Influence of size, shape, heteroatom content and dispersive contributions on guest binding in a coordination cage.

A halide-triggered metallosupramolecular host was systematically studied for the uptake of small neutral molecules using NMR and MS experiments. Starting from benzene, cyclic guests were screened with respect to size (ring count), shape (flatness, 3D structure, substitution pattern, flexibility) and hetero atom content (number, position, donor character). 5-Rings and substituted 5/6-rings bind only weakly, while oversized (e.g. naphthalene, adamantane, ferrocene) and linear alkanes do not bind at all. Bridged 6-rings of the norbornane type and in particular DABCO bind strongly, likewise other guests with oppositely arranged hetero atoms. For the DABCO complex, a single crystal X-ray structure was obtained. The contribution of dispersive interactions to binding was derived from electronic structure calculations. Together, experimental and theoretical data deepen the understanding of guest selectivity and encapsulation driving force towards application of the host as a switchable receptor and reaction chamber.