Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host.

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd 6 L 4 coordination host previously reported to exist exclusively as a tube isomer ( T ). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host ( C ) in quantitative yield. Extracting the guest affords empty C , which is metastable and spontaneously relaxes to T , and the T ⇄ C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".