Self-assembly and anion encapsulation properties of cavitand-based coordination cages

Two novel classes of cavitand-based coordination cages 7a-j and 8a-d have been synthesized via self-assembly procedures. The main factors controlling cage self-assembly (CSA) have been identified in (i) a P-M-P angle close to 90 degrees between the chelating ligand and the metal precursor, (ii) Pd and Pt as metal centers, (iii) a weakly coordinated counterion, and (iv) preorganization of the tetradentate cavitand ligand. Calorimetric measurements and dynamic H-1 and F-19 NMR experiments indicated that CSA is entropy driven. The temperature range of the equilibrium cage-oligomers is determined by the level of preorganization of the cavitand component. The crystal structure of cage 7d revealed the presence of a single triflate anion encapsulated. Guest competition experiments revealed that the encapsulation preference of cages 7b,d follows the order BF4- > CF3SO3_ >> PF6- at 300 K. ES-MS experiments coupled to molecular modeling provided a rationale for the observed encapsulation selectivities. The basic selectivity pattern, which follows the solvation enthalpy of the guests, is altered by size and shape of the cavity, allowing the entrance of an ancillary solvent molecule only in the case of BF4-.