Cocrystallization of diiodine with a series of heteroaromatic Noxides produced three types of halogen-bonded associates: (a) alternating chains comprising I 2 molecules bridged by oxygen atoms of N-oxides of pyridine, quinoline, or 4-methylpyridine; (b) discrete 2:1 complexes, in which diiodine links a pair of acridine N-oxide molecules; and (c) amphoteric 1:1 adducts, in which one end of each diiodine molecule is halogen-bonded to Noxide of 4-methoxypyridine or 4-chloroquinoline, and another diiodine's end forms contacts with the other I 2 molecules. In all cases, halogen bonds between diiodine and N-oxides are characterized by nearly linear I−I•••O angles, close to perpendicular dihedral C−N−O•••I angles and N−O•••I angles of about 110°.The halogen bond length in the 1:1 adducts is about 0.3 Å shorter than those in the 2:1 associates and in the infinite chains. Computational analyses confirmed that the variations in I•••O separations are related predominantly to the distinct effects of halogen bond competition in different supramolecular associates. Experimental and computational data also indicated that coordination of the second diiodine to the same oxygen atom of N-oxide has a smaller effect on the halogen bond length and energy than coordination of the second N-oxide to another iodine atom in the I 2 molecule.

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.

Structural and spectral features of the novel supramolecular [DÁNXOÁBF 3 ] complexes formed via simultaneous n-coordination of a Lewis acid (BF 3 ) and p-complexation of organic donors (D) to polyfunctional nitrosubstituted N-oxide molecules (NXO = 4-nitropyridine-N-oxide or 4-nitroquinoline-N-oxide) are reported. X-Ray studies of [pyreneÁNPOÁBF 3 ]ÁCH 2 Cl 2 and [(pyrene) 2 ÁNQOÁBF 3 ] salts revealed that the Lewis acid is coordinated to the oxygen atom of the N-oxide group with O-B distance of B1.52 A ˚similar to that in the separate [NXOÁBF 3 ] adducts. Aromatic rings of N-oxide molecules in the ternary systems are p-stacked with pyrene moieties, and their interplanar separations of B3.35 A ˚are common for conventional charge-transfer complexes. In dichloromethane, associations between [NXOÁBF 3 ] adduct and organic donors are characterized by higher formation constants (and charge-transfer bands of [DÁNXOÁBF 3 ] complexes are red-shifted) as compared to the complexes between the same donors and NXO acceptors. Spectral data and electrochemical measurements point out enhanced acceptor abilities of [NXOÁBF 3 ] adducts relative to the separate N-oxides which correspond to B0.5 V positive shift of reduction potentials of acceptors. Synergetic oxidation of strong organic donors by [NXOÁBF 3 ] dyads (which results in the formation of the corresponding cation radicals and products of transformation of both BF 3 and NXO components) is discussed.

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