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••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 in UNOMIV and from 30.1 to 33.9 kcal/mol in ZAQZOK.Natural Bond Orbital analysis confirms that charge transfer from the central iodine's lone pairs to the antibonding σ* orbitals of 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.