Exploring the Synthesis of Metalloporphyrin Equipped Pd₁₂L₂₄ Supramolecular Spheres

The range of metalation states within porphyrins provide a facile approach to tunable optical, electrochemical and catalytic properties. Spatial organization of metalloporphyrin units can yield additional emergent properties, with precise organization in Pd₁₂L₂₄ (L = Ligand) supramolecular nanosphere systems made possible by metal-mediated self-assembly. Here, we explore synthetic approaches to form porphyrin-appended Pd₁₂L₂₄ nanospheres, featuring porphyrins with different metal centers. New synthetic strategies are devised and explored en route to porphyrin-equipped nanosphere building blocks. We determine that Pd²⁺, Cu²⁺, Fe³⁺, and Zn²⁺ metalated porphyrins can withstand the nanosphere formation process, while Mg²⁺ porphyrins experience transmetalation under nanosphere formation conditions. The free base porphyrins are easily metalated by the metal precursor [Pd(MeCN)₄](BF₄)₂, required for Pd₁₂L₂₄ nanosphere formation, but free base porphyrin-containing nanospheres are readily formed under kinetic control when the precise stoichiometry is employed. With these newly developed synthetic methods, porphyrin-equipped building block compounds can form Pd₁₂L₂₄ nanosphere assemblies with porphyrins placed at both the exohedral and endohedral sites of the supramolecular construct. The cage-appended porphyrins retain metallation state dependent optical properties, although transient absorption measurements reveal much faster photophysics than molecular controls. Therefore, porphyrin-containing supramolecular nanospheres can now be explored as energy transduction systems with potential applications in artificial photosynthesis.