Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N ‐Oxide Complexes

A study of the strong N−X⋅⋅⋅ − O−N + (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X⋅⋅⋅O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X⋅⋅⋅O XBs. The XB interaction energies vary from −47.5 to −120.3 kJ mol −1 ; the strongest N−I⋅⋅⋅ − O−N + XBs approaching those of 3‐center‐4‐electron [N−I−N] + halogen‐bonded systems (ca. 160 kJ mol −1 ). 1 H NMR association constants (K XB ) determined in CDCl 3 and [D 6 ]acetone vary from 2.0×10 0 to >10 8   m −1 and correlate well with the calculated donor×acceptor complexation enthalpies found between −38.4 and −77.5 kJ mol −1 . In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R XB ) between 0.65–0.67 for the N−I⋅⋅⋅ − O−N + halogen bond.