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 CDCl₃ and [D₆ ]acetone vary from 2.0×10⁰ to >10⁸  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.