Cooperative Supramolecular Engineering: Dual-Mode Halogen and Hydrogen Bonding for Enhancement of Exchange Interactions in Nitronyl Nitroxide Systems

This work presents a novel cooperative supramolecular engineering strategy based on the simultaneous utilization of halogen bonding (I···N) and hydrogen bonding (H···N) interactions for the directed self-assembly of three structurally distinct nitronyl nitroxide radicals: 2-(4-iodophenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide ( 1 ), 2-(4-iodoethynylphenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide ( 2 ), and 2-(2,3,5,6-tetrafluoro-4-iodophenyl)-4,4,5,5-tetramethylimidazolin-1-oxyl-3-oxide ( 3 ) with 1,4-diazabicyclo[2.2.2]octane (DABCO). We synthesized and characterized cocrystals ( 1 – 3 )·DABCO containing these iodine-substituted nitronyl nitroxide radicals with varied electronic properties. The primary novelty lies in demonstrating that cooperative dual-mode noncovalent assembly significantly outperforms single-interaction approaches, achieving quantitative enhancement of magnetic exchange interactions by nearly two orders of magnitude from approximately 0 K for unassociated radicals to −78 K for supramolecular assemblies. The 3 ·DABCO system approaches the literature benchmark for purely organic nitronyl nitroxide materials, representing a substantial advancement in metal-free magnetic coupling strength. Comprehensive theoretical analysis using DFT, energy decomposition analysis, natural bond orbital analysis, and quantum theory of atoms in molecules elucidated the mechanistic basis for cooperative enhancement, revealing orthogonal energetic profiles where halogen bonds exhibit predominantly electrostatic character with significant orbital contributions, while hydrogen bonds show dispersive dominance with minimal orbital involvement. This complementary nature enables additive stabilization without competitive interference between interaction modes. The methodology addresses inherent limitations of single-interaction approaches, providing enhanced predictability and tunability compared with serendipitous discoveries.