Abstract
The co-crystallization of para-diiodotetrafluorobenzene (p-DITFB) with ammonium and phosphonium halide (Cl- and Br-) salts afforded four new compounds, [(n-Bu4PCl)(p-DITFB)] (2), [(n-Bu4NBr)(p-DITFB)] (3), [(n-Bu4PBr)(p-DITFB)] (4), and [(EtPh3PBr)(2)(p-DITFB)] (5), that exhibit moderately strong halogen bonding interactions. They have been characterized by single-crystal X-ray diffraction and C-13 solid-state nuclear magnetic resonance (SSNMR) spectroscopy in magnetic fields of 9.4 and 21.1 T. The X-ray crystallography shows that in 2, 3, and 4, the halide is ditopic and forms long polymeric zigzag chains, whereas the bromide in 5 forms a dianionic species when involved in halogen bonding interactions. The NMR data, when combined with zeroth-order regular approximation density functional theory (ZORA-DFT) calculations, provide insight into the relationship between the strength of the halogen bond and the C-13 isotropic chemical shift. When the carbon-iodine bond length increases, the C-13 chemical shift also increases. Further insights into the relationship between halogen bonding and the C-13 chemical shifts are obtained through additional systematic ZORA-DFT calculations as a function of the halogen bonding environment.