Halogen bonding in 5-iodo-1-arylpyrazoles investigated in the solid state and predicted by solution 13C-NMR spectroscopy

Abstract

X-ray crystallography revealed the presence of halogen bonding in the crystal supramolecular structure of three highly substituted 1-arylpyrazoles. However the compounds 1–3 present different halogen bonding motifs that feature C–I⋯N (1), C–I⋯O (2) and C–I⋯π (3) contacts respectively. The magnitudes of the σ-hole corresponding to the iodine atom in the 5-iodo-1-arylpyrazoles 1–3 were calculated by DFT methods and the importance of halogen bonding as a significant stabilizing force within the crystal lattice was evaluated. The halogen bonding of 1-aryl-5-iodopyrazoles with several Lewis bases (Et₃N, pyridine, DABCO or DMSO) was investigated by ¹³C NMR spectroscopy in the solution phase to confirm the halogen bonding affinity of the iodine atom. The most suitable reporting atom for the formation of the halogen bond is C-5 of the pyrazole ring, which is directly bonded to the iodine atom. The C-5 atom is significantly deshielded by as much as 6–7 ppm upon interaction with the Lewis bases in solution revealing the strong halogen bonding character of the iodine atom attached to C-5 of the pyrazole ring.