[O─I─O] ⁺ Halogen‐Bonded Complexes of Pyridine <i>N</i> ‐Oxides

ABSTRACT 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 (3 c 4 e ) 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 1 H, 15 N 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 3 c 4 e halogen‐bonded systems and highlights their potential utility in the rational design of organic reactions and supramolecular materials.