Unraveling the Impact of Fluorine Positioning in Crystal Packing: Halogen-Bonded Cocrystals of 4-Styrylpyridines with Pentafluoroiodobenzene

This study investigates how the position of fluorine substitution on 4-styrylpyridine (4SPY) controls the crystal packing of its halogen-bonded cocrystals with pentafluoroiodobenzene (FIB). Three fluorinated derivatives, 2′-fluoro- (2F-4SPY), 3′-fluoro- (3F-4SPY), and 4′-fluoro- (4F-4SPY) along with the parent 4SPY as a halogen bond acceptor were combined with FIB, a halogen bond donor, to examine substituent effects on supramolecular assembly. Single-crystal X-ray structural analyses revealed that the position of fluorine substitution critically governs supramolecular assembly: [(4SPY)(FIB)] (1), [(2F-4SPY)(FIB)] (2), and [(4F-4SPY)(FIB)] (4) form an isostructural one-dimensional zigzag arrangement, whereas [(3F-4SPY)(FIB)] (3) adopts a distinct linear arrangement. Packing similarity analysis (CrystalCMP and unit cell similarity index) confirmed these relationships, highlighting the structural divergence of 3. Importantly, all four cocrystals undergo reversible melt recrystallization, a solvent-free route that not only enables clean crystallization but also demonstrates their phase stability. Together, these results highlight the significant influence of substituent positioning on supramolecular organization and offer insights into the structure–property relationships of halogen-bonded cocrystals.