posted on 2019-08-01, 15:11authored byWilliam Borley, Brandon Watson, Yakov P. Nizhnik, Matthias Zeller, Sergiy V. Rosokha
Halogen bonding (XB)
in complexes of diiodine with heteroaromatic N-oxides
was examined via a combination of UV–vis
spectral and X-ray structural measurements, as well as computational
analysis. While all of these associates were formed by analogous I···O
bonds, they showed considerable variations of formation constants
(5–1500 M–1) and intermolecular I···O
bond length (2.3–3.2 Å). In the solid state, both atoms
of I2 molecules were involved in XB, and the I···O
separations were determined by the electron-donor abilities of N-oxides and the strength of the bonding on the opposite
side of the ditopic XB donor. The solution-phase formation constants
of 1:1 complexes, K, as well as magnitudes of the
calculated interaction energies, ΔE, increased
with the shift of the values of the most negative potentials on the
surfaces of N-oxides’ oxygen atoms, Vmin, toward more negative values. Yet, the interatomic
contacts consistently deviated from the locations of Vmin. Instead, the structures of complexes were well suited
for highest occupied molecular orbital/lowest unoccupied molecular
orbital interactions of reactants. The values of K, ΔE, and the intermolecular distances dI···O in the calculated complexes
were highly correlated with the charge-transfer interaction energies
derived from the natural bond orbital analysis. This indicated that,
besides electrostatic, molecular orbital interactions play a substantial
role in XB between diiodine and N-oxides. This conclusion
was supported by the analysis of the complexes using the quantum theory
of atoms in molecules, noncovalent interaction index, and density
overlap region indicator, which showed that the covalent character
of I···O bonding increases with the rise of interaction
energies in the complexes.