Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Cage

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, rigid capsules with small windows can efficiently protect their 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 Pd6L4 coordination cage previously reported to tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a capsule-shaped host (C) in a quantitative yield. Extracting the guest affords the 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 towards controlling functional properties of coordination cages “on demand”.