MOLECULAR DESIGN OF CROWN-ETHERS - 16 - CALORIMETRIC TITRATION OF COMPLEXATION OF LIGHT LANTHANOID NITRATES WITH N-BENZYLAZA-16-CROWN-5 AND15,15-DIMETHYL-16-CROWN-5 IN ACETONITRILE - ENHANCED SELECTIVITY FOR PR3+

Calorimetric titrations have been performed in anhydrous acetonitrile at 25 degrees C to give the complex stability constants (K-s) and the thermodynamic parameters (Delta G degrees, Delta H degrees, and Delta S degrees) for the stoichiometric 1:1 complexation of light lanthanoid (III) nitrates (La-Gd) with N-benzylaza-16-crown-5 (1) and 15,15-dimet hyl-16-crown-5 (3). Using the present and previous data, the effects o f substitution and ring enlargement of crown-5 derivatives (1-4) upon complexation behavior are discussed comparatively from the thermodynam ic point of view. Possessing a less-symmetrical skeleton and the same N-substitution as compared with N-benzylaza-15-crown-5 (2), N-benzylaz a-16-crown-5 (1) gave lower complex stabilities by 1-2 orders of magni tude for all light lanthanoids examined, but exhibited significantly a ltered relative cation selectivity with much enhanced preference up to 8 for Pr3+ over the other light lanthanoids. The lower complex stabil ities for 3 than for the parent 16-crown-5 4 are attributed to the red uced enthalpic gains caused by the steric hindrance between the axial methyl group introduced at C-15 and the accommodated cation, but the m ethyl substitution appears to help lock the ring conformation such tha t complexation can occur, as indicated by the positive entropic change s for most light lanthanoids. These results indicate that the introduc tion of a nondonating sidearm to C- and N-pivot 16-crown-5 alters Sign ificantly not only the binding constant but also the relative cation s electivity.