Correlation between 13C chemical shifts and the halogen bonding environment in a series of solid para-diiodotetrafluorobenzene complexes

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 ¹³C 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 ¹³C isotropic chemical shift. When the carbon–iodine bond length increases, the ¹³C chemical shift also increases. Further insights into the relationship between halogen bonding and the ¹³C chemical shifts are obtained through additional systematic ZORA–DFT calculations as a function of the halogen bonding environment.