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

## TL;DR

This study explores how substituents on porphyrin structures affect the formation and stability of capsules involving antimony and bismuth.

## Contribution

The paper introduces a new strategy for controlling capsule conformation and reactivity through pnictogen bonding and para-substitution effects.

## Key findings

- Bulky para-substituents hinder complete capsule formation, leading to semicapsules or oligomers.
- Perfluorinated porphyrins enable full 1:3 capsule formation stabilized by intramolecular pnictogen bonds.
- DFT calculations show that remote coordination enhances Lewis acidity and stabilizes capsule structures.

## Abstract

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.