Effects of a triangular nanocage structure on the binding of neutral and anionic ligands to Co^II and Zn^II porphyrins

Interactions of neutral pyridine ligands and anionic benzoate ligands with the metalloporphyrin walls of M6L3 nanocages (M = [(tmeda)Pt]2+, L = tetra(3-pyridyl)porphyrin-M′, M′ = CoII, Co3-1; ZnII, Zn3-1) were evaluated in CD3CN. The binding of ligands to simple tetra(N-methyl-3-pyridinium)porphyrin-M′ complexes Co-2 and Zn-2 was also evaluated. It was found that pyridine ligands bind with generally similar strength to the monomeric porphyrins as to the outer faces of the corresponding cage. Coordination is enhanced inside the cages for the binding of one equivalent of pyridine, 3-methylpyridine, or 3,5-dimethylpyridine. In contrast, 4-(4′-methoxyphenyl)pyridine cannot bind inside the cages, and 4-methylpyridine shows weaker binding inside than outside. Steric effects play a complex role in influencing ligand binding since 4-tert-butylpyridine binds more strongly in the cages than 4-methylpyridine. These latter two ligands also reveal a subtle allosteric effect in which binding of ligands inside Zn3-1 becomes increasingly favored over external binding once some of the external sites are occupied. It was confirmed that benzoate and 3,5-di-tert-butylbenzoate can also coordinate to the walls of M3-1, but quantitative analysis of these interactions was complicated by competing affinity of the anions for other sites of the cages, limiting detailed insight.