Sb- and Bi-Based Salphen Capsules: Pnictogen Bonding, Macrocycle Formation, and ON/OFF Catalysis

Abstract

In this work, we explore the use of heavier tris(3-pyridyl) pnictogen linkers E(3-py)3 (E = Sb, Bi) with metallosalphen building blocks for the construction of supramolecular capsules and macrocycles. The incorporation of these heavier pnictogen centers has a pronounced influence on both self-assembly and reactivity. Depending on the substitution pattern of the salphen and the nature of the metal center (M = Zn, Mg), 1D coordination polymers, discrete 1:3 supramolecular capsule-like assemblies or hierarchically assembled multicapsule macrocycles can be formed. Combining Bi(3-py)3 with unsubstituted Zn-salphen gives an unprecedented polymeric architecture supported by synergistic pyridyl coordination and intermolecular Bi···O pnictogen bonding (PnB). In contrast, Zn salphens with bulky substituents suppress PnB-mediated aggregation and favor capsule formation. Replacing Zn(II) with Mg(II) in the salphen unit redirects the assembly toward heterobimetallic macrocycles {[E(3-py)3]6·(MgSal-tBu)10} derived from capsule-like subunits. Finally, we show that encapsulation of Sb and Bi within Zn-salphen capsules modulates Sb- and Bi-mediated catalytic oxidation of α-hydroxyketones and organic diols, and the reversible encapsulation of Sb(3-py)3 enables supramolecular ON/OFF control of the catalysis.