Lanthanide ion complexes of deprotonated p-isopropylcalix(n)arenes in dipolar aprotic solvents

The binding of lanthanide(III) cations with p-isopropylcalix[n]arenes (n = 4, 6, 8) after partial deprotonation has been investigated using UV-Vis absorption spectrophotometry. The ease and degree of deprotonation of the calixarenes closely parallels the behaviour of their t-butyl analogues in the same solvents. The conditional stability constants of the praseodymium, europium, ytterbium and yttrium complexes were determined in DMF and in acetonitrile. The effect of water on the stability of lanthanide complexes in DMF was also treated. The results show that the cation binding selectivity, stoichiometry and stability of the complexes depend on various factors such as the degree of condensation (n) and the consequent number of O-donor atoms as well as the nature of the cation solvation. The stability of the Y(III), Pr(III), Eu(III) and Yb(III) complexes in DMF increased in all cases with increase of the ring size. DFT calculations were used to estimate the influence of the solvent on both the acidity and the conformations of the calixarene anions.