σ-Hole Engineering Enables Selective Supramolecular Isomer Formation in Sb(III) Hosts.

Pnictogen bonding (PnB) has emerged as a directional and tunable noncovalent interaction, yet the presence of multiple σ-holes on trivalent pnictogen centers introduces a question: can individual σ-holes be selectively addressed for binding, and do their relative reactivities remain unchanged upon sequential binding? This work demonstrates that nonsymmetric Sb(III) catecholate hosts exhibit intrinsic selectivity among σ-holes that is dependent on the identity of the directly bonded substituents. Structural studies with polydentate guests reveal distinct and reproducible supramolecular isomers with chloro-substituted hosts favoring a mixed σO/σCl pnictogen bonding motif, while phenyl-substituted hosts favor σO/σO. Density functional theory calculations reproduce these preferences but show that the electrostatic potential maxima of the isolated host fail to predict the observed selectivity. Instead, initial binding at the site with the highest electrostatic potential leads to a redistribution of electron density that reshapes the remaining σ-hole landscape, leading to an inversion in the relative strengths of the remaining σ-holes in the case of the chloro-substituted hosts, consistent with asymmetric negative cooperativity. These findings establish that σ-hole reactivity in pnictogen-bonding systems evolves in response to guest binding, providing a framework for selectively addressable and adaptive supramolecular hosts.