Selective Guest Encapsulation by a Cobalt‐Assembled Cage Molecule

Metal‐assembled resorcinarene‐based cages enclose space and entrap organic molecules from water. Addition of cobalt( II ) ions to a neutral, aqueous solution of a resorcinarene that has iminodiacetic acids attached to its upper rim results in the formation of cages. These cages not only entrap organic molecules, but they do so in a selective manner. Guests with optimum size, shape, and polarity are preferentially entrapped. For example, selection of p ‐xylene is twenty thousand times more favorable than that of m ‐xylene. The enthalpy of resorcinarene deprotonation and cage formation was calculated by performing calorimetry studies and ranged from −305 to −348 kJ mol −1 . The change in enthalpy of guest encapsulation varied by as much as 43 kJ mol −1 . The differences in change in free energy of guest encapsulation varied by −16 kJ mol −1 . The changes in enthalpy and free energy of guest encapsulation were used to calculate the changes in entropy, which ranged from −97 to +37 J mol −1  K −1 . An enthalpy–entropy compensation of guest encapsulation was observed.