Toward Molecular Recognition: Three-Point Halogen Bonding in the Solid State and in Solution

A well-defined three-point interaction
based solely on halogen
bonding is presented. X-ray structural analyses of tridentate halogen
bond donors (halogen-based Lewis acids) with a carefully chosen triamine
illustrate the ideal geometric fit of the Lewis acidic axes of the
former with the Lewis basic centers of the latter. Titration experiments
reveal that the corresponding binding constant is about 3 orders of
magnitude higher than that with a comparable monodentate amine. Other,
less perfectly fitting multidentate amines also bind markedly weaker.
Multipoint interactions like the one presented herein are the basis
of molecular recognition, and we expect this principle to further
establish halogen bonding as a reliable tool for solution-phase applications.