MOLECULAR DESIGN OF CROWN-ETHERS .12. COMPLEXATION THERMODYNAMICS OF 12-CROWN TO 16-CROWN-4 - THERMODYNAMIC ORIGIN OF HIGH LITHIUM SELECTIVITY OF 14-CROWN-4

Calorimetric titrations have been performed in methanol at 25-degrees- C to give the complex stability constants (K(s)) and the thermodynamic parameters for the stoichiometric 1:1 complexation of lithium and sod ium ions with 12- to 16-crown-4: 1,4,7,10-tetraoxacyclododecane (12-cr own-4), 1,4,7,10-tetraoxacyclotridecane (13-crown-4), 1,4,8,11-tetraox acyclotetradecane (14-crown-4), 1,4,8,12-tetraoxacyclopentadecane (15- crown-4), and 1,5,9,13-tetraoxacyclohexadecane (16-crown-4). Possessin g a less-symmetrical structure, 14-crown-4 gave the highest K(s) for L i+ and the lowest for Na+ among the crown-4 series, eventually showing the highest cation selectivity for Li+ over Na+. The highly selective binding of Li+ by 14-crown-4 has shown to originate from the cooperat ive entropic and enthalpic contribution at this ring size, bath of whi ch may be attributable to the least conformational changes and the mos t extensive desolvation caused by the size-fitted combination of catio n and cavity diameters.