Confinement Alters Amine Chemistry

Amine groups confined in nanosized cages exhibit chemical behavior distinct from their unconfined counterparts, according to Northwestern University chemical engineers (J. Am. Chem. Soc., DOI: 10.1021/ja806179j). Harold H. Kung, Mayfair C. Kung, Juan D. Henao, and coworkers prepared porous siloxanes containing 2-nm-diameter cavities in which about eight aminopropyl groups are tethered to the interior surfaces. The team proposed that in near-neutral solutions repulsive electrostatic interactions in the confined space would shift the amine groups' affinity for protons and limit protonation to only one of the eight amine groups. In contrast, about half of the free-floating amine groups in solution would be protonated. The researchers tested this hypothesis by probing the way AuCl₄– binds to the amino groups inside the cavity—the binding mode to gold depends on the amines' protonation state. On the basis of spectroscopy studies, the group concludes that even in acidic solution the confined amines form gold complexes that are characteristic of unprotonated amines. The shift in proton affinity thus leaves a large fraction of neutral amines in the cavity available to mediate base-catalyzed reactions even in neutral or acidic media, the team points out.