Complexation thermodynamics of crown ethers. Part 4. Ring-enlargement effects upon cation complexation with dibenzo-18 to dibenzo-22-crown-6

Calorimetric titrations have been performed in methanol and acetonitrile at 25°C to give the complex stability constants (K) and the thermodynamic parameters for the complexation of sodium thiocyanate and potassium thiocyanate with the dibenzo-crown ethers: 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene (dibenzo-18-crown-6) (1); 2,3,12,13-dibenzo-1,4,7,10,15,18-hexaoxacycloeicosa-2,12-diene(dibenzo-20-crown-6) (2); 2,3,12,13-dibenzo-1,5,8,11,15,18-hexaoxacycloeicosa-2,12-diene) (dibenzo-20-crown-6) (3); and 3,4,14,15-dibenzo-1,6,9,12,17,20-hexaoxacyclodocosa-3,14-diene) (dibenzo-22-crown-6) (4). Data analyses assuming 1:1 stoichiometry were successfully applied to all of the crown ether-cation combinations employed. The complex stability constants, reaction enthalpies and entropies were calculated directly by using a calorimeter connected to a CA-033 microcomputer. The thermodynamic parameters obtained and the examination of CPK molecular models reveal that the less-symmetrical arrangement of donor oxygen is induced by increasing methylene in the dibenzo-18-crown-6 molecule and leads to an unfavorable conformation for complexation compared with the parent crown ether. The complex stability constants are lower than those of dibenzo-18-crown-6 (1) for the ligands (2)–(4) with Na+ and K+, but the relative cation selectivity for K + Na + is increased respectively. The effects of the molecular structure of crown ether and the cation diameter, and of solvent upon complex stability are discussed from a viewpoint of thermodynamics.