Noncovalent chemical chameleons in action: positive cooperativity of trifurcated halogen bonds in 2I⋯I⋯Nu assemblies

Abstract

While multicenter halogen bonds typically exhibit negative cooperativity in donor-only systems, crystallographic surveys reveal robust hetero-atomic assemblies featuring the 2I⋯I⋯Nu motif (Nu = N, O, S, and C). Here we present a comprehensive theoretical investigation of this bonding pattern to evaluate its intrinsic electronic synergy. We initially performed a systematic density functional theory (DFT) analysis on fully optimized model systems involving diatomic iodine (I₂) and the iodine trimer ((I₂)₃) acting as σ-hole donors, interacting with a series of small, linear Lewis bases (HF, CO, HCN, OCN⁻, SCN⁻, and SeCN⁻). Our calculations on these optimized models demonstrate that the (I₂)₃ trimer functions as a significantly stronger σ-hole donor than the isolated I₂ molecule, confirming pronounced positive cooperativity. To validate these findings in the solid state, we extended the analysis to two representative crystal structures: a homotrimeric 2,4,5-triiodoimidazole assembly (UNOMIV) and a cocrystal of 1,3,5-triiodo-2,4,6-trifluorobenzene with 1,4-dithiane (ZAQZOK). Energy decomposition analysis and quantum theory of atoms in molecules results for these crystallographic systems mirror the trends observed in the model complexes. Specifically, the formation of flanking I⋯I contacts amplifies the σ-hole depth of the central iodine atom, increasing it from 21.9 to 24.8 kcal mol⁻¹ in UNOMIV and from 30.1 to 33.9 kcal mol⁻¹ in ZAQZOK. Natural bond orbital analysis confirms that charge transfer from the central iodine's lone pairs to the antibonding σ* orbitals of the flanking molecules drives this synergistic behavior. These findings validate the chemical chameleon property as a foundation for crystal engineering, establishing the 2I⋯I⋯Nu motif as a robust supramolecular synthon.