbon atoms of the ring. Their maximum electron density of 0.065 e Å−3 was reproduced by quantum chemical modelling. Energy of the interaction is estimated to be −11.16 kcal mol−1, which is comparable to the strength of moderate hydrogen bonding (about −10 kcal mol−1); it is dominantly of electrostatic nature, with a considerable dispersion component.