Halogen Bonding Interactions in Halopyridine-Iodine Monochloride Complexes

Halogen bond interactions are investigated in four halopyridine iodine monochloride complexes nXPy(N)-ICl in the solid state, solution and in silico, where nXPy = n-halopyridine, X = I or Br and n = 2 or 3. Two types of halogen bonding interactions were observed: stronger three-center Py(N)-ICl halogen bonds and weaker two-center Py(C)-X center dot center dot center dot Cl halogen bonds. Theoretical calculations (thermodynamic data and optimized bond distances) and solution NMR studies indicated that Py(N)-ICl halogen bonding interactions are stronger in 3XPy(N)-ICl complexes than in 2XPy(N)-ICl complexes. In the solid state, relying on bond distances, nBrPy(N)-ICl complexes show the same behavior to that in solution and in silico. In contrast, nIPy(N)-ICl halogen bonding interactions are stronger in 2IPy(N)-ICl than in 3IPy(N)-ICl. The ionic and covalent characters of I-Cl and N-I bonds were studied by the analysis of electron density topology using Quantum Theory of Atoms in Molecules (QTAIM), and Natural Population Analysis (NPA)/Natural Bond Orbital (NBO) analysis. According to the NBO analysis of the solid state structure, the ionic character of the I-Cl bond is higher in 2IPy(N)-ICl than in 3IPy(N)-ICl.