Anion binding properties of a hollow PdL-cage

The hollow [PdL][BAr^F]₂ complex 1 of a tetra-pyridyl (py) ligand (L) has a [Pd(py)₄]²⁺ coordination environment. Addition of coordinating anions resulted in the formation of a neutral species with Pd(py)₂(anion)₂ coordination environment (1^2A). These species bind further to the coordinating anions in the order Cl^- > N₃- > Br- > I- > AcO- with K_a1:1 < 414 M^-1. With relatively non-coordinating anions 1 remains intact and displays 1:2 binding behaviour dominated by the 1:1 stoichiometry in the order NO₃- (∼10⁵ M⁻¹) ClO₄^- and BF₄^- (∼10³ M⁻¹). As evidenced by crystal structure data, DFT calculations and {¹H-¹⁹F}-HOESY NMR with BF₄^-, the anions are bound by charge assisted [C-H]^+...center dot center dot center dot anion interactions.