Understanding the bifurcated halogen bonding N⋯Hal⋯N in bidentate diazaheterocyclic compounds

The nature of bifurcated halogen bonding N1⋯Hal2⋯N2 (Hal2 = Cl2, Br2, I2) in complexes with bidentate heterocycles has been studied through an analysis of the electron density and the electrostatic potential. It has been established, that the halogen atom can be specified as an electron acceptor for both the strong primary N1⋯Hal interaction and the weak secondary N2⋯Hal interaction in all the examined complexes. Consideration of two-atomic electrostatic and exchange contributions to the total energy, defined in terms of the Interacting Quantum Atom (IQA) method, reveals that the strengthening of the primary bonding with Cl2 is due to an increase in the exchange energy component. In contrast, the electrostatic contribution dominates both primary and secondary interactions in the complexes with I2. Considering series of bifurcated halogen bonds with two different electron donor centers, we have found simple quantitative correlations between the electronic kinetic and potential energy densities at the bond critical points and the IQA two-atomic exchange energy contribution. Relations linking the delocalization indices and two-atomic exchange energy contributions with the local halogen atom contribution from a source function to the N1⋯Hal2⋯N2 halogen bond critical points have also been established.