Isostructurality of quinoxaline crystal phases: the interplay of weak hydrogen bonds and halogen bonding

Tailoring the physical properties of molecular crystals though the construction of solid solutions requires the existence of isostructural crystals. Simple substitutions of a given molecular framework can give a range of different crystal structures. A set of quinoxaline derivatives, C8H4N2(C6H4X)(2), Q3,3 ' X-2, has been investigated (X = F, Cl, Br, I and Me) where kinetic factors generated a set of isostructural crystals for the lighter halogens (F, Cl, Br) alone. Computational analysis shows that the stabilising interactions are maximal for Cl, while DSC studies demonstrate the existence of more stable polymorphs for both F and Br containing systems. Steric factors appear to have a lower contribution than the balance of weaker hydrogen and halogen bonding shown by the Me and I containing systems displaying different packing driven by CHMIDLINE HORIZONTAL ELLIPSISN/CHMIDLINE HORIZONTAL ELLIPSIS pi bonds and IMIDLINE HORIZONTAL ELLIPSISI bonds respectively.