Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal-Organic Nanocages

Affinities of six anions (mesylate, acetate, trifluoroacetate, p-toluenecarboxylate, p-toluenesulfonate, and perfluorooctanoate) for three related Pt²⁺-linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M₆L₃¹²⁺ (1b) and M₄L₂⁸⁺ (2) composition (M=(en)Pt²⁺, L=(3-py)₄porphyrin) were prepared in a one-pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N−H hydrogen-bond donor ability. Comparisons of isostructural hosts that differ in hydrogen-bonding ability were made between 1b and a related M₆L₃¹²⁺ nanoprism (1a, M=(tmeda)Pt²⁺) that lacks N−H groups. Considerable variation in association constants (K₁=1.6×10³ M⁻¹ to 1.3×10⁸ M⁻¹) and binding mode (exo vs. endo) were found for different host–guest combinations. Strongest binding was seen between p-toluenecarboxylate and 1b, but surprisingly, association of this guest with 1a was only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity between p-toluenecarboxylate and 1a could be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.