Halogen-Bonded Driven Tetra-Substituted Benzene Dimers and Trimers: Potential Hosts for Metal Ions

Cyclic dimers and trimers of tetra-substituted benzenes, ((HOOC)2-C6H2-(NHI)2), are selected as convenient model systems for investigating NI…O=C halogen bond strength and cooperativity. The four substituents in benzene are chosen so that two of them act as halogen bond acceptors (COOH) and two act as halogen bond donors (NHI), as shown in the graphical abstract below. The potential for metal ion binding by each of the halogen-bonded aggregates is also investigated using the monoatomic sodium ion, Na+. Density functional theory calculations performed using the wB97XD functional and the DGDZVP basis set confirmed the ability of halogen bonding to drive the formation of the cyclic dimers and trimers of the model system chosen for this study. Evidence of halogen bond cooperativity is seen, for example, in a 9% shortening of each NI…O=C halogen bond distance with a corresponding 53% increase in the respective critical point density value, ρNI…O=C. Cooperativity also results in a 36% increase in the magnitude of the complexation energy per halogen-bond of the trimer relative to that of the dimer. The results of this study confirm the potential for binding a single Na+ ion by either the dimer or the trimer through their respective halogen-bond networks. Binding of two metal ions was shown to be possible by the dimer. Likewise, the trimer was also found to bind three metal ions. Lastly, the overall structure of the halogen-bonded dimer or trimer endured after complexation of the Na+ ions.