Directed Molecular Structure Variations of Three-Dimensional Halogen-Bonded Organic Frameworks (XBOFs)

In this study we have used a tetrahedral oligopyridine and four different fluoroiodides to obtain three-dimensional (3D) nanoporous halogen-bonded cocrystals. Many of the halogen-bonded cocrystals reported to date are one-dimensional chains or two-dimensional sheet-like structures; these new cocrystals possess multiple channels of 300-800 Å ³ volume per unit cell. The extended 3D channels can be designed by varying the molecular structure of the halogen bond donor and were found to occupy 20-40% of the unit cell volume. The N···I distances in our cocrystals are ∼80% of the sum of the van der Waals radii of the nitrogen and iodine atoms, and the N···I-C angles are nearly linear. Noncovalent stacking (π-π) interactions as well as H-bonding to solvents were also observed in some of the cocrystals. The supramolecular structures obtained in this study are effectively derived out of different donor-acceptor XB interactions, solvent and other noncovalent interactions. The weak nature of halogen bonds as well as the existence of multiple interactions make these cocrystal structures and their supramolecular organization difficult to predict. Even though this work does not attempt to single out the individual contributions of different factors affecting the supramolecular assemblies, we show here how the structure and hence the potential porosity of the halogen-bonded organic frameworks can be varied via careful design and combination of structurally different donors and acceptors.