¹³C and ¹⁹F solid-state NMR and X-ray crystallographic study of halogen-bonded frameworks featuring...

Halogen bonding is a noncovalent interaction between the electrophilic region of a halogen (σ-hole) and an electron donor. We report a crystallographic and structural analysis of halogen-bonded compounds by applying a combined X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) approach. Single-crystal XRD was first used to characterize the halogen-bonded cocrystals formed between two fluorinated halogen-bond donors (1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene) and several nitrogen-containing heterocycles (acridine, 1,10-phenanthroline, 2,3,5,6-tetramethylpyrazine, and hexamethylenetetramine). New structures are reported for the following three cocrystals, all in the P2₁/c space group: acridine-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C₆F₃I₃·C₁₃H₉N, 1,10-phenanthroline-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C₆F₃I₃·C₁₂H₈N₂, and 2,3,5,6-tetramethylpyrazine-1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C₆F₃I₃·C₈H₁₂N₂. ¹³C and ¹⁹F solid-state magic-angle spinning (MAS) NMR is shown to be a convenient method to characterize the structural features of the halogen-bond donor and acceptor, with chemical shifts attributable to cocrystal formation observed in the spectra of both nuclides. Cross polarization (CP) from ¹⁹F to ¹³C results in improved spectral sensitivity in characterizing the perfluorinated halogen-bond donor when compared to conventional ¹H CP. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations of magnetic shielding constants, along with optimization of the XRD structures, provide a final set of structures in best agreement with the experimental ¹³C and ¹⁹F chemical shifts. Data for carbons bonded to iodine remain outliers due to well-known relativistic effects.