A Model Halogen-Bonded Network as a Potential Tube-like Host for Li+: A DFT Study

The formation of a halogen-bonded network using four NHX-(CH2)3-NX-(CH2)3-NHX molecules (X = Cl, Br, or I) is investigated using DFT. The self-assembly of the four basic motifs results in a tube-like structure with C4h symmetry, with one halogen-bonded network located at each end of the structure and one at its center. Each halogen-bonded network has four quasi-planar N-X···N interactions with binding energies that increase with the size of X. The structure is found to bind Li+ at each of the halogen-bonded networks, albeit more strongly at its center. The binding of Li+ is driven by halogen atom lone pairs that produce a rich electron density orthogonal to the halogen bond. The presence and strength of the interactions are further examined using AIM and NBO calculations. Lastly, IRC calculations are performed to examine the transitions between the Li+ complex minima and, thus, the potential for transporting the metal ion from one end of the tube to the other. Based on the tetrameric structure, a model intramolecular structure is built and considered as a potential host for Li+. In this case, the central intermolecular N-X···N network is replaced by an intramolecular Si-C≡C-Si network. Interestingly, both intermolecular and intramolecular structures exhibit similar Li+ binding abilities.