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

Herein, we investigate the interplay between the heavy pnictogen bridgehead atom (E) in the tris(3-pyridyl) linkers E(3-py)₃ (E = Sb (1), Bi (2)), and meso-aryl substituents on the metalloporphyrin scaffolds MTPPX (M = Zn, Mg; TPPX = substituted tetraphenylporphyrin) with respect to capsule formation and conformational control. Coordination of 1 and 2 to para-substituted zinc porphyrins ZnTPPOMe and ZnTPPBr yielded partially encapsulated semicapsules {[E(3-py)₃]·(ZnTPPX)₂}, while MgTPPBr produced oligomeric structures, showing that relatively bulky para-substituents disfavor complete 1:3 capsule formation. In contrast, coordination of 1 and 2 to perfluorinated ZnTPPF₅ promotes the formation of full 1:3 capsules {[E(3-py)₃]·(ZnTPPF₅)₃}, stabilized by three intramolecular E···F pnictogen bonds (PnBs) that give rise to a unique "blocked" conformation. DFT calculations indicate that distal porphyrin coordination enhances Lewis acidity at E, deepening its σ-holes and strengthening E···F interactions, thus overcoming the negative cooperativity typically associated with multiple PnBs. This remote coordination effect offers a new supramolecular strategy to fine-tune σ-hole depth and Lewis acidity. The steric shielding of the bridgehead in this conformation markedly affects reactivity, as shown by the inhibition of Sb-catalyzed α-hydroxyketone oxidation. These studies illustrate the crucial role of PnBs in stabilizing capsules of this type and modulating their reactivity through conformational control.