The C–I···^–O–N⁺ Halogen Bonds with Tetraiodoethylene and Aromatic N‑Oxides

The nature of C–I···^–O–N⁺ interactions, the first of its kind, between nonfluorinated tetraiodoethylene halogen bond (XB) donor and pyridine N-oxides (PyNO) are studied by single-crystal X-ray diffraction and density functional theory (DFT) calculations. Despite the nonfluorinated nature of the C₂I₄, the I···O halogen bond distances are similar to well-known perfluorohaloalkane/-arene donor-PyNO analogues. With C₂I₄, oxygens of the N-oxides adopt exclusively μ₂-XB coordination in contrast to the versatile bonding modes observed with perfluorinated XB donors. The C₂I₄ as the XB donor forms with PyNO’s one-dimensional chain polymer structures in which the C₂I₄···(μ-PyNO)₂···C₂I₄ segments manifest two bonding motifs, namely, side-by-side (vicinal di-iodo) and head-to-head (geminal di-iodo), due to the nearly symmetric square-planar structure of the C₂I₄. While the attractive nature between I and O atoms is mainly electrostatic, the narrow range of C···O bond parameters demonstrates that the π-bond between four iodine atoms also plays an important role in enhancing the σ-hole strength. DFT-Based monodentate XB interaction energies, ΔE_int, in 13 1:1 XB complexes vary between 31.9 and 46.5 kJ mol^–1, the strongest remarkably exceeding the value reported for I–I···^–O–N⁺ = 42.0 kJ mol^–1. In the case of C₂I₄·(pyridine N-oxide) [31.9 kJ mol^–1], the monodentate XB energy is on a par with perfluorinated donor complexes, namely, CF₃I·(pyridine N-oxide) [31.1 kJ mol^–1] and C₆F₅I·(pyridine N-oxide) [32.3 kJ mol^–1].