Anion-templated aqueous self-assembly resulted in the formation of an endohedrally functionalized Fe$\mathrm{II}$$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$L$4$ tetrahedron from azaphosphatrane-based subcomponents. This new water-soluble cage is flexible and able to encapsulate anions with volumes ranging from 35 to 219 Å$3$ via hydrogen bonding and electrostatic interactions. It structurally adapts in response to the size and shape of the template anions, dynamically adopting a conformation either where all four azaphosphatrane $\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$P-H vectors point inward, or else where one points outward and the other three inward. The two cage isomers can coexist in solution and interconvert. A shape memory phenomenon was observed during guest displacement because guest exchange occurs more rapidly than structural reconfiguration.