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

Abstract

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.