Inherently Unstable Carbonyl Hypoiodites as Extremely Strong Neutral Halogen Bond Donors

Abstract

A series of neutral halogen-bonded O–I–N complexes were prepared from in-situ formed carbonyl hypoiodites and aromatic organic bases (pyridine, DMAP, 4-morpholinopyridine and 4-piperidinopyridine). The carbonyl hypoiodites exhibit a very strongly polarized iodine atom with larger s -holes than any known uncharged halogen bond donor. The hypoiodite synthesized from trifluoroacetic acid has a s -hole of +234.55 kJ/mol, which surpasses the previously strongest XB donor N -iodosaccharin that had a value of +226.36 kJ/mol. Modulating the Lewis basicity of the selected pyridine derivatives and carboxylates leads to halogen-bonded complexes where the classical O‒I···N halogen bond transforms more into a halogen-bonded COO – ···I – N + ion-pair (salt) with a strongly asymmetric O – I – N moiety. Detailed single crystal X-ray analyses, 1 H- 15 N HMBC NMR studies, and DFT calculations reveal the halogen bonding geometries of the carbonyl hypoiodite-based O – I – N complexes, confirming that in the solid-state the iodine atom is located much closer to the N-atom of the pyridine derivatives than its original position at the carboxylate O-atom.