Complexation Thermodynamics of p-tert-Butylcalix[4]arene Derivatives with Light Lanthanoid Nitrates in Acetonitrile

• Published in: Supramolecular chemistry 2001-07, Vol.13 (4), p.529-537
• Main Authors: Liu, Yu, Li, Shang, Bai, Xiao-Peng, Wada, Takehiko, Inoue, Yoshihisa
• Format: Article
• Language: English
• Subjects: Calixarene derivatives; Calorimetric titration; Complexation thermodynamics; Ions selectivities; Light lanthanoid cations
• ISSN: 1061-0278; 1029-0478
• DOI: 10.1080/10610270108028299

Calorimetric titrations have been performed for the first time in anhydrous acetonitrile at 25°C to give the complex stability constant (K S ) and thermodynamic quantities for the complexation of light lanthanoid(III) nitrates (La-Gd) with 5,11,17,23-tetra-tert-butyl-26,28-bis(cyanomethoxy)-25,27-dihydroxy-calix[4]arene (1) and 5,11,17,23-tetra-tert-butyl-26, 28-bis(2-aminoethoxy)-25,27-dihydroxycalix[4]arene (2). X-ray crystallographic structures of 1 and 2 were also determined and compared. Possessing the cyanomethoxy and aminoethyl substituents, 1 and 2 displayed strikingly different cation binding abilities and selectivity profiles with much higher K S values for La 3+ and Ce 3+ , which may be related to the original structures in the solid state. Thus, the binding profile for 2 showed a rapid decrease in K S with decreasing ionic diameter from La 3+ to Pr 3+ and then became flat up to Gd 3+ , while 1 gave a very flat profile which is superimposable with that for 2 between Pr 3+ -Gd 3+ . Thermodynamically, the complexation is driven absolutely by enthalpy which compensates the entropic loss arising from the structural freezing of the calix[4]arene derivatives upon simultaneous binding of lanthanoid ion by the phenolic oxygen and acetonitrile molecule in the cavity. The general validity and the meaning of the compensative enthalpy-entropy relationship observed are discussed.