[O─I─O]⁺ Halogen-Bonded Complexes of Pyridine N-Oxides

Eight [O─I─O]⁺ halogen-bonded complexes derived from pyridine N-oxides have been characterized by single-crystal x-ray diffraction analysis. In these 3-center-4-electron (3c4e) halogen-bonded systems, iodine(I), in its simplest halogen bond (XB) donor form, functions as a bidentate electron acceptor, symmetrically engaging two monodentate N-oxide O-atoms. The I⁺─O bond lengths of 2.172(17)-2.227(6) Å represent the shortest XBs reported for N-oxide-based systems. Solution complexation was studied by ¹H,¹⁵N HMBC NMR spectroscopy. With the crystallographic verification of the previously elusive [O─I─O]⁺ motif, Density Functional Theory calculations were performed to compare the electronic and bonding characteristics of [N─I─N]⁺, [O─I─O]⁺, and [S─I─S]⁺ systems. The computations reveal a distinct behaviour for the [O─I─O]⁺ complexes. Unlike nitrogen and sulfur counterparts, the N-oxide O-atom in [O─I─O]⁺ complexes retain an active electron residual viz. second lone pair, rendering these complexes susceptible to secondary interactions such as hydrogen bonds. This distinctive bonding environment establishes [O─I─O]⁺ complexes as a unique class within 3c4e halogen-bonded systems and highlights their potential utility in the rational design of organic reactions and supramolecular materials.