Covalency of the Strong Br···N Halogen Bonds in Neutral and Ionic Complexes

Halogen bonding (HaB) between bromine and nitrogen was investigated in a series of quinuclidine (QN) complexes with various bromo-substituted electrophiles and in bromine(I) species QN-Br-QN⁺ and Sac-Br-Sac^- (Sac = saccharin residue). X-ray crystallography and density functional theory (M0-2X/def2-TZVPP) calculations reveal that the Br···N interactions span a continuum from weak supramolecular contacts to covalent bonds, depending on the HaB donor strength. In complexes with the strongest HaB donors such as Br₂, the Br-N bond lengths are only 10-15% longer than fully developed covalent bonds. Electron densities of about 0.1 au and negative energy densities at these bond critical points, as well as values of electron localization function (ELF) and Mayer bond orders (MBOs) of ∼0.5-0.6 point to covalency of this bonding. Also, these strong Br-N bonds significantly weaken adjacent Br-X covalent bonds of HaB donors. Electron and energy densities at bond critical points, ELF and MBOs values of both Br-N and Br-X bonds in a series of complexes indicate that the transition toward partial covalency occurs at the Br-N distances about halfway between van der Waals contacts and covalent bonds. Energy decomposition analysis suggests that this transition is primarily driven by the increasing orbital (charge-transfer) contribution in the strong complexes. Overall, this study demonstrates that strong Br···N halogen bonds exhibit partial covalent character comparable to that in cationic and anionic bromine(I) complexes, providing insight into the continuum between supramolecular and covalent bonding.