Halide-triggered assembly and selective bisulfate recognition in a quadruply interlocked coordination cage

Interlocked coordination cages are a class of multi-cavity architectures with applications in selective anion recognition, adaptive sensing, and catalysis. Controlling the partitioning of their cavities through ligand design and appropriate anion templates is critical to their guest binding scope, yet remains a challenge. Here, we present a thermodynamically stable [Pd₂L₄](BF₄)₄ cage assembled from a bis-monodentate ligand featuring a non-coordinating bis-pyrazole methane backbone. As a result of its idealized dimensions, NMR, ESI-MS, and X-ray analyses reveal that halides can trigger the interpenetration of this cage into a [X@Pd₄L₈]⁷⁺ dimer (X = Cl⁻ or Br⁻) where the halide template resides only in the central pocket. The anion-cation pattern of this interlocked host facilitates exceptional binding affinity for the bisulfate anion in its two outer pockets (up to 10⁶ M⁻¹ in MeCN), strongly outcompeting other tetrahedral anions of similar size.