A THERMODYNAMIC STUDY OF ENANTIOMERIC RECOGNITION OF ORGANIC AMMONIUMCATIONS BY PYRIDINO-18-CROWN-6 TYPE LIGANDS IN METHANOL AND A 1 1-METHANOL-1,2-DICHLOROETHANE MIXTURE AT 25.0-DEGREES-C/

Log K, DELTAH, and T DELTA S values for interactions of (R) and (S) en antiomers of alpha-(1-naphthyl)ethylammonium perchlorate (NapEt), alph a-phenylethylammonium perchlorate (PhEt), and the hydrogen perchlorate salt of 2-amino-2-phenylethanol (PhEtOH) with a series of chiral and achiral pyridino-18-crown-6 type ligands and 18-crown-6 (18C6) were de termined from calorimetric titration data valid in methanol and in a 1 : 1 (v/v) methanol-1,2-dichloroethane (MeOH-1,2-DCE) mixture at 25.0- degrees-C. In the MeOH-1,2-DCE solvent mixture, the chiral macrocyclic ligands exhibit excellent recognition for enantiomers of the three or ganic ammonium cations as shown by large differences in log K values ( DELTA log K) which range from 0.4 to 0.6 (2.5- to 4.0-fold difference in binding constants). The DELTA log K values in the solvent mixture M eOH-1,2-DCE are increased by 0.1-0.5 log K units over those in absolut e methanol, indicating a favorable effect of 1,2-dichloroethane on ena ntiomeric recognition. In 1,2-dichloroethane, however, the interaction s are too strong (log K > 6) to observe the degree of recognition by a direct calorimetric method. Complexation of organic ammonium cations by these macrocyclic ligands is driven by favorable enthalpy changes. The entropy changes are unfavorable in all cases. The thermodynamic or igin of enantiomeric recognition for NapEt in 1 : 1 (v/v) MeOH-1,2-DCE is enthalpic, but those for PhEt and PhEtOH are entropic. Effects of the ligand structure and flexibility and of the organic cation structu re on recognition and complex stability are discussed on the basis of the thermodynamic quantities. Different thermodynamic behaviors of ach iral 5 and 18C6 from those of chiral macrocyclic ligands indicate a di fference between chiral and achiral macrocycle interactions with the c hiral organic ammonium cations. The different thermodynamic behavior o f NapEt enantiomers compared to those of PhEt and PhEtOH enantiomers s upports the idea that the solution complex conformation of NapEt is di fferent from those of PhEt and PhEtOH. pi-pi interaction is absent for the PhEt and PhEtOH complexes with diesterpyridino-18-crown-6 ligands in solution. Therefore, the higher degree of enantiomeric recognition for NapEt than for either PhEt or PhEtOH by these chiral macrocyclic ligands is a result of the presence of pi-pi interaction in the NapEt system.