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

Abstract

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.