Novel co-crystals with π-hole interactions between iodide anions and quinoid rings involving charge transfer

Six novel co-crystals of tetrabromoquinone with iodide salts of organic cations displaying short contacts between iodide anions and the quinoid rings have been structurally characterised. Sandwich-like groups I-MIDLINE HORIZONTAL ELLIPSISquinoneMIDLINE HORIZONTAL ELLIPSISI- are present in all of them; competition of this motif with halogen bonding has been described for the first time. The black colour of the crystals indicates charge transfer from the iodide to the quinoid ring, which is interpreted as n -> pi*. Intermolecular interactions were analysed using Hirshfeld surfaces and Voronoi-Dirichlet polyhedra.