The C–I･･･⁻O–N⁺ Halogen Bonds with Tetraiodoethylene and Aromatic N-oxides

Abstract

The nature of C–I⋯⁻O–N⁺ interactions, first of its kind, between non-fluorinated tetraiodoethylene XB-donor and pyridine N-oxides (PyNO) are studied by single-crystal X-ray diffraction (SCXRD) and Density Functional Theory (DFT) calculations. Despite the non-fluorinated 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 2-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 manifesting 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 demonstrate that the -bond between four iodine atoms also plays an important role in enhancing the -hole strength. DFT-based monodentate XB interaction energies, ΔEint, in thirteen 1:1 XB complexes vary between 31.9 – 46.5 kJ mol–1, the strongest remarkably exceeding the value reported for I–I⋯⁻O–N⁺ = 42.0 kJ mol–1. In 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].