Enhancing Effects of Electron-Withdrawing Groups and Metallic Ions on Halogen Bonding in the YC6F4X•••C2H8N2 (X = Cl, Br, I; Y = F, CN, NO2, LiNC+, NaNC+) Complex

Halogen-bonding interactions are highly directional intermolecular interactions that are often important in crystal engineering. In this work, the second-order Moller-Plesset perturbation theory (MP2) calculations and the quantum theory of "atoms in molecules" (QTAIM) and noncovalent interaction (NCI) studies were carried out on a series of X center dot center dot center dot N halogen bonds between substituted haloperfluoroarenes C6F4XY (X = Cl, Br, I; Y = F, CN, NO2) as bond donors and 1,2-diaminoethane as bond acceptor. Our research supports earlier work that electron-withdrawing substituents produce an enhancement effect on the size of the sigma-hole and the maximum positive electrostatic potentials (V-s,V-max), which further strengthens the halogen bonding. The metallic ion M+ (M+ = Li+, Na+) has the ability to enhance the size of both the sigma-hole and V-s,V-max value with the formation of [MNCC6F4X](+), resulting in more electronic charge transfer away from the halogen atom X and an increase in the strength of the halogen bond. It is found that the values of V-s,V-max at the sigma-holes are linear in relation to the halogen-bonded interaction energies and the halogen-bonding interaction distance, indicating that the electrostatic interaction plays a key role in the halogen-bonding interactions. The values of V-s,V-max a at the sigma-holes are also linear in relation to the electron density rho(b), its Laplacian del(2)rho(b), and -G(b)/V-b of XB, indicating that the topological properties (rho(b), del(2)rho(b)) and energy properties (G(b), V-b) at the BCPs are correlated with the electrostatic potentials.