Complexes of Diiodine with Heteroaromatic N-Oxides: Effects of Halogen-Bond Acceptors in Halogen Bonding

Abstract

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, V min, toward more negative values. Yet, the interatomic contacts consistently deviated from the locations of V min. 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 d I···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.

In Situ Assessment of Intrinsic Strength of X-I⋯OA-Type Halogen Bonds in Molecular Crystals with Periodic Local Vibrational Mode Theory

“…Figure 3 shows that all Cl-I covalent bonds are shorter than I-I bonds by at least 0.15 Å and the Cl-I bonds in crystals have generally larger local stretching force constant values than I-I bonds although they overlap in the range of 0.95-1.35 mdyn/Å. All X-I covalent bonds in crystal models are longer and weaker than their molecular counterparts in gas phase, which is a result of delocalization of lone pair electrons into the σ * (X-I) anti-bonding orbital [97] upon halogen bonding in crystals. Furthermore, we found that when the halogen bond acceptor is an ether group (G, H, J, K) the X-I bond is stronger than those when halogen bonds have N-oxides (L, M, N, O, P, Q) as acceptors.…”

“…with N-oxide acceptors via crystallography and theoretical calculations [96,97]. Their analysis based on molecular dimer models suggests that the charge transfer is a key factor in the I• • • O halogen bonding besides electrostatic attraction.…”

“…Within either type of halogen bond, the intrinsic strength is mostly larger when the acceptor is N-oxide (L, M, N, O, P, Q) than those halogen bonds with ether group (G, H, J, K) as acceptors. This can be explained in terms of the charge transfer character (i.e., orbital interaction) [97] that the oxygen atom in N-oxide group could provide more lone pair electrons to be delocalized into the σ * (X-I) anti-bonding orbital for halogen bonding than the oxygen in an ether group.…”

“…The acceptor molecules L through Q are heteroaromatic N-oxides, where the N + -O − bond is linked to an aromatic ring [97].…”

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

“…have shown using N‐iodosaccharin (NISac) and highly nucleophilic 4‐ N , N ‐dimethylaminopyridine (DMAP) as the XB acceptor, that the N−I bond of the N‐haloimide can be dissociated forming an iodo‐pyridinium cation and N‐saccharinate anion, that is, forming a salt . While strong XBs with N‐heterocycles are common, the development of (strong) halogen bonds using other Lewis bases for example, ( N ‐oxide) oxygen, has remained largely unexplored …”