When halogen bonding isn't enough: solvation behavior in ionic cocrystals of benzyltrimethylammonium halides and 1,4-diiodotetrafluorobenzene

Abstract

The role of halide anion identity and the influence of reaction solvent on the resulting halogen-bonded assembly was explored by combining 1,4-diiodo-tetrafluorobenzene (p-F₄DIB) with trimethylbenzyl ammonium halides (NMe₃BzX, X = Cl, Br, I) in diverse organic solvents. Iodide salts predominantly yielded solvated crystalline products when the salt cocrystallized in an equimolar ratio with p-F₄DIB. In solvent systems where the iodides did not crystallize as solvates, the salt:organoiodine ionic cocrystal ratio departed from the 1 : 1 reaction stoichiometry, producing 8 : 3, 4 : 5, or 2 : 3 cocrystals. In contrast, bromide and chloride analogues favored unsolvated forms, with chloride consistently producing a single 1 : 1 motif across multiple solvents. A small number of solvated forms were isolated in the Br and Cl series, typically at matched donor : acceptor ratios. Notably, chloride and bromide salts formed nearly indistinguishable halogen-bonded networks, apart from differences attributable to anion size. These results emphasize the delicate balance between solvent, stoichiometry, and halide identity in directing halogen-bond-driven crystallization.