Calorimetric titration studies of adduct formation between lanthanide β-diketonates and organic bases in benzene solution

Abstract

The technique of titration calorimetry has been used to study the reactions of lanthanide shift reagents with organic substrates in benzene solution. Stability constants and enthalpies and entropies of formation have been determined for adducts of tris(2,2,6,6-tetramethylheptane-3,5-dionato)-europium(III) and -ytterbium(III) and tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyloctane-3,5-dionato)-europium(III) and -praseodymium(III) with pyridine, 2-methyl-, 3-methyl-, and 4-methyl-pyridine, 2,4,6-trimethylpyridine, dimethyl sulphoxide, and pyridine N-oxide. Adduct-formation constants mostly lie in the range 2 < log K < 4 and enthalpies of adduct formation in the range 15 < –ΔH^⊖ < 35 kJ mol^–1 with some variations which can be related to steric effects. Unusual positive entropy changes associated with the formation of adducts of the fluorinated reagents can be correlated with their unusually high solubility.