Tunable Half-Sandwich Iridium Coordination Cages for PAH Guest Encapsulation

Gaining control over the confinement dimensions and nature is key for a rational design of cages for specific guest molecules. In this work we synthesized three new iridium coordination cages enabling highly tunable cavities by systematically expanding the ligands. These cages have been fully characterized by the means of NMR spectroscopy, DOSY, HR-ESI-MS, and SCXRD. Density functional simulations indicate that the size expansion of the linkers leads to structurally analogous cages with a systematic variation of cavity size and shape. The large π-systems of the ligands enable guest binding of several polycyclic aromatic hydrocarbons (PAH), including the well-known carcinogen Benzo[a]pyrene. ¹H NMR spectroscopic host-guest titrations and mass spectrometric experiments were carried out and revealed a 1:3 binding with pyrene. Density functional calculations further proved the 1:3 binding stoichiometry of the adducts, consistent with the symmetry of the cages. The corresponding binding constants decrease with expanding cage size, and therefore decreasing the packing coefficient, providing an example for Rebek's rule of 55%.