Cyclic networks of halogen-bonding interactions in molecular self-assemblies: a theoretical N—X...N versus C—X...N investigation

The geometries and energetics of molecular self-assembly structures that contain a sequential network of cyclic halogen-bonding interactions are investigated theoretically. The strength of the halogen-bonding interactions is assessed by examining binding energies, electron charge transfer (NBO analysis) and electron density at halogen-bond critical points (AIM theory). Specifically, structural motifs having intramolecular N—X...N (X = Cl, Br, or I) interactions and the ability to drive molecular self-assembly via the same type of interactions are used to construct larger self-assemblies of up to three unit motifs. N—X...N halogen-bond cooperativity as a function of the self-assembly size, and the nature of the halogen atom is also examined. The cyclic network of the halogen-bonding interactions provides a suitable cavity rich in electron density (from the halogen atom lone pairs not involved in the halogen bonds) that can potentially bind an electron-deficient species such as a metal ion. This possibility is explored by examining the ability of the N—X...N network to bind Na⁺. Likewise, molecular self-assembly structures driven by the weaker C—X...N halogen-bonding interactions are investigated and the results compared with those of their N—X...N counterparts.