Design Rules for Sb and Bi Porphyrin Capsules: Para-Substitution Effects and Pnictogen Bond Conformational Control

Area of Science:

• Supramolecular Chemistry
• Porphyrin Chemistry
• Organometallic Chemistry

Background:

• Tris(3-pyridyl) linkers with heavy pnictogen bridgehead atoms (E = Sb, Bi) are explored.
• Metalloporphyrins (M = Zn, Mg) with substituted tetraphenylporphyrin scaffolds (TPPX) are utilized.
• Capsule formation and conformational control are key areas of investigation.

Purpose of the Study:

• To investigate the interplay between pnictogen bridgehead atoms and porphyrin substituents on capsule formation.
• To understand the role of pnictogen bonds (PnBs) in stabilizing supramolecular structures.
• To explore how conformational control influences the reactivity of these systems.

Main Methods:

• Coordination chemistry involving tris(3-pyridyl) linkers and metalloporphyrins.
• Synthesis and characterization of partially encapsulated semicapsules and full capsules.
• Density Functional Theory (DFT) calculations to analyze electronic properties and interactions.
• Assessment of catalytic activity, specifically Sb-catalyzed oxidation.

Main Results:

• Para-substituted zinc porphyrins yielded semicapsules, while bulky substituents disfavored full capsule formation.
• Perfluorinated zinc porphyrins promoted full 1:3 capsules stabilized by intramolecular E···F pnictogen bonds.
• DFT calculations revealed enhanced Lewis acidity and deepened σ-holes due to distal porphyrin coordination.
• The blocked conformation inhibited Sb-catalyzed α-hydroxyketone oxidation, demonstrating altered reactivity.

Conclusions:

• Heavy pnictogen bridgehead atoms and porphyrin substituents critically influence supramolecular assembly.
• Intramolecular pnictogen bonds are crucial for stabilizing full capsules and inducing specific conformations.
• Remote coordination effects provide a strategy to tune Lewis acidity and reactivity.
• Conformational control via PnBs offers a method to modulate catalytic activity.