Fe$^{II}$$_4L_6$ tetrahedral cage $extbf{1}$ undergoes post-assembly modification (PAM) via a Diels-Alder cycloaddition of the anthracene panels of the cage with tetracyanoethylene (TCNE). The modified cage $extbf{2}$ possesses an enclosed cavity suitable for encapsulation of the fullerene C$_{60}$, whereas original cage $extbf{1}$ forms a unique covalent adduct through a Diels-Alder cycloaddition of three of its anthracene ligands with C$_{60}$. This adduct undergoes further PAM via reaction of the remaining three ligands with TCNE, enabling the isolation of two distinct products depending on the order of addition of C$_{60}$ and TCNE. Modified cage $extbf{2}$ was also able to bind an anionic guest, [Co(C$_2$B$_9$H$_{11}$)$_2$]$^{-}$, which was not encapsulated by the original cage, demonstrating the potential of PAM for tuning the binding properties of supramolecular hosts.
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