COMPLEXATION THERMODYNAMICS OF CROWN-ETHERS .4. RING-ENLARGEMENT EFFECTS UPON CATION COMPLEXATION WITH DIBENZO-18 TO DIBENZO-22-CROWN-6

Calorimetric titrations have been performed in methanol and acetonitri le at 25 degrees C to give the complex stability constants (K) and the thermodynamic parameters for the complexation of sodium thiocyanate a nd potassium thiocyanate with the dibenzo-crown ethers: nzo-1,4,7,10,1 3,16-hexaoxacyclooctadeca-2,11-diene (dibenzo-18-crown-6) (1); zo-1,4, 7,10,15,18-hexaoxacycloeicosa-(2),12-diene) (dibenzo-20-crown-6) (2); enzo-1,5,8,11,15,18-hexaoxacycloeicosa-2,12-diene) (dibenzo-20-crown-6 ) (3); and enzo-1,6,9,12,17,20-hexaoxacyclodocosa-3,14-diene) (dibenzo -22-crown-6) (4). Data analyses assuming 1:1 stoichiometry were succes sfully applied to all of the crown ether-cation combinations employed. The complex stability constants, reaction enthalpies and entropies we re calculated directly by using a calorimeter connected to a CA-033 mi crocomputer. 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-c rown-6 molecule and leads to an unfavorable conformation for complexat ion compared with the parent crown ether. The complex stability consta nts are lower than those of dibenzo-18-crown-6 (1) for the ligands (2) -(4) with Na+ K+, but the relative cation selectivity for K+/Na+ is in creased respectively. The effects of the molecular structure of crown ether and the cation diameter, and of solvent upon complex stability a re discussed from a viewpoint of thermodynamics.